B2 - Organisation

The structures in the body can be classified into: cells, tissues, organs and organ systems (from smallest to largest).

Cell - The smallest functional unit of all organisms.

• e.g. Ciliated epithelial cells have tiny hair-like structures called cilia to sweep away foreign bodies or secretions, such as mucus.

Tissue - A group of cells with similar structure working in co-ordination to perform a specific function

• e.g. Glandular tissue contains secretory cells which produce substances such as enzymes.

Organ - A group of tissues which adapt to perform a specific function

• e.g. The pancreas produces insulin used to control blood sugar levels#

Organ System - A group of organs working together to perform a function in the body

• e.g. Circulatory system consists of organs such as arteries and the heart to pump blood around the body.

The mouth is where food is first broken down.Salivary glands release saliva, which contains enzymes such as amylase which breaks down starch into sugar. 

The oesophagus pushes food down into the stomach by a process called peristalsis, which involves the contraction and relaxation of muscles in a wave-like motion down the throat.

The liver produces bile (an alkali) to neutralise the strong stomach acid, as well as emulsifying fat (the seperation of fat into smaller particles to give them a higher surface area).The bile is stored in the gall bladder.

The pancreas releases enzymes such as lipase, which breaks down lipids and fats into fatty acids.

The small intestine absorbs glucose, fatty acids, amino acids and glycerol. Contains villi with folded membranes to increase surface area for substances to diffuse into the bloodstream quickly.

The large intestine is where undigested waste is passed through. It absorbs water from the waste, and the waste is then stores in the rectum to be excreted.

Enzymes are protein molecules which act as biological catalysts - they speed up reactions (by lowering the activation energy) without being used up or changed themselves.

• Enzymes bond and synthesise molecules, which happens in the active site
• Enzymes are specific about which reactions they catalyse. Only molecules with exactly the right shape (called substrates) can bind to the active site of the enzyme.
• This can be shown using the lock and key model, as shown below:

Temperature

As temperature increases, so does the rate of reaction/ enzyme activity. As the temperature is increased, the enzymes and molecules (or substrates) gain more kinetic energy, which reults in the enzyme colliding with the substrate more frequently.

pH

The pH at which a particular enzyme is most efficient depends on the type of enzyme in question. For example, pepsin, an enzyme found in the stomach, has an optimum pH of 2,which corresponds with its acidic environment where the pH of stomach acid is 2.

Optimum Conditions

The optimum conditions of a particular enzyme refers to the temperature and pH at which the enzyme works best. However, if the temperature or pH has exceeded the optimum value for a particular enzyme, the enzyme will denature. This means that the shape of the enzyme will be deformed so that the substrate will no longer bind to the active site of the enzyme. 

Types of enzymes which aid digestion:

• Amylase breaks down starch into glucose and is found in the salivary glands, the pancreas and the small intestine.
• Protease/ Pepsin breaks down protein into amino acids and is found in the stomach, the pancreas and the small intestine.
• Lipase breaks down lipids into fatty acids and glycerol and is found in the pancreas and the small intestine.

Ways in which optimum conditions are created for enzymes during digestion:

• The pH of stomach acid is 2 - the optimum pH for enzymes in the stomach such as pepsin.
• Digestive enzymes in the small intestines are denatured by stomach acid, so bile is released into the small intestine to neutralise it.
• Bile emulsifies fats, which means that there is a larger surface area for lipase to break down the fat into glycerol and fatty acids.

There are four components of blood:

Red blood cells - Contain the protein haemoglobin to pick up and transport oxygen. They also deliver carbon dioxide to the lungs to be exhaled. The equation for when haemoglobin binds to oxygen in the lungs is: oxygen + haemoglobin --> oxyhaemoglobin. (Note: Not vital to know this)

White blood cells - Concerned with immunity and protection against infectious diseases. They do this by: releasing anti-toxins to counteract toxins produced by pathogens, producing anti-bodies to stick to and clump pathogens, and by engulfing pathogens. (Phagocytosis)

Platelets - Fragments of cells which make tiny fibres that form a net at the side of a cut to form a scab. This reduces blood loss and prevents infection.

