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

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

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 OSMOSIS  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% Water70% Water 80% Water

H2OH2O H2OH2O H2OH2O H2OH2O 30% Sugar (70% H 2 O) 15% Sugar (85% H 2 O) Cell Environment Water moves from high concentration of water to lower concentration of water

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

Na – K Pump

ENERGY !

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