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Biology
Unit Two

Definitions
• Gene: a section of DNA that contains coded information for making
polypeptides
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• Locus: position of a gene on a chromosome or DNA molecule
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• Mitosis: produces two daughter nuclei, with the same number of chromosomes
as the parent cell
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• Interspecific variation: variation between different species
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• Species: a group of similar organisms that can breed together to produce fertile
offspring
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Phosphate group
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These combine during a condensation reaction to give a mononucleotide
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Continued linking: polynucleotide
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Adenine pairs with thymine by two hydrogen bonds
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Quantities of pairs are the same – ratio varies between species
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Properties of DNA
• Very stable: pass down generations without change
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• Extremely large: immense amount of genetic information
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The Triplet Code
• Genes determine the proteins of an organism
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• Scientists suggested there must be a minimum of three bases that code for
each amino acid:
• Only 20 amino acids regularly occur in proteins
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• Only four different bases
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• Using pairs, only sixteen types
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• In eukaryotes, much of the nuclear DNA does not code for amino acids – these
sections are called introns and can occur within genes and as multiple repeats
between genes
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Eukaryotic cell: has linear DNA, occur in association with proteins to form
chromosomes
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Two threads (chromatin) join at a single point – centromere
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DNA helix is wound around proteins – this DNA-protein complex is then coiled – coil
is looped and coiled into a chromosome
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Diploid number: 46 chromosomes
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Same genetic characteristic: something like eye colour
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Meiosis
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In sexual reproduction, two gametes fuse to give rise to new offspring
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During meiosis, one chromosome from each pair enters each gamete
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Recombination by crossing over
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Crossing over:
The chromatids of each pair become twisted around each other
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The broken portions then rejoin with the chromatids of its homologous partner
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Meiosis
1
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3
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5
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7
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9
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The chromosomes replicate
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The alignment pattern is random
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Homologous chromosomes separate and are attached to spindle fibres
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As the chromosomes reach the spindle poles, the nuclear membranes enclose
the separated chromosomes
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There are now two cells, each with the same
number of chromosomes as the parent cell
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The nuclear envelope breaks down and a new spindle forms
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The nuclear envelope reforms around the sets of daughter chromosomes
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Mitosis
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1
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3
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5
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Interphase: chromatids aren’t visible – cell is not dividing
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Prophase: mitosis begins
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Metaphase: chromosomes arrange themselves at the cell centre
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Telophase: the nuclear envelope reforms and two cells are produced
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Differentiation – these cells differentiate to give groups of specialised cells
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Repair – if cells are damaged or die, identical cells need to be produced
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Interphase: occupies most of the cell cycle and is known as the resting phase
as no division takes place
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2
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3
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A mammalian cell takes about 24 hours to complete the cycle – around 90% is
interphase
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It is caused by the damage to the genes that regulate mitosis and the cell cycle
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DNA Replication
• Before a nucleus divides, during interphase, the DNA must first be replicated
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The enzyme DNA helicase breaks the hydrogen bonds linking the base pairs of
DNA
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The double helix separates into two polynucleotides
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Each exposed polynucleotide acts as a template to which complementary
nucleotides from the cell cytoplasm are attracted
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Energy is used to activate the nucleotides
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The activated nucleotides are joined by the enzyme DNA polymerase and the
hydrogen bonds reform
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One new polynucleotide and one original join together – so each of the new
molecules contain one original DNA strand
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Similar cells are grouped into tissues  tissues into organs  organs into systems
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All cells are derived by mitotic divisions of the fertilised egg
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Examples of tissues 
Epithelial tissues: consist of sheets of cells that line the surfaces of organs,
Xylem: used to transport water and mineral ions throughout the plant and give mechanical
support
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Blood capillaries aren’t organs as they’re only made of one tissue – arteries and veins are
organs as they’re made up of many tissues
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Haemoglobin
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Primary structure: consists of amino acids in four polypeptide chains
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Tertiary structure: each polypeptide is folded into a precise shape
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One haemoglobin can contain four oxygen molecules
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Readily dissociates from oxygen at the tissues requiring it – in the presence of carbon dioxide,
the new shape of the haemoglobin molecule binds more loosely to oxygen
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Haemoglobin with low affinity for oxygen: takes up oxygen less easily – releases more readily
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Organisms with high metabolic rates need to release oxygen readily into the tissues –
