B2.1 Membrane and membrane transport

COMPLETE

What do phospholipids form in cells ?

Bi-layer of phospholipids forms a continuous sheet that controls the passages of substances

Where are lipid bilayers found in cells ?

Plasma membrane - border between cell and it's environment
Inside Eukaryotes to divide cytoplasm into compartments

What is the primary function of the cell membrane ?

A semi- permeable cell structure that controls which substances can enter or leave the cell.

Outline location of aqueous solutions in relations to the lipid bilayer

Aqueous solutions on either side of the cell membrane.

Solutes nearest to the membrane may enter between the hydrophilic phosphate heads but if they reach the hydrophobic core they are drawn back by the water outside the membrane

Why does the hydrophobic core have low permeability ?

The larger the molecule the lower the permeability

Define diffusion

The net movement of particles from an area of high concentration to an area of low concentration.

- passive process

What causes simple diffusion?

Due to particles passing between phospholipids in the membrane
- only occurs if phospholipid is permeable to particles

Explain how oxygen diffuses

As oxygen is non-polar it can diffuse easily
If the oxygen concentration inside a cell is reduced due to aerobic respiration and the O2 concentration is higher outside, oxygen will pass through the plasma membrane by passive diffusion

What effect does polarity have on rate of diffusion ?

Non polar molecules will diffuse quicker than polar molecules

What effect does particle size have on rate of diffusion ?

Larger molecules will take longer to diffuse as they'll require more kinetic energy

What effect does the concentration gradient have on the rate of diffusion ?

Larger concentration gradients allows for an increased rate of diffusion

What are the 2 groups of membrane proteins ?

Integral and peripheral

What is a transmembrane protein?

A protein which goes all the way through both layers of the membrane

Where are integral proteins found?

embedded in the hydrocarbon chains in the centre of the membrane because at least part of their surfaces are hydrophobic

Where are peripheral proteins located?

on the surfaces of the cell membrane as they are hydrophilic on their surfaces. Some have a single hydrocarbon chain anchoring the protein to the membrane surface

What are the functions of membrane bound proteins ?

Membrane proteins can act as hormone binding sites, electron carriers, pumps for active transport, channels for passive transport and also enzymes

Define osmosis

the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)

How do substances dissolve ?

Through the formation of intermolecular bonds with water molecules. These bonds restrict the movement of water molecules.

What direction will water travel based on differences in solute concentration ?

Regions with a higher solute concentration will have a lower concentration of water molecules that are free to move which is why the net movement is from a low to high concentration.

What are aquaporins?

Channel proteins which facilitate water diffusion through the membrane

What do aquaporins do ?

They increase membrane permeability to water. It allows water molecules to pass through in single file.

What is a channel protein?

an integral transmembrane protein that connects the cytoplasm to the aqueous solution outside the cell

What does a channel protein do?

Allows the transport of ions and polar molecules through the plasma membrane

Define facilitated diffusion

Net movement from a high to low concentration of specific molecules across cell membranes through protein channels

Outline the specificity of channel proteins

The diameter of the pore and the chemical properties of it's sides ensure that only one type of particle passes through

What do pump proteins do?

Pump substances out of the cell

How do pump proteins differ from channel proteins ?

- they use energy so carry out active transport rather than diffusion
- Only move particles in 1 direction whereas in channel proteins they can go in either
- Moves particles against the concentration gradient

What does it mean that pump proteins are inconvertible ?

Conformation 1: The transported particle can enter the pump from one side to reach the central chamber or binding site
Conformation 2: ion or molecule is allowed to pass out on the other side

What does the pump protein use to carry out the conformational change ?

Energy is used to change it from the more stable to the less stable state.
- ATP used for active transport

How specific are pump proteins ?

Ech pump protein carries one specific molecule across the membrane.

What does highly specific pump proteins allow ?

- allows cell to control contents of cytoplasm
- allows specific solutes required by cell to be absorbed even in low concentrations

Define semi-permeable

allowing certain small substances to pass through it
- freely permeable to the solvent

Define selectively permeable

Allows the passage of particular particles but not others

How do channel proteins and pumps allows for selective permeability ?

They are both specific to particular particles and only allows those ions to pass through

Why is simple diffusion not selectively permeable ?

only depends on the size and polarity of particles. Small hydrophobic particles cannot be prevented from passing across cell membranes

Outline the structure of glycoproteins

Conjugated proteins with carbohydrates as the non-polypeptide component
They are part of plasma membrane, Protein section is embedded into the membrane and carbohydrates project outwards

Outline the structure of glycolipids

Molecules consisting of carbohydrates linked to lipids.
Carbohydrate: usually a single monosaccharide or a short chain of between 2-4 sugar unite ( projects out of membrane )
Lipid: 1/2 hydrocarbon chains that fit into hydrophobic core of membrane

What is the role of glycolipids ?

Cell recognition: help immune system distinguish between self/ non-self cells so foreign tissues can be recognized and destroyed.

What do glycoproteins and glycolipids form ?

a carbohydrate rich layer on the outer layer of the plasma membrane with an aqueous solution in gaps between carbohydrates.

What is the layer that the glycoproteins + lipids form called ?

Glycocalyx

What can the glycocalyx do?

Adjacent cells glycocalyx can fuse, binding the cells together which prevents the tissue from falling apart.

