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AQA A-Level Biology > [3.2] Cells > Flashcards

[3.2] Cells Flashcards

Cell Structure, All Cells Arise from other Cells, Transport Across Cell Membranes, Cell Recognition & the Immune System

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1
Q

What are the distinguishing factors of eukaryotic cells?

A

Cytoplasm containing membrane-bound organelles so DNA enclosed in a nucleus.

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2
Q

Describe the general structure of eukaryotic cells.

A
  • Cell-surface membrane.
  • Mitochondrion.
  • Nucleus.
  • Ribosomes.
  • Rough endoplasmic reticulum.
  • Smooth endoplasmic reticulum.
  • Golgi apparatus.
  • Lysosome.
  • Chloroplast (in plants & algae).
  • Cell wall (in plants, algae & fungi).
  • Cell vacuole (in plants).
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3
Q

Describe the structure and function of the cell-surface membrane.

A

STRUCTURE

  • Phospholipid bilayer.
  • Hydrophilic phosphate heads - attracted to water.
  • Hydrophobic fatty acid tails - repelled from water.
  • Proteins.

FUNCTION

  • Selectively permeable - enables control of passage of substances in and out of the cell.
  • Molecules/receptors/antigens on the surface - allow cell recognition/signalling.
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4
Q

Describe the structure and function of the nucleus.

A

STRUCTURE

  • Nuclear envelope - double membrane with nuclear pores.
  • Nucleoplasm.
  • Nucleolus/nucleoli.
  • Contains chromosomes consisting of protein/histone-bound, linear DNA.

FUNCTION

  • Holds/stores genetic information which codes for polypeptides.
  • Site of DNA replication (transcription).
  • Site of transcription, producing mRNA.
  • Nucleolus makes ribosomes/rRNA.
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5
Q

Describe the structure and function of a ribosome.

A

STRUCTURE

  • Made of ribosomal RNA (rRNA) and protein.
  • Not a membrane-bound organelle.

FUNCTION

  • Site of protein synthesis (translation).
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6
Q

Describe the structure and function of the rough endoplasmic reticulum.

A

STRUCTURE

  • System of membranes.
  • Covered in ribosomes.

FUNCTION

  • Ribosomes on surface synthesise proteins.
  • Proteins processed/folded/transported inside rER.
  • Proteins packaged into vesicles for transport e.g. to Golgi apparatus.
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7
Q

Describe the structure and function of the smooth endoplasmic reticulum.

A

STRUCTURE

  • System of membranes.

FUNCTION

  • Synthesises and processes lipids such as cholesterol and steroid hormones.
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8
Q

Describe the structure & function of Golgi apparatus and Golgi vesicles

A

STRUCTURE

  • Golgi apparatus - flattened membrane sacs.
  • Golgi vesicle - small membrane sac.

FUNCTION OF GOLGI APPARATUS

  • Modifides proteins e.g. adds carbohydrates to produce glycoproteins.
  • Modifies lipids e.g. adds carbohydrates to make glycolipids.
  • Packages proteins and lipids into golgi vesicles.
  • Produces lysosomes.

FUNCTION OF GOLGI VESICLE

  • Transports proteins and lipids to their required destination e.g. moves to and fuses with cell-surface membrane.
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9
Q

Describe the structure and function of lysosomes.

A

STRUCTURE

  • Membrane-bound organelle.
  • Contains hydrolytic enzymes.

FUNCTION

  • Release hydrolytic enzymes (lysozymes) to hydrolyse pathogens or worn-out cell components.
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10
Q

Describe the structure and function of mitochondria.

A

STRUCTURE

  • Double-membrane (outer and inner membrane).
  • Cristae - inner membrane fold.
  • Matrix containing small (70S) ribosomes and circular DNA.

FUNCTION

  • Site of aerobic respiration to produce ATP for energy release for protein synthesis, vesicle movement, active transport etc.
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11
Q

Describe the structure and function of chloroplasts in plants and algae.

