Cellular Respiration & Photosynthesis

Organelles & Bioenergetics

This section explores the fundamental differences between autotrophs and heterotrophs, the anatomy of mitochondria and chloroplasts, and the principles of redox reactions and coenzymes. Understanding these concepts is essential for grasping how cells produce and utilize energy.

• Autotrophs vs. Heterotrophs:

  • Autotrophs synthesize their own organic food from inorganic materials, serving as primary producers in ecosystems. Photoautotrophs (e.g., green plants) use photosynthesis to convert elements into organic molecules.

  • Heterotrophs must consume other organisms or organic matter for energy and oxygen, performing cellular respiration (e.g., humans).

  • Example: Plants (autotrophs) produce glucose via photosynthesis; animals (heterotrophs) consume plants or other animals for energy.

• Mitochondrion Anatomy:

  • Enclosed by two phospholipid bilayer membranes with embedded proteins.

  • The inner membrane encloses the mitochondrial matrix, which contains DNA, ribosomes, and enzymes.

  • Folds of the inner membrane, called cristae, increase surface area for more electron transport chains (ETCs) and ATP production.

  • Example: The increased surface area of cristae allows for efficient ATP synthesis during cellular respiration.

• Chloroplast & Leaf Anatomy:

  • Stomata: Small pores on leaves for gas exchange (CO2 in, O2 out).

  • Chloroplasts are located in mesophyll tissue and are the site of photosynthesis.

  • Stroma: Fluid inside chloroplasts, containing thylakoids (stacked into grana), which house chlorophyll and carotenoids.

  • Pigments:

    • Chlorophyll a: Absorbs blue-violet and red light.

    • Chlorophyll b: Absorbs blue and orange light; transfers energy to chlorophyll a.

    • Carotenoids: Absorb blue-green light; provide photoprotection by dissipating excess energy.

  • Color perception arises from pigment molecules absorbing specific wavelengths and reflecting/transmitting others.

  • Example: Carotenoids protect chlorophyll from damage by absorbing excess light energy.

• Redox Reactions & Coenzymes:

  • OIL RIG: Oxidation Is Loss (of electrons or H+), Reduction Is Gain (of electrons or H+).

  • During cellular respiration: is oxidized to ; is reduced to .

  • Dehydrogenase enzymes remove two hydrogen atoms (oxidation), transferring electrons and a proton to NAD+ to form NADH (reduction).

  • Oxidation and reduction are coupled in redox reactions.

  • Example: NAD+ acts as an electron carrier, becoming NADH during glycolysis and the citric acid cycle.

Cellular Respiration

Cellular respiration is a multi-step process by which cells extract energy from glucose. It includes glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

• Glycolysis:

  • Occurs in the cytosol; breaks down glucose () into two 3-carbon pyruvate molecules.

  • Produces 2 net NADH and 2 net ATP through substrate-level phosphorylation.

  • Steps:

    1. Glucose is split into two 3-carbon groups; 2 ATP are used to phosphorylate intermediates.

    2. Each pyruvate is further phosphorylated and reduces NAD+ to NADH.

    3. 2 ADP are converted to 2 ATP; final products are two pyruvates ().

  • Example: Glycolysis is the first step in both aerobic and anaerobic respiration.

• Pyruvate Oxidation:

  • Each pyruvate enters the mitochondrial matrix, loses CO2, gains CoA, and reduces NAD+ to NADH, forming Acetyl CoA.

  • Acetyl CoA enters the citric acid cycle.

  • Example: Pyruvate oxidation links glycolysis to the citric acid cycle.

• Citric Acid (Krebs) Cycle:

  • Occurs in the mitochondrial matrix; runs once per pyruvate.

  • Acetyl CoA (2C) combines with oxaloacetate (OAA, 4C) to form citric acid (6C).

  • Citric acid undergoes a series of reactions, losing CO2, reducing NAD+ to NADH, converting ADP to ATP, and reducing FAD to FADH2.

  • Cycle regenerates OAA for the next round.

  • Example: The citric acid cycle produces high-energy electron carriers for the electron transport chain.

• Oxidative Phosphorylation & Chemiosmosis:

  • NADH and FADH2 deliver electrons to the electron transport chain (ETC) in the inner mitochondrial membrane.

  • Electrons move through protein carriers to oxygen, forming water.

  • Energy released pumps H+ into the intermembrane space, creating a concentration gradient.

  • H+ flows back through ATP synthase, driving the conversion of ADP to ATP (about 28 ATP per glucose).

  • Example: Chemiosmosis is the mechanism by which most cellular ATP is produced.

Photosynthesis

Photosynthesis is the process by which autotrophs convert solar energy into chemical energy. It consists of light reactions and the Calvin cycle, both occurring in the chloroplast.

• Light Reactions, Water Splitting, and Photosystems:

  • Occur on thylakoid membranes; convert solar energy to chemical energy.

  • Photons excite chlorophyll in Photosystem II; water is split into O2 and H+ ions.

  • H+ ions create a gradient, powering ATP synthase to convert ADP to ATP.

  • Electrons move through the ETC to Photosystem I, where they are re-excited and used to reduce NADP+ to NADPH.

  • Example: Light reactions produce ATP and NADPH for the Calvin cycle.

• The Calvin Cycle (Sugar Assembly):

  • Occurs in the stroma; uses ATP and NADPH to fix carbon and synthesize sugars.

  • Steps:

    1. Carbon Fixation: Rubisco attaches 3 CO2 to 3 RuBP (5C each), forming 6 molecules of 3C with phosphate.

    2. Reduction: 6 NADPH are oxidized to 6 NADP+, phosphates are removed, forming 6 high-energy G3P sugars; one G3P exits to build glucose.

    3. Regeneration of RuBP: ATP rearranges remaining G3P to regenerate 3 RuBP, completing the cycle.

  • Example: The Calvin cycle is responsible for synthesizing glucose and other organic molecules from CO2.

Summary Table: Cellular Respiration vs. Photosynthesis

This table compares the main features of cellular respiration and photosynthesis.

Key Equations

• Cellular Respiration:

• Photosynthesis:

Additional info: Academic context was added to clarify steps, terminology, and provide examples for each process. Table and equations were inferred and expanded for completeness.