Cell Transport A Cell Membrane 1 Selectively Permeable

Cell Transport A. Cell Membrane 1. Selectively Permeable: regulates substances entering and leaving the cell a. Small uncharged molecules (O 2, H 2 O, CO 2) can pass, but most large molecules cannot (must be digested)

2. Fluid-Mosaic Model a. Double lipid layer (phospholipid bilayer) containing floating protein molecules

Hydrophilic head (“water-loving”) Phospholipid Hydrophobic tail (“water-fearing”)

http: //learn. genetics. utah. edu/content/cells/membranes/

b. Membrane proteins: • Transport proteins: Facilitated diffusion or active transport • Receptor proteins: Receive chemical messages (hormones) http: //learn. genetics. utah. edu/content/cells/membranes/

B. Passive Transport (no energy is used): 1. Diffusion: movement of molecules from areas of high concentration to areas of low concentration (down the concentration gradient) a. Concentration gradient: the difference between areas of high and low concentration

b. Molecules in motion will eventually reach dynamic equilibrium (evenly distributed, while free to move back and forth) Concentration Gradient Dynamic Equilibrium

High Low

2. Facilitated Diffusion: protein channels allow specific molecules (i. e. ions, sugars, salts) to pass through the membrane a. Gated Channels : a chemical or electrical message causes a protein channel to open or close Cytoplasm

3. Osmosis a. The diffusion of water across a membrane. b. The concentration of water is highest (100%) in pure (distilled) water. • The concentration of water decreases as more solute is dissolved in it. *Which of the following has the greatest concentration of water? = solute A B C

c. Effects of osmosis on cells in solutions: Type of Solution Isotonic solution: Solute concentration is the same as cytoplasm *Same concentration of water as the cytoplasm Hypotonic solution: Solute concentration is less than cytoplasm *Higher concentration of water than cytoplasm (distilled water) Hypertonic solution: Solute concentration is greater than cytoplasm *Lower concentration of water than cytoplasm (salt solution) Animal Cell Plant Cell Stays the same Water diffuses in Swells and Possible Bursts Swells Water diffuses out Shrinks (not cell wall)

C. Active Transport 1. Cellular energy (ATP) is needed to move substances across a membrane 2. Three types: a. Substances are moved across a membrane against the concentration gradient (low to high concentration) Low Conc. High Conc. *Uses Protein Pumps

ex: Sodium-Potassium Pump – ATP alters the shape of the protein in order to pump sodium ions (Na+) out of the cell and potassium ions (K+) in. *3 Na+ are pumped out for every 2 K+ in

b. Endocytosis – Cell takes in molecules by forming a membrane vesicle around the molecules § Pinocytosis (“cell drinking”): membrane pinches in and forms small vesicles inward to trap small dissolved molecules

§ Phagocytosis (“cell eating”): membrane reaches out to engulf large undissolved molecules and then forms a vesicle

Food Pseudopods

Phagocytosis of red blood cell by macrophage Red arrows point to edges of pseudopods that macrophages extend to engulf cells

c. Exocytosis – cell secretes molecules § Transport vesicle from golgi body brings molecules to the membrane, fuses with the membrane, and the molecules spill out.