Passive Transport across Plasma Membrane

Plasma Membrane Functions Membrane Transport Cells bathed in interstitial fluid – rich in nutrients (amino acids, sugars, fatty acids, vitamins, hormones & neurotransmitters, salts, wastes) Selectively Permeable – allows some things into cell, keeps others out, also keeps substances in the cell but wastes can exit. Damaged cells (burns) – ruin permeability – fluids, proteins, ions exit dead or damaged cell.

2 types of membrane transport Passive transport – move across membrane without energy input Diffusion – transport for every cell. Filtration – across capillary walls Active Transport – cell provides ATP to move substances across membrane.

Diffusion Molecules & ions scatter evenly throughout - due to kinetic energy. Movement from high concentration to low concentration (down or along the concentration gradient) Faster diffusion with greater difference in gradient Molecules diffuse through physical barrier if… Lipid soluble Small enough to pass through channels Assisted by carrier molecule

Passive Membrane Transport: Diffusion Simple diffusion – nonpolar and lipid-soluble substances Diffuse directly through the lipid bilayer Substances Diffuse through channel proteins Oxygen, carbon dioxide, fat-soluble vitamins. PLAY Diffusion

Passive Membrane Transport: Diffusion Facilitated diffusion Transport of glucose, amino acids, and ions Not transported through lipid bilayer Transported substances bind carrier proteins or pass through protein channels Follows concentration gradient

Carrier Proteins – Part of Facilitated Diffusion Are integral transmembrane integral proteins Show specificity for certain polar molecules too large to pass through channels (including sugars and amino acids ) Oxygen, water, glucose, ions = passive transport to save ATP.

Diffusion Through the Plasma Membrane Extracellular fluid Small lipid- insoluble solutes Lipid- soluble solutes Lipid-insoluble solutes Water molecules Lipid bilayer Cytoplasm (a) Simple diffusion directly through the phospholipid bilayer (b) Carrier-mediated facilitated diffusion via protein carrier specific for one chemical; binding of substrate causes shape change in transport protein (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge (d) Osmosis, diffusion through a specific channel protein (aquaporin) or through the lipid bilayer Figure 3.7

Diffusion Through the Plasma Membrane Extracellular fluid Lipid- soluble solutes Cytoplasm (a) Simple diffusion directly through the phospholipid bilayer Figure 3.7

Diffusion Through the Plasma Membrane Lipid-insoluble solutes (b) Carrier-mediated facilitated diffusion via protein carrier specific for one chemical; binding of substrate causes shape change in transport protein Figure 3.7

Diffusion Through the Plasma Membrane Small lipid- insoluble solutes (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Figure 3.7

Diffusion Through the Plasma Membrane Water molecules Lipid bilayer (d) Osmosis, diffusion through a specific channel protein (aquaporin) or through the lipid bilayer Figure 3.7

Diffusion Through the Plasma Membrane Extracellular fluid Small lipid- insoluble solutes Lipid- soluble solutes Lipid-insoluble solutes Water molecules Lipid bilayer Cytoplasm (a) Simple diffusion directly through the phospholipid bilayer (b) Carrier-mediated facilitated diffusion via protein carrier specific for one chemical; binding of substrate causes shape change in transport protein (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge (d) Osmosis, diffusion through a specific channel protein (aquaporin) or through the lipid bilayer Figure 3.7

Passive Membrane Transport: Osmosis Occurs when the concentration of a solvent is different on opposite sides of a membrane Diffusion of water across a semipermeable membrane Osmolarity – total concentration of solute particles in a solution Tonicity – how a solution affects cell volume PLAY Osmosis

Effect of Membrane Permeability on Diffusion and Osmosis Figure 3.8a

Effect of Membrane Permeability on Diffusion and Osmosis Figure 3.8b

Passive Membrane Transport: Filtration The passage of water and solutes through a membrane by hydrostatic pressure (still passive but through pressure gradient) Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area Application – water forced out of capillaries, urine forced out of glomerulus. (Concentration gradient may bring molecules back into capillaries)

Effects of Solutions of Varying Tonicity Isotonic – solutions with the same solute concentration as that of the cytosol Not net gain of loss of water in or out of cell Hypertonic – solutions having greater solute concentration than that of the cytosol Cell in strong saline solution, cell loses water, shrinks Hypotonic – solutions having lesser solute concentration than that of the cytosol Cell in Distilled Water, cell fills up, can burst