Biology GCSE topic 2 revision notes (includes triple/higher tier).
Includes drawings + diagrams + required practicals.
Subtopics included :
The digestive system
Digestive enzymes
Bile
Effect of temp and pH on enzyme activity
Required practical 4
Absorption in the small intestine
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Muscle cells can contract (get shorter). They contain special protein
fibres that can change their length. Muscle cells also contain a lot of
mitochondria which provide the energy needed for contractions
Definitions :
A Tissue = A group of cells with a similar structure or function.
An Organ = A group of tissues working together for a specific function
(e.g. the stomach). The stomach contains muscle tissue and glandular
tissue (which release enzymes).
Organ systems = groups of organs which work together to form organisms.
The digestive system :
Food contains three main nutrients - carbohydrates (starch), protein and
lipids (fats).
These are all large molecules, they are too large to be absorbed into the
bloodstream so they have to be digested.
During digestion large food molecules are broken down into small molecules
by enzymes. The small molecules can then by absorbed into the bloodstream.
1. Food is chewed in the mouth. Enzymes in the
saliva begin to digest the starch into smaller
sugar molecules.
2. The food then passes down the oesophagus into
the stomach. In the stomach, enzymes begin the
digestion of proteins. The stomach contains HCL
which helps the enzymes digest proteins. The food
spends several hours in the stomach. The churning
action of the stomach muscles turns the food into a
fluid increasing the surface area for enzymes to
digest.
3. The fluid passes into the small intestine.
Chemicals are released into the small intestine
from the liver and pancreas. The pancreas releases
enzymes which continue the digestion of starch and
protein. They also start the digestion of lipids.
The liver releases bile which helps speed up the
digestion of lipids. It neutralises the acid
released from the stomach. The walls of the small
intestine release enzymes to continue the digestion
of protein and lipids. In the small intestine the
small food molecules produced by digestion are
absorbed into the bloodstream either by diffusion
or active transport.
Now the fluid travels through the large intestine,
where water is absorbed into the bloodstream and
,the faeces is released from the body.
Summary :
In the digestive system, large food molecules are digested into smaller
molecules and then the products of digestion are absorbed into the
bloodstream.
Now the products of digestion are used by the body to build new
carbohydrates, lipids and proteins.
Some of the glucose produced is used in respiration.
, Digestive enzymes :
Enzymes catalyse (speed up) chemical reactions.
Enzymes are large protein molecules and they have a groove on their
surface called the active site.
The active site is where the substrate attaches to.
substrate
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The enzyme now breaks
The substrate fits down the substrate
perfectly into the into the products.
active site.
This substrate does not fit into the active
site. The enzyme cannot break down this
substrate. Enzymes are specific because the
substrate must fit into the active site
perfectly (the lock and key theory).
Enzymes in the digestive system :
• Proteins are broken down by enzymes called proteases, these are found in
the stomach, pancreas and small intestine.
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Proteins are long chains PE
of chemicals
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amino acids. When we digest
proteins, the protease enzymes convert the protein back to the individual
amino acids, which are then absorbed into the bloodstream.
When the amino acids are absorbed by the body cells, they are joined
together in a different order to make human proteins.
• Starch consists of a chain of glucose molecules.
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Carbohydrates are pay
broken down by enzymes called carbohydrases. In the case
of starch this is called amylase. When carbohydrates like starch are
digested we produce simple sugars. Amylase is found in the saliva and
pancreatic fluid.
• A lipid molecule consists of a molecule of glycerol attached to three
molecules of fatty acids.
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Lipid molecules are digested by the enzyme lipase. This produces glycerol
and fatty acids. Lipase is found in the pancreatic fluid and small
intestine.
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