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All living organisms need nutrients, which provide materials and energy for survival. These nutrients need to be broken down before they can be used by cells. Plants make their own food using energy from the sun.

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Muscles in the organs of the digestive system also aid in mechanical digestion by pushing and squeezing food through a series of contractions as it moves through the body.

Chemical digestion occurs when the chemicals in juices such as saliva or stomach juice act on food. These chemicals are called enzymes. They break large food molecules down into simpler molecules that are water soluble so that they can enter the bloodstream. The molecules need to be small enough to pass through the cell membranes lining the intestines.

The human digestive systemThe first process your food has to undergo after it enters your mouth is mechanical digestion. As your teeth mechanically break down your food, saliva is added from your salivary glands. Saliva contains amylase, which is an enzyme that begins the process of chemical digestion by starting to break down the starch in food. After your food has been broken into small enough pieces to swallow and combined with your saliva, your tongue will roll the food into a bolus (a small ball of food made smooth by water and mucus) to the back of your mouth where it can be swallowed.

After leaving your mouth food passes through the oesophagus—this is the tube that connects the mouth to your stomach, passing your epiglottis on the way. You can see this in Figure 4.2.3. The epiglottis is a small flap of tissue that covers your trachea (windpipe) as food passes down your oesophagus so that your food doesn’t choke you. Muscle contractions then move the food down to your stomach. These contractions are like a series of waves and enable you to swallow food in any position, even upside down! These involuntary muscle contractions are called peristalsis.

DigestionDigestion is the process of breaking the food you eat down into simpler substances. For food to be used for energy, growth and repair by your cells it must be broken down so that it can be carried by the bloodstream. Your digestive system is a collection of organs and tissues where food undergoes these chemical and physical changes. The main digestive tract, called the alimentary canal, begins at the mouth and ends at the anus. There are other associated organs attached to the alimentary canal that provide chemicals to break down food.

There are two types of digestion: mechanical digestion and chemical digestion. Mechanical digestion is a physical process in which your teeth chew, grind, crush and tear food into smaller pieces.

Animals, however, need to eat plants or other animals that have eaten plants to obtain the nutrients they require for survival.

Fig 4.2.1 A meal may sit in the digestive tract of a snake for weeks while it is being digested. Some snakes from the python family may feed only once per year.

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After passing through the oesophagus food enters the stomach, where it will stay for about four hours while muscles churn the food into a soup-like consistency. At this stage it is called chyme. The stomach lining has many folds so that it can expand with every mouthful. Different enzymes are also added to begin the process of chemical digestion of protein.

Food entering and exiting the stomach is controlled by muscles called sphincters. These muscles protect the digestive tract above from rising acid and stomach contents. The sphincter muscle at the bottom of the stomach protects the next part of the stomach from acid, and allows some partly digested food (chyme) to move through to the intestines. The stomach is lined with mucus to prevent the acids and enzymes from attacking the stomach lining. Acid also kills any harmful bacteria that may enter your digestive tract.

The liver, gall bladder and pancreas all produce chemicals to aid digestion. You can see them in Figure 4.2.2. The liver makes bile, which acts a bit like a detergent and helps fats mix with the digestive juices. Bile is stored in the gall bladder. It moves to your intestines after a message from your brain tells it to do so.

The pancreas produces pancreatic juice to neutralise stomach acid. The pancreas also produces very important enzymes that break down proteins, fats and carbohydrates.

The duodenum is the start of the small intestine, which then connects to the ileum, the longer part of the small intestine. Like in the stomach, enzymes are added to continue the process of digestion while muscles churn food to break it down enough to move through the walls of the intestine so it can be absorbed into the bloodstream. The walls of the small intestine are covered in small ‘finger-like’ bumps called villi. These villi increase the surface area of the lining of the small intestine and, as they contain tiny blood vessels, enable more nutrients to be absorbed into the bloodstream as peristalsis pushes food by. The large surface area allows digested materials to pass rapidly through the cell membranes into the bloodstream. The blood then carries the nutrients to all of the cells of the body.

