CHAPTER 8CHAPTER 8

MEMBRANE STRUCTURE AND MEMBRANE STRUCTURE AND FUNCTIONFUNCTION

STRUCTURE OF STRUCTURE OF MEMBRANESMEMBRANES

Ingredients of cell membranes are lipids Ingredients of cell membranes are lipids and proteins (some carbohydrates also)and proteins (some carbohydrates also)

PHOSPHOLIPIDSPHOSPHOLIPIDS

- Are AMPHIPATHIC- having both a Are AMPHIPATHIC- having both a hydrophobic and hydrophilic regionhydrophobic and hydrophilic region

- Proteins are also amphipathicProteins are also amphipathic

HOW ARE LIPIDS AND PROTEINS HOW ARE LIPIDS AND PROTEINS ARRANGED?ARRANGED?

FLUID MOSAIC MODELFLUID MOSAIC MODEL- Membrane is a fluid structure with Membrane is a fluid structure with

proteins embedded in or attached to a proteins embedded in or attached to a double layer of phospholipidsdouble layer of phospholipids

- This model was developed in 1972 by This model was developed in 1972 by Singer and Nicolson (contributions by Singer and Nicolson (contributions by many other scientists)many other scientists)

PROOF FOR FLUID PROOF FOR FLUID MOSAIC?MOSAIC?

FREEZE FRACTURE- method of preparing FREEZE FRACTURE- method of preparing cells for electron microscopycells for electron microscopy

- Cell membrane is split along the middle of the Cell membrane is split along the middle of the phospholipid bilayerphospholipid bilayer

- Interior of the bilayer appears cobblestoned, Interior of the bilayer appears cobblestoned, with proteins interspersed in a smooth matrixwith proteins interspersed in a smooth matrix

MODELS ARE CONSTANTLY BEING MODELS ARE CONSTANTLY BEING REVISEDREVISED

MEMBRANES ARE FLUIDMEMBRANES ARE FLUID

Membranes are held together by weak Membranes are held together by weak hydrophobic interactionshydrophobic interactions

- Lipids and some proteins can drift laterallyLipids and some proteins can drift laterally- They do not usually flip from one side of They do not usually flip from one side of

the membrane to the otherthe membrane to the other- Membranes remain fluid as temperature Membranes remain fluid as temperature

decreases, until a certain temperature is decreases, until a certain temperature is reachedreached

Temperature at which a membrane solidifies Temperature at which a membrane solidifies depends on the types of lipids it is made of:depends on the types of lipids it is made of:

- Membranes with unsaturated tails will remain Membranes with unsaturated tails will remain liquid at lower temperaturesliquid at lower temperatures

- The kinks in the tails (double bonds) prevent The kinks in the tails (double bonds) prevent the lipids from packing close togetherthe lipids from packing close together

- Cholesterol can make membranes less fluid by Cholesterol can make membranes less fluid by restraining the movement of phospholipids, but restraining the movement of phospholipids, but also lowers the temperature required to solidify also lowers the temperature required to solidify because phospholipids cannot pack togetherbecause phospholipids cannot pack together

PROTEINS AND PROTEINS AND CARBOHYDRATESCARBOHYDRATES

2 types of proteins:2 types of proteins:

INTEGRAL- penetrate the hydrophobic INTEGRAL- penetrate the hydrophobic core of the lipid bilayercore of the lipid bilayer

- Many are Many are transmembrane proteinstransmembrane proteins that that completely span the membranecompletely span the membrane

PERIPHERAL- not embedded in the lipid PERIPHERAL- not embedded in the lipid bilayer at all; loosely bound to the surface bilayer at all; loosely bound to the surface of the membraneof the membrane

- Some proteins are attached to the Some proteins are attached to the cytoskeleton and the extracellular matrix to cytoskeleton and the extracellular matrix to give the cell a stronger frameworkgive the cell a stronger framework

- Carbohydrates on the surface of the cell Carbohydrates on the surface of the cell membrane function in cell-to-cell recognitionmembrane function in cell-to-cell recognition

- Some are bonded to lipids (glycolipids) but Some are bonded to lipids (glycolipids) but most are bonded to proteins (glycoproteins)most are bonded to proteins (glycoproteins)

