Movement of molecules through a membrane
Cytoplasm = mixture of particles (molecules) in water
Particles = solute
Water = solvent

Concentration = relative number of a particular molecule compared to total number of
molecules present
10 % sucrose =10 % of molecules present are sucrose
90% of molecules present are not sucrose (are water molecules)

Diffusion = passive movement of molecules from a region of high concentration to a
region of  lower concentration
passive means no energy is spent on the process
molecules move along a concentration gradient — from high to low
some molecules move through a membrane by simple diffusion
occurs until concentration of molecules is the same on both sides of the membrane
cell is now at equilibrium

at equilibrium molecules still move through the membrane but rate of
movement is equal in both directions (into and out of the cytoplasm)
Cell membrane does not allow all molecules to pass through it
Cell membrane is semi-permeable
allows some molecules to move freely
does not allow other molecules to move

OSMOSIS = diffusion (passive) of water through a semi-permeable membrane from a
region of high water concentration to a region of low water concentration
rate of water movement related to the difference in water concentration inside and
outside
if water concentration equal inside and outside, there is no NET movement of water
if 10% outside and 9% inside — water slowly enter
if 15% outside and 9% inside — water enters more rapidly
if 30% outside and 9% inside — water enters still more rapidly, etc.
as water enters cell, it expands
as water leaves a cell it shrinks

TONICITY  -  refers to difference in solute concentration on the two sides of the membrane
Isotonic = solute concentration are the same on both sides
Hypotonic = solute concentration is lower outside than inside
(thus, solvent concentration is higher outside than inside) and water
will enter the cell (move DOWN the concentration gradient)
Hypertonic = solute concentration is higher outside than inside
(thus, solvent concentration is lower outside than inside) and water
will leave the cell (move DOWN the concentration gradient)

Cell in a hypotonic solution will swell due to water entering the cell
To keep from expanding and bursting, living cells in hypotonic solutions
May have contractile vacuoles to pump water out continuously
Cell in a hypertonic solution will shrivel due to water leaving the cell
To keep from losing water, living cells in hypertonic solutions (salt water)
Increase solute concentration inside the cell

Plant cell in hypotonic solution
plant cell wall keeps cell from expanding indefinitely
(like blowing up a balloon inside a coke bottle
Turgor pressure = pressure exerted INWARD by cell wall to counteract the
OUTWARD pressure of water trying to enter the cell by osmosis

Osmosis and excretion

Too little water in body
Osmoreceptors (cells) in brain shrink when body fluids are too concentrated (too little
water in body)
Osmoreceptors signal pituitary gland to release antidiuretic hormone (ADH)
ADH carried by blood stream to kidney tubule cells, causing them to alter their
permeability so they release water that goes back into blood capillaries
= body conserves water that would, without ADH be released in the urine

Too much water in body
Osmoreceptors in brain expand when body fluids too dilute
Inhibits release of ADH by pituitary
Kidney tubule cells remain impermeable to water, ridding body of excess water
Aquaporins = proteins in kidneys control water reabsorption

Facilitated diffusion
polar or ionic molecules move through channels within carrier proteins in membrane
passive process
Carrier proteins have a channel allowing molecule to move through the hole to enter or
leave the cell
carrier protein like a doughnut embedded in a cell membrane  -
doughnut = carrier protein
hole         = channel
transports  glucose, amino acids, iron

Active transport
Active process (requires expenditure of energy, i.e., uses ATP)
Moves molecules AGAINST a concentration gradient (from region of low concentration
to a region of high concentration)
example = sodium-potassium pump
high concentration of potassium inside cell
1. cell carrier protein binds Na on inner surface of cell membrane
2. molecule of ATP is split, causing change in shape of carrier protein.
Causing Na to be released to outside of the cell, and phosphate to attach to
protein
3. this altered shape of carrier protein allows K to enter the cell as protein
resumes normal shape because of release of phosphate group.
4. carrier protein is now ready to start over again

Vesicle transport
Exocytosis      transports large particles out of cells
Endocytosis   transports large particles into cells
Receptor-mediated endocytosis  transports selected large particles into cells

Exocytosis —
Membrane-bound vesicle forms inside of cell containing large molecules (e.g., from ER,
or Golgi apparatus)
Vesicle moves to the cell membrane and fuses with it (fluid mosaic membrane model)
Releasing molecules to the outside of the cell
Release of neurotransmitters from pre-synaptic neuron.
Botulism and tetanus toxins block neuronal endo- and exocytosis resulting in fatal
paralysis

Endocytosis —
Membrane-bound vesicle forms by invagination of cell membrane around particles
being taken up
Membrane-bound vesicle released from cell membrane and moves into cytoplasm
After contents used, the membrane is recycled within the cytoplasm
Receptor-mediated endocytosis —
Receptor proteins on surface of cell membrane bind a specific protein (lock and key
recognition)
Cell membrane invaginates to form a vesicle

For additional information, please contact Dr. Robert K. Reid in the Department of Biology (919) 760-8409.
Page created by Bob Reid