The large intestine is made up of five

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salivary glands

liver

gall bladder

appendix

anus

pancreas

stomach

oesophagus

mouth

small intestine

large intestine

Fig 4.2.2 The human digestive system

Food and water are kept moving through the oesophagus by peristalsis, which is the contraction and relaxation of muscles.

oesophagus

circular musclescontracted

circular musclesrelaxed food mass

Fig 4.2.3

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The digestive systems of other animalsAnimals of different species have different diets and therefore have different digestive systems. Animals that eat only plants need bacteria to help them to digest the cellulose in the plant matter. Cows and sheep have a four-part stomach. After chewing and swallowing the food once, they regurgitate it and chew it again before swallowing. The food is then moved to another part of the stomach, where bacteria do most of the chemical digestion.

Some other animals, like snakes, swallow their food whole. Their jaws can become unhinged so they can swallow animals which may look too large to enter their jaws. The snake has very strong digestive juices that can digest bone, teeth and fur. Animals such as worms, which live off decomposing material, have strong muscular walls in their digestive tract to grind food. Sand consumed with food also helps to grind up plant matter. Digested matter is absorbed into the bloodstream and wastes are passed out the anus.

different sections—caecum, appendix, colon, rectum and anus. All the undigested material passes into the large intestine, where water, salts and vitamins are absorbed to be used by your body. Your faeces are then formed, ready to be expelled from the body through the anus. The smell of your faeces comes from intestinal bacteria that make up about one-third of your faeces. They help to break down fibre and decrease the amount of faeces, and some help to make the vitamins we need.

Homework book 4.2 Digestive system

BurpsA burp—sometimes called

a belch—is nothing but gas. When you eat or drink, you don’t just swallow food or liquid. You also swallow air at the same time. The air we breathe contains

gases such as nitrogen and oxygen. Sometimes when you swallow these gases, they need to get

out. That’s where burping comes in! Extra gas is

forced out of the stomach, up through the oesophagus and out of the mouth as a burp. You may also hear other animals burp, such

as dogs or cows.

The cow has a four-part stomach that contains bacteria to break down cellulose. Fig 4.2.5

Digestion and respirationDigestion and respiration

AntibioticsWhen you take antibiotics

to kill bacteria in your body, the bacteria in your large intestine are also killed. Since they make important vitamins for

your health you may suffer vitamin deficiency after

taking antibiotics for a long period of time.

Fig 4.2.4 Villi in the small intestine increase the surface area through which digested food can enter the bloodstream.

Launch your lunchEaten too much chocolate this

year? When you eat or drink too much of the wrong type of food,

messages from your stomach travel to your brain. The brain then sends messages back to

the muscle walls of the stomach, telling it to forcefully eject

the contents through reverse peristalsis. Vomiting may also be caused by the stomach wall being irritated by infection or

chemical irritation. You may also vomit if you experience extreme

pain or stress.

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glucose is broken down into lactic acid and water, and the reaction produces a small amount of energy. Oxygen is not involved in this type of respiration, so it is called anaerobic respiration. This reaction can be expressed by the following word equation:

glucose ! carbon dioxide + lactic acid + energy

Fig 4.2.6 Aerobic respiration occurs in the mitochondria of cells. As energy is released it is transported to sites where it is needed.

RespirationAll living organisms, from bacteria to elephants, need energy for survival. The energy you use in everyday life for growth, reproduction and to carry out daily activities is obtained from food. This energy is provided through the process of aerobic or anaerobic respiration, which occurs in every living cell. As you have just read, the digestive system breaks down food into small water-soluble molecules that are absorbed into your circulatory system and transported around the body in the bloodstream. One type of molecule that is absorbed from your digestive system is glucose. Glucose is a simple sugar made up of the elements carbon, hydrogen and oxygen. It is this molecule that cells use to carry out aerobic respiration.

