T2: Cells Flashcards
1
Q
Define the term eukaryotic and prokaryotic cell.
A
eukaryotic: DNA is contained in a nucleus; membrane bound specialised organelles
prokaryotic: DNA is ‘ free ‘ in cytoplasm, no organelles e.g. bacteria & archaea
2
Q
Describe the structure and function of the cell membrane.
A
structure: phospholipid bilayer with extrinsic and intrinsic proteins embedded function:
- isolates cytoplasm from extracellular environment
- selectively permeable to reguglate transport of substances
- involved in cell recognition
3
Q
Explain the role of cholesterol, glycoproteins & glycolipids in the cell surface membrane
A
cholesterol: steroid molecule connects phospholipids & reduces fluidity
glycoproteins: cell signalling, cell recognition ( antigens) & binding cell together
glycolipids: cell signalling & cell recognition
4
Q
Describe the structure of the nucleus
A
- surrounded by nuclear envelope , a semi-permeable double membrane with pores
- nuclear pores allow substances to enter/exit
- dense nucleolus made of RNA & proteins assembles ribosomes
- the nucleoplasm ( equivalent of cytoplasm that exists in the nucleus.)
5
Q
Describe the function of the nucleus.
draw it
A
- contains DNA / genetic info
- site of transcription producing mrna
- site of DNA replication
6
Q
Describe the structure of a mitochondrion.
A
- surrounded by double membrane
- folded inner membrane forms cristae which is the site of electron transport chain
- fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins
7
Q
Describe the structure of a chloroplast and how its structure relates to its function. (4)
draw it
A
**- Starch grains / lipid droplets store products of photosynthesis;
**
- double membrane - provides large surface for light absorption.
- Thylakoids: flattened discs stack to form grana; contain photosystems with chlorophyll.
- Intergranal lamellae: tubes attach thylakoids in adjacent grana.
- Stroma: fluid-filled matrix.- vesicular plastid
**- Permeable membrane allows diffusion of gases /carbon dioxide;
**- Stacking / arrangement of grana/thylakoids maximises light catchment
**
8
Q
State the function of mitochondria and chloroplasts.
A
mitochondria : site of aerobic respiration to produce ATP
chloroplasts: site of photosynthesis to convert solar energy to chemical energy
9
Q
Describe the structure and function of the Golgi apparatus
A
- series of flat membrane-bound sacs
function: - proteins transported from RER to golgi apparatus via vesicles and fuse with cis face
- modifies & packages proteins for export
- at transface, , golgi vesicles bud off fuse with cell membrane releasing their contents by exocytosis.
10
Q
Describe how lactose is formed and where in the cell it would be attached to a polypeptide to form a glycoprotein (3)
A
- Glucose and galactose;
- Joined by condensation (reaction);
- Joined by glycosidic bond;
- Added to polypeptide in Golgi (apparatus)
11
Q
describe the structure and function of a lysosome
A
- membrane bound
- contains hydrolytic enzymes
- acidic environment
- involved in cell death and digestion
12
Q
describe the structure and function of a ribosome
A
- formed of rRNA+ protein
- site of protein synthesis via translation
large subunit: joins amino acids
small subunit: contains mRNA binding site
13
Q
describe the structure and function of the endoplasmic reticulum (ER)
A
structure : network of cisternae and flattened sacs .
rough ER: ribosomes on suface synthesise proteins. Proteins transported inside RER. Proteins packaged into vesicles for transport
smooth ER: Involved in the production, processing and storage of lipids, E.g steroids + cholesterol
14
Q
Describe the general structure of prokaryotic cells
A
15
Q
Describe the structure of the cell wall in plants and bacteria
A
- bacteria: made of the polysaccharide murein
- plants: made of cellulose
16
Q
state three functions of the cell walls
A
- mechanical strength and support
- physical barrier against pathogens
- part of apoplast pathway (plants) to enable easy diffusion of water
17
Q
Describe the structure and function of the cell vacuole in plants.
