7
Membrane Chapter Structure and Function
PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Plasma Membrane
The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cellular membranes are fluid mosaics of lipids and proteins
The plasma membrane is a thin (5-8 nm) film of lipids and proteins. Many have linked carbohydrates = glycolipds, glycoproteins Membrane lipids include phospholipids and sterols (cholesterol and phytosterols) Membrane lipids and membrane proteins are amphipathic, which means they have both hydrophobic and hydrophilic regions.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Review- Phospholipid Structure
R = Fatty acids, saturated and unsaturated. hydrophobic region of the molecule.
X = Many types of polar molecules. Hydrophilic region of the molecule.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 7-2
WATER Hydrophilic head Hydrophobic tail
WATER
A simple membrane is a phospholipid bilayer
A stable boundary between inside and outside of cell Hydrophilic heads Outside of the cell Hydrophobic interactions stabilize the lipid bilayer. Hydrophobic tails
Phospholipids
Hydrophilic heads Phospholipid bilayer Inside of the cell
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Membrane Models:
Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer In 1935, H. Davson and J. Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins Later studies found problems with this model, particularly the placement of membrane proteins, which have both hydrophilic and hydrophobic regions In 1972, Singer and Nicolson proposed that the membrane is a mosaic of proteins dispersed and individually inserted into the phospholipid bilayer, with only the hydrophilic regions exposed to "water"; cytosol and extracellular fluids
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The fluid mosaic model
Membrane proteins are embedded in the lipid bilayer, and float freely.
Membranes have a mosaic of proteins in a phospholipid bilayer
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Freeze-fracture and freeze-etch
Freezefracture experiments show that the proteins transverse the lipid bilayer.
Freeze-fracture studies of the plasma membrane supported the fluid mosaic model Freeze-facture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What does fluid mosaic mean?
Phospholipids can move laterally within the plasma membrane.
1.
Membrane fluidity: membrane lipids drift laterally, and even "flip-flop" (rarely, because flip-flop involves transitions between hydrophobic and hydrophilic environments and, thus, requires a lot of energy).
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
How does one visualize the lateral movement of lipids?
Step 1
Fluorescence Photobleaching:
1. Label lipids with a fluorescent "tag"
Fluorescent tag
Step 2
2. Focus a strong beam on a cell surface to bleach the label 3. Watch how fast the label comes back from unbleached parts
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
ng b Stro
eam
Step 3
Lipid movement ~2 m/sec
(Bacterial cell size ~1-5 m Eukaryotic cell size ~10-100 m)
How Membranes Remain Fluid Membranes must be fluid to work properly; they are usually about as fluid as salad oil As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Cold-blooded animals and hibernating mammals increase the amount of unsaturated lipids and cholesterol in their membranes to prevent freezing.
Unsaturated lipids and cholesterol prevent tight packing
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Membrane Fluidity
The steroid cholesterol has different effects on membrane fluidity at different temperatures.
Increase in temperature
At warm temperatures (37C), cholesterol restrains movement of phospholipids. At cool temperatures, it maintains fluidity by preventing tight packing. Cholesterol behaves like a buffer.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
How do we visualize protein movement?
Fuse both types of cells
Label with rhodamine (red fluorescence) Label with fluoresceine (green fluorescence)
Mixed proteins after a few hours
Proteins move also, but at a much slower rate. Many are bound to the cytoskeleton, which promotes or inhibits their movement.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The mosaic of proteins
Membrane Proteins = integral + peripheral Integral Proteins
Are at least partly inserted into membranes; most completely span it (even several times)
Peripheral Proteins
Are attached to the membrane surface, cytoskeleton, ECM, but not inserted
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transmembrane Hydrophobic domains
Hydrophilic domains
The structure of a transmembrane protein
Bacteriorhodopsin has seven transmembrane helices.
N-term
Integral proteins that span the membrane are called transmembrane proteins. The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
C-term
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Sidedness (asymmetry) of the plasma membrane
Membranes have distinct inside and outside faces. The asymmetrical distribution of proteins, lipids and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus Molecules on the inside surface of the ER, Golgi, and vesicles end up on the outside surface of the cells, and the vesicle outer membrane becomes in inner plasma membrane, etc.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Some functions of membrane proteins
Selective Channels for ions. Act as pumps to transport proteins utilizing ATP. Membrane can organize a series of enzymes in a pathway.
Form junctions between cells.
Sorting of cells into tissues and organs. (also "self")
Receptors
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
Example: blood groups (A, B, AB, O) reflect variations in cell surface oligosaccharides (short polymers of sugars)
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Membrane structure results in selective permeability
A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell's molecular traffic Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Selective Permeability of Lipid Bilayers
Permeability scale (cm/sec)
Size and charge affect the rate of diffusion across a membrane.
