3.2.1 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 and 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 and 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 and glycolipids in the cell-surface membrane.

Answer 4

Cholesterol: steroid molecule connects phospholipids and reduces fluidity.
Glycoproteins: cell signalling, cell recognition (antigens) and binding cells together.
Glycolipids: cell signalling and 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 and 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 and 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 stacked 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 and 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 and rRNA free in the 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 and flattened sacs extends from the cell membrane through cytoplasm and connects to nuclear envelope:
- Rough ER: many ribosomes attached for protein synthesis and transport
- Smooth ER: lipid synthesis

Question 14

Describe the structure of the cell well.

Answer 14

• Bacteria: made of the polysaccharide murein.
• Plants: made of cellulose microfibrils plasmodesmata allow the molecules to pass between cells, middle lamella acts as boundary between adjacent cell walls.

Question 15

State the functions of the cells 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

• Prevents desiccation
• Acts as food reserve
• Provides mechanical protection against phagocytosis and external chemicals.
• Sticks cells together.

Question 21

Compare eukaryotic and prokaryotic cells.

Answer 21

Both have:
- Cell membrane
- Cytoplasm
- Ribosomes

Question 22

Contrast eukaryotic and prokaryotic cells.

Answer 22

Prokaryotic
- Small cells and always unicellular
- No membrane-bound organelles or nucleus
- Circular DNA not associated with proteins
- Small ribosomes (70S)
- Binary fission (always asexual reproduction)
- Murein cell walls
- Capsule, sometimes plasmids and cytoskeleton
Eukaryotic
- Larger cells and often multicellular
- Always have organelles and nucleus
- Linear chromosomes associated with histones
- Larger ribosomes (80S)
- Mitosis and Meiosis (sexual and/or asexual reproduction)
- Cellulose cell walls (plants)/chitin (fungi)
- No capsule, no plasmids, always cytoskeleton.

Question 23

Why are viruses referred to as particles instead of cells?

Answer 23

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

Question 24

Describe the structure of a viral particle.

Answer 24

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

Question 25

Describe the structure of an enveloped virus.

Answer 25

- Simple virus surrounded by matrix protein - Matrix protein surrounded by envelope derived from cell membrane of host cell - Attachment proteins on surface

Question 26

State the role of the capsid on viral particles.

Answer 26

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

Question 27

State the role of attachment proteins on viral particles.

Answer 27

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

Question 28

Describe how optical microscopes work.

Answer 28

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 29

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

Answer 29

1. Obtain than section of tissue. 2. Place 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 30

Suggest the advantages and limitations of using an optical microscope.

Answer 30

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

Question 31

Describe how a transmission electron microscope works.

Answer 31

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 32

Suggest the advantages and limitations of using a transmission electron microscope.

Answer 32

Advantages - electrons have shorted wavelength than light = high resolution, so ultrastructure visible. - high magnification (x500000) Disadvantages - 2D image - requires a vacuum = cannot show living structures - extensive preparation may introduce artefacts - no colour image

Question 33

Describe how a scanning electron microscope works.

Answer 33

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 produced an image on a photographic plate.

Question 34

Suggest the advantages and limitations of using a scanning electron microscope.

Answer 34

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

Question 35

Define magnification and resolution.

Answer 35

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 36

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

Answer 36

1. Place micrometre on stage to calibrate eyepiece graticule. 2. Line up scales on graticule and micrometre. Count how many graticule divisions are in 100 micrometres on the micrometre. 3. Length of 1 eyepiece division = 100 micrometres divided by the number of divisions. 4. Use calibrated values to calculate actual length of structures.

Question 37

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

Answer 37

actual size = image size divided by magnification

Question 38

Outline what happens during cell fractionation and ultracentrifugation.

Answer 38

1. Mince and homogenize tissue to break open cells and 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 39

State the order of sedimentation of organelles during differential centrifugation.

Answer 39

most dense -> least dense nucleus -> mitochondria -> lysosomes -> RER -> plasma membrane -> SER -> ribosomes

Question 40

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

Answer 40

Cold: Slow action of hydrolase enzymes. Buffered: Maintain constant pH. Isotonic: Prevent osmotic lysis/shrinking of organelles.