PHOTOSYNTHESIS [CSEC BIOLOGY & HSB]

SYLLABUS REFERENCE 

HSB

• [A8] explain the process of photosynthesis;
• [A9] investigate the effect of light and chlorophyll on the production of starch; 

CSEC BIOLOGY 

• [B2.2] describe the process of photosynthesis in green plants;
• [B2.3] relate the structure of the leaf of a flowering plant to its function in photosynthesis; 
• [B2.4] explain how environmental factors affect the rate of photosynthesis;

THE PROCESS OF PHOTOSYNTHESIS 

Photosynthesis is essentially the conversion of simple inorganic molecules (carbon dioxide) to complex, organic molecules (glucose), using light energy. 

The process involves several chemical reactions. The summary equation outlines the initial reactants and the final products. 

THE STAGES OF PHOTOSYNTHESIS 

Even though the process consists of multiple reactions, it can be divided into two stages. 

• LIGHT STAGE (aka Light-dependent Stage) - Light is required. 
• Water is split into hydrogen and oxygen. Excess oxygen diffuses out of the leaf as a waste product. The hydrogen participates in the Dark Stage. 
• DARK STAGE (aka Light-independent Stage) - Light is not required. 
• The hydrogen from the Light Stage reacts with carbon dioxide to produce glucose, a carbohydrate. 

DOWNSTREAM PRODUCTS OF PHOTOSYNTHESIS 

Glucose is the direct, final product of photosynthesis. 

However, it does not remain for long, as it is immediately converted to other products. The majority of it is converted to starch (for storage) and sucrose (for transport). 

WHY IMMEDIATE CONVERSION TO STARCH FOR STORAGE?

Glucose is soluble and can thus affect the water balance in a plant. If the concentration of glucose were to vary in different parts of the plant, then osmosis would take place, moving water around the plant. This would disrupt the entire organism. 

Starch, however, is insoluble in water, and so it has no effect on the concentration of solutions. This means that it can be stored in different parts of the plant without having any effect on its water balance. 

Starch is also a very compact molecule, and it is easily broken down to glucose when needed by plant cells. 

ADAPTATIONS FOR PHOTOSYNTHESIS 

LEAVES ARE THE MAIN SITE FOR PHOTOSYNTHESIS. THEREFORE, MOST OF THE ADAPTATIONS OCCUR HERE, IN BOTH ITS EXTERNAL AND INTERNAL STRUCTURE. 

Leaves are green, and flat, with a large surface area. 

• Leaves are green due to the presence of the pigment chlorophyll in the chloroplasts. This is what absorbs the light energy. 
• The leaf blade is flat with large surface area to maximise exposure of the cells to the sunlight or other light source. Note that, in order to maintain a large surface area, narrow leaves tend to be long, and vice versa. 

Leaves are thin. 

 The leaves tend to be about 1 mm thick. This allows rapid diffusion of carbon dioxide and oxygen in and out of the leaf. 

Strengthened veins. 

The veins transport water and other nutrients into and out of the leaf. They tend to be strengthened with materials such as lignin, as they also support the very thin leaves. 

Stomata 

• These are pores found mostly on the underside of the leaf. They allow the uninterrupted flow of gases into and out of the leaf. 
• Guard cells on the sides of each stoma can close them, to prevent excessive water loss via evaporation out of the leaf whenever conditions are too hot. 

INTERNAL LEAF ADAPTATIONS 

MICROGRAPH OF LEAF CROSS-SECTION 

DIAGRAM OF LEAF CROSS-SECTION 

• Waxy cuticle is waterproof and thus protects the leaf from excessive water loss. It is also transparent, so light can pass through it to the cells below. 
• Upper epidermis. The cells of this tissue are relatively transparent, so that light can get to the cells below. 
• Palisade mesophyll is the main photosynthetic tissue in the leaf. It is a tightly packed layer of long, rectangle-shaped cells. These cells are the most active in photosynthesis, and are thus packed with chloroplasts. When light levels are low, the chloroplasts cluster close to the surface of the leaf. 
• Spongy mesophyll layer includes more rounded cells, loosely arranged with many air spaces between them. The air spaces allows the diffusion of carbon dioxide and oxygen through the leaf. These cells are less active with respect to photosynthesis, so they have less chloroplasts than the palisade cells. 
• Vascular bundle consists of xylem and phloem vessels. 
• The xylem transports water and dissolved minerals throughout the plant from the roots. 
• The phloem transports food substances from the leaf to other parts of the plant. 
• Lower epidermis protects the leaf and contains most of the stomata. 

ENVIRONMENTAL FACTORS 

EXPERIMENTAL INVESTIGATIONS OF PHOTOSYNTHESIS 

CONDITIONS FOR PHOTOSYNTHESIS

Photosynthesis requires light, carbon dioxide, and water. 

This is now common knowledge, but this had to be determined via experimentation. 

Coincidentally, these experiments are a perfect study in experimental design. Therefore, CXC gives a lot of attention to these experiments. 

BASIC EXPERIMENTAL COMPONENTS 

• Destarching - One destarches one or more plants of the same species, in each experiment. This is a widely known technique, so one does not have to describe it in detail unless asked to do so. 
• How to destarch a plant? - Place the plant in a dark space such as a cupboard for at least 24 hours. Ideally, if the plant is health, leave it in darkness for about 48 hours. 
• Manipulated variable - Example would be removing carbon dioxide from the air around the plants being investigated.
• Control - Example would be allowing another plant to have access to natural levels of carbon dioxide. 
• Starch Test - This is also a widely known technique. Therefore, one does not have to describe it in detail, unless asked to do so. 

STARCH TEST 

One cannot simply add a few drops of iodine solution to the leaves of a plant to test for starch. Here's why:

• The waxy cuticle forms a waterproof layer, preventing the iodine from penetrating to the cells. 
• The green colour of the leaf would mask any slight colour change of the iodine. 

Here's a video demonstrating the starch test on a leaf.  

ANALYSIS OF RESULTS 

Starch in a plant comes only from glucose, which is a product of photosynthesis. 

If photosynthesis does not occur, there will not be any glucose available to convert to starch. 

PHOTOSYNTHESIS EXPERIMENTS 

IS CHLOROPHYLL NECESSARY FOR PHOTOSYNTHESIS?

In this case, the manipulated variable is a leaf with parts that are white, and therefore does not have any chlorophyll. 

This type of leaf is called a variegated leaf. 

The green parts of the leaf will be the control, as chlorophyll is present.

The leaf can be drawn before conducting the starch test, showing white and green regions to compare. 

Another option would be to cut discs out of the white and green parts of the leaf. Those discs would then be tested separately for the presence of starch. 

IS CARBON DIOXIDE NECESSARY FOR PHOTOSYNTHESIS?

In this case, the plants are placed in a controlled space, under sealed bell jars. 

The manipulated variable is the removal of carbon dioxide from the surrounding air. This is accomplished by adding a beaker of potassium hydroxide (soda lime) to one of the bell jars ('Y' in the diagram). This extracts carbon dioxide from the air under the bell jar. 

The control would be bell jar 'X' in the diagram, in which there is a beaker of distilled water instead of potassium hydroxide. The carbon dioxide remains within the bell jar. 

Leave the two plants for a few hours under sunlight, then conduct the starch test on leaf samples from both plants. 

IS LIGHT NECESSARY FOR PHOTOSYNTHESIS?

In this case, the control will be a plant left in the light for a few hours. 

The manipulated variable will be leaving another plant in a dark space for the same amount of time. 

Another option would be to use one plant but cover some of the leaves with foil or black paper.