- Surface carbohydrates vary from cell to cellSurface carbohydrates vary from cell to cell

PERMEABILITY OF PERMEABILITY OF MEMBRANESMEMBRANES

PERMEABILITY OF THE BILAYER PERMEABILITY OF THE BILAYER ITSELF AFFECTS THE SELECTIVE ITSELF AFFECTS THE SELECTIVE PERMEABILITY OF THE MEMBRANE:PERMEABILITY OF THE MEMBRANE:

- Hydrophilic molecules such as ions and - Hydrophilic molecules such as ions and polar molecules have difficulty crossing polar molecules have difficulty crossing the hydrophobic core of the membranethe hydrophobic core of the membrane

THE PRESENCE OF TRANSPORT THE PRESENCE OF TRANSPORT PROTEINS AFFECTS THE SELECTIVE PROTEINS AFFECTS THE SELECTIVE PERMEABILITY OF THE CELL PERMEABILITY OF THE CELL MEMBRANE:MEMBRANE:

- Ions and polar molecules avoid the lipid Ions and polar molecules avoid the lipid bilayer by passing through transport bilayer by passing through transport proteinsproteins

- Some proteins have channels that act like Some proteins have channels that act like tunnels for hydrophilic molecules to pass tunnels for hydrophilic molecules to pass throughthrough

- Some transport proteins hold onto their Some transport proteins hold onto their passengers and physically move them passengers and physically move them across the membraneacross the membrane

WHAT DETERMINES THE DIRECTION WHAT DETERMINES THE DIRECTION OF TRAFFIC ACROSS A MEMBRANE?OF TRAFFIC ACROSS A MEMBRANE?

PASSIVE TRANSPORTPASSIVE TRANSPORTDIFFUSION- tendency for molecules of any DIFFUSION- tendency for molecules of any

substance to spread out into the available substance to spread out into the available spacespace

- Each molecule moves randomly, but a Each molecule moves randomly, but a population of molecules may move in a population of molecules may move in a certain directioncertain direction

- A SUBSTANCE WILL DIFFUSE FROM A SUBSTANCE WILL DIFFUSE FROM WHERE IT IS MORE CONCENTRATED WHERE IT IS MORE CONCENTRATED TO WHERE IT IS LESS CONCENTRATEDTO WHERE IT IS LESS CONCENTRATED

- Any substance will diffuse down its Any substance will diffuse down its CONCENTRATION GRADIENTCONCENTRATION GRADIENT

- Diffusion is spontaneous because it Diffusion is spontaneous because it decreases free energydecreases free energy

- Diffusion is PASSIVE TRANSPORT because Diffusion is PASSIVE TRANSPORT because no energy must be used in order for it to no energy must be used in order for it to happenhappen

- The concentration gradient represents The concentration gradient represents potential energy and drives diffusionpotential energy and drives diffusion

In comparing 2 solutions of unequal solute In comparing 2 solutions of unequal solute concentrations:concentrations:

- The solution with the higher The solution with the higher concentration of solutes is HYPERTONICconcentration of solutes is HYPERTONIC

- The solution with the lower concentration The solution with the lower concentration of solutes is HYPOTONICof solutes is HYPOTONIC

- Solutions with equal concentrations of Solutions with equal concentrations of solute are ISOTONICsolute are ISOTONIC

OSMOSIS- diffusion of water across a OSMOSIS- diffusion of water across a selectively permeable membraneselectively permeable membrane

- Water moves from a hypotonic solution to - Water moves from a hypotonic solution to a hypertonic solution (in hypotonic a hypertonic solution (in hypotonic solution there is a greater concentration solution there is a greater concentration of of WATERWATER))

DRAW A DIAGRAM DEMONSTRATING DRAW A DIAGRAM DEMONSTRATING OSMOSISOSMOSIS

WATER BALANCE IN WATER BALANCE IN LIVING CELLSLIVING CELLS

WATER BALANCE IN CELLS WITHOUT WATER BALANCE IN CELLS WITHOUT CELL WALLS (ANIMAL CELLS):CELL WALLS (ANIMAL CELLS):

