U1T3 & U1T4 - Viruses & Cells Flashcards
1
Q
What ideas does cell theory embrace?
A
Cells form the building blocks of living organisms, they arise by cell division, their genetic material is passed from parent to daughter cells in cell division + the cell is the functioning unit of life, metabolism takes place in them.
2
Q
How have we been able to investigate cell structure in more detail?
A
The development of the electron microscope.
3
Q
What does cytoplasm contain?
A
A large number of organelles which each have a specific function.
4
Q
What are the 2 different types of cell structure found in living organisms?
A
Prokaryotic + eukaryotic cells
5
Q
What are the characteristics of a prokaryotic cell?
A
Small (1-10um diameter), no nuclei/organelles (mitrochondria/endoplasmic reticulum) + has DNA in a single circular strand in cytoplasm which is naked DNA.
6
Q
Give an example of prokaryotic cells?
A
Bacteria + cyanobacteria (blue-green algae)
7
Q
What kind of role does bacteria play?
A
Some are harmful + responsible for diseases (cholera/plague) + others form the basis of much of modern biotechnology. Play an important role in ecological processes. Nutrient cycles rely on action of bacteria in making elements locked up in complex molecules available to plants once again.
8
Q
How might eukaryotic cells have evolved?
A
Evolved from prokaryotic cells some 1000 million years ago.
9
Q
What are the characteristics of a eukaryotic cell?
A
Large (10-100um), nucleus separated from cytoplasm by nuclear membrane, nucleus contains chromosomes with helical histone bound chromatins, cytoplasm is compartmentalised by membranes into organelles, contains eukaryotic flagella + cilia made of tubes sometimes with a cell wall.
10
Q
What can eukaryotic cells develop during cell division?
A
A mitotic spindle
11
Q
What are eukaryotic flagella + cilia made of?
A
A series of tubes arranged in a cylindrical manner.
12
Q
What does the cell wall of eukaryotic cells usually contain?
A
Cellulose or chitin
13
Q
What are plasma membranes made up of?
A
50:50 lipid and protein by weight, but since proteins are larger than lipids there are about 50 lipids for every protein. Mostly phospholipids, but also cholesterol + glycolipids. Cholesterol only in animal cells, not plant cells.
14
Q
Why is cholesterol necessary in an animal cell membrane? Why not in plant cells?
A
It stabilises the cell membrane. The cell wall does this job instead.
15
Q
Why is the phospholipid bilayer considered a fluid configuration? What do bilayers do?
A
The molecules can move about freely in their own layers. Provide the structural basis for all cell membranes + gives membrane its selectively permeable property.
16
Q
What is the main function of a cell’s plasma membrane?
A
Acts as a selective barrier between inside of cell and extracellular fluid. Regulates transfer of substances into and out of the cell. This keeps it supplied with raw materials and removed waste products. Compartmentalises cell.
17
Q
What mechanisms are involved with the function of the cell membrane?
A
Diffusion + active transport.
18
Q
What do cells lose and gain thanks to diffusion?
A
Dissolved oxygen, carbon dioxide and water molecules.
19
Q
When can diffusion work?
A
When there is a difference in concentration. Substances always move down the concentration gradient.
20
Q
Where can diffusion occur in the cell membrane?
A
Directly through the lipid bilayer or proteins present in membrane can act as carriers/channels.
21
Q
What does active transport require in the cell membrane?
A
Membrane proteins. Energy is used up and substances transported against their concentration gradient.
22
Q
How do surface carbohydrates work?
A
Attach to proteins to form glycoproteins or attached to phospholipids to form glycolipids groups. The composition + branching pattern of them varies. They may be involved in cell-cell communication.
23
Q
What are most glycoproteins?
A
Intrinsic proteins
24
Q
What do membrane proteins and glycoproteins determine?
A
What functions a cell can perform.
25
What are the main functions of membrane proteins?
