Competitive Exams Botany Notes: Photosynthesis

What is Photosynthesis?

Photosynthesis is a chemical reaction in which carbon dioxide and water is changed to glucose by the action of chlorophyll and with sunlight energy.

Photosynthesis is performed by plants, green algae, and plant-like protists such as the

Euglena. To photosynthesize, a plant, or other heterotrophic organism, needs Carbon dioxide, water, light and chlorophyll.

Plants store food as starch. Thus, after producing glucose, the plant transforms glucose into starch, which is an insoluble polysaccharide, to be stored. Glucose goes down the stem towards the roots in the Phloem vessels in the vascular bundles, while water goes upwards the stem from the roots through the xylem vessels in the vascular bundles.

To find out if the plant has performed photosynthesis, you must do a starch test on a leaf. If the leaf has starch, then it must have photosynthesized but if the leaf has no light

Testing a Leaf for Starch

  1. Cut a leaf from a plant and boil it in a beaker with water to soften it.

  2. Dip it in alcohol (ethanol) to decolorize it. The leaf must be put in a boiling tube dipped in warm water. Don't heat up the boiling tube with alcohol because it is flammable.

  3. Put the decolorized leaf again in the warm water to soften it again.

  4. Put the leaf on a white tile and add two drops of iodine on the leaf.

Results for Iodine test

If the iodine turns blue-black, then the leaf has starch, hence it has photosynthesized.


De-starching occurs when the plant doesn't make any photosynthesis (e. g. Because it is in the dark) and so the plant uses its stored starch stored for energy. It turns starch into glucose and uses it up:

The Importance of Photosynthesis

Photosynthesis is the process in which plants get the energy from. Without it, plants wouldn't exist. Thus photosynthesis is indirectly useful for other animals, which eat plants.

Photosynthesis releases oxygen as a by-product of its reaction. Oxygen is used by almost all living organisms for the breakdown of glucose and release of energy.

Inside a Leaf

Photosynthesis happens in plants, exactly in the chloroplasts that are found in leaves.

The green part of the plant is usually the leaf, and this is because chloroplasts have a special green chemical called chlorophyll that converts sunlight into chemical energy.

The waxy cuticle is the uppermost part of the leaf. It makes the leaf waterproof and protects the leaf from losing water. It is transparent.

The upper epidermis is the second layer of the leaf, but the first layer that is made up of living cells. The cells in this layer don't have chloroplasts, so that light passes directly into next layer.

The palisade layer is a thick layer of elongated cells packed with chloroplasts. It is here that most photosynthesis takes place.

The spongy layer is characterized by air spaces between the cells, so diffusion of gases takes place efficiently, as photosynthesis uses carbon dioxide and produces oxygen. The cells in the spongy layer also have chloroplasts.

The palisade and the spongy layer are made up of cells called mesophyll cells.

The lower epidermis is similar to the upper epidermis, with the cells making it up that don't have chloroplasts, but this layer has stomata; tiny holes from which exchange of gases takes place. Stomata are surrounded by two guard cells, which are the only cells in the lower epidermis that have chloroplasts. These cells have thin cell walls on the outer side but wide cell walls on the inner side.

The following picture shows the structure of guard cells:

In the leave there are also vascular bundles (plant veins) that are made up of xylem and phloem vessels. Water and soluble minerals pass from the xylem vessels while sugars pass from the phloem vessels.

How are leaves adapted for photosynthesis

  • Leaves have numerous adaptations to ease photosynthesis.
  • They have a large surface area, for absorbing light and carbon dioxide.
  • Leaves are arranged so that they don't over-shadow each other, and all of them receive light.
  • They have a lot of stomata in the lower epidermis for gas exchange, carbon dioxide gets in and oxygen does out while photosynthesis takes place.
  • Leaves are thin to allow fast diffusion of carbon dioxide.
  • The waxy cuticle and epidermis are transparent to allow light passage throughout the leaf.
  • The place were most photosynthesis takes place; the palisade layer, is found near the upper side of the leaf, were most of the light comes.
  • The palisade layer is made up of palisade mesophyll cells, which are packed with chloroplast, and these organelles move around the cell so as to find the best position to find light.
  • There are air spaces around the spongy mesophyll cells to allow gas circulation.

Glucose and sugars

In the chemical reaction of photosynthesis, glucose and other sugars are produced.

With these, the plant can do a number of things:

  1. Respiration: Like any other living thing, plants need energy. Plants and animals do this my oxidizing glucose in the process called respiration, releasing water and carbon dioxide.

