Plant Nutrition

Photosynthesis

Background

Photosynthesis is the process by which plants manufacture carbohydrates from inorganic materials using energy from light.

Chlorophyll is where photosynthesis happens. It transfers light energy into chemical energy for the synthesis of carbohydrates (i.e. glucose). The glucose is then either converted to sucrose for transport around the plant or starch for storage. Glucose is too reactive to be transported around the plant on its own. There it has to be converted to sucrose first. It also cannot be stored due to its reactivity, so therefore it must be converted to starch first.

Limiting Factor

The term limiting factor is something present in the environment in such a short supply that it restricts life processes.

For instance, if there was a shortage of carbon dioxide but all other raw materials were in abundance, then carbon dioxide would be the limiting factor for photosynthesis. If there was a shortage of chlorophyll, then that would become the limiting factor instead.

Investigations

Necessity of Chlorophyll, Light, and Carbon Dioxide

The equation for photosynthesis shows that chlorophyll, light, and carbon dioxide (and water) are required, otherwise it won’t work.

To prove it, get a functioning plant and deprive it of each of these factors individually and prove that the plant stops photosynthesizing when these factors are absent.

The way to do that is pretty simple. First of all, de-starch all our plants by leaving the plants in the dark for 48 hours. During this period the plants will be unable to photosynthesize and therefore use up all its starch for respiration. This means that in the beginning of the experiments, all plants (test and control) will have absolutely no starch in them.

This means that after the experiment, if a starch test (iodine test) is conducted and starch is present, it would indicate that photosynthesis had occurred.

NOTE: We can’t just add iodine onto a fresh leaf and expect results. First of all, we need to break the leaf so that iodine can seep in to begin with. Moreover, we need to remove the chlorophyll to decolorize the leaf so that the colour change from iodine is easier to see. So, here are the steps we need to take:

Boil the leaf in water to kill the leaf making it permeable
Boil the leaf in ethanol as chlorophyll dissolves and the leaf decolours
Rinse the leaf in water
Spread the leaf out on a white tile
Add iodine solution

To investigate the effect of light on photosynthesis, we need to partially cover the leaves of the plant and leave it under sunlight. The covered areas will be deprived of light whereas the rest will be exposed. A starch test is then carried out after a few hours (steps above). The results should show that the covered areas have a negative starch test (i.e. no photosynthesis) whereas the exposed areas have a positive starch test.

To investigate the effect of carbon dioxide on photosynthesis, we place a test plant in a container with a carbon dioxide absorber (i.e. sodium hydroxide) and a control plant without the absorber. The absorber will remove the carbon dioxide. A starch test is then carried out after several hours. The results should show that the test plant has a negative result whereas the control plant has a positive one.

To investigate the effect of chlorophyll on photosynthesis, we need to use a plant with variegated leaves. This means some parts of the leaf have chlorophyll whereas other parts do not (and are whiter thus). After several hours, a starch test is carried out. The results should show that parts of the leaf without chlorophyll will show negative results whereas the parts that do have chlorophyll will show a positive result.

Effect of Light intensity, Carbon Dioxide Concentration, and Temperature on Rate of Photosynthesis

You need to be aware of a couple of different graphs. They are quite simple so don’t worry.

Graph 1: Rate of photosynthesis increases with light intensity until it plateaus. The graph plateaus because something else becomes the limiting factor (i.e. carbon dioxide). This means that even with a stronger light intensity, there may not be enough carbon dioxide to make the rate of photosynthesis even faster.
Graph 2: Rate of photosynthesis increases with increasing carbon dioxide concentration. Again, at a certain point the graph will plateau. In this case, the light may become the limiting factor.
Graph 3: The rate of photosynthesis increases with temperature until the graph reverses and eventually drops down to zero. This is because high temperatures will denature enzymes that are required for photosynthesis.

Leaf Structure

You need to know the structure of a leaf, and how this structure is adapted for photosynthesis.

The leaf consists of a broad, flat part called the lamina, which is joined to the rest of the plant by a leaf stalk or petiole. Running through the petiole are vascular bundles, which then form the veins in the leaf.

We will go through the functions of each of the structures in the diagram above:

Cuticle
- Made of wax which waterproofs the leaf
- Secreted by cells of the upper epidermis
Upper Epidermis
- A barrier against disease organism
- The cells are thin and transparent to allow light to enter the leaf
- No chloroplasts
Palisade Mesophyll
- Main site of photosynthesis
- Cells are long and packed with chloroplasts to trap light energy
- Receive CO₂ via diffusion from air spaces in the spongy mesophyll
Spongy Mesophyll
- Cells are spherical and loosely packed
- Contain chloroplasts but not as many as the palisade layer
- Air spaces allow gas exchange (i.e. carbon dioxide to the cells, and oxygen from the cells)
Vascular Bundle
- Contains xylem and phloem
- Xylem vessels bring water and minerals to the leaf
- Phloem vessels transport sugars and amino acids away from the leaf to the rest of the plant (translocation)
Lower Epidermis
- Acts as a protective layer
- It contains the stomata to regulate the loss of water vapour (transpiration)
- Site of gaseous exchange
Stomata
- Gaps in the underside of the leaf, surrounding by a pair of guard cells
- Guard cells control whether the stoma is open or closed
- CO₂ diffuses into the leaf and O₂ diffuses out
- Water vapor is lost through this structure in transpiration.

Adaptations of the Leaf

Adaptation	Function
Supported by stem and petiole	Exposes as much of the leaf as possible to sunlight and air
Large Surface Area	Exposes large area to sunlight and air
Thin	Allows sunlight to penetrate to all cells; quick diffusion of CO₂ & O₂
Stomata in lower epidermis	Quick diffusion of CO₂ & O₂
Air spaces in spongy mesophyll	Quick diffusion of CO₂ & O₂ from all cells
No chloroplasts in epidermal cells	Allows sunlight to penetrate to mesophyll layer
Chloroplasts present in mesophyll layer	Absorbs energy from sunlight so CO₂ combines with H₂0
Palisade cells arranged end on	Keeps as few cell walls between sunlight and chloroplasts
Chloroplasts arranged broadside on	Exposes as much chlorophyll as possible to sunlight
Chlorophyll arranged on flat membranes	Exposes as much chlorophyll as possible to sunlight
Xylem vessels near every mesophyll cell	Supplies water to cells (which may be used in photosynthesis)
Phloem vessels near every mesophyll cell	Takes away sucrose and other organic products of photosynthesis

Uses of Glucose

Used for Energy – Glucose may be broken down to release energy through respiration.

Stored as Starch – As a monosaccharide and a soluble sugar, it may dissolve in water and hence needs to be converted to starch, which can then be turned into granules and stored in chloroplasts.

Used for Proteins and Other Organic Substances – Glucose can be used as a starting point to make various substances. Sugars from photosynthesis can be used to make amino acids. For this nitrate ions will be important for plants as they are used in building amino acids (which eventually become proteins). A nitrate ion deficiency would slow down the growth of the plant, the stem would weaken. Lower leaves will turn yellow-ish and the upper leaves will become pale green as they die off. Nitrate ions combine with glucose to form amino acids

To make chlorophyll, plants require nitrate ions and magnesium ions. If a plant has a magnesium ion deficiency then they will lack chlorophyll. Leaves turn yellow from the bottom of the stem upwards and plant growth will slow down due to reduced photosynthesis.

Changed to Sucrose for Transport – Sucrose is small, like glucose, but less reactive which is why they dissolve in the sap in the phloem vessels and are distributed to whatever parts of the plant needs them.