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Cells
Structure of prokaryotic and eukaryotic cells

Organelle
Nucleus

Structure
• Largest organelle
• Nucleolus inside
• Has nuclear pore
• Composed of 2 fluid filled
membranes

Smooth endoplasmic
reticulum
Rough endoplasmic reticulum

•

Golgi Apparatus

•

•

Function
• Contains genetic material
• Chromatin consists of DNA and
proteins
• Nucleolus takes RNA and
makes ribosomes

Flattened membrane
bound sacs
Flattened membrane
bound sacs

•

Involved in making lipids

•

Transports proteins made on
attached ribosomes

Stack of membrane
bound flattened sacs

•
•
•

Receives proteins from the ER
Modifies them e
...
add sugar
Packages proteins into vesicles
to be transported inside cell or
to the outside
Produces secretory enzymes

•

Mitochondria

•
•
•

Chloroplasts

•
•

2 membrane separated
by a fluid filled space
Inner membrane is folded
to form cristae
Central part is called
matrix
2 membranes separated
by fluid filled space
Inner membrane is
continuous with a
network of thylakoids

•

Site where ATP is produced
during aerobic respiration

•
•

Site of photosynthesis
Light energy used to derive
carbohydrates from carbon
dioxide

•
•

Lysosome

•
•

Vacuole

•

Ribosome

•

Centrioles

•
•
•

A stack of thylakoids is
called a granum
Chlorophyll molecules are
present on these
membranes
Spherical sacs
surrounded by a single
membrane
Formed by Golgi
apparatus
Membrane bound
organelle in the
cytoplasm of most cells,
especially plant cells,
containing water and
dissolved substances
such as salts, amino
acids, sugars and
enzymes
...

Different differentiations include:
•
•
•
•

Change in number of organelles
Change in shape of cell
Change some contents of the cell
Some all the above

Structure of Prokaryotic Cells
Prokaryotic – (before the nucleus)
e
...
bacterial cells

•

•
•

Contain digestive enzymes
which break down materials
e
...
specialised lysosomes in
the head of sperm helps it
penetrate egg
...


•

Form fibres in cell division
known as spindles which
separate chromosomes
...

Help bacteria stick to areas of high nutrients etc

Ribosomes
Cytoplasm
Capsule
Cell wall
Fimbriae / Pilli

Viruses
•
•
•
•

Viruses are acellular
Simply particles of nucleic acid
enclosed in a protein coat
...


Viral Envelope
Surface proteins
Virus genetic material

Methods of Studying cells
Magnification = image size / actual size
Actual size = image size x magnification
Relative sizes
Nanometre (nm)
1

Micrometre (µm)
0
...
000001

Microscopes
Light microscope
•
•
•
•

Beams of light are used to form an image
Maximum resolution 200-250nm which means you can only really see the nucleus
Maximum useful magnification is around x1500 beyond this you cannot make out any detail
the image is just bigger
They are cheaper and easier to look after, and you can also view live specimens

Electron microscopes
Pros
-

Uses a beam of electrons which have a much smaller wavelength than light
Magnification x500,000
Resolution of 1-20nm so can be used to look at all cell organelles

Cons
-

Needs a vacuum (electrons are absorbed by air particles; specimen has all water removed to
allow it to be sectioned)
Specimen needs to be extremely thin
Complex staining is needed for TEM
Image may contain artefacts
Image is black and white

Transmission Electron Microscope (TEM)
• Beam of electrons focused on specimen
– some are absorbed (appear dark),
some pass through (appear bright)
• Image is projected on a screen
• Image is 2D
• Image is black and white
• Needs extremely thin samples
• Section then impregnated with a heavy
metal stain
• Produces ‘electron micrograph’
• You get a high resolution allowing you
to see detail of organelles

Scanning Electron Microscope (SEM)
• A beam of electrons is passed across a
specimen
• Electrons are scattered and gathered in
a cathode ray tube; the pattern
depends upon the surface
• The specimen can be thicker but still
not a living cell
• Computer analysis can build up a 3D
model
• Resolution is 20nm which is lower than
TEM

Cell Fractionation
Cell fractioning – cell breaking
Homogenisation – breaking up the cells
Step 1 – Homogenisation
The aim is to break open the membrane and release the organelles
...
The solution also needs a pH buffer and needs to
be isotonic
...
Organelles pass through the gaps
Step 3 – ultracentrifugation
The cell fragments are poured into a tube and spun first at a low speed in a centrifuge
...

Supernatant is then poured off, spun again at a higher speed to collect smaller organelle
...


Cell cycle and Mitosis
Chromosome structure
•
•

Each chromosome consists of two chromatids joined
somewhere along its length with a centromere
Genetic information carried on each chromatid is
identical

Cell cycle – sequence of cell growth and division
Interphase
-

Cell growth
Synthesis of organelles
DNA copying and checking of genetic information

Mitosis
-

Chromosomes divide

Cytokinesis
-

Cytoplasm divided between the daughter cells

Interphase is the longest phase and is split into G1, S, G2
G1

First growth phase involves the manufacture of
proteins to form cell organelles
...

