Energy Harvesting and Photosynthesis

Chapter 07: How Cells Harvest Energy

This chapter covers the essential processes of cellular respiration, including glycolysis, the citric acid cycle, and the electron transport chain, leading to energy production in cells. It also briefly explores fermentation and the metabolism of other macromolecules.

Cellular Respiration

  • Definition: Cellular respiration is a metabolic process that converts glucose into energy that can be utilized by living organisms.

  • Importance of Stepwise Energy Release:

    • Energy is released in small, manageable steps to prevent energy wastage and to enable the efficient capture of energy in the form of ATP (adenosine triphosphate).

  • Metabolic Pathways:

    • Cellular respiration consists of four distinct pathways:

    • Glycolysis

    • Preparatory Reaction

    • Citric Acid Cycle (Krebs Cycle)

    • Electron Transport Chain (ETC) with Oxidative Phosphorylation (Chemiosmosis)

Aerobic Respiration Overview

  • Key Molecules Involved:

    • NADH: Electron carrier that transports electrons to the electron transport chain.

    • ATP: Primary energy carrier for cellular processes.

    • Pyruvate: Product of glycolysis, precursor for further processes.

    • Acetyl-CoA: Product of pyruvate oxidation, enters the citric acid cycle.

    • CO2: By-product released during the citric acid cycle and pyruvate oxidation.

    • Visual summary includes mitochondrial structures like the outer and inner membranes and the mitochondrial matrix.

Glycolysis

  • Function: Glycolysis is the initial step for the breakdown of glucose, utilized by almost all living organisms.

  • Location: Occurs in the cytoplasm of both prokaryotic and eukaryotic cells.

  • Process:

    • Glucose is converted into two molecules of pyruvate, generating a small amount of ATP directly and producing NADH as a by-product.

    • Key Reaction Steps:

    • Conversion involves multiple substrates including fructose diphosphate and ATP.

    • Overall conversion of glucose leads to the production of:

      • 2 NADH

      • 2 ATP (net gain)

      • 2 molecules of pyruvate

  • Diagram of Glycolysis:

    • Key intermediates: Glyceraldehyde 3-phosphate, addition of NAD+, and conversion of ADP to ATP.

Fermentation

  • Purpose: Provides energy under low-oxygen (anaerobic) conditions.

  • Types:

    • Lactic Acid Fermentation: Used by animals and certain bacteria.

    • Alcohol Fermentation: Produces ethanol and CO2 as by-products.

Preparatory Reaction

  • Location: Pyruvate is transported into mitochondria for further processing.

  • Process:

    • If oxygen is present, it is converted into acetyl-CoA, which enters the citric acid cycle.

    • By-products include CO2 and NADH.

The Citric Acid Cycle

  • Definition: A closed loop of chemical reactions that processes acetyl-CoA.

  • Products per Cycle Turn:

    • 2 molecules of CO2

    • 1 ATP (or equivalent)

    • Reduced NADH and FADH2

  • Requirement for Glucose Breakdown: Two turns of the cycle are necessary for each molecule of glucose metabolized.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

  • Function: Major ATP production occurs through the electron transport chain where electrons are passed along a series of proteins, ultimately reducing oxygen to water.

    • Chemiosmosis drives ATP synthesis via ATP synthase, thanks to a proton gradient created across the inner mitochondrial membrane.

  • Process Highlights:

    • High energy electrons (from NADH and FADH2) are transferred through carrier molecules leading to ATP formation.

ATP Synthase Structure and Function

  • Made up of a rotor and a catalytic head.

  • Facilitates ATP synthesis from ADP and inorganic phosphate (Pi) through the movement of protons from the intermembrane space back into the mitochondrial matrix.

Catabolism of Macromolecules

  • General Overview: Metabolism connects the breakdown and synthesis of carbohydrates, proteins, and lipids.

  • Proteins:

    • Amino acids undergo deamination to remove the amino group, converting to intermediates entering glycolysis or the citric acid cycle (e.g., alanine → pyruvate).

  • Fats:

    • Broken down into fatty acids and glycerol with fatty acids converted to acetyl groups through β-oxidation.

    • Respiartion of a 6-carbon fatty acid yields approximately 20% more energy than that of a 6 carbon glucose molecule.

Chapter 08: Photosynthesis

Photosynthesis is the process by which solar energy is converted into chemical energy, producing carbohydrates.

Photosynthetic Basics

  • Definition: Photosynthesis converts solar energy into chemical energy stored in carbohydrates.

  • Producers:

    • Autotrophs: Organisms that produce their own food (e.g., plants).

    • Heterotrophs: Organisms that consume other organisms for energy.

  • Output: Oxygen and sugars are produced using sunlight, carbon dioxide, and water.

Photosynthetic Reactions

  • Light-Dependent Reactions: Capture sunlight, producing ATP and NADPH.

    • Takes place in the thylakoid membranes of chloroplasts.

  • Calvin Cycle (Light-Independent Reactions): Utilizes ATP and NADPH to synthesize organic molecules from CO2.

    • Occurs in the stroma of chloroplasts.

Historical Development of Photosynthesis Understanding

  • Key Figures:

    • Jan Baptista van Helmont: Tested that plant mass comes from water, not soil.

    • Joseph Priestly: Discovered plants release oxygen into the air.

    • Jan Ingenhousz: Proposed sunlight splits CO2 into carbon and oxygen.

Chloroplast Structure

  • Composed of both outer and inner membranes, stroma, and thylakoids.

  • Pigments:

    • Molecules that absorb specific wavelengths of light, key components in the process of photosynthesis.

    • Chlorophyll a: Main pigment, absorbs red and blue light.

    • Chlorophyll b: Accessory pigment that offsets wavelengths chlorophyll a does not absorb.

    • Carotenoids: Absorb a broad spectrum of light, providing protection and antioxidant functions.

Light-Dependent Reactions

  • Energy Conversion: Light energy is converted into chemical energy through absorption by pigments.

  • Processes Involved:

    • Noncyclic Electron Pathway: Generates ATP and NADPH.

    • Cyclic Electron Pathway: Produces additional ATP.

    • Key events include photolysis of water, creation of an electrochemical gradient, and ATP production through chemiosmosis.

The Calvin Cycle

  • Process Overview: Fixation of CO2 to build carbohydrate molecules occurs in three stages:

    • Carbon Fixation: RuBP and CO2 combine to form PGA.

    • Reduction: PGA is converted to G3P using ATP and NADPH.

    • Regeneration of RuBP: G3P is used to regenerate RuBP to continue the cycle.

  • Energy Requirements: Requires 12 ATP and 12 NADPH for reduction, and 6 ATP for regeneration in six cycles to yield one glucose molecule.

Outputs of the Calvin Cycle

  • G3P Production: G3P serves as a precursor for glucose and other carbohydrates.

  • Conversion: Used in the formation of sucrose (transport sugar) and glycogen (storage form of glucose).

Review Questions

  • Which are the two basic types of metabolic pathways? Energy-producing vs energy-consuming.

  • Define open vs closed systems in thermodynamics.

  • Relate the First and Second Laws of Thermodynamics to biological systems.

  • What are the two main types of energy?

  • Discuss major pathways of cellular respiration and their products.

  • What are the two types of fermentation?

  • How is light wavelength related to energy?

  • Describe steps of light-dependent reactions and the Calvin cycle.