Obtaining Cellular Energy

Photosynthesis

Photosynthesis is the process of converting carbon dioxide, water and light energy into oxygen and high-energy sugar molecules. The chemical equation representing this process is 6CO2 + 6 H2O + light C6H12O6 + 6 O2. Plants, algae and some bacteria can use the sugar molecules produced during photosynthesis to make complex carbohydrates such as starch of cellulose for food. The process of photosynthesis consists of two basic stages: Light-dependent reactions and light-independent reactions (also called the Calvin cycle).

Photosynthesis takes place inside an organelle called a chloroplast. A chloroplast is one type of organelle called a plastid. Plastids engage in photosynthesis and store the resulting food. The chloroplast is a specific organelle with a double membrane that contains stacks of sac-like membranes called thylakoids. The thylakoid membrane contains a green pigment called chlorophyll. Pigments are substances that absorb light. Light-dependent reactions take place inside the thylakoid membrane. Light-independent reactions take place in the stroma. In the light-dependent phase, sunlight hits the leaf of the plant where it is absorbed by the pigments in the leaf. There are several plant pigments, but the most familiar is chlorophyll, the green pigment. Chlorophyll is stored in the chloroplasts of the plant cell.

When light hits the chlorophyll, electrons absorb the energy, become excited and leave that chlorophyll molecule. Carrier molecules transport the electrons which follow an electron transport chain. Electron acceptor molecules pick up the electrons in a series and pass them from one molecule to another. As this occurs, energy is released, and ATP is formed. The final electron acceptor is NADP+.

Splitting a molecule of water replaces the electrons released from the chlorophyll. These electrons, now available, combine with the NADP+ to form NADPH. The next stage of photosynthesis uses the NADPH, while oxygen leaves as an end product of the reaction.

The end products of the light-dependent reactions are ATP, oxygen and NADPH. The ATP and NADPH are used in the light-independent reactions, and the oxygen is released into the atmosphere.

The next phase, the light-independent or carbon fixation reactions, uses the ATP formed during the light-dependent reaction as an energy source. In this phase, carbon, from carbon dioxide, and NADPH are used to form glucose. To accomplish this, a five-carbon sugar (called a pentose) uses a carbon atom from carbon dioxide to create a six-carbon sugar (a hexose). Glucose is the end result, after several conversions have taken place. The glucose can then be used as food to enter cellular respiration, or it can be converted to other carbohydrate products such as sucrose or starch.

Cellular Respiration

Cellular respiration is the process of breaking down food molecules to release energy. Plants, algae, animals and some bacteria use cellular respiration to break down food molecules. There are two basic types of cellular respiration: aerobic and anaerobic. Aerobic respiration occurs in the presence of oxygen and is represented by the chemical equation below. The energy released through cellular respiration is used to create ATP. Cellular respiration occurs in three phases: glycolysis, Krebs cycle, and electron transport. The process starts with a molecule of glucose and the reactions of cellular respiration occur with the use of enzymes. Respiration is the primary means by which cells obtain usable energy.

Glycolysis is the first phase in cellular respiration. This step occurs in the cytoplasm of the cell, and it can occur whether or not oxygen is present. In this phase, the glucose molecules (a 6-carbon sugar) is broken in half through a series of reactions. The energy released by breaking down the glucose is used to produce ATP. Additionally, some high-energy electrons are removed from the sugar during glycolysis. These electrons pass on to an electron carrier called NAD+, converting it to NADPH. These electrons will alter b used to create more energy.

In aerobic respiration, the 3-carbon sugars produced from glycolysis enter the mitochondria along with oxygen. As the sugars enter the mitochondria they convert to citric acid in phase two of cellular respiration. The citric acid cycle, or Krebs cycle, is the cyclical process that breaks down the citric acid through a series of reactions The citric acid cycle produces more ATP, as well s some GTP (a high-energy molecule similar to ATP). More high-energy electrons are released, forming NADH from NAD+.

The last phase of cellular respiration is the electron transport chain, which occurs on the inner mitochondrial membrane. In this phase the NADH releases the hydrogen ions and high-energy electrons it picked up during glycolysis and the citric acid cycle. The energy from these electrons is used to convert large quantities of ADP into ATP. The electrons transfer through a series of carrier proteins. At the end of the electron transport chain, the free electrons and H+ ions bond with oxygen. The oxygen and H+ ions form water, which is released from the cell as waste product. Each electron transfer releases energy. The multi-step processes carried out during aerobic cellular respiration are very efficient. From the input of one glucose molecule, 36 ATP molecules can be produced.