A
structure:
- surrounded by a single membrane: tonoplast
- contains cell sap : mineral ions, water, enzymes , soluble pigments
function:
- controls turgor pressure
- absorbs and isolates toxic substances
18
Q
Describe and explain features you would expect to find in a cell specialised for absorption. (5)
A
- Folded membrane/microvilli so large surface area (for absorption);
- Large number of co-transport/carrier/channel proteins so fast rate (of absorption) / for active transport
/ facilitated diffusion; - Large number of mitochondria so make (more) ATP (for aerobic respiration) to release energy for active transport;
- Membrane-bound (digestive) enzymes so maintains
concentration gradient (for fast absorption);
Accept named examples of digestive enzymes.
19
Q
State the role of plasmids in prokaryotes
A
- small ring of DNA that carries non-essential genes
20
Q
State the role of flagella in prokaryotes
A
- rotating tail propels ( usually unicellular) organism
21
Q
state the role of the capsule in prokaryotes
give 3
A
- a polysaccharide layer:
- prevents desiccation
- provides mechanical protection against phagocytosis & external chemicals
- attaches the cell to surfaces
22
Q
Give three structural properties shared by eukaryotic and prokaryotic cells
similar structures
A
- cell membrane
- cytoplasm
- ribosomes
23
Q
Contrast eukaryotic and prokaryotic cells
A
- EC smaller and always unicellular, vs larger and often multicellular.
- PC lack membrane-bound organelles and a nucleus, vs membrane-bound organelles and a distinct nucleus.
- PC have circular DNA that is not associated with proteins, vs linear chromosomes associated with histone proteins.
- Prokaryotic cells reproduce by binary fission, which is asexual, whereas eukaryotic cells reproduce by mitosis and meiosis.
- Prokaryotic cells have cell walls made of murein (peptidoglycan), while eukaryotic plant cells have cell walls made of cellulose and fungi have cell walls made of chitin.
24
Q
Give4 reasons why are viruses referred to as ‘ particles’ instead of cells?
A
- acellular & non-living
- no cytoplasm
- can’t self-reproduce
- no metabolism
25
Describe the structure of a viral particle
- linear genetic material (DNA or RNA ) & viral enzymes e.g. reverse transcriptase
- nucleic acid surrounded by capsid ( protein coat made of capsomeres)
- contains attachment proteins and lipid envelope
26
Describe the structure of an enveloped virus.
- simple virus surrounded by matrix protein
- matrix protein surrounded by envelope derived from cell membrane of host cell
- attachment proteins on surface
27
state the role of the capsid on viral particles.
- protect nucleic acid from degradation by restriction endonucleases
28
state the role of attachment proteins on viral particles.
- enable viral particle to bind to complementary sites on host cell
29
Describe how optical microscopes work
1. lenses focus rays of light and magnify the view of a thin slice of specimen
2. different structures absorb different amount and wavelengths of light
3. reflected light is transmitted to the observer via the objective lens and eyepiece
30
outline how a student could prepare a temporary mount of tissue for an optical microscope
1. Obtain thin section of tissue e.g. using ultra tome or by maceration.
2. Place plant tissue in a drop of water.
3. Stain tissue on a slide to make structures visible.
4. Add coverslip using mounted needle at 45° to avoid trapping air bubbles.
31
Suggest the advantages and limitations of using an optical microscope.
| 3/2
+ colour image
+ can show living structure
+ affordable
limit:
- 2D image
- lower resolution than electron microscopes = cannot see ultrastructure
32
Describe how a transmission electron microscope (TEM) works.
1. Focus electromagnets on specimen
2. more dense structures appear darker since they absorb more electrons
3. Focus image onto fluorescent screen using magnetic lenses
33
Suggest the advantages and limitations of using a TEM.
| 2/4
+ electrons have shorter wavelength than light = high resolution , so ultrastructure visible
+ high magnification ( x 500,000)
limitations:
- 2D image
- requires a vacuum so cannot show living structures
- extensive preparation may introduce artefacts
- no colour image
34
Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells. (6)
1. TEM use electrons and optical use light;
2. TEM allows a greater resolution;
3. (So with TEM) smaller organelles / named cell structure can be observed/ greater detail in organelles
4. TEM view only dead / dehydrated specimens and optical (can) view live specimens;
5. TEM does not show colour and optical (can);
6. TEM requires thinner specimens;
7. TEM requires a more complex/time consuming preparation;
8. TEM focuses using magnets and optical uses (glass) lenses;
35
Describe how a scanning electron microscope (SEM) works.
