Cell Metabolism Study Notes

Cell Metabolism: Synthesis and Degradation of Biological Molecules

Course Information

• Course: BSC2010

• Lecture Number: 12

• Instructor: Meiyan Jin

• Date: February 11, 2026

• Project Involvement: SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE

Learning Outcomes

Cellular Respiration and Photosynthesis

• Comparison of Oxidation/Reduction Reactions:

  • Analyze the oxidation and reduction reactions in cellular respiration in contrast with those in photosynthesis.

  • Discuss the relation of these reactions to the carbon pathway throughout each of these processes.

• Synthesis of High-Energy Molecules:

  • Explain the process by which high-energy molecules ATP and NADH are synthesized from lower-energy substrates ADP and NAD+.

• Stages of Aerobic Cellular Respiration:

  • Summarize the four stages of aerobic cellular respiration:

  1. Glycolysis

  2. Pyruvate Oxidation

  3. Citric Acid Cycle (Krebs Cycle)

  4. Oxidative Phosphorylation

  • Identify the reactants and products of each stage and elaborate on the physical locations where these stages occur.

• Generalized Citric Acid Cycle:

  • Draw and outline the citric acid (Krebs or TCA) cycle highlighting:

  • Points where electron carriers, CO2, and ATP are produced.

  • The molecule that is recycled at the end of the cycle.

Photosynthesis

• Role of Oxygen in Aerobic Respiration:

  • Discuss the critical role of oxygen as a terminal electron acceptor in aerobic respiration.

• Glucose Fermentation Pathways:

  • Compare and contrast the two primary pathways for glucose fermentation.

• Pathways of Photosynthesis:

  • Summarize the two pathways involved in photosynthesis along with their reactants and products and specify the physical locations.

• Role of Key Molecules in Photosynthesis:

  • Describe the functions of water (H2O), NADPH, ATP, and the enzyme Rubisco in the process of photosynthesis.

• Photosynthetic Pigments:

  • Detail the location and main functions of photosynthetic pigments in capturing light energy.

• Generalized Calvin Cycle:

  • Draw the Calvin cycle showing:

  • Points of electron carriers, glucose, and ATP production.

  • The recycling of molecules within the cycle.

Key Concepts

Introduction to Cellular Metabolism

1. ATP and Reduced Coenzymes as Energy Currency:

   • Understand the role of ATP and reduced coenzymes such as NADH in biosynthesis and energy transfer.

2. Energy Release During Carbohydrate Catabolism:

   • Discuss how carbohydrate catabolism in the presence of oxygen results in a large release of energy.

3. Interconnected Catabolic Pathways:

   • Explore how catabolic pathways for carbohydrates, lipids, and proteins are interconnected in cellular metabolism.

4. Anabolic Pathways Energy Demand:

   • Identify the large energy demands of anabolic pathways for biosynthesis.

5. Role of Light Energy:

   • Elaborate on how life on Earth is sustained through the conversion of light energy during photosynthesis, converting carbon dioxide (CO2) into carbohydrates.

Overview of Cellular Respiration and Photosynthesis

Cellular Respiration

• Chemical Reaction of Glucose Oxidation:

  • The balanced equation representing cellular respiration is:
    $C<em>6H</em>{12}O<em>6 + 6 O</em>2 <br>ightarrow 6 CO<em>2 + 6 H</em>2O + ext{chemical energy}$

Photosynthesis

• Chemical Reaction of Photosynthesis:

  • The balanced equation representing photosynthesis is:
    $6 CO<em>2 + 12 H</em>2O + ext{light energy} <br>ightarrow C<em>6H</em>{12}O<em>6 + 6 O</em>2 + 6 H_2O$

• Relationship Between Photosynthesis and Respiration:

  • Photosynthesis is considered the reverse of aerobic respiration.

Overview of Photosynthesis

Photosynthetic Organisms

• Autotrophs:

  • Organisms that can perform photosynthesis (e.g., plants, algae, cyanobacteria) and primarily utilize the energy captured for their growth and reproduction.

• Heterotrophs:

  • Organisms that cannot photosynthesize and rely on autotrophs for their energy needs (e.g., animals, fungi).

Pathways of Photosynthesis

1. Light Reactions:

   • Convert light energy into chemical energy, generating ATP and the reduced electron carrier NADPH.

