Cells (Topic 2) Flashcards
1
Q
Define the terms eukaryotic and
prokaryotic cell.
A
Eukaryotic: DNA is contained in a
nucleus, contains membrane-bound
specialised organelles.
Prokaryotic: DNA is ‘free’ in cytoplasm,
no organelles e.g. bacteria & archaea.
2
Q
State the relationship between a system
and specialised cells.
A
Specialised cells → tissues that perform
specific function → organs made of
several tissue types → organ systems
3
Q
Describe the structure and function of
the cell-surface membrane.
A
‘Fluid mosaic’ phospholipid bilayer with extrinsic &
intrinsic proteins embedded
• Isolates cytoplasm from extracellular environment.
• Selectively permeable to regulate transport of
substances.
• Involved in cell signalling / cell recognition.
4
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 cells together.
Glycolipids: cell signalling & cell recognition.
5
Q
Describe the structure of the nucleus.
A
• Surrounded by nuclear envelope, a
semi-permeable double membrane.
• Nuclear pores allow substances to
enter/exit.
• Dense nucleolus made of RNA & proteins
assembles ribosomes.
6
Q
Describe the function of the nucleus.
A
• Contains DNA coiled around chromatin into
chromosomes.
• Controls cellular processes: gene
expression determines specialisation & site
of mRNA transcription, mitosis,
semiconservative replication.
7
Q
Describe the structure of a
mitochondrion.
A
• Surrounded by double membrane folded
inner membrane forms cristae: site of
electron transport chain
• Fluid matrix: contains mitochondrial DNA,
respiratory enzymes, lipids, proteins.
8
Q
Describe the structure of a chloroplast.
A
• Vesicular plastid with double membrane.
• Thylakoids: flattened discs stack to form
grana; contain photosystems with chlorophyll.
• Intergranal lamellae: tubes attach thylakoids
in adjacent grana.
• Stroma: fluid-filled matrix.
9
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
10
Q
Describe the structure and function of
the Golgi apparatus.
A
Planar stack of membrane-bound, flattened sacs
cis face aligns with rER.
Molecules are processed in cisternae
vesicles bud off trans face via exocytosis:
• modifies & packages proteins for export
• synthesises glycoproteins
11
Q
Describe the structure and function of a
lysosome.
A
Sac surrounded by single membrane
embedded H
+
pump maintains acidic conditions
contains digestive hydrolase enzymes
glycoprotein coat protects cell interior:
• digests contents of phagosome
• exocytosis of digestive enzymes
12
Q
Describe the structure and function of a
ribosome.
A
Formed of protein & rRNA
free in cytoplasm or attached to ER.
• 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
Cisternae: network of tubules & flattened sacs
extends from cell membrane through cytoplasm &
connects to nuclear envelope:
• Rough ER: many ribosomes attached for protein
synthesis & transport.
• Smooth ER: lipid synthesis.
14
Q
Describe the structure of the cell wall.
A
• Bacteria:
Made of the polysaccharide murein.
• Plants:
Made of cellulose microfibrils
plasmodesmata allow molecules to pass between
cells, middle lamella acts as boundary between
adjacent cell walls.
15
Q
State the functions of the cell wall.
A
• Mechanical strength and support.
• Physical barrier against pathogens.
• Part of apoplast pathway (plants) to
enable easy diffusion of water.
16
Q
Describe the structure and function of
the cell vacuole in plants.
A
Surrounded by single membrane: tonoplast
contains cell sap: mineral ions, water, enzymes,
soluble pigments.
• Controls turgor pressure.
• Absorbs and hydrolyses potentially harmful
substances to detoxify cytoplasm.
17
Q
Explain some common cell adaptations.
A
• Folded membrane or microvilli increase
surface area e.g. for diffusion.
• Many mitochondria = large amounts of ATP for
active transport.
• Walls one cell thick to reduce distance of
diffusion pathway.
18
Q
State the role of plasmids in prokaryotes.
A
• Small ring of DNA that carries
non-essential genes.
• Can be exchanged between bacterial
cells via conjugation.
19
Q
State the role of flagella in prokaryotes.
A
Rotating tail propels (usually unicellular)
organism.
20
Q
State the role of the capsule in
prokaryotes.
A
polysaccharide layer:
• Prevents desiccation.
• Acts as food reserve.
• Provides mechanical protection against
phagocytosis & external chemicals.
• Sticks cells together.
21
Q
Compare eukaryotic and prokaryotic
cells.
A
both have:
• Cell membrane.
• Cytoplasm.
