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Diagram of eukaryotic cells

Structure and function – cell-surface membrane
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Phospholipid bilayer with embedded proteins etc
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Matrix containing small 70S
ribosomes, small circular DNA and enzymes involved in aerobic respiration (glycolysis)
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Not membrane bound
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Site of protein synthesis, specifically, translation

Structure and function – rough endoplasmic reticulum

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Ribosomes bound by a system of membranes
Folds polypeptides to secondary / tertiary structure
Packages to vesicles, transport to the Golgi apparatus etc
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These sit in the stroma (fluid) and are surrounded by a double membrane
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(Chlorophyll) absorbs light for photosynthesis to produce organic substances

Structure and function – cell wall (plants, algae and fungi)
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Made mainly of cellulose in plants and algae, and of chitin in fungi
Rigid structure surrounding cells in plants, algae and fungi
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Surrounding membrane is called the
tonoplast
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Villi
and microvilli increase surface area
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one or more plasmid, a capsule, and/or one or more flagella

Viruses

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Acellular → not made of or able to be divided into cells
Non-living → unable to exist/reproduce without a host cell

Principles and limitations of optical microscopes, transmission electron
microscopes and scanning electron microscopes
Optical microscope
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Use light to form a 2D image
Visible light longer
wavelength so lower
resolution 200nm
Low magnification x1500

 2D image
 Only used on thin specimens
 Low resolution; can’t see
internal structures of organelles
or organelles smaller than 200nm
e
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ribosomes
 Low magnification
☺ Can see living organisms
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2nm)
- High magnification x1500000

Transmission electron
microscope
- Use electrons to form a 3D
image
- Electromagnets focus beam of
electrons onto specimen,
transmitted, more dense =
more absorbed = darker
appearance
- Electrons shorter wavelength
(so higher resolution 0
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g
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g
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Homogenise tissue using a blender
- Disrupts cell membrane / break open cell
- Release contents / organelles
2
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Filter homogenate
- Remove large, unwanted debris e
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whole cells, connective tissue
4
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Eukaryotic cells that do
retain the ability to divide show a cell cycle
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Stages - ‘PMAT’

Stages of mitosis (the behaviour of chromosomes and the role of spindle
fibres attached to centromeres in the separation of chromatids)
Prophase
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Chromosomes condense, becoming shorter and thicker = appear as two sister chromatids
joined by a centromere
Nuclear envelope breaks down and centrioles move to opposite poles forming spindle
network

Metaphase
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Chromosomes align along equator

-

Spindle fibres attach to chromosomes by centromeres

Anaphase
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Spindle fibres contract, pulling sister chromatids to opposite poles of the cell
Centromere divides

Telophase
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Chromosomes uncoil, becoming longer and thinner
Nuclear envelope reforms = two nuclei
Spindle fibres and centrioles break down

Cytokinesis
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The division of the cytoplasm, usually occurs, producing two new cells

The importance of mitosis in the life of an organism
Parent cell divides to produce 2 genetically identical daughter cells for…
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Growth of multicellular organisms by increasing cell number
Repairing damaged tissues / replacing cells
Asexual reproduction

Recognising the stages of the cell cycle and explaining the appearance of cells
in each stage of mitosis

(source:
kerboodle textbook online)
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Interphase – C → no chromosomes visible (visible nucleus)
Prophase – B → chromosomes visible but randomly arranged
Metaphase – D → chromosomes lined up on the equator
Anaphase – E → chromatids (in two sets) being separated to opposite poles by spindles, V
shape shows sister chromatids have been pulled apart at their centromeres
Telophase – A → chromosomes in two sets, one at each pole

Uncontrolled cell division can lead to the formation of tumours and of cancers
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Malignant tumour – cancer – spreads and affects other tissues / organs
Benign tumour – non-cancerous

Many cancer treatments are directed at controlling the rate of cell division
Disrupt the cell cycle – cell division / mitosis slows – tumour growth slows
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Prevent DNA replication → prevent / slows down mitosis

-

Disrupts spindle activity / formation → chromosomes can’t attach to spindle by their
centromere → sister chromatids can’t be pulled to opposite poles of the cells →
prevent/slow mitosis

 Disrupt cell cycle of normal cells too, especially rapidly dividing ones e
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cells in hair follicles
☺ Drugs more effective against cancer cells because dividing uncontrollably / rapidly

Prokaryotic cells replicate by binary fission

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Circular DNA and plasmids replicate (circular DNA replicates once, plasmids can be
replicated many times)
Cytoplasm expands (cell gets bigger) as each DNA molecule moves to opposite poles of the
cell
Cytoplasm divides
= 2 daughter cells, each with a single copy of DNA and a variable number of plasmids

Viral replication
Viruses don’t undergo cell division because they are non-living
1
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Inject nucleic acid (DNA/RNA) into host cell
3
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Sodium ions actively transported out of epithelial cells lining
the ileum, into the blood, by the sodium-potassium pump
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of
sodium in lumen than epithelial cell)
2
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Creating a concentration gradient of glucose – higher conc
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Glucose moves out of cell into blood by facilitated diffusion
through a protein channel

Movement across membranes by osmosis and factors
affecting rate
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Net movement of water molecules across a selectively permeable membrane down a water
potential gradient
Water potential is the likelihood (potential) of water molecules to diffuse out of or into a
solution; pure water has the highest water potential and adding solutes to a solution lowers
the water potential (more negative)
Passive
Factors affecting rate – surface area, water potential gradient, thickness of exchange surface
/ diffusion distance

How might cells be adapted for transport across their internal or external
membranes
-