1. focus a beam of electrons onto a specimen's surface using electromagnetic lenses
2. electrons reflected off surface
3. hit a collecting device and are amplified to produce an image on a photographic plate
36
Suggest the advantages and limitations of using an SEM
| 2/3
+ 3D image
+ electron shave a shorter wavelength than light + high resolution
- requires a vacuum = cannot show living structures
- no colour image
- only shows outer surface
37
Before the cell was examined using the electron microscope, it was stained. This stain caused parts of the structure of the cell-surface membrane to appear as two dark lines.
Suggest an explanation for the appearance of the cell-surface membrane as two dark lines. (3)
1. Membrane has phospholipid bilayer;
2. Stain binds to phosphate / glycerol;
3. On inside and outside of membrane.
38
Define magnification and resolution.
Magnification: factor by which the image is larger than the actual specimen.
Resolution: smallest separation distance at which 2 separate structures can be distinguished from one another.
39
Explain how to use an eyepiece graticule and stage micrometer to measure the size of a structure.
1. Place micrometer on stage to calibrate eyepiece graticule.
2. Line up the scales.
3. Count how many eyepiece graticule divisions fit into a known distance.
4. Find out one graticule division length
4. Use calibrated values to calculate actual length of structures.
40
State an equation to calculate the actual size of a structure from microscopy.
actual size = image size / magnification
- triangle : IAM
41
Outline what happens during cell fractionation and ultracentrifugation.
1. Blend and homogenize tissue to break open cells & release organelles.
2. Place cold, buffered, isotonic solution
3. Filter homogenate to remove debris.
4. Perform differential centrifugation:
a) Spin homogenate in centrifuge.
b) The most dense organelles in the mixture form a pellet.
c) decant the supernatant and spin again at a higher speed.
42
state the order of sedimentation of organelles during differential centrifugation.
most dense ----> least dense
nucleus , chloroplast, mitochondria , lysosomes, RER , plasma membrane , SER, ribosomes
| "Nice Cute Mice Love Eating Rice"
43
Explain why fractionated cells are kept in a cold, buffered, isotonic solution.
cold: slow action of hydrolase enzymes.
buffered: maintain constant pH.
isotonic: prevent osmotic lysis/ shrinking
of organelles.
44
Describe how you would use cell fractionation techniques to obtain a sample of chloroplasts from leaf tissue (3)
1. Macerate / homogenise / blend / break tissues / cells (in solution);
2. Centrifuge;
3. At different / increasing speeds until chloroplast fraction obtained;
45
Explain how ultracentrifugation separates CENP-W from other molecules. (2)
1. Spin (liquid / supernatant) at (very) high speed / high g;
*(Need context of high, not just ‘faster’, ‘higher’ in context of use of bench centrifuge)*
Accept high centrifugal force
2. Molecules / CENP-W separates depending on (molecular) mass / size / density
| Accept high centrifugal force
46
state what the cell cycle is and outline its stages
- cycle of division with intermediate growth periods
1. interphase
2. mitosis or meiosis ( nuclear division)
3. cytokinesis ( cytoplasmic division)
47
Explain why the cell cycle does not occur in some cells.
- After differentiation, some types of cell in multicellular organisms
- no longer have the ability to divide.
48
What is the difference between the cell cycle and mitosis?
