Transporting Substances Across the Cell Membrane B3, B2.1, B3.6

Recall: the cell membrane is “semi-permeable” Two types of transport: Passive transport Active transport B3, B2.1, B3.6

Passive transport is the movement of molecules through the membrane in which -no energy is required -molecules move in response to a concentration gradient -three types: 1. Diffusion is the movement of molecules from high concentration to low concentration Crossing a biological molecule: issue is the membrane’s hydrophobic interior-repels polar molecules but not non-polar molecules. i.e. O2 is non-polar so it can move across if there is a concentration gradient. B3, B2.1, B3.6

B3, B2.1, B3.6

Factors affecting the rate of diffusion: Molecule Size: rate of diffusion decreases as the size of the molecule increases Molecule Polarity: small polar molecules can cross, but at a lower rate as same-sized non-polar molecules Molecule or Ion Charge: Usually charged molecules and ions cannot diffuse across a cell membrane Temperature & Pressure: Inc. T &/or P, inc. rate B3, B2.1, B3.6

-dissolved substances are the solutes In an aqueous solution -water is the solvent -dissolved substances are the solutes 2. Osmosis is the movement of water from an area of high to low concentration of water across a semi-permeable membane -movement of water toward an area of high solute concentration Membrane is impermeable to the SOLUTE, but permeable to the SOLVENT (water) B3, B2.1, B3.6

When 2 solutions have different osmotic concentrations When 2 solutions have the SAME osmotic concentrations isotonic solution When 2 solutions have different osmotic concentrations -the hypertonic solution has a higher solute concentration -the hypotonic solution has a lower solute concentration Osmosis moves water through aquaporins toward the hypertonic solution. Hyper: “more than” Hypo: “less than” Iso: “same as” B3, B2.1, B3.6

B3, B2.1, B3.6

3. Facilitated diffusion is movement of a molecule from high to low concentration across a cell membrane with the help of a carrier protein. a. Channel proteins have a polar interior allowing polar molecules or ions to pass through. b. Carrier proteins bind to a specific molecule (e.g. glucose, aa) to facilitate its passage. P75: Fig. 2.16 Channel proteins: tubular, composed of helixes, (coiled springs), Exterior of a channel protein is usually composed of aa with non-polar side chains that interact with the non-polar interior of the cm, anchoring the pr in place. Shape and size of hole determines the shape and size of particles that can pass through it. Some remain open all the time, others are gated-may open/close in response to hormones, electric charges, pressure, light, etc. Cystic fibrosis: defective Cl ion channel protein Carrier Proteins: change shape while transporting molecules. Slower rate of diffusion, since bind to only a few molecules at a time. Exterior is usually composed of n-polar aa that interact with the n-polar interior of the membrane. Cystinurea: disease caused by the inability of carrier pr to remove cystine and some other aa from urine-cystine will then crystallieze into painful stones that can block flow of urine in the urinary tract. B3, B2.1, B3.6

Active Transport -requires energy – ATP is used directly (primary active transport) or indirectly (secondary active transport to fuel active transport -moves substances from low to high concentration -requires the use of carrier proteins B3, B2.1, B3.6

E.g. Primary Active Transport: Sodium-potassium (Na+-K+) pump Both Na+ & K+ move to opposite sides of cell membrane against a conc. gradient, due to the usage of ATP P76 Fig 2.18 E.g. Secondary Active Transport: Hydrogen-Sucrose pump Uses an electrochemical gradient as a source of energy to transport molecules or ions across a cell membrane P77 Fig. 2.19 B3, B2.1, B3.6

Hmk: P81: #1,2,4-7,9-11 B3, B2.1, B3.6