A

STRUCTURE

  • Double membrane
  • Stroma containing thylakoid membranes, small (70S) ribosomes, circular DNA and starch granules/lipid droplets.
  • Grana - stacks of thylakoid.
  • Lamella - thylakoid linking grana.

FUNCTION

  • Absorbs light energy for photosynthesis to produce organic substances like carbohydrates and lipids.
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12
Q

Describe the structure and function of the cell wall in plants, algae and fungi.

A

STRUCTURE

  • Composed mainly of cellulose in plants and algae.
  • Composed of chitin in fungi.

FUNCTION

  • Provides mechanical strength to cell so prevents cell changing shape or bursting under pressure due to osmosis.
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13
Q

Describe the structure and function of the cell vacuole in plants.

A

STRUCTURE

  • Tonoplast membrane.
  • Cell sap.

FUNCTION

  • Maintains turgor pressure in cell which stops the plant from wilting.
  • Contains cell sap which stores sugars, amino acids, pigments and any waste chemicals.
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14
Q

Describe how eukaryotic cells are organised in complex multicellular organisms.

A
  1. Tissue - group of specialised cells with a similar structure working
    together to perform a specific function, often with the same origin.
  2. Organ - aggregations of tissues performing specific functions.
  3. Organ system - group of organs working together to perform specific functions.
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15
Q

What are distinguishing features of prokaryotic cells?

A

Cytoplasm lacking membrane-bound organelles so genetic material not enclosed in a nucleus.

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16
Q

Describe the general structure of prokaryotic cells.

A

ALWAYS PRESENT

  • Cell-surface membrane.
  • Cell wall containing murein, a glycoprotein.
  • Cytoplasm lacking membrane-bound organelles.
  • Small ribosomes (70S).
  • Circular DNA that’s free in cytoplasm and not associated with proteins.

SOMETIMES PRESENT

  • Capsule surrounding cell (provide protection and help with adhesion).
  • Plasmids - small rings of DNA.
  • Flagella.
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17
Q

Compare and contrast the structure of eukaryotic and prokaryotic cells.

A

EUKARYOTIC CELL

  • Has membrane-bound organelles.
  • Has a nucleus containing DNA.
  • DNA is long, linear and associated with histone proteins.
  • Larger (80S) ribosomes.
  • Cell wall only in plants, algae and fungi containing cellulose or chitin.
  • Plasmids and capsule never present.
  • Larger overall size.

PROKARYOTIC CELL

  • No membrane-bound organelles.
  • No nucleus, DNA is free in the cytoplasm.
  • DNA is short, circular and not associated with proteins.
  • Smaller ribosomes (70S).
  • Cell wall in all prokaryotic cells containing murein, a glycoprotein.
  • Plasmids and capsule sometimes present.
  • Much smaller overall size.
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18
Q

Explain why viruses are described as acellular and non-living.

A
  • Aceullar - not made of cells, no cell membrane/cytoplasm/organelles.
  • Non-living - have no metabolism, cannot independently
    move/respire/replicate/excrete.
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19
Q

Describe the general structure of a virus particle.

A
  • Nucleic acids (DNA or RNA) surrounded by a capsid (protein coat).
  • Attachment proteins allow attachment to specific host cells.
  • No cytoplasm, ribosomes, cell wall, cell-surface membrane etc.
  • Some also surrounded by a lipid envelope such as HIV.
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20
Q

Describe the difference between magnification and resolution.

A

Magnification = number of times greater image is than size of the real (actual) object.

Resolution = minimum distance apart 2 objects can be to be distinguished as separate objects.

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21
Q

What is the equation for magnification?

A

Magnification = size of image / size of real object

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22
Q

Describe the principles, advantages and disadvantages of optical microscopes.

A

PRINCIPLES

  • Light focused using glass lenses.
  • Light passes through specimen, different structures absorb different
    amounts & wavelengths which generates a 2D image of a cross-section.

