Cell Structure

Question 1

Define the term eukaryotic cell

Answer 1

DNA is contained in a nucleus, contains membrane-bound specialised organelles

Question 2

Define the term prokaryotic cell

Answer 2

DNA is ‘free’ in cytoplasm, no membrane-bound organelles

Question 3

State the relationship between a system and specialised cells

Answer 3

Specialised cells -> tissues that perform a specific function -> organs made of several tissue types -> organ systems

Question 4

Describe the structure and function of the csm

Answer 4

‘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 5

Explain the role of cholesterol in the csm

Answer 5

Steroid molecule connects phospholipids & reduces fluidity

Question 6

Explain the role of glycoproteins in the csm

Answer 6

Cell signalling, cell recognition & binding cells together

Question 7

Explain the role of glycolipids in the csm

Answer 7

Cell signalling & cell recognition

Question 8

Describe the structure of the nucleus

Answer 8

• 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 9

Describe the function of the nucleus

Answer 9

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

Question 10

Describe the structure of a mitochondrion

Answer 10

• surrounded by double membrane
• folded inner membrane forms cristae: site of ETC
• fluid matrix: contains mitochondrial DNA, respiratory enzymes, lipids, proteins

Question 11

Describe the structure of a chloroplast

Answer 11

• 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 12

State the function of mitochondria

Answer 12

Site of aerobic respiration to produce ATP

Question 13

State the function of chloroplasts

Answer 13

Site of photosynthesis to convert solar energy to chemical energy

Question 14

Describe the structure and function of the Golgi apparatus

Answer 14

Planar stack of membrane-bound, flattened sacks
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 15

Describe the structure and function of a lysosome

Answer 15

Sac surrounded by single membrane
Embedded H+ pump maintains acidic conditions
Contains digestive hydrolase enzymes
Glycoprotein coat protects cell interior
- digest contents of phagosome
- exocytosis of digestive enzymes

Question 16

Describe the structure and function of a ribosome

Answer 16

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 17

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

Answer 17

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 18

Describe the structure of the cell wall

Answer 18

• bacteria: made of polysaccharide murein
• plants: made of cellulose microfibrils
  plasmodesmata allow molecules to pass between cells, middle lamellae acts as boundary between adjacent walls

Question 19

State the functions of the cell wall

Answer 19

• mechanical strength and support
• physical barrier against pathogens
• part of apoplast pathway (plants) to enable easy diffusion of water

Question 20

Describe the structure and function of the cell vacuole in plants

Answer 20

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 21

Explain some common cell adaptations

Answer 21

• folded membrane or microvilli increase SA e.g for diffusion
• many mitochondria = large amounts of ATP for AT
• walls one cell thick to reduce diffusion distance pathway

Question 22

State the role of plasmids in prokaryotes

Answer 22

• small ring of DNA that carries non-essential genes

- can be exchanged between bacterial cells via conjugation

Question 23

State the role of flagella in prokaryotes

Answer 23

Rotating tail propels organism

Question 24

State the role of the capsule in prokaryotes

Answer 24

Polysaccharide layer:

• prevents desiccation
• acts as food reserve
• provides mechanical protection against phagocytosis & external chemicals
• sticks cells together

Question 25

Compare eukaryotic and prokaryotic cells

Answer 25

- cell membrane - cytoplasm - ribosomes (not membrane-bound)

Question 26

Why are viruses referred to as particles instead of cells?

Answer 26

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

Question 27

Describe the structure of a viral particle

Answer 27

- linear genetic material & viral enzymes e.g reverse transcriptase - surrounded by capsid (protein coat made of capsomeres) - no cytoplasm

Question 28

Describe the structure of an enveloped virus

Answer 28

- simple virus surrounded by matrix protein - matrix protein surrounded by envelope derived from cell membrane of host cell - attachment proteins on surface

Question 29

State the role of the capsid on viral particles

Answer 29

- 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 30

State the role of attachment proteins on viral particles

Answer 30

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

Question 31

Describe how optical microscopes work

Answer 31

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 32

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

Answer 32

1. Obtain thin section of tissue 2. Place plant tissue on a drop of water 3. Stain tissue on a slide to make structures visible 4. Add coverslip using mounted needle at 45 degrees to avoid trapping air bubbles

Question 33

Suggest the advantages and limitations of using an optical microscope

Answer 33

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

Question 34

Describe how a TEM works

Answer 34

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 35

Suggest the advantages and limitations of using a TEM

Answer 35

+ electrons have shorter wavelength than light = high resolution so ultrastructure visible + high magnification (x500000) - 2D image - requires a vacuum = cannot show living structures - long preparation time - no colour image

Question 36

Describe how a SEM works

Answer 36

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 37

Suggest the advantages and limitations of using an SEM

Answer 37

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

Question 38

Define magnification

Answer 38

Factor by which the image is larger than the actual specimen

Question 39

Define resolution

Answer 39

Smallest separation distance at which 2 points can be distinguished from one another

Question 40

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

Answer 40

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 41

How would you calculate actual size of a structure from microscopy?

Answer 41

actual size = image size / magnification

Question 42

Outline what happens during cell fractionation and ultracentrifugation

Answer 42

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) most dense organelles in mixture form a pellet at the bottom c) filter off the supernatant and spin again at a higher speed

Question 43

What is the order of sedimentation of organelles during differential centrifugation?

Answer 43

nucleus -> mitochondria -> lysosomes -> RER -> plasma membrane -> SER -> ribosomes

Question 44

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

Answer 44

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