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