ADVANTAGES

  • Can view living organisms.
  • Simple preparation.
  • Can show colour.

DISADVANTAGES

  • Low resolution due to long wavelength of light.
  • Can’t see internal structure of organelles or ribosomes.
  • Specimen has to be thin.
  • Low magnification (x1500).
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23
Q

Describe the principles, advantages and disadvantages of transmission electron microscopes (TEM).

A

PRINCIPLES

  • Electrons focused using electromagnets.
  • Electrons pass through specimen, denser parts absorb more and appear darker which generates a 2D image of a cross-section.

ADVANTAGES

  • Very high resolution due to short wavelength of electrons.
  • Can see internal structures of organelles and ribosomes.
  • High magnification (x 1,000,000)

DISADVANTAGES

  • Specimen has to be very thin.
  • Can only view dead/dehydrated specimens as uses a vacuum.
  • Complex preparation so artefacts often present.
  • Does not show colour.
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24
Q

Describe the principles, advantages and disadvantages of scanning electron microscopes (SEM)

A

PRINCIPLES

  • Electrons focused using electromagnets.
  • Electrons deflected/bounce off specimen surface which generates 3D image of surface.

ADVANTAGES

  • High resolution due to short wavelength of electrons.
  • High magnification (x 1,000,000)
  • Specimen does not need to be thin.

DISADVANTAGES

  • Can’t see internal structures.
  • Can only view dead/dehydrated specimens as uses a vacuum.
  • Complex preparation so artefacts often present.
  • Does not show colour.
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25
Q

List the steps in calculations involving magnification, real size and image size.

A
  1. Note formula and rearrange if necessary.
  2. Convert units if necessary - image and actual size must be in the same units.
  3. Calculate answer and check units required or if standard form is required.
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26
Q

Describe how to convert between different units.

A

Metre (m) to centimetre (cm) = x100
Centimetre (cm) to millimetre (mm) = x10
Millimetre (mm) to micromere (µm) = x1000
Micrometre (µm) to nanometre (nm) = x1000

(To convert back, use the same numbers, but divide instead)

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27
Q

Describe how the size of an object viewed with an optical microscope can be measured.

A
  1. Line up scale of eyepiece graticule with scale of stage micrometre.
  2. Calibrate eyepiece graticule by using stage micrometre to calculate size of divisions on eyepiece graticule.
  3. Take micrometre away and use graticule to measure how many divisions make up the object.
  4. Calculate the size of object by multiplying number of divisions by size of division.
  5. Recalibrate eyepiece graticule at different magnifications.
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28
Q

Describe and explain the principles of cell fractionation and ultracentrifugation as used to separate cell components.

A
  1. Homogenise tissue/use a blender.
    • Disrupts cell membrane, breaking open cells and releasing
      contents/organelles.
  2. Place in cold, isotonic, buffered solution.
    • Cold to reduce enzyme activity so organelles not broken
      down/damaged.
    • Isotonic so water doesn’t move in or out of organelles by osmosis so they don’t burst.
    • Buffered to keep pH constant so enzymes don’t denature.
  3. Filter homogenate
    • Remove large, unwanted cell debris.
  4. Ultracentrifugation - separates organelles in order of density/mass.
    • Centrifuge homogenate in a tube at high speed.
    • Remove pellet of heaviest organelle and respin supernatant at a higher speed.
    • Repeat at increasing speeds until separated out, each time pellet made of lighter organelles (nuclei -> chloroplasts/mitochondria -> lysosomes -> ER -> ribosomes)
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29
Q

Describe the stages of the cell cycle in eukaryotic cells.