Glucose and oxygen molecules react together in your cells to produce carbon dioxide and water. Enzymes control the rate of the reaction and energy is released. Aerobic respiration takes place in specialised organelles of cells called mitochondria and can be represented by the following equation:

glucose + oxygen ! carbon dioxide + water + energy

This reaction can also be represented symbolically as:

C6H12O6 + 6O2 ! 6CO2 + 6H2O + energy

Put simply, sugar is broken down in cells to provide energy for the organism. Carbon dioxide and water are waste products and are released back into the ecosystem through excretion.

Stretching is important before exercise to increase blood flow around muscles. Fig 4.2.7

Anaerobic respiration is also used by many microorganisms that do not use oxygen from the air to respire. Microscopic organisms such as yeast and bacteria use anaerobic respiration to release energy. This reaction uses stored glucose to produce energy, with the waste products being alcohol and carbon dioxide. This example of anaerobic respiration is called fermentation. Fermentation is used in the production of beer, wine, cider and bread. The waste product of fermentation, carbon dioxide, makes the bread rise.

Fig 4.2.8Yeast is used in many of the products you and your family eat or drink every day. The process of fermentation releases carbon dioxide to put

the bubbles in champagne and beer, and to make bread rise.

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Although aerobic respiration releases the greatest energy in your cells, there are other energy-releasing reactions, although they are not as efficient. Glucose that is not broken down in aerobic respiration is stored in your muscles to use when doing strenuous exercise and you need extra energy. In this reaction,

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[ Questions ]Use your book

Digestion 1 What is digestion? 2 Why does food have to be digested? 3 a What are the two types of digestion?

b What is the function of each type of digestion?

The human digestive system 4 What digestive processes occur in the mouth? 5 Why does food travel down your oesophagus and not

your trachea? 6 Why do you have sphincter muscles at the entrance and

exit of the stomach? 7 How are harmful bacteria killed in the stomach? 8 Why are enzymes such important chemicals in digestive

juices? 9 Where in your body is digested food absorbed into your

bloodstream?10 What causes the smell of faeces?

The digestive systems of other animals 11 Why do animals that eat plant matter have more than

one stomach?12 How do snakes eat small animals whole without the aid

of mechanical digestion?

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Protein digestionPurposeTo investigate the action of a protein-digesting enzyme.

Requirements (per group)Four test tubes, texta, two 2 mL pipettes, egg white suspension, bottle 2M HCl, Bunsen burner, matches, 250 mL beaker, 1% pepsin solution, bottle distilled water, test tube rack.

Procedure 1 Label four test tubes 1 to 4. 2 Into each tube add 2 mL egg white suspension. Put the

used pipette into cold water in the tray at the teacher’s desk.

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3 To tubes 2 and 3 add three drops of 2M HCl. 4 Make a water bath by mixing hot and cold water to a

temperature of 35°C. Have the beaker about half full of water.

5 Use a pipette to add 1 mL of pepsin solution to tubes 1 and 3.

6 Add 1 mL of distilled water to tubes 2 and 4. 7 Place all four tubes in the water bath. 8 After 10 minutes remove the tubes and place in a test

tube rack. 9 Copy the following table into your notes and record

your results.

Respiration13 What is aerobic respiration?14 What type of respiration is fermentation?

Use your head15 Why is it necessary for the body to break down the

food you eat into such small particles?16 What would happen if the small intestine was smooth

rather than covered with villi? 17 What would happen to your cells if they were not able

to get any oxygen for a long time?

Investigating questions18 Research the digestive system of an animal of your

choice and then compare what you have found to the human digestive system. Present your findings as a poster.

19 Research the similarities and differences between aerobic and anaerobic respiration. Give examples of when and where each occurs.

20 Design an experiment to investigate the effect of temperature on fermentation. You may also choose to conduct this investigation to test your experimental design. DYO

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Digestion and respirationDigestion and respiration