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what do green plants and algae use to make their own food
light energy , they obtain the raw materials they need to make this food from the air and the soil .
the conditions in which plants are grown can be
changed to promote growth
what is the main site of photosynthesis
photosynthesis is summarised by the equation
carbon dioxide + water ----light energy----> glucose + oxygen
during photosynthesis light energy is absorbed by a green substance called
clorophyll , which is found in chlorplasts in some plant cells and algae
the light energy is used in photosynthesis to convert
carbon dioxide (from the air) and water (from the soil) into sugar (glucose)
what is realsed as a by-product during photosynthesis
the rate of photsynthesis can be limited by
- shortgae of light
- low temperature
- shortage of carbon dioxide
the glucose produced in photosynthesis can be converted into
insoluble starch for storage , also plant cells use some of the glucose produced duruing photosynthesis for respiration
some glucose in plants and algae is used to
- produce fat or oil for storage
- produce cellulose , which strengthens the cell wall
- produce proteins
to produce proteins , plants also use
nitrate ions that are absorbed from the soil
leaves are well adapted for
how are leaves adapted for photosynthesis
they are thin with a large flat surface , this gives them a large surface area for absorbing sunlight
the carbon dioxide needed for photosnythesis enters the leaf through
tiny pores called stomata, which are mainly in the lower surface of the leaf.
The leaves are thin so diffusion happens quickly, because the distance through which the gases have to diffuse is short.The carbon dioxide in the air spaces of the leaf is quickly absorbed into the cells and used for photosynthesis. Therefore a concentration gradient is maintained between the air outside the leaf and the air in the spaces between the leaf cells.This ensures that carbon dioxide continues to enter the leaf by the passive process of diffusion.Oxygen, which is produced as a by-product of photosynthesis, is also released into the atmosphere via diffusion through the open stomata.
a green pigment that absorbs light.
leaf cells contain many
chloroplasts, which in turn contain chlorophyll This is because the palisade cells are nearer the upper surface of the leaf, which therefore receives a greater intensity of sunlight.
Palisade cells have more chloroplasts than the cells
in the spongy mesophyll layer.
The only cells of the epidermis that contain chloroplasts
are the guard cells.The epidermal cells are therefore transparent and this allows light to pass through to the chloroplasts of the cells below.
Leaves have a waxy cuticle to
prevent excessive water loss due to evaporation as well as some protection against being eaten by insects
the water needed for photosynthesis enters the plant via its
Plant roots are covered in millions of microscopic root hairs.
these root hairs
increase the surface area of the root for water and mineral uptake.The water and dissolved minerals then travel from the root hair cells, through the root cells to the xylem.
Xylem is a
specialised tissue adapted to transport water and dissolved minerals up the stem to the leaves.
Soluble products of photosynthesis, like glucose, are transported from the leaves around the plant in
a series of chemical reactions by which green plants make or synthesise glucose from carbon dioxide and water. Enzymes in the chloroplasts of the cells combine these two reactants using light energy from the sun.
Oxygen gas is a
by-product of the process and is released into the atmosphere.
The glucose produced in the initial stages of photosynthesis is rapidly turned into
starch, which is insoluble.The insoluble starch granules can be stored in the leaves without affecting the osmotic balance of the cells.
To discover if photosynthesis has taken place,
the easiest method is to identify whether starch has been produced in the leaves.
how is a leaf tested for the presence of startch
- th leaf is placed in boiling water for two minutes to stop all reactions and break open the cell walls
- the leaf is placed in warm ethanol to remove chlorophyll so the final results are easier to observe
- the leaf is dipped in warm water to sofetn it and remove ethanol
- the leaf is covered in iodine solution to test for the presence of startch , the starch turns iodine solution blue-black
In all the investigations for photosynthesis, the plant used must first be
destarched, by placing it in the dark.In the dark the plant uses up any starch that has been stored in the leaves.
To show whether chlorophyll is really necessary for photosynthesis you need a leaf from a
variegated plant. A variegated leaf has areas of green and areas of white.
once the plant has been destarched by leaving it in the dark the
destarched plant can be now left in the light for 24 hours. A leaf of the plant can then be tested to see if starch has been formed.
To show that light is necessary for photosynthesis
, a piece of foil is wrapped around part of a leaf from a destarched plant. The plant is then left in the light for 24 hours.
To show carbon dioxide is necessary for photosynthesis
a leaf of the plant needs to be enclosed in a flask containing potassium hydroxide, which absorbs carbon dioxide.the destarched plant is then left in the light for 24 hours.The two leaves, X and Y, are then tested for the presence of starch.
The energy released in respiration is used
to build more complex organic molecules. For example, energy is used to convert glucose into insoluble starch, which is stored in tubers and seeds.
Some energy is used to combine
glucose with nitrogen from nitrates, sulphur from sulphates and phosphorus from phosphates to make amino acids and proteins needed for growth Proteins are needed to make enzymes, chlorophyll, cell membranes, the nuclei and cytoplasm.
Some energy is used to convert glucose into lipids - fats or oils
these lipids are needed to make cell membranes and some are stored in seeds.
The three main factors, which affect the rate of photosynthesis, are
light intensity, carbon dioxide concentration, and temperature. the overall rate of photosynthesis will be limited by the one factor that is in shortest supply. this is known as the law of limitating factors
Increasing light intensity will increase the rate of photosynthesis so long as
there is enough carbon dioxide and the temperature is not too cold.
The factor of light intensity can be easily investigated using an aquatic plant like Elodea - Canadian pondweed.The advantage of an aquatic plant is that
- the rate of photosynthesis can be measured as the rate at which oxygen gas is released. The bubbles of oxygen can be easily counted and the volume measured. By changing the distance between the lamp and the plant the light intensity can be varied.
- The volume of oxygen given off during a set period of time, at a variety of distances, can be measured
The measuring cylinder containing the Elodea is placed in a large beaker of water or a water bath to keep the temperature of the water constant.This ensures
that we are only measuring the effect of changing light intensity, since temperature is one of the factors that affects the rate of photosynthesis.
Increasing the carbon dioxide concentration increases the rate of photosynthesis, if
there is enough sunlight to provide the necessary energy and provided the temperature is not too cold.
However at temperatures above the optimum the shape of the enzyme molecules will be altered. The enzymes are said to be denatured.
this means the enzymes are no longer able to catalyse the reactions of photosynthesis and photosynthesis will slow down.
For healthy growth, plants need several mineral elements as well. These elements are dissolved
in soil water in the form of mineral ions, and are taken into the roots by two processes - diffusion and active transport.
f you follow the green line on the graph you can see that at first increasing the light intensity increases the rate of photosynthesis (the graph goes upwards) but after a while this levels off and it does not matter how much light is increased, the rate of photosynthesis does not increase as another factor is limiting the rate of photosynthesis.It could be the amount of heat that is limiting the rate of photosynthesis, if this is increased along with the light, you can see by the red line that the rate of photosynthesis reaches a new higher point than the green line. This line again reaches a maximum and curves off. This is because something else is still limiting the rate of photosynthesis such as carbon dioxide.The blue line shows the effect of increasing light, heat and carbon dioxide concentration - it increases the rate of photosynthesis to an even higher level, but is eventually limited by another factor.
Increasing the temperature will increase the rate of photosynthesis up to the optimum temperature for the enzymes involved, provided
there is still enough carbon dioxide and sunlight to provide the necessary energy.
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