GCSE Science/Photosynthesis

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Photosynthesis[edit | edit source]

Lilac tree

Like all living things, plants need food to live. This food is used for energy and to make new materials when plants grow. Plants are able to take two inorganic chemicals, carbon dioxide gas and water, to make an organic chemical, glucose. This simple food can be used as an energy source or converted into other useful organic molecules.

Photosynthesis takes place in chloroplasts: the proteins that participated in the reaction are located in the thylakoid membranes of the chloroplast.

Photosynthesis requires an input of energy. Plants have found a way to capture the energy from sunlight, using a pigment called chlorophyll. Once this light energy has been captured it can be used to create glucose, so converting the light energy into chemical energy. Oxygen gas is released as a waste chemical.

The formula for photosynthesis is:

6CO2 + 6H2O → C6H12O6 + 6O2
carbon dioxide + water → glucose + oxygen

The reaction:

  • reduces production of NADPH
  • generates a proton gradient for the formation of ATP
  • produces oxygen

Photosystems[edit | edit source]

There are two kinds of photosystems in photosynthesis that are called Photosystem I (PsI) and Photosystem II (PsII). During PsI, a light absorbing pigment called chlorophyll which is a molecule that absorbs red light of 700 nanometer wavelength most efficiently, we normally called it P700. On the other hand, the pigment in PsII reaction center chlorophyll absorbs called P680 because it absorbs light at around 680 nanometers. The reason photosynthesis has two system is that one alone can't capture enough energy to power the carbon-fixation reactions and to supply the rest of the energy requirements of plant metabolism. The combinations of these systems can cause different mechanisms to change the light energy to chemical energy, therefore provide the maximum energy that plants require.

The process, however, starts with PsII, not PsI. The energy absorbed by a P680 molecule of chlorophyll in the reaction center of PsII first converts the molecule to its excited state and then raises the energy level of an electron, and then the electron lose from the molecule, electron transport system accepts the electron, and immediately moves it down the transport chain, losing some energy in each transfer. We can tell that when The P680 molecule losing an electron, is unstable. To make it become stable again, we need to draw another electron from water. The process and continue over and over.

ATP known as a energy resource, produced from the energy that lost during electron transfer. the molecule universally used by organisms as a quick energy source.

The energy that reaches the end of the electron transport chain is transferred to the P700 chlorophyll in the reaction center of PsI. As the above said, both photosystem are similar except PsI captures light energy at 700 nm.

Cyclic Photophosphorylation[edit | edit source]

In the process between the photosystems discussed above, cyclic photophosphorylation occurs on the thylakoid membrane. During Cyclic Photophosphorylation the electrons are recycled. The excited electrons resulting from the absorption of light in PS I are received by the primary electron acceptor and then transferred to an electron transport chain(move from donor to acceptor). And then the electron return back to the reaction centre, repeat the process again. The electrons are using to translocate Protons which the ATPase uses to synthesise ATP. No reduction of NADP+ occurs in this process.

Accessory pigments are able to funnel energy into reaction center[edit | edit source]

Chlorophyll A & B can absorb light and funnel the energy to reaction center. Robert Emerson and William Arnold did an experiment on Chlorella cells in 1932. The experiment implies that only 1 molecule of O2 was produced for 2500 chlorophyll molecules excited. Both additional chlorophylls A and B (A has methyl group and B has formyl group) are closely associated with reaction centers.

Limiting Factors[edit | edit source]

Photosynthesis requires the following 3 factors:

  1. Carbon dioxide
  2. Water
  3. Light

There are a 4 factors which affect the rate at which a plant can make glucose by photosynthesis.

  1. The Concentration of Carbon Dioxide
  2. The Light Intensity
  3. The Temperature
  4. Any factor that influences the production of chlorophyll, enzymes, or energy carriers(ATP and NADPH).

We can achieve a better rate of photosynthesis, if more CO2 is in the air, and if its content was less than the process needed, photosynthesis couldn't happen normally. It is hypothesized that global warming due to increased CO2 might increase plant growth.