Form channels/pores, transporter, act as enzymes, act as receptors, antigens + antibodies, provide stability + support, adhesion sites where adjacent cells are held together. These are often glycoproteins.
26
Membrane protein functions:
| Form channels/pores
Allow substances in and out of cell. Proteins forming these are lined with hydrophilic groups allowing water soluble substances through. Gated channels are important in controlling passage of ions in and out of neurons during nervous conduction.
27
Membrane protein functions:
| Transporter (carrier)
These proteins transport large/polar molecules across membrane by flipping over/across membrane. Energy required.
28
Membrane protein functions:
| Act as enzymes
They catalyse a reaction in or out of the cell (depending where active site faces) e.g. lactase from epithelial cells in small intestine breaks down lactose.
29
Membrane protein functions:
| Act as receptors
Receptor recognises specific ligand. Relays signals between intra and extra cellular space. This can alter cell's function. e.g. Antidiuretic hormone (ADH) binds to receptors in cell membrane in kidneys which changes water permeability of certain plasma membranes.
30
Membrane protein functions:
| Antigens
Usually protein foreign substances on surface membrane of bacterial/viral cell. Once recognised by body, immune response activated.
31
Membrane protein functions:
| Antibodies
Proteins which recognise and destroy the antibodies.
32
What do most plasma membranes contain for ion movement? Examples of these ions?
Channels
| Na+, K+, Cl-.
33
Where do glycolipids appear?
5% of membrane lipids. Layer that faces the extracellular fluid (outer surface).
34
What are glycolipids important for?
Adhesion between cells, communication between cells + regulating cell growth + development.
35
Where are cholesterol molecules found?
20% of membrane lipids. Phospholipids in animal cells. Plant membranes don't have cholesterol.
36
What Is the purpose of cholesterol in membranes?
Stabilises the cell membrane, making it more fluid at low temp by acting like a wedge to stop phospholipids from sticking together. At high temp, cholesterol wedges restrict sideways movement of phospholipid so membrane doesn't become too fluid.
37
What do cells contain that divide them?
Internal membranes divide cell into diff compartments e.g. mitochondria and chloroplasts.
38
Describe the nucleus.
Largest organelle in eukaryotic cell (10-25um diameter). Surrounded by double membrane containing nuclear pores. Outer membrane is encrusted with ribosomes + is evaginated to form RER. Contains chromosomes known as chromatin. Also contains nucleolus, may be more than 1, darker staining body
39
What is the purpose of nuclear pores?
Important for communication + movement of large molecules between nucleoplasm + cytoplasm (transport of mRNA from nucleoplasm to cytoplasm)
40
Describe chromosomes.
Made of DNA which has been supercoiled. Only visible when cell is dividing, usually spread out.
41
Describe the nucleolus.
Found within nucleus. Darker staining body. Contains DNA with copies of genes that code for rRNA. rRNA is synthesised here and early stages of ribosomal formation takes place. (1 - 3 um)
42
Describe ribosomes.
Up to 30nm (Very small) Made of protein + RNA in equal quantities. Site of protein synthesis. mRNA is read here, Ribosomes free in cytoplasm are site of protein synthesis for proteins which stay within cell. Those which are attached to RER produce proteins which are to be secreted from the cell. Formed of large + small subunit, bonded with RNA, frequently occur as groups called polyribosomes. Only 1 molecule made at a time.
43
Describe the endoplasmic reticulum.
Network of membranes found in cell cytoplasm, consisting of complex system of pairs of membranes arranged parallel to each other enclosing cisternae. Some membranes are covered in ribosomes (RER) Those that aren't are called the SER.
44
Describe the Smooth Endoplasmic Reticulum?
Has no ribosomes but is the site where substances needed by the cell are synthesised, it is also important in the manufacture of lipids. A special form exists in the cytoplasm of voluntary muscle cells. Site of storage of calcium ions, which have an important role in the contraction of muscle fibres.
