concept 7.5: bulk transport across the plasma membrane occurs by exocytosis and endocytosis

how do large molecules cross the membrane

in bulk inside the vesicles requiring ATP

exocytosis

transport vesicles migrate to the membrane, fuse with it and release their contents outside the cell

what is exocytosis used to do

to move out large numbers of molecules

endocytosis

macromolecules are taken into the cell in vesicles

how does endocytosis occur

the plasma membrane forms a pocket that deepens and pinches off forming a vesicle around the material for transport into the cell

what are the three different types of endocytosis

phagocytosis, pinocytosis, receptor-mediated endocytosis

what happens in phagocytosis

a cell engulfs a particle and packs it into a food vacuole, and the vacuole fuses with a lysosome and digests the particle/bacterium

what happens in pinocytosis

typically done to take in H2O, molecules are taken up when extracellular fluid is gulped into vesicles that line the inner site of the plasma membrane

what happens in receptor mediated endocytosis

vesicle formation is triggered by solute binding to receptors, humans use it to take it in cholesterol

osmosis

the diffusion of free water, across a selectively permeable membrane

what is free water

water molecules that are not clustered around another substance

what direction do free water molecules diffuse

across the membrane from region of lower concentration to region of higher concentration

when does water stop moving during osmosis

water moves until the concentration is equal on both sides

tonicity

the ability of surrounding solution to cause a cell to gain or lose water

what does tonicity depend on

depends on the concentration of solutes in the solution that cannot cross the membrane relative to that inside the cell

isotonic solution

if the solute concentration is the same as that inside the cell, water diffuses at the same rate in both directions

hypertonic solution

not enough water inside the cell, water is leaving, the solute concentration is greater than that inside the cell

hypotonic solution

too much water inside the cell, water going in, solute concentration is less than that inside the cell

what does it mean to be lysed

hypotonic cell, too full of water

what does it mean to be shriveled

hypertonic cell, not enough water

when is hypotonic or hypertonic environments bad for cells

when the cells have no cell walls, they cannot tolerate excessive water loss or gain

what is osmoregulation

control of solute concentration and water balance

what happens when a plant cell is in a hypotonic solution

it will swell with water until the wall exerts back a pressure on the cell called surgor pressure

what is a healthy state for most plant cells

turgid or very firm

what happens if a plant cell and its surroundings are isotonic

there is no net movement of water and the cell becomes limp or flaccid

what happens if plant cell is in hypertonic environment

the cell loses water and the membrane will pull away from the cell in multiple locations called plasmolysis and the plant will wilt