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H2 BIOLOGY
CORE SYLLABUS II

Topics covered:
1
...
Diversity and evolution

2
...
Cell signaling

3
...
Nervous system

4
...
Principle of Segregation – based on Mendel’s work involving monohybrid crosses
2
...

Heterozygotes (CRCW) only has one copy of the allele per cell, thus produces inadequate enzymes
to synthesize red pigment
...

Codominance:
Both alleles are equally expressed in the phenotype of the heterozygote
...
Hence both products are present in
the phenotype of the heterozygote

Lethal genes:
Sometimes a gene coding for a certain character may be linked to a lethal gene
...
However, one gene only carries two
alleles, but there are more than 2 forms of the gene
...

• Since human is a diploid organism, only two of the three alleles can be present in an individual
...
Father with a recessive X-linked allele will
transmits allele to all daughters, but not sons
2
...
Females who are heterozygous are carriers
4
...
A reciprocal cross is a pair of crosses in which the character of the two
parents are reversed
Comments:
• Male transmits X
chromosome only to
female offspring
• Female transmits X
chromosome to both
offspring
• As male does not
transmit
X
chromosome to his
sons, he does not
contribute
to
the
appearance of their Xlinked phenotypes

If same results are obtained, the character is carried on an autosomal chromosome
If different results are obtained, the character is carried on a sex- chromosome

PEDIGREE ANALYSIS
A pedigree chart is a diagram of a family tree over several generations showing their relationships
and presence or absence of the trait in the members
...
Identify the affected offspring
2
...
Next check for whether x-linked or autosomal
2nd filter (x-linked?)
• X-linked dominant: All female offspring affected, all male offspring not affected
• X-linked recessive: All sons from affected mothers affected; if fathers not affected, daughters
definitely not affected
LINKAGE – MENDEL’S LAW DOESN’T APPLY
Two genes are linked when they are on the same chromosome and unlinked when they are situated
on different pairs of homologous chromosomes
When two gene loci are linked, the ratios of 9:3:3:1 and 1:1:1:1 will not be observed
...
Find out the relative distance between linked genes on a chromosome
2
...
5 or lower) – aluminum assumes a form easily absorbed by plant roots,
thus flowers are predominantly blue
• In alkaline soils (pH 6
...

Commonly the various gene products function in a metabolic pathway that contributes to the
development of one particular phenotype

Non-epistatic gene interactions:
Two independently assorting genes may interact to influence a single character
Although the basic ratio is the same in dihybrid inheritance involving unlinked genes, the gene
interaction below and dihybrid inheritance is different
• Dihybrid inheritance – studies two characteristics controlled by two genes
• Gene interaction – studies one characteristic controlled by two or more genes
Example (comb shape in chicken)
• P gene locus (when dominant R is absent) – dominant P produces pea-shaped comb and
recessive p results in single comb
• R gene locus (when dominant P is absent) – dominant R produces rose comb and recessive r
results in single comb
• When both dominant alleles are present, a walnut comb is observed
• The ratio 9 walnut comb : 3 pea comb : 3 rose comb : 1 single comb is observed
Biochemical basis of comb shapes in chicken
Different combinations of alleles from the two genes results in different phenotypes of a single
character, due to interaction of their gene products, each of which contributes to comb shape at the
biochemical or cellular level

EPISTATIC GENE INTERACTIONS
Epistasis is inferred when the expression of an allele of one gene suppress/inhibit the expression of
alleles of a different gene
• An epistatic gene is one which is able to suppress/inhibit the effect of a gene at a different locus
– it can either be recessive or dominant in their effects
• The suppressed gene is termed the hypostatic gene
Recessive epistasis:
Two recessive alleles at the epistatic gene locus will suppress/inhibit the effect of either allele of the
hypostatic gene at a different locus
Example (coat colour in Labrador retrievers)
• Labrador retrievers may either be black, brown or yellow and their different coat colours are
determined by interactions between genes at two loci
Gene locus
E: pigment deposition
B: pigment coloration

Dominant
Recessive
E allele allows deposition of dark e allele prevents deposition of
pigment (black or brown)
dark pigment, causing yellow hair
B allele codes for black pigment
b allele codes for brown pigment

Biochemical basis of coat colour in Labrador retrievers

Recessive epistasis shows 9 black : 3 brown : 4 white ratio in double heterozygous cross
Dominant epistasis:
One dominant allele at the epistatic gene locus will suppress/inhibit the effect of both alleles of the
hypostatic gene at a different locus
Example (fruit colour in summer squash)
• Fruit colour of summer squash comes in 3 colours: yellow, white or green
Gene locus
W: inhibition of
pigment production
Y: pigment production

Dominant
W allele codes for inhibitor for
pigment production
Y allele codes for yellow pigment
production

Recessive
w allele allows for pigment
production
y allele codes for green pigment
production

Biochemical basis of fruit colour in summer squash

Genotype

wwY_
wwyy
W_Y_
W_yy

Presence /
absence of
precursor
+
+
+
+

Inhibition
of
Ability to
Presence /
conversion
convert
absence of
of precursor
intermediate
intermediate
to
product
intermediate
No
+
Yes
No
+
No
Yes
Yes
Yes
No

Presence /
absence of
product

Phenotype

+
-

Yellow
Green
White
White

Dominant epistasis shows 12 white : 3 yellow : 1 green ratio with a double heterozygous cross
Duplicative recessive epistasis:
Sweet pea flowers are either purple (with anthocyanin pigment) or white (without pigment)
• Each dominant allele encodes an enzyme that controls a step in the synthesis of anthocyanin
from a biochemical precursor
• If the dominant allele is absent, its step in the biosynthetic pathway is blocked and anthocyanin
will not be produced
Biochemical basis of flower colour of sweet pea flowers

Genotype
C_P_
ccP_
C_pp
ccpp

Presence / absence
of precursor
+
+
+
+

Presence / absence
of intermediate
+
+
-

Presence / absence
of anthocyanin
+
-

Phenotype

Duplicative recessive epistasis shows 9 purple : 7 white ratio with a dihybrid cross

Purple
White
White
White

Summary of modified F2 Mendelian dihybrid ratios:
Gene A and B are two independently assorting gene loci, gene A is epistatic over gene B
Phenotypic
ratio
A_B_
9:3:3:1

9:3:4

Genotype
A_bb
aaB_

9

3

9

aabb

3

3

1

Type of interaction
Non-epistatic
Recessive epistasis in
double heterozygous cross
(a epistatic to B and b)

4

aa encodes non-functional
gene product
Dominant epistasis in
double heterozygous cross
(A epistatic to B and b)
12:3:1

12

3

1
A encodes functional gene
product acting as an
inhibitor
Duplicative recessive
epistasis in dihybrid cross
(a epistatic to B and b;
b epistatic to A and a)

9:7

9

7

aa encodes non-functional
gene product
bb encodes non-functional
gene product

Example
Comb shape in
chicken
Coat colour in
Labrador
retrievers
Coat colour in
mice

