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CIE IGCSE Biology Your notes
7.1 Human Diet & Digestion
Contents
7.1.1 Diet & Deficiencies
7.1.2 Digestive System
7.1.3 Physical Digestion
7.1.4 Teeth & Digestion
7.1.5 The Stomach
7.1.6 Emulsification of Fats & Oils: Extended
7.1.7 Chemical Digestion
7.1.8 Enzymes in Digestion
7.1.9 Hydrochloric Acid
7.1.10 Digestion of Starch: Extended
7.1.11 Digestion of Protein: Extended
7.1.12 Bile: Extended
7.1.13 Absorption
7.1.14 Adaptations of the Small Intestine: Extended
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7.1.1 Diet & Deficiencies
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Balanced Diet
A balanced diet consists of all of the food groups in the correct proportions
The necessary food groups are:
Carbohydrates
Proteins
Lipids
Vitamins
Minerals
Dietary Fibre
Water
Food Groups Table
Vitamin and Mineral Requirements Table
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Varying Dietary Needs of Individuals Table
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Scurvy & Rickets
Scurvy Your notes
Scurvy is the name for a severe vitamin C deficiency
It is caused by a lack of vitamin C in the diet for over 3 months
Its symptoms include:
Anemia
Exhaustion
Spontaneous bleeding
Pain in the limbs
Swelling
Gum ulcerations
Tooth loss
It is a condition that was commonly seen in sailors between the 15th to 18th centuries
Long sea voyages made it very hard to access a ready supply of fresh produce
Scurvy can be treated with oral or intravenous vitamin C supplements
Rickets
Rickets is a condition in children characterised by poor bone development
Symtpoms include:
Bone pain
Lack of bone growth
Soft, weak bones (sometimes causing deformities)
Rickets is caused by a severe lack of vitamin D
Vitamin D is required for the absorption of calcium into the body
Calcium is a key component of bones and teeth
Vitamin D mostly comes from exposure to sunlight but it can also be found in some foods (fish, eggs
and butter)
The treatment for rickets is to increase consumption of foods containing calcium and vitamin D
Alternatively vitamin D supplements can be prescribed
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7.1.2 Digestive System
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Identifying Organs of the Digestive System
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The human digestive system
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Organs of the Digestive System: Function
Stages of food breakdown Your notes
Food taken into the body goes through 5 different stages during its passage through the alimentary
canal (the gut):
Ingestion - the taking of substances, e.g. food and drink, into the body through the mouth
Mechanical digestion - the breakdown of food into smaller pieces without chemical change to the
food molecules
Chemical digestion - the breakdown of large, insoluble molecules into small, soluble molecules
Absorption - the movement of small food molecules and ions through the wall of the intestine into
the blood
Assimilation - the movement of digested food molecules into the cells of the body where they are
used, becoming part of the cells
Egestion - the passing out of food that has not been digested or absorbed, as faeces, through the
anus
Functions of the Digestive Organs Table
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7.1.3 Physical Digestion
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Physical Digestion
Physical digestion (sometimes referred to as mechanical digestion) is the breakdown of food into
smaller pieces without chemical change to the food molecules
The processes that take place during physical digestion help to increase the surface area of food for
the action of enzymes during chemical digestion
It is mainly carried out by the chewing action of the teeth, the churning action of the stomach and the
emulsification of fats by bile in the duodenum
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7.1.4 Teeth & Digestion
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Types of Human Teeth
Mechanical digestion is the breakdown of food into smaller pieces without chemical change to the
food molecules
It is mainly carried out by the chewing action of the teeth, the churning action of the stomach and the
emulsification of fats by bile in the duodenum
Teeth are held firmly in the bone of the jaw
They are used for chewing to increase the surface area of the food so that it can be exposed to
saliva and other digestive juices and broken down more quickly
The differing shapes and sizes of teeth enable them to perform slightly different functions:
Incisors - chisel-shaped for biting and cutting
Canines - pointed for tearing, holding and biting
Premolars and molars - larger, flat surfaces with ridges at the edges for chewing and grinding up
food
Types of teeth
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Structure of a Tooth
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Structure of a typical tooth
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7.1.5 The Stomach
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The Stomach
The stomach is one of a number of organs that make up the digestive system
The role of the digestive system is to break down large insoluble molecules into smaller, soluble food
molecules to provide the body with nutrients
The stomach lining contains muscles which contract to physically squeeze and mix the food with the
strong digestive juices that are present
Also known as "stomach churning"
Food is digested within the stomach for several hours
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Three types of tissue found in the stomach are muscular, epithelial and glandular. These tissues work
together to allow the stomach to carry out its role.
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7.1.6 Emulsification of Fats & Oils: Extended
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Emulsification of Fats & Oils: Extended
Cells in the liver produce bile which is then stored in the gallbladder
Bile production and secretion
Bile has two main roles:
It is alkaline to neutralise the hydrochloric acid which comes from the stomach
The enzymes in the small intestine have a higher (more alkaline) optimum pH than those in the stomach
It breaks down large drops of fat into smaller ones. This is known as emulsification. The larger surface
area allows lipase to chemically break down the lipid into glycerol and fatty acids faster
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Exam Tip
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Emulsification is the equivalent of tearing a large piece of paper into smaller pieces of paper.This is an
example of mechanical digestion, not chemical digestion – breaking something into smaller pieces
does not break bonds or change the chemical structure of the molecules which make it up, which is
the definition of chemical digestion.