A
  1. INTERPHASE
    • S Phase - DNA replicates semi-conservatively leading to 2 chromatids (identical copies) joined at a centromere.
    • G1/G2 - Number of organelles and volume of cytoplasm increase, protein synthesis.
  2. MITOSIS
    • Nucleus divides to produce 2 nuclei with identical copies of DNA produced by parent cell.
  3. CYTOKINESIS
    • Cytoplasm and cell membrane divide to form 2 new genetically identical daughter cells.
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30
Q

Name all the stages of mitosis, describe the behaviour of the chromosomes and the role of spindle fibres within these stages.

A
  1. PROPHASE
    • Chromosomes condense, becoming shorter/thicker and appearing as 2 sister chromatids joined by a centromere.
    • Nuclear envelope breaks down.
    • Centrioles move to opposite poles forming spindle network.
  2. METAPHASE
    • Spindle fibres attach to chromosomes by their centromeres.
    • Chromosomes align along equator.
  3. ANAPHASE
    • Spindle fibres shorten/contract.
    • Centromere divides pulling chromatids from each pair to opposite poles of the cell.
  4. TELOPHASE
    • Chromosomes uncoil, becoming longer and thinner.
    • Nuclear envelopes reform forming 2 nuclei.
    • Spindle fibres/centrioles break down.
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31
Q

Why do some eukaryotic cells not undergo the cell cycle?

A

Within multicellular organisms, not all cells retain the ability to divide such as neurones. Only cells that do retain this ability go through a cell cycle.

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32
Q

Explain the importance of mitosis in the life of an organism.

A

Parent cell divides to produce 2 genetically identical daughter cells for:

  • Growth of multicellular organisms by increasing cell number.
  • Replacing cells to repair damaged tissues.
  • Asexual reproduction.
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33
Q

Describe how tumours and cancers form.

A
  1. Mutations in DNA/genes controlling mitosis can lead to uncontrolled cell division (mitosis is a controlled process).
  2. Tumour formed if this results in mass of abnormal cells.
    • Malignant tumour = cancerous, can spread (metastasis).
    • Benign tumour = non-cancerous, doesn’t spread.
34
Q

Suggest how cancer treatments control rate of cell division.

A
  1. Some disrupt spindle fibre activity/formation.
    • So spindles do not attach to chromosomes by their centromere which means chromatids can’t be separated to opposite poles (no anaphase) so prevents/slows mitosis.
  2. Some prevent DNA replication during interphase.
    • So can’t make 2 copies of each chromosome (chromatids) so prevents/slows mitosis.

(Treatments more effective against cancer cells due to uncontrolled division, but disrupt cell cycle of rapidly dividing healthy cells too)

35
Q

Describe how prokaryotic cells replicate.

A

BINARY FISSION

  1. Replication of circular DNA.
  2. Replication of plasmids.
  3. Division of cytoplasm to produce 2 daughter cells that each contain a single copy of circular DNA and a variable number of copies of plasmids.
36
Q

Describe how viruses replicate.

A

Being non-living, viruses do not undergo cell division, instead:

  1. Attachment proteins attach to complementary receptors on host cell.
  2. Inject viral nucleic acid (DNA/RNA) into host cell.
  3. Infected host cell replicates virus particles:
    • a. Nucleic acid replicated.
    • b. Cell produces viral protein/capsid/enzymes.
    • c. Virus assembled then released.
37
Q

Describe the fluid-mosaic model of membrane structure.

A
  • Molecules free to move laterally in phospholipid bilayer.
  • Many components - phospholipids, proteins, glycoproteins and
    glycolipids.

(The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes is the same)

38
Q

Describe the arrangement of the components of a cell membrane.

A
  1. Phospholipids
    • Form a bilayer - fatty acid tails face inwards, phosphate heads face outwards.
  2. Proteins
    • Intrinsic/integral proteins span bilayer e.g. channel and carrier proteins.
    • Extrinsic/peripheral proteins on surface of membrane.
  3. Glycolipids
    • Lipids with polysaccharide chains attached to them found on exterior surface.
  4. Glycoproteins
    • Proteins with polysaccharide chains attached) found on exterior surface.
  5. Cholesterol (sometimes present)
    • Bonds to phospholipid hydrophobic fatty acid tails.
39
Q

Explain the arrangement of phospholipids in a cell membrane.

