The Plasma Membrane Maintaining a Balance

The Plasma Membrane The plasma membrane is a SELECTIVELY PERMEABLE membrane that allows nutrients and wastes to enter and exit the cell Homeostasis: maintaining balance in the cell

Selective Permeability Selective Permeability – allows some molecules to pass through and keeps others out E.g. Screen on a window Water can freely enter and exit the cell. Other particles such as Na+ and Ca+ ions are only allowed in at certain times (selective permeability)

Phospholipid The plasma membrane is composed of a PHOSPHOLIPID BILAYER (double layer) A phospholipid molecule is a lipid which is composed of a glycerol, a phosphate, and two fatty acids The two fatty acid tails of the phospholipid are NONPOLAR (not attracted to water) whereas the head of the phospholipid molecule is POLAR (attracted to water)

Polar Head (glycerol + phosphate) Nonpolar Tail (fatty acids)

Phospholipid Bilayer The two layers of phospholipid molecules make a sandwich with the fatty acid tails forming the interior of the membrane and the phospholipid heads facing the watery environments found inside and outside the cell

Cholesterol and Transport Proteins Cholesterol is mixed into the plasma membrane and helps keep the fatty acid tails from sticking together Transport proteins span across the plasma membrane and move larger molecules through the membrane

Cellular Transport

Diffusion Water can pass freely through the plasma membrane by DIFFUSION Diffusion – the movement of particles from an area of higher concentration to an area of lower concentration Like gravity  High to Low E.g. Perfume spreads from point of origin to evenly disperse around the room

Gas spreads out by diffusion!

Water In the cell, WATER always moves to reach an equal concentration of both sides of the membrane The diffusion of water across a semi- permeable membrane is called OSMOSIS

Osmosis If two solutions with different solute concentrations are separated by a selectively permeable membrane (i.e. the plasma membrane), the water molecules will move from the solution with HIGHER WATER (lower solute) CONCENTRATION to the solution with LOWER WATER (higher solute) CONCENTRATION. The solute doesn’t move; only the WATER moves

90% Water 70% Water 80% Water 80% Water

Water moves from high concentration of water to lower concentration of water 15% Sugar (85% H2O) H2O H2O 30% Sugar (70% H2O) Cell H2O H2O Environment

Facilitated Diffusion Facilitated Diffusion – transport of materials across a membrane with the aid of transport proteins (no energy) Proteins act like a gate or channel that opens and closes to let materials in and out

Osmotic Solutions Isotonic Solution – the concentration of water is EQUAL on both sides of the membrane. Water moves in and out of the cell equally Hypotonic Solution – the solute concentration is higher inside the cell than outside the cell. Water will move into the cell by osmosis and the cell will SWELL in size Hypertonic Solution – the solute concentration is lower inside the cell than outside the cell. Water will move out of the cell by osmosis and cell will SHRINK in size

Concentration Gradient CONCENTRATION GRADIENT - When there is an unequal concentration of solutes on different sides of a membrane WITH concentration gradient AGAINST concentration gradient

Passive Transport PASSIVE TRANSPORT - Particles naturally move WITH the concentration gradient High  Low NO ENERGY is needed Diffusion = passive Osmosis = passive Facilitated Diffusion = passive

Active Transport Active Transport – moves materials AGAINST the concentration gradient Low  High REQUIRES ENERGY! Active transport uses TRANSPORT PROTEINS + ENERGY to move materials from low concentration to high concentration

ENERGY!

Endocytosis/Exocytosis Endocytosis – the whole cell engulfs something Exocytosis – the whole cell ejects something