Energy and Life

Forms and Conservation of Energy

Energy is fundamental to all biological processes. It exists in various forms and can be converted from one form to another, but it cannot be created or destroyed. This principle is known as the law of conservation of energy.

• Kinetic Energy: The energy of motion. Example: A child sliding down a slide.

• Potential Energy: Energy stored due to an object's position or structure.

• Chemical Energy: A form of potential energy stored in the bonds of molecules. Living organisms release this energy by breaking chemical bonds.

• Conservation of Energy: Energy can be converted, not created or destroyed. Example: Power plants convert chemical energy in coal to electrical energy.

• Heat and Entropy: Energy conversions are never perfectly efficient; some energy is lost as heat, increasing the system's entropy (disorder).

Energy Flow in Ecosystems

Energy enters ecosystems primarily as sunlight. Producers capture this energy through photosynthesis, and consumers obtain energy by eating other organisms. Both producers and consumers release energy from food via cellular respiration.

• Producers: Organisms (like plants and algae) that absorb sunlight and convert it to chemical energy.

• Photosynthesis: The process by which producers convert CO2 and H2O into sugars using sunlight. By-products include O2 and heat.

• Consumers: Organisms that obtain energy by eating producers or other consumers.

• Cellular Respiration: The process by which cells release energy from sugars, producing ATP.

Photosynthesis

Chloroplasts and the Process of Photosynthesis

Photosynthesis occurs in chloroplasts, using CO2, H2O, and sunlight to produce sugars and O2. Chloroplasts contain internal membranes (grana and thylakoids) where these reactions take place.

• Chloroplast Structure: Contains stacks of disks called grana; each disk is a thylakoid.

• Inputs: Water (absorbed by roots), CO2 (enters via stomata), and sunlight.

• Chlorophyll: The primary pigment absorbing light, located in thylakoid membranes. Absorbs blue/violet and orange/red light; reflects green/yellow.

• Other Pigments: Become visible in autumn as chlorophyll breaks down.

Stages of Photosynthesis

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle.

• Light Reactions: Capture sunlight and produce high-energy molecules (ATP and NADPH).

• Calvin Cycle: Uses ATP and NADPH to produce sugar from CO2.

Stage 1: Light Reactions

• Occur in the thylakoid membranes.

• Sunlight excites electrons in chlorophyll within photosystems.

• Electrons are supplied by splitting H2O, releasing O2 as a by-product.

• Excited electrons pass through the electron transport chain, producing ATP from ADP and NADPH from NADP+.

Stage 2: Calvin Cycle

• Occurs in the stroma (fluid inside chloroplast).

• Uses ATP and NADPH (from light reactions) and CO2 to synthesize sugars.

• Produces a 3-carbon sugar, glyceraldehyde 3-phosphate (G3P), which is used to make glucose.

Photosynthesis Overall Equation

The overall chemical equation for photosynthesis is:

Cellular Respiration

Overview and Importance

Cellular respiration is the process by which cells harvest energy from food molecules, primarily glucose, using oxygen. The energy is stored in ATP, which powers cellular functions.

• Aerobic Respiration: Requires O2; occurs in mitochondria.

• Inputs: Glucose and O2.

• Outputs: ATP, CO2, H2O, and heat.

• ATP: Adenosine triphosphate, the cell's energy shuttle. Formed from ADP and a phosphate group.

• Kilocalories: Energy in food is measured in kilocalories (kcal), often called Calories.

Stages of Cellular Respiration

Cellular respiration is divided into three stages: glycolysis, citric acid cycle, and electron transport chain.

1. Glycolysis: Occurs in the cytoplasm. Splits one glucose molecule into two pyruvic acid molecules. Produces a small amount of ATP and NADH.

2. Citric Acid Cycle: Occurs in the mitochondrial fluid. Completes glucose breakdown, releasing CO2. Produces ATP, NADH, and FADH2.

3. Electron Transport Chain: Occurs in the inner mitochondrial membrane. High-energy electrons from NADH and FADH2 move through proteins, synthesizing ATP from ADP and phosphate. Electrons combine with O2 to form H2O.

Of these stages, the electron transport chain produces the most ATP.

Cellular Respiration Overall Equation

Fermentation: Anaerobic Energy Harvesting

Fermentation is an anaerobic process (does not require oxygen) that allows cells to harvest energy from glucose. It produces much less ATP than cellular respiration.

• Lactic Acid Fermentation: Occurs in human muscle cells during overexertion. Produces lactic acid and a small amount of ATP.

• Lactic Acid Fermentation by Bacteria: Many microorganisms use fermentation for all their energy needs.

• Alcohol Fermentation: Yeast ferments sugars in anaerobic conditions, producing CO2 and ethyl alcohol.

• NAD+ Recycling: All types of fermentation recycle NAD+ to allow glycolysis to continue.

Metabolic Pathways and ATP

Cellular respiration is central to metabolism. All food molecules feed into cellular respiration, and the ATP produced powers nearly all life processes.

• Various Food Molecules: Carbohydrates, fats, and proteins can all be used to produce ATP.

• ATP Usage: Powers muscle contraction, active transport, biosynthesis, and other cellular activities.

Summary Table: Photosynthesis vs. Cellular Respiration

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The main energy currency of the cell.

• ADP (Adenosine Diphosphate): Formed when ATP loses a phosphate group.

• Chlorophyll: The pigment responsible for absorbing light in photosynthesis.

• Stomata: Pores on leaf surfaces for gas exchange.

• Grana: Stacks of thylakoids in chloroplasts.

• Thylakoid: Disk-shaped membrane structure in chloroplasts.

• Photosystem: Cluster of proteins and pigments in thylakoid membranes.

• Calvin Cycle: Series of reactions in the stroma that produce sugars.

• Glycolysis: First stage of cellular respiration; splits glucose.

• Citric Acid Cycle: Second stage; completes glucose breakdown.

• Electron Transport Chain: Final stage; produces most ATP.

• Fermentation: Anaerobic process for energy production.

• Entropy: Measure of disorder in a system.