A
  • Bilayer with water present on either side.
  • Hydrophobic fatty acid tails repelled from water so point away from water/to interior.
  • Hydrophilic phosphate heads attracted to water so point to water.
40
Q

Explain the role of cholesterol in cell membranes.

A

Restricts movement of other molecules making up membrane so decreases fluidity and permeability/increases rigidity.

41
Q

Suggest how cell membranes are adapted for other functions.

A
  • Phospholipid bilayer is fluid so membrane can bend for vesicle formation/phagocytosis.
  • Glycoproteins/glycolipids act as receptors/antigens so involved in cell signalling/recognition.
42
Q

Describe how movement across membranes occurs by simple diffusion.

A
  • Lipid-soluble (non-polar) or very small substances (e.g. O₂, steroid hormones) move from an area of high concentration to an area of low concentration down a concentration gradient across phospholipid bilayer.
  • Passive - doesn’t require energy from ATP/respiration, only kinetic energy of substances.
43
Q

Explain the limitations imposed by the nature of the phospholipid bilayer.

A

Restricts movement of water-soluble (polar) and larger substances (e.g. Na⁺, glucose) due to hydrophobic fatty acid tails in interior of bilayer.

44
Q

Describe how movement across membranes occurs by facilitated diffusion.

A
  • Water-soluble (polar)/slightly larger substances move from an area of high concentration to an area of low concentration down a concentration gradient through specific channel/carrier proteins.
  • Passive - doesn’t require energy from ATP/respiration, only kinetic energy of substances.
45
Q

Explain the role of carrier and channel proteins in facilitated diffusion.

A
  • Shape/charge of protein determines which substances move.
  1. Channel proteins facilitate diffusion of water-soluble substances.
    • Hydrophilic pore filled with water.
    • May be gated - can open/close.
  2. Carrier proteins facilitate diffusion of slightly larger substances
    • Complementary substance attaches to binding site.
    • Protein changes shape to transport substance.
46
Q

Describe how movement across membranes occurs by osmosis.

A
  • Water diffuses from an area of high water potential to an area of low water potential down a water potential gradient through a partially permeable membrane.
  • Passive - doesn’t require energy from ATP/respiration, only kinetic energy of substances.
47
Q

What is water potential?

A
  • Water potential is a measure of how likely water molecules are to move out of a solution.
  • Pure (distilled) water has the maximum possible water potential (0 kPA).
  • Increasing solute concentration decreases water potential.
48
Q

Describe how movement across membranes occurs by active transport.

A
  • Substances move from an area of low concentration to an area of high concentration against the concentration gradient.
  • Requires hydrolysis of ATP and specific carrier proteins.
49
Q

Describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport.

A
  1. Complementary substance binds to specific carrier protein.
  2. ATP binds, hydrolysed into ADP + Pi, releasing energy.
  3. Carrier protein changes shape, releasing substance on side of higher concentration.
  4. Pi released - protein returns to original shape.
50
Q

Describe how movement across membranes occurs by co-transport.

A
  • Two different substances bind to and move simultaneously via a co-transporter protein (type of carrier protein).
  • Movement of one substance against its concentration gradient is often coupled with the movement of another down its concentration gradient.
51
Q

Describe the absorption of sodium ions and glucose (or amino acids) by cells lining the mammalian ileum.

A

CO-TRANSPORT

  1. Na⁺ actively transported from epithelial cells to blood by Na⁺/K⁺ pump establishing a concentration gradient of Na⁺ (higher in lumen than epithelial cell).
  2. Na⁺ enters epithelial cell down its concentration gradient with glucose against its concentration gradient via a co-transporter protein.
  3. Glucose moves down a concentration gradient into blood via facilitated diffusion.
52
Q

Describe how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes.

