Biology

Respiration is a chemical process in which energy is released from food substances, such as glucose - a sugar.

Aerobic respiration needs oxygen to work. Most of the chemical reactions involved in the process happen in tiny objects inside the cell cytoplasm, called mitochondria.

This is the equation for aerobic respiration:

glucose + oxygen → carbon dioxide + water (+ energy)

The energy released by respiration is used to make large molecules from smaller ones. In plants, for example, sugars, nitrates and other nutrients are converted into amino acids. Amino acids can then join together to make proteins. The energy is also used:

• To allow muscles to contract in animals
• To maintain a constant body temperature in birds and mammals

Blood carries oxygen and nutrients to the body's cells,and waste products away from them. The circulatory system consists of:

• The heart, which is the muscular pump that keeps the blood moving
• The arteries, which carry blood away from the heart
• The veins, which return blood to the heart
• The capillaries, which are tiny blood vessels that are close to the body's cell

A process called diffusion takes place in the capillaries. Diffusion is where particles of a high concentration move to an area of low concentration. Glucose and oxygen diffuse into the cells from the capillaries. Carbon dioxide diffuses out of the cells into the blood in the capillaries.

During exercise, the muscle cells respire more than they do at rest. This means:

• Oxygen and glucose must be delivered to them more quickly
• Waste carbon dioxide must be removed more quickly

This is achieved by increasing the breathing rate and heart rate. The increase in heart rate can be detected by measuring the pulse rate. The stroke volume also increases – this is the volume of blood pumped each beat. The total cardiac output can be calculated using the equation:

Cardiac output = stroke volume x heart rate

During hard exercise, the oxygen supply may not be enough for the needs of the muscle cells. When this happens, anaerobic respiration takes place, as well as aerobic respiration.

When exercising very hard, the heart cannot get enough oxygen to the muscles. Anaerobic respiration does not need oxygen. It releases energy from glucose but the amount is much lower. It happens when there is not enough oxygen for aerobic respiration. Here is the word equation:

glucose → lactic acid (+ energy)

Much less energy is released by anaerobic respiration than by aerobic respiration. The lactic acid that forms causes muscle fatigue and pain.

During hard exercise when anaerobic respiration occurs with aerobic respiration, an oxygen debt builds up. This is now known as Excess Post-exercise Oxygen Debt or EPOC. This is because glucose is not broken down completely to form carbon dioxide and water. Some of it is broken down to form lactic acid. Panting after exercise provides oxygen to break down lactic acid. The increased heart rate also allows lactic acid to be carried away by the blood to the liver, where it is broken down.

Farmers can use their knowledge of these limiting factors to increase crop growth in greenhouses. They may use artificial light so that photosynthesis can continue beyond daylight hours, or in a higher-than-normal light intensity. The use of paraffin lamps inside a greenhouse increases the rate of photosynthesis because the burning paraffin produces carbon dioxide, and heat too.

Transpiration explains how water moves up the plant against gravity in tubes made of dead xylem cells without the use of a pump.

Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what's lost. As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.

Factors that speed up transpiration will also increase the rate of water uptake from the soil. When water is scarce, or the roots are damaged, it increases a plant's chance of survival if the transpiration rate can be slowed down. Plants can do this themselves by wilting, or it can be done artificially, like removing some of the leaves from cuttings before they have chance to grow new roots.

FactorDescriptionExplanation Light In bright light transpiration increases The stomata (openings in the leaf) open wider to allow more carbon dioxide into the leaf for photosynthesis Temperature  
Transpiration is faster in higher temperatures Evaporation and diffusion are faster at higher temperatures Wind Transpiration is faster in windy conditions Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf Humidity Transpiration is slower in humid conditions Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air

TissueProcessWhat is movedStructure Xylem Transpiration Moves water and minerals     from roots to leaves

Columns of hollow,       dead reinforced cells

Phloem  Translocation  Moves food substances from leaves to rest of plant Columns of living cells

No heart, no blood and no circulation, but plants do need a transport system to move food, water and minerals around. They use two different systems – xylem moves water and solutes from the roots to the leaves – phloem moves food substances from leaves to the rest of the plant. Both of these systems are rows of cells that make continuous tubes running the full length of the plant.

Xylem cells have extra reinforcement in their cell walls, and this helps to support the weight of the plant. For this reason, the transport systems are arranged differently in root and stem – in the root it has to resist forces that could pull the plant out of the ground. In the stem it has to resist compression and bending forces caused by the weight of the plant and the wind.

Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by having a large surface area to speed up osmosis.

The absorbed water is transported through the roots to the rest of the plant where it is used for different purposes:

• It is a reactant used in photosynthesis
• It supports leaves and shoots by keeping the cells rigid
• It cools the leaves by evaporation
• It transports dissolved minerals around the plant

Leaves are adapted for photosynthesis by having a large surface area, and contain openings, called stomata to allow carbon dioxide into the leaf. Although these design features are good for photosynthesis, they can result in the leaf losing a lot of water. The cells inside the leaf have water on their surface. Some of this water evaporates, and the water vapour can then escape from inside the leaf by diffusion.

To reduce loss the leaf is coated in a wax cuticle to stop the water vapour escaping through the epidermis. Leaves usually have fewer stomata on their top surface to reduce this water loss.

Plants growing in drier conditions tend to have small numbers of tiny stomata and only on their lower leaf surface, to save water loss. Most plants regulate the size of stomata with guard cells. Each stoma is surrounded by a pair of sausage-shaped guard cells. In low light the guard cells lose water and become flaccid, causing the stomata to close. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.

Most plant cells are turgid at all times. This supports the weight of the plant, which is especially important where there is no woody tissue, such as leaves, shoot and root tip. If the plant loses water faster than it can be absorbed the cells lose turgor pressure and become flaccid. This causes the plant to wilt.

You should be able to explain why most plants will wilt if they get flooded by sea water. (Hint: sea water contains many chemicals in solution, such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the stronger solution.)

Osmosis is the movement of water molecules from an area of high concentration of water to an area of lower concentration of water through a partially permeable membrane. This can be the cell membrane. An example is the flooding of plants by sea water. Sea water contains many chemicals in solution, such as salt. Osmosis will move water across the plant cell membrane, from the weaker to the stronger solution.