Maintaining Balance
•Transport
across
Membranes
Why is this necessary?
As you will recall multi-cellular
organisms are made up of many
cells
 Each cell has requirements….
 Can you recall these?
 How do cells receive and remove
these substances?
Why is transport necessary?
 Cells need to be able to exchange substances
(requirements) between each other.
 Even though cells appear to be rubbing against each
other the immediate environment surrounding a cell
is known as tissue fluid. We call this fluid the
extracellular fluid (the fluid outside the cell)
Movement of chemicals into and out
of cells
 Some molecules can pass
through the cell membrane
because they are very
small
 Larger molecules must
enter and exit through the
protein transport channels
 We say the cell membrane
is selectively permeable or
semi-permeable
Passive transport of substances
across the cell membrane
(no energy required)
 Diffusion
 Osmosis.
Diffusion – free, unaided, random movement of
molecules from an area of high concentration to low
concentration until a dynamic equilibrium is reached.
Occurs in gases and liquids as the molecules are
always moving
Substances which can
easily enter cells via
diffusion include:
oxygen, carbon dioxide,
alcohol, steroids,
substances that
dissolve in fats
Osmosis – Osmosis is the spontaneous net
movement of water molecules (solvent) through a
semi-permeable membrane into a region of higher
solute concentration, in the direction that tends to
equalize the solute concentrations on the two sides
Diffusion of Oxygen
 Oxygen diffuses into cells because the cell is
constantly using oxygen for respiration. This means
there is always a lower concentration of oxygen in
the cell than outside the cell.
Diffusion of Carbon Dioxide
 Carbon dioxide diffuses out of cells because carbon
dioxide is constantly being produced via the
process of respiration. Therefore there is always a
higher concentration of carbon dioxide inside the
cells than outside the cell.
RESPIRATORY SYSTEM –
Diffusion in the Respiratory System
 Carbon dioxide and oxygen are exchanged
between the alveoli in the lungs and the blood
 When we breathe in:
 High concentration of oxygen in the alveoli
 Low concentration of carbon dioxide in the alveoli
 High concentration of carbon dioxide in the blood
 Low concentration of oxygen in the blood
 Oxygen diffuses from alveoli  blood
 Carbon Dioxide diffuses from blood  alveoli
Diffusion in the Circulatory System
 Substances needed by the cells are in high
concentration in the blood  diffuse into the cell via
capillaries
 Wastes produced by the cells are in high
concentration in the cell  diffuse into the blood via
capillaries
DIFFUSION IN THE
CIRCULATORY SYSTEM
Diffusion in the Digestive System
 Digested substances are in high concentration in the
intestines  diffuse into the blood through the villi
DIFFUSION IN THE
DIGESTIVE SYSTEM –
Features which assist in diffusion.
The alveoli, capillaries and villi are all specially
suited to perform the process of diffusion.
They all have the following features:
1. They are 1µm thin (one cell thick)
2. They are all moist
3. They all have a large surface area
4. They are all vascular
Interdependence of body systems
 All of the body systems rely on each other in order to
function correctly and maintain homeostasis
 If one system fails to work, every other system will
be affected
Interdependence of plant systems