Energy Usage by Living Things: ATP, Photosynthesis, and Respiration

Energy Usage by Living Things

Introduction

This section covers how living organisms obtain, use, and transform energy, focusing on ATP, photosynthesis, and cellular respiration. Understanding these processes is fundamental to biology, as they underpin metabolism and life itself.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

• Adenosine triphosphate (ATP) is the primary energy carrier in all living organisms.

• ATP consists of an adenine base, a ribose sugar, and three phosphate groups.

• The high-energy bonds between phosphate groups store potential energy.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Hydrolysis of ATP releases energy used for cellular work.

ATP/ADP Cycle

• Cells regenerate ATP from ADP and inorganic phosphate ($\text{P}_i$) using energy from food or sunlight.

• This cycle is continuous and essential for metabolism.

Enzymes and Energy Coupling

• Enzymes lower activation energy, allowing metabolic reactions to proceed efficiently.

• ATP hydrolysis is often coupled to endergonic (energy-requiring) reactions.

Photosynthesis

Overview of Photosynthesis

• Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy.

• Occurs in the chloroplasts of plant cells.

• General equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Produces glucose and oxygen from carbon dioxide and water.

Stages of Photosynthesis

• Light-dependent reactions: Occur in the thylakoid membranes; convert light energy to chemical energy (ATP and NADPH), releasing O2 as a byproduct.

• Light-independent reactions (Calvin Cycle): Occur in the stroma; use ATP and NADPH to fix CO2 into glucose.

Example: The light reactions of photosynthesis in prokaryotes and eukaryotes differ in their location and some details of electron transport.

Cellular Respiration

Overview of Cellular Respiration

• Cellular respiration is the process by which cells extract energy from glucose to produce ATP.

• Occurs in three main stages: glycolysis, Krebs cycle (citric acid cycle), and electron transport chain.

• General equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}$

Stages of Cellular Respiration

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into two molecules of pyruvate, producing a small amount of ATP and NADH.

• Krebs Cycle: Occurs in the mitochondrial matrix; processes pyruvate to produce CO2, ATP, NADH, and FADH2.

• Electron Transport Chain (ETC): Occurs in the inner mitochondrial membrane; uses NADH and FADH2 to generate a proton gradient, driving ATP synthesis via oxidative phosphorylation.

Fermentation

• Occurs when oxygen is not available.

• Allows glycolysis to continue by regenerating NAD+.

• Produces less ATP than aerobic respiration.

Example: Lactic acid fermentation in muscle cells during intense exercise.

Key Molecules: NAD+, NADH, FAD, FADH2

• NAD+ and FAD are electron carriers involved in redox reactions during respiration and photosynthesis.

• They shuttle electrons to the electron transport chain, enabling ATP production.

Summary Table: Comparison of Photosynthesis and Respiration

Process	Location	Reactants	Products	Energy Conversion
Photosynthesis	Chloroplasts (plants, algae)	CO2, H2O, light	Glucose, O2	Light energy to chemical energy
Cellular Respiration	Cytoplasm & Mitochondria	Glucose, O2	CO2, H2O, ATP	Chemical energy to ATP

Additional info:

• Labeling diagrams of mitochondria and chloroplasts is a common exam task. Be able to identify structures such as the matrix, cristae, thylakoid, and stroma.

• Understanding the flow of energy and electrons is crucial for mastering these topics.