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CELL TRANSPORT
Chapter 5
1. Cells must keep a biological balance with their environment to stay alive &
healthy. This condition is called homeostasis.
2. The cell membrane is able to maintain homeostasis by being selectively
permeable - allowing some molecules into cell while keeping others out.
3. Structure of the cell membrane:
a) there are 2 layers of phospholipid molecules - the fatty acid tails inside
the membrane are non-polar & avoid water (hydrophobic) & the
phosphate heads on the outer boundaries are polar & allow interaction
with cell’s watery environment (hydrophilic)
b) transport proteins within the phospholipid layers form channels to allow
materials to move across
4. When molecules can move easily across the cell membrane without the help of
cellular energy, this is called passive transport.
5. Diffusion requires only a concentration gradient. This means that molecules
move from an area of greater concentration to an area of lesser concentration
to reach equilibrium. The diffusion of water is called osmosis.
6. Sugars & amino acids use carrier proteins to cross the membrane. This form
of passive transport is called facilitated diffusion.
7. A solution is composed of a solute (sugar) dissolved in a solvent (water). The
direction water will move across a membrane depends on the concentration of
the 2 substances. A solution may be:
a) hypotonic - concentration of solute molecules outside cell is lower than in
cell - water will diffuse into cell
b) hypertonic - concentration of solute molecules outside cell is greater than
inside - water will diffuse out of cell
c) isotonic - concentration of solutes outside & inside cell are equal - water
will not diffuse in either direction
8. Ions (sodium, potassium) move across the membrane against the concentration
gradient (lesser to greater) with the help of carrier proteins. This requires
energy from ATP molecules in the cell & is called active transport.
9. Some cells use active transport to take in & release large molecules, groups of
molecules, or even whole cells.
a) endocytosis - cell surrounds material from environment & encloses it in a
vacuole where it’s digested by enzymes (food & microscopic organisms)
b) exocytosis - cell expels matter from its interior to the exterior
environment (waste, hormones, proteins)