• Ribosomes (don’t count as an
organelle since not membrane-bound).
22
Q
Contrast eukaryotic and prokaryotic
cells.
A
Eukaryotic:
• larger cells & often multicellular
• always have organelles & nucleus
• linear chromosomes associated with histones
• larger ribosomes (80S)
• mitosis & meiosis- sexual and/or asexual
• cellulose cell wall (plants)/ chitin cell wall (fungi)
• no capsule, no plasmids, always cytoskeleton
Prokaryotic:
• small cells & always unicellular
• no membrane bound organelles and no nucleus
• circular DNA not associated with proteins
• small ribosomes (70S)
• binary fission- always asexual
• murein cell wall
• capsule, sometimes plasmids and cytoskeleton
23
Q
Why are viruses referred to as ‘particles’
instead of cells?
A
Acellular & non-living: no cytoplasm,
cannot self-reproduce, no metabolism.
24
Q
Describe the structure of a viral particle.
A
• Linear genetic material (DNA or RNA) &
viral enzymes e.g. reverse transcriptase.
• Surrounded by capsid (protein coat
made of capsomeres).
• No cytoplasm.
25
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.
26
State the role of the capsid on viral
particles.
• Protect nucleic acid from degradation
by restriction endonucleases.
• Surface sites enable viral particle to
bind to & enter host cells or inject their
genetic material.
27
State the role of attachment proteins on
viral particles.
Enable viral particle to bind to
complementary sites on host cell : entry
via endosymbiosis.
28
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 amounts
and wavelengths of light.
3. Reflected light is transmitted to the observer
via the objective lens and eyepiece.
29
Outline how a student could prepare a
temporary mount of tissue for an optical
microscope.
1. Obtain thin section of tissue e.g. using ultratome 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.
30
Suggest the advantages and limitations
of using an optical microscope.
+ colour image
+ can show living structures
+ affordable apparatus
- 2D image
- lower resolution than electron microscopes =
cannot see ultrastructure
31
Describe how a transmission electron
microscope (TEM) works.
1. Pass a high energy **beam of electrons** through
thin slice of specimen.
2. More dense structures appear darker since they
absorb more electrons.
3. Focus image onto fluorescent screen or
photographic plate using magnetic lenses.
32
Suggest the advantages and limitations
of using a TEM.
+ electrons have shorter wavelength than light = high
resolution, so ultrastructure visible
+ high magnification (x 500000)
- 2D image
- requires a vacuum = cannot show living structures
- extensive preparation may introduce artefacts
- no colour image
33
Describe how a scanning electron
microscope (SEM) works.
1. Focus a beam of electrons onto a specimen’s
surface using electromagnetic lenses.
2. Reflected electrons hit a collecting device and
are amplified to produce an image on a
photographic plate.
34
Suggest the advantages and limitations
of using an SEM.
+ 3D image
+ electrons have shorter wavelength than light = high
resolution
- requires a vacuum = cannot show living structures
- no colour image
- only shows outer surface
35
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.
36
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 scales on graticule and micrometer. Count how
many graticule divisions are in 100μm on the micrometer.
3. Length of 1 eyepiece division = 100μm / number of
divisions
4. Use calibrated values to calculate actual length of
structures.
37
State an equation to calculate the actual
size of a structure from microscopy.
actual size
=
image size
/
magnification
38
Outline what happens during cell
fractionation and ultracentrifugation.
1. Mince and **homogenize** tissue to break open cells &
release organelles.
2. Filter homogenate to remove debris.
3. Perform **differential centrifugation**:
a) Spin homogenate in centrifuge.
b) The most dense organelles in the mixture form a **pellet**.
c) Filter off the **supernatant** and spin again at a higher speed.
39
State the order of sedimentation of
organelles during differential
centrifugation.
most dense → least dense
nucleus → mitochondria → lysosomes →
RER → plasma membrane → SER →
ribosomes
40
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.
41
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)
42
Explain why the cell cycle does not occur
in some cells.
After differentiation, some types of cell in
multicellular organisms (e.g. neurons) no
longer have the ability to divide.
43
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.
44
Outline what happens during interphase.
**G1**: cell synthesises proteins for replication e.g.
tubulin for spindle fibres & **cell size doubles**
**S**: DNA replicates = chromosomes consist of 2
sister chromatids joined at a centromere
**G2**: organelles divide
45
State the purpose of mitosis.
produces 2 genetically identical daughter cells
for:
• Growth
• Cell replacement/ tissue repair
• Asexual reproduction
46
Name the stages of mitosis.