45
What is the main function of the SER?
Synthesises, secretes + stores carbohydrates, lipids + other non-protein products.
46
What is the main function of the RER?
Provides scaffolding for ribosomes to make proteins and operates as a distribution network for these proteins.
47
What is the main function of the golgi apparatus?
It modifies proteins for export by exocytosis (adding carbohydrate group by changing them into glycoproteins). It is also involved in the production of lysosomes, digestive enzymes + transporting + storing lipids. Also may turn them into lipoproteins by adding lipid, add a prosthetic group/cofactor, form proteins with quaternary structure. They are labelled, packaged or sorted for export.
48
Why are the enzymes in lysosomes surrounded by a membrane?
Their enzymes would digest the proteins + lipids normally found in the cell and destroy them.
49
What is the function of lysosomes?
Digest engulfed bacteria enclosed in phagosome by phagocytosis. e.g. Food taken in by amoeba is digested by lysosomal enzymes.
They also hydrolyse worn out cell organelles by fusing with other vesicles.
e.g. metamorphosis, when a frog's tail disappears as it is gradually reabsorbed, this is done by lysosomes.
50
Describe the golgi apparatus.
Series of cisternae with a small vesicles entering + leaving the system.
51
Where are mitochondria found?
Cytoplasmic matrix (cytosol).
52
What happens when a cell requires energy?
Hydrolyses ATP usin ATP-ase to provide energy whenever it's needed in mitochondria.
53
Why might lots of energy be required in a cell?
Active transport (e.g. Absorption of digestive products in small intestine) , movement (e.g. Contraction of muscles) + chemical reactions (e.g. Producing new molecules like liver cells)
54
Describe the membranes of mitochondria.
Outer layer - smooth surface layer enclosing it.
Inner layer - Convoluted to form cristae which extend into matrix.
Separated by inter membrane space.
55
Describe cristae.
Provide mitochondria with large surface area. Their membranes are densely studded with stalked particles made of protein which contain enzymes involved in reactions for respiration (ATP production)
56
Describe spindle fibres.
Important in movement of chromosomes during mitosis + meiosis, formed of microtubules.
57
What is the function of centrioles?
Assembly of spindle fibres during cell division, formed of microtubules. Also important part of cilia + flagella
58
Describe centrioles.
Contain microtubules in structure. Animal cells have a pair of centrioles (centrosome) which usually lie at right angles to each other + adjacent to nuclear membrane. Each consists of 9 groups (triplets) of microtubules in circular arrangement.
59
What happens during cell division in terms of centrioles?
Centrioles separate + move to opposite ends of cell where they act as a focus for formation of a spindle. The fibres of the spindle play an important part in cell division by pulling diff groups of chromosomes apart to form nuclei of daughter cells.
60
What organelles are present in animal cells?
Nucleolus, nuclear membrane, cell surface membrane, free ribosomes, SER, glycogen granules, enchromatin, mictochondria, heterochromatin, RER, centriole, vesicles, nucleus + golgi apparatus.
61
What organelles are present in plant cells?
Nucleus, RER, cell wall (cellulose), vacuole, tonoplast, plasmodesmata, SER, mitochondria, chloroplast, middle lamella + golgi apparatus.
62
What organelles are present in fungal cells?
SER, nucleus, vacuole, RER, cell wall (chitin), mitochondria, glycogen granules, nucleus (some have none whilst some are multinucleate) + lysosomes.
63
What organelles are present in prokaryotic cells?
Cell wall (glycoprotein), small ribosomes, slime capsule, storage granules, plasmid, cytoplas + free DNA.
64
Describe the plant cell wall.
Immediately outside the cell surface membrane, 1um thick, made of cellulose which is laid down as microfibrils. Fully permeable, doesn't control entry + exist of substances.
65
What does the cellulose cell wall allow?
