2.1 Cell Structure

Question 1

Define the terms eukaryotic and prokaryotic cell

Answer 1

Eukaryotic: DNA is contained in a nucleus, contains membrane-bound specialised organelles.
Prokaryotic: DNA is ‘free’ in cytoplasm, no organelles e.g. bacteria & archaea.

Question 2

State the relationship between a system and specialised cells

Answer 2

Specialised cells → tissues that perform specific function → organs made of several tissue types → organ systems

Question 3

Describe the structure and function of the cell-surface membrane

Answer 3

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.

Question 4

Explain the role of cholesterol, glycoproteins & glycolipids in the cell- surface membrane

Answer 4

Cholesterol: steroid molecule connects phospholipids & reduces fluidity.
Glycoproteins: cell signalling, cell recognition (antigens) & binding cells together.
Glycolipids: cell signalling & cell recognition.

Question 5

Describe the structure of the nucleus

Answer 5

Surrounded by nuclear envelope, a semi-permeable double membrane.
● Nuclear pores allow substances to enter/exit.
● Dense nucleolus made of RNA & proteins assembles ribosomes.

Question 6

Describe the function of the nucleus

Answer 6

Contains DNA coiled around chromatin into chromosomes.
● Controls cellular processes: gene expression determines specialisation & site of mRNA transcription, mitosis, semiconservative replication

Question 7

Describe the structure of a mitochondrion

Answer 7

Surrounded by double membrane folded inner membrane forms cristae: site of electron transport chain
● Fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins.

Question 8

Describe the structure of a chloroplast

Answer 8

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

Question 9

State the function of mitochondria and chloroplasts

Answer 9

● Mitochondria: site of aerobic respiration to produce ATP.

● Chloroplasts: site of photosynthesis to convert solar energy to chemical energy.

Question 10

Describe the structure and function of the Golgi apparatus

Answer 10

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

Question 11

Describe the structure and function of a lysosome

Answer 11

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

Question 12

Describe the structure and function of a ribosome

Answer 12

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

Question 13

Describe the structure and function of the endoplasmic reticulum (ER)

Answer 13

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.

Question 14

Describe the structure of the cell wall

Answer 14

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.
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Question 15

State the function of cell wall

Answer 15

● Mechanical strength and support.
● Physical barrier against pathogens
● Part of apoplast pathway (plants) to
enable easy diffusion of water.

Question 16

Describe the structure and function of the cell vacuole in plants

Answer 16

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.

Question 17

Explain some common cell adaptations

Answer 17

● 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.

Question 18

State the role of plasmids in prokaryotes

Answer 18

● Small ring of DNA that carries non-essential genes.

● Can be exchanged between bacterial cells via conjugation

Question 19

State the role of flagella in prokaryotes

Answer 19

Rotating tail propels (usually unicellular) organism

Question 20

State the role of the capsule in prokaryotes

Answer 20

polysaccharide layer:
● Prevents desiccation.
● Acts as food reserve.
● Provides mechanical protection against
phagocytosis & external chemicals.
● Sticks cells together.

Question 21

Compare eukaryotic and prokaryotic cells

Answer 21

both have:
● Cell membrane.
● Cytoplasm.
● Ribosomes (don’t count as an
organelle since not membrane-bound)

Question 22

Why are viruses referred to as ‘particles’ instead of cells?

Answer 22

Acellular & non-living: no cytoplasm, cannot self-reproduce, no metabolism

Question 23

Describe the structure of a viral particle

Answer 23

● Linear genetic material (DNA or RNA) & viral enzymes e.g. reverse transcriptase.
● Surrounded by capsid (protein coat made of capsomeres).
● No cytoplasm.

Question 24

Describe the structure of an enveloped virus

Answer 24

● Simple virus surrounded by matrix protein.
● Matrix protein surrounded by envelope derived from cell membrane of host cell.
● Attachment proteins on surfac

Question 25

State the role of the capsid on viral particles

Answer 25

● Protect nucleic acid from degradation by restriction endonucleases.
● Surface sites enable viral particle to bind to & enter host cells or inject their genetic material.

Question 26

State the role of attachment proteins on viral particles

Answer 26

Enable viral particle to bind to complementary sites on host cell : entry via endosymbiosis.

Question 27

Describe how optical microscopes work

Answer 27

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

Question 28

Outline how a student could prepare a temporary mount of tissue for an optical microscope

Answer 28

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.

Question 29

Suggest the advantages and limitations of using an optical microscope

Answer 29

\+ colour image
\+ can show living structures
\+ affordable apparatus
- 2D image
- lower resolution than electron microscopes =
cannot see ultrastructure

Question 30

Describe how a transmission electron microscope (TEM) works

Answer 30

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

Question 31

Suggest the advantages and limitations of using a TEM

Answer 31

+ 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
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Question 32

Describe how a scanning electron microscope (SEM) works

Answer 32

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

Question 33

Suggest the advantages and limitations of using an SEM

Answer 33

+ 3D image
+ electrons have shorter wavelength than light = high
resolution
- requires a vacuum = cannot show living structures
- no colour image
- only shows outer surface

Question 34

Define magnification and resolution

Answer 34

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

Question 35

Explain how to use an eyepiece graticule and stage micrometer to measure the size of a structure

Answer 35

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

Question 36

State an equation to calculate the actual size of a structure from microscopy

Answer 36

actual size = image size/magnification

Question 37

Outline what happens during cell fractionation and ultracentrifugation

Answer 37

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

Question 38

State the order of sedimentation of organelles during differential centrifugation

Answer 38

most dense → least dense

nucleus → mitochondria → lysosomes → RER → plasma membrane → SER → ribosomes
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Question 39

Explain why fractionated cells are kept in a cold, buffered, isotonic solution

Answer 39

cold: slow action of hydrolase enzymes. buffered: maintain constant pH.
isotonic: prevent osmotic lysis/shrinking of organelles.