1. **P**rophase
2. **M**etaphase
3. **A**naphase
4. **T**elophase
47
Outline what happens during prophase.
1. **Chromosomes condense**, becoming **visible**.
(X-shaped: 2 sister chromatids joined at
centromere)
2. **Centrioles** move to opposite poles of cell (animal
cells) & **mitotic spindle fibres** form.
3. **Nuclear envelope & nucleolus break down** =
chromosomes free in cytoplasm.
48
Outline what happens during metaphase.
Sister chromatids line up at **cell equator**,
attached to the mitotic spindle by their
centromeres.
49
Outline what happens during anaphase.
requires energy from ATP hydrolysis
1. Spindle fibres contract = **centromeres divide**.
2. Sister chromatids separate into 2 distinct
chromosomes & are pulled to opposite poles of
cell (looks like ‘V’ shapes facing each other).
3. Spindle fibres break down.
50
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.
51
Explain the procedure for a root tip
squash experiment.
1. Prepare a temporary mount of root tissue.
2. Focus an optical microscope on the slide. Count
total number of cells in the field of view and number
of cells in a stage of mitosis.
3. Calculate **mitotic index** (proportion of cells
undergoing mitosis).
52
Outline how to prepare a temporary
mount of root tissue.
1. Place root in hydrochloric acid 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. Avoid trapping air
bubbles.
53
Name 2 dyes that bind to chromosomes.
• toluidine blue (blue)
• acetic orcein (purple-red)
54
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.
55
What are tumour suppressor genes &
proto-oncogenes?
Genes that code for proteins to trigger
apoptosis (programmed death of
damaged cells)/ slow cell cycle (e.g. p53
acts between G1 & S in interphase so
damaged DNA cannot replicate).
56
What are proto-oncogenes?
Genes that code for proteins to stimulate
cell cycle to progress from one stage to
the next.
57
How can mutation to tumour suppressor
genes & proto-oncogenes cause cancer?
• **Tumour suppressor**: no production of a protein
needed to slow the cell cycle.
• **Proto-oncogenes**: form **permanently-activated**
oncogenes.
• Disruption to cell cycle → uncontrolled cell division →
tumour.
58
Suggest how cancer treatments control
the rate of cell division.
Disrupt the cell cycle:
• prevent DNA replication
• disrupt spindle formation = inhibit metaphase
/ anaphase
NB: can also damage healthy cells
59
How do prokaryotic cells replicate?
**Binary fission**:
1. DNA loop replicates. Both copies stay attached to cell
membrane. Plasmids replicate in cytoplasm.
2. Cell elongates, separating the 2 DNA loops.
3. Cell membrane contracts & septum forms.
4. Cell splits into 2 identical progeny cells, each with 1 copy
of the DNA loop but a variable number of plasmids.
60
Estimate the exponential growth of
bacteria within 8 hours. Assume binary
fission occurs once every 20 minutes &
there is 1 bacterium at the start.
8 x 60 = 480 mins
480 / 20 = 24 divisions
2
^24
61
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.**
62
How do new viral particles leave the host
cell?
a) Bud off & use cell membrane to form
envelope.
b) Cause lysis of host cell.
63
Why is it so difficult to develop effective
treatments against viruses?
Replicate inside living cells = difficult to
kill them without killing host cells.
64
Describe the fluid mosaic model of
membranes.
**Fluid: phospholipid bilayer** in which
individual phospholipids can move =
membrane has flexible shape.
**Mosaic: extrinsic & intrinsic proteins** of
different sizes and shapes are embedded.
65
Explain the role of cholesterol &
glycolipids in membranes.
• **Cholesterol**: steroid molecule in some plasma
membranes; connects phospholipids &
reduces fluidity to make bilayer more stable.
• **Glycolipids**: cell signalling & cell recognition.
66
Explain the functions of extrinsic and
transmembrane proteins in membranes.
**extrinsic**:
• binding sites/ receptors
e.g. for hormones
• antigens (glycoproteins)
• bind cells together
• involved in cell signalling
**intrinsic**:
• electron carriers
(respiration/photosynthesis)
• channel proteins (facilitated
diffusion)
• carrier proteins (facilitated
diffusion/ active transport)
67
Explain the functions of membranes
within cells.
• Provide internal transport system.
• Selectively permeable to regulate passage
of molecules into / out of organelles.
• Provide reaction surface.
• Isolate organelles from cytoplasm for
specific metabolic reactions.
68
Explain the functions of membranes
within cells.
