Module 1

Question 1

Why can phospholipids be described as amphipathic molecules?

Answer 1

Because they have both hydrophobic and hydrophilic parts.

Question 2

What does the fluid mosaic model mean?

Answer 2

That the membrane is fluid (i.e. moving and not sitting rigid) with a “mosaic” of proteins embedded in it.

Question 3

How do amphipathic phospholipids form the cell membrane?

Answer 3

DRAW A DIAGRAM Phospholipids align to form a bilayer whereby the phophate (hydrophilic) heads line the outsides of the membrane, while the lipid (hydrophobic) tails form the inside of the membrane

Question 4

What are the two types of lipid tails that can be formed and how do their shapes aid in their abilities to form an effective barrier?

Answer 4

• DRAW*
  1) Unsaturated hydrocarbons: Has a double bond along the chain, leading to a bend in its configuration. This bending prevents tight packing.
  2) Saturated hydrocarbons: Comprised entirely of single bonds, leading to straight (BUT FLEXIBLE) tails that move easily and can thus be packed closer together.

Question 5

Describe phospholipid movement in the membrane

Answer 5

1) Movement of phospholipids
- Lateral movement (10 million times per second)
- Flip flop (once per month)
- Increasing temperature increases speed

Question 6

How can membrane fluidity be changed?

Answer 6

• Lots of unsaturated hydrocarbon tails = fluid membrane DRAW
• Lots of saturated hydrocarbon tails = viscous membrane
• Increasing temperature increases fluidity.

Question 7

**How does cholesterol within an animal cell membrane aid in fluidity?

Answer 7

*DRAW**
Cholesterol balances out cell fluidity and keeps it from extremes.
- It makes the membrane more fluid at low temperature and less fluid at high temperatures.

Question 8

What is the difference between peripheral and integral proteins near a cell membrane?

Answer 8

Peripheral proteins line only one side of the membrane whilst integral proteins exist across the membrane and allow certain materials to pass through them.

Question 9

Describe the structure of an integral protein and how this structure enables it to exist across the phospholipid bilayer

Answer 9

**DRAW
An integral protein is comprised of a continuous chain of amino acids.
- The N-terminus exists on the extracellular side, while the C-terminus exists on the cytoplasmic side of the membrane.
- The alpha helix proteins form the middle of the membrane and have hydrophobic side chains, causing this region to be hydrophobic (similar to the lipid tails surrounding it).
- Beta-pleated sheet proteins form the ends of the protein and have hydrophilic side chains, causing the region to be hydrophilic (similar to the phosphate heads around it).

Question 10

How do we know it is a fluid (CONSTANTLY MOVING) mosaic?

Answer 10

DRAW
Mouse cell and human cells with differing membrane proteins were joined to form a hybrid cell. After an hour, the proteins had mixed.

Question 11

What are the 6 major functions of membrane proteins?

Answer 11

• DRAW*
  1. Transport
  2. Enzymatic activity (catalyse important functions)
  3. Signal transduction
  4. Cell-to-cell recognition
  5. Intercellular joining
  6. Attachment to the cytoskeleton and the extracellular matrix (ECM)

Question 12

What are lipid bilayers permeable to? What does this mean?

Answer 12

Few water molecules, and a few small, uncharged, molecules (e.g. oxygen and carbon dioxide).
This means that these particles can pass directly through the lipid bilayer without the need for other transport mechanisms.

Question 13

What are lipid bilayers impermeable to?

Answer 13

HIM

• Small hydrophilic molecules (e.g. glucose)
• Ions (e.g. Na+, Ca2+,HCO3-, etc)
• Macromolecules (e.g. proteins and RNA)

Question 14

Define passive transport and how it occurs

Answer 14

Passive transport = diffusion of a substance across a membrane with no energy investment.

Question 15

Define diffusion and its relation to passive transport

Answer 15

Diffusion refers to the random motion by which molecules become equally distributed (thus eliminating a concentration gradient, provided the molecules can cross the membrane).
- No work is done in this process

Question 16

Define osmosis and explain why it occurs.

Answer 16

The diffusion of water through a selectively permeable membrane into another aqueous compartment containing solute at a HIGHER concentration.
TL;DR: The movement of water across a selectively permeable membrane to equalize concentrations and establish equilibrium.

