Phases of Photosynthesis

Phases of photosynthesis

The process of photosynthesis is carried out in stages in two stages:

  • The light-dependent stage (or light stage);

  • The carbon fixation stage of which the Calvin cycle is a part.

Plants, algae, and cyanobacteria are photosynthetic organisms. Photosynthesis occurs inside plant cells in small objects called chloroplasts. 

Chloroplasts contain a green substance called chlorophyll. The chlorophyll from the green plants absorbs the light energy needed to make photosynthesis happen. 

Stage one of the photosynthesis: Light Reactions

The light stage is a biological process in which energy from sunlight is converted into chemical energy. Chlorophyll and other photosynthetic pigments such as carotene absorb energy from light and break up water molecules, thus releasing oxygen and carbohydrate molecules.

Photosynthetic Processes

The photosynthetic process takes place within chloroplasts. Within these, a system of membranes form stacks of flattened bags (thylakoid membranes).

Within these membranes, we find chlorophyll molecules. Chlorophyll molecules are aggregated to form so-called photosystems. Photosystem I and photosystem II can be distinguished.

The reaction center contains a unique pair of chlorophyll molecules, often called “special pair.” Once the energy reaches the “special pair,” it will no longer be passed on to other pigments through resonance energy transfer. Instead, when the “special pair” is exited, it can lose an electron.

The electron passes to another molecule in the complex called the primary electron acceptor. With this transfer, the electron will begin its journey through an electron transport chain.

What Are Photosystems?

Photosystems are a collection of pigment molecules arranged to surround a particular "trap" molecule of chlorophyll. The light energy of the photon passes from molecule to molecule until it reaches the special chlorophyll.

Photosystem I

In photosystem I, the trapped molecule is excited by a wavelength of 700 nm, in photosystem II of 680 nm.

Photosystem I is composed of an LHC (light-capturing complex). The LHC comprises about 70 chlorophyll molecules, 13 different polypeptide chains, and a reaction center containing about 130 chlorophyll molecules.

Photosystem II

Photosystem II is also made up of an LHC. This LHC comprises approximately 200 chlorophyll molecules, different polypeptide chains, a reaction center formed from around 50 chlorophyll molecules.

All of these molecules are capable of capturing energy from solar radiation. However, only those of chlorophyll can move to an excited state that activates the photosynthetic reaction.

Molecules that only have the capture function are called antenna molecules. Those that activate the photosynthetic process are called reaction centers.

How Is Energy Captured?

The light stage is dominated by chlorophyll a. Chlorophyll molecules selectively absorb light in the visible spectrum's red and blue-violet parts. The absorption is carried out through a series of other pigments. The energy captured by the chlorophyll molecules allows the promotion of electrons from lower energy atomic orbitals to higher energy orbitals through electron transporters.

The cleavage of water molecules replaces these.

The electrons released by the chemical reaction of chlorophyll II are fed into a transport chain. During the transport chain, they lose energy and move to a lower energy level. The lost power pumps protons from the stroma into the thylakoid space, creating a proton gradient.

In the end, the electrons arrive at the photosystem I.

Photosystem I, in turn, has lost other electrons due to light. Electrons lost are transferred to ferredoxin, which reduces NADP + to NADPH.

It is possible to form one ATP molecule for every two electrons lost by photosystems.

Stage Two of the Photosynthesis: Dark Reactions

The dark stage is a series of light-independent reactions that convert carbon dioxide and other compounds into glucose. This conversion is not directly dependent on light.

These dark reactions take the light stage products, mainly ATP (adenosine triphosphate; the energy currency of cells.) and NADPH (nicotinamide adenine dinucleotide phosphate), and carry out more chemical processes.

The dark stage converts the inorganic matter into organic matter.

These reactions do not require light to occur. These reactions take the products from the light stage and perform other chemical processes on them.

There are two dark reactions: carbon fixation and the Calvin Cycle.

Carbon Fixation Stage and the Calvin Cycle

The Calvin cycle uses the energy of short-lived electronically excited electron carriers to convert carbon dioxide and water into organic compounds that can be used by the body. This set of reactions is also called carbon fixation.

The Calvin cycle enzymes are functionally equivalent to most of the enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway. However, Calvin cycle enzymes are found in the chloroplast stroma rather than the cell cytosol, separating the reactions.

The light and light-dependent reaction's products activate these enzymes. These regulatory functions prevent the Calvin cycle from breathing in carbon dioxide.


Published: October 6, 2020
Last review: October 6, 2020