- In an isotonic environment, water moves In an isotonic environment, water moves into and out of the cell at equal ratesinto and out of the cell at equal rates

- In a hypertonic environment, water will In a hypertonic environment, water will leave the cell, the cell will shrivel and dieleave the cell, the cell will shrivel and die

- In a hypotonic environment, water will In a hypotonic environment, water will enter the cell and the cell will burstenter the cell and the cell will burst

Most cells live in isotonic environments so Most cells live in isotonic environments so water balance is maintainedwater balance is maintained

- Animal cells living in hypertonic or Animal cells living in hypertonic or hypotonic environments must have hypotonic environments must have adaptations for OSMOREGULATION, the adaptations for OSMOREGULATION, the control of water balancecontrol of water balance

Ex: Paramecium has a contractile vacuole Ex: Paramecium has a contractile vacuole that pumps excess water out of the cellthat pumps excess water out of the cell

WATER BALANCE IN CELLS WITH CELL WATER BALANCE IN CELLS WITH CELL WALLS (PLANTS, PROKARYOTES, WALLS (PLANTS, PROKARYOTES, FUNGI):FUNGI):

- As water enters these cells, the cells As water enters these cells, the cells swell but will not burstswell but will not burst

- The elastic cell wall will expand only so The elastic cell wall will expand only so much before it exerts a back pressure much before it exerts a back pressure that opposes uptake of more waterthat opposes uptake of more water

- At this point the cell is TURGID or firm, a At this point the cell is TURGID or firm, a healthy state for most plant cellshealthy state for most plant cells

- If plant cells are in an isotonic solution, If plant cells are in an isotonic solution, water will not enter and cells become water will not enter and cells become FLACCID (limp) and the plant wiltsFLACCID (limp) and the plant wilts

- If plant cells are in a hypertonic If plant cells are in a hypertonic environment water will leave the cell and environment water will leave the cell and PLASMOLYSIS occurs; the cells usually diePLASMOLYSIS occurs; the cells usually die

PASSIVE TRANSPORT PASSIVE TRANSPORT CONT.CONT.

FACILITATED DIFFUSION- diffusion with FACILITATED DIFFUSION- diffusion with the help of transport proteinsthe help of transport proteins

TRANSPORT PROTEINS BEHAVE LIKE TRANSPORT PROTEINS BEHAVE LIKE ENZYMES:ENZYMES:

- Transport proteins are specific for the - Transport proteins are specific for the solutes they transport and may have a solutes they transport and may have a binding site similar to the active site of an binding site similar to the active site of an enzymeenzyme

- Transport proteins may become Transport proteins may become saturatedsaturated

- Transport proteins may be inhibited by Transport proteins may be inhibited by molecules that resemble the normal molecules that resemble the normal solutesolute

TRANSPORT PROTEINS TRANSPORT PROTEINS FACILITATE DIFFUSIONFACILITATE DIFFUSION

- Many transport proteins provide corridors Many transport proteins provide corridors for molecules or ions to cross the for molecules or ions to cross the membrane- CHANNEL PROTEINSmembrane- CHANNEL PROTEINS

- Some function as GATED CHANNELS- Some function as GATED CHANNELS- some stimulus causes them to opensome stimulus causes them to open

- Some transport proteins undergo a Some transport proteins undergo a change in shape that moves the solute-change in shape that moves the solute-binding site across the membranebinding site across the membrane

ACTIVE TRANSPORTACTIVE TRANSPORT

Some transport proteins can move solutes Some transport proteins can move solutes against their concentration gradients from against their concentration gradients from less concentration to greater concentrationless concentration to greater concentration

ACTIVE TRANSPORT- movement against ACTIVE TRANSPORT- movement against the concentration gradient requiring the concentration gradient requiring energy from the cellenergy from the cell

Ex: Sodium-potassium pump- Na+ pumped Ex: Sodium-potassium pump- Na+ pumped out, K+ pumped inout, K+ pumped in

ION PUMPS GENERATE ION PUMPS GENERATE VOLTAGEVOLTAGE

All cells have voltages across their plasma All cells have voltages across their plasma membranesmembranes

