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 membrane bound organelles

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.
- 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 & glycoproteins in the cell-surface membrane.

Answer 4

Cholesterol: steroid molecule connects phospholipids & reduces fluidity.
Glycoproteins: cell signalling, cell recognition & 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 and semi-conservative replication

Question 7

Describe the structure of a mitochondrion.

Answer 7

• surrounded by double membrane folded inner membrane form cristae: site of electron transport chain
• fluid matrix: contains mitochondrial DNA, respitory 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 stach 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 translate:
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 encelope:
- 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 polysaccharide murein
• Plants: made of cellulose microfibrils plasmodesmata allow molecules to pass between cells, middle lamella acts as boundary between adjacent cell walls.

Question 15

State the functions of the 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 hydrolyse potentially harmful substances to detoxify cytoplasm

Question 17

Explain some common cell adaptations.

Answer 17

• folded membrane or microvilli increase surface area
• 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 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
- ribosome

Question 22

Contrast eukaryotic and prokaryotic cells.

Answer 22

Prokaryotic:
- small cells & always unicellular
- no membrane-bound organelles
- circular DNA not associated with proteins
- 70s ribosomes
- binary fission
- murein cell walls
- capsule
Eukaryotic:
- larger cells & often multicellular
- always have organelles & nucleus
- linear chromosomes associated with histones
- 80s ribosomes
- mitosis and meiosis
- cellulose cell wall
- no capsule

Question 23

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

Answer 23

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

Question 24

Describe the structure of a viral particle.

Answer 24

• linear genetic material & viral enzymes
• surrounded by capsid
• 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 capsid on viral particles.

Answer 26

- 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 27

State the role of attachment proteins on viral particles.

Answer 27

Enable viral particles 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

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 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 (500000) - 2D image - requires a vacuum = cannot show living structures - extensive preparation may introduce artefacts - no colour image

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 photograph plate

Question 33

Suggest the advantages and limitations of using a 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

Outline what happens during cell fractionation and ultracentrifugation.

Answer 35

1. Homogenise 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 high speed

Question 36

State the order of sedimentation of organelles during differential centrifugation.

Answer 36

Most dense —> least dense Nucleus —> mitochondria —> lysosomes—> RER —> plasma membrane —> SER—> ribosomes

Question 37

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

Answer 37

Cold: slow action of hydrolase enzymes Buffered: maintain constant pH Isotonic: prevent osmotic lysis/ shrinking of organelles