Plant cells can have high internal pressure due to water uptake without cells bursting, Protoplasm passes through pores in cellulose walls. Provides support.
66
What do pores in the plant cell wall allow?
Plant cells can be connected to their neighbours.
67
When a plant cell is dividing, what happens with the cell wall?
Cell wall is developed across old cell, divides contents of original cell.
68
What is between plant cell walls of adjacent cells?
Middle lamella, made of calcium pectate, the first layer to be deposited.
69
What gets trapped across the middle lamella? What does it do?
Endoplasmic reticulum, forms connections in cytoplasms of 2 new cells. These are known as plasmodesmata.
70
Describe chloroplasts.
Bound by a double membrane, both of which are smooth. Well organised membrane structure (lamellae). Inside each are grana + lamellae, each granum is made of thylakoids which are stacked together + contain chlorophyll. Lamellae form interconnecting network between grana + contain chlorophyl, which absorb light for photosynthesis (light dependent reactions). Starch grains + lipid globules found in fluid interior, known as stroma/matrix, contains enzymes for light independent reactions, contains small ribosomes/circular DNA. Between grana, membranes are less concentrated + referred to as inter grana.
71
What are the similarities between chloroplasts + mitochondria?
Both contain own DNA + ribosomes + can direct own protein synthesis, can also divide + reproduce independently of rest of cell. Could be intracellular descendants of prokaryotic cells which were incorporated into eukaryotic cells by phagocytosis.
72
What is the function of the large permanent vacuole?
Storage of ions + water + develops turgor for support.
73
Kingdom:
Prokaryotic
Eukaryotic
Prokaryotae
| Protoctista, plantae, fungi, animalia.
74
Size:
Prokaryotic
Eukaryotic
0.5 - 10 um diameter
| 10 - 100 um diameter
75
Genetic Material:
Prokaryotic
Eukaryotic
Most DNA is naked, some bacteria have plasmids, no nucleus.
Most DNA is linear + incorporated with proteins + RNA in chromosomes, nuclear bound by envelope. Circular DNA in mitochondria + chloroplasts.
76
Cell Division:
Prokaryotic
Eukaryotic
Usually binary fission but sometimes conjugation, no spindle formation.
Mitosis, meiosis, or both, spindle formation occurs.
77
Ribosomes:
Prokaryotic
Eukaryotic
70S only free in cytoplasm.
| 80S either free in cytoplasm/bound to RER
78
ER:
Prokaryotic
Eukaryotic
None.
| SER + RER
79
Organelles:
Prokaryotic
Eukaryotic
Few, none bound by double membrane.
| Many, bound by double membrane.
80
Hair-like structures:
Prokaryotic
Eukaryotic
Flagella + pili simple, not extracellular + lack internal system of microtubules.
Flagella + cilia complex extensions of membrane + contain intracellular arrangement of microtubules.
81
Cell Wall:
Prokaryotic
Eukaryotic
Rigid, made of protein + poylsaccharide, made of peptidoglycan.
Rigid for plant + fungi, containing cellulose/chitin, animals don't have cell walls.
82
Respiratory Membranes:
Prokaryotic
Eukaryotic
Mesosomes act as respiratory surface in some bacteria, no mitochondria.
Cristae act as main surface for aerobic respiration.
83
Photosynthetic Membranes:
Prokaryotic
Eukaryotic
Intracellular membranes not organised into grana, no chloroplasts.
Membranes within chloroplasts, plants + some algae membranes form thylakoids stacked into grana.
84
Specialised cells don't have the ability to do other things. What are the resulting differences between cells due to?
Division of labour
85
1m = millimetres
1000
86
1mm = micrometres
1000
87
1um = nanometres
1000
88
How might you calculate magnification?
Size of image / size of specimen
89
Describe the light microscope.
Resolving power of 0.5m. Compound microscope focuses light rays by condenser onto specimen on microscope slide. Objective lens forms image which is further magnified by eyepiece lens. Used to examine living + unstained specimens.