Colour in
summer squash

Flower colour in
sweet pea

VARIATION
Describes the recognizable differences in characteristics between organisms of the same natural
population or species
• There are two main forms of variation – discontinuous (qualitative) & continuous (quantitative)
• Studies of the variation involve measuring the expression of that characteristic in a large number
of organisms within the population
Discontinuous variation vs
...
of genes Variation controlled by a single or few Controlled by combined effect of
controlling
genes (may have two or more alleles)
multiple additive genes and known as
phenotypic
polygenic inheritance
variation
Genes act on phenotype in an additive
manner – thus combined effect can
produce individuals with infinite
phenotypic varieties
Effect
of There is little or no environmental Phenotypes can be modified by
environment effects on phenotypic expression of the cumulative
effects
of
varying
on phenotype gene
environmental factors acting on
different genotypes
Degree of expression allowed to this
genetic
potential
hinges
on
environmental
factors
during
development of organism
Mode
of Phenotypic measurements normally Phenotypic measurements form a
phenotypic
represented on bar graphs – phenotypic normal distribution curve – represented
measurement classes are distinct
on histograms

Examples

Qualitative: analyzed through counts Quantitative: statistical analyses give
and ratios
estimates of population parameters
Blood type
Height, weight, intelligence in humans

Causes of genetic variations in a population:
In asexually reproducing organisms
• DNA replication is highly accurate and almost error free – little possibility of variation in
genotype
• Any apparent variation is almost always the result of environmental influences
In sexually reproducing organisms
• There is ample opportunities for genetic variation to arise
• Crossing over between non-sister chromatids of homologous chromosomes during prophase I of
meiosis
• Independent assortment of bivalents at the metaphase plate during metaphase I of meiosis
• Random fertilization
• The above 3 merely lead to new combination of alleles – which is the basis of continuous
variation
• A mutation is a change in structure or number of chromosomes (chromosomal mutation) or in
the structure of a gene (gene mutation) and only those occurring during formation og gametes
can be inherited
CHI-SQUARE (χ2) TEST
Introduction:
The chi-square (χ2) test is a statistical test for the significance of data that consists of discontinuous /
discrete variables
• When observed phenotypic classes deviate from expected ratios, there is a need to test whether
this deviation or difference is due to chance or due to incorrect prediction of expected ratio
• The larger the difference between observed and expected results, the more likely that the
hypothesis is incorrect and hence, is not due to chance alone
• The smaller the difference between observed and expected results, the more likely that the
hypothesis is correct and hence, is due to chance alone
• The calculate χ2 value is compared against a chi-square distribution table to assess if the
difference between observed and expected results is statistically significant

Steps to solve chi-square (χ2) test questions:

Sample example:

~ END ~

Chapter 3: Photosynthesis
INTRODUCTION
Photosynthesis essentially involves trapping light energy, converting it to chemical energy and
synthesizing an organic compound (CO2)

Adenosine triphosphate (ATP):
All forms of life temporarily store energy in the chemical bonds (phosphoanhydride bond) of
adenosine triphosphate (ATP)
This source of energy is quickly released to the organism by a hydrolytic reaction that breaks the
phosphoanhydride bond, converting ATP to ADP, releasing large amounts of energy (30
...
Photophosphorylation – utilizing light energy in photosynthesis
2
...
Oxidative phosphorylation – occurs in final stage of aerobic respiration with oxygen, releasing
energy trapped in ATP

The chloroplast:
Site where photosynthesis occurs
Structure & Function
Shape
Chloroplast envelope
• Controls movement of water, CO2 and sugar
Stroma
• Receives products of light dependent reaction
• Site of Calvin cycle (fixation of CO2)
Thylakoids
• Site of light dependent reactions
Granum
• Produces ATP and NADPH

Description
Lens-shaped
Double membrane – outer membrane
selectively permeable but inner membrane
highly impermeable
Gel-like matrix, contains circular DNA, 70S
ribosomes, starch granules, enzymes
Photosynthetic pigments and electron carriers
embedded in thylakoid membrane
Stack of thylakoids, increase surface area and
amount of pigment for light dependent reaction

Photosynthetic pigments:
There are two basic classes of photosynthetic pigments in plants – chlorophyll (main photosynthetic
and most abundant pigment) and carotenoids (type of accessory pigment)
Most important pigment is chlorophyll a
Chlorophyll absorbs mainly red (650-700) and blue-violet (400-520) light, and reflects green
• Only chlorophyll a can participate directly in the light-dependent reaction, converting light
energy to chemical energy
• Other pigments can absorb light and transfer the energy to chlorophyll a
• Absorption spectrum – is a graph of the amount of light absorbed at different wavelengths
• Action spectrum – is a graph of the effectiveness of different wavelengths of light in driving
photosynthesis

LIGHT HARVESTING STAGE
The plant uses a mixture of pigments – chlorophyll, to capture light energy
Excitation of chlorophyll by light:
When a molecule of chlorophyll absorbs light, it becomes excited and this energy is used to boost
electrons to a higher energy level
An excited electron is unstable and returns to its ground state in two main ways:
1
...
By transferring the high-energy electron – to another nearby molecule, an electron acceptor,
and then returning back to its original state by taking up a low-energy electron from another
molecule, an electron donor (occurs in light dependent stage)
Usually water serves as a weak electron donor – when it is oxidized, it will release oxygen along
with two protons
Photosystems: Present in the thylakoid membranes
Photosynthetic pigments are arranged into photosystems, and they catalyze the conversion of light
energy captured by excited chlorophyll molecules to useful forms
...
Light harvesting complexes
• Light is collected by the 200-300 pigment molecules bound to light-harvesting protein
complexes in thylakoid membrane
• They absorb light and transfer the light energy to the reaction center
2
...
Primary electron acceptor
• Found in reaction center, involved in electron transfer (ETC)

ELECTRON TRANSPORT CHAIN
Electron carriers play an important role in redox reactions by transferring electrons from one
molecule to another
• Electron carriers can be coenzymes or protein molecules
• In the ETC, electrons are passed down carriers by a series of redox reactions

•
•
•

Each carrier molecule receives an electron (reduction), and in turn donates it (oxidation) to the
next carrier down the chain
An ETC allows transfer of electrons to be done in several energy-releasing steps instead of 1
An electron thus progressively loses energy down the chain

Coenzyme electron carriers include
• FAD, flavin adenine dinucleotide
• NAD, nicotinamide adenine dinucleotide
• NADP, nicotinamide adenine dinucleotide phosphate
These coenzymes can pick up a pair of electrons and a photon and thus reduced to FADH2, NADH
and NADPH
LIGHT DEPENDENT REACTION
The role of this stage is to synthesize reduced NADPH and ATP using captured light energy from
the light-harvesting stage – where they are used in the light-independent reaction to fix CO2 and
finally trap energy in the form of glucose
There are two ways by which the light dependent reaction can proceed
1
...
Cyclic photophosphorylation
• PSI can act alone to build the chemiosmotic gradient for ATP synthesis
• Occurs when Calvin cycle occurs too slowly to regenerate NADP

Non-cyclic photophosphorylation:
1) A photon of light strikes a pigment molecule in a light harvesting complex and energy is relayed
via resonance energy transfer until it reaches one of the two special chlorophyll molecules in the
PSII reaction center, exciting one of the P680 electrons to a higher energy state
2) Photoexcited electron is captured by the primary electron acceptor in the reaction center,
resulting in a deficit of electrons in the reaction center
3) Photolysis of a water molecule into 2 electrons, 2 H+ and one oxygen atom
...
The oxygen atom
then combines with another oxygen atom to release O2 as a by-product
4) Energized electron passes from PSII to PSI via a first ETC from plastoquine to cytochrome (b-f)
complex then to plastocyanin through redox reactions
5) The electron flow releases energy, used to generate proton gradient across thylakoid membrane
driving ATP synthesis (known as photophosphorylation) by driving H+ across the membrane via
active transport
6) Light energy strikes a pigment molecule in the light-harvesting complex of PSI, exciting an
electron of one of the two special chlorophyll a in the PSI reaction center
...
Two electrons are required
for its reduction to NADPH

