Photosynthesis, Respiration and Enzyme Action
Date: Photosynthesis, Respiration and Energy
Photosynthesis and Aerobic Respiration
Life cannot exist without the sun’s energy. Its light is converted into energy via photosynthesis. This is a biochemical process in which plants manufacture glucose using water, chlorophyll and carbon dioxide (Rao & Kaur, 2012). Photosynthesis takes place inside organelles called chloroplasts, in plant leaves. Photosynthesis is divided into two stages called the light and dark reactions. The light reaction takes place in the grana, which are inside chloroplasts. During the light reaction, light photons are trapped by chlorophyll molecules, which get excited thus generating small electric currents inside the chloroplast (Belk & Maier, 2007). These currents convert Adenosine Diphosphate (ADP) into Adenosine Triphosphate (ATP) by adding an inorganic phosphate molecule. The light energy also causes water molecules to split into oxygen gas and hydrogen ions. The Oxygen is a waste product, which is discharged into the air while the hydrogen ion reduces nicotinamide adenine dinucleotide phosphate (NADP) to form reduced nicotinamide adenine dinucleotide phosphate (NADPH).
The Dark reaction is light independent. It takes place in the stroma located in chloroplasts. It consists of the synthesis of carbohydrates through the combination of hydrogen and carbon dioxide. This occurs through the reduction of carbon dioxide in the presence of NADPH and ATP molecules formed in the light reaction. The final products of photosynthesis are glucose, water and oxygen. Glucose can be used by the cell immediately or stored in the plant as starch, cellulose and sucrose. Plants also manufacture proteins and other foods from glucose. The oxygen is released in to the atmosphere where it is used by human beings and animals for respiration. The glucose molecules are converted into carbon dioxide and adenosine triphosphate. This process occurs through out the day in all live body cells.
Respiration is the process, which converts the energy stored in food during photosynthesis into a form that is beneficial to human beings. Glucose molecules are degraded into two pyruvate molecules in the cytoplasm of cells through glycolysis. If there is sufficient oxygen, anaerobic respiration then takes place in cell organelles called mitochondria. The pyruvate molecules are oxidized into acetyl alcohol. A series of reactions called Krebs cycle then produces hydrogen in the presence of the acetyl alcohol. The hydrogen triggers a chain reaction, which releases sufficient energy to make adenosine triphosphate. In anaerobic respiration, every molecule of glucose produces thirty-eight molecules of adenosine triphosphate- a form of energy.
After glycolysis, anaerobic respiration takes place in case of insufficient oxygen. A fermentation process occurs in the cytoplasm in which pyruvate molecules are broken down into lactic acid or ethanol, carbon dioxide and two molecules of adenosine triphosphate. Anaerobic respiration is wasteful in comparison to aerobic respiration due to the significantly lower yield of energy. Photosynthesis and respiration are the basis of the carbon/oxygen cycle that maintains balance in the environment. Photosynthesis is crucial, as it is the main source of oxygen and energy. Respiration is also important as it converts the energy formed in photosynthesis into a form that can be used by humans.
Role of Enzymes in Biochemical Reactions
Enzymes allow biochemical reactions to take place under conditions that can sustain life. Reactions such as respiration and photosynthesis are triggered and speeded up by enzymes, which are made of globular proteins (Honeysett & Parker, 2010). Enzymes interact with substrates in three main ways depending on the structure of the proteins.
The first interaction can be described as a lock and key mechanism. An active cavity or lock in an enzyme is bonded to a substrate key that fits the cavity in a precise way. The resulting complex speeds up the biochemical reaction in the active site. In catabolic reactions, enzymes degrade substrates in to various products. The reverse happens in anabolic reactions whereby enzymes combine several substrates to make a single product (Audesirk, Audesirk & Byers, 2008). The second interaction is called the “induced fit”. Here, the enzyme cavity changes shape to fit the substrate key as the substrate approaches the reaction site. After the biochemical reaction is over, the enzyme cavity then goes back to its normal shape. The third interaction usually occurs when enzymes reduce the energy of substrates during reactions. This speeds up the reaction as high activation energy usually reduces the rate of catalysis.
The enzyme substrate ratio usually affects the rate of reactions. When substrate molecules exceed the enzymes, the reaction is slowed down. Another factor influencing the enzyme reaction rate is the temperature at the reaction site. Excess heat denatures enzymes and reduces the reaction rate. Low temperatures have a similar effect as they reduce the interaction between enzymes and substrates. The acidity or alkalinity of the reactive site determines the bonding of enzymes in reaction sites. Different enzymes function in different conditions. For example, Pepsin functions best in acidic conditions while salivary amylase works in alkaline mediums.
Cells usually regulate the actions of enzymes by producing inhibitive chemicals that stop or reduce the rate of reactions. These chemicals attach themselves to the enzymes and change their shape thus preventing substrates from interacting with them. Some inhibitors such as poisons and do not originate in the cell. They have an effect similar to that of inhibitors released by the cell.
Audesirk, T., Audesirk, G., and Byers, B. (2008). Biology – Life on earth with physiology (8th Ed.). San Francisco, CA: Benjamin Cummings.
Belk, C. & Maier, V. G. (2007). Biology: science for life with physiology. Upper Saddle River, NJ: Pearson Prentice Hall.
Honeysett, I. & Parker, J. (2010). AQA biology: study guide. London: Letts and Lonsdale.
Rao, D. K. & Kaur, J. J. (2012). Living Science Biology 10. New Delhi: Ratna Sagar Publications.