S
Synthesis phase, when DNA is replicated, and
chromosomes duplicated
G2
Second growth phase, organelles grow and
divide, energy stores are increased
...
E
...
death
Mitosis – refers to the process of nuclear division that occurs before a cell physically divides in two
...
In plant cells it
involves the formation of a cell plate

Mitosis
Stages of mitosis

IPMAT

•
•
•
•

Prophase – chromosomes appear, nucleus disappears
Metaphase – chromosomes at equator, spindle forms
Anaphase – chromatids pulled to poles
Telophase – chromatids at poles, nucleus reforms

-

Chromatids are separates by contraction of spindle fibres
Chromatids pulled to opposite poles of the cell
Cell then divides
Each chromatid contains identical genetic information, so each daughter cell also contains
identical information

Early Prophase

•

Late Prophase

•
•
•
•
•

Metaphase

•
•
•
•
•
•

Anaphase

•
•
•

Telophase

•

Cytokinesis

•
•
•
•
•

During prophase the chromosomes
become more distinct, they coil up,
shorten and thicken, and take up stain
easier
The centriole divides
Nucleolus becomes less prominent
The chromosomes have become more
distinct and are seen to consist of two
chromatids joined by a centromere
Centrioles migrate to opposite poles of
the cell
The nucleolus continues to shrink and
disappears
The nuclear envelope disintegrates
Each centriole is at a pole
Centriole grow/produce spindle fibres
Spindle fibres attach to the centromere
of the chromosomes
Each centromere is attached to both
poles
Chromosomes pulled to the metaphase
plate or equator
Spindle fibres contract
...
They begin the uncoil and they
become less distinct
Nuclear envelope starts to reform
The cell divides
In animal cells
...
A cell wall is laid down
Daughter cells have the same number
of chromosomes and the parent cell –
DNA replication precedes mitosis

Tumours occur when apoptosis (cell death) is slower than cell growth
...

Cancer – mutated forms of proto–oncogenes called oncogenes cannot be controlled by tumour
suppressor genes, so they continue to promote cell division resulting in a tumour
...
Some
rogue cells survive creating a malignant tumour
...
Also inhibit cell death
...

Tumour suppressor genes – prevent cell division, also lead to cell death
...

Mutated forms of tumour suppressor genes cannot control proto-oncogenes resulting in
uncontrolled cell growth
...
When DNA doesn’t
copy itself correctly, a gene mutation occurs
...

Neighbouring cells also communicate with dividing cells to regulate their growth also

Mutations can be caused by:
•
•
•
•
•

Smoking
Radiation
Pollutants
Chemicals
Viruses

-

Normal cells stop dividing when there is a mutation, cancer cells do not
...

Due to mutations cancer cells cannot communicate with neighbouring cells
...
Circular DNA and plasmids duplicate
2
...
Cytoplasm begins to divide, and new cell
walls begin to form
4
...
Each daughter cell has one copy of
the circular DNA although there can be a
variable number of plasmids
...

2
...

4
...

6
...


Mitotic Index
When mitosis is occurring actively in a tissue it is possible to make stained squashes to view to
process
...

Hydrophobic protein areas are anchored in their hydrophobic inner part of the membrane
...
Model of membranes accepted at present to be the fluid mosaic
model
...

Component of membrane
Phospholipids

Function
Acts as a barrier to most substances
...

They fit between phospholipid molecules and
help maintain the fluidity and stability of the
membrane
...
They
stabilise the membrane and act as receptor
molecules for hormones and
neurotransmitters
...
/ length of diffusion path

PERMEABLE

H+, Na+, K+,
Ca2+, Cl-,
HCO3-

PERMEABLE

Carbon dioxide is polar but its small enough to
pass through rapidly
...


IMPERMEABLE

Amino Acids
Glucose
Nucleotides

Oxygen is uncharged and non-polar
...


IMPERMEABLE

All these can only cross the membrane
through hydrophilic channels created by
protein molecules
...


Proteins in Membranes – Facilitated diffusions
Gated channel proteins – open and close in response to specific messenger
Carrier proteins – particle binds through carrier proteins, which changes shape
-

Proteins make up 45% by mass of a cell membrane typically
Integral or intrinsic or transmembrane proteins span the whole width of the membrane
...


Integral proteins:
•
•
•

Many carrier molecules or channels
Help transport substances, such as ions, sugars and amino acids, that cannot diffuse across
the membrane but are still vital to cells functioning
...


Peripheral proteins:
•
•
•

May be free on membrane surface or bound to an integral protein
Peripheral proteins on the extracellular side of the membrane act as receptors for hormones
or neurotransmitters or are involved in cell recognition
...

