GCSE Photosynthesis

• The process where plants use the simple inorganic molecules carbon dioxide and water in the presence of chlorophyll and light to make glucose and oxygen It is important as it converts light energy to chemical energy. Chlorophyll absorbs the light energy needed for the reaction to take place. (As the products contain more energy than the carbon dioxide and water, the chlorophyll has to absorb the light energy and convert it into chemical energy so the products can be formed)

• Upper epidermis * Outer layers of cell * Thin, transparent layer so light can pass through to the cells below. * Covered by a waxy cuticle which reduces water loss by evaporation and acts as a barrier against pathogenic microorganisms. * Very few chloroplasts * Transparent to let light through • Lower Epidermis • Has many holes/pores called stomata. They allow CO2 to diffuse into the leaf to reach the photosynthetic tissue • Allow oxygen and water vapour to diffuse out • Palisade mesophyll • Layer of long, tightly packed cells just under the upper epidermis. • Contains many chloroplasts → main site of photosynthesis. • Positioned at the top of the leaf to get the most light as the upper epidermis is relatively transparent. • Spongy mesophyll • Loosely packed cells with air spaces that allow gases to diffuse in and out of the mesophyll. • Have chloroplasts but less than the palisade layer. • Are the main gas exchange surface of the leaf. Allows rapid diffusion of CO₂ to photosynthesising cells. • Also allows oxygen and water vapour to diffuse out. • Stomata and guard cells • Mostly on the lower epidermis. • Stomata allow CO₂ in so they can reach the photosynthetic tissues and O₂ + water vapour out. • Guard cells open and close stomata to control gas exchange and water loss. • Xylem • Brings water and minerals from the roots to the leaf. • Brings it up in the transpiration stream. • Water is needed for photosynthesis. In the leaves, the water leaves the xylem and supplies the mesophyll cells. • Phloem • Carries sugars (products of photosynthesis) away from the leaf to the rest of the plant. • It carries it to the other parts of the plant so that the tissues and organs that cannot make their own food can receive food.

• Light, carbon dioxide and temperature • Chlorophyll uses light energy to perform photosynthesis. It can only do it as quickly as the light is arriving. If the light intensity is increased, the rate of photosynthesis will increase steadily, but only up to a certain point, beyond that point temperature or CO2 levels will be the limiting factor. The graph then levels off. • Only 0.04% of the air is CO2 so it is quite scarce. Like light intensity, increasing the concentration of CO2 will only increase the rate of photosynthesis up to a certain point. After this point, the graph flattens out as it isn’t the limiting factor. • Temperature affects the rate of photosynthesis as it affects the enzymes involved. As the temp increases, so does the rate of photosynthesis up to a point as the enzymes and particles move faster. If the temp is too high, the enzymes get denatured and the rate of photosynthesis decreases. The graph flattens out after a certain point as temperature isn’t the limiting factor anymore.

• Nitrate ions are used for making amino acids, proteins, chlorophyll, DNA and other compounds. Deficiency = limited growth of plant and its older leaves turn yellow (chlorosis) • Phosphate ions are used to make DNA and many other compounds. They are also used for part of the cell membrane. Deficiency = poor root growth. Younger leaves turn purple. • Potassium ions are needed for enzymes of respiration and photosynthesis to work. Deficiency = leaves turn yellow with dead spots. • Magnesium ions are needed to make part of the chlorophyll molecules. Deficiency = leaves turn yellow.

• Although plants make glucose in photosynthesis, leaves cannot be tested for its presence as the glucose is quickly used, converted into other substances and transported or stored as starch. • Starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesising. • Leaves can be tested for starch using the following procedure: ○ A leaf is dropped in boiling water to kill the cells and break down the cell membranes ○ The leaf is left for 5-10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll so colour changes from iodine can be seen more clearly ○ The leaf is dipped in boiling water to soften it ○ The leaf is spread out on a white tile and covered with iodine solution ○ In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf ○ This method can also be used to test whether chlorophyll is needed for photosynthesis by using a variegated leaf (one that is partially green and partially white) ○ The white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contain chlorophyll stain blue-black ○ The areas that had no chlorophyll remain orange-brown as no photosynthesis is occurring here and so no starch is stored

• Have a light source and a ruler, a clamp with a test tube with Canadian pondweed, water and sodium hydrogen carbonate and a gas syringe connected to the test tube by a capillary tube. • Canadian pondweed can be used to measure the effect of light intensity on the rate of photosynthesis. The rate at which the pondweed produces oxygen corresponds to the rate at which it is photosynthesising. • Set the light source up at a set distance from the test tube. The gas syringe should be empty and sodium hydrogen carbonate is added to the water to make sure that the plant has carbon dioxide as it released CO2 into the solution. • Let the pondweed photosynthesise for a set amount of time. As it photosynthesises, oxygen will collect in the capillary tube. • At the end of the experiment, the syringe is used to draw the gas bubble in the tube up alongside a ruler and the length of the gas bubble is measured. This is proportional to the volume of O2.

○ Only the green part of variegated leaves have chlorophyll. 1. Take a variegated leaf from a plant that has been exposed to light for a big. Make sure to record which bits are green and which bits aren’t. 2. Test the leaf for starch 3. Test the leaf for starch and you’ll see that only bits that were green turn blue-black suggesting only the part of the leaf with chlorophyll can undergo photosynthesis.

○ Destarch two plants by placing in the dark for a prolonged period of time ○ Place one plant in a bell jar which contains a beaker of sodium hydroxide (which will absorb carbon dioxide from the surrounding air) ○ Place the other plant in a bell jar which contains a beaker of water (control experiment), which will not absorb carbon dioxide from the surrounding air ○ Place both plants in bright light for several hours ○ Test both plants for starch using iodine ○ The leaf from the plant placed near sodium hydroxide will remain orange-brown as it could not photosynthesise due to lack of carbon dioxide ○ The leaf from the plant placed near water should turn blue-black as it had all necessary requirements for photosynthesis

○ Before starting the experiment the plant needs to be destarched by placing in a dark cupboard for 24 hours ○ This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment ○ A leaf can then be removed and tested for starch using iodine ○ The leaf should remain orange-brown as it has not been able to photosynthesise. ○ This proves that light is necessary for photosynthesis and the production of starch