Cell Structure

Question 1

Explain the procedure before cell fractionation (4)

Answer 1

1. Tissue is placed in a cold, buffered, Isotonic solution
2. Cold - reduces enzyme activity that might breakdown organelles
3. Buffered - change in pH could alter structure of organelles
4. Isotonic - of same water potential to prevent change in cell structure

Question 2

Explain procedure of cell fractionation (6)

Answer 2

1. Cells broken up by a homogeniser (homogenisation)
2. Resultant fluid (homogenate) filtered
3. Ultra centrifugation - filtrate placed in centrifuge and spun at low speed
4. Heaviest organelles (nuclei) collect at bottom forming pellets, and fluid (supernatant) is collected
5. Supernatant is spun again at higher speeds and next heaviest organelles (mitochondria) collect below
6. Process is repeated until all organelles are seperated

Question 3

Explain the functions of nucleus (3)

Answer 3

1. Control centre of cell through production mRNA and tRNA and protein synthesis
2. Retain genetic material
3. Manufactures rRNA and ribosomes

Question 4

Explain structure of nucleus (5)

Answer 4

1. Nuclear envelope - double membrane, controls entry and exit of materials into nucleus
2. Nuclear pores - allow passage of large molecules such as mRNA out of nucleus
3. Nucleoplasm - cytoplasm of nucleus
4. Chromosomes - contains protein bound DNA
5. Nucleolus - manufactures rRNA and ribosomes

Question 5

Explain structure of mitochondria (3)

Answer 5

1. Double membrane - control passage of materials into and out
2. Cristae - extension of inner membrane increase surface area for attachment of enzymes and proteins involved in respiration
3. Matrix - space containing protein, lipids, ribosomes and DNA that allows mitochondria to control production of own proteins

Question 6

Explain structure of chloroplasts (4)

Answer 6

1. Chloroplast envelope - double plasma membrane controls entry and exit of materials
2. Grana - stacks of thylakoid, increase surface area for attachment of chlorophyll
3. Thylakoid - contain chlorophyll, (where 1st stage photosynthesis takes place)
4. Stroma - fluid filled matrix (2nd stage photosynthesis) with enzymes

Question 7

Explain structure of Rough endoplasmic reticulum (2)

Answer 7

1. Ribosomes attach to the surface of membrane which provides a large surface area for protein synthesis
2. Provides pathway for transport of materials especially proteins, through cell

Question 8

Explain the function of Smooth endoplasmic reticulum (1)

Answer 8

1. Synthesis, storage and transport of lipids and carbohydrates

Question 9

Explain structure of The Golgi apparatus (5)

Answer 9

1. Stack of membranes that make up flattened stacks called Cisternae
2. Proteins and lipids produced by ER pass through Golgi which modifies the proteins by adding other components such as carbohydrates
3. The Golgi then ‘labels’ them to be accurately sorted and sent to the correct destinations
4. Once sorted, modified proteins and lipids are transported in vesicles that are pinched off from the Golgi cisternae
5. Vesicles may move to the cell surface where they fuse with membrane releasing contents to the outside

Question 10

Explain structure and function of Lysosomes (5)

Answer 10

1. Lysosomes are formed when vesicles from the Golgi contain enzymes such as proteases & lipases & lysozymes
2. Lysozymes hydrolyse material ingested by phagocytic cells i.e. bacteria
3. Lysosomes can release enzymes outside cell (exocytosis) to destroy material around cell
4. Lysosomes digest worn out organelles and re-use useful chemicals
5. Lysosomes completely break down cells after death (autolysis)

Question 11

Explain structure and function of Ribosomes (4)

Answer 11

1. 80S Ribosomes found in eukaryotes
2. 70S (smaller) ribosomes found in prokaryotes, mitochondria and chloroplast
3. Formed of one large and one small sub-unit
4. Site of Protein synthesis

Question 12

Explain structure and function of Cell wall (3)

Answer 12

1. Cellulose microfibrils provides mechanical strength to prevent cell from bursting under pressure by osmosis
2. Middle Lamella - a thin layer which cements adjacent cell walls and cells together
3. Cell wall allows for passage of water contributing to movement of water in plants

Question 13

Explain structure and function of Vacuoles (5)

Answer 13

1. Fluid-filled sac bounded by a single-membrane called the ‘Tonoplast’
2. Plant vacuole contains solution of mineral salts, sugars, amino acids, wastes, and pigments
3. Aid in keeping the cell turgid
4. Sugars and amino acids act as temporary food source
5. Pigments may be used to colour petals to attract pollination

Question 14

Explain the structure of a bacterial cell (6)

Answer 14

1. ‘Murein’ Cell wall
2. Slime capsule - protection against toxins and helps bacteria stick together
3. 70S Ribosomes
4. Genetic material in circular strand of DNA - not associated with proteins
5. Smaller circular DNA Plasmids can be passed between bacteria and may give resistance to harmful chemicals such as antibiotics
6. May have flagellum for motion

Question 15

Explain the structure of a Virus (4)

Answer 15

1. Contains DNA or RNA but can only reproduce in living host cells
2. Nucleic acids contained within a protein coat ‘Capsid’
3. Some viruses have a further lipid envelope
4. Attachment proteins (on lipid envelope or capsid) allow virus to identify and attach to host cells