The digestive system

Chapter 4: “Food and digestion”

Page 42
Your body is made up of chemicals. You get these chemicals from the food you eat. Your body needs this for:
• For energy
 You need food to work your muscles and other organs. Your food is the fuel that keeps you going. Your body won’t work without food.
• For growth and repair
 As you grow you make new cells. You also need to replace old or damaged cells. You make new cells from chemicals in your food.
• To stay healthy
 Lots of reactions take place in the cells of your body. Chemicals in your food are needed for these reactions.
So nutrition means getting food to provide energy and substances needed for growth and repair.
In our food are the nutrition’s:
• Proteins
• Vitamins
• Carbohydrates
• Minerals
• Fats
• Fibre
• Water

If we want to stay healthy our body must have:
• Enough food
• A variety of food – so that our body gets all the different things it needs.
• Enough water
We need a healthy balanced diet.

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Your body is made up of billions of cells. These cells are made mainly from proteins.
When you grow your body needs protein to make new cells. Your body may need to replaced damaged or old cells. For this you also need protein.
Proteins are made up of amino acids. There are 20 different types of amino acids.
Haemoglobin, enzymes, antibodies and some hormones are soluble proteins. Keratin is a fibrous protein found in the skin and hair.

Page 44
Sugars are carbohydrates.
Sugar for in your tea is called sucrose
Sugar in you milk is called lactose
Sugar that our body uses the most is glucose
Starch is also a carbohydrate. This is a large molecule. It is made up of lots of smaller glucose molecules joined together.
Glycogen is a large carbohydrate just like starch. This is also made up of glucose molecules joined together. We store glycogen in our liver and muscles. Our bodies change glycogen to glucose when we need it.

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Different types of fats. Fats are made up of three different fatty acids joined together. There are different kind of fatty acids.
Fats give us energy. It contains more energy than carbohydrates. Our body uses fats as an energy store. We store fat under the skin and around the heart and kidneys. When we are short of energy our body uses this.
Fats are good insulators. They cut down heat loss.
Fats also give buoyancy.  Be able to float on water.

We need:  Proteins for growth and repair of cells.
  Carbohydrates for energy
  Fats as a store of energy and for warmth.

There are two types of main fat:
• Saturated fat (from animals)
• Unsaturated fat (from plants)
Saturated fats increase the level of cholesterol in our blood.
Cholesterol is a chemical made in liver and found in the blood. The amount of cholesterol that we produce depends on your diet, but also inherited factors. High levels of cholesterol are linked to an increased risk of heart disease and an increase in blood pressure due to a narrowing of the blood vessels. 

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Food gives us energy. We need this for all the activities that we do (even lying down!!!!). Carbohydrates and fats are high- energy foods.
Energy in food is measured in kilojoules (kJ). 1 kilojoule= 1000 joules.
We can measure the amount of energy in some foods by burning it. As the food burns it gives out energy. We can use this energy to heat up some water. The hotter the water gets the more energy is in the food.

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Most foods have their energy content on the label. The amount of energy is given in kilojoules per 100g of food.
The amount of food that you need depends on how much energy you use up every day.
The amount of energy you need depends on:
• Your body size
• How active you are in a day
• How fast you grow

Page 48
The food you eat in a day should provide you with enough energy to get through the day.
When you’re lying on your bed you still use a lot of energy to keep:
• Your heart beating
• Your lungs breathing
• Your body temperature constant
• Keeping all the chemical reactions in your body going.
This “ticking over” speed at which our bodies works is the basal metabolic rate (BMR).
When you eat more food than you need, your body stores the extra as fat.
Your energy intake is the amount of energy you get in your food in a day.
Your energy output if our energy intake is greater than our energy output, we put on weight.

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In countries like Britain and the USA more people suffer from overeating than from eating too little. But when people eat too much they become obese. Being too fat
Major causes of obesity include:
• High intake of fatty foods and refined foods containing a lot of added sugar. Junk food
• Too little exercise
• Social and emotional stress, leading to “comfort eating” eating because you’re feeling sad for example.
There are 2 ways in which people can identify being obese:
• Being 20% above the recommended weight for his/ her height
• Having a body mass index (BMI) greater than 30.
 
Body mass index=

A person with a BMI of <20 is underweight, between 25 and 30 is overweight, and >30 is obese. Between 20 and 24 is perfect.
Waistband measurements are easier to calculate than BMI.
Overweight people are much more likely than thinner people to have the following health problems:
• Heart disease
• High blood pressure
• Diabetes (high blood sugar)
• Arthritis (worn joints)

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A few people eat so little that they suffer from anorexia. They worry so much about fat that their body mass drops dangerously low. They keep eating because there is a “voice” in their head that says they are fat
Most people that suffer from this are between the ages 15 and 25. They have health problems like:
• Pale, papery skin
• Reduced resistance to infection getting ill quickly
• Irregular periods
Vegetarian don’t eat meat. But you can’t just leave the meat on the side of you plate. They have to replace it by other forms of proteins, like cereals, seeds or nuts.
There are different kind of vegetarians. Some don’t eat meat but they do eat fish. Others no meat or fish but dairy products.
Vegans don’t eat animal products at all. No milk, no meat, no fish.