• Provide internal transport system.
• Selectively permeable to regulate passage
of molecules into / out of organelles.
• Provide reaction surface.
• Isolate organelles from cytoplasm for
specific metabolic reactions.
69
Explain the functions of the cell-surface
membrane.
• Isolates cytoplasm from extracellular
environment.
• Selectively permeable to regulate
transport of substances.
• Involved in cell signalling/cell recognition.
70
Name and explain 3 factors that affect
membrane permeability.
• **Temperature**: high temperature denatures
membrane proteins / phospholipid molecules
have more kinetic energy & move further apart.
• **pH**: changes tertiary structure of membrane
proteins.
• Use of a **solvent**: may dissolve membrane.
71
Outline how colorimetry could be used to
investigate membrane permeability.
1. Use plant tissue with soluble pigment in vacuole. Tonoplast &
cell-surface membrane disrupted = ↑ permeability = pigment
diffuses into solution.
2. Select colorimeter filter with complementary colour.
3. Use distilled water to set colorimeter to 0. Measure
absorbance/ % transmission value of solution.
4. high absorbance/ low transmission = more pigment in
solution.
72
Define osmosis.
**Water diffuses** across **semi-permeable
membranes** from an area of higher
**water potential** to an area of lower
water potential until a dynamic
equilibrium is established.
73
What is water potential (ψ)?
• pressure created by water molecules
measured in kPa
• Ψ of pure water at 25℃ & 100 kPa: 0
• more solute = ψ more negative
74
How does osmosis affect plant and
animal cells?
• osmosis **INTO cell**:
**plant**: protoplast swells = cell turgid
**animal**: lysis
• osmosis **OUT of cell**:
**plant**: protoplast shrinks = cell flaccid
**animal**: crenation
75
Suggest how a student could produce a
desired concentration of solution from a
stock solution.
• volume of stock solution = required concentration x
final volume needed / concentration of stock
solution.
• volume of distilled water = final volume needed -
volume of stock solution.
76
Define simple diffusion.
•** Passive process** requires no energy from ATP
hydrolysis.
• **Net movement** of **small, lipid-soluble**
molecules directly through the bilayer from an
area of high concentration to an area of lower
concentration (i.e. **down a concentration
gradient**).
77
Define facilitated diffusion.
**Passive process**
Specific **channel or carrier proteins** with
complementary binding sites transport **large and/
or polar molecules/ ions** (not soluble in
hydrophobic phospholipid tail) **down
concentration gradient**
78
Explain how channel and carrier proteins
work.
**Channel**: hydrophilic channels bind to specific ions =
one side of the protein closes & the other opens
**Carrier**: binds to complementary molecule =
conformational change releases molecule on other side
of membrane; in facilitated diffusion, passive process; in
active transport, requires energy from ATP hydrolysis
79
Name 5 factors that affect the rate of
diffusion.
• Temperature
• Diffusion distance
• Surface area
• Size of molecule
• Difference in concentration (how steep the
concentration gradient is)
80
State Fick’s law.
surface area x difference in
concentration / diffusion distance
81
How are cells adapted to maximise the
rate of transport across their
membranes?
• many carrier/ channel proteins
• folded membrane increases surface
area
82
Explain the difference between the
shape of a graph of concentration
(x-axis) against rate (y-axis) for simple vs
facilitated diffusion.
**Simple diffusion**: straight diagonal line; rate of
diffusion increases proportionally as concentration
increases.
**Facilitated diffusion**: straight diagonal line later levels
off when all channel/ carrier proteins are saturated.
83
Define active transport.
**Active process**: ATP hydrolysis releases phosphate
group that binds to carrier protein, causing it to change
shape.
Specific carrier protein transports molecules/ ions from
area of low concentration to area of higher concentration
(i.e. **against concentration gradient**).
84
Compare and contrast active transport
and facilitated diffusion.
• Both may involve carrier proteins.
• Active transport requires energy from ATP
hydrolysis; facilitated diffusion is a passive
process.
• Facilitated diffusion may also involve channel
proteins.
85
Define co-transport.
Movement of a substance **against** its concentration gradient
is **coupled** with the movement of another substance **down**
its concentration/ electrochemical gradient.
Substances bind to complementary intrinsic protein:
**symport**: transports substances in same direction
**antiport**: transports substances in opposite direction e.g.
sodium-potassium pump.
86
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 lower in epithelial cells than lumen of gut.
3. Transport of glucose/ amino acids from lumen to epithelial cells
is ‘coupled’ to facilitated diffusion of Na+
down electrochemical
gradient.
