Membranes Cell BILD 1 The Cell
Halpain, Lecture 7, October 9, 2009 Campbell & Reece, Chapter 7 Course announcements: CLASS WEB SITE: http://www.biology.ucsd.edu/classes/bild1.FA09 Midterm 1 is on Friday October 16, 12:00 12:50pm
Review Session: Thursday, October 15, 7:00 8:20pm, York 2722
Campbell & Reece, Chapter 7
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Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The plasma membrane exhibits selective permeability
Membranes form a boundary between aqueous compartments Protein transporters allow only selected substances to cross
Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids
Are the most abundant lipid in the plasma membrane Are amphipathic, containing both hydrophobic and hydrophilic regions
Figure 7.1
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The fluid mosaic model of membrane structure
States that a membrane is a fluid structure with a "mosaic" of various proteins embedded in a phospholipid bilayer
Membrane proteins are free to move laterally in the phospholipid bilayer (unless anchored to the
cytoskeleton)
Hydrophilic region of protein
WATER
Hydrophilic head Hydrophobic tail Figure 7.2
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Phospholipid bilayer
WATER
Figure 7.3
Hydrophobic region of protein
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The Fluidity of Membranes
Phospholipids in the plasma membrane
Can move laterally within the bilayer but cannot easily flip sides
Proteins in the plasma membrane can drift laterally within the bilayer
EXPERIMENT Researchers labeled the plasma membrane proteins of a mouse cell and a human cell with two different markers and fused the cells. RESULTS Membrane proteins
+
Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour
Lateral movement (~107 times per second) (a) Movement of phospholipids
Flip-flop (~ once per month)
Figure 7.5 A
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Figure 7.6
CONCLUSION Membrane proteins move within the plane of the plasma membrane.
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Unsaturated hydrocarbon tails increase fluidity
The steroid cholesterol
Acts as a "temperature buffer" of membrane fluidity
Fluid
Viscous
Unsaturated hydrocarbon tails with kinks (b) Membrane fluidity
Saturated hydroCarbon tails
Cholesterol (c) Cholesterol within the animal cell membrane
Figure 7.5 B
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Figure 7.5
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Membrane Proteins and Their Functions
A animal cell biological membrane
Fibers of extracellular matrix (ECM) EXTRACELLULAR SIDE OF MEMBRANE Glycoprotein Carbohydrate
Integral proteins
Penetrate the hydrophobic core of the lipid bilayer Are often transmembrane proteins
EXTRACELLULAR SIDE N-terminus
Glycolipid
EXTRACELLULAR SIDE
CYTOPLASMIC SIDE OF MEMBRANE
Microfilaments of cytoskeleton
Cholesterol
Peripheral protein
Integral protein
C-terminus
Figure 7.7
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Figure 7.8
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Helix
CYTOPLASMIC SIDE
3
Six major functions of membrane proteins
Peripheral proteins
Are bound to the surface of the membrane
(a)
Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. ATP
EXTRACELLULAR SIDE OF MEMBRANE
Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate
(b)
Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway.
Enzymes
Glycolipid (c)
Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell.
Signal
CYTOPLASMIC SIDE OF MEMBRANE
Microfilaments of cytoskeleton Cholesterol
Peripheral protein
Figure 7.7
Integral protein Figure 7.9
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Receptor
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six major functions of membrane proteins
(d)
Cell-cell recognition. Some glycoproteins serve as identification tags that are specifically recognized by other cells.
Glycoprotein
The Role of Membrane Carbohydrates in Cell-Cell Recognition
Membrane carbohydrates
Interact with the surface molecules of other cells, facilitating cell-cell recognition Glycolipids: carbohydrates bonded to lipids Glycoproteins: carbohydrates bonded to proteins
(e)
Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.31).
(f)
Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29).
Figure 7.9
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Synthesis and Sidedness of Membranes
ER
Membranes have distinct inside and outside faces Membrane proteins and lipids are synthesized in the ER and Golgi apparatus transport vesicles fuse with target membranes
1 Transmembrane glycoproteins
Concept 7.2: Membrane structure results in selective permeability A cell must exchange materials with its surroundings, a process controlled proteins in the plasma membrane
Secretory protein Glycolipid 2 Golgi apparatus
Vesicle
3 Plasma membrane: Cytoplasmic face 4 Transmembrane glycoprotein Extracellular face
Secreted protein
Figure 7.10
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Membrane glycolipid
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The Permeability of the Lipid Bilayer
Hydrophobic molecules
Are lipid soluble and can pass through the membrane rapidly
Transport Proteins
Transport proteins
Allow passage of hydrophilic substances across the membrane
Polar molecules
Do not cross the membrane rapidly
Ions
Do not cross the membrane
Ions and polar molecules require transport proteins to enter or leave the cell
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Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment
Diffusion
Is the tendency for molecules of any substance to spread out evenly into the available space
(a)
Diffusion of one solute.