A
  • Increasing surface area of membrane increases rate of movement.
  • Increasing number of channel/carrier proteins increases rate of facilitated diffusion/active transport.
  • Increasing concentration gradient increases rate of simple/facilitated diffusion and osmosis.
  • Increasing concentration gradient increases rate of facilitated diffusion until number of channel/carrier proteins becomes a limiting factor as all in use/saturated.
  • Increasing water potential gradient increases rate of osmosis.
53
Q

Explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes.

A
  1. Membranes folded (e.g. microvilli in ileum) increase surface area.
  2. More protein channels/carriers for facilitated diffusion (or active transport - carrier proteins only)
  3. Large number of mitochondria to make more ATP by aerobic respiration for active transport.
54
Q

What is an antigen?

A

Foreign molecule/protein/glycoprotein/glycolipid that stimulates an immune response leading to production of an antibody.

55
Q

How are cells identified by the immune system?

A
  • Each type of cell has specific molecules on its surface (cell-surface membrane/cell wall) that identify it.
  • Often they are proteins which have a specific tertiary structure (or glycoproteins/glycolipids).
56
Q

What types of cells and molecules can the immune system identify?

A
  1. Pathogens (disease-causing microorganisms) e.g. viruses, fungi,
    bacteria.
  2. Cells from other organisms of the same species e.g. organ transplants.
  3. Abnormal body cells e.g. tumour cells or virus-infected cells.
  4. Toxins (poisons) released by some bacteria.
57
Q

Describe phagocytosis of pathogens (non-specific immune response).

A
  1. Phagocyte attracted by chemicals/recognises (foreign) antigens on pathogens.
  2. Phagocyte engulfs pathogen by surrounding it with its cell membrane.
  3. Pathogen contained in vesicle/phagosome in cytoplasm of phagocyte.
  4. Lysosome fuses with phagosome producing a phagolysosome and releases lysozyme (hydrolytic enzymes).
  5. Lysozymes hydrolyse/digest pathogen.
  6. Phagocytosis leads to presentation of antigens, from the pathogen, where antigens are displayed on the phagocyte cell-surface membrane, stimulating the specific immune response (cellular and humoral response).
58
Q

Describe the response of T lymphocytes to a foreign antigen (the cellular response)

A

Specific helper T cells with complementary receptors on their cell surface bind to antigen on antigen-presenting cells (this could be infected cells, phagocytes, transplanted cells, tumour cells etc.) causing it to divide by mitosis to form clones which stimulate:

  1. Cytotoxic T cells - kill infected cells/tumour cells (by producing perforin).
  2. Specific B cells (humoral response).
  3. Phagocytes - engulf pathogens by phagocytosis.
59
Q

Describe the response of B lymphocytes to a foreign antigen (the humoral response).

A
  1. Clonal selection:
    • Specific B lymphocyte with complementary receptor (antibody on cell surface) binds to antigen.
    • This is then stimulated by helper T cells (which release cytokines).
    • So B lymphocyte divides rapidly by mitosis to form clones.
  2. Some differentiate into B plasma cells and secrete large amounts of monoclonal antibodies.
  3. Some differentiate into B memory cells and remain in blood for secondary immune response.
60
Q

What are antibodies?

A
  • Quaternary structure proteins (4 polypeptide chains).
  • Secreted by B lymphocytes e.g. plasma cells in response to specific antigens.
  • Bind specifically to antigens forming antigen-antibody complexes.
61
Q

Describe the structure of an antibody.

A
62
Q

Explain how antibodies lead to the destruction of pathogens.

A
  1. Antibodies bind to antigens on pathogens forming an antigen-antibody complex.
    • Specific tertiary structure so binding site/variable region binds to complementary antigen.
  2. Each antibody binds to 2 pathogens at a time causing agglutination of pathogens.
  3. Antibodies attract phagocytes.
  4. Phagocytes bind to the antibodies and phagocytose many pathogens at once.
63
Q

Explain the differences between the primary and secondary immune response.