Question 17

What are osmotica and what does this mean?

Answer 17

Osmotica refers to substances that cannot move across the membrane by passive diffusion and thus water moves instead to establish equilibrium.
Osmotica: SPIN- sugars, ions, proteins and nutrients.

Question 18

Define tonicity and the three forms it can take

Answer 18

Tonicity- the ability of a solution to cause a cell to gain or lose water

1) Isotonic: Solute concentration =cell concentration; no net water movement across the plasma membrane.
2) Hypertonic: Solute conc > cellular conc; cell loses water
3) Hypotonic: Solute conc < cellular conc: cell gains water (hyPOHtonic)

Question 19

Explain how osmosis can affect cell volume in hypertonic and hypotonic solutions

Answer 19

If water moves OUT of the cell, the cell volume DECREASES.

If water moves IN to the cell, the cell volume INCREASES.

Question 20

Explain the effect of placing an animal or plant cell into hypertonic, isotonic or hypotonic solutions:

Answer 20

Hypotonic:

• Animal: cell volume increases to a point after which it may lyse (burst)
• Plant: Cell volume is normal as the rigid cell walls prevent lysis. (Plant cells are generally healthiest in this environment- uptake of water eventually balanced by the wall pushing back on the cell).

Isotonic:

• Animal: cell volume is normal
• Plant: Cell appears flaccid (limp)

Hypertonic:

• Animal: Cell volume decreases, cell undergoes crenation (appears shriveled)
• Plant: Cell volume decreases, cell is plasmolysed- protoplast (inner membrane of cell) contracts

Question 21

What is the osmolarity of RBCs and how are they affected in different solutions?

Answer 21

Osmolarity= 300 osmol
Hypotonic: Cells lyse (hyPOHtonic), actual volume of cells decrease
Hypertonic: Cells crenate.

Question 22

Define facilitated diffusion. Does it still use concentration gradients or require energy?

Answer 22

In facilitated diffusion, transport proteins speed up the passive movement of molecules across the plasma membrane.
- The aim is still to only EQUALIZE concentration gradients, and thus it occurs along the solute concentration gradient and does NOT require energy.

Question 23

What are transport proteins and what types are there?

Answer 23

Transport proteins: Proteins that allow the passage of HYDROPHILIC substances across the membrane
Types DRAW:
a) Channel proteins, including aquaporins and ion channels: Open or close in response to a stimulus and are thus GATED CHANNELS

b) Carriers (or transporters) that bind to SPECIFIC molecules and change shape to shuttle them across the membranes, e.g. glucose transporters; amino acid transporters

Question 24

What is the difference between passive and active transportation? What forms of transport proteins exist in each

Answer 24

PASSIVE = DOWN conc gradient (inc. diffusion and facilitated diffusion)
ACTIVE = AGAINST conc gradient

Carrier proteins exist in both passive and active transportation, but ion channels ONLY exist in passive diffusion

Question 25

How does active transport work?

Answer 25

Carrier proteins mediate the active transport of hydrophilic substances across the membrane and AGAINST an existing concentration gradient. This process requires work and uses energy from ATP

Question 26

What is an electrogenic pump? What are the main types in animals/plants and explain how they work.

Answer 26

Electrogenic pump = transporter that generates voltage (potential difference in charges) across a membrane
Animals: Sodium-potassium pump (Na+/K+-ATPase
Plants/Fungi/Bacteria: Proton Pump- creates high conc outside of cell

Question 27

What is cotransport and how is it commonly used?

Answer 27

Cotransport occurs when the active transport of a solute indirectly drives the transport of another solute.
Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell.

Question 28

Explain how the proton pump in plants works and is used in cotransport.

Answer 28

DRAW DIAGRAM
A proton pump uses energy from ATP to push hydrogen ions (protons) outside the cell (creating a higher proton concentration outside the cell). Sucrose (which has a higher concentration outside the cell) aims to move across its concentration gradient and into the cytoplasm. It simultaneously binds with a proton to the proton/sucrose cotransporter, which is a carrier that allows both to enter back into the cytoplasm via diffusion**

Question 29

How can the Na+/K+-ATPase (Sodium-potassium pump) work together with the Na+-glucose (Sodium-Glucose) cotransporter to drive glucose uptake?

