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How Plants Produce Food

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How  plants produce food

Photosynthesis

Plants use the process of photosynthesis to make food.

In this process sunlight energy is absorbed by chlorophyll (a green pigment) contained in chloroplasts. (Which give plants their green colour.) This energy is then used to convert carbon dioxide (from the atmosphere) and water (from the soil) to produce oxygen and glucose.

Glucose can be used for energy, converted to starch and stored or used to construct structural material such as cellulose in cell walls.

Oxygen is a waste gas and is given off through the stomata ( pores in the underside of the leaves.)

Water vapour is also lost through the stomata.

Carbon dioxide enters the leaf through the stomata.

Carbon dioxide + water                             Glucose + Oxygen

image00.jpgimage01.jpg

Leaves

These are adapted to do their job in a number of ways-

  1. Broad to give a large surface area for absorbing sunlight
  2. Flat and thin with air spaces between cells to allow gases to circulate between cells easily
  3. They contain large numbers of chloroplasts to trap sunlight energy
  4. They have networks of veins to allow water to reach all parts of the plant
  5. They are thin and flexible to allow them to bend rather than break

Limiting factors

Anything that slows photosynthesis is a limiting factor, since plants need carbon dioxide and light and require warmth for reactions to occur these can all be limiting factors.

A lack of light means there is not enough energy for photosynthesis to proceed.

A low temperature means enzymes and so reactions are slow and this limits the rate of photosynthesis

If a plant uses the carbon dioxide in the air around it this may become a limiting factorimage02.pngimage03.png

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In the shade, evening, early morning or winter light or temperature might be limiting. (Sun low in the sky) During the day when there is no wind carbon dioxide may be limiting (because the air surrounding the plant will have its carbon dioxide used up by the plant).  

Respiration

This uses the glucose from photosynthesis and oxygen from the atmosphere to produce energy. Respiration is the reverse of photosynthesis and happens 24 hours a day (Photosynthesis only happens in daylight)

Glucose + oxygen                              Energy + carbon dioxide + water

The glucose was produced during photosynthesis

The oxygen is taken in through stomata

Carbon dioxide is given off as a waste gas

The energy is used to build large molecules needed by the plant from smaller ones ie glucose into starch or amino acids from nitrates and sugars.

Transport

Plants need to transport substances from one place to another ie water from the roots to the leaves and glucose from the leaves to the roots.

Special tissues are responsible for this-

Structure

Description

Function

Xylem

Dead thick walled cells with no divisions between them

Transports water from the roots to all parts of the plants

Phloem

Thin walled living cells with dividing walls between them

Transports sugars from the leaves to wherever it is needed

Cambium

Cells between the xylem and phloem

Divides to form new phloem and xylem cells

These tissues form straw like structures running up the stem between the roots and the

leaves.

image05.pngimage06.jpg

Glucose

Glucose is soluble in water and so cannot be stored by the plant. A high concentration of glucose would cause water to move towards it by osmosis and this would disrupt the balance of water within the plant. Instead it is converted into starch which is insoluble and stored tubers and bulbs ie carrots and potatoes.

Minerals

Photosynthesis produces the energy that plants need but they also need minerals in small amounts for making amino acids to make proteins or magnesium to make chlorophyll for photosynthesis.

Minerals are taken up through the roots dissolved in the water from the soil

Energy uses

The suns solar energy is converted into glucose (chemical energy) Glucose can be immediately used for energy within the plant or used to make cellulose.

Glucose can be converted to starch or oils for storage.

Glucose can be used along with nitrates (dissolved in soil water and taken in through the roots) and used to make proteins.

Exam questions in this topic often focus on factors effecting plant growth. You need to know the photosynthesis equation as a way of remembering what the process needs and what it produces. Experiments measuring photosynthesis will measure what is needed (carbon dioxide, light warmth) or what is produced (glucose, oxygen).

In questions showing or describing a situation affecting plant growth the above factors should be considered with respect to the environment described, (light levels-shade etc), (CO2 levels- how close the plants are to each other, wind etc), (warmth-position of the sun, time of day/year etc )  but don’t forget the need for minerals and the effects of pollution, drainage etc.  

This student written piece of work is one of many that can be found in our GCSE Green Plants as Organisms section.

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