Light-Dependent Reaction – Photosynthesis Ep 1

Photosynthesis is the process used by plants and other photosynthetic organisms to make their own glucose. The glucose can then go on to be used in respiration, stored as starch, or used in other reactions. Photosynthesis requires light energy, but it is actually only the first stage which is light-dependent. We will focus on the light-dependent reaction today, after a brief introduction.

Photosynthesis equation and photosystems

Photosynthesis has the following equation:

6CO2 + 6H2O (+energy) → C6H12O6 + 6O2

The reaction takes place in organelles called chloroplasts. Photosystems are found in the thylakoid membranes within chloroplasts. We will come across photosystem I (PSI) and photosystem II (PSII) as we go through this topic. Photosystems are a combination of protein and photosynthetic pigments. It is the photosynthetic pigments which absorb light of a certain wavelength – the main three found in chloroplasts are chlorophyll a, chlorophyll b, and carotene. Chlorophyll is what give leaves their green colour because it absorbs red and blue light but reflects green light.

The mix of photosynthetic pigments in photosystems contains primary pigments (reaction centres where electrons are excited) and accessory pigments (light-absorbing compounds which transfer light energy to the reaction centre).

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The Light-Dependent Reaction

The first stage of photosynthesis requires light energy and takes place in the thylakoid membranes where the photosystems are located. The two photosystems are linked by electron carrier proteins, meaning that electrons can be transferred between the two. (In reality there is a series of electron carrier proteins creating an electron transport chain, but to keep things simpler I’ve just drawn one in my diagrams.)

The light-dependent reaction involves two types of photophosphorylation – using light energy to add a phosphate group to a molecule (in this case Pi is added to ADP to form ATP). So unfortunately there are two processes to learn but the principles are very similar.

Cyclic Photophosphorylation

This is the simpler of the two types of photophosphorylation and only uses PSI. It does not need water, does need produce oxygen, and does not produce reduced NADP (see next section).

  1. Light energy (wavelength 700nm) is absorbed by photosynthetic pigments (e.g. chlorophyll) in PSI, which excites electrons so they move to a higher energy level and are released from the pigments. This is called photoionisation (removing electrons using light energy).
  2. The electrons move to the electron carrier proteins and lose energy. The energy is used to actively transport H+ ions from the stroma into the thylakoid to create a concentration gradient across the thylakoid membrane.
  3. The electrons are passed back to PSI (which is why it is a cyclic process).
  4. H+ ions diffuse down the concentration gradient into the stroma through ATP synthase. This drives phosphorylation of ADP to ATP. This is called chemiosmosis, the same process which is found in oxidative phosphorylation.
Cyclic photophosphorylation

Non-cyclic photophosphorylation

This version is a little more complicated. But it essential for this version to be carried out because it produces reduced NADP. NADP is a coenzyme, similar to NAD and FAD in respiration. The reduced form is needed for the light-independent reaction, so it must be produced here otherwise photosynthesis would grind to a halt.

Here is how non-cyclic photophosphorylation works. Remember that light energy causes three things to happen at the same time here – it excites electrons in PSI, excites electrons in PSII, and splits water in a process called photolysis.

  1. Light energy (wavelength 680nm) is absorbed by the photosynthetic pigments (e.g. chlorophyll) in PSII, which excites electrons so they move to a higher energy level and are released from the pigments (photoionisation).
  2. The electrons move to the electron carrier proteins and lose energy. The energy is used to actively transport H+ ions from the stroma into the thylakoid to create a concentration gradient across the thylakoid membrane.
  3. The electrons are passed to PSI to replace electrons which have been lost during photoionisation of the chlorophyll in PSI.
  4. The electrons lost from PSII are replaced by photolysis of water. Light energy causes water inside the thylakoid to split into electrons (which move to PSII), H+ ions, and oxygen. Water is oxidised to oxygen.
  5. H+ ions diffuse down the concentration gradient created in step 2 into the stroma through ATP synthase. This drives phosphorylation of ADP to ATP. (This is chemiosmosis.)
  6. The excited electrons lost from PSI are accepted by NADP (along with a H+ ion) to form reduced NADP in the stroma (so formation of reduced NADP requires light energy).
Non-cyclic photophosphorylation

Note: remember that H+ ions are in effect just protons, so a concentration gradient of H+ ions could be called a proton gradient.

In the next article we will see what the reduced NADP and ATP produced during the light-dependent reaction are used for.

Summary

I know it’s a lot. But it’s pretty cool don’t you think? If it wasn’t for this process, animals wouldn’t be able to exist because photosynthesis is a crucial first step in the food chain. Here is a few summary points but it’s important to get to grips with the details for this one.

  • The light-dependent reaction occurs across the thylakoid membranes found within chloroplasts.
  • Photosystems contain photosynthetic pigments (including chlorophyll) which can absorb light energy.
  • Light energy excites electrons in photosynthetic pigments, enables reduction of NADP to reduced NADP, and enables photolysis of water.
  • Photophosphorylation requires electron carrier proteins and can be cyclic or non-cyclic.
  • Chemiosmosis allows synthesis of ATP by ATP synthase.
  • Photolysis of water produces oxygen – water is oxidised to oxygen.

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