2. The Cell Membrane
The boundary of all living cells is a cell
membrane which controls the entry of
dissolved substances into and out of the
cell.
A cell membrane consists of both lipid and
protein.
The membrane is formed of a double –
layer of lipids.
Proteins are embedded in this layer
forming channels that allow water –
soluble substances pass through
5. Crossing The Membrane
All cells must be able to take in &
expel various substances in order to
survive, grow & reproduce.
As a cell membrane only allows
some dissolved substances to cross
it, it is described as being semi –
permeable OR partially –
permeable.
6. Main Types of Transport
Passive Transport: diffusion & osmosis
Active Transport
Endocytosis and Exocytosis
Passive – movement across membranes
that does not require any energy
Active – movement across membranes
that does require energy
7. Diffusion
Net movement of particles from a
region of high concentration to a
region of low concentration.
8. Diffusion
Difference in concentration between
two regions = concentration gradient
Diffusion occurs when a gradient
exists and continues until equilibrium
Factors can effect the rate of diffusion
when the concentration gradient is large
when heat is applied
when molecules are smaller
when movement occurs through a
gaseous membrane
9. Diffusion
Water, carbon dioxide and other
small uncharged particles
10. Osmosis
Net movement of a
solvent, (usually water)
across a differentially
permeable membrane from
a region of low solute
concentration to high
solute concentration.
That is, water molecules
move from a region of high
solvent concentration to
low concentration.
12. Osmosis in Animal Cells
Isotonic – when the extracellular fluid
and intracellular fluids are at equal
concentrations.
Hypotonic – when the extracellular
fluid has a higher water concentration
than the intracellular fluid.
Hypertonic – when the intracellular
fluid has a higher water concentration
than the extracellular fluid.
13. Osmosis
High Water Low water
Concentration concentration means
means the solution the solution is
is Hypotonic Hypertonic
Loses water by Gains water by
osmosis osmosis
Solute molecules cannot
Water molecules
pass through the
partially permeable
Partially membrane
permeable
membrane
There is a net movement of water molecules
towards the less concentrated side (in terms
of water molecules) until water
concentrations equalize.
15. Osmosis in Animal Cells
Isotonic solution
No net movement of solvent =
equilibrium
Red blood cell =
0.9% saline
Solution = 0.9%
saline
16. Osmosis in Animal Cells
Hypotonic solution
Solvent moves into the cell
Red blood cell =
0.9% saline
Solution = 0.4%
saline
17. Osmosis in Animal Cells
Hypertonic solution
Solvent moves out of the cell into the solution
Red blood cell =
0.9% saline
Solution = 1.2%
saline
18. Osmosis in Plant Cells
Different to animal cells because of the cell wall
19. Osmosis in Plant Cells
Isotonic solution
No net movement of solvent =
equilibrium
Cell placed in solution
that has the same
concentration as the cell
sap
20. Osmosis in Plant Cells
Hypotonic solution
Plant cell placed in solution that has a lower
concentration than the cell sap
•Water enters the vacuole
•The vacuole expands
•Cytoplasm is pushed
outwards
•‘Turgid’ cell
21. Osmosis in Plant Cells
Hypertonic solution
Plant cell placed in solution that has a higher
concentration than the cell sap
•Water exits the vacuole
•Vacuole shrinks
•Cytoplasm moves inwards
•‘Plasmolysis’
22. Active Transport
Diffusion is very slow & it moves substances
down a concentration gradient.
Many substances are needed by organisms
in much greater amounts than what can be
provided by diffusion alone. Additionally,
some substances need to be accumulated in
cells against the concentration gradient.
In order for this to occur, energy must be
used to move the needed substances across
the cell membrane.