90
Describe the transmission electron microscope.
Resolving power of 0.1nm. Shows cell ultrastructure. Increased resolution achieved as it uses beam of electrons instead of light rays. (Electrons have a shorter wavelength than light) Beam of electrons is directed at specimen in high vacuum as molecules in air would absorb electrons so vacuum is required. Screen is dark where electrons are absorbed by specimen - electron dense. Screen is bright where electrons penetrate screen - electron transparent. Needs thin samples, must be dead, b + w + images may contain artefacts.
91
What does the degree of electron scattering depend on?
Nuclear masses of atoms in specimen + thickness of specimen.
92
Describe the scanning electron microscope.
Used to produce 3D images of surface of specimens. Electrons are reflected from surface of specimen stained with heavy metal enabling it to produce images of whole specimens, tissues or even organisms. Resolution + magnification are lower than TEM. (20nm+) B + w, dead specimens.
93
How might we separate an organelle from the rest of the contents of the cell?
Homogenisation + centrifugation.
| Altering speed of rotation +/or time of centrifugation.
94
What is differential centrifugation?
Common procedure in microbiology + cytology used to separate certain organelles from whole cells for further analysis of specific cell parts. Tissue sample is lysed to break cell membranes + mix up cell contents (homogenisation) + lysate is subjected to repeated centrifugations, each time removing pellet + increasing centrifugal force.
95
Describe centrifugation.
Separation based on size + density of diff organelles (larger = lower force)
96
What order can cell components be separated in?
Whole cells + nuclei; mitochondria, chloroplasts + lysosomes; microsomes; ribosomes + cytosol.
97
```
Light Microscope:
Resolving Power
Magnification
Specimens
Image Produced
Cost
Limitations
```
200nm, x15,000, Alive + may need stain, can be coloured, cheap + low resolve power.
98
```
Transmission Electron Microscope:
Resolving Power
Magnification
Specimens
Image Produced
Cost
Limitations
```
0.1nm, x10,000,000, dead + complex prep, black and white with 2D ultrastructure, expensive, vacuum so it must be dead + may need training, also may contain artefacts.
99
```
Scanning Electron Microscope:
Resolving Power
Magnification
Specimens
Image Produced
Cost
Limitations
```
10nm, x1,000,000, dead + cooked in heavy metal, 3D black and white images, expensive, lower resolving power with complex prep and needs heavy metal.
100
Number, standard form + symbol:
| deci
0.1, 10^-1, d.
101
Number, standard form + symbol:
| centi
0.01, 10^-2, c.
102
Number, standard form + symbol:
| milli
0.001, 10^-3, m.
103
Number, standard form + symbol:
| micro
0.000001, 10^-6, μ.
104
Number, standard form + symbol:
| nano
0.000000001, 10^-9, n.
105
Number, standard form + symbol:
| pico
0.000000000001, 10^-12, p.
106
What is the pyramid for measuring magnification, image size and actual size?
I
--
A|M
107
Millimetre:
| Symbol + size in metres
mm, 10^-3m.
108
Micrometre:
| Symbol + size in metres
μm, 10^-6m.
109
Nanometre:
| Symbol + size in metres
nm, 10^-9m.
110
Describe the process of protein synthesis.
mRNA takes a copy of the DNA for protein synthesis which is stored in the nucleus. It moves into ribosomes where it is read. Here, tRNA allows the protein to be synthesised as one molecule is progressively assembled from amino acids and a polypeptide chain is created. It then passes through the membrane into the cisternae space of the RER. Here they are prepared for export from the cell, then transferred into the forming face/cisternae of the golgi apparatus by way of a vesicle which completes the modification + packaging of the protein. Finally, a vesicle buds off the mature face of the golgi + takes the final protein to where it will perform its function. (Incorporated into membrane as receptor site)
111
What are 4 structures of viruses?