- cell cycle includes growth period between divisions ; mitosis is only 10% of the cycle & refers only to nuclear division
49
Describe the process of Interphase
1. G1 (Gap 1): The cell synthesises proteins and organelles, and increases in size.
2. S (Synthesis): DNA replication occurs, so each chromosome is copied.
3. G2 (Gap 2): continues to grow, and checks for DNA damage; proteins required for mitosis are produced.
50
State the purpose of mitosis
produces 2 genetically identical daughter cells
- Growth of multicellular organisms
- tissue repair /cell replacement
- asexual reproduction
51
Give the stages of mitosis
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
52
Outline what happens during prophase
1. Chromosomes condense , becoming visible.
2. centrioles move to opposite poles of cell ( animal cells ) & mitotic spindle fibres form
3. Nuclear envelope & nucleolus break down = chromosomes free in cytoplasm
53
Outline what happens during metaphase
- sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres
54
Outline what happens during anaphase
- it requires energy from ATP hydrolysis
1. Spindle fibres contract and centromeres divide
2. Sister chromatids separate into 2 distinct chromosomes & are pulled to opposite poles of cells
3. Spindle fibres break down
55
Outline what happens during telophase
1. Chromosomes decondense , becoming invisible again
2. New nuclear envelopes form around each set of chromosomes = 2 new nuclei, each with 1 copy of each chromosome
56
Describe and explain the changes in DNA mass shown in the graph during one cell cycle.
- At the start, DNA mass stays constant **during the G1 phase** as the cell grows
- DNA mass doubles during the S phase when DNA replication occurs.
- DNA mass is constant during G2 phase and mitosis, as the cell prepares for division.
- During cytokinesis, DNA mass halves back to 2 units when the cell splits into two genetically identical daughter cells.
57
Explain how to prepare squashes of cells from plant root tips
| temporary root tip mount
1.Cut a thin slice of root tip (5mm from end) using scalpel and mount onto a slide.
2. Soak root tip in hydrochloric acid then rinse to halt cell division & hydrolyse middle lamella.
2. Stain root tip with a dye that binds to chromosomes.
3. Macerate tissue in water using mounted needle.
4. Use mounted needle at 45° to press down coverslip & obtain a single layer of cells.
5. Avoid trapping air bubbles.
58
Describe how to use an optical microscope to determine mitotic index
1. Clip slide onto stage and turn on light
2. Select lowest power objective lens
3. Use Coarse focusing dial to move stage
4. Adjust fine focusing dial to get a clear image
5. Swap to higher power objective lens, then refocus
6. Calculate mitotic index
59
Describe how to calculate mitotic index
- number of cells with visible chromosomes/ total number of cells in the sample
60
Name two dyes that bind to chromosomes
- toluidine blue ( blue)
- acetic orcein ( purple-red)
61
Why is only the root tip used when calculating a mitotic index
- meristematic cells at root tip are actively undergoing mitosis
- cells further from root tip are elongating rather than dividing
62
common questions:
1. Why is a stain used?
2. Why squash / press down on cover slip?
3. Why not push cover slip sideways?
4. Why soak roots in acid?
1. To distinguish chromosomes
2. creates a single layer of cells. So light passes through to make chromosomes visible
3. Avoid rolling cells together / breaking chromosomes
4. * Separate cells / cell walls
* To allow stain to diffuse into cells
* To allow cells to be more easily squashed
* To stop mitosis
63
What are tumour suppressor genes?
- Genes that code for proteins to trigger
apoptosis
- slow down cell cycle to prevent tumour formation
- (e.g. p53 acts between G1 & S in interphase so damaged DNA cannot replicate).
64
How can mutation to tumour suppressor genes & proto-oncogenes cause cancer?
· Tumour suppressor: no production of a protein needed to slow the cell cycle.
· Disruption to cell cycle -> uncontrolled cell division ->tumour.
65
Suggest how cancer treatments control the rate of cell division.
- Disrupt the cell cycle:
- prevent DNA replication during interphase
- disrupt spindle formation = inhibit metaphase / anaphase
66
Name and Describe the process of which prokaryotes replicate
- replication of circular DNA
- replication of plasmids
- Cell elongates, separating the 2 DNA loops.
- Cell membrane contracts & divides
- produces 2 daughter cells with
- 1 copy of the DNA loop
- but a variable number of plasmids.
| Binary Fission
67
Why are viruses classified as non-living?
They are acellular: no cytoplasm, no
metabolism & cannot self-replicate.
68
Outline how viruses replicate.
1. Attachment proteins attach to receptors on host cell membrane.
2. Enveloped viruses fuse with cell membrane or move in via endocytosis & release DNA/ RNA into cytoplasm OR viruses inject DNA/ RNA.
