Photosynthesis - AL only (3.5.1)
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Producers, such as plants and algae, carry out photosynthesis to make their own glucose. This glucose is used for cellular processes such as polysaccharide synthesis or for respiration (to release energy).
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:
Leaves have many adaptations to maximise photosynthesis:
- Large surface area to absorb sunlight.
- Leaves are usually thin to reduce the diffusion distance of gases.
- Transparent cuticle and upper epidermis to allow light to pass through.
- Columnar upper mesophyll cells contain many chloroplasts.
- Stomata open and close depending on light intensity to balance water loss and gas exchange.
- Air spaces in the lower mesophyll layer allow diffusion of gases.
- Xylem and phloem to transport water to the leaves and remove sugars produced during photosynthesis.
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.
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, chlorophyll is photoionised by absorbing light energy. This excites a pair of electrons to a higher energy level, causing the electrons to leave the chlorophyll molecule.
The electrons move to an electron carrier, which gains the electrons, converting to its reduced form. The electrons flow along the four electron carriers in the thylakoid membrane in a series of oxidation-reduction reactions. The electrons gradually lose energy as they move along the electron carriers. The energy lost in this process is used to make ATP.
Photolysis occurs as part of the light-dependent stage in the thylakoid membrane. An enzyme uses light energy to split water into hydrogen ions, electrons and oxygen.
Overall yield:
The electrons are used to replace the electrons lost from the chlorophyll during photoionisation.
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 in the stroma.
The concentration gradient produced by the photolysis of water is used for the process of chemiosmosis.
Hydrogen ions flow from the thylakoid space into the stroma through the ATP synthase channel in the thylakoid membrane. This flow facilitates ATP synthase combining ADP and Pi to produce ATP.
Once the hydrogen ions have entered the stroma, they are combined with the electrons from the electron transport chain and coenzyme NADP to produce reduced NADP.
The thylakoid membranes contain the pigment chlorophyll, electron carriers and enzymes involved in the light-dependent reaction. The thylakoid membranes have a large surface area to increase light absorption for the light-dependent reaction.
The light-independent stage of photosynthesis occurs in the stroma and uses the reduced NADP produced in the light-dependent stage.
The ATP produced in the light-dependent stage provides energy for the reactions in the light-independent stage, which produce simple sugars.
The light-independent reaction occurs in the stroma, which surrounds the grana. This means the products from the light-dependent stage only have a short diffusion distance from the thylakoid membrane to the stroma.
Carbon dioxide diffuses into the cell, then into the stroma of a chloroplast. The carbon dioxide is then used for a cycle of enzyme-controlled reactions called the Calvin cycle.
In the Calvin cycle, carbon dioxide is joined with ribulose bisphosphate (RuBP), a 5C compound, by the enzyme ribulose bisphosphate carboxylase (rubisco). This produces 2 × 3C glycerate 3-phosphate molecules.
Reduced NADP from the light-dependent stage of photosynthesis is used to convert each glycerate 3-phosphate to a 3C triose phosphate molecule. This produces NADP, which can be reused in the light-dependent stage. The ATP produced during the light-dependent stage provides the energy for this reaction.
Triose phosphate can then either be recycled into RuBP or used to make organic molecules such as glucose, lipids and amino acids.
Most of the triose phosphate produced in the Calvin cycle is recycled to produce RuBP, which requires ATP and allows the Calvin cycle to continue. Some of the triose phosphate is used to produce organic molecules, such as glucose.
As light intensity increases, the rate of photosynthesis increases. The rate of photosynthesis will level off when a different factor limits the process.
As light intensity increases, the rate of the light-dependent stage of photosynthesis increases. This leads to increased ATP and reduced NADP production.
The point at which the rate of oxygen production from photosynthesis and the rate of oxygen consumption due to respiration are equal is called the compensation point.
As the concentration of carbon dioxide increases, the rate of photosynthesis increases. The rate of the reactions in the Calvin cycle will increase which increases the rate at which RuBP is used and triose phosphate is produced.
Carbon dioxide accounts for 0.04 of atmospheric gases and is often the limiting factor in photosynthesis.
The vast majority of the reactions involved in photosynthesis are catalysed by enzymes. As temperature increases, the rate of photosynthesis initially increases because particles have more kinetic energy.
Above the enzymes’ optimum temperature, the rate of photosynthesis will decrease due to enzyme denaturation.
Various practices are used in agriculture to maximise the rate of photosynthesis and therefore maximise the rate of plant growth.
Practically, carbon dioxide levels are difficult to control as it is challenging to stop gases diffusing. Light intensity can be increased using lights which emit the wavelengths absorbed by the pigments involved in photosynthesis.
In colder climates, temperature can be controlled using heating and greenhouses to trap heat. The greenhouses must have transparent roofs or use lighting so the light levels do not limit the rate of photosynthesis.