Light intensity is another rate factor for photosynthesis. We can increase the light intensity in order to achieve more photosynthesis. But we need to know that where the plant has reached maximum photosynthesis levels, since any increase in light intensity will not affect the plant further.

Photosynthesis will occur faster in area with high temperature. But on the other hand, humidity can also affect the photosynthesis. When the air is saturated with water vapour, photosynthesis is limited. This is because at saturation levels, the air cannot absorb more moisture from the plants when they open the stoma to exchange gases.

The Structure of a leaf[edit | edit source]

Photosynthesis takes place in the leaves of plants. The leaves have many adaptations to make sure that as much photosynthesis goes on as possible. The more a plant can photosynthesize, the more food it can make and the faster it can grow.

  1. The leaf has a waxy cuticle to stop it from losing water and drying out.
  2. The epidermis is a protective layer of cells and contains no chloroplasts.
  3. The palisade layer contains the most chloroplasts as it is near the top of the leaf. The chloroplasts contain the pigment chlorophyll.
  4. The palisade cells are arranged upright. This means the light has to pass through the cell lengthways and so increases the chance of light hitting a chloroplast and being absorbed.
  5. The spongy layer contains fewer chloroplasts, enough to catch what the palisade layer cannot absorb.
  6. The spongy layer has air spaces to make it easier for gases to circulate in the leaf.
  7. The vascular bundle provides the leaf with water via the xylem vessels. Food, such as sugar, made in the leaf is transported in the phloem vessels to the rest of the leaf.
  8. The stomata (stoma - singular) are tiny pores that allow carbon dioxide to enter the leaf while oxygen leaves the leaf.
  9. Guard cells can open or close the stomatal pores to regulate how much gas can enter or leave the leaf. At night the pores close, opening in the daytime.

Respiration in plants[edit | edit source]

Respiration is the production of energy from glucose and oxygen with the release of carbon dioxide and water as waste products.Photo respiration occurs in many plants under sunlight. It differs from cellular respiration, which occurs in the mitochondria. The difference between photo respiration and cellular respiration is photo respiration does not release energy when it occurs. On the other hand, cellular respiration releases energy. The common part of both respiration is both of them absorb oxygen and release CO2. Photo respiration occurs at the same time with photosynthesis and uses some of the newly made carbohydrate for energy, thereby reducing the yield of photosynthesis to make it more efficiency.

The most important enzyme for the respiration is rubisco, it can make the respiration efficient. Because rubisco lacks the ability of reorganization, it accepts the O2 rather than CO2 and catalyzes a series of different reactions, resulting in carbon being released and energy being expended for no net energy gain.


C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy released
Glucose + Oxygen → Carbon dioxide + water + Energy released


6CO2 + 6H2O + Energy → C6H12O6 + 6O2
Carbon dioxide + water + Energy → Glucose + Oxygen

Plants use the energy from respiration to power the processes involved in growth.

Uses of glucose[edit | edit source]

The energy obtained from respiration is used to turn glucose into many other substances. Typical uses of glucose are:

  • Storage products
- glucose is used to make starch which can be converted back to glucose as required. Potatoes and rice are examples of parts of plants that contain starch.
- glucose is converted into lipids, especially in seeds. Sunflower oil and rapeseed oil, which are used in cooking, come from sunflower and oilseed rape seeds.
  • Structural products
- glucose is converted to cellulose to make cell walls.
  • Other products
- glucose and nitrates are used to make amino acids which are used to make proteins.
- glucose is also a basic raw material for making chlorophyll.
- glucose is used in respiration.

The ability to convert light into chemical energy is ancient[edit | edit source]

The photosystems have common structure which implies a common evolutionary origin. Geological evidence suggests that oxygenic photosynthesis became important approximately 2 billion years ago. Prior to this, green and purple sulfur bacteria existed which carried out anoxygenic reactions. And both of them are similar to oxygenic photosysthesis. And because no archaea was found in photosynthetic organism, concluded that photosynthesis did not evolve immediately at the origin of life.