  2. Translocation: This means that the excess sugars produced by the leaves are transported into other parts of the plant, through the phloem vessels, that cannot make photosynthesis, such as roots, to supply their needs.

  3. Production of cell material: From sugars, the plant can make other important chemical and material such as proteins, fats and oils. In order to make some of these materials, the plants must also have other minerals absorbed from the soil such as nitrogen, sulphur and potassium. For instance, the plant must have a supply of nitrogen in order to produce proteins.

  4. Conversion to starch: Enzymes in the plant convert glucose into starch. This is done so that glucose can be stored. Since glucose is soluble, it cannot be stored; it can only be used straight away or transported. Thus the plant converts it into starch, which is insoluble and stores it. Starch is stored in special storage organs, which are formed by part of the plant swelling up. These storage organs can be formed in roots, leaves or stems. When energy is needed and no glucose is formed by photosynthesis, such as when it is dark, the chain of glucose molecules, which makes starch, uncoils back into single glucose molecules in a process called hydrolysis. When a plant performs hydrolysis, starch is mobilised, which means it can now be moved or transported in a solution since glucose is water-soluble.

  5. Storage in germination structures: The plant stores some food for the next generation by storing starch or fat in their seeds and fruits. When a seed germinates, food passes from the seed to the new growing plant until it can make its own food by photosynthesis. Some plants store food in tubers or bulbs that can also germinate.

If the soil is deficient in some of these important nutrients, one must add fertilizers in order to replenish the soil with vital minerals. Fertilisers can be either artificial, such as NPK (Nitrogen, Phosphorous, Potassium), super phosphates or natural, such as humus and manure. These increase crop yield, however, they are expensive and can decrease soil fertility in the long run.

Important terms in Botany

Water cultures: A full water culture is a solution, which has all the necessary minerals so that a plant to grow healthy.

Hydroponics: It is the method to grow plants without soil; in fact, it is sometimes called soil-less culture. Plants are grown with water cultures. Some advantages that this method has are that the crop yield is increased and the soil doesn't have to be fertilized each year.

Limiting Factors

Limiting factors stop the rate of photosynthesis from increasing further. The rate of photosynthesis is affected by water, temperature, level of carbon dioxide, and light. The relationship between each and every one of these factors and photosynthesis are described below:

If light increases, photosynthesis increases.

If water is plenty, photosynthesis increases.

If carbon dioxide is plenty, photosynthesis increases.

When temperature increases photosynthesis increases, up to a certain point, or else, above 35oC, photosynthesis halts completely in most plants.

Despite this, when one factor is increasing, the other factors cause the rate of photosynthesis to stay constant anyway.

Food Chains and Food Webs

There are various ways to represent who animals feed in a particular habitat. A food chain is one such a way to show what eats what. The following is an example of a simple food chain.

The first organism in a food chain is always a producer. Producers make their own food from the sun by photosynthesis. Plants are an example of a producer. The other organisms in the chain are called consumers because they consume (eat) the organism before them. The first consumer is called the primary consumer, then there is the secondary consumer and so on. The last organism in a food chain is always called the top carnivore.

The primary consumer is always a herbivore because it eats plants or another producer. The secondary consumer is a carnivore because it eats other animals. If an organism eats both plants and animals, then it is called an omnivore.

The arrows in the food chain represents the flow of energy or the phrase is eaten by:

The ultimate source of energy is always the sun, but it is usually not included in a food chain.

More often than not, an organism doesn't eat only one type of food, i.e.. Any animal eats more than one species of organism. In order to represent this situation, a food web is produced. A food web is a collection of food chains mixed together to get a clearer picture of what animals eat what.

A food web gives us more information about the feeding of animals than food chains.

Despite this though, it doesn't give us the number of organisms involved. To show the number of organisms involved in a food chain, a Pyramid of Numbers.

The first (bottom) layer in the pyramid is always the producer. Then following it are the primary consumer, then the secondary and so on. Two examples of a pyramids of numbers are shown here below:

In order to show the dry mass of the organisms in a food chain, a Pyramid of biomass is produced.

Short note on Xylem and Phloem Vessels

Plant veins are called vascular bundles. These are present in the leaves, in the stem and in the roots; it is the important for the transport of materials throughout the plant.

The vascular bundles are made up of two vessels namely the Xylem and the Phloem vessels. The xylem vessels carry water and minerals up from the roots to the leaves while the phloem vessels carry sugars solutions from the leaves to the rest of the plants.

The xylem vessels are made up of strong tubes of lignin. Lignin is a strong material formed from dead cells.

Phloem vessels are made up of sieve plates with sieve tubes supported by companion cells.