B8.1 & B8.2 Photosynthesis

Plants make their own food by photosynthesis which occurs in the leaves of a plant, where chloroplasts are present. Chloroplasts contain a substance called chlorophyll which absorbs light. The equation for photosynthesis is: Carbon Dioxide + Water → Glucose + Oxygen

Photosynthesis is an endothermic reaction as it needs energy from its surroundings. This energy is transferred from the light absorbed by the chloroplasts. Because glucose is water soluble, the glucose that is not used immediately is stored a starch, which is not soluble in water, so that it can be used when there is no sunlight. If stored as glucose, there could be an imbalance through out the plant, affecting the way water moves into and out of the cells through osmosis. Plants have been adapted to make photosynthesis more efficient. These adaptations include: broad leaves for an increased surface area, thin leaves so the distance that diffusion distances for gas exchange are short,more chloroplasts so that more light can be absorbed, air spaces that allow gases to enter and leave cells and guard cells on the stomata that regulate gas exchange.

 The rate of photosynthesis can be increased by increasing the light intensity, increasing the concentration of carbon dioxide, increasing the concentration of water,making the temperature at optimum for the rate of photosynthesis and increasing the levels of chlorophyll in the leaf. If any of these factors are low, they limit the rate of photo synthesis. These are called limiting factors.

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B8.3 How Plants Use Glucose

As all living things respire, plants need to use the glucose made by photosynthesis for respiration. The glucose is broken down with oxygen to create waste carbon dioxide and water. Chemically, respiration is the opposite of photosynthesis.

Some of the energy created by respiration is used to build up smaller molecules into larger molecules, such as glucose into cellulose which is used to strengthen the plant's cell walls.Some glucose is used to make amino acids by combining it with nitrate and other mineral ions taken up from the soil by active transport in the roots.

Using energy, amino acids can be built up into protiens that have many different uses, such as for enzymes and protien synthesis. For this reason, plants in low-nutrient soils struggle to survive, unless they are carniverous and obtain their nutrients from animals. Carniverous plants produce enzymes to digest these trapped animals.

Energy transferred from respiration can be used in the build up of fats and oils (lipids) to be used in the plant's cells as an energy store or as a cell wall strengthener. Fats and oils are also used as an energy store in their seeds. These stores can be broken down into sugars, used in respiration, to provide energy for the seed to germinate when it is unable to photosynthesise.

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B9.1 Aerobic Respiration

Aerobic respiration is one of the most important enzyme-controlled processes in living things and takes place all of the time. Inside the human body, the digestive, circulatory and respiratory system work together to provide cells with the reactants needed for aerobic respiration to occur. aerobic respiration involves lots of different chemicals reactions, each controlled by a different enzyme.

Oxygen + Glucose → Carbon Dioxide + Water + [Energy]

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Aereobic respiration is an exothermic reaction because it transfers energy to the environment. Some of the energy transferred from aerobic respiration is used in the cells so that they are able to undergo all of the vital chemical reactions needed. Aerobic respiration takes place in the mitochondria (one of the many organelles in a cell). They have a folded inner membrane, creating a larger surface area and more active cells have more mitchondria so that these cells have enough energy.

On average girls need a lower energy intake than boys because they are generally smaller and boys have more muscle cells. More muscle cells result in more mitochondria which need glucose for aerobic respiration.

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B9.1 The Need for Respiration

Respiration creates the energy needed for lots of different processes in the body, these include:

  • The build up of smaller molecules into larger ones for making new cell material and the reverse in the digestion of food and within the cells themselves
  • In animals, energy is used to make muscles contract, allowing their bodies to move. Breathing and the beating of the heart require muscle contaractions, so this energy is needed all the time. All muscular activities require energy.
  • Mammals and birds maintain a constant internal body temperature, so on colder days,energy from respiration is used to warm the body. On hotter days, they sweat and energy is transferred to the surroundings to keep your body cool.
  • Energy from respiration is used in active transport to move dissolved nutrients, gases and water against the concentration gradient.
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B9.2 The Response to Exercise

The muscles in our bodies require a lot of energy to carry out their functions, move us around and support our bodies against gravity. Muscle tissues are made up of protien fibres which are able to contract when energy from respiration is transferred to them, groups of contracting and relaxing muscles allow us to move. Muscle cells contain lots of mitochondria to provide this energy. Muscles store glucose as the carbohydrate glycogen which can be converted back into glucose quickly during exercise.

When we exercise, our muscles are contracting harder and faster, meaning that they require more energy. The muscles need to undergo more aerobic respiration to meet this demand. During exercise, the muscles also produce excess carbon dioxide that needs to be removed to maintain the muscle's effectiveness. As a result of these needs, our breathing rate increases and becomes more deep to allow for their to be more oxygen supplied to the body and a more efficient removal of carbon dioxide. the heart rate alos increases to transport gases and glucose to where they are needed. To increase the amount of glucose avaliable, the glycogen stores in the muscles are broken down.

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B9.3 Anaerobic Respiration

When muscle cells become too short of oxygen to respire aerobically, they must find another way of creating the energy they need to function. A shortage of oxygen can be due to the blood not being able to supply the oxygen to where it is needed fast enough. The body creates this energy by breaking down glucose. This is called anaerobic respiration, meaning without oxygen.

Anaerobic respiration is less efficient than aerobic respiration because the glucose molecules are not broken down fully. The only product of anaerobic respiration is lactic acid which a build up of causes muscle fatigue because it is poisonous to the muscles. Lactic acid is transported to the liver to be broken down into carbon dioxide and water. This has to be done using oxygen. Oxygen debt is the the amount of oxygen needed to break the lactic acid down, however the body needs to "pay off" the oxygen debt before it can be used in other places of the body. Fitter people have a more efficient respiratory and circulatory sytem so respire aerobically for longer than those whgo do not exercise, they are also more efficient at "paying off" their oxygen debts.

Plants can respire anaerobically, but do not produce lactic acid, they produce carbon dioxide and ethanol. Some microorganisms (bacteria, fungi etc) produce lactic acid, where as others produce carbon dioxide and ethanol. Aerobic respiration in yeast cells is called fermentation and is used in making bread and alcohol.

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B9.4 Metabolism and the Liver

An organism's metabolism is the sum of all the reactions that take place within the cells or the body. There are many metabolic reations, including:

  • The conversion of glucose to starch, glycogen and cellulose
  • The formation of lipid molecules (from 1 glycogen molecule and 3 fatty acid molecules)
  • The formation of amino acids which are used to make protiens
  • The series of reactions in the mitochodria resulting in respiration
  • Photosynthesis in the chloroplasts of a plant
  • The break down of excess protiens in the liver to create urea, excreted by the kidneys as urine

The liver is a vital organ that carries out many difeerent metabloic functions. Its cells regenerate and grow very rapidly. The liver detoxifies the body of poisons and toxins, such as ethanol which comes from alcoholic drinks, passes broken down products into the blood so that it can be excreted through the kidneys and it breaks down old blood cells, storing the iron for when it is needed to syntheise more blood cells. The liver breaks down the toxic lactic acid produced by anaerobic respiration.

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