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Unit 7 Working with Complex Food Systems
Digestion and Metabolism
Semipermeable membranes and osmosis play a role in the way
your body uses the nutrients from food. Food scientists need to
understand how the human body uses food. This knowledge helps
them maintain a healthy and nutritious food supply.
Before your body can use the food you eat, the food must be
separated into nutrient parts. Digestion is the process of breaking down
food into substances that are usable by the body cells. See 23-10.
Digestion begins in the mouth where chewing grinds the food into
small pieces. The ground food and saliva form a solution. One of
the components of saliva is the digestive enzyme amylase. Chewing
distributes the amylase throughout the food so breakdown of starches
into sugars can begin.
From the mouth, food travels to the stomach. The stomach’s
churning action mixes the food with hydrochloric acid and proteases.
The acid denatures the protein molecules. This opens up the molecular
structure. The proteases can then begin breaking the peptide bonds
and separating the amino acids.
Cell Walls Versus
Cell Membranes
Plants have cell walls and
animals have cell membranes.
Cell walls and membranes react
differently to osmotic pressure.
When the osmotic pressure is higher
outside a cell wall, water will move into the
cell until it becomes turgid. Turgid means
swollen and hard. Other pressures from
within the cell stop any more water from
entering the cell. This is true even if the
osmotic pressure is still higher outside the
cell. Turgidity is what gives plants a crisp,
fi rm texture that holds the plant upright.
If plant cells are placed in a sugar solution,
the osmotic pressure becomes greater inside
the cell. A salt solution would have the same
effect. This is because the sugar or salt solution
has a high concentration as compared to the
fl uids in the cells. Water moves through the
semipermeable cell wall from the weak
solution inside the cells to the strong
solution outside. The cell walls
become fl accid, or limp, because
of loss of water. The plant develops
a wilted look. If noticed soon enough,
wilting can be reversed by increasing the
water content in the environment around
the plant.
Animal cells have only a thin membrane.
If animal cells have a higher osmotic pressure
outside the membrane, the cell will swell
with water until it bursts. It is also possible
when the reverse pressure exists to dehydrate
an animal cell to the point of death. One of
the problems with high-sugar and high-salt
diets is that fl uids surrounding the cells
develop a low osmotic pressure. The result
is that cells dehydrate and become stressed.
Animal cells need the osmotic pressure of
fl uids around the cells to equal the pressure
of fl uid in the cells.
STEM