Cellular Processes

How was a membrane split and imaged

freeze-fracture electron microscopy

What is the membrane structure

1. 8nm barrier that surrounds the cytoplasm of cell

2. 50% lipid 50% protein

What bond holds the membrane together

hydrogen bonds

Lipid is barrier to entry or exit of _____ substances

polar

What is the compositition of the lipid bilayer of the cell membrane

• two back-to-back layers of 3 types of lipid molecuels

• Cholesterol and glycolipids scattered among a double row of phospholipid molecules

What is the composition of phospholipids in the cell membrane

• 75% lipids

• 2 parallel layers of molecules

• Amphipathic, has both a polar and non-polar region

How is the bilayer of the phospholipids formed

Having the non-charged sides facing towards each other is the lowest energy state, so it is spontaneous

What is a leaflet

one layer of the bilayer of phospholipid

What is membrane fluidity

• Lipids can move around within the plane of the membrane leaflet

• Lipids rarely flip flop between membrane leaflets

What is membrane fluidity determined by

1. Lipid tail length - the longer the tail, decreases fluidity

2. Number of double bonds - more double bonds, increases fluidity

3. Amount of cholesterol - more cholesterol, decreases fluidity

What are the two types of membrane proteins

• Integral: extend completely across cell membrane

• Peripheral: attached to inner or outer surface of cell membrane and are easily removed

What is needed to remove integral membrane proteins

detergent

What are the features of integral membrane proteins

1. amphipathis

2. have hydrophobic regions that span the hydrophobic core

   1. usually consists of nonpolar amino acid helices

3. Hydrophillic ends of protein interact with aqueous solution

Functions of membrane proteins

1. Receptor

2. Cell identity

3. Linkers

4. Enzymes

5. Ions channels

6. Transporter proteins

What is it meant by the membrane being selectively permeable

The molecular organisation of the membrane means that is allows some substances but excludes others

What is the lipid bilayer permeable to

1. Uncharged molecules (O2, benzene) DIFFUSION

2. lipid soluble molecules (steroids, vitamins, fatty acids)

3. small uncharged polar molecules (Water, CO2)

What is the lipid bilayer impermeable to

1. large uncharged polar molecules (glucose, amino acids)

2. ions (K+, Na+, Cl-)

What do membrane proteins do in terms of transport

mediate the transport of substances across the membrane that cannot permeate the hydrophobic core of the lipid bilayer

What is diffusion

1. Random mixing of particles in a solution as a result of the particle’s kinetic energy

2. the movement of molecules from an area of high concentration to lower concentration until equilibrium is reached

What are some things that affect diffusion

1. Greater difference in concentration - faster rate

2. Higher temperature - faster rate

3. Increase surface area - faster rate

4. Larger size of substance - slower rate

5. larger distance - slower rate

What are the limits of cells because of diffusion

1. Cells are limited to 20um

2. Membrane area can be increased

3. Thinner membrane

4. Smaller cells

What are the different gradients across the cell membrane

1. Concentration gradient

2. Electrical gradient

• ions will be influenced by membrane potential in addition to conc gradient

What enables concentration gradient

the selective permeability of the membrane

How enables electrical gradient

Cells can maintain a difference in charged ions between the inside and outside of the membrane

How much energy is used to maintain concentration and electrical gradients

30%

What do the concentration and electrical gradients

stored energy

What is osmosis

Movement of water through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration

What is osmotic pressure

the pressure applied to a solution to prevent the inward flow of water across a semi-permeable membrane

What are the different processes of cross membrane transport

1. Non-mediated (no protein)

2. Mediated (protein)

3. Passive

4. Active

What is diffusion through the lipid bilayer

A non-mediated passive transport that is diffusion down the concentration gradient. For nonpolar hydrophobic molecules

What is diffusion through the ion channels

A non-mediated passive transport through an integral transmembrane protein that is a water filled pore that shields ions from the hydrophobic core of the lipid bilayer

What is the structure of the ion channel

What are the properties of ion channels

1. Ion selectivity

2. Gating

3. Electrical current

What is the ion selectivity of ion channels

• Specific amino acids lining the pore determine selectivity of the channel ions

• By being selective to a particular ion, the channel can harness the energy stored in the different ion gradients

What is the gating of ion channels

• Channels contain gates that control opening and closing of the pore

• Different stimuli control the channel opening and closing

• Stimuli inculde

  • Voltage

  • Ligand

  • Cell volume

  • pH

  • phsophorylation

What is the electrical current in ion channels

• Diffusion over 1mill ions/s generates current

• The current flowing can be measured by the patch clamp

• Current fluctuations represent opening and closing

• Channel gating is the conformational changes

What is carrier mediated transport

An active or passive trnasport where the substrate to be transported directly interacts with the transporter protein. The substrate binds and causes a conformational change in the protein

What does carrier mediated transport exhibit

1. Specificity

2. Inhibition

3. Competition

4. Saturation

What is the saturation of carrier mediated transport

There are only a limited number of binding sites, so once all the sites are saturated, rate of transport cannot increase

What is the facilitated diffusion of glucose

A passive mediated transport which

• glucose binds to transport protein (GLUT)

• GLUT changes shape and glucose moves down conc. gradient

• Kinase enzyme reduces glucose conc inside by phosphorylating to glucose-6-phosphate

• Conversion of gluoces meaintains concentration gradient

What is active transport

An energy requiring process that moves molecules and ions against the gradient

What are the two forms of active transport

1. Primary

2. Secondary

What are examples of primary active transporters

1. Na/KATPase

2. Ca/KATPase muscle

3. H/KATPase stomach

How does Na/KATPase work

1. 3 Na+ binds

2. ATP splits, phosphorylates and 3 Na+ is pushed out

3. K+ binds, phosphate released

4. K+ pulled in

What does Na/KATPase achieve

1. The pump generates a current and is electrogenic

2. maintains a low concentration of Na+ and a high concentration of K+ in the cytosol

Wht us maintaining a difference in ion conentration important

1. Maintain membrane potential of 80mV

2. Electrical excitability

3. Contraction muscles

4. Cell volume maintenance

5. Uptake of nutrients via secondary active transporters

6. maintain intracellular pH

What is the pump-leak hypothesis

Because Na and K are continually leaking back in to cell down the gradient, the