3.8 – Photosynthesis 3.8.1 - State that photosynthesis involves the conversion of light energy into chemical energy Photosynthesis takes place in the chloroplast. These are found in the green leaves of plants, algae and some bacteria. Chlorophyll is the pigment found in the chloroplast, and is what causes plants to appear green. Photosynthesis synthesises the compounds required more life, mainly glucose. Photons of light are trapped in the chlorophyll, which is then converted into chemical energy. There are both prokaryotic and eukaryotic organisms undergo photosynthesis to obtain their food. The reaction requires CO2 and H2O, and produces sugar and O2. The entire process is divided into the light-dependent reactions and the light-independent reactions. →
3.8.2 - State that light from the Sun is composed of a range of wavelengths (colours) The white light that comes from the sun is a combination of many different wavelengths of light, which blur to white light. They are found on a continuous spectrum, each wavelength representing a different colour. Light itself is simply electromagnetic radiation. Along the entire electromagnetic spectrum, humans are only able to see a small portion of it. Chlorophyll absorbs light of specific wavelengths to varying degrees, so not all light is absorbed equally. In fact, some wavelengths are reflected altogether.
3.8.3 - State that chlorophyll is the main photosynthetic pigment In green plants, there are a number of pigments that are able to help in photosynthesis; however chlorophyll is the predominant one. It is not soluble in water.
3.8.4 - Outline the differences in absorption of red, blue and green light by chlorophyll Chlorophyll is able to absorb red and blue light, and reflects green light. This is why leaves and plants appear green. Its chemical structure is what allows it to absorb some colours, but not others.
3.8.5 - State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen The chlorophyll molecules on the chloroplast membrane absorb and trap photons of light. This energy causes them to release electrons, or chemical energy, with oxygen released as a waste product. Water molecules are then split using this energy, through the process called photolysis. The hydrogen atoms are retained on hydrogen acceptor molecules. The light energy is also used to generate ATP from ADP and phosphate.
3.8.6 - State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules Once the water molecules have undergone photolysis, their electrons combine with carbon dioxide to form organic compounds. This process is called fixing CO2. The energy from ATP is used to form bonds between the carbon, hydrogen and oxygen, making sugars and carbohydrates.
3.8.7 - Explain that the rate of photosynthesis can be measured directly by the production of oxygen or the uptake of carbon dioxide, or indirectly by an increase in biomass There are a number of ways we can experimentally measure how much is produced during photosynthesis. One of these is through measuring the increasing biomass of the plant. Samples are taken at time intervals and the rate of increase in biomass is used to determine the rate photosynthesis However, this is very inaccurate due to the number of factors that affect photosynthesis, and energy is used or lost. A more accurate way of measuring it is destarching the leaves. This involves leaving them in the dark for 48 hours. Alternatively, the amount of oxygen produced can be measured. For water plants, the volume of oxygen bubbles produced is measured, while a data logger with a sensor is used for land plants. Another way is by measuring how much CO2 has been taken up by the plant using pH change in the water. A pH probe is placed in the water and any increase in pH can be detected.
3.8.8 - Outline the effects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis Temperature As the temperature in the environment increases, the rate of photosynthesis increases. However, each plant has an optimum temperature, after which the rate of photosynthesis falls off steeply. This is because the enzymes and chlorophyll denature or become unstable.
Light Intensity Since light is essential for photosynthesis to occur, the rate of photosynthesis increases as light intensity increases. However, the relationship plateaus when a certain temperature is reached. On the other hand, if there is no light, the plant can only respire. Chlorophyll can actually become damaged if the light is too intense.
Carbon Dioxide Concentration As the concentration of CO2 increases, the rate of photosynthesis also increases. Like light intensity, this relationship reaches a plateau. The relationship is exactly like the relationship of a substrate and enzyme. The optimum CO2 concentration varies between plants.