Physiology of Cells

Passive Transport1. Diffusion

– Tendency of small particles to spread out evenly within a given space

– Occurs down a concentration gradient until equilibrium is reached• Measurable difference between one area to

another

– Diffusion or permeability dependent upon presence of specified channels

– Selectively permeable – channels open and close based on cell’s needs

Diffusion

Passive Transport2. Dialysis (fig 4-4, pg. 94)

– Selectively permeable membrane separates smaller and larger particles

Passive Transport3. Osmosis (fig 4-5, pg. 95)

– Diffusion of water through a selectively permeable membrane

Osmotic Pressure

• Osmotic pressure – pressure that develops in a solution as a result of osmosis– Important concept for maintaining homeostasis

• Isotonic – two fluids have the same osmotic pressure

• Hypotonic – lower concentration of solute outside of the cell (ECF)

• Hypertonic – higher concentration of solute outside of cell (ECF)

Effects of Osmosis on a Cell

Passive Transport4. Facilitated

Transport – (also carrier-

mediated passive transport)

– Movement of a molecule is facilitated by a carrier mechanism in the cell membrane

– Still requires concentration gradient

Passive Transport

5. Filtration– Passing of water and permeable solutes

through a membrane by the force of hydrostatic pressure

– Hydrostatic pressure: force or weight of a fluid pushing against a surface

– Occurs down a hydrostatic pressure gradient

– Most often associated with movement across a sheet of cells (ex: capillaries)

Active Transport

• Carrier-mediate process• Moves molecules “uphill” (against

concentration gradients)• Ex: Sodium-potassium pump

– Operates in all human cells

Sodium-Potassium Pump

• Transports Na+ out of the cell and K+ into the cell

• Requires energy (sodium-potassium ATPase) – made in mitochondria

• 3 Na+ bind on the intracellular side and are exchanged for 2 K+ from the ECF

Sodium-Potassium Pump

Endocytosis 1. Receptors in the plasma membrane bind

to molecules in the ECF2. Cytoskeleton pulls a portion of the

membrane inward, creating a pocket3. Edges of the “pocket” fuse forming a

vesicle4. Vesicle is pulled inside the cell by the

cytoskeleton 5. Vesicle fuses with the membrane walls of

a lysosome

Exocytosis

Process by which large molecules (proteins) exit the cell

1. Wastes/proteins enclosed by a membrane vesicle

2. Cytoskeleton moves vesicle to plasma membrane

3. Fuses with membrane & releases contents to ECF