2. Carbon-Fixation Reactions:

   • Utilize ATP and NADPH to synthesize carbohydrates from CO2.

   • Both pathways occur in the chloroplasts of eukaryotic cells.

Role of NADPH and NADH

Definitions of Electron Carriers

• NADH:

  • Nicotinamide adenine dinucleotide, a key electron carrier in cellular respiration.

• NADPH:

  • Nicotinamide adenine dinucleotide phosphate, used in photosynthesis.

  • The phosphate group does not significantly impact the redox chemistry compared to NADH.

Light Energy and Pigments

Excitation in Pigment Molecules

• Nature of Light:

  • Light is a form of electromagnetic radiation, propagating as a wave yet exhibiting particle-like behavior (photons).

  • Energy in radiation inversely proportional to its wavelength.

• Pigment Absorption:

  • Specific receptor molecules absorb photons leading to their excitation to a higher energy state.

Structure of Chlorophyll

• Pigments:

  • Molecules that absorb specific wavelengths within the visible spectrum.

• Chlorophyll:

  • Absorbs blue and red wavelengths while reflecting green light.

  • Main types in plants are chlorophyll a and b with accessory pigments like beta-carotene capturing additional energy.

Photosystems in Light Reactions

Photosystem Structure and Function

• Photosystems:

  • Comprise the thylakoid membrane in chloroplasts, involve multiple antenna systems capturing light energy and transforming it to chemical energy at the reaction center.

• Antenna Complexes:

  • Pigments are bound to membrane proteins in complex arrangements transferring excitation to the reaction center.

Energy Transfer Mechanism

• Excitation Process:

  • When absorbed by chlorophyll (Chl), photons elevate chlorophyll to an excited state (Chl*), transferring energy to neighboring pigments.

• Energy Transfer at Reaction Center:

  • Chl* passes energy to an electron, oxidizing itself (Chl+), and transferring the excited electron to an acceptor:
    $ext{Chl*} + ext{acceptor} <br>ightarrow ext{Chl}^+ + ext{acceptor}^-$

  • This reaction symbolizes the conversion of light energy into chemical energy.

Electron Transport Chain During Photosynthesis

Function of Photosystems

• Role of Photosystem I:

  • Absorbs light at 700 nm, reducing NADP+ to NADPH using excited electrons.

• Role of Photosystem II:

  • Absorbs light at 680 nm, initiates ATP production, and oxidizes water, releasing oxygen as a byproduct.

Electron Transport Steps

1. Photosystem II Activation:

   • Light energy activates chlorophyll (P680), exciting electrons which are lost and replaced by splitting water:
     $2 ext{P680} + ext{H}<em>2 ext{O} ightarrow 2 ext{P680}^+ + rac{1}{2} ext{O}</em>2 + 2 ext{H}^+$

2. Electron Transport Chain:

   • Electrons pass through a series of molecules embedded in thylakoid membrane generating ATP through chemiosmosis (photophosphorylation).

   • The proton gradient established from water splitting is used, similar to mitochondria.

3. Photosystem I Re-Excitation:

   • Chlorophyll (P700) absorbs light (700 nm), re-exciting electrons to be passed to NADP+:
     $ext{NADP}^+ + ext{H}^+ + 2 ext{e}^- <br>ightarrow ext{NADPH}$

Calvin Cycle Overview

General Principles

• Purpose of the Calvin Cycle:

  • Use ATP and NADPH to fix CO2 into carbohydrates in reduced form.

  • The cycle occurs in the stroma of chloroplasts, with each step catalyzed by specific enzymes.

Steps of the Calvin Cycle

1. Carbon Fixation:

   • CO2 combines with ribulose 1,5-bisphosphate (RuBP) to create 3-phosphoglyceric acid (3PG) catalyzed by Rubisco.

2. Reduction and Sugar Production:

   • 3PG is reduced to glyceraldehyde 3-phosphate (G3P) using ATP and NADPH, some of which is utilized to form glucose.

3. Regeneration of RuBP:

   • The carbon acceptor RuBP is regenerated from leftover G3P, requiring additional ATP.

Summary

• Understanding cellular respiration and photosynthesis reveals critical metabolic pathways vital to life.

• Key molecules and processes support energy transformation within cells, underscoring the relationship between autotrophs and heterotrophs in energy dynamics.