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Human Nutrition
(Chapter 29)
 Autotrophic means that an organism can make its own food (e.g. plants)
 Heterotrophic means that an organism cannot make its own food (e.g. animals)
There are 3 main types of heterotrophic organisms:
(1) Herbivore: an animal that feeds only on plants (e.g. cattle, sheep, deer)
(2) Carnivore: an animal that feeds on other animals (e.g. lions, tigers, seal)
(3) Omnivore: animals that feed on both plants and animals (e.g. human, bear, badger)
DIGESTION
Digestion: the mechanical and chemical breakdown of food.
Mechanical – e.g. by teeth
Chemical – e.g. by enzymes
Why do we need to digest food?
 Digestion must occur so that food can enter the cells of the body (needed for respiration).
 If digestion did not occur, these food particles would be too large to pass into the
bloodstream (absorption) and then into the cells.
What do we need a digestive system?
 A digestive system allows the materials needed to process food to be localised in a single
place, e.g. teeth in the mouth, acid in the stomach.
 If we did not have a digestive system, each cell would have to be capable of digestion and
contain digestive enzymes etc.
The Stages of Digestion:
(1) Ingestion: the taking of food into the mouth
(2) Digestion: the mechanical and chemical breakdown of food
(3) Absorption: the passage of food out of the digestive system and into the bloodstream
(this mainly happens in the small intestine)
(4) Egestion: the removal of the unabsorbed and undigested material from the digestive
system
The
Digestive
System
Salivary Gland: produces saliva. Saliva contains amylase which digests starch.
Oesophagus: Carries food from mouth to stomach by peristalsis
Stomach: food is churned and chemically digested by enzymes
Duodenum: first part of small intestine. Chemical digestion continues here.
Liver: produces bile for the digestion of fats. This bile goes to the duodenum.
Pancreas: Produces enzymes to help digestion. These enzymes go to the duodenum.
(Note: food never enters the liver or pancreas)
Ileum: digested nutrients are absorbed into the bloodstream
Colon: water is taken back into the bloodstream
MOUTH
Function: Mechanical breakdown of food by the teeth (chewing) and chemical breakdown by
amylase in saliva.
Human teeth:
Incisor: cutting or biting food
Canine: tearing food
Premolar: crushing and chewing food
Molar: crushing and chewing food
A full set of 32 human teeth contains:
2 ( I2
C1
PM 2
M3 )
2
1
2
3
Salivary glands: are located under the tongue, at the back of the jaws and in the cheeks.
Saliva contains water, salts, mucous and the enzymes amylase and lysozyme.
Amylase: breaks down starch into maltose.
Lysozyme: helps to kill microorganisms.
 Food in the mouth forms into a ball called a bolus and is pushed back into the pharynx.
 When we swallow, a flap called the epiglottis closes over the trachea to prevent food from
going down into the lungs.
OESOPHAGUS
Function: Food is carried through the oesophagus by
peristalsis.
Peristalsis: the contraction of muscles in the walls of
the alimentary canal in order to move food through the
canal
Peristalsis also helps with the mechanical breakdown
of food.
Note: Peristalsis also occurs in the small and large
intestines.
STOMACH
Function: Food is mechanically digested by being churned
and chemically by enzymes and acid.
Cardiac
sphincter
 Food enters the stomach through the cardiac sphincter
muscle which opens and closes.
 The contraction of the stomach muscles churns the food.
The lining of the stomach (mucosa) contains glands which
produce the following gastric juices:
(1) Mucous: coats the stomach and prevents self digestion
(2) Pepsinogen: an inactive enzyme. When pepsinogen is converted into pepsin by acid, it
digests proteins.
(3) Hydrochloric Acid: kills bacteria, loosens fibrous and cellular foods, activates pepsinogen
and denatures amylase
 At this stage the soupy food mixture is called chyme.
 It leaves the stomach through the pyloric sphincter muscle.
The stomach is prevented from self digestion because:
(1) Mucous lines the stomach.
(2) Pepsinogen is released in an inactive form, so active pepsin doesn’t come into contact with
the stomach cells as it is released from the glands.
(3) Mucous is alkaline (basic) and so reduces acidity near the stomach wall.
SMALL INTESTINE: DUODENUM
The first part of the small intestine is called the duodenum.
Main function: Completing chemical digestion
 The cells lining the duodenum itself produce digestive enzymes.
 Digestive juices from the liver and pancreas also arrive here to aid digestion.
 The lining of the small intestine contains many foldings called villi (singular villus).
 Intestinal glands are located between these villi, which produce digestive juices.