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7.1.7 Chemical Digestion
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Chemical Digestion
Stages of food breakdown
Food taken into the body goes through 5 different stages during its passage through the alimentary
canal (the gut):
Ingestion - the taking of substances, e.g. food and drink, into the body through the mouth
Mechanical digestion - the breakdown of food into smaller pieces without chemical change to the
food molecules
Chemical digestion - the breakdown of large, insoluble molecules into small, soluble molecules
Absorption - the movement of small food molecules and ions through the wall of the intestine into
the blood
Assimilation - the movement of digested food molecules into the cells of the body where they are
used, becoming part of the cells
Egestion - the passing out of food that has not been digested or absorbed, as faeces, through the
anus
The role of chemical digestion is to produce small soluble molecules that can be absorbed
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7.1.8 Enzymes in Digestion
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Enzymes in Digestion
Amylases
Amylases are produced in the mouth and the pancreas (secreted into the duodenum)
Amylases digest starch into smaller sugars
The digestion of starch
Proteases
Proteases are a group of enzymes that break down proteins into amino acids in the stomach and
small intestine (with the enzymes in the small intestine having been produced in the pancreas)
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The digestion of proteins
Lipases
Lipase enzymes are produced in the pancreas and secreted into the duodenum
They digest lipids into fatty acids and glycerol
The digestion of lipids
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7.1.9 Hydrochloric Acid
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Hydrochloric Acid
The stomach produces several fluids which together are known as gastric juice
One of the fluids produced is hydrochloric acid
This kills bacteria in food and gives an acid pH for enzymes to work in the stomach
How is a low pH helpful in the stomach?
The low pH kills bacteria in food that we have ingested as it denatures the enzymes in their cells,
meaning they cannot carry out any cell reactions to maintain life
Pepsin, produced in the stomach, is an example of an enzyme which has a very low optimum pH -
around pH 2
The hydrochloric acid produced in the stomach ensures that conditions in the stomach remain within
the optimum range for pepsin to work at its fastest rate
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7.1.10 Digestion of Starch: Extended
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Digestion of Starch: Extended
Amylases are produced in the mouth and the pancreas (secreted into the duodenum)
Amylases digest starch into smaller sugars
The digestion of starch
Amylase is secreted into the alimentary canal in the mouth and the duodenum (from the pancreas) and
digests starch to maltose (a disaccharide)
Maltose is digested by the enzyme maltase into glucose on the membranes of the epithelium lining of
the small intestine
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7.1.11 Digestion of Protein: Extended
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Digestion of Protein: Extended
Proteases are a group of enzymes that break down proteins into amino acids in the stomach and
small intestine (with the enzymes in the small intestine having been produced in the pancreas)
The digestion of proteins
Protein digestion takes place in the stomach and duodenum with two main enzymes produced:
Pepsin is produced in the stomach and breaks down protein in acidic conditions
Trypsin is produced in the pancreas and secreted into the duodenum where is breaks down
protein in alkaline conditions
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7.1.12 Bile: Extended
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Bile: Extended
Cells in the liver produce bile which is then stored in the gallbladder
Bile production and secretion
Bile has two main roles:
It is alkaline to neutralise the hydrochloric acid which comes from the stomach
The enzymes in the small intestine have a higher (more alkaline) optimum pH than those in the stomach
It breaks down large drops of fat into smaller ones. This is known as emulsification. The larger surface
area allows lipase to chemically break down the lipid into glycerol and fatty acids faster
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Exam Tip
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Emulsification is the equivalent of tearing a large piece of paper into smaller pieces of paper. This is an
example of mechanical digestion, not chemical digestion – breaking something into smaller pieces
does not break bonds or change the chemical structure of the molecules which make it up, which is
the definition of chemical digestion.
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7.1.13 Absorption
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Absorbing Nutrients
Absorption is the movement of digested food molecules from the digestive system into the blood
(glucose and amino acids) and lymph (fatty acids and glycerol)
Nutrients are absorbed in the small intestine
Absorbing Water
Water is absorbed in both the small intestine and the colon, but most absorption of water (around
80%) happens in the small intestine
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7.1.14 Adaptations of the Small Intestine: Extended
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Adaptations of the Small Intestine: Extended
The ileum is adapted for absorption as it is very long and has a highly folded surface with millions of
villi (tiny, finger like projections)
These adaptations massively increase the surface area of the ileum, allowing absorption to take place
faster and more efficiently
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Adaptations of the small intestine
Microvilli on the surface of the villus further increase surface area for faster absorption of nutrients
Wall of the villus is one cell thick meaning that there is only a short distance for absorption to happen by
diffusion and active transport
Well supplied with a network of blood capillaries that transport glucose and amino acids away from
the small intestine in the blood
Lacteal runs through the centre of the villus to transport fatty acids and glycerol away from the small
intestine in the lymph
Exam Tip
The way in which the structure of a villus is related to its function comes up frequently in exam
questions so it is worth ensuring you have learned these adaptations.
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