Permeability of the Plasma Membrane

Permeability of the Plasma Membrane

the plasma membrane is selectively permeable or semi-permeable

Semi-permeable membrane

allows some molecules to pass through, and not others

Permeability of the Plasma Membrane

• small non-charged molecules (alcohol, oxygen) pass through the membrane freely

• small polar molecules (carbon dioxide, water) easily pass along a concentration gradient

Concentration gradient

from a region of high concentration to a region of low concentration

Permeability of the Plasma Membrane

• ions and charged molecules have difficulty crossing the hydrophobic phase of the bilayer

• ions and charged molecules usually cross the membrane through carrier proteins

Diffusion

molecules move from a region of higher concentration to lower concentration e.g. down their concentration gradient

a solution contains…

• a solute - usually a solid

• a solvent - usually a liquid

in the case of a dye diffusing in water…

• dye is a solution

• water is the solvent

Passive transport

is where molecules cross a membrane without expenditure of energy by cell

Passive transport includes…

diffusion and facilitated transport

Facilitated transported

the chloride ion channel

Passive transport

moves molecules across membrane without expenditure of energy by cell

Active transport

requires a carrier protein and uses energy (ATP) to move molecules across a plasma membrane

Osmosis

the diffusion of water across a semipermeable membrane

Osmosis

the movement of water from a region of low solute concentration to a region of high solute concentration across a semi-permeable membrane

Osmotic pressure

the pressure that develops in such system due to osmosis

Osmosis steps 1-4

1. a semipermeable membrane separates two solutions

2. the beaker has more water and the thistle tube has less water

3. the membrane does not permit passage of the solute; water enters but the solute does not exit

4. the membrane permits passage of water with a net movement of water from the beaker to the inside of the thistle tube

Tonicity

is strength of a solution in relation to osmosis

Iso

means ‘the same as’

Isotonic solutions

occur where the relative solution concentrations of two solutions are equal;

• a 0.9% salt solution is used in injections because it is isotonic to red blood cells RBCs)

Hypo

means ‘less than’

Hypotonic solution

has a solute concentration that is less than another solution therefor, when a cell is placed in a hypotonic solution, water enters the cells and they may undergo lysis (burst)

Hyper

means ‘more than’

Hypertonic solution

has a solute concentration that is more than another solution therefor, it has a higher percentage of solute than a cell; as a result, water may leave the cells

Crenation

solutions that cause cells to shrink are hypertonic solutions;

red blood cells placed in salt solutions above 0.9% shrink and wrinkle, a condition known as crenation

Plasmolysis

is shrinking of the cytoplasm due to osmosis in a hypertonic situation, often in a plant cell