Bio 178 Lecture 11 Biological Membranes (Cntd.) http://www.cellsalive.com/channels.htm
Feb 25, 2016
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 Proteins1. Single-Pass Transmembrane ProteinsProtein passes through membrane once - one non-polar region.
2. Multiple-Pass Transmembrane ProteinsProtein 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 ProcessesRequire 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 CellsSubstances 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 - CarriersTransport 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 MethodTransmembrane 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.AquaporinsMembrane 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 PressurePressure 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.
• TurgorCytoplasm 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.