Bio 178 Lecture 11Biological Membranes (Cntd.)

http://www.cellsalive.com/channels.htm

Reading

• Chapter 6

Quiz Material

• Questions on P 124

• Chapter 6 Quiz on Text Website (www.mhhe.com/raven7)

Outline• Biological Membranes

Proteins (Cntd.)

Membrane Transport

Transmembrane Proteins

1. Single-Pass Transmembrane ProteinsProtein passes through membrane once - one non-polar region.

2. Multiple-Pass Transmembrane Proteins

Protein passes through membrane several times using helices Channel

Example

• Bacteriorhodopsin - 7 pass protein Channel for protons to pass during photosynthesis.

Multiple-Pass Transmembrane Protein

http://www.enzim.hu/hmmtop1.1/doc/model.html

Channel Protein in Halobacterium halobium

Transmembrane Proteins

3. Pores

Large non-polar regions that form ß-pleated sheets, which form a barrel structure within the membrane.

Example

Porin proteins of bacteria.

Structure of a Pore Protein

Porin - Barrel

http://fig.cox.miami.edu/~cmallery/255/255etc/porin.htm

Membrane Transport1. Passive ProcessesDo not require an input of energy. Include:

• Diffusion

• Osmosis

2. Active Processes

Require an input of energy. Include:

• Endocytosis

• Exocytosis

• Active Transport

Diffusion

Diffusion

Movement of molecules and ions down a concentration gradient until they are evenly distributed, eg. O2.

Description

Selective Diffusion in Cells

Substances that do not cross the membrane by simple diffusion (polar substances) can cross via specific transporters.

Selective Diffusion

Ion Channels

Polar groups line the channel, allowing ions to pass through the membrane down their concentration gradient.

Each ion channel is specific.

Direction of Transport

Dependent on:

• Ion Concentration

• Voltage across the membrane

Selective Diffusion (Cntd.)Facilitated Diffusion - Carriers

Transport of molecules and ions down their concentration gradient that is achieved by a protein that physically binds them.

Example: Glucose transporter in RBCs

• Concentration Gradient

Maintained by addition of phosphate to glucose - prevents it from crossing back through membrane.

• Transportation Method

Transmembrane protein transports glucose by conformational change.

Facilitated Diffusion

Facilitated Diffusion (Cntd.)

Carrier Saturation

Occurs when all the carriers are saturated - increased concentration gradient does not increase transport rate.

Key Features of Facilitated diffusion

• Passive

• Specific

• Saturates

OsmosisDescriptionDiffusion of water across a selectively permeable membrane.

Aquaporins

Membrane channels for water.

Mechanism

• Different concentrations of solutes on the 2 sides of the membrane Different concentrations of free water.

• Free water moves down its concentration gradient ( higher [solute].

Osmosis

Osmotic ConcentrationDetermined by the concentration of all solutes in a solution.• Hyperosmotic Solution

Solution with the higher concentration (of solute).

• Hypoosmotic Solution

Solution with the lower concentration (of solute).

• Isosmotic solution

Concentration of solutes in the 2 solution is equal.

Direction of TransportHypoosmotic Hyperosmotic

Osmotic Pressure

Pressure that must be applied across a membrane to stop the osmotic movement of water across a membrane.

Hydrostatic Pressure

Pressure exerted by the cytoplasm pushing against the plasma membrane (increases as water flows in).

Counteracts osmotic pressure (water flowing into the cell).

Osmotic Pressure (What is the Ideal Extracellular Concentration for Cells?)

Methods Used to Maintain Osmotic Balance• Isosmotic CytoplasmIsosmotic with environment, eg. certain marine organisms.

• Turgor

Cytoplasm is hyperosmotic to environment, eg. Plant cells.

Plants - hydrostatic pressure (turgor pressure) pushes plasma membrane against cell wall.

• ExtrusionWater is pumped out of the organism to the hypoosmotic environment, eg. Contractile vacuole of Paramecium.

Bulk Transport

Bulk Transport

2 types:• Endocytosis

• ExocytosisEndocytosis

Utilization of a membrane to take material into a cell.

• PhagocytosisUptake of solid material, eg. Neutrophil uptake of bacteria.

• PinocytosisUptake of liquid material.

Use of a membrane to envelope material to be transported.

Phagocytosis

Pinocytosis

Endocytosis (Cntd.)• Receptor Mediated Endocytosis (RME)1. Specific molecules bind to specific receptors in the PM.

2. These accumulate in coated pits (clathrin).

3. The clathrin then causes a vesicle to form (only when the target molecule binds to the receptor) endocytosis.

Example - LDL (low density lipoprotein)• Means of transportation of cholesterol. When cholesterol is required for membranes the LDL is taken up by RME.

• Hypercholesterolemia - LDL receptors lack tails LDL not taken up by RME cholesterol remains in blood atherosclerosis.