87
What is an antigen?
• Cell-surface molecule which stimulate immune response.
• Usually (glyco)protein, sometimes (glyco)lipid or
polysaccharide.
• Immune system recognises as “self” or “non-self” =
enables identification of cells from other organisms of
same species, pathogens, toxins & abnormal body cells.
88
How does phagocytosis destroy
pathogens?
1. Phagocyte moves towards pathogen via **chemotaxis**.
2. Phagocyte engulfs pathogen via endocytosis to form
a **phagosome**.
3. Phagosome fuses with lysosome (**phagolysosome**).
4. Lysozymes **digest pathogen**.
5. Phagocyte absorbs the products from pathogen
hydrolysis.
89
Explain the role of antigen-presenting
cells (APCs).
Macrophage displays antigen from pathogen on
its surface (after hydrolysis in phagocytosis).
Enhances recognition by TH
cells, which cannot
directly interface with pathogens/ antigens in
body fluid.
90
Give 2 differences between specific and
nonspecific immune responses.
**nonspecific** (inflammation, phagocytosis) = same for all
pathogens
**specific** (B & T lymphocytes) = complementary pathogen
**nonspecific** = immediate
**specific** = time lag
91
Name the 2 types of specific immune
response.
• cell-mediated
• humoral
92
Outline the process of the cell-mediated
response.
1. **Complementary** TH
lymphocytes bind to foreign
antigen on APC.
2. Release cytokines that stimulate:
a) clonal expansion of complementary TH
cells (rapid
mitosis): become **memory cells** or trigger **humoral
response**.
b) clonal expansion of **cytotoxic T cells** (TC
): secrete
enzyme **perforin** to destroy infected cells.
93
Outline the process of the humoral
response.
1. **Complementary** TH
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.
94
What is an antibody?
proteins secreted by plasma cells
**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**.
95
How do antibodies lead to the
destruction of a pathogen?
Formation of **antigen-antibody
complex** results in **agglutination**, which
**enhances phagocytosis**.
96
What are monoclonal antibodies?
Antibodies produced from a single clone
of B cells.
97
What are memory cells?
• Specialised TH
/ 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.
98
Contrast the primary and secondary
immune response.
**secondary response**:
• Faster rate of antibody production.
• Shorter time lag between exposure & antibody production.
• Higher concentration of antibodies.
• Antibody level remains higher after the secondary
response.
• Pathogen usually destroyed before any symptoms
Note:*any of these points in inverse related to primary response also count*
99
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.
100
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.
101
Compare passive and active immunity.
Give examples of both types.
• both involve antibodies
• can both be natural or artificial
**passive natural**: antibodies in breast milk/ across placenta
**passive artificial**: anti-venom, needle stick injections
**active natural**: humoral response to infection
**active artificial**: vaccination
102
Contrast passive and active immunity.
**Passive**:
• no memory cells & antibodies not replaced when broken = short-term
• immediate
• antibodies from external source
• direct contact with antigen not necessary
**Active**:
• memory cells produced = long-term
• time lag
• lymphocytes produce antibodies
• direct contact with antigen necessary
103
Explain the principles of vaccination.
1. Vaccine contains dead/ inactive form 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.
104
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.
105
Suggest some ethical issues surrounding
the use of vaccines.
• production may involve use of animals
• potentially dangerous side-effect
• clinical tests may be fatal
• compulsory vs opt-out
106
Describe the structure of HIV.
• Genetic material (**2 x RNA**) & **viral enzymes**
(integrase & reverse transcriptase) surrounded by
**capsid**.
• Surrounded by viral **envelope** derived from host
cell membrane.
• GP120 **attachment proteins** on surface.
107
How does HIV result in the symptoms of
AIDS?
1. Attachment proteins bind to complementary CD4
receptor on TH
cells.
2. HIV particles replicate inside TH
cells, killing or damaging
them.
3. AIDS develops when there are too few TH cells for the
immune system to function.
4. Individuals cannot destroy other pathogens & suffer from
secondary diseases/ infections.
108
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 cells = difficult to
destroy them without damaging normal body
cells.
109
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.
110
Explain the principle of a direct ELISA
test.
detects presence of a specific antigen
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.
111
Explain the principle of an indirect ELISA
test.
detects presence of an antibody against a specific antigen
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.
112
Suggest some ethical issues surrounding
the use of monoclonal antibodies.
• Production involves animals.
• Drug trials against arthritis &
leukaemia resulted in multiple organ
failure.