Peripheral proteins on the cytosolic side of the membrane are involved in cell signalling or
chemical reactions
...


Osmosis
Osmosis – diffusion of water molecules that are free to move from an area of high-water potential to
an area of low water potential through a partially permeable membrane
...
so water potential is more negative
A solution with a high-water potential has many water molecules that are free to move
...
Binding of cytoplasmic Na+ to the protein stimulates phosphorylation by ATP
2
...
The conformational change expels Na+ to the outside and extracellular K+ bonds
4
...
Loss of phosphate restores original conformation

6
...

Glucose & sodium are co-transported in
the ileum:

Cell Recognition and the immune system
•
•
•
•
•

Antigen – proteins and glycoproteins on cells surface
...

Antibody – also called immunoglobins, Y shaped proteins produced to attack pathogens
...
Chemoattractant: chemical products of the foreign
body
2
...
Pseudopodia: false feet that extent around foreign
body
4
...
Lysosome: vesicle bound enzyme packets

The Specific response: T and B lymphocyte cells
B Lymphocytes mature in Bone marrow
T Lymphocytes mature in the Thymus gland
B Lymphocytes:
•
•
•

Produce antibodies
Respond to foreign materials outside body cells
Respond to bacteria and viruses

T Lymphocytes:
•
•
•

Involved in cell mediated immunity
Respond to own cells altered by viruses etc
Respond to foreign materials outside body cells

Cell mediated response involves highly specialised cells targeting body cells

Macrophage ingests and displays
antigen

T lymphocyte meets specific antigen

T lymphocyte
divides to
become T
helper cells

T helper or T killer display antibody
specific to antigen on its surface

These secrete
cytokines to
stimulate
phagocytic cells
and B lymphocytes

T Helper cells:
•
•

Activate macrophage and
T killer cells
Create memory cells

T (cytotoxic) killer cells:
•

Kill abnormal cells and
infected body cells via
perforin

T memory cells:
•

Remain in the blood for
years and provide long
term protection

T lymphocyte
divides to
become T killer
cells

These attach to cells infected by
viruses, which displays the antigen
on its surface
T lymphocytes kill the cell they
attach to
...
When skin is cut, bacteria penetrates wound
...
The presence of bacteria to local white blood cells called mast cells, triggers a
release of histamine granules to surrounding tissue
3
...
Since local blood vessels are dilated from histamine, plasma flow to an area is
increased, causing redness, swelling and pain
...

5
...

Roles of T cells and B cells
Helper T cells:
•
•

Act as co-ordinates of immune response
When pathogen detected, helper T cells produce a chemical signal
...
Helper T cells also activate B
cells and help to produce memory cells
...


Effector B cells:
•

Cells multiply and give rise to plasma cells when activated by Helper T cells
...
If stimulated, they
divide and rapidly produce a secondary immune response
...


Humoral Immunity:

Surface antigens of the invading
pathogen are taken up by B cells
...

T helper cells activated in the
process, attach to the processed
antigens on the B cells thereby
activating them
...

The antibodies attach to antigens
on the pathogen and destroy
them
...

Some B cells develop into
memory cells
...

This is the secondary
immune response
...
Delay of the B
cells to release the antibodies
...
The
levels of antibody’s in the plasma never reaches zero after
primary response as the body is prepared for the return of
the pathogen which is why the response is much quicker
...


Vaccinations and immunity

Immunity

Artificially acquired

Naturally Acquired

Active

Passive

Active

Antigens enter body
naturally

Antibodies pass on
from mother to
foetus
...
Stable with no live components
...

They grow in a vaccinated individual although they cause no harm
...
Rare cases of
weakened pathogen regaining full strength

Sub-unit conjugated:
-

Vaccines contain specific pieces of the germ such as protein/sugar
...


Sub-unit Recombinant:
-

Vaccine that has been generated using recombinant DNA technology
...


Health and Disease – H
...
V
1
...
Viral
membrane then fuses with cell
membrane
...
DNA produced using reverse
transcriptase
...

3
...
This is the latent phase
...
HIV takes over control of the cell
which produces HIV
...


Monoclonal antibodies (mAbs)

Treating cancer:
1
...
Presence of mAbs attached to cells activates cytotoxic drugs
...
Targeted cellular destruction
...

Transplant antirejection
•
•
•

T cells recognise foreign tissue
Cause rejection
mAbs made to recognise T cells and Knock them out

Immunoassays
•
•
•

drug testing kits
pregnancy tests
detecting AIDS

Pregnancy Tests
1
...
HCG molecules bind to mobile antibodies
to form HCG/antibody complex
3
...

4
...
Due to the antigen bonds to it
whether it has formed a complex with
the HCG or not, this line will appear even
if the woman isn’t pregnant

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