Cells Flashcards
(96 cards)
1
Q
Define the term eukaryotic cell
A
DNA is contained in a nucleus, contains membrane-bound organelles.
2
Q
Define the term prokaryotic cell
A
DNA is “free” in cytoplasm, no organelles
3
Q
Describe the structure and function of the cell-surface membrane
A
‘Fluid mosaic’ phospholipid bilayer with extrinsic & intrinsic proteins embedded.
4
Q
Explain the role of cholesterol in the cell-surface membrane
A
Cholesterol: steroid molecule connects phospholipids & reduces fluidity
5
Q
Explain the role of glycoproteins in the cell-surface membrane
A
Cell signalling, cell recognition & binding cells together
6
Q
Explain the role of glycolipids in the cell-surface membrane
A
Cell signalling & cell recognition
7
Q
Describe the structure of the nucleus
A
Surrounded by nuclear envelope.
Nuclear pores allow substances to enter/exit
Dense nucleolus made of RNA & proteins assembles ribosomes.
8
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, semi-conservative replication.
9
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
10
Q
Describe the structure of a chloroplast
A
Vesicular plastid with double membrane.
Thylakoids: flattened discs stack to form grana
Intergranal lamellae: tubes attach thylakoids in adjacent grana.
Stroma: fluid-filled matrix
11
Q
State the function of mitochondria
A
Site of aerobic respiration to produce ATP.
12
Q
State the function of chloroplasts
A
site of photosynthesis to convert solar energy to chemical energy
13
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
14
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
15
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
16
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.
17
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.
18
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.
19
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.
20
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.
21
Q
Why are viruses referred to as ‘particles’ instead of cells?
A
Acellular & non-living: no cytoplasm, cannot self-reproduce, no metabolism.
22
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.
23
Q
Describe how optical microscopes work.
A
- Lenses focus rays of light and magnify the view of a thin slice of specimen.
- Different structures absorb different amounts and wavelengths of light.
- Reflected light is transmitted to the observer via the objective lens and eyepiece
24
Q
Outline how a student could prepare a temporary mount of tissue for an optical microscope.
A
- Obtain thin section of tissue e.g. using ultratome or by maceration.
- Place plant tissue in a drop of water.
- Stain tissue on a slide to make structures visible.
- Add coverslip using mounted needle at 45° to avoid trapping air bubbles.
25
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
26
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.
27
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
28
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.
29
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
30
Define magnification
factor by which the image is larger than the actual specimen
31
Define resolution.
smallest separation distance at which 2 separate structures can be distinguished from one another
32
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.
33
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.
34
State the order of sedimentation of organelles during differential centrifugation.
most dense → least dense
nucleus → mitochondria → lysosomes → RER → plasma membrane → SER → ribosomes
35
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.
36
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)
37
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.
38
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
39
State the purpose of mitosis.
produces 2 genetically identical daughter cells for:
● Growth
● Cell replacement/ tissue repair
● Asexual reproduction
40
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.
41
Outline what happens during metaphase.
Sister chromatids line up at cell equator, attached to the mitotic spindle by their centromeres.
42
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.
43
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.
44
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).
45
Explain how to prepare a temporary root tip mount.
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.
46
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.
47
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.
48
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
49
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.
50
How do new viral particles leave the host cell?
a) Bud off & use cell membrane to form envelope.
b) Cause lysis of host cell.
51
Why is it so difficult to develop effective treatments against viruses?
Replicate inside living cells = difficult to kill them without killing host cells.
52
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
53
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.
54
Explain the function of extrinsic proteins in membranes.
binding sites/ receptors
e.g. for hormones
antigens (glycoproteins)
bind cells together
involved in cell signalling
55
Explain the function of intrinsic proteins in membranes.
electron carriers (respiration/photosynthesis)
channel proteins (facilitated diffusion)
carrier proteins (facilitated diffusion/ active transport)
56
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.
57
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.
58
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.
59
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.
60
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
61
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
62
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).
63
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
64
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
65
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)
66
State Fick’s law.
surface area x difference in concentration / diffusion distance
67
How are cells adapted to maximise the rate of transport across their membranes?
many carrier/ channel proteins
folded membrane increases surface area
68
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.
69
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).
70
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.
71
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.
72
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.
73
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
74
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.
75
Explain the role of antigen-presenting cells (APCs).
Macrophage displays antigen from pathogen on its surface (after hydrolysis in phagocytosis).
Enhances recognition by T-helper cells, which cannot directly interface with pathogens/ antigens in body fluid.
76
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
77
Name the 2 types of specific immune response.
● cell-mediated
● humoral
78
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.
79
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.
80
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.
81
How do antibodies lead to the destruction of a pathogen?
Formation of antigen-antibody complex results in agglutination, which enhances phagocytosis.
82
What are monoclonal antibodies?
Antibodies produced from a single clone of B cells.
83
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.
84
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.
85
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.
86
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
87
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
88
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.
89
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
90
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.
91
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.
92
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.
93
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.
94
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.
95
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.
96
Suggest some ethical issues surrounding the use of monoclonal antibodies.
● Production involves animals.
● Drug trials against arthritis & leukaemia resulted in multiple organ failure.
