Cellular Respiration & Photosynthesis

Cellular Respiration & Photosynthesis

10. Describe the similarities and differences between the biochemical pathways of aerobic respiration and photosynthesis in eukaryotic cells. Include in your discussion the major reactions, the end products, and energy transfers.

Both photosynthesis and cellular respiration are used by living things to produce energy. Photosynthesis and cellular respiration are used by plants. The food for the plant is formed in photosynthesis and then this food is used to make energy in cellular respiration. Animals only use cellular respiration which is used to break down the food the animals ingest into energy. Photosynthesis and cellular respiration are complementary reactions. They are basically the same reactions in reverse from one another. Animals give off carbon dioxide from cellular respiration which is needed by the plants for photosynthesis. And plants give off oxygen from photosynthesis which is needed by the animals for cellular respiration.

Photosynthesis is a reaction that, in the presence of sunlight, converts carbon dioxide into organic compounds. And cellular respiration involves reactions that convert food into ATP (energy) and carbon dioxide. Photosynthesis requires sunlight, carbon dioxide, and water (these are the reactants). And it gives off organic compounds and oxygen (these are the products). Cellular respiration requires organic compounds (food) and oxygen (these are the reactants). And it gives off carbon dioxide and water (these are the products). The chemical reaction of photosynthesis can be summarized as: carbon dioxide and water combine in the presence of light and produce glucose and oxygen (carbon dioxide is absorbed and oxygen is released). The chemical reaction of cellular respiration can be summarized as: glucose and oxygen are broken down into water and carbon dioxide (oxygen is absorbed and carbon dioxide is released).

Photosynthesis takes place in the chloroplasts of plant cells and cellular respiration occurs in the mitochondria of all living cells. Both produce ATP (energy) needed by all living organisms for growth, development, and cellular functions. Photosynthesis can only occur in the presence of light, but cellular respiration occurs at all times (light is not required). Photosynthesis requires a catalyst, sunlight. Cellular respiration does not require a catalyst. Photosynthesis has two stages: a light dependent stage and a light independent stage. And its main function is the production of food and energy capturing. Its final electron receptor is NADP+, its electron carrier is NADPH, and its electron source is the oxidation of water at PSII. Cellular respiration has three stages: glycolysis, link reaction, and Krebs cycle. And its main function is the breakdown of food and releasing energy. Its final electron receptor is oxygen, its electron carrier is NADH, and its electron carrier is NADH and FADH2.

Photosynthesis and cellular respiration are both reactions that produce energy. Both are essential for life. Photosynthesis and cellular respiration are used by plants and cellular respiration is required for animals. And both reactions are at cellular level. Both involve organic molecules, water, oxygen, and carbon dioxide. But their reactions are the same reaction occurring in reverse. And photosynthesis requires a catalyst, sunlight, while cellular respiration does not. But each one produces a component needed by the other: photosynthesis produces oxygen needed in cellular respiration and cellular respiration produces carbon dioxide needed in photosynthesis. And in the end, both reactions are used essentially to provide energy for the cell.

12. Describe the light reactions of photosynthesis and, for both a C3 and a C4 plant, trace the path of a carbon dioxide molecule from the point at which it enters a plant to its incorporation into a glucose molecule. Include leaf anatomy and biochemical pathways in your discussion of each type of plant.

Photosynthesis occurs in two stages: a light dependent stage and a light independent stage. Here the light dependent stage will be discussed. In the light reactions of photosynthesis, solar energy is converted into ATP and NADPH. In other words, the plant uses the energy from sunlight to excite the chlorophyll molecules, split water releasing oxygen, and make energy in the forms of ATP and NADPH needed for the light independent stage of photosynthesis. And all of this occurs in the thylakoid membrane stacks inside of the chloroplasts.

Sunlight is absorbed by the chlorophyll and this excites the chlorophyll molecules so that they give up electrons (H+). These electrons (H+) are carried down the electron transport chain. Molecules of water are split to replace the electrons (H)+ lost by the chlorophyll. And this releases oxygen as a waste product. In the electron transport chain, the electrons (H+) are passed around to a series of proteins which produces chemical energy in the forms of ATP and NADPH. Then this energy enters the Calvin cycle.

CO2 enters photosynthesis at the Calvin cycle. The CO2 is absorbed from the air. It undergoes carbon fixation, then reduction, and then regeneration. In these steps, one molecule of G3P (a three-carbon sugar) exits the Calvin cycle for every three molecules of CO2 fixed. And this is converted into sugar and other organic molecules. Therefore, to synthesize one G3P molecule, the cycle uses three molecules of CO2, nine molecules of ATP, and six molecules of NADPH. One cycle looks like this: three CO2 molecules enter the plant through the stomata in the leaves. Then each carbon is attached with the help of the enzyme rubisco to a five-carbon sugar (RuBP). This forms a highly unstable six-carbon sugar that immediately splits in half forming two molecules of 3-phosphoglycerate (two molecules of 3-phosphoglycerate are formed for each original CO2 molecule). This step is called carbon fixation. Then the reduction step occurs where a phosphate group is added to each 3-phosphoglycerate molecule (the phosphate comes from ATP molecules which changes ATP into ADP). This produces 1,3-biphosphoglycerate. Then NADPH donates a pair of electrons to reduce 1,3-biphosphoglycerate and cause it to lose a phosphate group. Now 1,3-biphosphoglycerate becomes G3P. (For every three CO2 molecules that enter the Calvin cycle, six G3P molecules are formed. This causes a net gain of one three-carbon molecule or G3P.) One molecule of G3P is then available for use by the plant's cells. The other five G3P molecules go onto step three, the regeneration of the CO2 acceptor (RuBP). Here the five G3P molecules each accept another phosphate group from ATP (the phosphate comes from ATP molecules which changes ATP into ADP). Then