B2.1 Membranes + membrane transport

Another name for plasma membrane

Cell membrane

Basis of cell membranes

Lipid bilayers

Structure of a phospholipid

• Hydrophobic fatty acid tail

• Hydrophilic phosphate head

Are phospholipids hydro phobic or phyilic?

Amphipathic

Amphipathic

Contains polar, hydrophilic regions AND non-polar, hydrophobic regions

What do phospholipids + other amphipathic lipids naturally form in water?

Naturally form continuous sheet-like bilayers in water

Function of cell membrane (lipid bilayer)

Control what materials enter + leave the cell

• Selectively permeable

• Act as barriers

Loction of lipid bilayers

1. Plasma membrane

2. Organelle membranes (eg mitochondria)

3. Vesicles + lysosomes

Membranes are effective barriers between…

Aqueous solutions

Permeability

Ability of a molecule to pass thru a membrane

What is permeability based on?

• Size of molecule

• Charge of molecule

What forms the core of the membrane (lipid bilayer)?

Hydrophobic hydrocarbon chains

• Fatty acid tails

Permeability of hydrocarbon chains (FA tails)

Low permeability to large molecules + hydrophilic particles (incl ions + polar molecules)

How do lipid bilayers make effective barriers?

Core of membrane = hydrophobic hydrocarbon chains

• Have low permeability to large molecules + hydrophilic particles (ions, polar molecules)

What is the membrane not permeable to?

These have low permeability:

• Large molecules

• Hydrophilic particles- ions, polar molvules

What types of molecules can pass thru the membrane (high permeability)?

Small, non-polar molecules

• Eg O2

Why can’t glucose go thru the membrane?

Small but polar

• Needs alternative method

Qualities of cell membranes

1. Semi-permeability

   • Only certain materials can freely cross the CM

2. Selectivity:

   • Cell can control the passage of any material that cannot freely cross the membrane

Passive transport

No energy (ATP) required

• Relies on CG

Methods of passive transport

1. Simple diffusion

2. Facilitated diffusion

3. Osmosis

Simple diffusion

Passive net movement of particles from an area of high to low conc down a CG until a dynamic equilibrium is reached

• Equilibrium = same conc on either side of the membrane

Does simple diffusion require a membrane?

No

• Can happen w or w/o a membrane

What type of molecules can simple diffusion occur for?

Small, non-polar

• Eg O2, CO2

Example of simple diffusion across membranes

Movement of O2 + CO2 molecules betw phospholipids

Concentration gradient

Shows the difference in conc betw 2 areas

Factors affecting diffusion

1. Temp

   • Higher temp = more KE

2. Steepness of CG

   • Higher CG = faster diffusion

3. Molecule size + charge

   • Faster for small, uncharged molecules

4. SA

   • Higher SA = faster diffusion

5. Diffusion distance

   • Shorter DD = faster diffusion

Do all membrane proteins have the same structure, location, function?

No

• Diverse structure, location, function

2 types of membrane proteins

1. Integral

2. Peripheral

Integral proteins

Embedded in 1 or both of the lipid layers of a membrane

• Transmembrane

• Amphipathic

Examples of integral proteins

1. ATP synthase

2. Channel proteins

3. Protein pumps

Peripheral proteins

Attached to 1 or other surface of the bilayer

• Inner or outer membrane

• Hydrophyllic

Examples of peripheral proteins

1. Glycoproteins

2. Cytochrome c (electron transport chains)

6 functions of membrane proteins (JETRAT)

1. Junction

2. Enzymes- ATP synthase

   • Immobilised enzymes

3. Transport- channel proteins, protein pump

4. Recogniton- glycoproteins

5. Attachment

6. Transduction of hormone signals

   • Hormone binding sites (receptors)

What does diffusion happen due to?

The random movement of particles

Movement of water molecules across membranes by osmosis

Osmosis

Passive net movement of water from an area of high water conc to an area of low water conc thru aquaporins thru a partially permeable membrane

• Until equilibrium is reached

• Low solute conc → high solute conc

3 reasons for osmosis

1. Random movement of particles

2. Impermeability of membranes to solutes

3. Differences in solute concentration

Aquaporins

Channel proteins which specifically allow water to pass thru

• Greatly increases the permeability of water

• Integral protein

Where are lots of aquaporins found?

• Kidneys

• Root hair cells

Define osmosis based on hypo / hypertonic

Water moves from low solute (hypotonic) to high solute (hypertonic) environments

In osmosis, why is the net movement of water from low to high solute conc?

• Substance dissolves, it is surrounded by water molecules

• → forms H bonds → restricts movement of water molecules

• So areas w high solute conc have fewer water molecules free to move to areas w lower solute conc

Water is polar but it can pass thru membranes (lipid bilayer)- how?

It is small enough

Osmolarity

A measure of solute concentration.

• High osmolarity = lots of solute

• Low osmolarity = little solute

Water potential

The tendency of water to move from one area to another due to osmosis

What type of transport are channel proteins needed for?

Facilitated diffusion

Facilitated diffusion

Passive net movement of particles from an area of high to low conc thru a channel protein embedded in the membrane

What types of particles use facilitated diffusion to pass thru the membrane?

Polar, charged molecules

• Eg glucose

Are channel proteins generic to all molecules?

No- specific to molecules

• Eg K+ channels

• Most allow only 1 type of molecule to diffuse thru

How does the structure of channel proteins make it selectively permeable?

Allows specific ions to diffuse thru when channels are open but not when they are closed

• Diameter + specific shape allows for transport of specific molecules

Can protein channels open + close?

Yes

• They are gated

• Eg voltage gated

• Allows for selective permeability w/o using energy

Similarities betw simple + facilitated diffusion

Both:

1. Passive (no energy)

2. Move particles down CGs

Differences betw simple + facilitated diffusion

1. Molecule charge

2. Molecule size

3. Need channel proteins?

Active transport

Active movement of particles from an area of low to high conc thru pump proteins

• Needs ATP (produced by mitochondria in AR)

• Against CG

Why does AT need energy?

Moves particles against the CG

Role of pump proteins in AT

1. Pumps use energy ATP to transfer specific particles across membranes

2. So they can move particles against a CG

In how many ways can protein pumps move particles in?

Only 1

Are protein pumps specific or general?

Specific to diff molecules

• Eg Na pump

What types of molecules can be actively transported?

• Molecules

• Ions

How do pump proteins work?

1. Input of energy (ATP)

2. → conformational change (pump protein changes shape)

3. So particle moves thru + transported against CG

What makes membranes selectively permeable?

1. Facilitated diffusion (channel proteins)

2. Active transport (protein pumps)

Examples of molecules that are always permeable

• Water

• Small, non-polar eg O2

Is permeability by simple diffusion selective?

No

• Only depends on the size + hydrophilic / hydrophobic properties of particles.

Glycoprotein

Membrane proteins with carbohydrate chains attached (on the outside)

Glycolipids

Membrane lipids with carbohydrate chains attached (on the outside)

Where is the carbohydrate chain attached on glycoproteins + glycolipids?

On the extracellular side of membranes

Function of glycoprotein + glycolipid

Role in:

1. Cell adhesion

2. Cell recognition

In what cells are glycolipids found?

Eukaryotes

• Recognition (immune system)

Fluid mosaic model of membrane structure

Include:

1. Peripheral + integral proteins

2. Glycoproteins

3. Phospholipids

4. Cholesterol

5. Indicate hydrophobic + hydrophilic regions

Are membranes flexible?

Yes