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 functions -> organs made of several tissue types -> organ system

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 a 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 of bacteria and plants

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

Question 15

Sate 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

• Control 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

Sate 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

Contrast eukaryotic and prokaryotic cells

Answer 22

• Prokaryotic, small cells & always unicellular/Eukaryotic, larger cells & often multicellular
• Prokaryotic, no membrane-bound organelles & no nucleus/Eukaryotic, always have organelles & nucleus
• Prokaryotic, binary fission (always asexual reproduction)/Eukaryotic, mitosis & meiosis

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 (DNA or RNA) & 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

Sate 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 & 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 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 30

Suggest the advantages and limitations of using an optical microscope

Answer 30

``` + Colour image + Can show living structures + Affordable apparatus - 2D image - Lower resolution than electron microscopes = cannot see ultrastructure ```

Question 31

Describe how a transmission electron microscope (TEM) 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 TEM

Answer 32

+ 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 (experimental error) - No colour image

Question 33

Describe how a scanning electron microscope (SEM) 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 produce an image on a photographic plate

Question 34

Suggest the advantages and limitations of using an SEM

Answer 34

+ 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 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 seperate structures can be distinguished from one another

Question 36

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

Answer 36

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 37

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

Answer 37

- Actual size = image size / magnification - Image = actual size x magnification - Magnification = image size / actual size

Question 38

Outline what happens during cell fractionation and ultracentrifugation

Answer 38

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 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 cation of hydrolase enzymes - Buffered: maintain constant pH - Isotonic: prevent osmotic lysis/shrinking of organelles