Cyclic photophosphorylation: No NADPH is produced
1) Light absorbed by the light-harvesting complex of PSI is passed on to a chlorophyll a (P700)
molecule in the reaction center of PSI
• Causes chlorophyll a molecule to emit an energized electron which is raised to a higher
energy level and picked up by the primary electron acceptor
2) Energized electron is passed to FD, cycled back to cytochrome (b-f) complex on the first ETC
and back to PSI
3) As these electrons are passed along the first ETC, enough energy is released to synthesize ATP

Chemiosmosis:
Electrons flow along the ETC from PSII to PSI
• Energy is released to pump the H+ ions against their concentration gradient from the stroma,
across the thylakoid membrane, into the thylakoid space/lumen
• Also photolysis of water produces H+ ions, contributing to the proton concentration in the
thylakoid space
• The thylakoid membrane is impermeable to H+, thus as light reaction proceeds, there is
accumulation of H+ in the thylakoid space, setting up a proton gradient
• The H+ ions diffuse down the gradient from the thylakoid space across the thylakoid membrane
into the stroma through the ATP synthase complex
• Drives the formation of ATP catalyzed by ATP synthase one ATP for every 2 H+

LIGHT INDEPENDENT REACTION

The Calvin Cycle:
The reaction occurs in the stroma of chloroplasts and its purpose is to reduce CO2 using ATP and
NADPH, produced in the light-dependent reaction as the energy source and reducing power
The reaction of the Calvin cycle can be classified into 4 phases
1
...
Reduction
3
...
Product synthesis and sugar formation
For net synthesis of 1 G3P
CO2
ATP
NADPH

Number of moles used
3
6+3
6

Phase
1
2,3
2

Phases involved in the Calvin Cycle:
CO2 uptake and fixation
• CO2 diffuses through the stomata and into the cytoplasm of the plant cells and into the
chloroplast
• CO2 is fixed when it combines with a 5C CO2 acceptor, ribulose bisphosphate (RuBP) to form
an unstable 6C intermediate, catalyzed by Rubisco
• The unstable 6C intermediate breaks down into two molecules of 3C compound called
phosphoglyceric acid (PGA) / glycerate-3-phosphate (GP) / 3-phosphoglycerate
Reduction and sugar formation
• Each molecule of GP is phosphorylated by ATP forming 1,3-biphosphateglycerate
• A pair of electrons donated from NADPH further reduces 1,3-biphosphateglycerate to form
glyceraldehyde-3-phosphate (GALP or G3P) / triose phosphate (TP)
Regeneration of CO2 acceptor
• For every 3 molecules of CO2 that enter the Calvin cycle, 3 molecules of RuBP are carboxylated
and 6 molecules of TP are formed
• Only one molecule of TP can be counted as net gain as the other 5 are used to regenerate the 3
molecules of RuBP
• Hence 3 more molecules of ATP are used
• RuBP is regenerated and the Calvin cycle continues
Product synthesis
• TP becomes starting material for metabolic pathways that synthesize other organic compounds
• 2 molecules of TP are used to produce 1 molecule of hexose sugar, thus formation of one
molecule of hexose sugar requires 6 turns of Calvin cycle
FATE OF PHOTOSYNTHETIC PRODUCTS
Although TP is the end product of light-independent reaction, it does not accumulate in large
quantities, as both GP and TP are also intermediates in glycolysis
Synthesis of carbohydrates:
Large proportion of TP is converted to hexose sugar (glucose and fructose), which are respiratory
substrates for ATP production
...
Glycerol is made from TP
...
Plants then use these molecules to make other nitrogen-containing
compounds, such as nucleotides for DNA and RNA synthesis

FACTORS AFFECTING PHOTOSYNTHESIS
The rate of photosynthesis is affected by many factors – which determines the yield of material
produced by the plant
The principle of limiting factors states that
• The rate of a biochemical process, which consists of a series of reactions, is limited by the
slowest reaction in the series
• When it is affected by several factors, its rate is limited by the factor that is nearest its minimum
value – it directly affects the biochemical process if its quantity is changed
Light intensity:
At low light intensity, rate of photosynthesis increases linearly with increasing light intensity until it
reaches light saturation point
• Gradually rate of increase falls off as other factors become limiting
• Illumination on a clear summer’s day is 100,000 lux, but light saturation point is 10,000 lux –
thus light intensity is usually not a limiting factor
• Very high light intensity may damage the chlorophyll and decrease rate of photosynthesis
• The compensation point is point where rate of photosynthesis is equal to the rate of respiration –
there is no net gaseous exchange
Below compensation point, rate of P < rate of R: CO2 is evolved and O2 is taken in
Above compensation point, rate of P > rate of R: CO2 is taken in and O2 is evolved

Sun and shade plants:
Some plants are adapted to positions of full sunlight and others to permanently shaded positions –
these are known as sun and shade plants respectively
Shade plants have lower rate of respiration than sun plants
• They have thinner leaves, less palisade mesophyll layers
• Smaller number of cells require less energy for maintenance
• Shade plants reach compensation point at lower light intensity and much quickly as well

Sun plant
Multiple layers of canopy
Thick leaves with several cell layers
Small chloroplasts
High PSI/PSII ratio
High maximum photosynthetic rate
High light saturation point
High light compensation point
High respiration rate

Shade plant
Single layer canopy
Thin leaves with few cell layers
Large chloroplasts
Low PSI/PSII ratio
Low maximum photosynthetic rate
Low light saturation point
Low light compensation point
Low respiration rate

Light wavelength:
Red light has less energy than blue light
...
03%)
• Greenhouse crops like tomatoes are grown in CO2 enriched environment for greater yield

Temperature:
The light-independent reaction, and to a certain extent, light-dependent reaction are enzymecontrolled, and thus temperature sensitive
• For temperate plants, the optimum temperature is about 25oC
• Rate of reaction doubles for every 10oC increase up to about 35oC – as temperature increases,
rate of reaction increases as molecules involved move more quickly and have greater chance of
colliding
• Rate of photosynthesis decreases at higher temperatures as enzymes start to denature
Chlorophyll concentration:
Chlorophyll concentration is normally not a limiting factor, but reduction in chlorophyll levels can
be induced by several factors
• Diseases such as mildew, rusts and viral diseases
• Mineral deficiency (iron, magnesium and nitrogen are needed during chlorophyll synthesis –
that latter two elements are part of chlorophyll structure)
• Normal ageing process (senescence)
• Lack of light as light is needed for final stage of chlorophyll synthesis
Specific inhibitors:
Two main examples where photosynthesis is inhibited
• DCMU (dichlorophenyl dimethyl urea) that short-circuits non-cyclic electron flow and thus
inhibit light-dependent reactions
• Cyanide that disrupts photosynthesis by disrupting bond interactions between R-groups of
proteins in ETC
Water:
Periods of temporary wilting can lead to severe losses in crop yield
• Plants usually close their stomata in response to wilting, and this prevents access of CO2 for
photosynthesis
• Abscisic acid, a growth inhibitor, has been shown to accumulate in water deficient leaves of
some species
Pollution:
Low levels of certain gases of industrial origin (ozone and sulfur dioxide) are very damaging to
leaves of some plants
• Lichen is very sensitive to sulfur dioxide
• Soot can block stomata and reduce transparency of the leaf epidermis
Oxygen:
As a by-product of photosynthesis, it is still able to affect the rate of photosynthesis
• High [O2] inhibits photosynthesis
• O2 completes with CO2 for the active site of RuBP carboxylase oxygenase (Rubisco) which
combines CO2 with RuBP during light-independent reaction
• Photorespiration occurs when there is high [O2] relative to CO2, thus ATP is not produced,
reducing rate of photosynthesis
~ END ~

Chapter 4: Cellular Respiration
INTRODUCTION
Most organisms have an absolute requirement for oxygen and are called obligate aerobes
...
In the presence of oxygen [Aerobic Respiration]
• Results in complete oxidation of glucose to carbon dioxide and water
• Process yields a maximum of 36/38 ATP molecules
2
...
Glycolysis
2
...
Krebs Cycle
4
...

