Photosynthesis in higher plants encompasses additional processes, yet its fundamental nature remains unchanged. It is a physicochemical process that utilizes sunlight to synthesize organic compounds. During this process, oxygen is emitted into the atmosphere.
Photosynthesis takes place in the chloroplasts located within the mesophyll cells of the leaves. There are four pigments that play a role in photosynthesis:
- Chlorophyll a
- Chlorophyll b
- Xanthophylls
- Carotenoids
We are all familiar with the process of photosynthesis in smaller plants. Let us delve into the notes on photosynthesis in higher plants to thoroughly examine the process.
Processes of Photosynthesis in Higher Plants
Photosynthesis in higher plants includes the following processes:
- Light Reaction
- Dark Reaction
Light Reaction
This phenomenon occurs when light is present.
The pigment captures light and generates energy in the form of ATP.
The process involves the absorption of light, the splitting of water, the release of oxygen, and the formation of ATP and NADPH.
The protein-bound pigment molecules create the light-harvesting complexes found within two photosystems—PS-I and PS-II. Each photosystem contains a reaction center made up of a chlorophyll a molecule, along with antennae that house accessory pigments.
The reaction center for PS-I is designated as P-700 due to the absorption peak for chlorophyll a being at 700 nm, while PS-II is referred to as P-680 because the absorption peak for chlorophyll a is at 680 nm.
Dark Reaction
Dark reaction is called so because it is a light-independent process in which carbohydrate molecules are formed from carbon dioxide and water molecules. It is also known as the carbon-fixing reaction. The dark reaction occurs in the chloroplast’s stoma, utilizing the light reaction’s products.
Mechanism of Dark Reaction
It is a light-independent or biosynthetic phase of photosynthesis. It assimilates the carbon dioxide with the help of ATP and NADPH. The dark reactions occur through the Calvin cycle and CAM cycle.
Photophosphorylation
The process of ATP formation in the presence of sunlight is termed photophosphorylation. It can be categorized into two types:
- Non-cyclic photophosphorylation
- Cyclic photophosphorylation
Non-cyclic Photophosphorylation
PS-II absorbs light at a wavelength of 680 nm, leading to the excitation of electrons. These excited electrons are accepted by an electron acceptor and subsequently transferred to the electron transport system.
The electrons from the electron transport system are then passed on to PS-I.
Early Experiment of Photosynthesis Done by Joseph Priestley
- Joseph Priestley, in 1774, discovered that plants replenish oxygen in the air, which respiratory animals and burning candles eliminate.
- Julius von Sachs discovered in 1854 that the green parts of plants are where glucose is produced, which is usually stored as starch.
- W Engelmann (1843-1909) carried out a fantastic experiment. He used a prism to separate light into its spectrum components before lighting a green alga named Cladophora that was dissolved in an anaerobic bacteria solution. The microorganisms were used to discover where O2 originated. He found that germs gathered primarily in the split spectrum’s blue and red light areas. So, the first photosynthesis action spectrum was thus defined, resembling the absorption spectra pigments of chlorophyll a and b.
- The empirical equation then depicted the entire process of photosynthesis for oxygen-evolving organisms: Where CH2O is a carbohydrate like glucose (6 carbon sugar).
- Cornelius van Neill, a microbiologist, made a notable participation in the understanding of photosynthesis by demonstrating that photosynthesis is a light-dependent reaction in which H2 from an appropriate hydrogen compound, i.e., H2O, reduces carbon dioxide to carbohydrates.
- The equation can be expressed as,
- H2O is the H2 donor in green plants and is oxidized to O2. Some life forms do not produce O2 during photosynthesis. As a result, it is deduced that O2 created by plants (green) comes from H2O rather than CO2.
- As a result, the photosynthesis equation is as follows:
Photosynthesis occurs in the chloroplasts of mesophyll cells of the green leaves, specifically in the green leaves of plants.
Pigments Involved in Photosynthesis
Pigments are materials capable of absorbing light at specific wavelengths.
The pigments present in leaves indicate that the color we perceive is not attributed to a single pigment but rather to four distinct pigments:
Chlorophyll a, which is the most prevalent plant pigment globally (appearing bright or blue-green in chromatograms), chlorophyll b (yellow-green), xanthophylls (yellow), and carotenoids (ranging from yellow to yellow-orange).
Photosystem II Photosynthetic pigments are arranged into two separate photochemical light-harvesting complexes (LHC) located within Photosystem I (PS I) and (PS II).
The LHC consists of hundreds of pigment molecules that are attached to proteins. Each photosystem contains all the pigments (with the exception of one chlorophyll a molecule) that together form a light-harvesting system, commonly referred to as antennae.
The individual chlorophyll molecule constitutes the reaction centre.
The reaction centre varies between the two photosystems. In PS I, the reaction centre chlorophyll a exhibits an absorption peak at 700 nm; thus, it is designated as P700, whereas in PS II, it has absorption maxima at 680 nm and is referred to as P680.
Chlorophyll is a porphyrin derivative that is complexed with a magnesium ion, found in plant chloroplasts, algae, and cyanobacteria. Chlorophyll plays a crucial role in the photosynthesis process. It captures light in the blue and red regions of the visible spectrum and transfers the energy from the absorbed photons to an electron, which is subsequently utilized to generate ATP.