Answer 29

Create low concentration of sodium outside the cell membrane then use cotransporter to bring glucose into the cell.

Question 30

What are the relative concentrations of K+ and Na+ inside and outside of the cell?

Answer 30

There is a higher concentration of Na+ OUTSIDE the cell and a higher concentration of K+ INSIDE the cell.

Question 31

What are the two mechanisms by which bulk transport can occur across the plasma membrane?

Answer 31

• Exocytosis

- Endocytosis

Question 32

What are the three types of endocytosis? Explain each and provide a diagram

Answer 32

• DRAW*
  1. Phagocytosis: Engulfing particles by the formation of small vesicles from the cell membrane
  2. Pinocytosis: Cells drinking- ingestion of liquid into the cell by the formation of small vesicles from the cell membrane
  3. Receptor-mediated endocytosis: The ingestion of specific ligands as determined by specific receptors to form small vesicles from the cell membrane.

Question 33

Explain the process of exocytosis

Answer 33

The release of the contents of a vesicle produced in endocytosis. The walls of the vesicle merge with the walls of the cell membrane, releasing the substances within the vesicle.

Question 34

List and explain the types of cell-to-cell communication.

Answer 34

DRAW
LOCAL:
a) Paracrine signalling- A signalling cell acts on nearby target cells by secreting molecules of a local regulator.

b) Synaptic signalling- A nerve cell, upon receiving electrical stimulus, releases NT molecules into a synapse, stimulating a target cell (another neuron/cell)

LONG-DISTANCE:
c) Endocrine (hormonal) signalling- Specialised endocrine cells secrete hormones into body fluids (often blood). The hormones reach most body cells, but only affect certain cells.

d) Neuron signalling

Question 35

What are the 3 stages of cell signalling? Explain each briefly

Answer 35

1) Reception: A signalling molecule binds to a receptor (e.g. transmembrane protein)
2) Transduction: Rela molecules transduce the signal along a pathway
3) Response: A cellular response is activated.

Question 36

Explain how hydrophilic and hydrophobic signalling molecules differ in the way they act on receptors

Answer 36

For hydrophilic signalling molecules (which are soluble and easily carried in water), they act on cell-surface receptors to produce a response.

For hydrophobic signalling molecules, they bind to carrier proteins which carry them into the cell and allow them to bind to intracellular receptors and act on the nucleus of the cell.

Question 37

List the receptor types in order of fastest to slowest, and where each may be used.

Answer 37

Plasma Membrane Receptors

1) Ion Channel Receptors (msec): E.g. Na+ channel is opened via the binding of a ligand. Allow for fast neurotransmission
2) G protein-coupled Receptors (sec): 7 Transmembrane-spanning regions. Used in all aspects of physiology and pharmacology
3) Tyrosine kinase linked receptors (mins/hrs). E.g. insulin receptors. Used in metabolism, cell growth, cell reproduction.

Intracellular receptors:
4) Steroid receptors.

Question 38

Explain the process by which messages are transduced by GPCRs

Answer 38

SEE ONE NOTEThe first messenger (a signalling molecule-ligand) binds to the surface of a GPCR changing its conformation. This change in shape allows

Question 39

List the three most common secondary messengers used in GPCRs

Answer 39

cAMP, IP3, Ca2+

Question 40

List characteristics of GPCRs:

Answer 40

• Always 7 transmembrane-spanning domains (i.e. one continuous chain of amino acids that pop out of the membrane 7 times)
• Largest family of receptors
• Activated by a variety of stimuli (e.g. light, ions, odourants, NTs, hormones, etc.

Question 41

Explain how tyrosine kinases work

Answer 41

*see one note

Question 42

What are the 3 roles of protein phosphorylation? What do they result in?

Answer 42

• Conformational change
• Protein-protein interactions
• Change in cellular location

These changes lead to activation/inactivation of the target proteins.

Question 43

**Explain how insulin receptors work (tyrosine kinase receptors)

Answer 43

*

Question 44

Explain how steroid receptors.

Answer 44

A hormone enters the cytoplasm via diffusion and binds to a receptor protein, creating a hormone-receptor complex. This complex then enters the nucleus of the cell and effects the DNA, causing various proteins to be made.