3. Host cell uses viral genetic information to synthesise new viral proteins/ nucleic acid.
4. Components of new viral particle assemble.
69
How do new viral particles leave the host cell?
a) Bud off & use cell membrane to form
envelope.
b) Cause lysis of host cell.
70
Why is it so difficult to develop effective treatments against viruses?
- Replicate inside living cells
- thus difficult to
kill them without killing host cells.
71
Describe the fluid mosaic model of membranes.
fluid: phospholipid bilayer so molecules free to move laterally
mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded.
72
Explain the role of cholesterol & glycolipids in membranes
cholesterol: connects phospholipids, restricing movement so decreases fluidity for stability
glycolipids: cell signalling & cell recognition
73
explain the functions of extrinsic and intrinsic proteins in membranes.
extrinsic:
- binding sites receptors e..g. for hormones
- antigens
- bind cells together
- involved in cell signalling
intrinsic:
- electron carriers (respiration/photosynthesis)
- channel proteins ( facilitated diffusion)
- carrier proteins ( facilitated diffusion/ active transport)
74
Explain the arrangement of phospholipids in a cell membrane
- bilayer with water present on either side
- hydrophobic fatty acid tails repelled from water so point away from water
- hydrophilic phosphate heads attracted to water so point towarrd water
75
Suggest how cell membranes are adapted for other functions
- phospholipid bilayer is fluid - membrane can bend for vesicle formation / phagocytosis
- glycoproteins act as receptors /antigens - invlolved in cell signalling and recognition
76
Describe how movement across membranes occurs by simple diffusion
- lipid-soluble (non-polar) or small substances
- move from an area of higher concentration to lower concentration
- across the phospholipid bilayer
- passively- no ATP required
77
Explain the limitations imposed by the phospholipid bilayer
- resticts movemnt of water soluble (polar) and larger substances e.g. Na+ / glucose
- due to hydrophobic fatty acid tails
78
Describe how the movement across membranes occurs via **faciliated diffusion**
- water-soluble (polar), larger substances
- move down a concentration gradient
- through specific channel/carrier proteins
- passively - NO ATP
79
Explain the role of carrier and channel proteins in facilitated diffusion
- shape/charge of protein determines which substances move
- channel proteins facilitate diffusion of water soluble substances
- carrier proteins faciliate diffusion of larger substances:
- complementary substance attaches to binding site
- protein changes shape to transport substance
80
Describe how movement across membranes occurs by osmosis
- water diffuses/moves
- from an area of high to low water potential
- through a partially permeable membrane
- passive: ATP
81
Describe how movement across membranes occurs by active transport
- substance move from an area of lower to higher concentration
- requiring hydrolysis of ATP and specific carrier proteins
82
Describe the role of carrier proteins and ATP in active transport
- complementary substance binds to specific carrier protein
- ATP binds , hydrolyses into ADP + Pi, releasing energy
- Carrier protein changes shape, releasing substance on side of higher concentration
- Pi released - protein returns to original shape
83
Describe how movement across membranes occurs by co-transport
- two different substances bind and move via a co-transport protein
- movement of one substance against its concentration gradient is coupled with the movement of another down its conc gradient
84
Explain how co-transport is involved in the absorption of glucose/amino acids in the small intestine.
1. Na+ actively transported out of epithelial cells & into bloodstream.
2. Na+ concentration is lower in epithelial cells than lumen of gut.
3. Na + and glucose/ amino acids moves down from lumen into epithelial cells via a carrier protein down electrochemical gradient.
