WJEC Biology A-level Topic 3.1-3.3: Importance of ATP, Photosynthesis and Respiration Notes

ATP (Adenosine Triphosphate)

• Definition: ATP is a nucleotide derivative composed of ribose, adenine, and three phosphate groups.
• Energy Release:
  • Energy is liberated when ATP is hydrolyzed to form ADP (adenosine diphosphate) and an inorganic phosphate molecule.
  • This hydrolysis is catalyzed by the enzyme ATP hydrolase.
• Phosphorylation:
  • The liberated inorganic phosphate can be utilized to phosphorylate other molecules, increasing their reactivity.
• ATP Production:
  • The condensation of ADP and inorganic phosphate to synthesize ATP is catalyzed by ATP synthase during photosynthesis and respiration.

Photosynthesis

• Definition:
  • Photosynthesis is a biochemical reaction wherein light energy is harnessed to break the strong bonds in water molecules through a process called photolysis, leading to the combination of hydrogen with carbon dioxide. This results in the production of glucose, a form of fuel.
  • Oxygen is released as a waste product of this reaction into the atmosphere.
• Factors Influencing Rate:
  • The photosynthetic rate is influenced by the concentrations of carbon dioxide, light intensity, and temperature.
• Location:
  • The chloroplast is the organelle responsible for photosynthesis, adapted in various ways:
  • Contains stacks of thylakoid membranes called grana, which house photosynthetic pigments like chlorophyll arranged in photosystems.
  • Contains stroma, the fluid surrounding the grana, rich in enzymes required for the light-independent stage of photosynthesis.
• Stages of Photosynthesis:
  • Light-Dependent Reactions:
  • Light energy excites electrons in the chlorophyll molecules within the thylakoid membranes, raising them to a higher energy state.
  • These high-energy electrons are passed down an electron transport chain (ETC), generating ATP from ADP and inorganic phosphate through photophosphorylation.
  • Reduced NADP is generated as electrons are transferred to NADP along with protons.
  • Light-Independent Reactions (Calvin Cycle):
  • The final stage of photosynthesis utilizing ATP and reduced NADP to synthesize glucose. The steps are:
    1. Carbon fixation: Combines RuBP (Ribulose bisphosphate) with carbon dioxide, catalyzed by RUBISCO.
    2. Conversion of RuBP into two glycerate 3-phosphate (GP) molecules.
    3. Reduced NADP and ATP convert GP into glyceraldehyde 3-phosphate (TP).
    4. Some TP molecules transform into glucose, which can further form polysaccharides, lipids, amino acids, and nucleic acids.
    5. Remaining TP molecules are used to regenerate RuBP with ATP assistance.

Respiration

• Definition:
  • Aerobic respiration involves the breakdown of respiratory substrates to produce carbon dioxide as a waste product while combining hydrogen with oxygen, releasing significant energy.
  • In contrast, anaerobic respiration occurs without air.
• Multi-Step Process:
  • Respiration comprises multiple stages, each mediated by specific intracellular enzymes.
• Glycolysis:
  • The first stage in both aerobic and anaerobic respiration occurring in the cytoplasm.
  • Glucose is phosphorylated to yield two pyruvate molecules, two ATP molecules, and two NADH molecules.
• Anaerobic Pathway:
  • In anaerobic conditions, pyruvate is further processed into lactate with the assistance of NADH. Lactate can subsequently be converted back into pyruvate in the liver.
• Aerobic Pathway:
  • The next step following glycolysis in aerobic respiration is the link reaction, where pyruvate transforms into Acetyl-CoA with the help of NADH.
• Krebs Cycle:
  • Acetyl-CoA enters this cycle, leading to the oxidation of glucose, production of carbon dioxide, ATP, reduced NAD, and reduced FAD.
• Oxidative Phosphorylation:
  • A crucial process for ATP synthesis occurring within the electron transport chain located in the mitochondria. The process unfolds as follows:
  • Reduced coenzymes transport hydrogen ions and electrons to the electron transport chain on the inner mitochondrial membrane.
  • Electrons undergo a series of redox reactions, where the electron carrier donating the electron is oxidized, while the receiving one is reduced.
  • High concentration of hydrogen ions builds up in the intermembrane space.
  • Hydrogen ions diffuse back into the mitochondrial matrix along their electrochemical gradient, driving ATP synthesis via ATP synthase.
  • The hydrogen atoms combine with oxygen, forming water.

Importance of Water and Inorganic Ions

• Water Functions:
  • Essential for photosynthesis, maintaining structural rigidity, transporting substances, and thermoregulation.
• Magnesium Ions:
  • Vital for chlorophyll production and activate certain plant enzymes.
• Nitrate Ions:
  • Provide nitrogen necessary for synthesizing DNA, RNA, proteins, and chlorophyll.
• Calcium Ions:
  • Constituent of plant cell walls, forming calcium pectate; they play a crucial role in plant growth.