By an increase in surface area
Increase in number of protein channels / carriers

Antigen definition
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Molecules which, when recognised as non-self/foreign by the immune system, can
stimulate an immune response and lead to the production of antibodies
Often proteins on the surface of cells
Note: proteins have a specific tertiary structure / shape allowing different proteins to act as
specific antigens

Antigens are specific so allow the immune system to identify…
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Pathogens (disease causing organisms) e
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viruses, fungi, bacteria
Cells from other organisms of the same species e
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organ transplant, blood transfusion
Abnormal body cells e
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cancerous cells / tumours
Toxins released from bacteria

Phagocytosis of pathogens – non-specific immune response
1
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g
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Phagocyte engulfs pathogen by surrounding it with its cell surface membrane / cytoplasm
3
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Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes) into the
phagosome
5
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Phagocyte becomes antigen presenting and stimulates specific immune response

The cellular response (the response of T lymphocytes to a foreign antigen e
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infected cells, cells of the same species)
1
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Specific T helper cell with receptor complementary to specific antigen binds to it, becoming
activated and dividing rapidly by mitosis to form clones which:
a) Stimulate B cells for the humoral response
b) Stimulate cytotoxic T cells to kill infected cells by producing perforin
c) Stimulate phagocytes to engulf pathogens by phagocytosis

The humoral response (the response of B lymphocytes to a foreign antigen e
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in blood/tissues)
1
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Some become B plasma cells for the primary immune response – secrete large amounts of
monoclonal antibody into blood
3
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Scientists have questioned the value of
the research because there may be differences between human and money
responses / immune systems, and a vaccine developed against SIV may not
work against HIV / may be (significant) differences between SIV and HIV
- Potential bias?

The use of monoclonal antibodies
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Monoclonal antibody = antibody produced from a single group of genetically identical
(clones) B cells / plasma cells
- Identical structure
Bind to specific complimentary antigen
- Have a binding site / variable region with a specific tertiary structure / shape
- Only one complementary antigen will fit

Why are monoclonal antibodies useful in medicine?
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Only bind to specific target molecules / antigens because…
Antibodies have a specific tertiary structure (binding site / variable region) that’s
complementary to a specific antigen which can bind/fit to the antibody

Monoclonal antibodies: targeting medication to specific cell types by
attaching a therapeutic drug to an antibody
Example: cancer cell
1
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Anti-cancer drug attached to antibody
3
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Delivers attached anti-cancer drug directly to specific cancer cells so drug accumulates →
fewer side effects e
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fewer normal body cells killed
Exam question example: some cancer cells have a receptor protein in their cell-surface membrane
that binds to a hormone called growth factor
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Scientists
have produced a monoclonal antibody that stops this stimulation
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Two species that cause malaria are Plasmodium falciparum and Plasmodium vivax
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It can also be used to find which species of Plasmodium they are infected by
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This antibody has a coloured dye
attached
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The mixture moves up the test strip by capillary
action to an absorbent pad
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The position of these
antibodies and what they bind to is shown in figure 1
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(2
marks)
✓ Antibody has tertiary structure
✓ Complementary to binding site on protein
(b) Antibody B is important is this test shows a person is not infected with Plasmodium
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(2 marks)
✓ Prevents false negative results
✓ (Since) shows antibody A has moved up strip / has not bound to any Plasmodium
protein
(c) One of these test strips was used to test a sample from a person thought to be infected
with Plasmodium
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What can you conclude from this result? Explain how you reached your conclusion
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lambia
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(i)
(ii)
(iii)

Explain why the antibodies used in this test must be monoclonal antibodies
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(1 mark)
✓ Has complementary 9shape) / due to (specific) tertiary structure / variable region
(of antibody)
The plate must be washed at the start of step 4, otherwise a positive result could be
obtained when the Giardia antigen is not present
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(2 marks)
✓ Enzyme / second antibody would remain / is removed by washing
✓ Enzyme can react with substrate (when no antigen is present)

Exam question example: A test has been developed to find out whether a person has antibodies
against the mumps virus
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(a) Explain why this test will detect mumps antibodies, but not other antibodies in the blood
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(2 marks)
✓ Removes unbound 2nd antibodies
✓ Otherwise enzyme may be present / get a colour change anyway / false positive
(c) Explain why there be no colour change if mumps antibodies are not present in the blood
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e
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HIV infects T helper cells (host cell)
- HIV attachment protein (GP120) attaches to a receptor on the helper T-cell
membrane
2
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Viral DNA is made from viral RNA
- Reverse transcriptase produces a complementary viral DNA strand from viral RNA
template
- Double stranded DNA is made from this (DNA polymerase)
4
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This remains latent for a long time in host cell until activated
6
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New virus bud from cell (taking some of cell surface membrane as envelope)
8
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Most host cells are infected and process repeat

How HIV causes the symptoms of AIDS – acquired immune deficiency
syndrome
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Infects and kills helper T cells (host cell) as it multiplies rapidly
- T helper cells then can’t stimulate cytotoxic T cells, B cells and phagocytes →
impaired immune response
- E
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B plasma cells can’t secrete antibodies for agglutination and destruction
of pathogens by phagocytosis
Immune system deteriorates
- More susceptible to infections
- Diseases that wouldn’t cause serious problems in healthy immune system are deadly
(opportunistic infections) e
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pneumonia

Why antibiotics are ineffective against viruses
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Antibiotics can’t enter human calls – but viruses exists in its host cell (they are acellular)
Viruses don’t have own metabolic reactions e
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ribosomes (use of the host cell’s) which
antibiotics target
If we did use them… act as a selection pressure + gene mutation = resistant strain of
bacteria via natural selection → reducing effectiveness of antibiotics and waste money



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