4. glucose/amino cids moves into blood via facilitated diffusion
85
Name and describe five ways substances can move across the cell-surface membrane into a cell. (5)
1. (Simple) diffusion of small/non-polar molecules down a concentration gradient;
2. Facilitated diffusion down a concentration gradient via
protein carrier/channel;
3. Osmosis of water down a water potential gradient;
4. Active transport against a concentration gradient via
protein carrier using ATP;
5. Co-transport of 2 different substances using a carrier protein;
## Footnote
For ‘carrier protein’ accept symport OR cotransport
protein
86
The movement of substances across cell membranes is affected by membrane structure. Describe how. (5)
1. Phospholipid (bilayer) allows movement/diffusion of nonpolar/lipid-soluble substances;
2. Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances
3. Carrier proteins allow active transport;
4. Channel/carrier proteins allow facilitated diffusion/co-transport;
5. Shape/charge of channel / carrier determines which
substances move;
6. Number of channels/carriers determines how much movement;
7. Membrane surface area determines how much
diffusion/movement;
8. Cholesterol affects fluidity/rigidity/permeability;
87
Describe how to calculate dilutions
| required practical 3
- Use the formula. C1 x V1 = C2 x V2
- Cl = concentration of stock solution
- V1 - volume of stock solution used to make new
concentration
- C2 = concentration of solution you are making
- V2 = volume of new solution you are making
- V2 = V1 + volume of distilled water to dilute with
88
Describe a method to produce of a calibration curve with which to identify the water potential of plant tissue
1. Create a series of dilutions using a 1 mol
dm-3 sucrose solution
2. Use scalpel / cork borer to cut potato into
identical cylinders
3. Blot dry with a paper towel and measure / record initial mass of each piece
4. Immerse one chip in each solution and leave for a set time (20-30 mins) in a water bath at 30C
5. Blot dry with a paper towel and measure /
record final mass of each plece.
6. Calculate % change in mass
89
Give 5 control variables for the method in identifying the water potential of plant tissue
- Volume of solution
- Size, shape and surface area of plant tissue
- Source of plant tissue le variety or age
- Length of time in solution
- Temperature
90
Describe how to determine the water potenial of an unknown solution using a calibration curve
- Plot a graph with concentration on x axis and percentage change in mass on y axis (calibration curve)
- Identify concentration where line of best fit intercepts x axis (0% change)
- Water potential of sucrose solution = water potential of potato cells
91
common RP2 questions:
- Why calculate % change in mass?
- Why blot dry before weighing?
1. Enables comparison / shows proportional change
. As plant tissue samples had different initial masses
2. Solution on surface will add to mass; only want to measure water taken up or lost
92
Explain the changes in plant tissue mass when placed in different concentrations of solute
1. increase: water moved into cell via osmosis as Ψ of soltuion higher then cell
2. decrease: water moved out of cell by osmosis as Ψ of solution lower then inside cell
3. no change: Ψ of solution= Ψ of cell
93
describe a method to investigate the effect of a named variable on the permeability of cell surface membrane
1. Cut equal sized cubes of plant tissue using a scalpel
2. Rinse/ blot on towel to remove pigment released during cutting
3. Add same number of cubes to 5 different test tubes containing same volume of water (eg. 5cm3)
4. Place each test tube in a water bath at a different temperature ( or named variable e.g. diff ph / in a solvent)
5. Leave for same amount of time
6. Remove beetroot and measure intensity of colour of surrounding solution:
7. **colour standards**: Use a known concentration of extract & distilled water to prepare a dilution series (colour standards). Compare results with colour standards to estimate conc.
94
describe how to measure intensity of colour of a solution using a colourimeter
- Measure absorbance (of light) of known concentrations using a colorimeter
- Draw a calibration curve -. plot a graph of absorbance (y) against conc. of extract (x)
- draw a line / curve of best fit
- Absorbance value for sample read off calibration curve to find associated extract conc.
95
Common questions
1. What are the issues with comparing to a colour standard?
2. Why wash the beetroot before placing it
in water?
3. Why regularly shake each test tube containing cubes of plant tissue?
4. Why control the volume of water?
5. How could you ensure beetroot cylinders were kept at the same temperature throughout the experiment?
1. matching is subjective
2. wash off any pigment to show release is only due to variable being tested
3. to esnure all surfaces of cube contact liqic to maintain conc gradient for diffusion
4. too much water dilutes pigment - appear lighter. Controlling allows for results to be comprable
5. take temp readings in intervals using a digital temperature. use corrective measure if temp has changed
96
define antigen (2)
- Foreign protein or polysaccharide
- (that) stimulates an immune response / production of antibody;
97
how does phagocytosis destroy pathogens?
