14/8/2011
Biology revision
Topic:Digestion

Why do we eat?
Our diet is made up of food and drink we consume everyday.

Reasons for eating:

• Grow new cells and tissues
• Repair damaged tissues
• Provide energy for work,movement and to maintain body temperature
• Maintain body functions such as breathing and keeping the heart beating

The substances in our diet that keep us healthy are called nutrients.They comprise of carbohydrates,proteins and fats.

What are Carbohydrates
Definition:
Carbohydrates provide body cells with the energy they need.

Carbon,hydrogen and oxygen make up carbohydrates.Foods rich in carbohydrates are starch and sugars.
Examples:Bread,rice,beans,pasta,potatoes(starch);apples,bananas,grapes,raisins,chocolates and soft drinks(sugars)

Excess carbohydrates in our body turns into fats and stored.Our fats are used for providing energy only if our carbohydrates intake is low.

What are Proteins
Definition:
Proteins are the main substances used for growth of new cells and the repair of damaged tissues.They also provide energy when the body lacks carbohydrates and fats.

Carbon,hydrogen,oxygen and nitrogen(and sulfur) make up proteins.
Examples:Fish,meat,eggs,yoghurt,cheese,beans

• Proteins are used to replace damaged cells,repair and growth of damaged tissues.
• Forms structures of cells
• Form enzymes
• Strengthens teeth and bones
• Source of energy if body do not get enough carbohydrates and fats

What are Fats
Definition:
Fats are important sources of energy that can provide twice as much energy as carbohydrates.

Carbon,hydrogen and oxygen make up fats
Examples:Butter,margarine,milk,cheese,nuts and fatty parts of meat

• Sources of energy once carbohydrates have been depleted
• Provides insulation against the cold(keep warm)
• Cushion our internal organs
• Part of cell membrane

Why Digestion
Digestion is the process whereby large,insoluble food molecules are broken down into smaller,soluble molecules in the body.

The smaller molecules can then by absorbed into cells where they are used for growth,repair and for respiration to provide energy.

How are the large molecules changed into smaller molecules?
The body produces special substances called digestive enzymes to digest food.These enzymes cut up the large molecules into smaller molecules and speed up the process of digestion without being used up.

Complex,large food molecules-->digestion--->simpler,smaller food molecules

Each enzymes digests one type of food only

• Carbohyrates--Carbohydrase-->Glucose
• Proteins--Protease--> Amino Acids
• Fats--Lipase-->Fatty Acids and Glycerol
• Vitamins & Mineral & Water---------->Absorbed into blood without digestion

The Human Digestive System
The digestive system comprises two major parts:Organs of the alimentary canal and Organs joined to the alimentary canal.

Organs of the alimentary canal :Mouth,oesophagus,stomach,small intestine,large intestine,anus
Organs joined to the alimentary canal:Liver,gall bladder,pancreas

What happens once food enters the digestive system
In the mouth

• Food stimulates our teeth to chew
• Food is broken down into smaller pieces-increase its surface area for enzymes activity
• Food also stimulates salivary glands to secrete saliva
• Saliva softens food
• Contains enzyme carbohydrase(Salivary amylase) to digest starch to maltose

The proteins and fats have not been digested chemically yet.The partly digested food rolls into small balls and is swallowed into the oesophagus.

Oesophagus

• No digestion of food here
• Elastic,muscular walls to push food down to stomach by the process peristalsis

In the stomach

• Food mixes with the gastric juice secreted by the stomach walls when the stomach churns
• Gastric juice contains hydrochloric acid and enzyme protease
• Hydrochloric acid makes the stomach acidic and kills bacteria in food
• Enzyme protease digest complex proteins into simpler proteins
• No digestion of carbohydrates and fats in the stomach

In the small intestine
3 digestive juices present
Bile(emulsification)

• Greenish liquid made in liver
• Released in small intestine
• Functions to break fats into small droplets to make them easier to digest
• (Increase surface area)

Pancreatic juice

• Made in pancrease
• Flows into the small intestine
• Contains digestive enzymes carbohydrase,protease and lipase

Intestinal juice

• Produced in the walls of small intestine
• Also Contains digestive enzymes carbohyrase,protease,lipase

The digestive enzymes present digest fats and complete the digestion of maltose and simpler proteins.

• Maltose--Carbohydrase in pancreatic juice and intestinal juice->Glucose
• Simpler proteins--Protease  in pancreatic juice and intestinal juice->Amino acids
• Fats--Lipase  in pancreatic juice and intestinal juice->Fatty acids and glycerol

In the small intestine

• Digestion of food completes
• Large complex molecules digested to simple soluble end products
• Food stays in the small intestine for about 5 hours
• Digested food pases through the walls of the small intestines and absorbed into the blood
• Water and indigestible food move to the large intestine

In the Large intestines

• Only food that cannot be digested reaches the large intestines
• As food passes through,a lot of water is removed and reabsorbed into the body
• Undigested food passes out of the body through the anus as faeces

How does Food pass through the digestive system
Digested food is made up of:

• Small soluble molecules-absorbed through the walls of small intestines
• Enters the large intestines and removed through the anus as faeces

Making absorption easier

• The finger-like parts,called villi,provide a large surface area for quicker absorption of digested food into the blood

Structure of small intestine that make absorption easier

• Long 7 meters-Increase the surface area for quicker absorption of food and more time for the absorption of food
• Wall only 1-celled thick-Allows digested food to pass through easily and quickly
• Many blood capillaries-Carry digested food away quickly from the small intestines