Metabolism, Photosynthesis, and Cellular Respiration

Posted on Mar 6, 2025 in Biology

Key Definitions in Metabolism and Energy

Metabolism: The process by which your body converts what you eat and drink into energy.

Energy: The capacity for doing work. It may exist in potential, kinetic, thermal, electrical, chemical, nuclear, or other forms.

Chemical Energy: Energy stored in the bonds of chemical compounds. It may be released during a chemical reaction, often in the form of heat (exothermic reactions).

Potential Energy: Stored energy that depends upon the relative position of various parts of a system. It is a property of a system, not of an individual body or particle.

Kinetic Energy: Energy that an object or a particle has by reason of its motion. It depends on both its motion and its mass.

Anabolism/Catabolism:

  • Anabolism: A metabolic process where complex molecules are synthesized from simpler ones, storing energy.
  • Catabolism: Enzyme-catalyzed reactions where large molecules are broken down, releasing energy. Some of this energy is conserved in energy-rich compounds like ATP.

Mitochondria: Membrane-bound cell organelles that generate most of the chemical energy needed to power the cell’s biochemical reactions, stored as ATP.

Plant Leaf Structures

Epidermis: Covers the upper and lower surfaces of the leaf. It’s a single layer of tightly-packed cells that mediates exchanges between the plant and its environment, controlling water loss, gas exchange, and sunlight transmission.

Mesophyll: Two layers of cells inside the leaf. The palisade parenchyma cells, located under the epidermis, contain many chloroplasts and are primarily responsible for photosynthesis.

Chloroplasts: Occur in all green tissues, concentrated in the mesophyll. They have a double membrane and a third inner membrane (thylakoid membrane) and contain the stroma.

Stroma: The innermost matrix of chloroplasts, containing metabolic enzymes and multiple copies of the chloroplast genome.

Stomata: Microscopic openings or pores in the epidermis of leaves and young stems, more numerous on the underside of leaves. They open and close in response to the internal pressure of guard cells.

Chlorophyll: The substance that gives plants their green color and absorbs light energy. The internal structure of the leaf is protected by the leaf epidermis.

Laws of Thermodynamics

The First Law of Thermodynamics states that energy cannot be created or destroyed; the total quantity of energy in the universe stays the same.

The Second Law of Thermodynamics states that there is a natural tendency of any isolated system to degenerate into a more disordered state (entropy increases).

Photosynthesis and Cellular Respiration

Photosynthesis Equation: 6CO2 + 6H2O → C6H12O6 + 6O2

Reactants: Carbon dioxide and water. Products: Glucose and oxygen.

Cellular Respiration Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O

Reactants: Glucose and oxygen. Products: Carbon dioxide and water.

Roles of Photosynthesis and Cellular Respiration

Photosynthesis: Plants (autotrophs) produce their own food, transforming water, sunlight, and carbon dioxide into oxygen and sugars.

Cellular Respiration: Occurs in the mitochondria, breaking down sugar with oxygen to release energy as ATP, producing carbon dioxide and water as waste.

Locations of Processes

Photosynthesis: Takes place in chloroplasts.

Cellular Respiration: Most steps take place in the mitochondria.

Fate of Glucose from Photosynthesis

Glucose can be:

  • Converted into starch for storage.
  • Broken down during respiration, releasing stored energy.
  • Used to build other organic molecules.

ATP Production and Structure

ATP Production: Created from ADP using sunlight during photosynthesis, or during cellular respiration in mitochondria. Aerobic respiration produces ATP, CO2, and water from glucose and oxygen.

ATP Structure: A nucleoside triphosphate with adenine, ribose sugar, and three phosphate groups. The bond between the second and third phosphate groups provides readily releasable energy.

Aerobic Respiration vs. Fermentation

Aerobic respiration is more efficient, producing the most ATP per energy input. Fermentation (anaerobic) occurs without oxygen.

Stages of Photosynthesis

Photosynthesis has two stages:

  • Light-dependent reactions (in the thylakoid membrane): Use light energy to make ATP and NADPH.
  • Calvin cycle (in the stroma): Uses ATP and NADPH to make GA3P from CO2.

Inputs and Outputs

Light Reactions: Inputs: Light, water, ADP, P, NADP+. Outputs: Oxygen, ATP, NADPH.

Calvin Cycle: Inputs: CO2, ATP, NADPH. Outputs: GA3P (an energy-rich sugar).

Stages of Cellular Respiration

Three main stages:

  1. Glycolysis (cytoplasm)
  2. Krebs Cycle (mitochondrial matrix)
  3. Electron Transport Chain (mitochondrial membrane)

Inputs, Outputs, and ATP Production

  • Glycolysis: Inputs: Glucose, 2 ATP, 2 NAD+. Outputs: 2 pyruvate, 4 ATP (net gain of 2 ATP), 2 NADH.
  • Citric Acid Cycle (Krebs Cycle): Inputs: Acetyl-CoA, 3 NAD+, 1 FAD, 1 ADP + Pi. Outputs: 2 CO2, 3 NADH, 1 FADH2, 1 ATP (per cycle, two cycles per glucose molecule, so 2 ATP total).
  • Electron Transport Chain: Inputs: NADH, FADH2, O2. Outputs: H2O, ~32-34 ATP.

Fermentation Products

Lactic Acid Fermentation: Produces lactic acid.

Alcoholic Fermentation: Produces ethanol and carbon dioxide.