- phagocyte moves towards pathogen via chemotaxis
- phagocyte engulfs pathogen via endocytosis to form a phagosome
- phagosome fuses with lysosome to form a phagolysosome
- lysosome releases hydrolytic enzymes digesting the pathogen
- phagocyte absorbs the products from pathogen hydrolysis.
98
Explain the role of antigen-presenting cells ( APCs)
- macrophage displays antigen from pathogen om its surface ( after hydrolysis in phagocytosis )
- enhances recognition by Th cells which cannot directly interface with pathogens/antigens in body fluid.
99
Name the 2 types of specific immune response.
- cell mediated
- humoral
100
Outline the process of the cell-mediated response.
1. Complementary T lymphocytes bind to a foreign antigen on MHC molecules found on APCs.
2. These T cells become activated T cells.
3. T helper cells release cytokines that stimulate both:
4. Clonal expansion of helper T cells (rapid mitosis), which either become memory cells or help trigger the humoral response by activating B cells.
5. Clonal expansion of cytotoxic T cells (Tc cells), which secrete the enzyme perforin to destroy infected cells.
101
Outline the process of the humoral response.
1. Complementary T lymphocytes bind to foreign antigen on antigen-presenting T cells.
2. Release cytokines that stimulate clonal expansion (rapid mitosis) of complementary B lymphocytes.
3. B cells differentiate into plasma cells.
4. Plasma cells secrete antibodies with
complementary variable region to antigen.
5. B memory cells are also produced.
102
what is an antibody (2)
- protein / immunoglobulin specific to an antigen;
- Produced by B cells
OR
Secreted by plasma cells;
103
Describe the structure of an antibody.
- Quaternary structure: 2 'light chains' held together by disulfide bridges, 2 longer 'heavy chains'.
- Binding sites on variable region of light chains have specific tertiary structure complementary to an antigen.
- The rest of the molecule is known as the constant region.
104
Explain how antibodies lead to the destruction of pathogens
1. Antibodies bind to antigens on pathogens forming an antigen-antibody complex
2. Specific tertiary structure so binding site / variable region binds to complementary antigen
3. Each antibody binds to 2 pathogens at a time causing agglutination (clumping) of pathogens
4. Antibodies attract phagocytes
5. Phagocytes bind to the antibodies and phagocytose many pathogens at once
105
what are memory cells?
· Specialised T/ B cells produced from
primary immune response.
. Remain in low levels in the blood.
. Can divide very rapidly by mitosis if
organism encounters the same pathogen
again
106
Explain the differences between the primary & secondary immune response
**Primary** - first exposure to antigen
- Antibodies produced slowly & at a lower conc.
- 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 & at a higher conc.
- B memory cells rapidly undergo mitosis to
produce many plasma cells which produce
specific antibodies.
107
what causes antigen variability?
1. Random genetic mutation changes DNA base sequence.
2. Results in different sequence of codons on mRNA
3. Different primary structure of antigen = H-bonds, ionic bonds & disulfide bridges form in different places in tertiary structure.
4. Different shape of antigen.
108
Explain how antigen variability affects the incidence of disease.
- memory cells no longer complementary to antigen = individual not immune = can catch the disease more than once.
- many varieties of a pathogen = difficult to develop vaccine containing all antigen types.
109
Describe the differences between active and passive immunity
110
DESCRIBE vaccination.
1. vaccine contains dead/inactive from of a pathogen or antigen
2. triggers primary immune response
3. memory cells are produced and remain in the bloodstream , so secondary response is rapid & produces higher concentration of antibodies
4. Pathogen is destroyed before it causes symptoms
111
what is herd immunity?
- vaccinating large proportion of population reduces available carriers of the pathogen.
- protects individuals who have not been vaccinated e.g. those with a weak immune system
112
Suggest some ethical issues surrounding the use of vaccines
- production may involve use of animals
- potentially dangerous side-effects
- clinical tests may be fatal
- compulsory vs opt-out
113
what are monoclonal antibodies
- antibodies which have the same tertiary structure
- they are complementary to specific antigens .
- they are produced from a single clone of B cells.
114
115
Describe the structure of HIV.