Selectively permeable membrane which is highly folded to form cristae which
increase surface area for embedding electron transport chain & ATP synthase
• Not permeable to NADH
• Contains proteins for transporting anions, ATP, ADP
• Contains members of ETC and ATP synthase complexes
• Site of oxidative phosphorylation
Compartment enclosed by the inner membrane of the mitochondrion
• Site of Link Reaction and Krebs Cycle

Stages of aerobic respiration:

STAGE 1: GLYCOLYSIS
Process of glycolysis is a ten-step reaction sequence, divided into 2 major phases
1
...
Energy-payoff phase (ATP formation)
C6H12O6 (glucose) + 2ADP + 2NAD ! 2C3H4O3 (pyruvate) + 2ATP + 2NADH + 2H+
Overview of process
Location
Metabolism

Substrates
Products

!

Cytosol / Cytoplasm
One molecule of 6-C glucose is split into two 3-C molecules, each of which
is rearranged to form a 3-C compound, pyruvate
Both substrate level phosphorylation (ADP " ATP) and dehydrogenation
(NAD " NADH) are key reactions
• Glucose or other hexose sugars, ADP, Inorganic phosphates (Pi), NAD
Products formed per glucose molecule
• 2 molecules of pyruvate
• 2 ATPs (net gain)
• 2 NADH
• H2O (Waste product)

Process of glycolysis
Step 1 to 5: Energy-investment phase
Steps 1 to 3: Activation of glucose
• Conversion (and hence activation) of unphosphorylated glucose to a phosphorylated fructose
1,6-bisphosphate
• Involves hydrolysis of 2 ATPs to provide phosphate groups and energy
• Step 3 – rate limiting step of glycolysis involving the enzyme phosphofructokinase
Step 4 to 5: Cleavage / Lysis
• Cleavage of fructose 1,6-bisphosphate to two 3-carbon sugars (glyceraldehyde-3-phosphate /
G3P)
Steps 6 to 10: Energy-payoff phase
Step 6: Reduction of NAD (Dehydrogenation)
• Each G3P is oxidized and NAD is reduced to NADH, with 2 NADH per glucose molecule
• 1 NADH supplies 2 energized electrons to drive most ATP production by oxidative
phosphorylation
Steps 7 and 10: Substrate-level phosphorylation
• Substrate-level phosphorylation of ADP occurs at steps 7 and 10, coupled to the
dephosphorylation of an organic substrate
• Produces 4 ATP per glucose molecule
• Results in overall net gain of 2 ATPs per glucose molecule
Importance of glycolysis
1
...
Supplies cells with essential biosynthetic precursors
• Liver carries out glycolysis to provide precursors for molecules it synthesizes (fats)
• Glycolysis associated with supplying initial steps of fat biosynthesis with substrate, rather than
acting as a source of ATP
• For microorganisms, both energy and necessary biosynthetic precursors are obtained from
glycolysis
Regulation of glycolysis
Phosphofructokinase is an allosteric enzyme inhibited by ATP and stimulated by AMP
• As ATP accumulates, inhibition of enzyme slows down glycolysis
• Enzyme becomes active as cellular work converts ATP to ADP (and AMP) faster than ATP
being regenerated
• It is also sensitive to citrate, first product of Krebs Cycle – if citrate accumulates in
mitochondria, some passes into cytosol and inhibits phosphofructokinase
• Thus it helps to synchronize rates of glycolysis and Krebs Cycle

STAGE 2: LINK REACTION
If molecular oxygen is present, the pyruvate enters the mitochondrion where enzymes of the Krebs
cycle complete the oxidation of the organic fuel
...
Carbons in pyruvate is liberated as CO2
2
...
An acetyl group, becomes attached to coenzyme A, yielding acetyl CoA
This occurs in the mitochondrial matrix, catalyzed by pyruvate dehydrogenase
...
Oxidative decarboxylation – occurs at 2 steps in the cycle (TCA-3 and TCA-4), so input of 2
carbons as acetate is balance by loss of 2 carbons as CO2
2
...
Production of reduced coenzymes – by dehydrogenation at 4 steps (TCAs 3,4,6 and 8), with
NAD as electron acceptor (in 3,4 and 8) and FAD (in 6)
!

STAGE 4: OXIDATIVE PHOSPHORYLATION
Reduced coenzymes NADH and FADH2 transfer the electrons in an electron transport chain
...

• It comprises a sequence of electron carriers that have the ability to be reversibly reduced and
oxidized as electrons from NADH and FADH2 are passed down mitochondrial ETC
• Each subsequent member of the ETC has greater affinity for electrons than its predecessor,
allowing one-way transport of electrons down the ETC as it moves along electron carriers in
order of increasing electron affinity
• Electrons eventually passed onto final electron acceptor, O2 and is reduced in the mitochondrial
matrix to produce a molecule of water
Chemiosmotic coupling model
As electrons are transferred along the ETC, energy is released, driving the proton pumps to actively
pump H+ unidirectional across the inner mitochondrial membrane to intermembrane space
• Proton pumping generates electrochemical proton gradient with 2 components
o Concentration gradient of H+
o Electrical gradient, voltage across membrane because of higher concentration of
positively charged protons on one side of membrane
• With the build up of H+ concentration in the intermembrane space, there is thus a tendency for
H+ to diffuse back into the matrix, creating a proton motive force
• Yet as the inner mitochondrial membrane is impermeable to H+, protons can only re-enter the
matrix through ATP synthase complex

ATP Synthase Complex
It couples the exergonic passage of H+ to the endergonic phosphorylation of ADP to form ATP
For each pair of electrons stripped from NADH
and passed down the ETC:
5 pairs (10 H+) are pumped into inner
mitochondrial membrane, re-entry generates 3
ATP molecules

For each pair of electrons stripped from FADH2
and passed down the ETC:
3 pairs (6 H+) are pumped into inner
mitochondrial membrane, re-entry generates 2
ATP molecules

Comparing substrate-level phosphorylation and oxidative phosphorylation:
Substrate-level phosphorylation
Enzymatic endergonic phosphorylation of ADP
with a 5’ monophosphate, is coupled to
exergonic dephosphorylation of an organic
substrate
During glycolysis (in cytoplasm)
During Krebs cycle (in mitochondrial matrix)
Produces only small amount of ATP

