Transport through the Cell
Membrane
A. Passive Transport
• passive transport is the movement of molecules through the
cell membrane without cellular energy
• there are 3 types: diffusion; osmosis;
and facilitated diffusion
1. Diffusion
• molecules are in constant motion
• diffusion is the process by which particles move naturally from an
area of HIGH concentration to an area of LOW concentration
until a dynamic equilibrium is reached (no net movement)
[high]
[low]
• this is called moving “with the concentration gradient”
• small molecules diffuse through the phospholipids of the cell
membrane (e.g. O2, CO2, H2O, alcohol, small lipids)
Factors that affect diffusion
• concentration. of particles ( conc.,  rate of diffusion)
temperature. of particles ( temp.,  rate of diffusion)
pressure ( pressure,  rate of diffusion)
agitation ( movement,  rate of diffusion)
2. Osmosis
• osmosis is the diffusion of water through a semi- permeable
membrane
Three types of membranes:
impermeable
permeable
semi-permeable
Which membrane is most like a cell membrane?
Three terms for solutions:
• Solute: the dissolved substance, e.g. salt
• Solvent: able to dissolve things, e.g. water
• Solution: mixture of solvent and solute
Three types of solutions:
• Hypertonic solution: the solution surrounding the cell has a higher
[solute] than the cell’s cytoplasm, water moves out of the cell
• Hypotonic solution: the solution surrounding the cell has a lower
[solute] than the cell’s cytoplasm, water moves into the cell
• Isotonic solution: the [solute] is the same in the cell’s cytoplasm
and in the solution surrounding the cell
What will happen to red blood cells
placed in a salt water solution?
It depends on the type of solution they are placed in.
Hypertonic
More water diffused
out of the cells
Isotonic
Water diffused
equally in and out
of the cells
Hypotonic
More water diffused
into the cells
Turgor pressure
• the rigid cell wall of plant cells prevents them from bursting
when they are filled with water
• water in vacuoles causes an outward push called turgor
pressure
• plant cells swollen with water are said to be turgid
Plasmolysis
• when plant cells are placed in a salt solution, the cells shrink,
this is called plasmolysis
3. Facilitated Diffusion
• some molecules are too large or are hydrophilic, and cannot
pass through the phospholipids
• transport proteins assist these molecules through the cell
membrane (e.g. glucose)
transport protein
• transport occurs with
the concentration
gradient (no energy is
required)
B. Active Transport
• active transport is the movement of molecules through the cell
membrane against the concentration gradient using transport
proteins
[low]
[high]
• this process requires the use of
cellular energy (ATP)
• transport proteins are highly
selective (e.g. Na+/K+ pump)
Active vs. Passive Transport
Facilitated
Diffusion
Diffusion
Active
Transport
C. Transport by Vesicles
a) Endocytosis:
• transport of material into a cell by means of vesicles
• cell engulfs material by folding a portion of its membrane
around it
• energy is required
• 3 types of endocytosis:
1. phagocytosis, cell eating;
movement of large
molecules and whole cells
into the cell’s interior
2. pinocytosis: cell drinking;
transport of liquids into
vesicles inside cell
3. receptor-mediated endocytosis:
molecules bind to receptors
on cell’s surface and are
folded into vesicles within
the cell
b) Exocytosis:
• transport of macromolecules (e.g. hormones) out of a cell by
means of vesicles made by the Golgi complex;
• energy is required
Work on review in the booklet