Stages of photosynthesis and chloroplast structure

In a nutshell

Photosynthesis occurs in the chloroplasts and it is the process of producing glucose and oxygen from carbon dioxide and water. Chlorophyll will absorb light energy from the sun and this energy is then transferred into chemical energy through the sugars that are produced. Photosynthesis is a two-stage process that will be explored in this summary.

Photosynthesis

Plants require energy for many cellular processes like cell division, protein synthesis, DNA replication and active transport. Unlike other organisms, they are autotrophs which means they can use inorganic compounds to produce organic compounds. They achieve this through photosynthesis. The equations for photosynthesis can be seen below.

$carbon\ dioxide\ + water\xrightarrow[chlorophyll]{light}{glucose\ + oxygen}$

$6CO_2 + 6H_2O\rightarrow C_6H{_1}{_2}O_6 + 6O_2$

The energy will be stored in the glucose that is produced through photosynthesis. The plants can release this energy through respiration. Animals can also eat the plants and carry out out respiration to break down the glucose and release the stored energy.

Photosynthesis is a complex metabolic pathway with a lot of intermediate products. The two main stages of photosynthesis are the light-dependent reaction and the light-independent reaction.

Chloroplasts

Each plant cell contains between $10-50$ chloroplasts. Photosynthesis occurs within chloroplasts and it is very important to learn the structure and function of the components within the chloroplast.

1.	Granum (plural is grana)	That grana are stack of thylakoids. This is the site of the light-dependent stage of photosynthesis.
2.	Stroma	The stroma is the fluid surrounding thylakoid membranes. It contains all of the enzymes needed to carry out the light-independent reaction. *Note:* Starch grains are also found in the stroma. The starch grains store the products of photosynthesis.
3.	Outer membrane	The outer membrane is freely permeable to molecules such as $CO_2$ and $H_2O$ that are needed for photosynthesis.
4.	Inner membrane	The inner membrane contains membrane proteins that are used to transport substances in and out of the chloroplast.
5.	Intermembrane compartment	The intermembrane compartment is the space between the outer and inner membranes.
6.	Intergranal lamellae	Integranal lamellae are membranes that join adjacent thylakoids. *Note:* These are also called intergranal thylakoids.
7.	Thylakoids	The thylakoids are a system of interconnected flattened, fluid filled sacs. They contain proteins, including the photosynthetic pigment chlorophyll and the electron carriers that are needed for the light-dependent reaction.

Redox reactions

Reduction and oxidation reactions form an important part of photosynthesis. Reduction reactions are a gain of electrons, gain of hydrogen or loss of oxygen. Oxidation reactions are a loss of electrons, loss of hydrogen or gain of oxygen.

Light-dependent reaction

The light-dependent reaction occurs in the grana and requires light in order to occur. This stage of photosynthesis is explained below.

Biology; Energy for biological processes; KS5 Year 12; Stages of photosynthesis and chloroplast structure — 1. Stroma, 2. Thylakoid space, 3. Stalked particle, 4. Thylakoid membrane, A. Transmembrane complex containing chlorophyll, B. Proton pump, C. Electron carrier, D. ATP synthase channel

1.	Light will hit photosystem II of a chlorophyll molecule. This will cause a pair of electrons to be 'excited' as they are raised to a higher energy level. They leave the chlorophyll molecule. *Note:* This process is called photoionisation as the chlorophyll molecule is ionised due to the effects of the light.
2.	The electrons are picked up by a series of electron carriers that are embedded in the thylakoid membrane. The electron carriers are reduced as they gain electrons and oxidised when they lose the electrons.
3.	The movement of electrons through the successive carriers is called the electron transport chain. Each carrier is at a slightly lower energy level than the one before it. This means that energy is lost from the electrons with each transfer. Some of this energy is used in the synthesis of ATP from ADP and inorganic phosphate. *Note:* This process is called photophosphorylation (see below).
4.	Light energy excites electrons in photosystem I of a chlorophyll molecule. Two electrons are lost from photosystem I and they are replaced by the electrons from photosystem II.
5.	Photolysis occurs in the thylakoid space. This is the breakdown of water to give oxygen gas and hydrogen ions ( $H_2O\rightarrow 2H^+ + \frac{1}{2}O_2$ ). The oxygen atoms will combine and $O_2$ is released as a by-product.
6.	The $2H^+$ ions are accepted by the coenzyme NADP. This forms NADPH (or reduced NADP). *Note:* A coenzyme is not an enzyme. It is a molecule that aids the function of an enzyme, often by transferring a chemical group from one molecule to another.
7.	The ATP and reduced NADP are then used in the light-independent reaction.

Non-cyclic photophosphorylation

As the electrons lose energy, this energy is used to transport protons into the thylakoids. This means that the proton concentration in the thylakoid is greater than in the stroma. As a result, protons will travel down their concentration gradient through a protein channel, called ATP synthase, and enter the stroma. This movement transfers energy to combine ADP and inorganic phosphate to regenerate ATP.

At the final electron acceptor, the electrons are accepted by NADP to form reduced NADP. This process is non-cyclical.

Cyclic photophosphorylation

Cyclic photophosphorylation occurs in photosystem I and it only produces ATP, not NADP or oxygen like non-cyclic photophosphorylation does. This is because the electrons are recycled and passed back via an electron carrier.

Light-independent reaction

As the name suggests, the light-independent reaction does not require light. The light-independent reaction occurs in the stroma and uses the ATP and reduced NADP that was generated in the light-dependent reaction.

Biology; Energy for biological processes; KS5 Year 12; Stages of photosynthesis and chloroplast structure — 1. Ribulose bisphosphate (RuBP), 2. Unstable intermediate, 3. Glycerate-3-phosphate (GP), 4. Triose phosphate (TP), 5. Hexose sugars, lipids and amino acids

1.	Carbon dioxide from the air combines with a five-carbon compound called ribulose bisphosphate (RuBP). The reaction joining these two molecules is catalysed by ribulose bisphosphate carboxylase (RuBisCo).
2.	An unstable six-carbon compound is formed which immediately breaks down to form two three-carbon compounds called glycerate $3$ -phosphate (GP).
3.	Glycerate $3$ -phosphate is reduced to glyceraldehyde $3$ -phosphate (GALP) using the reduced NADP and ATP from the light-dependent reaction. NAD is reformed and will go back to the light-dependent reaction to accept more protons. *Note:* GALP is also known as triose phosphate (TP).
4.	Two of every twelve GALPs that are produced will create a six-carbon sugar called hexose. This will then be converted into other organic compounds such as glucose or starch.
5.	The remaining ten GALPs will recreate RuBP. They will rearrange to form six five-carbon molecules of RuBP. ATP from the light-dependent reaction will be used at this stage.