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The process of the photosynthesis

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Photosynthesis is the process described by this equation This equation shows the complex 2 steps process that takes place in the chloroplast of green plants. The end product is glucose, but the complex organic molecule such as carbohydrates, amino acid, lipids, and nucleic acids. Photosynthesis is important because it is the biological process that produces > it produces complex organic molecules that are needed for growth It produces oxygen which is used for respiration

Energy for the processes in the organism

When plants are eaten, the organic molecules are used to provide energy to the organisms higher in the food chain. The oxygen which is produced is released into the atmosphere and is available for other organisms.

Structure of chloroplast- Thylakoid-: This is the 2 membrane that forms the envelope, chloroplast that contains a third internal membrane. The inside portion of the thylakoid is called thylakoid lumen, this contains plastocyanin and other molecules that are required for the transport of electrons. Thylakoid is a collection of membranes that are stacked together and these stacks are called grama. Granum this is a flat membrane that increases the surface area and vol ratio and small internal volumes quickly accumulate ions. Intergranular thylakoid: Stroma – the stroma is an aqueous matrix that is present inside the double membrane envelope. the inside components, as well as other solutes, are dispersed into the stroma. The stroma is rich in proteins and it contains several enzymes that are necessary for the vital cellular processes. The DNA in chloroplast is also present in the stroma along with the ribosomes and other molecules that are required for protein synthesis. The starch synthesized through photosynthesis is stored in the stoma in the form of granules.

Photosynthetic pigments, this is a colored biological compound that is present in the chloroplast and photosynthetic bacteria and this captures light energy for photosynthesis. In plants, the two types of pigments are chlorophylls and carotenoids. These are colored because they absorb particular wavelengths of light and reflect others. The reason plants are green is because of the chlorophyll pigments, it gives the plants the green color by reflecting green light. The carotenoids reflect red, orange or yellow light. ATP is an important molecule that is found in all living organisms. It diffuses around the cell and provides energy for cellular processes. Adenosine triphosphate is made in the light-dependent reaction in photosynthesis from Adenosine diphosphate and organic phosphate group p this requires energy. ATP releases energy in the light-independent reaction and forms a bond between inorganic phosphate groups, which then produces ADP and an inorganic phosphate group.

NADP and NADPH is the coenzyme that is involved in the photosynthesis reactions. The compound is a nucleotide that contains an adenine base and a nicotinamide base. The nucleotides are joined through the phosphate groups. There is an extra phosphate on the ribose of the adenine containing nucleotide. NADP can accept electrons that are reduced to NADP these are often called NADPH. This is oxidized back to NADP releasing electrons. In photosynthesis, the phosphorylation of ADP to form ATP using energy from sunlight and this is called photophosphorylation. There are only 2 sources of energy available to living organisms these are sunlight, and reduction and oxidation redox reactions. All organism produces ATP. There are two stages of photophosphorylation and these are Cyclic and Noncyclic photophosphorylation. Stages of photophosphorylation In the process of photosynthesis and the phosphorylation of ADP to form ATP, this uses the energy of sunlight and this is called phtotphosplation. Photophosphorylation light energy is used to create a high energy electron donor and a lower energy electron acceptor. Cyclic Photophosphorylation only involves photosystem 1 and does not use reduction of NADP+ When the light is absorbed by photosystem 1 the electrons will enter into the electron transport chain to produce ATP. the de-energized electron will return to the photosystem restoring the electron supply. The electron will then return to the NADP+ which means it hasn’t been reduced and water is not needed to replenish the electron supply Noncyclic photophosphorylation is in two-stage involving two different photosystems. Photosystem II and photosystem I and it does require the reduction NADP+. The noncyclic occurs in the frets of the stroma. When the light is absorbed by photosystem II the electrons that have excited will enter the electron transport chain to produce ATP while the photoactivation of photosystem I result in the release electrons which reduces NADH+ to form NADPH. The photolysis of water will release the electrons which then replaces the electrons that are lost by photosystem II. Photolysis is the splitting of chemical compounds by light energy or photons. There are two steps to photosynthesis: this is light dependent and light independent.


The light-dependent reaction uses photosynthetic pigments that are organized into photosystems which converts light energy into chemical energy Eg. ATP and NADPH. The situated membranes are light harvesting systems called photosystems. There are 2 photosystems and these are Photosystem I and Photosystem II both of these have chlorophyll at their centers. The light-dependent reaction of photosynthesis is the first major process in photosynthesis as it uses light energy which is then converted into chemical energy such as ATP and NADP. This takes place across the chloroplast thylakoids membranes, this is between the chloroplast stroma and thylakoid space. In the thylakoids, there are 3 steps involved in the reaction that occurs in the specialized membrane discs in the chloroplast and these are Excitation of photosystems by light energy.