Molecules of dye Membrane (cross section)
Figure 7.11 A
Net diffusion
Net diffusion
Equilibrium
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Effects of Osmosis on Water Balance
Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another
(b) Diffusion of two solutes.
Osmosis
the Is movement of water across a semipermeable membrane Is affected by the concentration of solutes because they inversely affect the concentration of free water molecules
Figure 7.11 B
Net diffusion Net diffusion
Net diffusion Net diffusion
Equilibrium Equilibrium
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Osmosis
Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar
Water Balance of Cells
Tonicity
Is the ability of a solution to cause a cell to gain or lose water
Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can More free water molecules (higher concentration) Osmosis
Water molecules cluster around sugar molecules
Fewer free water molecules (lower concentration)
Figure 7.12
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Water moves from an area of higher free water concentration to an area of lower free water concentration
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Water Balance of Cells
Isotonic solutions:
The concentration of solutes is the same as inside the cell no net movement of water The concentration of solutes is greater than inside the cell The cell will lose water The concentration of solutes is less than it is inside the cell The cell will gain water
Water Balance of Cells Without Walls
Water balance in cells without walls
Hypertonic solutions
Hypotonic solution
(a) Animal cell. An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.
Isotonic solution
H2O H2O
Hypertonic solution
H2O
H2O
Hypotonic solutions
Lysed Figure 7.13
Normal
Shriveled
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Water Balance of Cells With Walls
Water balance in cells with walls
Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion
Transport proteins speed the movement of molecules across the plasma membrane into and out of the cell
Hypotonic solution
(b) Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell.
Isotonic solution
H2O H2O
Hypertonic solution
H2O
no input of energy
H2O
Turgid (normal)
Flaccid
Plasmolyzed
Figure 7.13
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Facilitated Diffusion: Passive Transport Aided by Proteins Channel proteins
Allow a specific molecule or ion to cross the membrane down its gradient
EXTRACELLULAR FLUID
Facilitated Diffusion: Passive Transport Aided by Proteins Carrier proteins
Undergo a subtle change in shape that translocates the solute across the membrane down its concentration gradient
Channel protein
Solute CYTOPLASM
Carrier protein
Solute
(a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 7.15
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(b) A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net Figure 7.15 movement being down the concentration gradient of the solute.
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The Need for Energy in Active Transport
Concept 7.4: Active transport uses energy to move solutes against their gradients Active transport
Moves substances against their concentration gradient Requires energy: in the form of ATP or the concentration gradient of other molecules
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Active Transport
1
Cytoplasmic Na+ binds to the sodiumpotassium pump.
EXTRACELLULAR FLUID
[Na+] high [K+] low Na+ Na+ Na+ [Na+] low [K+] high P ADP ATP
2
Na+ Na+
Na+ binding stimulates phosphorylation by ATP.
Review: Passive and active transport compared
Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.
The sodiumpotassium pump
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Na+ CYTOPLASM
Na+ Na+ Na+
K+ is released and Na+ sites are receptive again; the cycle repeats.
K+
3
K+
P
Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside.
5
Figure 7.16
Loss of the phosphate restores the protein's original conformation.
K+
ATP
Diffusion. Hydrophobic
4
K+ K+ K+
P Pi
Extracellular K+ binds to the protein, triggering release of the phosphate group.
molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.
Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins.
Figure 7.17
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Maintenance of Membrane Potential by Ion Pumps
Membrane potential
Is the voltage difference across a membrane Voltage is a measure of electromotive force
An electrogenic pump
Is a transport protein that generates voltage across a membrane by pumping ions
An electrochemical gradient
Is caused by the combined concentration and electrical gradients of ions across a membrane
ATP + + EXTRACELLULAR FLUID H+ H+ H+ CYTOPLASM Figure 7.18
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Proton pump + + + + H+ H+ H+
Cotransport: Coupled Transport by a Membrane Protein Cotransport: active transport driven by a concentration gradient of another molecule
ATP + +
Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Large molecules
enter or leave the cell in vesicles
H+ H+
+
H+
H+
Proton pump
H+
+
Sucrose-H+ cotransporter
+ +
H+ Diffusion of H+ H+
Figure 7.19
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Sucrose
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Exocytosis
In exocytosis
Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents Used by cells for secretion
Endocytosis
In endocytosis
The cell takes in macromolecules by forming new vesicles from the plasma membrane
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Endocytosis
Three types of endocytosis
PHAGOCYTOSIS
EXTRACELLULAR FLUID CYTOPLASM Pseudopodium 1 m
Endocytosis
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein Receptor Coated vesicle
Pseudopodium of amoeba
"Food" or other particle Food vacuole
Ligand: any molecule that binds to a receptor protein
Ligand Coated pit A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs).
Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM).
PINOCYTOSIS
Plasma membrane 0.5 m Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM).
Coat protein
Vesicle
Plasma membrane 0.25 m
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Figure 7.20
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