Chapter 7: Human nutrition

Diet

Diet is the food eaten in one day

Balanced diet is a diet that contains all of the required nutrients, in suitable proportions, and the
right amount of energy which varies depending on:
Age
Gender
Activities per day
Pregnancy
In a diet, humans need:
Carbohydrates
Vitamins
Proteins
Minerals
Water
Lipids
Fibres

Nutrients
Carbohydrates
Carbohydrates are needed for energy
Sources of Carbohydrates: Potato, Beetroot, bread, rice, noodles, pasta and sweet foods etc.
Fats and Oils(lipids)
Fats and Oils are needed for energy and to make cell membranes
We store excess fat and oil under the skin, in adipose tissue which acts as an insulator
This can also form a layer around body organs such as the kidneys which provides mechanical
protection
Sources of Fats: Cooking oil, butter, meat, eggs, dairy products and oily fish etc.
Proteins
Proteins are needed to build new cells, for growth and repair.
They are also used to make proteins, including haemoglobin, insulin(a hormone) and
antibodies(which help destroy pathogens)
Sources of proteins: Meat, fish, dairy products, peas and beans, nuts and seeds etc
Vitamins
Minerals

Fibre
Fibres are needed to provide bulk(roughage) for food to move through the alimentary canal.
Fibres keep the digestive system working in good order and prevent constipation
Peristalsis is the rhythmic muscular contractions that move food through the alimentary canal
which is stimulated mostly by harder, less digestible food e.g fibre
Sources of fibres: Fruits, vegetables, cereal grains such as oats, wheat and barley, whole grains
etc.
Water
More than 60% of the human body is water
Water is an important solvent
Water are needed for metabolic/chemical reactions to take place in cells
We get rid of waste products because of water through urine which contains urea dissolved in
water
We transport substances throughout the body because of water through the blood which
contains many substances dissolved in water
The Human Digestive System
The digestive system is a group of organs that break down the food that we eat into smaller,
simpler substances so that it can be absorbed into the blood and delivered to the cells.
The digestive system includes the alimentary canal, the liver and the pancreas.
The processes of the digestive system
Ingestion- the process of taking food and drink into the mouth. Ingestion uses the lips, teeth and
tongue.
Digestion- The process of breaking down large pieces of food into smaller pieces of food
(Physical Digestion) and breaking large molecules into smaller, simpler molecules (Chemical
Digestion).
Absorption- The movement of small nutrient molecules and mineral ions through the wall of the
intestines into the blood.
Assimilation- The process of the cell absorbing and using the small nutrient molecules or ions.
Egestion- The process of expelling waste products that the body cannot digest out of the
intestine.
The alimentary canal
The alimentary canal is a long tube that runs from the mouth to the anus.
The muscles on the walls of the alimentary canal contract and relaxes to make the food move
down the tube (peristalsis).
There are muscles along the alimentary canal that closes and opens according to the body's
needs aka keeping the food in one place before it can move along further. These muscles are
called sphincter muscles.
In order for the food to move more easily, The body lubricates the food with mucus. Mucus is
made in goblet cells all throughout the alimentary canal.
The alimentary canal consists of the mouth, the oesophagus, the stomach, the small and large
intestine.
The mouth
The mouth is where ingestion happens using the teeth, lips and tongue.
The lips help in holding the food in the mouth.
The teeth bite and grind the food into small pieces(increasing its surface area).
The tongue mixes the food with saliva and rolls it into a small ball.
The salivary glands produces saliva which is a mixture of:
Water: to dissolve the substances in the food and let us taste it.
Mucus: to help the food bind into a ball and lubricate the food at the same time.
Amylase: to begin the digestion of starch.
Structure of teeth
The teeth is made out of:
Enamel- Is the hardest substance made by animals but is susceptible to acids. (Sweet foods on
the enamel are feasted on by bacteria producing acid. This is why you need to brush your teeth
in order to neutralise the acid.)
Dentine- Are hard (but not as hard as enamels). It has channels that contain living cytoplasm.
There are nerves and blood vessels in the middle of the dentine called the pulp cavity that
supply the cytoplasm with nutrients and oxygen.
Cement- The part of the tooth that is embedded in the gum. Contains fibre that connects to the
jawbone. Allows the teeth to move slightly when chewing or biting.