Production of ATP by an electron transport chain

Reduction of NADP+ and photolysis of water first step is the Excitation of photosystems by light energy. This is when the photosystems are transferred in groups of photosynthetic pigments which includes chlorophyll is embedded in the thylakoid membrane. Then the photosystems that are classed according to the maximum absorption wavelengths Photosystem I equals 700 nm and photosystem II equals 680nm. When the photosystems absorb the light energy they are delocalized electrons in the pigments that become energized or excited. Then these electrons that have been excited are transferred to carrier molecules in the thylakoid membrane. 2. The second stage of a light dependant is the Production Of ATP by The Electron Transport Chain. The electrons that existed from the photosystems II P680 are transferred to an electron transport chain in the thylakoid membrane. Then as the electrons pass through the chain they lose their energy, this is then translocated into H+ ions into the thylakoid. This then builds up the protons in the thylakoid which creates an electrochemical gradient or a proton motive force. The H+ ions will return to the stroma which is along the proton gradient by the transmembrane enzyme ATP synthase chemiosmosis The ATP synthase uses the passage of H+ ions to catalyze the synthesis of ATP from ADP+Pi. This process is called Photophosphorylation as the light provided the initial energy source for ATP production. The de-energized electrons from Photosystem II will be taken up by Photosystem I. 3. This is the last step of the Light Dependant. It is the Reduction of NADP+ and the Photolysis of water. The electrons that have excited from Photosystem I can be transferred to a carrier molecule and used to reduce NADP+This then forms NADPH, which is needed in the conjunction with ATP for the light-independent reactions. The electrons that are lost from the photosystem I am replaced by the de-energized electrons from the Photosystem II The electrons lost from Photosystem II are replaced by electrons that are released from water by Photolysis. The water is split by the light energy into H+ ions, that are used in chemiosmosis, and oxygen is released as a by-product. Light-independent the light-independent reaction, the reactions use chemical energy derived from the light-dependent reaction to form organic molecules. In the light-independent reaction happen in the stroma, this is the fluid/ filled space of the chloroplast. The light-independent reaction is also known as the Calvin cycle and it involves the 3 steps:

Carboxylation of ribulose bisphosphate

Reduction of glycerate

PhosphateRegeneration of ribulose bisphosphate

  1. The first step is the Carbon Fixation. The Calvin Cycle is a chemical reaction that takes place in the chloroplast during photosynthesis. The cycle is a light-independent reaction because it requires sunlight so it takes place after the energy has been captured from the sunlight. The reaction begins when 5C compound of ribulose biphosphate (RuBP)An enzyme,RuBP carboxylase, catalyses the attachment of CO2 molecule to Rupp This then results in 6C compound being unstable and when leads to the compound breaking down into two 3C compounds called glycerate 3 phosphate GPThen the cycle involves 3 molecules of RuBP which combines with the 3 molecules of CO2 to make six molecules of GP2.
  2. Step 2 is the Reduction of Glycerate PhosphateGlycerate 3 phosphate GP is converted in the triose phosphate using NADPH and ATP The reduction of NADPH transfers hydrogen atoms to the compound, while the hydrolysis of ATP provides energy Then the GP will need one NADPH and one ATP to form a triose phosphate. A single cycle will require six of each molecule.
  3. The third stage is Regeneration of RuBPOut of the six molecules of TP produced per cycle, one TP molecule may be used to form half of a sugar molecule 2 cycles will be required to produce a single glucose monomer and more will be needed to produce polysaccharides such as starch. The ramininfg5 TP molecules will be combined with the regenerate stocks of RuBP 5* 3C= 3*5C The regeneration of RuBP will require energy derived from the hydrolysis of ATP. Calvin Cycle limiting Factors affecting photosynthesis The main factors that affect the rates of photosynthesis are LIght Intensity, Carbon Dioxide Concentration, and temperature. Light Intensity Light intensity increases the rate of the light-independent reaction which increases photosynthesis, therefore, the reaction is being photoactivated. More photons of light that fall on the leaf, the greater the number of chlorophyll molecules that are ionized and the more ATP and NADPH are generated. The light-dependent reaction uses light energy and so it’s not affected by the changes in the temperature. When light intensity increases there are limiting factors of the rate of photosynthesis. The rate of the plateau as all available chlorophyll will be saturated with the light. Chlorophyll will be damaged when the rate drops steeply. Chlorophyll is used in both of the Photosystems. The different wavelength of light has different effects on the rate of the photosystems. The green light is reflected. Photosystem I will absorb energy most efficiently at 700 nm and PSII at 680nm. The higher the light energy concentrated in the wavelength the higher rate of photosynthesis. CO2 Concentration When there’s an increase in carbon dioxide concentration, there’s an increase in the rate of carbon when its combined with carbohydrates in the light-dependent reaction and so the rate of photosynthesis will increase until there’s another limiting factor. Carbon dioxide is involved in the fixation of carbon atoms to form organic molecules. There is a low amount of carbon dioxide in the air about 0.04%, increasing carbon dioxide concentration causes a rapid rise in the rate of photosynthesis will plateau as the enzymes are responsible for carbon fixation saturated.


Photosynthesis is controlled by enzymes which are sensitive to temperature fluctuations although the light-dependent reaction of photosynthesis is not affected by changes in temperature. Light is dependant on temperature and the reactions are catalyzed by enzymes. When the enzymes reach their optimum temperature the whole reaction increases as the temperature increases the rate of reaction increases, as the reactant have greater kinetic energy, this results in more collision. Once it’s reached its peak the rate begins to decrease as enzymes become denatured and the reaction stops.

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