Oxidative phosphorylation
Enzymatic endergonic phosphorylation of ADP
with a 5’ monophosphate, is coupled to
exergonic electron transport from substrate to
final electron acceptor O2 and exergonic passage
of protons along a proton / electrochemical
gradient
During ETC (in inner mitochondrial membrane)
Produces almost 90% of ATP generated in
respiration

Comparing photophosphorylation and oxidative phosphorylation:
Photophosphorylation
In cellular aerobic respiration
Inner mitochondrial membrane
Chemical energy in organic respiratory substrate
Molecular O2 as final electron acceptor
H+ from matrix into intermembrane space
Forms NAD and FAD

Oxidative phosphorylation
In photosynthesis
Thylakoid membrane of chloroplast
Light energy from the sun
NADP / P700 as final electron acceptor
H+ from stroma to thylakoid lumen
Forms NADPH

Respiratory poison / inhibitors:
Effects of 3 classes of respiratory poisons provide evidence for chemiosmosis and dependence on
structural organisation of mitochondrial membrane
1
...
Poisons that inhibit ATP synthase
• Poisons (like oligomycin) directly inhibits ATP synthase
• Although proton gradient becomes larger than normal, its potential energy cannot be tapped to
make ATP
3
...
Lactic acid fermentation – pyruvate converted to lactate
2
...
CO2 released from pyruvate,
converted to acetaldehyde
2
...
Homologies and Divergent Evolution (Homologous structures, early embryonic development,
vestigial structures, molecular/biochemical homologies)
2
...
Biogeography (Island and Continent biogeography)
4
...
When they have no clear function and are no longer subject to natural selection, they will
remain unchanged through a lineage
Example: Whales and vestigial hind limb bones
Modern whales have vestigial hind limb bones as whales actually evolved from a land animal
• Presence of vestigial pelvic girdle and legs
• Presence of fossil records that link modern whales and dolphins to land ancestors
• However these structures are severely reduced in modern whales

Molecular / Biochemical homologies:
Includes universal genetic code & conserved sequences of nucleotides in DNA/RNA and amino
acids in proteins
...
6% similarity)
Hemoglobin beta chain used as standard molecule for comparing the precise sequence of amino
acids in different species
• Chimpanzees had identical sequence and those less-closely related had a greater number of
differences (suggesting close genetic relationships between us and chimpanzees)
FOSSIL EVIDENCE
Fossils are the preserved remains of once-living organisms and they serve to:
1
...
Study the history of particular groups
3
...
They belong to a sexually reproducing
species, where individuals share a common gene pool and random mating occurs
...
Majority of offspring die before they
reach sexual maturity
Competition is inevitable and organisms face a constant struggle
for existence and few survive to reproduce
Sexually produced offspring show individual variation such that
no two offspring are identical

Variations are absolute pre-requisites for evolution by natural
selection as the environment merely selects those individuals that
happen by chance to be better adapted
Survival of the fittest by Some will be better-adapted (fitter) to survive in the struggle for
natural selection [I]
existence
Natural selection is the process by which the environment or
nature selects for those well-adapted individuals with inherited
traits that are best suited to the local environment
Differential reproduction Those that survive to breed are likely to breed offspring similar to
[O]
themselves – leading to adaptation (evolutionary modification that
improves the chances of survival and reproductive success in a
given environment)
Formation of new species
Accumulation of modifications and adaptations " speciation

Modern Theory of Evolution:
It incorporates principles of Mendelian genetics and knowledge of molecular biology
1
2
3
4
5

Organisms have great
potential to reproduce [O]
Environmental restrictions
/ Constancy in numbers
[O]
Struggle for existence /
survival [I]
Variation
within
a
population [O]
Survival of the fittest by
natural selection [I]

Same as Darwin’s Theory
Same as Darwin’s Theory
Same as Darwin’s Theory

Variations within a population arise as a result of spontaneous
mutations and is controlled by genes
Individuals with genetic variations best adapted to the new
environment are more likely to survive to sexual maturity and
reproduce
reproduction With each succeeding generation, proportion of individuals who
possess the favorable genes / alleles and are at a selective
advantage increases

6

Differential
[O]

7

Formation of new species

Leads to change in allele and genotype frequencies
Natural selection alone is not adequate to explain how new
species are formed
Speciation can happen only if populations are separated so they do
not interbreed

SECTION 3: EVOLUTIONARY PROCESSES
There are 5 main processes that can bring about evolutionary changes:

Note: All examples in this section are important and must be memorized!

MUTATIONS
It is a permanent change in the nucleotide sequence of an organism’s DNA; it immediately brings
about changes in the gene pool of a population by substituting one allele for another
Thus random mutations with pre-existing genes are a source of new alleles that are new heritable
variations
• Mutation can obviously change the allele frequency in the gene pool of a population
• Mutations are slow, random and rare
GENE FLOW
It is the movement of genes / alleles from one population to another through migration of fertile
individuals to another and breeding in that new population
A population may therefore gain or lose alleles by gene flow, but it reduces differences between
populations that have accumulated as a result of natural selection or genetic drift
Example: Copper tolerance in grass plants
Demonstrates gene flow opposing natural selection
• Concentration of metals ions in soil is relatively high, thus we expect resistant allele to grow on
soil with heavy metals ions to occur with 100% frequency on mine sites and 0% elsewhere
• However this thus not occur due to prevailing winds blowing pollen containing non-tolerant
alleles onto mine site and tolerant alleles beyond the site’s borders
GENETIC DRIFT
It is the random change of allele frequencies as a result of chance alone, differing from generation
to generation in a small gene pool, resulting in less genetic variation within the small population
Effects of genetic drift:
1
...
Causes random change of allele frequencies
3
...
Can cause harmful alleles to become fixed
Founder effect:
Occurs when one or few individuals colonize a habitat isolated from their place of origin or new to
that species
Example 1: Amish population in the US / Example 2: Finches on the Galapagos Island
About 200 members of the Amish religion migrated from Switzerland to Pennsylvania between
1720 and 1770
• Discovered they had an allele frequency for Ellis-van Creveld syndrome of about 0
...
001 in the general population
• One couple who immigrated in 1744 carried the allele and inbreeding passed the allele along to
their descendants
• Also, by chance the Ellis-van Creveld had more children than the average Amish, further
increasing the allele frequency by genetic drift

Bottleneck effect:
Occurs when natural disasters, diseases, or predators (by chance) kill large numbers of individuals,
resulting in drastic short-term reductions of a population size
Alleles may be under-represented, over-represented or even eliminated
• Portion of its original diversity is permanently lost
• Leads to reduction and restriction in genetic variability
Example: Greater prairie chicken
Millions of greater prairie chicken used to live on the prairies of Illinois
• These prairies were converted to farmland and caused the number of chickens to plummet such
that by 1993, only two populations remained (fewer than 50 birds)
• Also had low levels of genetic variation and less than 50% of their eggs hatched
• Hence this suggest that genetic drift during the bottleneck led to a loss of genetic variation and
increase in frequency of harmful alleles
• 1993 Illinois greater prairie chicken population had lost nine alleles present in the museum
specimen
NON-RANDOM MATING / ARTIFICIAL SELECTION
Non-random selective breeding is a process where changes in allele and genotype frequencies are
determined by deliberate human actions – man exerts a directional selection
Artificial is a fast and rapid process
• Alleles favored by humans increase in frequency at the expense of less favorable alleles
• There are two kinds of artificial selection: inbreeding and outbreeding
Inbreeding:
Involves breeding of closely related individuals – most extreme form is self-fertilization
• Increases proportion of individuals that are homozygous at many gene loci
• Tends to maintain desirable characteristics and allows production of crop with uniformity in
characteristics
• However, genetic diversity may be reduced to an extent where every individual has identical
alleles for every gene (complete homozygosity) and cause reduced fertility and lowered disease
resistance
Outbreeding:
Involves breeding between members of genetically distant populations – forming hybrids
• These hybrids often have characters which are superior to that of either parents, phenomenon
known as hybrid vigor / heterosis
Example: Domestication of brassica
Artificial selection by humans has produced six separate vegetables from this single species:
Brassica oleracea
• Kale, Brussels sprouts, Cabbage, Broccoli, Kohlrabi, Wild mustard
Example: Domestication of breeds of dogs
Many dogs also descended from the Gray Wolf

