Cell Energetics II - Photosynthesis
In laboratory 4 the process of respiration was examined. All autotrophic and heterotrophic organisms carry on cellular respiration as the method of converting the energy in complex organic compounds such as glucose, to a form usable by the organism. In addition to respiration these organisms also require organic compounds in the synthesis of new protoplasm.
The functioning of ecosystems is based on the fixation of energy and the production of organic compounds through the process of photosynthesis by autotrophs. Photosynthesis is carried out primarily by green plants and algae, both aquatic and terrestrial. It also occurs in certain pigmented bacteria and in the cyanobacteria. The ultimate energy source for this synthesis of organic compounds is sunlight. The raw materials for photosynthesis are carbon dioxide and water. These two compounds supply the carbon, oxygen and hydrogen that are the predominant elements in organic molecules.
The classical equation indicating the overall process of photosynthesis is:
It is unfortunate that the above equation tends to give the erroneous impression that photosynthesis is a single step process with carbohydrate synthesis as its major feature. In fact it is a complex process involving a large number of different chemical reactions. For the sake of study, these reactions are normally divided into two groups as follows:
In eukaryotic cells photosynthesis takes place within chloroplasts. Chloroplast morphology is quite variable, particularly among the unicellular and filamentous algae. The chloroplast of higher plants is typically lens shaped and measures between 4 and 6 micrometers in diameter. Each chloroplast is bounded by an envelope consisting of two unit membranes. The internal structure of the chloroplast is complex. A fairly homogeneous matrix, called the stroma, is traversed by an elaborate system of membranous thylakoids (lamellae). The stacks of disc-like thylakoids are called grana and the various grana are interconnected by intergranum lamellae. Starch granules will be present in the stroma of chloroplasts that are photosynthetically active.
Examine the electron micrograph of chloroplast.
Note the grana, stroma, intergranum lamellae and starch granules of the chloroplasts.
Where are the photosynthetic pigments located?
Where do the "light" reactions of photosynthesis occur?
Where do the "dark" reactions of photosynthesis occur?
Separation
and Identification of Pigments Involved in Photosynthesis
The three main groups of photosynthetic pigments are the chlorophylls, xanthophylls and carotenoids. Most photosynthetic pigments are insoluble in water but soluble in non-polar solvents such as ether, acetone and chloroform. One can therefore make use of this property in resolving a solution of pigments into its individual components. The technique of paper chromatography will be utilized to resolve a chloroplast preparation into its components. Consult the appendix of your lab manual and make sure you have a good knowledge of the principles of chromatography.
With the aid of a capillary tube carefully spot a tiny droplet of the pigment extract on the paper strip. The spot should be about 1.5 cm from the tip of the paper. Allow the spot to dry and repeat the application 30 to 40 times. (Be careful to keep the spot as small as possible to facilitate good resolution). Secure the strip of paper to the stopper provided by means of a paper clip. Place about 3 ml of the solvent (acetone-petroleum ether 10:90 v/v) in a test tube. Suspend the paper strip in the test tube making sure it does not touch the solvent or the sides of the tube. This can be achieved by raising the paper clip. Stopper tightly. After 5 minutes lower the paper clip so that the tip of the paper touches the solvent but the spotted area is well above the solvent.
After about 10-15 minutes, observe the chromatogram. You should be able to identify:
Carbon dioxide’s role in Photosynthesis
In this exercise we will be examining the uptake of CO2 by the fresh water plant Elodea while it carries out photosynthesis in a test tube. We will be doing this by looking at the pH change in the test tube as the CO2 gets taken up by the plant. This is possible because when CO2 dissolves into water it creates a weak acid, Carbonic acid, by disassociation.
Please, working in groups of 2-3, follow the instructions:
1. Pour 250ml of tap water into the 500ml beaker provided and add 15-20 drops of phenol red indicator. Phenol red indicator is a pH indicator that is yellow when in acidic solution and red in a basic solution. The water in the beaker should be red.
2. Pour some into the provided large test tube labeled 1 and place the large test tube in the rack on your lab bench.
3. Gently blow, using the straw provided, into the beaker water until it begins to change colour. Your breath contains lots of CO2 as a result of your cellular respiration. Gently swirl the flask until there is an even orange-yellow colour.
4. Pour the orange yellow water into 2 test tubes labeled 2 and 3. You should now have 3 test tubes. Tube 1 will have red water while tube 2 and 3 will have orange-yellow water.
5. Select a small, pre cut, snippit of Elodea from the finger bowl on the lab bench and place in test tube number 3.
6. Set the test tube rack aside, wait 30 minutes and record your observations on the table below
Formulate a hypothesis for this experiment:
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