Respiration and Photosynthesis

Aerobic Respiration

• The overall reaction is:

Glucose	+	Oxygen		Carbon Dioxide	+	Water	+	Energy
C6H12O6	+	O2		CO2	+	H2O	+	Energy

• Respiration is an exergonic reaction.
• There are 4 steps in the reaction:
  1. Glycolysis
     This occurs in the cytoplasm, where glucose breaks down to pyruvate (also called pyruvic acid), which then enters the mitochondrion. This occurs in both aerobic and anaerobic respiration.
  2. First Part of Krebs Citric Acid Cycle
     This occurs in the mitochondrion, where pyruvate is broken down to carbon dioxide in a process that does not use oxygen. A co-enzyme called NAD+ becomes NADH.
  3. Second Part of Krebs Citric Acid Cycle
     This occurs on the inner membranes of the mitochondrion. NADH is converted to NAD+ in a process that uses oxygen.
  4. ATP Synthesis
     This occurs in the mitochondrion. Hydrogen ions are pumped across the inner membrane of the mitochondrion. Then both the hydrogen ions and an enzyme called ATP Synthetase convert ADP plus a phosphate group to ATP.

Anaerobic Respiration

• Anaerobic Respiration is the breakdown of glucose (Glycolysis) to form energy-rich ATP, but does not require oxygen.
• It is less efficient than aerobic respiration.
• It occurs in 2 different ways in different organisms.

Photosynthesis Reaction

Carbon Dioxide	+	Water	Sunlight	Glucose	+	Oxygen
			Chlorophyll
CO2	+	H2O		C6H12O6	+	O2

• Photosynthesis is an endergonic reaction.
• The part of the visible light spectrum that is mostly used is blue and red light. These are absorbed by the green chlorophyll which reflects green light.
• There are 2 photosynthetic reactions:
  1. Photophosphorylation or Light-Dependent Reaction ('Light' Reaction)
     This occurs on the membranes in chloroplasts. Light that is 'trapped' by chlorophyll drives reactions that produce ATP and NADPH for the next 'dark' reaction. Oxygen is also produced but is released as a by-product.
  2. Carbon Fixation or Light-Independent Reaction ('Dark' Reaction)
     This is also called the Calvin-Benson Cycle, and does not require light. It occurs in the stroma of chloroplasts. The NADPH and ATP from the 'Light' Reaction are used to convert carbon dioxide to glucose.

Factors Affecting Photosynthesis

• Light Intensity - The rate of photosynthesis increases with light intensity until a certain level is reached.
• Light Wavelengths - Green light is least absorbed, since it is reflected by leaves, which is why leaves appear green. Red-orange and violet colours are those more readily absorbed by chlorophyll.
• Carbon Dioxide Levels - Increases in CO₂ concentration increase the rate of photosynthesis until it levels off.
• Temperature - As the temperature increases slightly, the rate of photosynthesis also increases. However, if the temperature drops too low, plants are unable to make chlorophyll, the leaves turn yellow as in autumn, and they are unable to photosynthesise. If the temperature rises too high, photosynthesis ceases.
• Amount of Water - As the photosynthesis reaction requires water, a lack of water decreases the plant's ability to photosynthesise.
• C₃ and C₄ Plants - In C₃ plants, up to 50% of CO₂ trapped by chlorophyll is released before being converted to sugar. C₄ plants have a different enzyme to trap CO₂ and are more efficient in conditions of higher CO₂ concentrations such as that formed by the Greenhouse Effect. Examples of C₄ plants include corn, sorghum and sugarcane.