NATURAL SELECTION
Defined as the process by which certain individuals that are better adapted to an environment
survive to reproduce (differential survival and reproduction)
...
Directional
2
...
Disruptive

Directional selection:
Directional selection favors one extreme of the phenotype range, and shifts the population mean for
the selected character
...
Strong selection pressure against the allele in the homozygous condition (HbSHbS) which results
in sickle-cell anemia
2
...
There are 4 main ways to define the term
species
...
Focus on reproductive barriers, directs attention to how speciation occurs – this help to
distinguish groups of individuals sufficiently different to be considered of different species
Limitations
1
...
Does not apply to organisms that reproduce asexually and self-fertilizing species
3
...
Distinguish groups of individuals sufficiently different to be considered separate species
2
...
Difficulty in determining the degree of difference to separate the species
Morphological Species Concept:
It classifies a species by body shape and other structural features
Members of a species appear to have similar anatomical traits
• We are forced to distinguish species this way as there is little or no information about their
mating capabilities
Advantage
1
...
Can be useful even without information on the extent of gene flow
Limitations
1
...
Difficult to analyze quantitative traits that vary in a continuous way in the same species
3
...
Accommodate both asexually and sexually reproducing species
2
...
Does not take into account the species’ morphology and reproductive compatibility

SPECIATION
Defined as the evolution / origin of species
...
leucurus
• Accumulation of sufficient reproductive isolating mechanisms (RIMs) and genetic diversity
• If individuals cannot produce viable, fertile offspring, then speciation has occurred

Example 2: Darwin’s finches on Galapagos Islands (Also for adaptive radiation)
There were different, though apparently related, species of finches on the different islands of the
Galapagos Islands – each species had a beak adapted to the particular food source
• One of the islands was colonized by a small population of finches from South America
• Natural selection resulted in adaptations to local conditions on the island and reproductive
isolating mechanisms (RIMs)
• Geographical isolation permitted additional speciation
• Later a few of these species reached neighboring islands and after diverging, a new species recolonized the island
• Selection pressure – types, availability and competition for food
Sympatric speciation:
A new species evolves within the same geographical region as the parental species or
geographically overlapping populations
Polyploidy
Instances in which organisms possess more than two of the haploid chromosome set
...
Systematists classify organisms based on data from
morphology, developmental biology, fossil records, behavioral biology and molecular biology
Taxonomy: It is the science of naming, describing and classifying the diversity of organisms
Classification: Act of systematically arranging organisms into groups based on particular
characteristics (mainly morphology) and their similarities – may not take into account evolutionary
relationship between species
Phylogeny: Organisation of species according to particular characteristics which takes into
consideration the evolutionary relationship between species

TAXONOMY
Binomial nomenclature:
It is a universal / internationally accepted system of naming organisms and each species is assigned
a unique two-part name
• First part of a scientific name is designated the genus
• Second part is called the specific epithet
Classification:
It is the act of systematically arranging organisms into groups based on particular characteristics
(mainly morphology) and their similarities
When moving up the hierarchical classification, each group is more inclusive, meaning it includes
more groups of organism (more dissimilarities amongst organism within each taxon)
• Basic unit of natural classification system is the species whereby closely related species are
assigned the same genus, and closely related genera grouped into a single family
• Families are grouped into orders, orders into classes, classes into phyla, phyla into kingdoms,
and kingdoms into domains
• May not reflect evolutionary relationships
Hierarchy
Domain
Kingdom
Phylum (animals) = Division (plants)
Class
Order
Family
Genus
Species

Mnemonic
Do
Keep
Pond
Clean
Or
Frog
Gets
Sick

Trends
Decrease in number of
organisms within each taxon
Increased similarities amongst
organisms within each taxon

PHYLOGENY AND PHYLOGENETIC TREES
Phylogeny – represents a hypothesis about patterns of evolutionary relationships among species
Phylogenetic tree:
It is an evolutionary tree showing a visual representation of a phylogeny to illustrate lineages and
their evolutionary relationships
...
Traces patterns of shared ancestry between lineages
2
...
Determines evolutionary relationships
Key points about a phylogenetic tree
(A) Sequence of branching in a tree does not necessarily indicate the actual (absolute) age of the
particular species
(B) A taxon on a phylogenetic tree does not evolve from the taxon next to / preceding it
Grouping of ancestors and descendants:
Within a phylogeny, a grouping of ancestors and descendants can be either monophyletic,
paraphyletic or polyphyletic
Monophyletic

Polyphyletic

Paraphyletic

Includes an ancestral species Consists several evolutionary Contains a common ancestor
and all its descendants
lines that do not share the same and some, but not all, of its
recent common ancestor
descendants
CONSTRUCTING PHYLOGENY USING SHARED CHARACTERS
In constructing a group’s evolutionary relationships, biologist first sort (1) homologous features
from analogous features then (2) infer phylogeny using these homologous characters
Cladistics:
It is an approach to systematics in which the common decent is the primary criterion used to
classify organisms by placing them into groups called clades
It is a group that includes an ancestral species and all its descendants (monophyletic group)
• They are nested within larger clades
• A phylogenetic tree inferred by synapomorphies (shared derived characters) is called a
cladogram
Shared ancestral characters:
Also known as plesiomorphies, they are features present in ancestral species and remain present in
all groups descended from that more distant ancestor
Due to descent with modification, organisms both share characteristics with and differ from their
ancestors
• For mammals, the backbone is a shared ancestral character

Shared derived characters:
When two populations are separated and evolve independently, some of their homologous traits
change as a result of mutation, natural selection and genetic drift – traits that evolve are thus known
as synapomorphies
Shared derived characters originate in a recent common ancestor and species that share these
characters form a clade
Key ideas about synapomorphies
1
...
They are nested, as each branching event adds one or more shared, derived traits
Example: Using shared derived characters to infer phylogeny

MOLECULAR SYSTEMATICS
When a new species evolves, it does not always exhibit obvious phenotypic differences when
compared to closely related species
Macromolecules that are functionally similar in two different types of organisms are considered
homologous if their primary sequence is similar
• This helps us to understand phylogenetic relationships that cannot be determined by nonmolecular methods such as comparative anatomy
Use of nucleic acid and amino acid sequence homologies:
Nucleotide base sequences in DNA or RNA and amino acid sequences in proteins change over time
due to accumulation of mutations
When comparing homologous genes in different organisms, the greater the degree of homology in
the primary sequence of macromolecules between the two species, the more closely related they are
• The differences reflect how much time has passed since the groups branched / diverged
• Two main macromolecules are used: (1) DNA sequences and (2) Amino Acid Sequences