LIVER
Functions:
(1) making bile
(2) storage of fat-soluble vitamins
(3) detoxification – breaking down poisons such as drugs and
alcohol
(4) deamination – breaking down of excess amino acids to
form urea
(5) making cholesterol – needed to form hormones
(6) making plasma proteins – e.g. fibrinogen, used in blood clotting
(7) heat generation – to warm the blood and body
 Bile is made in the liver and stored in the gall bladder.
 Bile enters the duodenum through the bile duct.
Functions of bile/bile salts:
(1) Emulsifies fats - breaks fats down into smaller droplets.
This increases their surface area for digestion by enzymes.
(2) Contains bicarbonate which helps neutralise chime form stomach
PANCREAS
The pancreas produces a pancreatic juice which contains the
following:
(1) Amylase – breaks down starch to maltose
(2) Lipase – breaks down lipids into fatty acids and glycerol
(3) Insulin – hormone that regulates glucose metabolism
(4) Sodium bicarbonate – neutralises acidic chyme
Pancreatic juice is made in the pancreas and enters the duodenum through the pancreatic
duct.
Summary of Chemical Digestion
Substance
Made In
Active In
Amylase
Pepsin
HCl
Salivary Glands
Pancreas
Stomach Lining
Stomach Lining
Mouth
Duodenum
Stomach
Stomach
Bile Salts
Sodium
Bicarbonate
Lipase
Liver
Liver
Pancreas
Pancreas
Duodenum
Duodenum
Duodenum
Preferred
pH
7-8
Breaks Down
/Works On
Starch
Into
2
-
Proteins
Bacteria &
Fibrous Foods
Lipids
Acid
Peptides
Kills bacteria and
softens foods
Lipid Droplets
Neutralises Acid
7-8
Lipids
Fatty
Acids
Glycerol
End Products of Digestion:
Carbohydrates
Monosaccharides (e.g. glucose)
Proteins
Amino acids
Lipids
Fatty Acids & Glycerol
Maltose
&
SMALL INTESTINE: ILEUM
Food is now mostly digested.
Main function: absorption – Nutrients are absorbed into the bloodstream by diffusion.
Absorption: the movement of food across a membrane into the blood (or lymph).
Like the duodenum, the lining of the ileum contains many tiny foldings called villi.
Function of villi: to increase surface area for the absorption of nutrients into the blood
 The villi walls are only one cell thick, so that nutrients
can easily pass through them.
 Inside each villus there are capillaries.
 The capillaries absorb water and nutrients such as
glucose, amino acids, vitamins and minerals.
 Each villus also contains a lacteal.
 Lacteals contain a liquid called lymph.
 Fats are absorbed into the lacteals.
 This lymph eventually will enter the bloodstream.
 The blood capillaries carry the nutrients in the blood to
the hepatic portal vein which carries blood to the liver.
To cells, heart,
kidneys
 The liver stores some nutrients and releases others into
the blood stream.
 Deamination also occurs in the liver.
 Urea and other substances leave the liver through the
hepatic vein.
 Nutrients are carried through the bloodstream to all the
nutrient-requiring cells.
 Waste products, such as urea are transported to the
kidneys and excreted.
 Oxygenated blood from the heart enters the liver through the
hepatic artery.
From heart
From small
intestine
LARGE INTESTINE
CAECUM & APENDIX: The function of these parts of the large intestine are not known.
It is believed they are vestigial organs: they once had a function in our ancestors, but have
now lost that function.
COLON
Function: to reabsorb water.
 The waste is thus converted to a semi-solid substance called faeces.
 Feaces is stored in the rectum before being egested from the body through the anus.
Note: Feaces are not excreted from the body. Excretion is the removal of waste products of
metabolism (made in the cells. E.g. urea, CO2, water vapour)
Symbiotic Bacteria in Large Intestine:
(1) Bacteria in the Colon feed in waste and produce some B vitamins and Vitamin K. These
vitamins are absorbed back into the bloodstream
(2) Bacteria break down some food (e.g. cellusose) which can be reabsorbed.
(3) The presence of good bacteria prevents the growth of more harmful (pathogenic)
bacteria in the digestive system.
Diarrhoea: when food moves too quickly through the colon and not enough water is
reabsorbed
Constipation: when food moves too slowly and too much water is reabsorbed.
Benefits of Fibre:
Fibre contains cellulose from plant cells walls.
 Fibre stimulates peristalsis in the large intestine.
 It therefore helps the movement of food through the colon, preventing constipation.
BALANCED DIET
A balanced diet consists of the correct amounts of carbohydrates, proteins, lipids, vitamins,
minerals, fibre and water needed by the body.
A good diet should contain a variety of different foods and the food pyramid should be used
as a guide to plan a balanced diet.
The Food Pyramid
The amount of food a person requires depends on:
(1) Age (young people need more food than older people)
(2) Activity levels (more active people require more food)
(3) Gender (males need more food than females)
(4) Health (unhealthy people may require more food)