Light-Dependent Reaction - Photosynthesis Ep 1

Photosynthesis is the process used by plants and other photosynthetic organisms to produce glucose. The glucose can then go on to be used in respiration, stored as starch, or used for other purposes. Photosynthesis requires light energy, but it is actually only the first stage which is directly 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 → C₆H₁₂O₆ + 6O₂

It is an endothermic reaction because it requires light energy. The reactions take place in organelles called chloroplasts. Photosystems are found in the thylakoid membranes within chloroplasts. We will come across photosystem 1 (PS1) and photosystem 2 (PS2) 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 centers where electrons are excited and lost) and accessory pigments (light-absorbing compounds which transfer light energy to the reaction center).

The Light-Dependent Reaction

Before going into this reaction it is worth mentioning that different A-level biology specifications require a different level of detail about this reaction. The OCR A specification requires details of both cyclic and non-cyclic photophosphorylation as described below, and therefore knowledge of both PS1 and PS2. Other specifications such as AQA and Edexcel Salters-Nuffield only require a more general overview, which is why I have simplified the diagram for this process slightly in my A-level biology notes for those specifications.

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 carriers, meaning that electrons can be transferred between the two.

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

Cyclic Photophosphorylation

This is the simpler of the two types of photophosphorylation and only uses PS1. It does not need water, does not produce oxygen, and does not produce reduced NADP (see next section).

Light energy is absorbed by photosynthetic pigments (e.g. chlorophyll) in PS1, which excites electrons so they get raised to to a higher energy level and are released from the pigments. This is called photoionisation (removing electrons using light energy).
The electrons are passed to the electron transport chain. As they are passed from one electron carrier to the next, the energy released from the electron transfer is used to pump H⁺ ions from the stroma into the thylakoid to create a proton gradient across the thylakoid membrane.
The electrons are passed back to PS1 (which is why it is a cyclic process).
H⁺ ions diffuse down the proton 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.

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 PS1, excites electrons in PS2, and splits water in a process called photolysis.

Light energy is absorbed by the photosynthetic pigments (e.g. chlorophyll) in PS2, which excites electrons so they get raised to a higher energy level and are released from the pigments (photoionisation).
The electrons are passed to the electron transport chain. As they are passed from one electron carrier to the next, the energy released from the electron transfer is used to pump H⁺ ions from the stroma into the thylakoid to create a proton gradient across the thylakoid membrane.
The electrons are passed to PS1 to replace electrons which have been lost during photoionisation of the chlorophyll in PS1.
The electrons lost from PS2 are replaced by electrons from photolysis of water. Light energy causes water inside the thylakoid to split into electrons (which move to PS2), H⁺ ions, and oxygen.
H⁺ ions diffuse down the proton gradient created in step 2 into the stroma through ATP synthase. This drives phosphorylation of ADP to ATP. (This is chemiosmosis.)
The excited electrons lost from PS1 are accepted by NADP (along with a H⁺ ion) to form reduced NADP in the stroma (so formation of reduced NADP requires light energy).

Note: remember that H⁺ ions are in effect just protons, which is why a concentration gradient of H⁺ ions can 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, and causes photolysis of water.
ATP and reduced NADP produced in this reaction are essential for the light-independent reaction.