Upload
margery-weaver
View
215
Download
0
Embed Size (px)
Citation preview
Membrane Transport
• Plasma membranes are selectively permeable• Some molecules easily pass through the
membrane; others do not
Types of Membrane Transport
• Passive processes– No cellular energy (ATP) required– Substance moves down its concentration gradient
• Active processes– Energy (ATP) required– Occurs only in living cell membranes
Passive Processes
• What determines whether or not a substance can passively permeate a membrane?
1. Lipid solubility of substance2. Channels of appropriate size3. Carrier proteins
PLAY Animation: Membrane Permeability
Passive Processes
• Simple diffusion• Carrier-mediated facilitated diffusion• Channel-mediated facilitated diffusion• Osmosis
Passive Processes: Simple Diffusion
• Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer
PLAY Animation: Diffusion
Figure 3.7a
Extracellular fluid
Lipid-solublesolutes
Cytoplasm
(a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer
Passive Processes: Facilitated Diffusion
• Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:– Exhibit specificity (selectivity)– Are saturable; rate is determined by number of
carriers or channels– Can be regulated in terms of activity and quantity
Facilitated Diffusion Using Carrier Proteins
• Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids)
• Binding of substrate causes shape change in carrier
Figure 3.7b
Lipid-insoluble solutes (such as sugars or amino acids)
(b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein
Facilitated Diffusion Using Channel Proteins
• Aqueous channels formed by transmembrane proteins selectively transport ions or water
• Two types:– Leakage channels
• Always open
– Gated channels• Controlled by chemical or electrical signals
Figure 3.7c
Small lipid-insoluble solutes
(c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge
Passive Processes: Osmosis
• Movement of solvent (water) across a selectively permeable membrane
• Water diffuses through plasma membranes:– Through the lipid bilayer– Through water channels called aquaporins (AQPs)
Figure 3.7d
Watermolecules
Lipidbillayer
Aquaporin
(d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer
Passive Processes: Osmosis
• Water concentration is determined by solute concentration because solute particles displace water molecules
• Osmolarity: The measure of total concentration of solute particles
• When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached
Figure 3.8a
(a) Membrane permeable to both solutes and water
Solute and water molecules move down their concentration gradientsin opposite directions. Fluid volume remains the same in both compartments.
Leftcompartment:Solution withlower osmolarity
Rightcompartment:Solution with greater osmolarity
Membrane
H2O
Solute
Solutemolecules(sugar)
Both solutions have thesame osmolarity: volumeunchanged
Figure 3.8b
(b) Membrane permeable to water, impermeable to solutes
Both solutions have identicalosmolarity, but volume of thesolution on the right is greaterbecause only water is free to move
Solute molecules are prevented from moving but water moves by osmosis.Volume increases in the compartment with the higher osmolarity.
Leftcompartment
Rightcompartment
Membrane
Solutemolecules(sugar)
H2O
Importance of Osmosis
• When osmosis occurs, water enters or leaves a cell
• Change in cell volume disrupts cell function
PLAY Animation: Osmosis
Tonicity
• Tonicity: The ability of a solution to cause a cell to shrink or swell
• Isotonic: A solution with the same solute concentration as that of the cytosol
• Hypertonic: A solution having greater solute concentration than that of the cytosol
• Hypotonic: A solution having lesser solute concentration than that of the cytosol
Figure 3.9
Cells retain their normal size andshape in isotonic solutions (same
solute/water concentration as insidecells; water moves in and out).
Cells lose water by osmosis and shrink in a hypertonic solution
(contains a higher concentration of solutes than are present inside
the cells).
(a) Isotonic solutions (b) Hypertonic solutions (c) Hypotonic solutions
Cells take on water by osmosis untilthey become bloated and burst (lyse)
in a hypotonic solution (contains alower concentration of solutes than
are present in cells).
Summary of Passive Processes
• Also see Table 3.1
Process Energy Source
Example
Simple diffusion
Kinetic energy
Movement of O2 through phospholipid bilayer
Facilitated diffusion
Kinetic energy
Movement of glucose into cells
Osmosis Kinetic energy
Movement of H2O through phospholipid bilayer or AQPs