DNA Sequences:
Mutations are assumed to have occurred at a fairly steady rate over millions of years
Thus if more differences occur in homologous DNA sequence of one species compared to another,
more time has elapsed since the two species diverged from a common ancestor
Amino Acid Sequences:
Homologous proteins are proteins that are evolutionarily related and they usually perform the same
function in different species
Examples: Cytochrome c
It is part of the electron transport chain where electrons are passed to oxygen during cellular aerobic
respiration and is found in the mitochondria of every aerobic eukaryote
• Number of residues that differ in homologous proteins from any two species is proportional to
phylogenetic difference between the species
• 48 aa residues differ in the cytochrome c molecules of the horse and the yeast which are widely
separated
• Only 2 residues differ in the cytochrome c molecules of the duck and the chicken which are
closely related
Advantages Of Molecular Methods:
1
...
Nucleic acid and amino acid sequence variation has a clear genetic
basis that is easy to interpret as compared to the relationship between genes and the trait
2
...
All living organisms possess nucleic acid
and proteins and so molecular data can be collected from any organism and for objective
comparisons to be made to establish evolutionary relationships through molecular homology
3
...
Enormous amount of
data can be accessed by molecular methods for objective comparison
4
...
All organisms have certain molecular
traits in common which allows for valid basis of comparison
5
...
Nucleic acid and amino acid sequence data is precise, accurate
and easy to quantify, facilitates objective assessment of evolutionary relationships
6
...

7
...
Can be used to make comprehensive
evolutionary comparisons between many groups of organisms
8
...

9
...
No gene marks time with complete precision and some portions of the genome appear to have
evolved in irregular fits
2
...
Same gene might also evolve at different rates in different groups of organisms
4
...
To mediate the maintenance of homeostatic bodily processes that are crucial to the survival and
perpetuation of a complex multicellular organism
2
...
Cells also communicate in order to collaborate and coordinate their activities such as growth,
differentiation and metabolism
SECTION 1: CELL SIGNALING
STAGES OF CELL SIGNALING
There are three main stages of cell signaling:

Signal reception:
Refers to the target cell’s detection of an extracellular signal molecule
...
g
...
Steroid hormone diffuses across plasma membrane into target cell
Signal Reception:
2
...
Activated hormone-receptor complex enters the nucleus and binds to hormone response
elements of a specific gene
Cellular Response:
4
...
G protein-coupled receptors (GPCRs)
2
...
Ion channel receptors (or ligand-gated ion channels)
They transmit extracellular signal information into the cell via conformational changes or subunit
aggregation
Protein phosphorylation:
Phosphorylation and dephosphorylation is a common cellular mechanism
...
Signal molecule binds to GPCR and causes change in receptor conformation, activating GPCR
Signal Transduction:
2
...
The activated G-protein dissociates from the GPCR and diffuses along the membrane
4
...
Change in target protein activity initiates a cascade of signal transduction events by triggering
next step in the transduction pathway
6
...
Last activated molecule in the transduction pathway triggers a cellular response
8
...
The inactive G-protein leaves the enzyme, returning it to its original inactive state, making it
available for reuse
Second messengers in GPCR Signaling
Cyclic Adenosine Monophosphate
• Target protein is adenylyl cyclase,
catalyzes synthesis of many cAMP
• Results in the activation of protein
kinase A, which then phosphorylates
various other proteins
• Number of cAMP does not persist for
long in the absence of the hormone as
phosphodiesterase, converts it back to
AMP

Ca2+ and IP3
• Target protein is phospholipase C
• It cleaves a plasma membrane called PIP2 into
DAG and IP3
• DAG functions as a second messenger in other
pathways while IP3 quickly diffuses through the
cytosol
• It then binds to IP3 gated calcium channel in the
ET membrane, cause it to open and let Ca2+ ions
diffuse out, lead to activation of calcium binding
proteins
• It then activates the next protein such as cadmulin

Maintenance of Ca2+ concentration
Low cytosolic Ca2+ concentration is usually maintained in the cell
...
Signal molecule binds to RTK resulting in receptor aggregation and dimerization
2
...
Each tyrosine kinase domain adds a phosphate from an ATP molecule to a tyrosine on the tail of
the other polypeptide subunit – RTK now fully activated
Signal Transduction:
4
...
The relay protein is now activated, undergoing a conformational change
6
...
Last activated molecule in the transduction pathway triggers a cellular response
Insulin receptor
No second messengers are involved

Ion-channel receptors:
They are membrane receptors that allow specific ions to pass through when the receptor changes
conformation
When a signal molecule binds as a ligand to the receptor protein, a region of it acts as a “gate” to
open or close, allowing or blocking the flow of specific ions through a channel / pore
• It is important in the functioning of nervous system, especially in the formation and
transmission of nervous impulses
• Such as receptors for neurotransmitters in synaptic signaling
Signal Reception:
1
...
Allows or blocks respectively the flow of specific ions through a channel / pore
3
...
Altered activity of membrane / cytosolic proteins triggers a cellular response
5
...
Signal amplification – allows for possibility of amplifying a signal, and the result can be a large
number of activated molecules, hence a small number of extracellular signal molecules can
produce a large cellular response
2
...
Contribute to specificity of the response – the same signal molecule can lead to a variety of
cellular responses via specific combination of signaling/relay proteins present in each cell
Signal amplification:
It is the process of enhancing the signal strength as the signal is relayed through a transduction
pathway
There are three key features of signal amplification
• At each catalytic step in the cascade, number of activated products is much greater than the
preceding step
• Small number of extracellular molecules is sufficient to elicit a cellular response
• Response at the target cell is large, as large number of activated molecules are produced
Amplification effect is possible for two reasons
• Presence of multiple steps in the transduction pathway (signal reception and cellular response)
• Persistence of proteins in the active form long enough to process many molecules of substrate

Regulation of cell signaling:
For a cell to remain sensitive to and continually respond to incoming signals, each molecular
change must only last a short time
This entails signal termination, where the components of the signal transduction pathway return to
their inactive states
...
Carry out vital cellular processes and functions
2
...
Any deviation from the set point
activates the control system to trigger responses to return the condition to its optimum level
Two types of feedback
1
...
Positive feedback – reinforcing the stimulus to allow the reaction to proceed at a faster rate

Negative feedback
The variable being regulated will stay within its steady state range of values and oscillates around
the set point since the response loop is switched off once the variable is restored to its set point
Thus negative feedback mechanisms are
• Associated with increasing stability of the system
• Brought about by disturbance to set in motion a sequence of events to counteract the disturbance
and restore the system to its original state
• Most common in homeostatic mechanism of organisms
SECTION 3: THE ENDOCRINE SYSTEM
Introduction to the Endocrine system:
It is a system made up of small endocrine glands that secrete chemical messengers known as
hormones
Endocrine glands have the following properties
• Secretes hormones directly into the blood stream
• Hormone-producing cells are embedded within a network of blood capillaries
• Are known as ductless gland
PROPERTIES AND FUNCTIONS OF HORMONES
Hormones are chemical messengers secreted by endocrine glands and cells that are transported to
and act on specific target cells
Properties of hormones:
• Effective in low concentration
• Effect on target cells may be slow due to time to travel through bloodstream and binding to
receptors
• Effect on target cells is sustained, prolonged and long-lasting
• Bind to receptors on target cells
Functions of hormones:
Hormones alter cellular operations by changing the types, activities, and/or quantities of important
enzymes and structural proteins by
• Stimulating the synthesis of an enzyme or structural protein by activating genes in the nucleus
• Increasing / Decreasing the rate of synthesis of a particular enzyme or protein by changing the
rate of gene expression
• Activating or inactivating an existing enzyme by altering its specific 3D conformation
Mechanisms of hormones:
Water-soluble hormones make use of cell surface / membrane receptor signaling while lipid-soluble
hormones makes use of intracellular receptor signaling

