B3

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

Animal tissues include:

• Muscular tissue, which can contract to bring about movement.
• Glanduar tissue, to produce substances such as enzymes or hormones.
• Epithelial tissue, which covers some parts of the body.

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 cober the outside and the inside of the stomach.

The pancreas is an organ. The pancreas has two types of glandular tissue producing:

• Hormones to control blood sugar 
• Some of the digestive enzymes 

Multicellular organisms are made up of organ systems that work together.

• Each organ is made up of several organs that work together to perform a particular function.
• Organ systems include the digestive system, circulatory system, and gas exchange system.
• These systems have organs that are adapted to be efficient exchange surfacces. These have large surface areas, short diffusion paths, rich blood supplies, and mechanisms for ventilating surfaces or for moving materials

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

All carbohydrates are made of units of sugar.

• Glucose has one unit of sugar. Sucrose has two 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.

Starch turns yellow-red iodine solution blue-black.

Lipids are molecules made of three molecules of fatty acids linked to a molecule of glycerol.

A cloudy white layer with the ethanol test indicates the presence of lipids in solutions.

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. If the protein is heated the shape is changed an dthe protein is denatured.

Each protein has a specific function. Some proteins are structural components of tissues such as muscles. Other proteins are hormones, antibodies, or enzymes.

Buiret reagent turns from blue to purple in the presence of proteins.

Catalysts increase the rate of chemical reactions without changing chemically themselves.

• Enzymes are biological catalysts. 
• Chemical reactions in cells are controlled by proteins called enzymes.
• The shape of enzyme is vital for its function. The enzyme has an area called the active ste where ts substrate molecule can fit.
• The substance is held in the active site and may be jined 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 molcules 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.

• Reactions take place faster when it's 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 th 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.

The effect of pH on enzyme action:

• 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 neutral 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 site changes.

Digestion involves the breakdown of large, insoluble molecules into soluble substances. These smaller molecules can then e absorbed into the blood across the wall of the small intestine.

Digestive enzymes:

• Are produced by specialised cells in glands and in thelining 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 its digested.

The digestive system is a long hollow, muscular tube. The digestive system:

• Breaks 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 diffrent 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.

Amylase (a carbohydrate) is produced by the salivary glands, the pancreas, and the small intestine. Amylase catalyses the digestion of starch into sugars in the mouth and small intestine.

Proteases are produced by the stomach, the pancreas, and thesmall intestine. Proteases catalyse the breakdown of proteins into amino acids in the stomach and small intestine.

Lipase is produced by the pancreas and the small intestine. Lipase catalyses the breakdown of lipids (fat and oils) to fatty acids and glycerol.

Human digestive enzymes work best at body temperature, 37 degrees celcius, so the temperature in the digestive system is optimum. Different enzymes have different optimum 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 the this the liver produces bile that is stored in the gall bladder and released into the small intestine when 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.