• The study of how energy is transformed and exchanged within living organisms, particularly at the cellular level.

1.1 Photosynthesis

• The biochemical process by which green plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy stored in glucose molecules.

1.1.1  Photosynthetic Reaction

• Photosynthesis is an endothermic reaction in which light transfers energy from the environment to the chloroplasts.

• The equation represents photosynthesis:

  • carbon dioxide + water →light→ glucose + oxygen

  • Chemical Symbols

    • Carbon Dioxide: CO₂

    • Water: H₂O

    • Oxygen: O₂

    • Glucose: C₆H₁₂O₆

1.1.2 Rate of Photosynthesis

Factors that affect the rate of Photosynthesis

• Temperature: An increase in temperature equates to an increased rate of photosynthesis, making the involved enzyme more active. 

  • Excessively high temperatures can denature enzymes and photosynthetic activity.

• Light Intensity: Low light intensities limit the rate of photosynthesis while increased light intensity increases the rate of photosynthesis until a point where another factor becomes limiting.

• Carbon Dioxide Concentration: Increased CO2 concentration in the environment equates to an increased rate of photosynthesis while environments with low CO2 concentrations limit photosynthesis.

• Amount of Chlorophyll: Plants with higher chlorophyll content generally have a greater capacity for photosynthesis as they can capture more light energy.

  • Factors that affect chlorophyll content, such as nutrient availability, plant health, and genetic factors, can indirectly influence the rate of photosynthesis.

Measuring the Rate of Photosynthesis

• Oxygen Production: Measures the rate of oxygen production during photosynthesis. 

  • Involves placing an aquatic plant in water and exposing it to light while collecting the oxygen bubbles released.

  • Oxygen can be measured by counting bubbles evolved from pondweed, or by using the Audus apparatus to measure the amount of gas evolved over a period of time.

Photosynthesis Rate Graph

• The factors mentioned above may become a limiting factor.

  • A graph with one line that levels off involves one limiting factor (horizontal axis) and rate of photosynthesis (vertical axis)

  • The rate of photosynthesis will increase as carbon dioxide concentration increases. The graph will level out at the point when another limiting factor prevents any further increase in the rate of photosynthesis.

1.1.3 Uses of glucose from photosynthesis

• The glucose produced in photosynthesis may be;

  • Used for respiration

  • Converted into insoluble starch for storage

  • Used to produce fat or oil for storage

  • Used to produce cellulose, which strengthens the cell wall

  • Used to produce amino acids for protein synthesis.

• To produce proteins, plants also use nitrate ions that are absorbed from the soil.

1.2 Respiration

1.2.1 Aerobic and Anaerobic Respiration

• Respiration: a metabolic process wherein, the living cells of an organism acquire energy by taking in oxygen and liberating carbon dioxide from the oxidation of complex organic substances.

• Cellular Respiration: the process by which cells acquire energy from glucose.

  • It is an exothermic reaction which is continuously occurring in living cells.

• The energy transferred supplies all the energy needed for living processes.

  • Respiration in cells works either aerobically (with oxygen) or anaerobically (without oxygen), to transfer energy.

Process of Aerobic and Anaerobic Respiration

1. Aerobic Respiration

   • Oxygen is essential for the complete breakdown of glucose into carbon dioxide and water.

   • The end products are carbon dioxide and water as byproducts.

   • Produces a much larger amount of ATP per glucose molecule compared to anaerobic respiration.

   • Equation: glucose + oxygen → carbon dioxide + water

2. Anaerobic Respiration

   • It can occur in the absence of oxygen.

   • The end products are carbon dioxide and ethanol or lactic acid.

   • The oxidation of glucose is incomplete which reduces the amount of energy (ATP) transferred compared to aerobic respiration. 

   • Fermentation refers to the anaerobic respiration in yeast which has economic importance in the manufacture of bread and alcoholic drinks.

   • Equation (in Muscles): glucose → lactic acid

   • Equation (in Plant and Yeast Cells): glucose ethanol + carbon dioxide

• Adenosine triphosphate (ATP) is the source of energy for use and storage at the cellular level.

• Organisms need energy for:

  • Chemical reactions to build larger molecules

  • Movement

  • Keeping warm

1.2.2 Response to Exercise

• The human body reacts to increased demand for energy during exercise.

  • The heart rate, breathing rate and breath volume increases during exercise to supply the muscles with more oxygenated blood.

  • Anaerobic respiration takes place in muscles when there is insufficient supply of oxygen. The incomplete oxidation of glucose causes a buildup of lactic acid and creates an oxygen debt. 

  • Muscles become fatigued and stop contracting efficiently during long periods of vigorous activity.

1.2.3 Metabolism

• Refers to the process where chemical reactions in the body's cells change food into energy.

• The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesises new molecules.

• Metabolism includes;

  • Conversion of glucose to starch, glycogen and cellulose

  • Formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids

  • Use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins

  • Respiration

  • Breakdown of excess proteins to form urea for excretion.

Importance of the following in the synthesis and breakdown of carbohydrates, proteins and lipids:

• Sugars

  • Serve as the primary building blocks for carbohydrates synthesis.

    • For example, in photosynthesis, sugars are synthesised into complex carbohydrates like starch and glycogen, which serve as energy storage molecules.

  • Sugars (glucose) are broken down through processes like glycolysis to produce ATP, the primary energy currency of cells.

    • Glucose can be converted into other molecules like pyruvate, which can enter various metabolic pathways to produce additional energy or serve as precursors for other molecules.

• Amino Acids

  • Serve as the building blocks of protein synthesis.

    • During protein synthesis, amino acids are linked together through peptide bonds to form polypeptide chains.

  • Protein catabolism is important for mobilising essential amino acids for absorption. (protein breakdown)

    • These amino acids can then be used for protein synthesis, energy production, or as precursors for other molecules.

• Fatty Acids and Glycerol

  • Are essential components of lipids (synthesis), including triglycerides (fats and oils) and phospholipids.

    • Fatty acids are linked to glycerol molecules that serve as energy storage molecules, providing insulation and protection for organs.

    • Phospholipids (composed of a glycerol backbone, two fatty acids, and a phosphate group) are crucial components of cell membranes.

  • Lipids are broken down through processes like lipolysis, where triglycerides are hydrolyzed into fatty acids and glycerol.

    • Fatty acids released from lipolysis can be oxidised in mitochondria to generate ATP through beta-oxidation or used as precursors for the synthesis of other lipid molecules.