A

PRIMARY - FIRST EXPOSURE TO ANTIGEN

  • Antibodies produced slowly and at a lower concentration.
  • Takes time for specific B plasma cells to be stimulated to produce
    specific antibodies.
  • Memory cells produced.

SECONDARY - SECOND EXPOSURE TO ANTIGEN

  • Antibodies produced faster and at a higher concentration.
  • B memory cells rapidly undergo mitosis to produce many plasma cells which produce specific antibodies.
64
Q

What is a vaccine?

A

Injection of antigens from attenuated (dead or weakened) pathogens stimulating formation of memory cells.

65
Q

Explain how vaccines provide protection to individuals against disease.

A
  1. Specific B lymphocytes with complementary receptor binds to antigen.
  2. Specific T helper cell binds to antigen-presenting cell and stimulates B cell.
  3. B lymphocyte divides by mitosis to form clones.
  4. Some differentiate into B plasma cells and release antibodies.
  5. Some differentiate into B memory cells.
  6. On secondary exposure to antigen, B memory cells rapidly divide by mitosis to produce B plasma cells.
  7. These release antibodies faster and at a higher concentration.
66
Q

Explain how vaccines provide protections for populations against disease.

A

Herd immunity - large proportion of population vaccinated, reducing spread of pathogen.

  • Larger proportion of population immune so do not become ill from infection.
  • Fewer infected people to pass pathogen on/unvaccinated people less likely to come in contact with someone with disease.
67
Q

Describe the differences between active and passive immunity.

A

ACTIVE IMMUNITY

  • Initial exposure to antigen e.g. vaccine or primary infection.
  • Memory cells involved.
  • Antibody produced and secreted by B plasma cells.
  • Slow; takes longer to develop.
  • Long-term immunity as antibodies can be produced in response to a specific antigen again due to memory cells.

PASSIVE IMMUNITY

  • No exposure to antigen.
  • No memory cells involved.
  • Antibody introduced from another organism e.g. breast milk/across placenta from mother.
  • Fast acting.
  • Short-term immunity as antibody hydrolysed.
68
Q

Explain the effect of antigen variability on disease and disease prevention.

A
  • Antigens on pathogens change shape/tertiary structure due to gene mutations creating new strains.
  • So no longer immune from vaccine or prior infection.
    • B memory cell receptors cannot bind to/recognise changed antigen on secondary exposure.
    • Specific antibodies not complementary/cannot bind to changed antigen.

(Examples: yearly new flu vaccines, no vaccine for HIV, catch a cold many times)

69
Q

Describe the structure of an HIV particle.

A
70
Q

Describe the replication of HIV in helper T cells.

A
  1. HIV attachment proteins attach to receptors on helper T cell.
  2. Lipid envelope fuses with cell-surface membrane, releasing capsid into cell.
  3. Capsid uncoats, releasing RNA and reverse transcriptase.
  4. Reverse transcriptase converts viral RNA to DNA.
  5. Viral DNA inserted/incorporated into helper T cell DNA.
  6. Viral protein/capsid/enzymes are produced
    • a. DNA transcribed into HIV mRNA.
    • b. HIV mRNA translated into new HIV proteins.
  7. Virus particles are assembled and released from cell via budding or bursting the cell.
71
Q

Explain how HIV causes the symptoms of acquired immune deficiency syndrome (AIDS).

A
  1. HIV infects and kills helper T cells as it multiplies rapidly.
    • So T helper cells can’t stimulate cytotoxic T cells, B cells and phagocytes.
    • So plasma cells can’t release as many antibodies for agglutination and destruction of pathogens.
  2. Immune system deteriorates so more susceptible to infections.
  3. Pathogens reproduce, release toxins and damage cells.
72
Q

Explain why antibiotics are ineffective against viruses.