Describe the fluidity of saturated fatty acids in lipid bilayers

Straight chains that are tightly packed together reduces the fluidity and its flexibility and permeability by simple diffusion

Describe the fluidity of unsaturated fatty acids in lipid bilayers

Have more kinks in chains so pack together loosely making membranes more fluid, flexible and permeable

Give an example of adaptations in membranes in relation to a habitat

Fish from Antarctic waters have a higher % of unsaturated fatty acids in their membrane compared to fish in warmer waters

Where are cholesterol molecules placed in a cell membrane ?

Between phospholipids in the membrane with its hydroxyl group facing outwards
- preferably intercalates between saturated fatty acids rather than unsaturated

Describe the structure of cholesterol

Hydrophobic hydrocarbon tails in the centre of the membrane. One end of the molecule has a hydroxyl group which is hydrophilic

How does temperature affect fluidity of the cell membrane ?

As increases the membranes become more fluid and cant control what substances passes through,. If too low the membrane is too viscous and cell movement becomes restricted and cell may burst.

How does cholesterol act as a modulator ?

It helps maintain the necessary orderly arrangement of phospholipids
- stabilizes membrane at higher temperatures which maintains it's impermeability to hydrophilic particles
- ensures saturated fatty acids do not solidify at low temperature preventing stiffening of the membrane

What's a vesicle?

membrane bound sac

Function of vesicles

Transportation of materials across the cell

What allows for formation of vesicles ?

Fluid properties of the cell membrane

How is a vesicle made ?

A small region of a membrane is pulled from the rest of the membrane and pulled off.
- carried out using proteins in the membrane and ATP energy

What is endocytosis ?

When a vesicle is made from plasma membrane pinching inwards taking in water and solutes from outside of the cell

What do vesicles formed by endocytosis often contain ?

large molecules that cannot enter through the plasma membrane

Outline 2 examples of materials bought into cells via endocytosis

Placenta: proteins from mother are absorbed into foetus via endocytosis
White blood cells: take in pathogens by endocytosis then kill them

What do secretory cells do?

Transport vesicle contents within a cell
The protein within will by synthesised by ribosomes in the rER and then vesicales will then bud off and transport them to the golgi apparatus

What is exocytosis?

When vesicle fuses with target membrane to transfer all contents across plasma membrane

Outline 2 examples of materials released from cells via exocytosis

Removal of excess water in unicellular organisms: Water loaded into vesicle and moved to plasma membrane for expulsion
Polypeptides carried to plasma membrane for exocytosis ( secretion )

When is the release of materials called secretion ?

When a useful product is being released

What's a contractile vacuole?

A vacuole that maintains cell volume by pumping out access water so that the cell doesn't lyse. It will contract to release access water.

How does endocytosis increase the area of the plasma membrane ?

Phospholipids are synthesised and inserted into the rER membrane. The ribosomes will then synthesize the proteins and move it to the plasma membrane. This increases the area slightly.

What do ion channels allow?

facilitated diffusion across the plasma membrane in either direction

What do gated ion channels do ?

open or close in response to stimuli which allows diffusion to be switched on or off

What do negative voltages suggest about neurons ?

indicates that there are more positive charges outside the neuron than inside

What causes voltages ?

an imbalance of positive and negative charges across the membrane

When will sodium and potassium channels open or close ?

If voltage is below -50mV the channels remain closed
If it rises above -50mV sodium channels open allowing sodium ions to diffuse in
If it rises above +40mV potassium channels open allowing potassium ions to diffuse out of the neuron

\what is the structure of the potassium channel ?

It has 4 subunits and a globular protein subunit that is attached by a flexible chain of amino acids

When the 4 subunits are in an open confirmation the ball can fit into an open pore.

How do potassium ions pass through the channel ?

The bonds between potassium ions and surrounding water molecules are broken and the bonds temporarily form between ions and a series of amino acids in the narrowest part of the pore

Once it's passed through it becomes associated with the shell of water molecules

How do nicotinic ACH receptors work?

ligand gated Na/K channels

They have receptors with 5 subunits on with a binding site between 2 subunits for acetylcholine. This binding causes a conformational change that opens a pore between subunits allowing cations to pass through. Sodium will then change the voltage causing voltage gates to open .

What do exchange transporter proteins do ?

It transports different ions in opposite directions across the membrane

Outline the role of the sodium potassium pump in maintaining neural resting potential

-The Na/K Pump maintains the cell's resting potential. It keeps the inside of the cell negative and the outside of the cell positive.
-For every 3 Na+ ions it removes, it brings 2 K+ ions into the cell

Describe the structure of a sodium potassium pump

The sodium-potassium pump is an integral membrane protein. It had binding sites for three sodium ions, two potassium ions and an inorganic phosphate group (which comes from ATP).

What is the function of cotransporters ?

To transfer 2 molecules. 1 against the concentration gradient and 1 down the gradient

When is sodium dependant cotransport used by cells ?

In the wall of the proximal tubule in the kidney - reabsorbs glucose that has been filtered out of blood to prevent it being lost in urine

What is indirect active transport ?

depends on the cotransport of two solutes, with the movement of one solute down the concentration gradient driving the movement of the other solute up its gradient

What is CAMS?

cell adhesion molecules

What are CAMs typically ?

proteins with domains embedded in the phospholipid bilayer and others protruding outwards into the outside environment

What forms junctions ?

The CAms in adjacent cells binding together in extracellular domains

How is an asymmetrical junction formed ?

When the CAMs that bind together are different

Outline the role of CAMs in tissue formation

The cell adhesion molecules binds adjacent cells leading to the structure of a tissue through the functional relationships