Membrane structure and function

Properties of biological membranes

• barrier, selectively permeable

• flexible

• continuous

• self-repairing

Amphipathic

Molecules that have both hydrophilic (polar) and hydrophobic (non polar) components

Term used to describe molecules that have both hydrophilic (polar) and hydrophobic (non polar) components

Amphipathic

Which tails in phospholipids result in unsaturated phospholipid?

cis C=C double bond (allows fatty acid to bend)

What 2 structures can phospholipids form?

Micelles and bilayer (have inner and outer leaflets)

Explain the self repairing function of membranes. How does it work?

Phospholipids are highly mobile. So they have natural tendency to seal themselves up to eliminate water-exposed edges.

1. Energy unfavourable: hydrophobic tails are exposed to water

2. Tendency to snap back together and hide hydrophobic tails from water

3. Energy favourable: sealed compartment formed

Ways in which phospholiipids can move

Happens all the time:

• rotate around their axis of symmetry (spin)

• exchange in the lateral plane of one leaflet <—>

Can occurs in an energy-dependent process, but doesn’t occur spontaneously:

• can move (very slowly) from one leaflet to another = flip-flop movememnt

Factors that affect membrane fluidity

Many cis C=C bonds + short fatty acid chains = less tightly packed —> greater fluidity (e.g. olive oil)

Length explained:

(Shorter fatty acyl chains —> easier for phospholipids to move between leaflets (flip-flop) —> higher fluidity

How can cold blooded animals always keep their membrane fluid at variable temperatures?

By adjusting membrane fluidity:

They agjust number of C=C bonds and length of acyl chains

How does cholesterol control membrane permeability?

At high temperatures:

• Membrane is more fluid (and more permeable)

• Cholesterol tucks itself in between 2 phospholipids

• This stiffens that area locally —> decreases fluidity (membrane becomes more rigid)

• less permeable membrane

At low temperatures or when there are too many C-C bonds (at high conc of saturated fatty acids):

• Membrane is less fluid (and less permeable)

• Cholesterol tucks itself in between 2 phospholipids

• Increases membrane fluidity

• less permeable membrane

Types of membrane proteins + their structure (+ their function)

+ how to purify them?

examples

Integral membrane proteins

• Have transmembrane domain that anchors the proteins in the membrane

• Have extracellular domain and Cytoplasmic domain (or intracellular domain)

~ (of transmembrane domain): unsure that they are always embedded in the membrane ~ maintains proteins orientation in the membrane

- To purify them, you need to use a very strong detergent to get them out of plasma membrane

Peripheral membrane portines

• associated reversible with membranes

• may have fatty acid modification that can flip in and out of the membrane

- To purify them we can use weaker detergent (e.g. salt)

~ regulators(cycle on and off —> used in regulatory processes),

~ membrane identity proteins,

~ signalling ligands,

~ intracellular transport

Microdomains =

+ their functions

= an area where you have a collection of phospholipids that have longer fatty acyl chains

— these areas usually have more cholesterol (to increase fluidity of that area, as longer chains make it less fluid —> ensures it doesn’t become solid(crystalline))

Cause cluster of signalling molecules —> higher efficiency of switching signalling on or off.

~ are important in bringing together proteins which will have a particular functions (e.g. signalling)

Why is membrane asymmetry important?

give example

Forms distinct difference in composition of inner and outer leaflets

~ blood clotting !!

— Phosphatidyl-serine (-ve charged) is found on platelets. It flips, and that’s a signal to start a cascade of blood clotting reactions

~ cell signalling

~ Recognition and clearence of dead cells / apoptosis !!

— Macrophages recognise amino phospholipids (e.g. phosphatidyl-serine), which are transferred to the outer leaflets of the plasma membrane of apoptotic cell —> phagocytosis starts

~ protein orientation and activity …..

e.g. Blood groups are determined by asymmetric topology of glycolipids and glycoproteins

Phosphatidyl-serine

= a type of phospholipid

Has serine component —> gives net -ve charge

Is VERY important — a major component of plasma membrane:

~ is displayed on the surface of platelets. When it flips, its a signal to start a cascade of reaction

~ apoptosis: is transferred to the outer leaflets of apoptotic cells —> macrophages recognise it, bind to it —> phagocytosis starts

Different solution types

Hypotonic (low salt conc., high wp) - water enters cells → cells burst = lysis

Isotonic (normal)

Hypertonic (high salt, low wp) - water exits the cell → cell becomes flaccid

How does the cytoskeleton help red blood cells maintain their shape?

*mutations??

Integral proteins connect plasma membrane to the Spectrin dimers and actin cytoskeleton (at the junctional complexes). This forms a lattice network on the inside of the cell membrane

→ provides structural support and flexibility → allows cells to change shape

Mutations in spectrin protein may cause change in cell shape

e.g. hereditary spherocytosis and hereditary elliptocytosis

What is a liposome

A mixture of phospholipids that assemble in special structures (water inside and outside)

→ we can use these model systems to think about diffusion

Types of carrier proteins used for active transport

• coupled transporter: some molecules are transported down their conc. graidient. This generates energy which can be used for active transport

  • Uniport

  • Symport — coupled transporter

  • Antiport — coupled transport

• ATP driven

• light driven

Why does the rate of transport using carrier proteins eventually plateaus?

There is a fixed number of carrier proteins, eventually there will be too much solute → rate of transport plateaus

Explain how bacteria transport lactose

Symport carrier protein uses H+ graidient to transport lactose into the cell down its conc. graidient

Explain how glucose uptake in epithelial cells lining gut happens

1. Apical surface: symport protein transports Na+ along with glucose (against conc. gradient) from gut lumen into the cell.

2. Basal surface: glucose is passively transported (by carrier proteins) into the blood stream down its conc. graidient

3. Na+/K+ pump working all the time at the Basal surface — is energy dependent: pumps Na+ out of the cell and K+ in the cell → maintains low conc. of Na+ inside the cell, so that 1. can happen

Explain how transport happens in photosynthesis across the thylakoid membrane

proton graidient, and movement of H+ through the electron transport chain facilitates ATP synthesis (when H+ moves through ATP synthase).