Membrane Transport Plasma membranes are selectively permeable Some

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 substance 2. Channels of appropriate size 3. 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

Extracellular fluid Lipidsoluble solutes Cytoplasm (a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer Figure 3. 7 a

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

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 Figure 3. 7 b

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

Small lipidinsoluble solutes (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Figure 3. 7 c

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)

Water molecules Lipid billayer Aquaporin (d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer Figure 3. 7 d

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

(a) Membrane permeable to both solutes and water Solute and water molecules move down their concentration gradients in opposite directions. Fluid volume remains the same in both compartments. Left compartment: Solution with lower osmolarity Right compartment: Solution with greater osmolarity Both solutions have the same osmolarity: volume unchanged H 2 O Solute Membrane Solute molecules (sugar) Figure 3. 8 a

(b) Membrane permeable to water, impermeable to solutes Solute molecules are prevented from moving but water moves by osmosis. Volume increases in the compartment with the higher osmolarity. Left compartment Right compartment Both solutions have identical osmolarity, but volume of the solution on the right is greater because only water is free to move H 2 O Membrane Solute molecules (sugar) Figure 3. 8 b

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 that of the cytosol • Hypotonic: A solution having lesser solute concentration that of the cytosol

(a) Isotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). (b) Hypertonic solutions Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells). (c) Hypotonic solutions Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of solutes than are present in cells). Figure 3. 9

Summary of Passive Processes Process Simple diffusion Facilitated diffusion Osmosis Energy Source Kinetic energy • Also see Table 3. 1 Example Movement of O 2 through phospholipid bilayer Movement of glucose into cells Movement of H 2 O through phospholipid bilayer or AQPs