Phospholipid bilayer
High permeability
O2, CO2 H2 O
O2, CO2, N2
H2O, urea, glycerol Glycerol, urea Glucose
Glucose, sucrose
Cl Low permeability K+ Na+ Cl , K+, Na+
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Passive transport is diffusion of a substance across a membrane with no energy investment
Diffusion is the net drift of molecules in the direction of lower concentration due to random thermal movement. As a result of diffusion, molecules will spread out evenly into the available space. Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction. At dynamic equilibrium, as many molecules cross one way as cross in the other direction.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Passive Transport
Passive transport is the diffusion of a substance across a membrane without the expenditure of cellular energy. The concentration gradient itself represents the potential energy that drives diffusion. Example - During respiration O2 diffuses across the cellular membrane as long as O2 is being consumed.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another
Net diffusion Net diffusion
Net diffusion Net diffusion
Equilibrium Equilibrium
Diffusion of two solutes No work must be done to move substances down the concentration gradient The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make happen
Effects it of Osmosis on Water Balance
Osmosis is the diffusion of water across a selectively permeable membrane The direction of osmosis is determined only by a difference in total solute concentration Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 7-12
Lower concentration of solute (sugar)
Higher concentration of sugar
Same concentration of sugar
H2 O
Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration
Osmosis
Water Balance of Cells Without Walls
Tonicity is the ability of a solution to cause a cell to gain or lose water Isotonic solution: solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: solute concentration is less than that inside the cell; cell gains water
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The water balance of living cells
Less solute More solute
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 7-14
Filling vacuole
50 m
Contracting vacuole
50 m
Water Balance of Cells with Walls (plants, prokaryotes, fungi, some protists)
Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the cell wall opposes uptake; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the cell wall, a usually lethal effect called plasmolysis
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 7-13
Hypotonic solution Animal cell H2O H2O H2O H2O Isotonic solution Hypertonic solution
Lysed Plant cell H2O H2O
Normal H2O
Shriveled H2O
Turgid (normal)
Flaccid
Plasmolyzed
Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane Channel proteins provide hydrophilic corridors that allow a specific molecule or ion to cross the membrane Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transport Proteins
Transport proteins allow passage of hydrophilic substances across the membrane EXTRACELLULAR FLUID
Channel protein
Solute CYTOPLASM
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel. Channel proteins called aquaporins facilitate the passage of water
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transport Proteins
Carrier protein
Solute
Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves
Gated Channel
A membrane channel whose permeability is regulated; facilitated or mediated transport system. There are two major types involve the opening of the channel: (1) Voltage-gated channel (response to changes in electrical potential). (2) Ligand-gated channel (response to the binding of a molecule, such as a neurotransmitter, to a specific receptor).
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Active Transport
Active transport is the movement of a solute across a biological membrane such that the movement is directed upward in a concentration gradient (against the gradient) and requires the expenditure of energy. Often the energy is supplied by the hydrolysis of ATP.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Active transport uses energy to move solutes against their gradients
Facilitated diffusion is still passive because the solute moves down its concentration gradient Some transport proteins, however, can move solutes against their concentration gradients. Active transport moves substances against their concentration gradient Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins embedded in the membranes
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 7-17
Passive transport
Active transport
ATP Diffusion Facilitated diffusion
LE 7-16
EXTRACELLULAR [Na+] high FLUID [K+] low
Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low [K+] high P ADP ATP P Na+ Na Na+
+
CYTOPLASM
Cytoplasmic Na+ bonds to the sodium-potassium pump
Na+ binding stimulates phosphorylation by ATP.
Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside.
K+
K+ K+ K+
K+ K+
P
P
Extracellular K+ binds to the protein, triggering release of the phosphate group.
Loss of the phosphate restores the protein's original conformation.
K+ is released and Na+ sites are receptive again; the cycle repeats.
The sodium-potassium pump: a specific case of active transport
An electrogenic pump.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
3 Na+ that go out, 2 K+ go in.
Review: passive and active transport compared
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Maintenance of Membrane Potential by Ion Pumps
All cells have voltages across their membranes (electrical potential energy from opposite charge separation). This is called the membrane potential. It is between -50 and -200 mV (negative because the cytoplasm is negative) Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane: A chemical force (the ion's concentration gradient) An electrical force (the effect of the membrane potential on the ion's movement)
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
An electrogenic pump is a transport protein that generates the voltage across a membrane (membrane potential)
Proton pumps are the main electrogenic pumps in plants, bacteria, and fungi. The Na/K ATPase is also electrogenic because 3 Na+ leave, only 2 K+ enter
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cotransport (coupled transport) occurs when active transport of a solute indirectly drives transport of another solute
Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of sugars from leaves towards the rest of the plant.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Rapid rehydration after strenuous exercise or disease. Hi levels of Na and glucose promote water into the bloodstream, and to tissues.
http://www.gatorade.ca/en/giguere/_images/gatorade.gif
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins, but large molecules, such as polysaccharides and proteins, cross the membrane via vesicles The cell secretes (exports) macromolecules by fusion of vesicles with the plasma membrane exocytosis. In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products (hormones, antibodies, etc)
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Endocytosis
In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Three types of endocytosis: Phagocytosis ("cellular eating"): Cell engulfs particle in a vacuole Pinocytosis ("cellular drinking"): Cell creates vesicle around fluid Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation (cholesterol + LDL bind LDLR, removes cholesterol from bloodstream)
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The three types of endocytosis in animal cells
A vacuole is formed and then fuses with a lysosome for digestion.
Soluble materials (nonspecific) are taken up into vesicles for digestion.
Receptormediated endocytosis is specific. A process by which specific substances are brought into the cell that are in low concentration or only to be used in specific cell types.
Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor Coat protein Coated vesicle
Coated pit Ligand Coat protein A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs). 0.25 m
Plasma membrane
Objectives
Describe a typical plasma membrane. Define an give examples of an amphipathic molecule. Discuss the functions of membrane proteins Discuss how unsaturated fatty acids and cholesterol affect the melting point and fluidity of membranes. Know what hypertonic, hypotonic, and isotonic mean. Define and give examples of: passive transport, Active transport, Osmosis, Facilitated diffusion, Cotransport Know the differences between exocytosis and endocytosis. Describe and give examples of phagocytosis, pinocytosis, and receptor-mediated endocytosis.