B3: Organisation and the digestive system

3.1: Tissues and organs

Tissues

A tissue is a group of cells with similar structure and function

Animal tissues include

Muscular tissue, which can contract to bring about movement

Glandular tissue, to produce substances such as enzymes or hormones

Epithelial tissue, which covers some parts of the body

Organs

Organs are made of tissues

The stomach is an organ made of

Muscular tissue to churn the stomach contents

Glandular tissue to produce digestive juices

Epithelial tissue to cover the outside and the inside of the stomach

The pancreas is an organ that has 2 types of glandular tissue, producing

Hormones to control the blood sugar

Some of the digestive enzymes

Organ systems

Multicellular organisms are made up of organ systems that work together

Each organ system is made up of several organs that work together to perform a particular function

Organ systems include

The digestive systems

The circulatory system

The gas exchange system

These systems have organs that are adapted to be efficient exchange surfaces - these have large surface areas, short diffusion paths, rich blood supplies, and mechanisms for ventilating surfaces or for moving materials

3.2: The human digestive system

The digestive system is responsible for changing the food you eat from insoluble molecules into soluble molecules, then absorbing them into the blood

The digestive system is a muscular tube that includes

Glands, such as the pancreas and salivary glands, that produce digestive juices containing enzymes

The stomach, where digestion occurs

The liver, which produces bile

The small intestine, where digestion occurs, and which also has a large number of villi where the absorption of soluble food occurs

The large intestine, where water is absorbed from the undigested food, producing faeces

The villi are adapted to absorb soluble food efficiently

Villi have

A very large surface area to absorb soluble food molecules by diffusion and active transport

A thin wall to provide a short diffusion path

A good blood supply to carry the food molecules away to maintain a concentration gradient

3.3: The chemistry of food

Carbohydrates

All carbohydrates are made of units of sugar

Glucose has 1 unit of sugar - sucrose has 2 sugar units linked together (these are simple sugars)

Starch and cellulose are made of long chains of simple sugar units that are bonded together (these are complex carbohydrates)

Lipids

Lipids are molecules made of 3 molecules of fatty acids linked to 1 molecule of glycerol

Proteins

Proteins are made of long chains of amino acids - the long chains are folded to form a specific shape - other molecules can fit into these specific shapes

Each protein has a specific function - some proteins are structural components of of tissues, such as muscles

Other proteins are

Hormones

Antibodies

Enzymes

If the protein is heated the shape is changed and the protein is denatured

Food tests

Carbohydrates

Iodine test for starch

Yellow-red iodine solution turns blue-black if starch is present

Benedict's test for sugars

Blue Benedict's solution turns brick red on heating if a sugar such as glucose is present

Protein

Biuret test

Blue biuret reagent turns purple if protein is present

Lipids

Ethanol test

Ethanol added to a solution gives a cloudy white layer if a lipid is present

3.4: Catalysts and enzymes

Chemical reactions in cells are controlled by proteins called enzymes

Enzymes are biological catalysts - they speed up reactions

Enzymes are large proteins - the shape of an enzyme is vital for its function - the enzyme has an area called the active site where its substrate molecule can fit

The substrate is held in the active site and may be joined to another molecule or may be broken down into smaller molecules

The lock and key theory is a simple model of how enzymes work

Enzymes can

Build large molecules from many smaller ones, such as building starch from glucose molecules

Change one molecule into another one, such as converting one type of sugar into another

Break down large molecules into smaller ones - the digestive enzymes do this

Metabolism is the sum of all the reactions that take place in a cell or in the whole body

3.5: Factors affecting enzyme action

The effect of temperature on enzyme action

The effect of pH on enzyme action

Reactions take place faster when it is warmer

At higher temperatures the molecules move around more quickly and so collide with each other more often, and with more energy

Enzyme-catalysed reactions are similar to other reactions - when the temperature is increased the rate of an enzyme-catalysed reaction increases

However, after increasing the temperature beyond a certain point the rate no longer increases

If the temperature gets too high the enzyme stops working because the active site changes shape - the enzyme becomes denatured

Each enzyme works best at a particular pH - some enzymes work best in acid conditions, such as those found in the stomach, while others need natural or alkaline conditions

The folded shape of the protein molecule that forms an enzyme is held together by forces - a change in pH affects these forces, which in turn changes the shape of the active site

At the optimum pH the active site has the best shape so that the enzyme works most efficiently

When the pH is too acidic or too alkaline, the enzyme becomes denatured as the shape of the active sight changes

3.6: How the digestive system works

The food you eat contains large, insoluble molecules such as starch (a carbohydrate), protein, and lipid

The large molecules must be digested into smaller, soluble molecules that can be absorbed into the blood

Digestive enzymes

Are produced by specialised cells in glands and in the lining of the small intestine

Pass out of the glands into the cavity of the digestive system - they work outside the cells, unlike most enzymes

Come into contact with the food so it is digested

The digestive system

Breaks down the food into smaller pieces to increase the surface area for enzymes to work on

Mixes the food with digestive juices that contain the enzymes

Has muscles to move the food along

Has areas with different levels of pH (e.g. the mouth and the small intestine are alkaline whilst the stomach is acidic)

Absorbs the small, soluble food molecules into the blood in the small intestine

Each type of food is digested by a specific enzyme

Amylase

Produced by the salivary glands, pancreas, and small intestine

Amylase catalyses the digestion of starch into sugars in the mouth and small intestine

Proteases

Produced by the stomach, pancreas, and small intestine

Proteases catalyse the breakdown of proteins into amino acids in the stomach and small intestine

Lipase

Produced by the pancreas and small intestine

Lipase catalyses the breakdown of lipids to fatty acids and glycerol

3.7: Making digestion efficient

Human digestive enzymes work best at body temperature (37 degrees C), so the temperature in the digestive system is optimum - different enzymes have different optimum pH levels

Protease enzymes in the stomach work best in acid conditions - glands in the stomach wall produce hydrochloric acid to create very acidic conditions

Other proteases, amylase, and lipase work best in the small intestine where the conditions are slightly alkaline

Food leaving the stomach is very acidic so its pH must be changed - to do this the liver produces bile that is stored in the gall bladder and released into the small intestine when the food enters

Bile

Neutralises the stomach acid

Makes the conditions in the small intestine slightly alkaline

Emulsifies fats (breaks large drops of fats into smaller droplets) to increase the surface area of the fats for lipase enzymes to act upon