Simple and
complex organisms
Why have
transport systems?
Exchange: Getting in and out |
Simple and complex organisms
The single-celled Amoeba lives in water. It exchanges nutrients
and wastes directly through its outer cell membrane to the surrounding
fresh water. These substances move by diffusion, from areas of high concentration
to areas of lower concentration.
The Amoeba's area of cell membrane is great enough to allow quick entrance of oxygen and small nutrients to maintain life, and quick removal of wastes to avoid poisoning.
As larger, multicellular organisms evolved many cells on the inside of
the organisms were no longer in direct contact with the environment (unlike
the Amoeba). Diffusion alone was not sufficient to move the substances
quickly enough through to the internal cells. Transport systems were needed!
Bigger organisms also require more complex systems for movement (skeleton
and muscles), communication (nerves and hormones), and food breakdown
(digestive). A system is a co-ordinated group of organs and tissues with
a common function to solve a particular problem.
Why have transport systems?
Transport systems in living things bring every cell in the body in contact
with the external environment.
Any method of transporting substances directly from the outside would
aid survival.
Insects have tiny tubes (spiracles) which open through their hard covering
(exoskeleton) and carry air via a system of smaller tubes directly to
the body cells.
However, endothermic (warm-blooded) organisms need an even more efficient
method to maintain energy supply.
Internal lungs provide an enormous area of alveoli (air sacs) for
exchange of gases with the external environment. Capillaries surround
these air sacs, and gases are then transported around the body by the
blood vessels.
Similarly food can be brought inside the body and digested by the digestive system but the simple food units still need to get to all individual cells. Again, the circulatory system is close to the area of absorption and carries out this role.
Wastes produced in the cells need to be carried via the blood system to the kidneys (excretory system) for removal.
Exchange: Getting in and out
The
following factors affect how quickly the substances pass across the cell
membrane.
- The bigger the surface area (S.A.) compared to volume for exchange
the better! All exchange areas are adapted for maximum surface area,
e.g. the millions of airsacs in the lung and the enormous capillary
network.
Similarly the walls of the small intestine are highly folded into
villi to increase surface area.
The placenta too has an enormous network of capillaries, some belonging to the mother's circulatory system and others belonging to the baby. They lie close together to aid exchange between the two systems.
- The faster the blood moves at the exchange surface the better. This
keeps the concentrations high on one side and low on the other speeding
up exchange via diffusion. Good blood pressure is essential. Heart problems,
for example, often result in death through kidney failure because wastes
cannot be removed efficiently enough.
- Molecules must be small enough to diffuse easily across the capillary walls, which is why food must be digested to simple units before transportation. Large blood cells and proteins cannot pass through the cell membrane via diffusion.
The capillary walls are only one cell thick to allow for easy exchange
of nutrients and wastes.
All internal body cells are surrounded by extra-cellular fluid (ECF), rather like the watery world of the Amoeba. This means, for example, that simple sugars will pass out of the capillary (where their concentration is high) into the extra-cellular fluid, then into the cells where the sugars are continually used up for energy so that he concentration inside the cell is low. Similarly, wastes leave the cell.
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Cellular exchange |
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