The light and dark reactions of Photosynthesis.
By: Peter Miles
Tags: Photosynthesis, Chloroplast, Chlorophyll, Climate Change, Biology

Including Photosystems, I and II.

Photosynthesis is a two-stage process, the light reactions using photons of light energy to drive molecules to a higher energy state and dark reactions which use the energy molecules to fix carbon dioxide into 6 carbon sugars.

The dark reaction doesn’t have to be in the dark and is also called the light independent reaction.

These reactions are sometimes referred to as photosynthesis 1 (light) and photosynthesis 2 (dark), not to be confused with photosystems I and II, see below.

Photosynthesis is an autotrophic process fundamental to all life on earth, converting solar energy into chemical energy and sugars or carbohydrates.

Diagrammatic representation of chloroplasts. Image — Pixy.org Creative Commons.

Photosynthesis occurs in specialized organelles within the cell called chloroplasts. Chloroplasts contain chlorophyll pigments which are responsible for the green colour of leaves. Light is captured and used to convert carbon dioxide from the atmosphere into sugars for use by the plant.

In photosynthesis the carbon dioxide CO2 is split into carbon C and oxygen 02. This occurs in the presence of light by one CO2 molecule combining with two water molecules H2O. Oxygen atoms in the water are released, along with one H2O and carbohydrate, generally triose (3 carbon) sugars are produced.

The abundance of life depends on this chemical reaction:

3 CO2 + 6 H20 + light -> C3H6O3 + 3 O2 + 3 H20

Carbon dioxide + water + light ->sugar + oxygen + water

It is also often written thus:

6CO2 + 12H20 + light -> C6H12O6 + 6O2 + 6H2O

This reaction occurs in three stages:

The first is absorption of light photons by the chlorophyll pigment molecules, with the energy of the light transferred to the photosystem reaction centres.

In the second stage electrons are released by splitting water, producing protons and oxygen. Electrons are then transported (only the energy is transported not the electron) on the electron transport chain in the thylakoid membrane of the chloroplast. This produces the high energy compounds NADPH from NADP+ and ATP from ADP ready for the third stage.

The reactions so far are the light reactions of photosynthesis as they require the input of light and its energy.

The third stage is the use of NADPH and ATP in the photosynthetic reduction of carbon cycle, the Calvin cycle, to assimilate CO2 and produce the triose sugars. The enzyme acting as a catalyst in this reaction is Rubisco.

This third stage can operate with or without light and is known as the dark reaction of photosynthesis.

Plants are grouped as having C3, C4 or CAM photosynthesis depending on the photosynthetic processes they use. C3 plants use the Calvin cycle described above with those stages occurring at the same time and includes many of the temperate plants. See my C3, C4, CAM Photosynthesis article.

Light reactions of photosynthesis shown as a representation of the structures involved inside the thylakoid lumen. Image — Wikimedia Commons.

The following explains the two light reactions in a little more detail.

Inside the chloroplast organelles are compartments called the thylakoid lumen.

The membrane of the chloroplast organelle is shown in the image above by two rows of yellow dots.

A photon of light, on the left, is captured in the antenna complex in the mesophyll cells of plants and called photosystem two, here labelled PSII.

The light energy is transferred to the reaction centre of the photosystem, this is a group of proteins with a pair of chlorophyll molecules called P680.

In the reaction centre an electron e- gets excited by the light energy and the energy moves to the right in the diagram along an electron transport chain. This eventually leads to an electron being removed from a water molecule, bottom left of diagram.

When two electrons are excited and removed from water, hydrogen ions, protons H+ and oxygen are produced.

Some of the excited electrons transfer their energy to the H+ ions, across bottom of diagram.

At PSI another photon of light enters exciting the electron to an even higher state, P700, then transferring its energy to the molecule NADPH, this is Photosystem I.

The numbering of photosystem II and I is historical with photosystem I discovered and named first. Even though it is shown occurring second in the diagram, in practice they operate at the same time. Photosystem II has optimal light absorption at 680 nanometres, hence P680 and photosystem I at 700 nanometres, P700.

The protons H+ move across the membrane through a protein gate using energy from PSII, this energy is then lost to ADP recharging it to ATP. This protein is called ATP synthase (synthesizes ATP).

Diagram of light energy reactions of photosynthesis. Image — Wikimedia Commons.

In the image above photons of light energy cause an electron to become excited at PSII P680, then it releases that energy as it moves along the electron transport chain and through the membrane synthesizing ATP, then the electron gets even more excited at PSI P700 and releases energy to form NADPH. It is referred to as the Z diagram (Z shape on its side).

Chlorophyll a and chlorophyll b, light wavelengths. Image — Wikimedia Commons.

Chlorophyll is a pigment and the main light absorbing pigment is ‘chlorophyll a’, which absorbs blue and red light. The other chlorophyll pigment, ‘chlorophyll b’, absorb light at other wave lengths, blue-green and yellow-orange, and in the reaction centre transfers the energy to ‘chlorophyll a’. This gives a broader spectrum of light wavelength being absorbed, mostly in the visible light range.

For further information this is a useful lecture:

https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-calvin-cycle-reactions/v/photosynthesis-calvin-cycle

Reference:

Knox, B., Ladiges, P., Evans, B., Saint, R., (2014). Biology: An Australian Focus (5th Ed.). NSW. Australia.: McGraw-Hill Education. Book.