PHOTOSYNTHESI S: A LABORATORY EXPERIMENT THEORETICAL BACKGROUND EXPERIMENT CONCLUSIONS introduction procedure results

THEORETICAL BACKGROUND NUTRITION IN LIVING ORGANISMS Nutrition is the process of obtaining energy and matter in order to drive metabolism in cells. There are two types of nutrition: 1.Heterotrophic nutrition occurs when food is eaten by an organism and is broken down to provide ready-made nutrients. Heterotrophic means “other-feeding” and is found in all animals, humans included. 2.Autotrophic nutrition occurs when nutrients are made by the organisms themselves from simple, inorganic chemicals and the input of energy. Autotrophic means “ self-feeding”. Plants use this method: photosynthesis.

THEORETICAL BACKGROUND Photosynthesis is a chemical process that takes place in the chloroplasts of green plants. Chloroplasts are organelles found in the cells of the green parts of the plant: leaves and, sometimes, stems. Photosynthesis takes place in daytime when it is light. Sunlight energy is absorbed by green chlorophyll and used to turn carbon dioxide and water into carbohydrate and oxygen gas. The carbon dioxide is absorbed by the leaves from the outside air while water enters the plant through the roots and travels upwards. This word equation sums up the whole process: Sunlight + carbon dioxide + water =(chlorophyll)* glucose + oxygen * chlorophyll acts as a catalyst for the process: so it helps photosynthesis take place but is not itself permanently changed or used up.

THEORETICAL BACKGROUND Photosynthesis is important because: the glucose formed is used for respiration and provides the plant with energy. some glucose can be stored in the form of starch for later use. some glucose can be converted to cellulose and used for the formation of cell walls for new cells. glucose, with the addition of minerals from the soil, can be converted into pigments, vitamins, fats, oils and proteins, all of which are required by the plant to grow and develop. some of the oxygen formed is used in plant respiration and is released into the air. AND …

THEORETICAL BACKGROUND Photosynthesis in plants provides food for all living organisms. Plants are at the bottom of all food chains. Sunlight energy is passed to all consumers through these chains. Photosynthesis provides oxygen for all living organisms. It is the only natural source of oxygen gas on the Earth. Photosynthesis reduces the level of carbon dioxide in the atmosphere. It is a vital part of the control of CO 2 levels. The destruction of young trees in rain forests has worsened the accumulation of carbon dioxide in the atmosphere.

EXPERIMENT: INTRODUCTION Remember: photosynthesis is a chemical process through which a plant produces an organic substance. Photosynthesis is represented in this equation: 6 CO H 2 O KCAL (sunlight energy) 6O 2 + C 6 H 12 O 6 Remember that the glucose C 6 H 12 O 6 is a monosaccharide sugar that usually polymerizes into complex sugars, such as starch, that is stored in amyloplasts (leucoplasts) inside leaves and in the plant’s roots.

EXPERIMENT: INTRODUCTION First prepare some geranium leaves by covering areas of some leaves with dark cardboard so as to block out all light. Photosynthesis does not take place when there is no light. Expose the leaves to strong light for 48 hours. In order to see whether photosynthesis has taken place in the areas exposed to light test for starch: starch is made during photosynthesis. We expect there will be no starch in the parts of the leaf that have not been exposed to light for 48 hours. To test for starch, boil the leaves in ethyl alcohol to remove some chlorophyll. Then colour the leaf with Lugol’s reagent, a colouring agent that reacts with complex sugars, i.e. with starch.

EXPERIMENT: AIM – RESOURCES – MATERIALS AIM: test for starch in leaves (if photosynthesis has taken place there will be starch in the leaves) RESOURCES: test tube - Petri dish – pliers – bunsen burner MATERIALS: geranium leaf / leaves Lugol’s reagent Ethyl alcohol

EXPERIMENT: PROCEDURE PROCEDURE: Expose a plant to sunlight for 48 hours with some leaves partly covered in black paperboard; Select a leaf and photograph it (to keep a record of chlorophyll distribution); Lift the black covering, roll up the leaf and put it in a test tube containing ethyl alcohol; Heat the test tube in a double boiler (in bain-marie), boiling the alcohol will decolorize the leaf; Take the leaf out of the test tube and dip it in water to soften it; Place the leaf in a Petri dish; Flood the whole leaf with Lugol’s reagent drops; Rinse the leaf and observe the colour.

EXPERIMENT: RESULTS RESULTS: Compare the photograph to the original, observe the differences. Lugol (which reacts with starch) has reacted in the leaf section exposed to sun (colour near violet), because glucose is one of the products of photosynthesis: glucose is a monosaccharide which easily polymerises into starch. The covered part of the leaf has changed colour because photosynthesis has not taken place so Lugol’s reagent cannot detect starch.

EXPERIMENT: CONCLUSIONS CONCLUSION: What happened in the leaf? The part of the leaf that was covered was not coloured by Lugol’s reagent, it is lighter. This means that there were no carbohydrates, starch, in these parts of the leaf for the Lugol’s reagent could react with as Lugol’s reagent only changes colour in the presence of starch. Starch is not a product of the photosynthesis, but when photosynthesis takes place, most of the glucose formed is stored in the leaf as starch. Since Lugol’s reagent didn’t find any carbohydrate in the part covered in cardboard, we can say that photosynthesis did not take place there. Thus we can conclude that sunlight is essential for photosynthesis, because without sun it does not take place.