Homeostasis
Why is homeostasis important for cells?
Living cells depend on the movement of chemicals around
the body. Chemicals such as oxygen, carbon dioxide and
dissolved food need to be transported into and out of cells.
This is done by the processes of diffusion and osmosis and
active transport. These processes depend on the body's
water and salt balance, which are maintained by
homeostasis.
DIFFUSION VS. OSMOSIS
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Diffusion
Diffusion
The process by which molecules spread from areas of high
concentration, to areas of low concentration. When the
molecules are even throughout a space - it is called
EQUILIBRIUM
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What is diffusion?
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Diffusion in action: one gas
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Diffusion and digestion
Carbohydrates, proteins and fats are made up of large
molecules that cannot be absorbed by the body.
Digestion breaks down large nutrient molecules into
simpler forms such as glucose, amino acids and fatty
acids that can be easily absorbed.
In which part of the digestive system is most food absorbed?
Small food molecules are usually absorbed in the small
intestine, diffusing across the intestine wall and into the
bloodstream.
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Diffusion and the small intestine
Only some food molecules are small enough to diffuse
through the villi and into the bloodstream.
Fats, fat-soluble vitamins and glucose can move into
the bloodstream by diffusion.
These small molecules
diffuse from the small
intestine into the blood,
moving from higher to
lower concentration.
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glucose
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Osmosis
Osmosis - the diffusion of water (across a membrane)
Water will move in the direction where there is a high
concentration of solute (and hence a lower concentration of
water.
A simple way to remember is:
Salt is a solute, when it is concentrated inside or outside the
cell, it will draw the water in its direction. This is also why you
get thirsty after eating something salty.
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Osmosis and animal cells
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Osmosis in action
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Osmosis and digestion
In order to remain healthy, animal cells need to maintain
homeostasis in water balance. This means that the
water concentration both inside and outside the cell are
equal.
In the large intestine water is
absorbed back in the body cells by
osmosis. This also makes the
feces solid.
Good bacteria in the large intestine produce vitamins like
Vitamin K which is also reabsorbed by osmosis.
These bacteria also produce flatus, a mixture of
nitrogen and carbon dioxide, with small amounts of the
hydrogen, methane, and hydrogen sulphide.
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Passive Transport
Diffusion and osmosis do not require energy so they are
called passive transport.
high
concentration
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low
concentration
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Active Transport
Sometimes dissolved molecules are larger or at a higher
concentration inside the cell than outside, but, because
the organism needs these molecules, they still have to
be absorbed.
Active transport is the process by
which dissolved molecules move
across a cell membrane from a
lower to a higher concentration. In
active transport, particles move
against the concentration gradient
and therefore require an input of
energy from the cell.
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Active transport and digestion
During digestion, the villi in the small intestine absorb the
soluble nutrients. Over time, the concentration of nutrients in
the villi reach an equilibrium with the concentration in the gut.
glucose
Active transport is
used to continue the
transport of the small
amounts of remaining
nutrients against the
concentration gradient.
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Active transport and digestion
Amino acids are also absorbed by active transport.
Active Transport
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What is active transport?
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Glossary
 active transport – The movement of molecules against
a concentration gradient, and which requires energy.
 concentration gradient – The difference in
concentration across a given area.
 diffusion – The movement of molecules from an area of
high concentration to low concentration.
 osmosis – The diffusion of water molecules from a dilute
solution to a more concentrated solution.
 partially-permeable membrane – A membrane that
allows only certain sized molecules to pass through it.
 villi – Tiny finger-like projections on the inner surface of the
small intestine, across which nutrients diffuse.
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Multiple-choice quiz
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