Icosahedral, enveloped, complex or helical.
112
What does HIV do in humans?
Invades helper T-cells which are a type of lymphocyte. Very important in immune system when protecting against disease. As more T-cells are destroyed, immune system becomes critically compromised + AIDS develops.
113
What are the similarities + differences between bacteriophages + HIV?
Very small, non-cellular + contain genetic material within protein coat.
Different shapes + phages have baseplate pins with tail fibres attached to base plate, HIV has capsid within phospholipid bilayer, glyproteins only in HIV, bacteriophages contain DNA whilst HIV contains RNA + only HIV contains reverse transcriptase. HIV is spherical whilst bacteriophage is icosahedral. Different proteins on exterior allow each virus to invade different cells.
114
Why do phages have tail fibres attached to a base plate?
Important in attaching virus to bacterial cells.
115
What 2 places would store glycogen?
Muscle + liver cells.
116
In what 2 ways is glycogen adapted to its function?
Branched so it is compact + has more terminal ends and therefore can be hydrolysed more quickly. Insoluble so it doesn't affect osmosis.
117
Explain why the nucleus might be pushed against the cell wall.
The large vacuole has pushed it to the side.
118
How do chloroplasts increase the amount of light energy absorbed?
They have a large surface area so more chlorophyll and more light absorbed.
119
Why might the nucleus not be visible in an image of a cell?
It is thin and it may not be in the plane of view.
120
Which organism has cristae + a matrix?
Mitochondria
121
How are cristae designed for their function?
Folded to increase surface area + contain embedded enzymes.
122
Why is the plasma membrane around mitochondria thin?
Easier diffusion of ATP + active transport as there is less distance to travel.
123
Which organelle has pores + why?
Nucleus, allows more transport of macromolecules between nucleoplasm + cytoplasm.
124
What is diffuse DNA called?
Chromatin
125
Why do pancreatic cells have many ribosomes?
They need to synthesise lots of protein.
126
Why are tiny folds of the cell surface membrane called?
Microvilli
127
Why do small intestine epithelial cells have lots of microvilli?
To increase the surface area for diffusion.
128
Why do small intestine epithelial cells have lots of mitochondria?
They need lots of ATP energy for active transport.
129
What cell part is responsible for autolysis?
What sort of enzymes do they contain?
What sort of reactions do they catalyse?
Lysosomes
Digestive
Hydrolysis
130
Which organelle stores + transports lipids + carbohydrates?
What flattened stacks is it made of?
Why do they have a large lumen?
What does it form?
Golgi apparatus
Cisternae
Need space for protein folding
Lysosomes
131
Why is the fluid mosaic model called this?
Phospholipids can move about laterally so it is fluid. Proteins are randomly placed within the membrane so it is a mosaic. The agreed structure is based upon experimental + chemical evidence so it's classed as a model.
132
How can you look at a virus?
SEM, not light microscope because they're too small.
133
Describe icosahedral viruses.
Appear spherical, but are icosahedral. Equilateral triangles fused into spherical shape. Optimal closed shell using identical protein sub-units.
134
How do icosahedral viruses work?
Released into environment when cell dies, breaks down + lyses, releasing the virions.
135
Give examples of icosahedral viruses. (3)
Poliovirus, rhinovirus + adenovirus.
136
Describe envelope viruses.
Conventional icosahedral/helical structure, surrounded by lipid bilayer membrane. Envelope is formed when virus is exiting cell via budding + infectivity depends on envelope.
137
Give examples of envelope viruses. (3)
Influenza virus, Hepatitis C + HIV.
138
Describe complex viruses.
May have complex outer wall/head-tail morphology which is unique to bacteriophages. Icosahedral head + helical tail.
139
How do complex viruses work?
Uses tail to attach to bacteria with protinaceous 'pins', tail contracts + tail plug creates hole in cell wall + membrane + inserts DNA using tail as channel.