A

Viruses do not have structures/processes that antibiotics inhibit:

  • Viruses do not have metabolic processes/ribosomes.
  • Viruses do not have bacterial enzymes/murein cell wall.
73
Q

What is a monoclonal antibody?

A

Antibodies produced from genetically identical/cloned B lymphocyte/plasma cells so have same tertiary structure.

74
Q

Explain how monoclonal antibodies can be used in medical treatments.

A
  1. Monoclonal antibody has specific tertiary structure/binding site/variable region.
  2. Complementary to receptor/protein/antigen found only on a specific cell type (e.g. cancer cell).
  3. Therapeutic drug attached to antibody.
  4. Antibody binds to specific cell, forming antigen-antibody complex, delivering drug.

(Some monoclonal antibodies are designed to block antigens/receptors on cells too)

75
Q

Explain how monoclonal antibodies can be used in medical diagnosis.

A
  1. Monoclonal antibody has a specific tertiary structure/binding site/variable region.
  2. Complementary to specific receptor/protein/antigen associated with diagnosis.
  3. Dye/stain/fluorescent marker attached to antibody.
  4. Antibody binds to receptor/protein/antigen, forming antigen-antibody complex.
76
Q

Explain the use of antibodies in ELISA test to detect antigens.

A

DIRECT ELISA

  1. Attach sample with potential antigens to well.
  2. Add complementary monoclonal antibodies with enzymes attached which bind to antigens if present.
  3. Wash well to remove unbound antibodies to prevent false positive.
  4. Add substrate - enzymes create products that cause a colour change which indicates positive result.

SANDWICH ELISA

  1. Attach specific monoclonal antibodies to well.
  2. Add sample with potential antigens, then wash well.
  3. Add complementary monoclonal antibodies with enzymes attached which bind to antigens if present.
  4. Wash well to remove unbound antibodies to prevent false positive.
  5. Add substrate - enzymes create products that cause a colour change which indicates positive result.
77
Q

Explain the use of antibodies in the ELISA test to detect antibodies.

A

INDIRECT ELISA

  1. Attach specific antigens to well.
  2. Add sample with potential antibodies, wash well.
  3. Add complementary monoclonal antibodies with enzymes attached which bind to antibodies if present.
  4. Wash well to remove unbound antibodies to prevent false positive.
  5. Add substrate - enzymes create products that cause a colour change which indicates a positive result.
78
Q

Suggest the purpose of a control well in the ELISA test.

A
  • Compare to test to show only enzyme causes colour change.
  • Compare to test to show all unbound antibodies have been washed
    away    .
79
Q

Discuss some general ethical issues associated with the use of vaccines and monoclonal antibodies.

A
  1. Pre-clinical testing on/use of animals - potential stress/harm/mistreatment.
    • But animals not killed and help produce new drugs to reduce human suffering.
  2. Clinical trials on humans - potential harm/side-effects.
  3. Vaccines - may still develop/pass on pathogen.
  4. Use of drug - potentially dangerous side effects.
80
Q

Suggest some points to consider when evaluating methodology relating to the use of vaccines and monoclonal antibodies.

A
  • Was the sample size large enough to be representative?
  • Were participants diverse in terms of age, sex, ethnicity and health
    status?
  • Were placebo/control groups used for comparison?
  • Was the duration of the study long enough to show long-term effects?
  • Was the trial double-blind (neither doctor/patient knew who was given drug or placebo) to reduce bias?
81
Q

Suggest some points to consider when evaluating evidence and data relating to the use of vaccines and monoclonal antibodies.

A
  • What side effects were observed and how frequently did they occur?
  • Was a statistical test used to see if there was a significant difference start and final results?
  • Was the standard deviation of final results large, showing some people did not benefit?
  • Did standard deviations of start and final results overlap, showing there may not be a significant difference?
  • What dosage was optimum? Does increasing dose increase effectiveness enough to justify extra cost?
  • Was the cost of production and distribution low enough?

AQA A-Level Biology (5 decks)

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