Photosynthesis and Cellular Respiration: Biochemical Pathways

Introduction to Biochemical Pathways

• Biochemical pathways are a series of linked biochemical reactions.
• These reactions start with an initial reactant and are converted step-wise to a final product.
• The product of one reaction serves as the reactant for the next step.
• The sum of all biochemical reactions in an organism is called metabolism.

KEY TAKEAWAY: Biochemical pathways are interconnected series of reactions, not isolated events, essential for life.

Photosynthesis: A Biochemical Pathway

Photosynthesis is the process where light energy is converted into chemical energy in the form of glucose. It occurs in two main stages: light-dependent and light-independent reactions.

Light-Dependent Reactions (in the Thylakoid Membranes)

• Location: Thylakoid membranes of the chloroplast.
• Inputs:
  • Light energy
  • Water (\(H_2O\))
  • ADP
  • \(NADP^+\)
• Process Overview:
  1. Light energy is absorbed by chlorophyll and other pigments.
  2. Water molecules are split through photolysis, releasing electrons, protons (\(H^+\)), and oxygen (\(O_2\)).
  3. Electrons move through the electron transport chain (ETC), generating a proton gradient.
  4. ATP is produced via photophosphorylation using ATP synthase.
  5. \(NADP^+\) is reduced to NADPH.
• Outputs:
  • Oxygen (\(O_2\)) - released as a byproduct.
  • ATP - energy currency for the next stage.
  • NADPH - reducing power for the next stage.

Light-Independent Reactions (Calvin Cycle in the Stroma)

• Location: Stroma of the chloroplast.
• Inputs:
  • Carbon dioxide (\(CO_2\))
  • ATP
  • NADPH
• Process Overview:
  1. Carbon fixation: \(CO_2\) is incorporated into an organic molecule - Ribulose-1,5-bisphosphate (RuBP), catalyzed by Rubisco.
  2. Reduction: The resulting molecule is reduced using ATP and NADPH to form glyceraldehyde-3-phosphate (G3P).
  3. Regeneration: Some G3P is used to regenerate RuBP, allowing the cycle to continue.
• Outputs:
  • Glyceraldehyde-3-phosphate (G3P) - a precursor to glucose and other organic molecules.
  • ADP
  • \(NADP^+\)

VCAA FOCUS: Understand the inputs and outputs of each stage, and where they occur within the chloroplast.

Cellular Respiration: A Biochemical Pathway

Cellular respiration is the process where glucose is broken down to release energy in the form of ATP. It involves three main stages: glycolysis, Krebs cycle, and the electron transport chain.

Glycolysis (in the Cytosol)

• Location: Cytosol of the cell.
• Inputs:
  • Glucose
  • 2 ATP (initial investment)
  • \(NAD^+\)
• Process Overview:
  1. Glucose is broken down into two molecules of pyruvate (3-carbon molecule).
  2. This process requires an initial investment of 2 ATP molecules.
  3. ATP and NADH are produced.
• Outputs:
  • 2 Pyruvate
  • 2 ATP (net gain)
  • 2 NADH

Krebs Cycle (Citric Acid Cycle in the Mitochondrial Matrix)

• Location: Mitochondrial matrix (in eukaryotes).
• Inputs:
  • Pyruvate (converted to Acetyl-CoA)
  • \(NAD^+\)
  • FAD
  • ADP
• Process Overview:
  1. Pyruvate is converted to Acetyl-CoA, which enters the Krebs Cycle.
  2. Acetyl-CoA combines with oxaloacetate to form citrate.
  3. Through a series of reactions, citrate is regenerated back into oxaloacetate, releasing \(CO_2\), ATP, NADH, and \(FADH_2\).
• Outputs (per pyruvate molecule):
  • \(CO_2\)
  • 1 ATP
  • 3 NADH
  • 1 \(FADH_2\)

Electron Transport Chain (ETC) and Oxidative Phosphorylation (in the Inner Mitochondrial Membrane)

• Location: Inner mitochondrial membrane (cristae in eukaryotes).
• Inputs:
  • NADH
  • \(FADH_2\)
  • \(O_2\)
  • ADP
• Process Overview:
  1. NADH and \(FADH_2\) donate electrons to the ETC.
  2. Electrons move through a series of protein complexes, releasing energy to pump protons (\(H^+\)) from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient.
  3. The proton gradient drives ATP synthesis via chemiosmosis through ATP synthase.
  4. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.
• Outputs:
  • ATP (approximately 32-34 ATP per glucose molecule)
  • \(H_2O\)

STUDY HINT: Create flowcharts for both photosynthesis and cellular respiration to visualize the sequence of reactions and the movement of molecules.

Anaerobic Fermentation

When oxygen is limited, cells can use anaerobic fermentation to regenerate \(NAD^+\) so that glycolysis can continue.

• Animals (Lactic Acid Fermentation): Pyruvate is reduced to lactic acid, regenerating \(NAD^+\).
• Yeast (Alcohol Fermentation): Pyruvate is converted to ethanol and \(CO_2\), regenerating \(NAD^+\).

COMMON MISTAKE: Don’t confuse the locations of the different stages of cellular respiration. Glycolysis occurs in the cytosol, while the Krebs cycle and ETC occur in the mitochondria (in eukaryotes).

Enzymes and Coenzymes

• Enzymes: Biological catalysts that speed up biochemical reactions by lowering the activation energy.
• Coenzymes: Non-protein organic molecules that assist enzymes in catalyzing reactions (e.g., NAD+, FAD, and CoA).

REMEMBER: Enzymes are specific to their substrates, meaning each enzyme catalyzes only one type of reaction.

Factors Affecting Rate of Photosynthesis and Cellular Respiration

• Photosynthesis:
  • Light availability
  • Water availability
  • Temperature
  • Carbon dioxide concentration
• Cellular Respiration:
  • Temperature
  • Glucose availability
  • Oxygen concentration

EXAM TIP: Be prepared to explain how these factors affect the rates of photosynthesis and cellular respiration, and why. For example, temperature affects enzyme activity, and lack of water can limit photosynthesis.

Summary of Pathways

• Photosynthesis: Anabolic pathway (builds glucose).
• Cellular Respiration: Catabolic pathway (breaks down glucose).

APPLICATION: Understanding these pathways is critical in biotechnology for improving crop yields, developing biofuels, and other applications.