PANCREAS AND THE ISLETS OF LANGERHANS
The pancreas is an endocrine gland responsible for the production and secretion of insulin and
glucagon to regulate blood glucose levels
Hormone
Structure
Target cells
Mode of action
Hormonal effects

Regulatory control

Glucagon α cells
Linear peptide hormone with 29 aa

Insulin β cells
Peptide hormone with 51 aa; consist A
and B chains held together by pair of
inter-chain disulfide bridges
Liver, adipose
Liver, adipose, muscle
GPCR
RTK
Elevates [blood glucose]
Facilitates uptake of glucose
Mobilizes lipid reserves
Accelerates glucose utilization and
Promotes glucose synthesis and storage
glycogen breakdown in liver
Stimulates glycogen, lipid and protein
synthesis
Stimulated by low [blood glucose]
Stimulated by high [blood glucose]

Control of blood glucose concentration:
When blood glucose concentration rises above the set point, a series of reactions is triggered to
lower it back to set point
...
Insulin secreted into and travels in the
blood stream
2
...
RTK phosphorylates intracellular
enzymes causing the following effects

[blood glucose] < 90mg/100ml
1
...
Binds to cell-surface glucagon
receptors
3
...
A
...
AA
Inhibits gluconeogenesis
absorbed in blood steam are converted
into glucose in liver ten released into the
blood stream

Effects

Outcome

[blood glucose] > 90mg/100ml
Stimulates lipogenesis in adipose tissue,
which is triglyceride formation by
increasing absorption of glucose into
adipocytes where excess glucose is stored
as triglycerides

[blood glucose] < 90mg/100ml
Stimulates lipolysis, which is triglyceride
breakdown in adipocytes
...
Uses millions of sensory receptors to monitor changes occurring both inside and outside the
body – gathered information known as sensory unit
2
...
Causes a response, known as motor output, by activating effector cells (muscle cells)
NEURONS
The neuron of interest to be studied would be the motor neuron
Structure of neurons:
A typical neuron has a large cell body with large nucleus and prominent nucleolus
...
Receptive segment
• Cell body and dendrites (input
zone)
• Receives inputs, if it is strong
enough, it will be transmitted to
the initial segment
2
...
Conductive segment
• Axon (conducting zone)
• Signals are propagated as action
potential down the axon
4
...
Leak Channels
• Allow constant ion flow down an electrochemical gradient by facilitated diffusion
2
...

•
•
•

Gated Channels
Channels requiring some form of stimulus to open for ion flow
Chemically-gated channels (ligand-gated) – open in response to binding of a ligand
Voltage-gated channels – open in response to changes in membrane potential and primarily
found in the membranes of axons

Resting membrane potential:
Refers to the difference in electrical charge (voltage) across a cell membrane and is the result of
uneven distribution of ions across the ion-impermeable phospholipid bilayer
Interior of a neuron always has a negative charge of -70 to -100mV
Extracellular fluid contains high concentration of Na+ whereas cytosol contains high concentration
of K+ and negatively charged proteins
...
Also the longer the
duration of the triggering event, the longer the duration of the graded potential

Fate of the graded potential is determined when it reaches the trigger zone (axon hillock)
• Graded potential below threshold potential of -55mV to -50mV (subthreshold) is not strong
enough to trigger an action potential
• Graded potential above threshold potential (suprathreshold) triggers an action potential
...
Absolute refractory period
• Membrane cannot respond to further stimulation when voltage-gated sodium channels open at
threshold until slow closing of sodium channel
• Due to the fact that all voltage-gated Na+ channels not yet reset to resting conformation
2
...
Action potential travels faster as it does not have to be regenerated at myelinated sections but
only at unmyelinated sections – thus less charge leaks out in myelinated sections and more
arrives at the node adjacent to the active node – allowing AP to be generated sooner
2
...

•
•
2
...
Presynaptic terminal / knob – an axon terminal of the transmitting neuron, with synaptic
vesicles, mitochondria and other cell organelles
2
...
Synaptic terminal cleft – space of average width of 20nm between presynaptic terminal and
postsynaptic endings
Cholinergic synapse:
They are chemical synapses that release the neurotransmitter acetylcholine
...
2-0
...
Entry of Ca2+ ions into the presynaptic cell
2
...
Diffusion of acetylcholine across synaptic cleft and binding of acetylcholine to receptor
molecules on postsynaptic membrane [Main Factor]
POSTSYNAPTIC POTENTIALS
Postsynaptic potential:
They are graded potentials that develop in the postsynaptic membrane in response to the binding of
neurotransmitters
There are two major types of potentials that can develop
•

Excitatory postsynaptic
potential (EPSP)
•
•
Inhibitory postsynaptic
potential (IPSP)

•
•

It is a graded depolarization resulting from opening of chemicallygated Na+ channels, leading to influx of Na+
It is referred to as excitatory as membrane potential moves towards
threshold, allowing it to generate action potentials
It is a graded hyperpolarization resulting from opening of chemicallygated K+ and/or Cl- channels, leading to efflux of K+ or influx of ClIt is referred to as inhibitory as membrane potential moves further
away from threshold, antagonizing the production of action potentials
While hyperpolarization continues, the neuron requires a greater than
normal depolarizing stimulus to bring membrane potential to
threshold

Summation:
An individual EPSP or IPSP may have a small effect on the postsynaptic membrane, but they can
combine their effects cumulatively through either temporal or spatial summation
Temporary summation
• Summation that occurs when action potentials are fired in rapid succession
• Occurs at a single synapse that is excited / stimulated repeated
• Every time an action potential arrives at a synaptic knob, a group of vesicles discharges
acetylcholine into the synaptic cleft
• Each time more acetylcholine molecules reach the postsynaptic membrane, more chemicallygated ion channels open and degree of depolarization increases!
!
Spatial summation
• Summation that occurs when simultaneous stimuli at different synapses have a cumulative
effect on the membrane potential
• As in temporal summation, action potential is generated if the threshold potential is reached

COMPARISON BETWEEN NERVOUS AND ENDOCRINE SYSTEM
Basis
Complexity

Endocrine system
Less structurally complex

Nervous system
More structurally complex; can integrate
vast amounts of information and stimulate
wide variety of responses
Nature of
Information transmitted as chemical Information transmitted as nerve impulse
transmission substances (hormones)
which is electrical in nature
Across a synapse, information transmitted
by chemical substances (neurotransmitters)
Mode of
By circulatory system via blood
By the nervous system via neurons / nerve
transmission
cells
Rate of
Sow transmission and slow-acting Rapid transmission and response (in
transmission (except for adrenaline)
milliseconds)
Duration of Often long-term changes / effects
Short-term effects
effect
Types of
Response may be very wide spread, Response often very localized
responses
and a single hormone may act on
target organs far from each other
Specificity
Not specific (blood circulation around Pathway is specific (through neurons / nerve
of pathway body) but target is specific
cells)
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