Cellular Transport

Diffusion• Maintaining

homeostasis– Cell membranes help

organisms maintain homeostasis by controlling what substances may enter or leave cells.

– Passive Transport- substances can cross the cell membrane without any input of energy

• Diffusion–Molecular motion•Molecules are in constant motion

because they have kinetic energy (brownian motion)•Molecular motion is random•Molecules tend to move from areas

where they are more concentrated to areas where they are less concentrated

• Diffusion- movement of molecules from area of higher concentration to area of lower concentration– Concentration gradient- difference in the

concentration of molecules across a distance– So, molecules tend to move “down” their

concentration gradient– Diffusion will eventually cause the molecules to be

in equilibrium– Equilibrium- concentration of molecules will be

the same throughout the space the molecules occupy

• Diffusion– Simple diffusion- diffusion of substances across

the cell membrane without the help of membrane proteins

– Facilitated diffusion- the movememt of molecules across the cell membrane is assissted by specific proteins in the membrane• Used to move large molecules, polar molecules, ions• Carrier proteins- used in facilitated diffusion

Osmosis• Osmosis- the process by which water

moleucles diffuse across a cell membrane from high to low concentration

• Form of passive transport (does not requir energy)

• Direction of Osmosis–Depends on relative concentration of solutes on

the two sides of the membrane• Hypotonic- describes solution with solute

concentration lower than concentration inside cell (water diffuses in)• Hypertonic- describes solution with solute

concentration higher than concentration in cell (water diffuses out)• Isotonic- describes solution with solute

concentration equal to concentration in cell (water diffuses in and out at same rate)

• How osmosis affects living cells

Active transport• Active transport- when cells move materials

from area of lower concentration to an area of higher concentration, “against” the concentration gradient (requires energy, ATP)

• Cell membrane pumps– Carrier proteins – “pumps”- moving from low to

high concentration– Sodium-potassium pump- transports Na+ ions and

K+ ions up their concentration gradient• Helps create electrical impulses that are important in

animal cells (nerve cells)

• Movement in vesicles– Endocytosis- process by which cells ingest external

fluid, macromolecules, and large particles, including other cells by forming a vesicle• Pinocytosis- involves transport of solutes or fluids• Phagocytosis- movement of large particles or cells

• Exocytosis- process by which a substance is released from the cell through a vesicle