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Photosynthesis and respiration

All organisms respire but only plants photosynthesise. Photosynthesis is a process by which green plants make their own food. It plays a vital role in making energy available to organisms through the food chain. Plants can only respire when there is light available and so at night, they only respire whereas during the day, they respire and photosynthesise. There will be a net output of oxygen though. 


Respiration is a process by which all organisms release energy from food. Organisms use energy for many other activities such as movement and producing larger molecules for growth.


Image result for respiration equation

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Enzymes are chemicals that speed up the rate of chemical reactions. They are proteins and consist of long chains of amino acids joined together. Chemicals assemble enzymes using instructions provided in genes. The chemicals that the enzymes work on are called substrates and the chemicals produced in the reaction are called products. The area of the enzyme that fits onto the substrate is called the active site. The order and types of amino acids that make up each enzyme give it a complex 3D shape that is essential for the enzyme to work. Each enzyme has an individual shape that allows it to fit neatly over the substrate like a lock and key.

Enzymes need a specific pH and temperature to function properly. They denature if the pH is wrong or the temperature becomes too high. The optimum temperature for most enzymes is 37c since this is the normal body temperature of mammals and birds. As the temperature increases, the enzyme activity increases because the reacction rate increases. High temperatures change the shape of the enzyme's active site. The enzyme becomes denatured at the point when the change in shape is permanent.

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Glucose: making it and using it

Light energy from the sun is required to drive the photosynthesis reaction between carbon dioxide and water to build up glucose. Sunlight is absorbed by the green chemical chlorophyll which, along with the enzymed for photosynthesis, is found in structures in the cell called chloroplasts. These are found in parts of the plant exposed to sunlight. 

The product of photosynthesis, glucose, can be used in...

  • cellulose for cell walls
  • proteins for growth
  • making chlorophyll
  • respiration to produce energy
  • oil for energy storage
  • starch for energy storage

Glucose from photosynthesis and nitrates taken up by the plant roots are used to synthesise amino acids, which are assembled into proteins which can then be used in the growth of the plant.

(Amino acids are used to make proteins in plant, animal and microbial cells)

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The cell as a factory

The basic unit of life is the cell. The cell wall allows gases and water to pass in and out of the cell freely but is a barrier to other chemicals. The nucleus contains DNA which stores the genetic code. This carries the information needed to make enzymes and other proteins. Mitochiondria are found in the cytoplasm and contain the enzymes required fro the release of energy by aerobic respiration.

Plant cells contain a nucleus, cytoplasm, cell membrane, mitochondria and a cell wall.

Animal cells contain a nucleus, cytoplasm, cell mebrane and mitochondria.

Microorganisms all contain cytoplasm, cell embrane and a cell wall. Bacteria have cells with no nucleus; thier circular DNA is in the cytoplasm. Yeast cells have a nucleus. Yeast cells also have mitochondira but bacteria do not.

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Moving chemicals in and out of plants by diffusion

Diffusion is the movement of chemicals from a high to a low concentration. 

Diffusion happens because of the random movement of molecules. This movement does not stop when the two concentrations are the same; there is just no overall movement.

Osmosis is the overall movement of water from an area of high concentration (lots of water=dilute solution) to an area of low concentration (less water=high concentration).

Osmosis is a special kind of diffusion involving water. It happens when chemicals are separated by a partially permeable membrane such as the cell membrane. The cell membrane lets water pass through but keeps other chemicals in or out. 

Active transport uses energy from respiration to go against the chemicals' natural movement and transport chemicals across cell membranes in the opposite direction to how they would normally move between concentrations.

Minerals taken up by plant roots are used to make chemicals essential to cells (eg. nitrogen taken up as nitrates is used to make proteins). Nitrates are normally in a higher concentration in plant cells than the soil, so root cells cannot take up nitrates by diffusion. Instead, the root cells use active transport.

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Factors that limit the rate of photosynthesis

The rate of photosynthesis can be limited by temperature, carbon dioxide levels and light intensity. Carbon dioxide is present in the air at only 0.04%, so commercial plant growers often increase levels in their greenhouses. 

If a factor such as light intensity or carbon dioxide concentration is increased, the rate of photosynthesis increases and then levels off. At the point where the graph levels off, something is limiting the rate. This is called the limiting factor. A rise in temperature increases the rate of photosynthesis up to a certain point. The rate then decreases because of the effect of temperature on enzyme activity.

rate of photosynthesis plotted against light intensity. the rate begins to slow as the light intensity continues to increase (http://www.bbc.co.uk/staticarchive/afa3f2b16b4d58d077943c96929c9a4020fea83a.gif)rate of photosynthesis plotted against carbon dioxide concentration. the rate begins to slow as the carbon dioxide concentration continues to increase (http://www.bbc.co.uk/staticarchive/020c4bb33ca143df87adc99bc1afa29bb95a1688.gif)rate of photosynthesis plotted against temperature. the rate begins to slow as the temperature continues to increase (http://www.bbc.co.uk/staticarchive/32b3b8ad49fe959ff58ac5611559c0c4480cc33b.gif)

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Investigating the effects of light on plant growth

When investigating the effects of light intensity on plant growth, ecologists need to:

  • Use an identification key to identify the plants they find eg. is the fruit spherical?
  • Measure how common the plants are
  • Use a light meter to measure the light intensity

Ecologists investigate the abundance and distribution of plants at different locations in an area and/or in different areas to make a comparison. By using the information collected, ecologists can relate the plants' distribution to the availability of light and other factors.

A sample of how many plants are in an area is taken using a quadrat (a metal or wooden frame that is put on the ground in random locations and a number of times so that an average can be calculated). Alternatively, the quadrat can be used at measured distances along a line called a transect.

Trees, bushes, hedges, or a group of the same species of plant, can cast shade on an area and therefore affect the growth of plants around them. There may therefore be a correlation between the distance from them and the growth of other plants grown at high and low densities.

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Aerobic respiration

All organisms respire (plants, animals and microorganisms). Respiration occurs in every cell in the body. The chemicals used for respiration is glucose (chemicals in our food are converted into glucose, or we eat glucose itself). Respiration using oxygen is called aerobic respiration and this takes place in animal and plant cells and some microorganisms. The process takes place as a series of enzyme-controlled reactions, with small amounts of energy being released in each stages (each step needs its own enzyme).

Organisms require the energy released from respiration for movement and the synthesis of large molecules. Plants use the food produced by photosynthesis for respiration and active transport. Excess glucose is stored as starch.

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Anaerobic respiration & fermentation

Anaerobic respiration:

Some organsisms obtain energy even when there is little or no oxygen. This is done through a process called anaerobic respiration. Anaerobic respiration occurs in human muscles during vigorous exercise, plant roots in water logged soil and bacteria in deep puncture wounds. Anaerobic respiration releases much less energy per molecule of glucose than aerobic respiration. 

Glucose ---> Lactic acid + ENERGY


Fermentation is a type of anaerobic respiration used by some microrganisms. Some fermentation products are useful to us such as the ethanol produced by yeast.

Some microorganisms, including yeast, and some plants produce alcohol (ethanol) by fermentation: Glucose --> Ethanol + Carbon dioxide + ENERGY                                            This is important in the production of alcoholic drinks and bread. Bubbles of carbon dioxide make bread rise and alcoholic drinks sparkle. Anaerobic respiration is also important in biogas generation.

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