3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Simple diffusion through the phospholipid bilayer Fig. 3-7a Simple diffusion through the phospholipid bilayer lipid-soluble molecules and O 2, CO 2, and H 2 O (extracellular fluid) (cytoplasm) O2O2 (a)

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Examples of passive transport (continued) – Facilitated diffusion: the diffusion of water-soluble molecules through a channel or carrier protein down a concentration gradient Channel protein: pores in the lipid bilayer through which ions or molecules can diffuse Carrier protein: membrane protein that grabs a specific molecule on one side of the membrane and carries it to the other side

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Facilitated diffusion via a channel protein Fig. 3-7b Facilitated diffusion through a channel protein H 2 O, ions Proteins form a hydrophilic channel channel protein (cytoplasm) (b) Cl –

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Facilitated diffusion via a carrier protein Fig. 3-7c Facilitated diffusion through a carrier protein amino acids, sugars, small proteins carrier protein (extracellular fluid) (cytoplasm) A carrier protein has a binding site for a molecule A molecule enters the binding site The carrier protein changes shape, transporting the molecule across the membrane The carrier protein resumes its original shape (c)

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Animation—Movement Across a Membrane PLAY

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Examples of passive transport (continued) – Water crosses membranes in response to molecular concentration differences on each side of the membrane.

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Isotonic extracellular fluid: the same molecular concentration outside the cell as inside the cell – There is equal movement of water across the cell membrane in each direction under this condition; there is no net water movement.

Isotonic solution has the same salt concentration as the cytoplasm Equal movement of water into and out of cells 10 micrometers (a) 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Isotonic solution Fig. 3-8a

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Hypertonic extracellular fluid: the molecular concentration outside the cell is greater than the molecular concentration inside the cell – Net water movement out of the cell; the cell shrinks.

Hypertonic solution has a higher salt concentration than the cytoplasm Net water movement out of cells; cells shrivel (b) 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Hypertonic solution Fig. 3-8b

3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Hypotonic extracellular fluid: the molecular concentration outside the cell is less than the molecular concentration inside the cell – Net water movement into the cell; the cell swells.

Hypotonic solution has a lower salt concentration than the cytoplasm Net water movement into cells; cells swell and burst (c) 3.6 How Do Diffusion And Osmosis Affect Transport Across The Plasma Membrane? Hypotonic solution Fig. 3-8c

3.7 How Do Molecules Move Against A Concentration Gradient? Energy-requiring transport processes – During active transport, the cell uses energy to move substances against a concentration gradient. – Membrane proteins regulate active transport. – Adenosine triphosphate (ATP) donates energy to the active transport processes.

3.7 How Do Molecules Move Against A Concentration Gradient? Active transport – One binding site on a protein binds a transported molecule and a second binding site binds ATP. – Energy from ATP moves the other molecule up a concentration gradient.

3.7 How Do Molecules Move Against A Concentration Gradient? Active transport Fig. 3-9 The protein releases the ion and the remnants of ATP (ADP and P) and closes ATP binding site recognition site ATP P ADP Ca 2+ (extracellular fluid) (cytoplasm) ATP The transport protein binds both ATP and CA 2+ Energy from ATP changes the shape of the transport protein and moves the ion across the membrane 1 2 3

3.7 How Do Molecules Move Against A Concentration Gradient? Endocytosis: moves fluid droplets or large particles across cell membranes Pinocytosis moves water into a cell. Phagocytosis moves solid material into a cell. Receptor-mediated endocytosis transports specific molecules across membranes.

3.7 How Do Molecules Move Against A Concentration Gradient? During endocytosis, a portion of the plasma membrane engulfs the extracellular fluid or particle and pinches off into the cytoplasm as a membranous sac, called a vesicle.

(extracellular fluid) (cytoplasm) vesicle containing extracellular fluid A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid. The membrane encloses the extracellular fluid, forming a vesicle. Pinocytosis (a) How Do Molecules Move Against A Concentration Gradient? Pinocytosis: movement of water into a cell Fig. 3-10a

The plasma membrane extends pseudopods toward an extracellular particle (food, for example). The ends of the pseudopods fuse, encircling the particle. A vesicle that contains the engulfed particle is formed. Phagocytosis food particle pseudopods vesicle containing the particle (extracellular fluid) (cytoplasm) (b) How Do Molecules Move Against A Concentration Gradient? Phagocytosis: movement of solid material into a cell Fig. 3-10b

3.7 How Do Molecules Move Against A Concentration Gradient? Receptor-mediated endocytosis: transports only specific molecules across membranes – The process depends on the many receptor proteins on the outside surface of a cell. – Receptors can be in depressions in the plasma membrane, called coated pits. – The transported molecule binds to receptors in the coated pits, starts the formation of a membrane vesicle that surrounds the bound molecule, and the vesicle enters the cell.

3.7 How Do Molecules Move Against A Concentration Gradient? Receptor-mediated endocytosis Fig. 3-10c nutrients receptors coated pit Receptor-mediated endocytosis Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites. The receptors bind the molecules and the membrane dimples inward. The coated pit region of the membrane encloses the receptor-bound molecules. A vesicle (“coated vesicle”) containing the bound molecules is released into the cytosol. (cytoplasm) (extracellular fluid) coated vesicle (c)

3.7 How Do Molecules Move Against A Concentration Gradient? Exocytosis: moves material out of the cell, including the waste products of digestion and secreted materials, such as hormones – During exocytosis, a vesicle carrying material to be expelled moves to the cell surface, where the vesicle fuses with the plasma membrane. – Following fusion, the vesicle opens to the extracellular fluid and its contents diffuse out.

Material is enclosed in a vesicle that fuses with the plasma membrane, allowing its contents to diffuse out plasma membrane (cytoplasm) vesicle 0.2 micrometer secreted material (extracellular fluid) 3.7 How Do Molecules Move Against A Concentration Gradient? Exocytosis Fig. 3-11

3.7 How Do Molecules Move Against A Concentration Gradient? Some plasma membranes are surrounded by cell walls. – Cell walls occur around the plasma membranes of plants, fungi, and some bacteria. – Cell walls provide support for the cells, making them capable of resisting gravity and blowing winds.

3.7 How Do Molecules Move Against A Concentration Gradient? Some plasma membranes are surrounded by cell walls (continued). – Cell walls are porous to small molecules, which can pass across these barriers to the plasma membrane. – Plasma membranes of these cells regulate the transport of molecules by the same processes as those that occur in other cells without cell walls.