The Cell - The smallest living human subunit

Question 1

Why are we interested in Compartmentalisation?

Answer 1

Allows for regions of different composition and function to co-exist (maintains low entropy)
Separators cause different regions to have different composition: different chemical reactions and different molecular composition.

Question 2

What makes a good separator?

Answer 2

Insoluble in water (does not dissolve in water)
Non-rigid; Flexible to accommodate expansion and contraction in response to cellular dynamics

Question 3

Why are lipids a good and bad separator?

Answer 3

Lipids are good separators as oil and water don’t mix
However, they have minimal surface area as parts of a lipid repels water

Question 4

Define amphipathic molecules

Answer 4

Molecules with a polar/hydrophilic region and a non-polar/hydrophobic region

Question 5

Describe the features of a phospholipid

Answer 5

Long chain of fatty acids (two per molecule)
A bridging organic molecule (typically glycerol or some form of sphingosine)
A phosphate group
A polar small organic molecule (typically choline or serine)

Question 6

What structures do phospholipids form?

Answer 6

Phospholipids (as amphipathic molecules) form lipid bilayers and hence cell membranes.
The hydrophobic tails repel water coming together exposing the hydrophilic region to the aqueous environment

Question 7

What characteristic does fatty acids determine in phospholipid?

Answer 7

Fluidity and Flexibility

Question 8

What characteristic does polar groups determine in a phospholipid?

Answer 8

Chemical properties of surface

Question 9

What characteristic do both fatty acids and polar groups determine in a phospholipid molecule?

Answer 9

Protein binding

Question 10

How does communication between compartments occur in a cell?

Answer 10

Exchange of small molecules
Exchange of large molecular assemblies
Signalling (without transit of molecules across compartments)

Question 11

How does exchange of small molecules occur?

Answer 11

1. Non-polar molecules diffuse through lipid bilayer
2. Polar molecules can’t diffuse through lipid bilayer. They use transporter proteins (usually contain hydrophobic regions embedded in bilayer and hydrophilic regions in contact with aqueous solution)

Question 12

What are the ways of classifying membrane transporters?

Answer 12

Specificity of the solute that they will allow through
Direction in which the solute passes across the membrane in relation to their concentration gradients

Question 13

Describe the features of a channel protein

Answer 13

Allow many solutes through such as ions
Moves solutes down concentration gradients (from higher concentration to lower concentration)
Carrier proteins also moves solutes down concentration gradients

Question 14

Describe the features of a pump

Answer 14

Doesn’t allow as many solutes through (more selective than channel and carrier proteins)
Pumps move solutes upwards from low concentration to high concentrations.
This requires energy in the form of ATP that lowers the entropy to make the system more organised
This means pumps cause different composition in their compartments

Question 15

What are the three ways in which large molecular assemblies are exchanged?

Answer 15

1. Vesicle Formation/membrane fusion
2. Nuclear Pore Complex (NPC)
3. Clathrin

Question 16

Describe the features of the mitochondria

Answer 16

Biochemistry is prokaryote, not eukaryote
Have own “bacterial” genome and replicates autonomously - this is another whole genome in our cells
Main function is respiration - depending on forming a H+ gradient across the mitochondrial membrane
Double Membrane

Question 17

How does the electron transport chain help with the formation of ATP

Answer 17

Electron transport chain pumps proton out the inner membrane and the pH gradient is maintained by the presence of the outer membrane
Then relaxation of the H+ gradient drives ATP synthesis

Question 18

Describe the features of the Endoplasmic Reticulum

Answer 18

Receives newly synthesised proteins (coated with ribosomes - “Rough” ER)
Ensures proper protein folding (chaperoning)
Performs protein quality control with misfolded proteins tagged for degradation - Endoplasmic Reticulum
Associated Degradation
From these two functions, the ER is a stress-response organelle and is actively researched as a drug target
Other compartmentalised synthetic functions apart from protein synthesis is the smooth ER are lipid synthesis and hepatic enzymatic activity

Question 19

Describe the features of the Golgi Apparatus

Answer 19

Initial step of the vesicular transport system that delivers proteins to specific compartments or to the cell exterior
Proteins are tagged, typically by glycosylation according to the compartment in which they must be delivered.
Both ER and Golgi display directional movement, from protein synthesis near the nucleus to vesicle formation and delivery to the cell membrane
Membranes move inside the cell, and do so with a well define direction

Question 20

Describe the features of microtubules

Answer 20

Microtubules are formed by the polymerisation of two proteins: alpha and beta tubulin
A microtubule grows in one direction only
Hence, microtubules are asymmetric with a + end and a - end.
The - end is usually attached to a structure e.g. a centriole or microtubule organising centre - MTOC containing y-tubulin while the + end is free to grow

Question 21

What are the name of ATP-dependent motor proteins?

Answer 21

Kinesins and dyneins

Question 22

What is the function of kinesins and dyneins

Answer 22

ATP-dependent motor proteins that drag cargo (e.g. vesicles) along microtubules towards either the + end (anterograde) or the - end (retrograde), respectively.
This ATP-powered motor system provides directional movement for membranes and vesicles

Question 23

How is cell motility achieved?

Answer 23

Cell motility is achieved by polymerisation of a protein called actin (G-actin when in globular, unpolymerized form)
Fibres of polymerised actin (F-actin) push cell membranes forward
Actin fibres also provide “tracks” for myosin, which cause contraction of muscle cells

Question 24

How is DNA packaged into chromatin?

Answer 24

DNA wraps around four histone molecules: H2a, H2b, H3 and H4 (plus a “linker” histone H1)
Histone acetylation determines chromatin opening and DNA accessibility for transcription

Question 25

What is the name of the membraneless organelle?

Answer 25

Nucleolus

Question 26

Describe the features of nucleolus

Answer 26

Synthesises ribosomal RNA (RNA polymerase I)
Process initial RNA into smaller fragments
Pre-assembles and exports ribosomes
Many viral proteins “piggyback” on nuclear export
Highly sensitive to stress - particular metabolic stress due to high energy demand of RNA synthesis

Question 27

Define Karyokinesis

Answer 27

The separation of chromosomes is termed karyokinesis

Question 28

What is the name of the motors that pull the daughter chromatids along the microtubules towards the daughter of the cells?

Answer 28

Kinetochore

Question 29

Define kinetochore

Answer 29

Motors that pull the daughter chromatids along the microtubules towards the daughter of the cells

Question 30

Where is the minus end of the microtubules situated in mitosis?

Answer 30

In the centriole of the chromosome