Basic Cell Structure

Question 1

What are the main differences between prokaryotes and eukaryotes?

Answer 1

Prokaryotes

• No nucleus; plasmids + free DNA
• No membrane-bound organelles
• Peptidoglycan cell wall
• 1-10 um

Eukaryotes

• Nucleus; several chromosomes
• Membrane-bound organelles
• No cell wall
• 10-100 um

Question 2

Why are chemical reactions more efficient in eukaryotic cells than prokaryotic cells?

Answer 2

• Eukaryotic cells are compartmentalised by internal membranes (membrane-bound organelles) - more efficient reactions.
• In prokaryotes, all biochemical processes occur in the same compartment.

Question 3

What is the largest organelle and describe its structure.

Answer 3

• Nucleus
• Surrounded by nuclear envelope containing nuclear pores.
• Contains nucleolus (forms ribosomes).

Question 4

How is DNA packaged inside the nucleus?

Answer 4

• Folded into nucleosomes using histones - chromatin.
  
  1. Heterochromatin (10%): remains packed after mitosis (solenoid), transcriptionally inactive.
  2. Euchromatin: beads-on-a-string, transcriptionally active.

Question 5

Which structures envelope the cell?

Answer 5

• Plasma membrane (plasmalemma)

- Glycocalyx

Question 6

What is the PM made up of?

Answer 6

• Specific proteins, lipids and carbs.
• Amphipatic phospholipids (hydrophilic heads and hydrophobic tails) assemble into a lipid bilayer - forms a relatively impermeable barrier to most water-soluble molecules.

Question 7

What is the fluid mosaic model?

Answer 7

• Proteins ‘swim’ in the fluid lipid layer and mediate most membrane functions.
• Lipid rafts = membrane regions that assemble specialised proteins and lipids to perform a certain task.

Question 8

What is the glycocalyx?

Answer 8

• Cell coat made up of oligosaccharide and polysaccharide side chains of glycoproteins on the outside of the plasmalemma.
• This glycosylation often changes the function of the associated protein.

Question 9

Give examples of how membrane proteins can be associated with the lipid bilayer in different ways.

Answer 9

• Integral membrane proteins: contain hydrophobic membrane-spanning domains (e.g. Alpha-helix) that interact with the phospholipid fatty acyl groups.
• Peripheral membrane proteins: bound indirectly to membrane via interactions with integral membrane proteins or with lipid polar head groups.

Question 10

Name the functions of the PM.

Answer 10

1. Selective permeability
2. Transport of materials along cell surface
3. Endo- and exo-cytosol
4. Intercellular recognition and adhesion
5. Signal transduction

Question 11

Which organelles are involved in protein synthesis? What is the structure of these?

Answer 11

• Ribosomes
• 2 large subunits made up of ribosomal RNA and proteins
• Located free in cytoplasm or associated with rER

Question 12

What is the ER?

Answer 12

• Membranous extension of nuclear envelope.
• Involved in: synthesis and transport.
• 2 types that are continuous.

Question 13

What are the different functions of the 2 types of ER?

Answer 13

1. Smooth ER (no ribosomes).
   - Involved in:
   A) lipid synthesis
   B) metabolism of carbohydrates
   C) detoxification of drugs/poisons
   D) calcium storage (for cell signalling)
   - Less common type
2. Rough ER (ribosomes)
   - Involved in: secretory enzyme processing

Question 14

In which cell types is sER more commonly found in?

Answer 14

• Liver cells and mammary glands (lipid biosynthesis)

- Ovary, testis and adrenal gland (steroidogenesis)

Question 15

What is the function of the Golgi apparatus?

Answer 15

Modifies (e.g. O-glycosylation), sorts, concentrates and packages proteins synthesised on the rER.

Question 16

What are the 2 faces of the Golgi called and where does vesicle budding take place?

Answer 16

• Cis

- Trans (vesicle budding)

Question 17

Name the components of the Golgi.

Answer 17

1. Cis Golgi network
2. Golgi stack: cis cisterna, medial cisterna and trans cisterna
3. Trans Golgi network

Question 18

What are the 3 fates of proteins exiting the Golgi?

Answer 18

1. Constitutive secretion - targeted to PM
2. Regulated secretion - signal-mediated diversion to secretory vesicles
3. Signal-mediated diversion to lysosomes

Question 19

What are lysosomes?

Answer 19

• Single-membrane organelles generated by the Golgi

- Degrade unwanted material, inc. pathogens

Question 20

How do lysosomes degrade material?

Answer 20

1. Contain many hydrolytic enzymes
   E.g. Nucleases, proteases, glycosidases, phosphatases and phospholipases.
2. Very acidic environment maintained by vacuolar H+ ATPase (proton pump)

Question 21

How is the membrane of lysosomes protected from enzyme damage?

Answer 21

Coated by a specialised glycocalyx.

Question 22

What are peroxisomes and what is their function?

Answer 22

• Multi-purpose single membrane-bound organelles that contain many enzymes (crystalline core).
• Functions include:
  
  • hydrogen peroxide (H2O2)/ROS metabolism
  • lipid metabolism (e.g. Fatty acid beta-oxidation, phytanic acid alpha-oxidation)
  • detoxification (oxidise) of a number of molecules inc. alcohol, phenols, formic acid and formaldehyde (in liver and kidney cells)

Question 23

What is the function of mitochondria?

Answer 23

Energy production: use sugars, fats and oxygen to produce ATP via oxidative phosphorylation using ATP synthase.

Question 24

Describe the structure of mitochondria.

Answer 24

• Double membrane (separated by intermembrane space):
  
  • inner membrane folds to form cristae. Contains enzymes for oxidation reactions of respiratory chain.
  • outer membrane
• Matrix: contains 100s of enzymes, ribosomes and mitochondrial DNA genome

Question 25

Name examples of MT-rich tissues.

Answer 25

- cardiac cells - muscle - liver - brown adipose

Question 26

Give evidence for the endosymbiosis theory.

Answer 26

1. Double-membrane 2. Own genetic info 3. Own ribosomes 4. Divide by fission

Question 27

Name the 3 components of the cytoskeleton.

Answer 27

1. Actin filaments 2. Intermediate filaments 3. Microtubules

Question 28

Where is each cytoskeleton filament found and what is their role?

Answer 28

1. Actin microfilaments (5-9nm diameter) - Underlies plasma membrane (cortex): provides strength and shape - Forms cell surface projections, e.g. Microvilli and stereocilia 2. Intermediate filaments (10nm diameter) - Forms star-like cytoplasmic array: tough supporting mesh work - Underlies inner nuclear membrane forming nuclear lamina 3. Microtubules (25nm diameter) - Found in sites where structures are moved, e.g. Nerve fibre axon, mitotic spindle, cilia/flagella.

Question 29

Describe the structure and arrangement in cilia of MTs.

Answer 29

- Long hollow cylinders made up of tubulin protein. | - 9+2 arrangement in cilia/flagella