Photosynthesis – The Biological Pathway

Photosynthesis is a process that takes place in the chloroplasts. Chloroplasts are found in many calls such as guard cells and mesophyll (both spongy and palisade) cells. Chloroplasts have a pigment, chlorophyll, which traps light and is able to produce energy from this trapped light. There are two different types of biological pathways energy can be produced and that is the light dependent stage and the light independent stage (The Calvin Cycle). Firstly the light dependent stage occurs within the membrane of the thylakoid within the chloroplast. This process splits water, by a process called photolysis, allowing energy to be released resulting in the production ATP, reduced NADP and also oxygen as a waste product. Meanwhile, the light independent stage (The Calvin Cycle) occurs within the stroma of the chloroplast. This is the site where carbon dioxide combines to a five-carbon acceptor and also gets phosphorylated and reduced by the produced from the light dependent stage.


These pigments generally absorb light in the red and violet areas of the absorption spectrum.


These pigments generally absorb light within the blue-violet region of the absorption spectrum.

Photosynthetic Pigments

Different pigments allow the absorption of different wavelengths giving rise to increased rate of photosynthesis. The main pigments found are carotenoids and chlorophyll. Each pigment absorbs a particular colour within the absorption spectrum. When there is absorption on a particular wavelength, an action spectrum can be produced. Absorption spectrum is a graph that shows how much light the particular pigment is absorbing at a particular wavelength. An action spectrum is a graph that illustrates the rate of photosynthesis used at different wavelengths of the absorption spectrum.

How Light Is Harvested In A Chloroplast

Upon the thylakoid membranes are thousands of specialised proteins associated with a particular pigment i.e. Chlorophyll a. This is known as the Antenna Complex. These specialised proteins assist with the funnelling of photons of light energy to the pigment in the middle, called the reaction centre. Each pigment molecule passes the photon of light energy to the next pigment molecule.

When light strikes the antenna complex, the photon is passed to the reaction centre, exciting an electron. This electron in an excited stage raises to a higher energy level resulting in a flow of electrons.

Light Dependent Stage

The stages occur on the thylakoid membrane of the chloroplast. And can be represented as a ‘Z Scheme’. There are two processes to Cyclic and Non-Cyclic phosphorylation. Part 5.(ii). Tells you how cyclic photophosphorylation occurs.

1. Light strikes the photosystem II and light is passed to chlorophyll a in the reaction centre. Two electrons get excited and rise to a higher energy level.

2. The electrons then pass to an electron acceptor.

3. The electrons are passed down a series of transport chains through a proton pump, resulting in ATP being formed by a process called Photophosphorylation. The electron then passes onto Photosystem I.

4. Since Photosystem II has lost an electron, it is very unstable therefore enzymes split water into protons, electrons and oxygen. This process is called photolysis as it uses light energy.

5.(i) Light then strikes Photosystem I resulting in an electron to be excited and move to a higher energy level where is passes to a final electron acceptor of NADP. The protons produced from photolysis of water also combine to produce NADPH2 (reduced NADP).

5.(ii). The electron can also be reused to replace the electron lost in Photosystem I in a process called Cyclic Photophosphorylation. The electron is fed back into the electron transport chain and then used to resynthesize ATP.

The ATP and Reduced NADP produced in this stage are used in the light independent stage (The Calvin Cycle).

Light Independent Stage – The Calvin Cycle

This process occurs within the stroma of the chloroplast and uses the products that were produced in the light dependent stage. The ATP is used as a source of energy. The reduced NADP is used as a source of reducing power to reduce hexose sugar and the carbon dioxide.

1. Ribulose Bisphosphate (RuBP) is a five carbon acceptor molecule that accepts carbon dioxide with the help of an enzyme, Rubisco, to form an unstable six carbon compound.

2. The unstable six carbon compound immediately splits into two molecules of three carbon compound called Glycerate-3-Phosphate (GP).

3. Glycerate-3-Phosphate (GP) is then phosphorylated by ATP and is then reduced by reduced NADP to form triose phosphate.

4. Only some of the triose phosphate is used and converted into hexose phosphate to be condensed into useful products such as starch. While the rest of the product is resynthesized by many reactions, using ATP for energy, to resynthesize Ribulose Bisphosphate (RuBP). This is what allows the cycle to continue.

Nutrition For Plants

Plants need nutrition in the form of minerals. These are normally taken up through the root from the soil, but can be gathered from insects with carnivorous plants such as the Venus flycatcher (Dionaea muscipula). Minerals are needed to synthesise the required compounds for growth of the plant. Macronutrients such as sodium, nitrate, magnesium and many more are needed in substantial amounts for a plant to function properly. On the other hand there are micronutrients such as manganese and copper that are only needed in small amounts for the plant to function efficiently.

Nitrogen as nitrates

The roots absorb nitrogen in the form of nitrates. Nitrates travel in the xylem vessels as nitrates while it travels in the phloem as amino acids. When there are scarce supplies of nitrogen, a yellow colour forms in the leaf called chlorosis. This is due to inadequate supply of nitrates for the production of chlorophyll. But this is only common in older leaved.


The roots absorb magnesium in the form of magnesium ion. The magnesium ions function as a way to help in the production of chlorophyll and activation of ATPase. It is transported around the plant in xylem vessels to places where new leaves are being made. This is due to almost all tissue requiring magnesium.


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