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You need small, regular amounts of vitamins and minerals. Vitamins and minerals are essential for good health. They must be present in our balanced diet. When we don’t have them we get ill.
Name Rich (food) sources Use in body Deficiency disease
Vit. A Carrots, milk, butter, liver Good eyesight, healthy skin Sore eyes, poor night vision, unhealthy skin
Vit. B1 Yeast, cereals, beans, egg yolk (ei geel)  Healthy nerves, growth Beri- beri (retarted growth) Not normal growth
Vit. C Citrus fruits Tissue repair, resistance to disease Scurvy (bleeding gum)
Vit. D Fish oil, milk, butter, made by body in sunlight Strong bones and teeth Rickets (soft bones)
Iron Liver, meat, cocoa Healthy red blood cells Anaemia (bloed tekort)
Calcium Milk, green vegetables Strong bones and teeth Rickets (soft bones)
Iodine Fish, iodised salt tablets Thyroid gland Goitre (enlarged thyroid)

Page 52
Your body consists of 2/3 out of water. You take in water when you eat and drink. Without food you could live for a few weeks. But you can only survive a few days without water.
Why do we need water?
• In our cells, chemical reactions take place in water
• Waste chemicals are passed out of our bodies in water
• Our blood transports substances dissolved in water
• Water in our sweat cools us down
Dietary fibre or roughage comes from plants. It is mainly cellulose from plant cell walls. Although it can’t be digested, it is an important part of your diet.
Fibre is important because:
• Fibre adds bulk to our food.
• Fibre absorbs poisonous wastes from digesting foods.
• It prevents constipation
• Many doctors believe that a high- fibre diet lowers the level of cholesterol in the blood. Fibre reduces the risk of heart disease and bowel cancer.

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When some foods are manufactured, chemicals are put in them. These chemicals are called food additives.
They are added because:
• Preservatives keep the food fresh
• Flavourings replace the flavour of the food that is often lost when it is processed.
• Colourings make the food look more attractive and appetising.
Some people avoid eating food additives because they think that they are harmful. Some may cause asthma and headaches. Others are blamed for over-activity in children. There is evidence that some additives destroy vitamins.

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Before your body can use the food you have eaten it must be broken up into very small molecules.
Large food molecules like proteins must be broken down into small amino acids. Proteins to amino acids
Large carbohydrate molecules like starch must be broken down into small sugar molecules. Carbohydrate to sugar
Large fat molecules must be broken down into small fatty acids. Fats to fatty acids

The breakdown of large food molecules into small food molecules is called digestion.
When the food has been digested, it is absorbed through the wall of the gut (digestive system) into the blood. But before they can go through they must be dissolved.
Large food molecules are insoluble: they will not dissolve.
Large food molecules can’t get through the gut wall.
Small food molecules are soluble: they will dissolve.
Small food molecules can get through the gut wall.

Enzymes are important chemicals. They help digestion by breaking down large food molecules into small food molecules.
The three main kind of enzymes in your gut:
• A protein molecule is made up of many different amino acids – protease breaks down protein molecules
• A starch molecule is made up of many glucose molecules – carbohydrase breaks down carbohydrate molecules
• A fat molecule is made up of fatty acids and glycerol molecules – lipase breaks down fat molecules

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Your gut (digestive system) is about 9 metres long. It starts at your mouth and ends at your anus. But your gut isn’t just a simple tube. There are many different parts.
Ingestion is the process of taking food and drinks into the body through the mouth.
The chewed food is mixed with saliva. Your salivary glands make saliva.
• Saliva contains a carbohydrase enzyme called amylase. This starts to digest starch to sugar.
• Mucus is a slimy substance in saliva. It helps the food slip down your throat.
Swallowing is a reflex- it happens without you noticing.

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The (o)esophagus (also known as gullet) passes the food down to your stomach.
The esophagus has circular muscles in its wall. These muscles contract and squeeze in behind the food to push it down.
In front of the food the muscles relax. This means it is pushed down
This way of moving food down your gut is called peristalsis.
Your stomach is a muscular bag that will hold up to 2 litres of food. When your food reaches your stomach it:
• The stomach makes digestive juices. These contain proteases which start the digestion of proteins to amino acids.
• The juices also contain hydrochloric acid (can burn a hole through the table, so sour is it). This is because stomach protease works best in an acid pH.
• Babies make the enzyme rennin. It makes milk solid so it stays in the stomach longer. The milk can then be digested.
• The muscular walls churn up the food making sure that it is mixed up well with the juices.
The stomach acid also kills germs.

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Your small intestine itself is about 6 meters long. The liquid food is squeezed gently along it. 3 important liquids are added to the food:
• Pancreatic juice (made by the pancreas) contains carbohydrases, proteases and lipases. These enzymes carry on digesting the food.
• Bile enters the small intestine from the bile duct. Bile is made by the liver and stored in the gallbladder. It has 2 important functions:
1. Bile is alkaline and neutralises acid which was added to the food in the stomach. This gives the best pH for enzymes in the small intestine to work.
2. Bile emulsifies fats (breaks down large drops of fats into small drops). This increases the surface area of fats for lipase enzymes to act upon.
• Intestinal juice is made by glands in the wall of the small intestine. It also has carbohydrases, proteases and lipases in it. These enzymes complete the digestion of the food:
Starch  carbohydrases  sugar
Proteins  proteases  amino acids
Fats  lipases  fatty acids and glycerol.

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The small intestine has another important job apart from digestion. Digested food has to pass through the wall into the blood.
The small intestine is well designed for absorption. It has:
• A thin lining
• A good blood supply
• A very large surface area
The surface area of the small intestine is about 9 m2. How does this large surface area fit into such a small space?
• It is very long (6 meters!!!)
• It has a folded inner lining
• It has millions of tiny, finger- like processes called villi
All this means that the digested food can pass easily through into the blood vessels.
By the time your food comes into the large intestine there’s not much useful food left. It’s mainly fibre, dead cells, bacteria and water. As it passes along some of the water is absorbed into the blood.
The solid waste (faeces) poop are stored in the rectum. The passing out of indigestible food through the anus, in the form of faeces, is called egestion.
Normally it takes 24-48 hours for food to pass through the whole body.