plant nutrition Flashcards
photosynthesis can be defined as
the process by which plants manufacture carbohydrates from raw materials using energy from light
Photosynthesis word equation
Balanced chemical equation for photosynthesis
light energy is conerted into
chemical energy in the bonds that are holding the atoms in the glucose molecules together
The Products of Photosynthesis
Plants use the glucose they make as a source of energy in respiration
They can also convert it into starch for storage, into lipids for an energy source in seeds, into cellulose to make cell walls or into amino acids (used to make proteins) when combined with nitrogen and other mineral ions absorbed by roots
The photosynthesis equation is the exact reverse of the
aerobic respiration equation
leaves cannot be tested for the presence of glucose
why?
Although plants make glucose in photosynthesis, leaves cannot be tested for its presence as the glucose is quickly used, converted into other substances and transported or stored as starch.
Starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesising.
Leaves can be tested for starch using the following procedure:
A leaf is dropped in boiling water to kill and break down the cell walls
The leaf is left for 10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll so colour changes from iodine can be seen more clearly
The leaf is dipped in boiling water to soften it
The leaf is spread out in a Petri dish and covered with iodine solution
In a green leaf, the entire leaf will turn blue-black as photosynthesis is occuring in all areas of the leaf
This method can also be used to test whether chlorophyll is needed for photosynthesis by using a variegated leaf (one that is partially green and partially white)
The white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contain chlorophyll stain blue-black
The areas that had no chlorophyll remain orange-brown as no photosynthesis is occurring here and so no starch is stored
Testing a variegated leaf for starch
Care must be taken when carrying out this practical as ethanol is extremely flammable – the safest way to heat the ethanol is in an electric water bath rather than using a beaker over a Bunsen burner with an open flame
The Need for Light in Photosynthesis
The same procedure as in the investigation above can be used to investigate if light is needed for photosynthesis
Before starting the experiment the plant needs to be destarched by placing in a dark cupboard for 24 hours
This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment
Following destarching, a leaf of the plant can be partially covered with aluminium foil and the plant placed in sunlight for a day
The leaf can then be removed and tested for starch using iodine
The area of the leaf that was covered with aluminium foil will remain orange-brown as it did not receive any sunlight and could not photosynthesise, while the area exposed to sunlight will turn blue black
This proves that light is necessary for photosynthesis and the production of starch
The Need for Carbon Dioxide in Photosynthesis
Destarch a plant
Tie a clear bag containing sodium hydroxide, which will absorb carbon dioxide from the surrounding air, around one leaf
Tie a clear bag containing water (control experiment), which will not absorb carbon dioxide from the surrounding air, around another leaf
Place the plant in bright light for several hours.
Test both leaves for starch using iodine
The leaf from the bag containing sodium hydroxide will remain orange brown as it could not photosynthesise due to lack of carbon dioxide
The leaf from the control bag containing water should turn blue black as it had all necessary requirements for photosynthesis
The Rate of Photosynthesis Using a Water Plant
The plant usually used is Elodea – a type of pondweed
As photosynthesis occurs, oxygen gas produced is released
As the plant is in water, the oxygen released can be seen as bubbles leaving the cut end of the pond weed
The number of bubbles produced over a minute can be counted to record the rate
The more bubbles produced per minute, the faster the rate of photosynthesis
A more accurate version of this experiment is to collect the oxygen released in a test tube inverted over the top of the pondweed over a longer period of time and then measure the volume of oxygen collected
This practical can be used to investigate the effect of changing light intensity (by moving a lamp different distances away from the beaker containing the pondweed), changing carbon dioxide concentration (by dissolving different amounts of sodium hydrogen carbonate in the water in the beaker) or changing temperature by changing the temperature of the water in the beaker
Care must be taken when investigating a condition to keep all other variables constant in order to ensure a fair test – for example, when investigating changing light intensity, a glass tank should be placed in between the lamp and the beaker to absorb heat from the lamp and so avoid changing the temperature of the water as well as the light intensity
What is a Limiting Factor?
So a limiting factor can be defined as something present in the environment in such short supply that it restricts life processes
There are three main factors which limit the rate of photosynthesis:
Temperature
Light intensity
Carbon dioxide concentration
temperature as a limiting factor
s temperature increases the rate of photosynthesis increases as the reaction is controlled by enzymes
However, as the reaction is controlled by enzymes, this trend only continues up to a certain temperature beyond which the enzymes begin to denature and the rate of reaction decreases
Effect of temperature on the rate of photosynthesis
as a limiting factor as a limiting factor
The more light a plant receives, the faster the rate of photosynthesis
This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply
At low light intensities, increasing the intensity will increase the rate of photosynthesis
At this point light intensity stops controlling the rate of photosynthesis
The rate becomes constant regardless of how much light intensity increases as something else is limiting the rate
The factors which could be limiting the rate when the line on the graph is horizontal include temperature not being high enough or not enough carbon dioxide
Effect of Light on Net Gas Exchange investigation
We can investigate the effect of light on the net gas exchange in an aquatic plant using a pH indicator such as hydrogencarbonate indicator
This is possible because carbon dioxide is an acidic gas when dissolved in water
Hydrogencarbonate indicator shows the carbon dioxide concentration in solution
The table shows the colour that the indicator turns at different levels of carbon dioxide concentration
Several leaves from the same plant are placed in stoppered boiling tubes containing some hydrogen carbonate indicator
The effect of light can then be investigated over a period of a few hours
Results from a typical experiment are shown in the table below:
Plants obtain elements such as nitrogen and magnesium in the form of
mineral ions actively absorbed from the soil by root hair cells