haemoglobin with low affinity for oxygen
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Oxygen Dissociation Curves
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At low concentrations, the four haemoglobin polypeptide are closely united –
difficult to absorb oxygen
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Very small decreases in partial pressure of oxygen lead to it being dissociated from
haemoglobin
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The further to the left the curve, the greater the affinity for oxygen
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Bohr effect: the greater the concentration of carbon dioxide, the more readily the
haemoglobin releases oxygen
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Dissolved carbon dioxide is acidic – low pH causes haemoglobin to change shape
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Starch & Glycogen
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Starch 
Occurs in seed and storage organs – only in plants
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Un-branched α-glucose chain wound into tight coil – makes it compact
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Doesn’t diffuse out of cells
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Hydrolysis forms α-glucose – easily transported and used in respiration
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Major carbohydrate storage product of animals
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More readily hydrolysed to form α-glucose
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• -OH group is above the glucose ring – to form glycosidic bonds, each β-glucose
must rotate by 180˚ compared to neighbour – position of CH2OH alternates
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• Cellulose molecules group together to form microfibrils which are arranged in
parallel groups called fibres
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• Prevents cell from bursting as water enters by osmosis – exerts inward pressure
that stops any further influx of water
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Plant Structure
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Leaf palisade cell 
Carries out photosynthesis
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Numerous chloroplasts arrange themselves in the best positions to collect maximum light
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Chloroplasts 
Organelles that carry out photosynthesis:
Chloroplast envelope: double plasma membrane – highly selective in what enters and leaves
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Stroma: fluid-filled matrix where second stage of photosynthesis takes place – contains starch
grains
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Fluid of stroma possess all enzymes needed to carry out second stage
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Plant Structure
• Cell Wall 
• Consists of polysaccharides like cellulose
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• Functions:
• Provides mechanical strength
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• Allows water to pass along it
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Materials needed to be interchanged:
Respiratory gases, nutrients, excretory products and heat
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Organisms have evolved to overcome the problem of diffusion rate with SA:V ratio:
A flattened shape so that no cell is ever far from the surface
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Features of specialised exchange surface:
Large SA:V ratio to increase rate of exchange
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Partially permeable – allows selected materials to cross
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Being thin, surfaces are easily damage and therefore often inside the organism
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To reduce water loss, terrestrial organisms have 
Waterproof coverings – insects have rigid outer skeletons covered with waterproof cuticles
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Insects cannot use body surfaces to diffuse gases – have internal network of tubes called
trachae that are supported by strengthened rings
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Respiratory gases move in and out of tracheal system in two ways 
Along a diffusion gradient – when cells respire, oxygen is used up so there is a low
concentration at the end of the tracheoles
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Ventilation – movement of muscles in insects creates mass movements of air in and out of
trachae – speeds up exchange
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Gas Exchange in Fish
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Gills are made up of gill filaments – these are stacked in a pile
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Water is taken in through the mouth and forced over the gills – leaves through the
operculum when the mouth closes
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Counter-current exchange principle 
Water and blood flow in opposite directions
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Blood that is already loaded well with oxygen meets water which has its maximum
amount of oxygen
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Fairly constant rate of diffusion across the entire length of gill lamellae
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A specialist exchange surface is therefore needed for gas exchange
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Whether or not there is a specialised transport medium, and whether or not it is circulated by
a pump depends on 
SA:V ratio and how active the organism is
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Form of mass transport – transports medium around in bulk
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Mechanism for moving medium – pressure difference in two parts of the system achieved by
muscular contraction (animals) or passive natural processes such as evaporation (plants)
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A means of controlling the flow of the transport medium to suit the changing needs of
different parts of the organism
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Blood Vessels and their Functions
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Arteries: carry blood away from heart into arterioles
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Capillaries: tiny vessels linking arterioles to veins
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Structure of arteries, arterioles & veins 
Tough outer layer – resists pressure changes
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Elastic layer – helps maintain blood pressure by stretching
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Lumen – central cavity of blood vessel
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Elastic layer thinner than artery – lower blood pressure
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Thin elastic layer – low blood pressure won’t cause them to burst – pressure too low to create
recoil action
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Valves – to prevent backflow – ensures pressure from contracting muscles directs blood to
heart
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Numerous and highly branched – large surface area
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Narrow lumen – red blood cells are squeezed flat to bring them closer to the cells and reduce
the diffusion pathway
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The final journey after the capillary for metabolic materials is a liquid solution called tissue
fluid
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• After supplying cells, it receives carbon dioxide and other waste
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• Formation 
• The pressure of blood from arteries to arterioles to capillaries is called
hydrostatic pressure at the end of the capillaries
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• The outward pressure is opposed by the hydrostatic pressure of the tissue fluid
outside of the capillaries which prevents outward movement of liquid and the