- Voltage is electrical potential energy, a Voltage is electrical potential energy, a separation of opposite chargesseparation of opposite charges

- Voltage across a membrane is called Voltage across a membrane is called MEMBRANE POTENTIAL (inside of the cell is MEMBRANE POTENTIAL (inside of the cell is negative compared to outside)negative compared to outside)

- ELECTROCHEMICAL GRADIENT- combination ELECTROCHEMICAL GRADIENT- combination of forces acting on an ion (concentration of forces acting on an ion (concentration gradient and membrane potential)gradient and membrane potential)

- The sodium-potassium pump does not The sodium-potassium pump does not translocate Na+ and K+ one for one, but translocate Na+ and K+ one for one, but pumps 3 sodium ions out for every 2 pumps 3 sodium ions out for every 2 potassium ions it pumps inpotassium ions it pumps in

- There is a net transfer of 1 positive There is a net transfer of 1 positive charge from inside the cell to the outside charge from inside the cell to the outside of the cell with each crank of the pumpof the cell with each crank of the pump

- This stores energy in the form of voltageThis stores energy in the form of voltage

ELECTROGENIC PUMP- a transport ELECTROGENIC PUMP- a transport protein that generates voltage across a protein that generates voltage across a membranemembrane

- The sodium-potassium pump is the main The sodium-potassium pump is the main electrogenic pump of animal cellselectrogenic pump of animal cells

- The PROTON PUMP is the main The PROTON PUMP is the main electrogenic pump of plants, bacteria, and electrogenic pump of plants, bacteria, and fungi (H+ ions are pumped out of the cell fungi (H+ ions are pumped out of the cell transferring positive charge to the outside transferring positive charge to the outside of the cell)of the cell)

COTRANSPORTCOTRANSPORTIn COTRANSPORT an ATP-driven pump In COTRANSPORT an ATP-driven pump

stores energy by concentrating a substance stores energy by concentrating a substance on one side of the membraneon one side of the membrane

- As the substance leaks back across the As the substance leaks back across the membrane through specific transport membrane through specific transport proteins, it escorts other substances into the proteins, it escorts other substances into the cellcell

WATER THAT HAS BEEN PUMPED UPHILL WATER THAT HAS BEEN PUMPED UPHILL PERFORMS WORK AS IT FLOWS BACK PERFORMS WORK AS IT FLOWS BACK DOWNDOWN

TRANSPORT OF LARGE TRANSPORT OF LARGE MOLECULESMOLECULES

EXOCYTOSIS- secretion of large molecules by EXOCYTOSIS- secretion of large molecules by the fusion of vesicles with the plasma the fusion of vesicles with the plasma membranemembrane

- A transport vesicle from the Golgi comes into A transport vesicle from the Golgi comes into contact with the plasma membranecontact with the plasma membrane

- The layers of the bilayer rearrange themselves The layers of the bilayer rearrange themselves so that the vesicle membrane and the plasma so that the vesicle membrane and the plasma membrane fusemembrane fuse

- The contents of the vesicle spill outside the cellThe contents of the vesicle spill outside the cell

ENDOCYTOSIS- cell takes in large ENDOCYTOSIS- cell takes in large molecules by forming new vesicles from molecules by forming new vesicles from the plasma membranethe plasma membrane

- A small area of the plasma membrane A small area of the plasma membrane sinks inward to form a pocketsinks inward to form a pocket

- The pocket pinches in, forming a vesicle The pocket pinches in, forming a vesicle that contains material from outside the that contains material from outside the cellcell

3 TYPES OF 3 TYPES OF ENDOCYTOSISENDOCYTOSIS

SEE DIAGRAMSSEE DIAGRAMS

1.1. PHAGOCYTOSIS- cell eatingPHAGOCYTOSIS- cell eating

2.2. PINOCYTOSIS- cell drinkingPINOCYTOSIS- cell drinking

3.3. RECEPTOR-MEDIATED RECEPTOR-MEDIATED ENDOCYTOSIS- very specificENDOCYTOSIS- very specific

- receptor sites on the surface of the cell - receptor sites on the surface of the cell bind ligands, and coated pits form bind ligands, and coated pits form vesicles (lined with proteins)vesicles (lined with proteins)