Plasma - A liquid in which all other blood components are suspended in, as well as transportation of dissolved substances around the body. Some of these substances include: nutrients such as water, glucose, amino acids, minerals and vitamins, hormones and waste products such as urea and carbon dioxide.

There are 3 types of blood vessel:

Arteries - Carry oxygenated blood away from the heart. (An exeption of this is the pulmonary artery, which carries de-oxygenated blood to the lungs).

• Arteries have thick walls of muscle and elastic fibres so that they can stretch and return to their original shape as blood is forced through them.

Veins - Carry de-oxygenated blood to the heart. (Again an exeption is the pulmonary vein, which carries blood from the lungs to the heart).

• Veins have valves to direct the blood flow
• Have thinner walls in comparison  to arteries - the blood is under less pressure

Capillaries - Form a huge network throughout the body, linking arteries and veins.

• Wall is only 1 cell thick to allow diffusion of substances
• Very close to cells and tissues for shorter difffusion distance
• Small diameter forces blood cells to pass in single file - ensures all red blood cells are involved in gaseous exchange.

One system carries oxygenated blood from the aorta around the body, and carries de-oxygenated blood back to the heart through the vena cava. The other system de-oxygenated blood to the lungs through the pulmonary artery, and oxygenated blood is brought back from the lungs through the pulmonary vein. This is called double circulation. 

Statins - Drugs which reduce the level of cholesterol in the blood

• Advantage: Reduces blood cholesterol level which reduces risk of diseases 
  
• Disadvantage: Risk of negative side effects

Stents - Metal grids which are inserted into a narrow artery to keep it open

• Advantages: Allows adequate bloodflow reducing the risk of heart attacks.
  
• Disadvantages: Risk of blood clotting in the area of the stent

Artificial heart - Man-made hearts which function as temporary substitutes for biological hearts while patients wait for a suitable donor

• Advantages: Are available straight away- the patients tissues do not need to match
• Disadvantages: Risk of blood clotting, and lots of machinary is required to keep it working

Heart Transplant: A donor heart is transplanted during an open-heart surgery

• Advantages: Gives people the chance to live a relatively normal, healthy life.
  
• Disadvantages: Patient must wait for donor heart to match, and immunodepressant medication must be taken

Examples:

Root hair cell: Absorb water from the soil by osmosis

• Long projections increase the surface area and the rate of diffusion
• Large permanent vacuole speeds up the movement of water from soil to the cell by osmosis (the vacuole/ root hair cell is hypertonic to the soil).

Spongy mesophyll: A tissue which allows gas exchange in the plant

• Air spaces within spongy mesophyll layer increases surface area - more CO2 can be absorbed

Leaf: An organ which allows the plant to photosynthesise

• Lots of chloroplasts enable rapis absoption of sunlight
• Large surface area also increases the amount of sunlight absorbed

Translocation is the movement of nutrients around the plant.

• It occurs in the Phloem
• The Phloem is made of columns of living cells, and transports the food in the form of sugar/ sap from the leaves to the rest of the plant.

Transpiration is the loss of water from the leaves of a plant, which causes the uptake of more water from the soil.

• It occurs in the xylem
• The xylem is made of columns of dead cells, and transports water and minerals from the roots to the leaves

• Light intensity - As light intensity increases, transpiration increases: when the light intensity is higher, more light can be absorbed for photosynthesis. This also causes the stomata to open for gaseous exchange. This increased rate of photosynthesis means that more water must be drawn from the soil, so the rate of transpiration  increases.

• Temperature - As temperature increases, transpiration decreases because the rate of diffusion and evaporation also increases at higher temperatures.

• Wind conditions - The rate of transpiration increases in windy conditions because wind sweeps away water vapour so that the diffusion of more water vapour out of the leaf is faster (diffusion is from high to low concentration of substances)

• Humidity - Transpiration decreases in humid conditions: the humid air surrounding the leaf is high in water concentration, so the rate of diffusion is decreased because substances diffuse from high to low concentrations.