Anaerobic Respiration

Anaerobic respiration, or fermentation, is the process by which sugars break down in the absence of oxygen. Our muscle cells, fungi and some bacteria are capable of carrying out anaerobic respiration. These cells convert the products of glycolysis into either alcohol or lactic acid. Glycolysis releases energy, while the production of alcohol or lactic acid provides NAD+, the electron carrier needed for glycolysis.

Yeast and some bacteria can carry out alcoholic fermentation. Yeast produces ethanol through alcoholic fermentation. The chemical equation representing this process is shown below. During the fermentation process, carbon dioxide gas is released. This carbon dioxide gas is responsible for the holes in bread. The fermentation of yeast cells produces carbon dioxide, which becomes trapped in the dough, forming small bubbles and causing the bread to rise. Other uses of alcoholic fermentation are the making of beer, wine, and liquor.

Animal cells use a form of anaerobic cellular respiration called lactic acid fermentation. This generally occurs when the organism is engaging in strenuous exercise. During this period, the organism is not able to take in enough oxygen to meet the demand of all the cells in the body. So in order to keep working, the cells begin to break down glucose in the absence of oxygen. The products of the reaction are energy and lactic acid.

Lactic acid fermentation proceeds at a very fast rate, but the amount of energy produced is far less than with aerobic respiration. Remember the 36 ATP molecules theoretically produced during aerobic respiration? By comparison, lactic acid fermentation produces only 2 ATP molecules.

So, can you tell if your cells are respiring aerobically or anaerobically? Well, you can assume that if you are doing anything that causes muscle fatigue-like weight lifting, those muscles have needed extra energy very quickly, and they obtained it through lactic acid fermentation. Your suspicions will be confirmed the next day if you are sore. Muscle soreness occurs because of lactic acid build-up in the muscle.

Comparing Photosynthesis and Cellular Respiration

All organisms must be able to obtain and convert energy to carry out life functions, such as growth and reproduction. Photosynthesis is one way that organisms can rap energy from the environment and convert it into a biologically useful energy source. Cellular respiration is a way that organisms can break down energy sources to carry out life’s processes. Photosynthesis takes place in plants, protists, algae and some bacteria. Cellular respiration takes place in all eukaryotic cells and some prokaryotic cells.

Photosynthesis / Cellular Respiration
Function / Energy storage / Energy release
Location / Chloroplast / Mitochondria
Reactants / Carbon dioxide and water / Glucose and oxygen
Products / Glucose and oxygen / Carbon dioxide and water
Chemical equation / 6CO2 +6 H2O C6H12O6 + 6O2 / C6H12O6 + 6O2 6CO2 + 6H2O

Section Review Questions

  1. Define the following terms:
  2. Photosynthesis
  3. Calvin cycle
  4. Chloroplast
  5. Thylakoid
  6. Chlorophyll
  7. Pigment
  8. Plastid
  9. Light-dependent phase
  10. Light-independent phase
  11. Carbon fixation
  12. Cellular respiration
  13. Aerobic respiration
  14. Anaerobic respiration
  15. Glycolysis
  16. Krebs cycle
  17. Electron transport chain
  18. Alcoholic fermentation
  19. Lactic acid fermentation
  20. chemosynthesis
  21. What form of energy is used by cells?
  22. Enzymes
  23. Cofactors
  24. ATP
  25. DNA
  26. The process of releasing energy from the chemical breakdown of compounds in a cell is
  27. Hesitation
  28. Expiration
  29. Elimination
  30. respiration
  31. What is released when ATP is broken down into ADP and one phosphate?
  32. Oxygen
  33. Water
  34. Energy
  35. hydrogen
  36. The process by which the energy from the Sun is used to create glucose molecules is known as
  37. Cellular respiration
  38. Photosynthesis
  39. Chemosynthesis
  40. fermentation
  41. What is a plastid’s function within a cell?
  42. Digest food and break down wastes
  43. Produce proteins
  44. Carry on cellular respiration
  45. Carry out photosynthesis and provide color
  46. Compare and contrast aerobic and anaerobic respiration.
  47. Write the chemical equation for photosynthesis and cellular respiration using words instead of chemical formulas.