lower water potential of the blood, due to plasma proteins, pulls water back
into the blood
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Tissue Fluid
• Once tissue fluid has exchanged materials, it returns to the blood plasma via
the capillaries 
• The loss of tissue fluid from the capillaries reduces hydrostatic pressure inside
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• Tissue fluid is therefore forced back in
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• Not all tissue fluid can return to capillaries, the rest is carried back via the
lymphatic system
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• Contents in system moved by 
• Hydrostatic pressure of fluid that has left the capillaries
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Gas Exchange in the Leaf
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When photosynthesis takes place, although some carbon dioxide comes from cell
respiration, most is take from the external air
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When photosynthesis doesn’t occur, oxygen diffuses into the leaf
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Adaptions for rapid diffusion:
Thin, flat shape – large surface area
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Numerous interconnecting air-spaces throughout mesophyll
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Pore can open and close – controls rate of gaseous exchange
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Movement of Water through Roots
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Plants need to conserve water – covered by waterproof layer
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Transpiration: the main force pulling water up the stem through evaporation of
water from leaves
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They provide a large surface area and have a thin surface layer
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Water continues its journey across the root in two ways:
The apoplastic pathway
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Movement of Water through Roots
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1
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3
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5
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Cohesion: the mutual attraction of molecules
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The symplastic pathway 
Water passes over plasmodesmata – openings filled with cytoplasm – continuous
column from root hair cells to the xylem
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Root hair cell has higher water potential than first cell in cortex
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This continues on to neighbouring cells
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Water potential gradient is therefore set up across all cells of the cortex
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Water is forced into the living protoplast of the cell – joins water that has arrived by
symplastic pathway
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Endodermal cells do this – requires energy so must occur within living tissue
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Active transport of mineral ions into the xylem lowers water potential – water
moves by osmosis into the xylem – this force is called root pressure
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Metabolic inhibitors prevent most energy released by respiration – root pressure
ceases
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Xylem sap excludes from the cut stems of certain places at certain times
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• When stomata open, water vapour diffuses out – water is replaced by
evaporation from cell walls of surrounding mesophyll cells
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Water movement occurs because:
Mesophyll cells lose water to air spaces – lowers water potential
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They, in turn, take in water from neighbours by osmosis
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Cohesion-tension theory:
Water evaporates from leaves due to transpiration
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Water forms a continuous pathway across mesophyll cells and down the xylem
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Water is hence pulled up the xylem – transpiration pull
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Evidence 
Change in diameter of tree trunks – during the day, transpiration is at its greatest so trunk
shrinks – more tension in the xylem
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When xylem vessel breaks, air is drawn in – consistent with being under tension
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Xylem vessels are dead where water moves – end walls can break down – xylem forms a
series of continuous, unbroken tubes from roots to leaves
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Stomata allow inward diffusion of carbon dioxide
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Benefits 
Mineral ions, sugars and hormones are moved around by dissolving in water
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Factors affecting it 
Light: photosynthesis occurs in light – stomata open in light
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Humidity: the measure of water molecules in the air – when air outside the leaf has high
humidity, the water potential gradient is reduced – lower transpiration rate
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Energy for transpiration comes from the evaporation of water from leaves
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Potometer
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It is almost impossible to measure transpiration because it is extremely difficult to condense
and collect all water vapour that leaves all plant parts
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The rate of water loss can be measured using a potometer
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The potometer is filled completely with water – make sure there’s no air bubbles
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The potometer is removed from under the water and all joints are sealed with waterproof
jelly
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The distance moved by the air bubble in a given time is measured a number of times and the
mean is calculated
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The volume of water lost against the time in minutes can be plotted on a graph
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The experiment can be repeated to compare the rates of water uptake under different
conditions
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• Thick waxy cuticle: wax on leaves forms waterproof barrier – thicker cuticle
means less water can escape
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• Hairy leaves: a thick layer of hair on leaves traps moist air next to leaf surface
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• Reduced SA:V ratio: leaves are small and roughly circular rather than flat and
broad – this surface area reduction must be balanced against need for sufficient
area for photosynthesis
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Intraspecific variation: variation within a species
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Chance
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• Random Sampling:
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2
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3
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Variation causes 
Genetic differences: due to mutations, meiosis and fusion of gametes
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Variation can occur due to both factors
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Represented on a bar chart of pie graph
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Due to environmental influences:
Some characteristics grade into one another, forms a continuum
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Environmental factors play a role in determining where on the continuum the
organism lies
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Mean: measurement at the maximum height of the curve