Type of teeth:
Incisors: are the chisel-shaped teeth in front of the mouth, used for biting pieces off food.
Canines: Pointed teeth in both sides of the incisors, used for helping to tear and cut food.
Premolars and molars: are the large teeth at the back of the mouth. Molars are larger and have
more broadened, rigid surfaces than Premolars. They are used to chew the food.
The oesophagus

There are two tubes leading down from the back of the mouth, the trachea (your windpipe) and
the oesophagus.
The space in the middle of any tube of the body is called a lumen. The lumen separates your
trachea from your oesophagus. It is also present in all parts of the alimentary canal including the
blood vessels.
The entrance between the stomach and oesophagus is guarded by a sphincter muscle. It opens
to let the food in and closes so the food won’t get out.

The stomach
The stomach has strong and muscular walls that contract and relax to mix the food with
enzymes and mucus.
The stomach contains cells that makes
Mucus- lubricates the food
Enzymes- Especially proteases. Proteases are enzymes that digest protein.
Hydrochloric acid (HCl)- Produces low pH (pH=2) which kills most microorganisms and helps
achieve optimum pH for the protease enzymes.
The stomach can store the food for one to two hours. After that, It releases the partly digested
food into the small intestine (duodenum).

The small intestine

The small intestine is the part between the stomach and the alimentary canal.
The first part of the small intestine near the stomach is called the duodenum. The second part
near the large intestine is called the ileum.
The pancreatic duct is a tube connecting the deodenum with the pancreas.
A fluid called the pancreatic juice that contains many different enzymes, flows through this duct
into the duodenum to continue the chemical digestion of the food.
The ileum is the place where absorption of the nutrients from the food takes place.

The large intestine

The large intestine is the final part of the alimentary canal.
It is made up of three parts:
Colon- where the remaining water is absorbed from the food into the blood.
Rectum- stores the undigested food called the faeces.
Anus- egests the faeces out of the body.

The Pancreas and the Liver

Even though they are part of the digestive system, They are not part of the alimentary canal.
The pancreas is a cream-coloured gland under the stomach.
The pancreas secretes a fluid called pancreatic juice. This juice contains many enzymes to
digest the food. It is sent from the pancreas to the small intestine(duodenum) through the
pancreatic duct.
The liver is a large organ that does many things but we will be only discussing its digestive
function. The liver secretes a fluid called the bile in the gall bladder. The bile flows along the bile
duct into the duodenum. The bile is a yellowish-green, alkaline watery liquid and has two
purposes:
To neutralise the acid from the stomach
To break down large drops of fat into smaller drops of fat. This is called emulsification.
Types of Digestion
Physical Digestion- turn the large pieces of the food into smaller pieces of the same food. This
is mostly done in the teeth and the churning movements from the stomach. Physical digestion
does not change the chemical components of the food. Emulsification is considered a physical
digestion process because it does not change the chemical components of the fat, only breaks it
into smaller pieces.
Chemical Digestion- breaks the large molecules of the food into smaller, simpler molecules. This
involves chemical reactions catalysed by enzymes. These smaller molecules are easier to be
absorbed into cells.

Absorption and Assimilation

When fully digested:
Carbohydrates->Maltose->Glucose (by amylase and maltase)
Fats->Fatty acids and glycerol (by lipase)
Proteins->Amino acids (by a type of protease called pepsin and trypsin where pepsin is the
protease in the stomach and trypsin is from the pancreas)
Others such as mineral ions, vitamins and water are already small enough to be absorbed into
the small intestine without being digested.
Villi
The walls of the small intestines are covered with tiny projections called villi. Each being 1 mm
long.
The cell membrane on the surface of the villus is folded to form microvilli. Maltase acts on these
microvilli to break down glucose. Microvilli are also responsible for absorbing glucose, amino
acids, glycerol,etc. Microvilli increases the surface area of the villi, decreasing the time for full
absorption.
Most of these substances pass from the villi to the blood capillaries and join up with the hepatic
portal vein which takes everything to the liver.
The liver absorbs these substances and assimilates them by:
Glucose-> Glycogen for storage
Amino acids->Protein If there is excess it will be turned to urea which is excreted from the body
through urine.
Fatty acid and Glycerol is deposited to blood vessels called lacteals. The lacteals will then
empty the fatty acids and ​ ​ glycerols into the blood.