Chapter 5
Homeostasis and Cell Transport
This is a
matter of life
and death in
some cases!
Homeostasis – Cells maintain internal
environment by controlling what goes in
and out of cell
They can do this by:
1. Passive Transport - movement of
substances across the membrane without
the input of energy
2. Active Transport - Movement of
materials up a concentration gradient
(requiring energy input)
How is this car
trying to
maintain a
steady speed
of 55 mph an
example of
homeostasis?
Types of Passive Transport:
A. Diffusion
- Movement of molecules
from area of high concentration
to area of low concentration
1. Concentration Gradient –
The difference in concentration
of molecules across a space
2. Requires no energy input
Diffusion cont.
3. Equilibrium – the concentration of
molecules is the same throughout a space
- When a solution has reached equilibrium,
movement still exists
- Equilibrium means the amount of substance
leaving is the same as the amount arriving.
Equilbrium in solution
Diffusion cont.
4. Molecules that can dissolve in lipids will
freely pass through the membrane
Other molecules must be transported across
the membrane, requiring energy
B. Osmosis – Movement of H2O
B. Osmosis
- Process of diffusion of water molecules
across a membrane
1. Always form high to low concentration
2. Does not require energy input
3. Direction of Osmotic flow:
A. Hypotonic – where a solution contains a higher
concentration compared to cell
(= lower concentration in cell)
B. Hypertonic – where a solution contains a lower
concentration compared to cell
(= higher concentration in cell)
C. Isotonic – where a solution contains an equal
concentration inside & outside of cell
D. Water flows from Hypotonic to Hypertonic
areas
Effect of placing cell in different
osmotic conditions
Osmosis cont.
4. Contractile Vacuoles – organelles that
pump water out of the cell
Necessary for organisms living in freshwater
Open vacuole
Closed Vacuole
Osmosis cont...
5. Plant’s cell walls prevent exploding due to
osmotic pressure
A. Turgor Pressure – the pressure exerted
by water on the inside of cell wall
B. When plants lose water, turgor pressure is
lost and cells wilt
- This wilting is called plasmolysis – cell
water loss
What happens if a cell is placed in an
extremely hypotonic environment?
- Cytolysis – bursting of cells
- Only occurs in cell that lack
protection (like cell wall or contractile
vacuoles) to prevent bursting
- i.e. Red Blood Cells