Chapter 13: Photosynthesis

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar. In photosynthesis, light energy is absorbed by chlorophyll pigments and converted to chemical energy, which is used to produce complex organic molecules. Oxygen is a waste product of this reaction and is released into the atmosphere.

Photosynthesis reaction involves two reactants, carbon dioxide and water. These two reactants yield two products, namely, oxygen and glucose. Hence, the photosynthesis reaction is considered to be an endothermic reaction. Following is the photosynthesis formula:

6CO2    +   6H2O  —> C6H12O6  + 6O2

Photosynthetic Pigments

There are four different types of pigments present in leaves:

  1. Chlorophyll a
  2. Chlorophyll b
  3. Xanthophylls
  4. Carotenoids

Chlorophyll

Chlorophyll is a green pigment found in the chloroplasts of the plant cell and in the mesosomes of cyanobacteria. This green colour pigment plays a vital role in the process of photosynthesis by permitting plants to absorb energy from sunlight. Chlorophyll is a mixture of chlorophyll-a and chlorophyll-b. Besides green plants, other organisms that perform photosynthesis contain various other forms of chlorophyll such as chlorophyll-c1, chlorophyll-c2,  chlorophyll-d and chlorophyll-f.

Chloroplast

  • A chloroplast is an organelle within the cells of plants and certain algae that is the site of photosynthesis, which is the process by which energy from the Sun is converted into chemical energy for growth. The main role of chloroplasts is to conduct photosynthesis. They also carry out functions like fatty acid and amino acid synthesis. Each chloroplast is surrounded by a double-membrane envelope
    • Each of the envelope membranes is a phospholipid bilayer
  • Chloroplasts are filled with a fluid known as the stroma
    • The stroma is the site of the light-independent stage of photosynthesis

Stages of Photosynthesis

There are two stages of photosynthesis and they are:

  1. Light Dependent Stage

The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem. Photosystems exist in the membranes of thylakoids. A pigment molecule in the photosystem absorbs one photon, a quantity or “packet” of light energy, at a time. It converts as follows:

  • The chlorophyll in the plants absorbs sunlight and transfers to the photosystem which are responsible for photosynthesis.
  • Water is used to provide hydrogen ions and electrons but also produces oxygen.
  • The electrons and protons are used to produce NADPH (the reduced form of nicotine adenine dinucleotide phosphoric acid) and ATP (adenosine triphosphate).
  • ATP and NADPH is energy storage and electron carrier/donor molecule. Both ATP and NADPH are used in the next stage of photosynthesis.
  • The chlorophyll molecule regains the lost electron from a water molecule through a process called photolysis, which releases dioxygen (O2) molecule.
  1. Light Independent Reaction

Light-independent reaction is a series of biochemical reactions in photosynthesis not requiring light to proceed, and ultimately produce organic molecules from carbon dioxide. The energy released from ATP (produced during the light reactions) drives this metabolic pathway.

The light-independent reactions of photosynthesis take place within the stroma. It contains enzymes that work with ATP and NADPH to “fix” carbon from carbon dioxide into molecules that can be used to build glucose. The chloroplasts own genetic material (separate from that of the cell) is also stored in the stroma. The energy and hydrogen are used during the light-independent reactions (known collectively as the Calvin cycle) to produce complex organic molecules, including (but not limited to) carbohydrates, such as:

    • Starch (for storage)
    • Sucrose (for translocation around the plant)
    • Cellulose (for making cell walls)
  • This stage of photosynthesis does not, in itself, require energy from light (hence light-independent) and can therefore take place in light or darkness. However, as it requires inputs of ATP and reduced NADP from the light-dependent stage, the Calvin cycle cannot continue indefinitely in darkness, as these inputs will run out
  • There are three main steps within the Calvin cycle:
    • Rubisco catalyzes the fixation of carbon dioxide by combination with a molecule of ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound
    • GP is reduced to triose phosphate (TP) in a reaction involving reduced NADP and ATP
    • RuBP is regenerated from TP in reactions that use ATP

Limiting factors of Photosynthesis

The major limiting factors for photosynthesis are light intensity, temperature, and carbon dioxide levels. If there is a shortage of any of these factors, photosynthesis cannot occur at its maximum possible rate.

Light intensity

Without enough light, a plant cannot photosynthesize very quickly – even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the rate of photosynthesis. The rate of photosynthesis increases as light intensity increases:

  • The greater the light intensity, the more energy supplied to the plant and therefore the faster the light-dependent stageof photosynthesis can occur
  • This produces more ATP and reduced NADP for the Calvin cycle (light-independent stage), which can then also occur at a greater rate

Carbon dioxide concentration

Even if there is plenty of light, a plant cannot photosynthesize if there is insufficient carbon dioxide. The rate of photosynthesis increases as carbon dioxide concentration increases:

  • Carbon dioxide is one of the raw materials required for photosynthesis
  • It is required for thelight-independent stage of photosynthesis, when CO2 is combined with the five-carbon compound ribulose bisphosphate (RuBP)
  • It means the more carbon dioxide that is present, the faster this step of the Calvin cycle can occur and the faster the overall rate of photosynthesis

Temperature

  • If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesize if it gets too hot. As temperature increases the rate of photosynthesis increases as the reaction is controlled by enzymes
  • the Calvin cycle is affected by temperature, as the light-independent reactions are enzyme-controlled reactions