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Genetic Diversity
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The greater the number of different alleles that all members of a species possess,
the greater the genetic diversity
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Selective breeding: identifying individuals with the desired characteristics and using
them to parent the next generation – reduced genetic diversity
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In time, the population may develop into a separate species
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Genetic bottleneck 
Population of a species may suffer from a drastic reduction in population due to a
natural or unnatural disaster
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Species Diversity
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Biodiversity: the variety in the living world
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Genetic diversity: the variety of genes possessed by the individuals that make up
any one species
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Measuring biodiversity 
The number of different species in a given area – species richness
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Simpson’s Index (measuring species diversity) 
N = total number of organisms of all species
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The higher the value of D, the greater the species
diversity
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Selective breeding – the number of species and the genetic variety of alleles they
possess is reduced to the few that exhibit desired features
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The species have to compete for remaining resources – most won’t survive
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Overall – reduction in species diversity
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Up to 50, 000 species are being lost each year
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Overall – major loss in species diversity
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Classification: the grouping of organisms
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Artificial classification 
This divides organisms according to differences that are useful
at the time
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Natural classification 
Based upon evolutionary relationships
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The groups are arranged into a hierarchy
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Courtship Behaviour
• Each individual tries to ensure their DNA is passed on
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Identify a mate that is capable of breeding
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Synchronise mating
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• This ritual proceeds as a stimulus-response chain – differs for members of
different species
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When one species gives rise to another, the DNA of the new species will be very similar – due
to mutations, the sequence of bases will change slightly – over time there will be more
differences
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DNA hybridisation 
When DNA is heated, the double strand separates – when cooled, the bases recombine – in
time, all strands pair up completely
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DNA from one is labelled with a radioactive or fluorescent marker
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The mixture is cooled, complementary bases recombine
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This is known as 50% of the DNA will be labelled
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At each stage, the degree to which strands are linked is measured
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The stronger the strand, the higher the temperature needed
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Genetic Comparisons using
Proteins and DNA
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Classifying plants 
Used to be based on physical features – based on whether they had a single seed
leaf or two seed leaves
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For each plant, the DNA sequence for all three genes were determined
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A phylogenetic tree for the families of flowering plants was devised based upon
DNA sequences of the species used
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Once the sequence is determined, you can count either the number of similarities
or the number of differences
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• Process 
• Serum from species A is injected into species B
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• Serum extracted from B – contains antibodies from A
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• The response is the formation of a precipitate
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• Species B is the intermediate
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Genetic Variation in Bacteria
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Adaption: through the process of natural selection, organisms adjust to suit the changing
environment
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Sexual reproduction – recombining the existing DNA of two individuals
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Any difference in the base sequence may result in difference in amino acids – different proteins will
be made – if protein is an enzyme then it will disrupt metabolic pathway
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• Conjugation (when one bacterial cell transfers DNA to another cell) 
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In conjugation, DNA in the form of genes is passed from one species to another
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Vertical gene transmission: where genes are passed down from one generation to the next
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In bacterial cells, water constantly enters by osmosis
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The wall is made of a tough material – not easily stretched
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Ways antibiotics work 
Certain antibiotics kill bacteria by preventing them forming cell walls – they inhibit
synthesis and assembly of peptide cross-linkages – this weakens the wall so that it
can’t withstand pressure – osmotic lysis can occur
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Viruses have different coverings – antibiotics are ineffective
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Mutations occur randomly and are very rare
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Only the mutant individual survives – divides to reproduce – gene is passed on –
vertical gene transmission
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Allele for resistance is carried in plasmids – can be transferred by conjugation –
horizontal gene transmission
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The more we use antibiotics, the greater the chance that the mutant bacterium
will gain an advantage over the normal variety
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Antibiotic Use and Resistance
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Tuberculosis 
Antibiotics are taken for 6-9 months – full course must be taken
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Selection pressure leads to the development of strains that are resistant – these interchange
genes by conjugation – multiple antibiotic-resistant strains of TB are developed
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MRSA 
Many people carry a bacterium belonging to the genus Staphylococcus in their throats – SA
causes minor symptoms in healthy individuals
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MRSA is the name given to strains that are resistant to one or more antibiotics
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Patients live close together and examined by people who touch other patients – transmission
of infections
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MRSA is very difficult to treat – some strains have developed resistance to almost every
known antibiotic

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