140
Give an example + describe a complex virus.
Poxvirus (Variola virus = smallpox) largest virus, complex structure with unique outer wall + capsid.
141
Describe helical viruses.
Capsid with central cavity/hollow tube made by proteins arrange in circular fashion, creating disc like shape. Disc shapes helically attached created tube with room for nucleic acid in centre. Filamentous viruses are helical. Usually 15-19nm wide + 300-500nm long depending on genome size.
142
Give an example of a helical virus.
Tobacco mosaic virus.
143
How might a virus be transmitted?
Within fluid e.g. mucus droplets from a sneeze
144
What happens once a virus has entered a living host?
They begin replication.
145
Why do viruses need a host cell?
They cannot synthesise proteins, don't have ribosomes, can't translate mRNA into proteins, can't generate/store energy in ATP so must derive energy + other metabolic functions from host cells. Also parasitise cell for basic building materials (Amino acids, nucleotides + lipids)
146
How are viruses classified?
By the organisms they infect (animals, plants or bacteria) Further classified into families genera based on structure (Type + size of nucleic acid, size + shape of capsid + where they have a lipid envelope)
147
How are plant viruses transmitted + why?
Insects/other organisms that feed on plants. Cannot penetrate plant cell walls.
148
What are the 2 main kinds of shapes found amongst viruses + describe them.
Rods/filaments and spheres. Rod shape is due to linear array of nucleic acid + protein subunits making up capsid. Sphere is actually a icosahedron.
149
What are the 3 functions of the capsid?
Protein nucleic acid from digestion by enzymes, contains special sites on surface allowing virion to attach to host cell + provides proteins which enable virion to penetrate host cell membrane + inject nucleic acid into cytoplasm.
150
What will happen to viral RNA in a liquid suspension of protein molecules in the right conditions?
Protein molecules in suspension will self assemble a capsid to become functional + infectious virus.
151
Describe the membrane of a virus.
Lipoprotein bilayer which may contain material from host cell. Obtains lipids from viral budding process. Replaces proteins with own proteins creating hybrid structure of cell derived lipids + virus derived proteins. Many also develop glycoprotein spikes on envelopes to help with attachment.
152
Describe the nucleic acid in a virus.
Usually maintain genetic material with RNA.
153
What are the 2 types of RNA based viruses?
In most, genomic RNA is termed a plus strand as it acts as mRNA for direct synthesis of viral protein. Some have negative RNA strands, these have enzyme called RNA-dependent polymerase (transcriptase) which catalyses production of mRNA from virion genomic RNA before protein synthesis occurs.
154
Why is the tissue sample homogenised in a cold, isotonic, buffered solution?
Cold - Reduces enzyme activity which may break down organelles.
Isotonic - Prevents bursting/shrinking of organelles due to osmosis.
Buffered - Maintains constant pH.
155
What structure is necessary for the formation of ribosomes?
Nucleolus
156
What structure shows protein secretion?
Vesicles
157
Why would microvilli increase the rate of absorption in a cell?
Gives a larger surface area.
158
Why is cell ultrastructure not visible using a light microscope?
Organelles are so tiny they need a high resolution. The wavelength of light is too long in the light microscope so resolution will not be high enough to make organelles visible.
159
What are the differences between animal and bacterial cells?
Bacterial cell is smaller, cell wall surrounds bacterial cell only and may have a capsule. Cell membrane of animal cell contains cholesterol and ribosomes in bacterial cell are smaller and its DNA is circular whilst animal DNA is linear. Bacterial DNA is naked and they may contain plasmids. Genetic material enclosed in nuclear membrane in animal cells and they contain no membrane bound organelles. Centrioles only present in animal cells.
160
What are the similarities between animal and bacterial cells?
Both have cell membrane/cytoplasm, cell membrane has fluid mosaic structure, both have ribosomes + DNA + may contain glycogen.