· Genetic material + viral enzymes
(integrase & reverse transcriptase) surrounded by capsid.
. Surrounded by viral envelope derived from host cell membrane.
. GP120 attachment proteins on surface.
116
How does HIV result in the symptoms of AIDS?
1. Attachment proteins bind to complementary CD4 receptor on TL cells.
2. HIV particles replicate inside T cells, killing or damaging them.
3. AIDS develops when there are too few T cells for the immune system to function.
4. Individuals cannot destroy other pathogens & suffer from secondary diseases/ infections.
117
Describe how HIV is replicated. (4)
1. Attachment proteins attach to receptors on helper T cell/lymphocyte;
2. Nucleic acid/RNA enters cell;
3. Reverse transcriptase converts RNA to DNA;
4. Viral protein/capsid/enzymes produced;
5. Virus (particles) assembled and released (from cell);
118
Describe how the human immunodeficiency virus (HIV) is replicated once inside helper T cells (TH cells). (4)
1. RNA converted into DNA using reverse transcriptase
2. DNA /inserted into (helper T cell) genome/nucleus;
3. DNA transcribed into (HIV m)RNA
4. (HIV mRNA) translated into (new) HIV/viral proteins (for assembly into viral particles);
119
Explain how HIV affects the production of antibodies when AIDS develops
in a person. (3)
1. Less/no antibody produced;
2. (Because HIV) destroys helper T cells;
Accept ‘reduces number’ for ‘destroys’
3. (So) few/no B cells activated / stimulated
OR
(So) few/no B cells undergo mitosis/differentiate/form plasma cells;
3
120
Why are antibiotics ineffective against viruses
- antibiotics often work by damaging murein cell walls to cause osmotic lysis. Viruses have no cell wall.
- viruses replicate inside host cell = difficult to destroy them without damaging normal body cells.
121
Suggest the clinical applications of monoclonal antibodies.
- pregnancy tests by detecting HCG hormones in urine
- Diagnostic procedures e.g. ELISA test
- targeted treatment by attaching drug to antibody so that it only binds to cells with abnormal antigen e.g. cancer cells due to specificity of tertiary structure of binding site
122
Explain how monoclonal antibodies can be used in medical treatments
- Monoclonal antibody has a specific tertiary structure / binding site / variable region
- Complementary to receptor / protein / antigen found only on a specific cell type (eg cancer cell)
- Therapeutic drug attached to antibody
- Antibody binds to specific cell, forming antigen-antibody complex, delivering drug
123
Explain how monoclonal antibodies can be used in medical diagnosis
- Monoclonal antibody has a specific tertiary structure / binding site / variable region
- Complementary to specific receptor / protein / antigen associated with diagnosis
- Dye / stain / fluorescent marker attached to antibody
- Antibody binds to receptor / protein / antigen, forming antigen-antibody complex
124
Explain the principle of the direct ELISA test
1. Monoclonal antibodies bind to bottom of test plate.
2. Antigen molecules in sample bind to antibody. Rinse excess.
3. Mobile antibody with 'reporter enzyme' attached binds to
antigens that are 'fixed' on the monoclonal antibodies. Rinse excess.
4. Add substrate for reporter enzyme. Positive result: colour change.
125
Explain the principle of an indirect ELISA test.
1. Antigens bind to bottom of test plate.
2. Antibodies in sample bind to antigen. Wash away excess.
3. Secondary antibody with 'reporter enzyme' attached binds
to primary antibodies from the sample.
4. Add substrate for reporter enzyme. Positive result: colour change.
## Footnote
detects presence of an antibody against a specific antigen
126
Suggest some ethical issues surrounding the use of monoclonal antibodies.
- production involves animals
- drug trials against arthritis & leukaemia resulted in multiple organ failure.
127
Describe the role of antibodies in producing a positive result in an ELISA
test. (4)
1. (First) antibody binds/attaches /complementary (in shape) to antigen;
2. (Second) antibody with enzyme attached is added;
3. (Second) antibody attaches to antigen;
Accept (second) antibody attaches to (first) antibody (indirect ELISA test).
4. (Substrate/solution added) and colour changes;
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