Photosynthesis (5.5, 5.7, 5.6, 5.8, 5.9)
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Respiration requires glucose and oxygen and releases carbon dioxide, water and energy in the form of ATP. Chemical energy is released during this process.
Overall equation:
Photosynthesis is an endothermic process which converts energy from sunlight into chemical energy. Energy from sunlight splits water molecules, which are then combined with carbon dioxide to form glucose. Oxygen is produced as a waste product.
Overall equation:
ATP provides an immediate energy source for biological processes. ATP is continuously hydrolysed (by ATP hydrolase) to form ADP + Pi and resynthesised within cells to support metabolic activities such as active transport, muscle contraction and protein synthesis.
The phosphate released from the hydrolysis of ATP can be used to phosphorylate other compounds.
ATP can be (re)synthesised by the condensation of ADP via a phosphorylation reaction requiring energy.
Phosphorylation of ADP is catalysed by the enzyme ATP synthase and occurs in mitochondria, during aerobic respiration (electron transport chain) and in chloroplasts, during photosynthesis (light-dependent reactions).
Coenzyme NADP can be represented in different ways. For example, coenzyme NADP can also be written as NADP+, whilst reduced NADP can be shown as NADPH or NADPH + H+.
The coenzymes and their reduced forms in the following notes are shown in the same way as in the exam board specification.
Photosynthesis occurs in two stages. The first is the light-dependent reaction where sunlight is used to split water molecules, producing ATP (and oxygen as a waste product). The coenzyme NADP is converted to reduced NADP by accepting hydrogen ions and electrons.
The second stage is the light-independent stage. This stage uses the ATP and reduced NADP made in the first stage. It also uses carbon dioxide, along with the hydrogen ions and electrons from reduced NADP, to build organic molecules such as glucose.
Electron transport chains are used in cell processes such as photosynthesis and respiration.
They transfer high-energy electrons through a series of four electron carriers (complexes) which are embedded in a membrane, such as the thylakoid membrane, in photosynthesis.
Each complex is reduced when it receives electrons and oxidised when it passes electrons to the next complex. The energy from the electrons is gradually transferred during the process and is used to create a hydrogen ion gradient.
In the light-dependent stage of photosynthesis, light energy is used to move two electrons from photosystem II to a higher energy level.
The two electrons then move along a series of four electron carrier molecules which form the electron transport chain. As the electrons move along the chain, a series of reduction and oxidation reactions occur which release energy.
This energy is used to convert ADP and to ATP in a process called photophosphorylation.
Photolysis occurs in the thylakoid space. An enzyme is used to split water into oxygen, hydrogen ions and electrons. These electrons are used to replace the electrons lost by photosystem II.
Overall yield:
The hydrogen ions stay within the thylakoid space, which produces a concentration gradient during photosynthesis, with the concentration of hydrogen ions higher in the thylakoid space than the stroma.
After photophosphorylation, the electrons are then passed to photosystem I. The electrons are excited by light energy again. They pass along another electron transport chain where they are then joined with coenzyme NADP and two hydrogen ions to produce reduced NADP.
The light-independent stage of photosynthesis occurs in the stroma of the chloroplast.
This stage reduces carbon dioxide in a cycle of reactions called the Calvin cycle. This stage also uses the ATP and reduced NADP from the light-dependent stage to produce organic molecules such as carbohydrates, lipids and proteins.
The light-independent stage remakes NADP which can then be used again in the light-dependent stage.
Carbon dioxide diffuses into the stroma for the light-independent stage of photosynthesis. This involves a series of enzyme-controlled reactions called the Calvin cycle.
In the Calvin cycle, carbon dioxide reacts with ribulose bisphosphate (RuBP), a 5C molecule. This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase (RuBISCO). An unstable 6C intermediate is formed which immediately decays to form 2 × 3C glycerate 3-phosphate.
Each glycerate 3-phosphate is converted to 3C glyceraldehyde 3-phosphate. This reaction uses reduced NADP and ATP to produce coenzyme NADP, ADP and Pi.
Glyceraldehyde 3-phosphate can then either be recycled to RuBP or used to make organic molecules such as glucose, lipids and amino acids.
Most of the glyceraldehyde 3-phosphate produced in the Calvin cycle is recycled to produce RuBP, which requires ATP and allows the Calvin cycle to continue. Some of the glyceraldehyde 3-phosphate is used to produce organic molecules.
Six Calvin cycles use 6 molecules of carbon dioxide and produce 12 glyceraldehyde 3-phosphate molecules. 10 of these molecules would be used to reproduce the original 6 molecules of RuBP. The remaining 2 glyceraldehyde 3-phosphate molecules can be used to make 1 hexose sugar.
Chloroplasts are specialised organelles found in plants and algae where photosynthesis occurs.
The light-dependent reactions occur in the thylakoid membrane. The thylakoids have a large surface area, increasing the membrane surface available for photosynthetic reactions. Chlorophyll, other photosynthetic pigments and enzymes involved in the light-dependent reaction are found within the thylakoid membrane.
The light-independent reaction occurs in the stroma. The stroma contains chloroplast DNA and ribosomes so is able to make some of its own proteins and enzymes. The stroma is where organic molecules are produced.