7 transport across cell membranes

what are the types of transport?

• simple diffusion

• facilitated diffusion

• active transport

• osmosis

what’s simple diffusion?

the net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. the process doesn’t require ATP

in simple diffusion, what must molecules be to diffuse across the membrane?

lipid soluble and small

what’s facilitated diffusion?

a passive process where proteins are used to transport molecules from an area of high concentration to an area of low concentration. the movement of ions and polar molecules (therefore not lipid soluble), which cannot simply diffuse, can be transported across membranes using protein channels and carrier proteins

what do protein channels do?

form tubes filled with water and this enables water soluble ions to pass through the membrane. this is still selective, as the channel proteins only open in the presences of certain ions when they bind to the protein

what do carrier proteins do?

bind with a molecule (the molecule has to be complementary in shape to the carrier protein), such as glucose, which causes a change in the shape of the protein. this shape change enables the molecules to be released to the other side of the membrane

what’s osmosis?

the movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane

define water potential

the pressure created by water molecules (measured in kPa)

what states can simple diffusion only be carried out in? and why?

liquids and gases as they have kinetic energy and solids do not

how do the molecules move in simple diffusion?

they do have energy due to the kinetic energy they already have to enable them to move in fluids

what water potential does pure water have?

zero

when solutes dissolve in water, what’s the water potential?

becomes negative

what does it having a more negative water potential mean?

the more solute has dissolved in it

define isotonic

when the water potential is the same in the solution and the cell within the solution

define hypotonic

when the water potential of a solution is more positive (closer to zero than the cell)

what happens in animal cells if they’re placed in a hypotonic solution such as pure water?

a lot of water will move into the cell by osmosis. as animal cells do not have a cell wall the pressure will cause the cell to burst

define hypertonic

when the water potential of a solution is not negative than the cell

why don’t plant cells burst in hypotonic solutions?

the have a cell wall and will instead become turgid

what will happen to both animal and plant cells if they’re placed in hypertonic solutions?

shrink and shrivel due to large volumes of water leaving the cell due to osmosis