2.4 Energy and Metabolism

Energy and metabolism are the powerhouses of life. They drive every biological process, from the tiniest cellular reactions to the grand cycles of ecosystems. Understanding these concepts is key to grasping how organisms function and interact with their environment.

This topic delves into the forms of energy, metabolic pathways, and crucial processes like cellular respiration and photosynthesis. It also explores how organisms balance energy input and output, highlighting the intricate dance of life at all levels.

Energy in Biological Systems

Forms of Energy

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• Energy is the capacity to do work or cause change in a system
  • Can be converted between different forms but cannot be created or destroyed (law of conservation of energy)
• Kinetic energy is the energy of motion
  • Examples include the movement of molecules in a cell or an organism moving through its environment
• Potential energy is stored energy that has the capacity to do work
  • Examples include the chemical energy stored in the bonds of molecules like glucose or ATP
• Thermal energy is the total kinetic energy of the molecules in a system, often measured as temperature
  • Heat is the transfer of thermal energy between systems
• Chemical energy is a form of potential energy stored in the bonds of chemical compounds
  • Released when compounds like glucose are broken down during cellular respiration
• Mechanical energy is the sum of an object's kinetic and potential energy
  • Involved in processes such as muscle contractions or the movement of fluids in the body
• Electromagnetic energy is a form of energy that travels in waves
  • Examples include visible light used in photosynthesis or the energy released during chemical reactions in the cell

Energy Transformations

• Energy can be converted from one form to another in biological systems
  • For example, light energy is converted to chemical energy during photosynthesis
  • Chemical energy in glucose is converted to kinetic and thermal energy during cellular respiration
• Energy transformations are governed by the laws of thermodynamics
  • First law: Energy cannot be created or destroyed, only converted from one form to another
  • Second law: In any energy transfer or transformation, some energy is lost as heat, increasing entropy
• Coupling of exergonic (energy-releasing) and endergonic (energy-requiring) reactions allows cells to harness energy for metabolic processes
  • ATP hydrolysis is often coupled with endergonic reactions to drive them forward
• Efficiency of energy transformations is important for organisms
  • Higher efficiency means more energy available for growth, reproduction, and other processes
  • Adaptations such as countercurrent exchange and insulation help minimize energy loss

Metabolism and Energy Transformations

Metabolic Pathways

• Metabolism refers to the sum of all chemical reactions involved in maintaining the living state of cells and organisms
  • Includes the breakdown of molecules to obtain energy (catabolism) and the synthesis of compounds needed by the cell (anabolism)
• Metabolic pathways are series of enzymatic reactions that convert an initial molecule into a final product through a series of metabolic intermediates
  • For example, the steps involved in breaking down glucose to release energy during cellular respiration
• Enzymes are biological catalysts that lower the activation energy of chemical reactions
  • Allow reactions to proceed more quickly and efficiently without being consumed in the process
• Coupled reactions involve the linking of an energetically unfavorable reaction (endergonic) with an energetically favorable reaction (exergonic)
  • Allows the overall process to be thermodynamically feasible
  • Example: coupling of ATP hydrolysis with the synthesis of cellular macromolecules

Regulation of Metabolism

• Regulation of metabolic pathways is crucial for maintaining homeostasis and responding to changing conditions
• Feedback regulation involves the use of products or intermediates of a metabolic pathway to regulate the activity of enzymes earlier in the pathway
  • Negative feedback inhibits the pathway when product accumulates, preventing overproduction
  • Positive feedback stimulates the pathway when more product is needed
• Allosteric regulation involves the binding of molecules at sites other than the active site of an enzyme
  • Can either enhance (allosteric activators) or inhibit (allosteric inhibitors) enzyme activity
• Compartmentalization of metabolic processes in organelles like mitochondria and chloroplasts allows for efficient regulation and control
  • Allows different conditions (pH, concentration of reactants) to be maintained in different parts of the cell
• Hormonal regulation integrates metabolic processes across different tissues and organs
  • For example, insulin promotes glucose uptake and storage, while glucagon stimulates glucose release from the liver

Cellular Respiration and Photosynthesis

Cellular Respiration

• Cellular respiration is the process by which cells break down organic molecules (usually glucose) to release energy in the form of ATP
  • Overall reaction: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy (ATP)
• Glycolysis is the first stage of cellular respiration, taking place in the cytoplasm
  • Breaks down glucose into two pyruvate molecules, releasing a small amount of ATP and NADH
• The citric acid cycle (Krebs cycle) takes place in the mitochondrial matrix
  • Oxidizes acetyl-CoA derived from pyruvate to release CO2, NADH, and FADH2
• Oxidative phosphorylation occurs in the inner mitochondrial membrane
  • Involves the electron transport chain and chemiosmosis to generate a proton gradient
  • Proton gradient powers ATP synthase to produce the majority of ATP in the cell

Photosynthesis

• Photosynthesis is the process by which plants, algae, and some bacteria use light energy to convert CO2 and H2O into organic compounds (usually glucose) and oxygen
  • Overall reaction: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
• The light-dependent reactions take place in the thylakoid membranes of chloroplasts
  • Light energy excites electrons in chlorophyll pigments
  • Electron transport chain and chemiosmosis generate ATP and NADPH
• The light-independent reactions (Calvin cycle) occur in the stroma of chloroplasts
  • Use ATP and NADPH from light-dependent reactions to fix CO2 into organic compounds
  • Produce glucose and other sugars as end products
• Photosynthesis is crucial for life on Earth
  • Provides energy for nearly all ecosystems and the oxygen needed for cellular respiration
  • Helps regulate atmospheric CO2 levels and mitigate climate change

Energy Input vs Output in Organisms

Energy Balance and Metabolic Rate

• Energy balance is the relationship between energy intake (through food or photosynthesis) and energy expenditure (through cellular processes, growth, and physical activity)
  • Positive energy balance (intake > expenditure) leads to energy storage and weight gain
  • Negative energy balance (expenditure > intake) leads to weight loss and can be unsustainable long-term
• Metabolic rate is the amount of energy an organism expends over a given time period
  • Influenced by factors such as body size, age, sex, physical activity, and environmental temperature
• Basal metabolic rate (BMR) is the minimum amount of energy required to maintain basic physiological functions at rest
  • Accounts for the majority of daily energy expenditure in most organisms
  • Influenced by factors such as body size, muscle mass, and thyroid hormone levels

Thermoregulation and Torpor

• Thermoregulation is the process by which organisms maintain a relatively constant internal body temperature
  • Can involve behavioral adaptations (seeking shade or sunlight) or physiological mechanisms (sweating or shivering)
• Endotherms (mammals and birds) maintain high, stable body temperatures through internal heat production
  • Requires high energy intake but allows for activity in a wide range of environmental conditions
• Ectotherms (reptiles, amphibians, fish) rely on external heat sources to regulate body temperature
  • Generally have lower metabolic rates and energy requirements than endotherms
• Torpor is a state of decreased physiological activity, usually marked by a reduced body temperature and metabolic rate
  • Allows animals to conserve energy during periods of food scarcity or harsh environmental conditions
• Hibernation is a type of torpor characterized by a significant drop in body temperature and metabolic rate
  • Lasts for an extended period (days to months) during winter
  • Examples include bears, ground squirrels, and some bats

Energy Transfer in Ecosystems

• Energy transfer between trophic levels in an ecosystem is inefficient
  • Only about 10% of the energy from one level is passed on to the next
  • Most energy is lost as heat or used for metabolic processes at each level
• Primary producers (autotrophs) convert light energy or chemical energy into organic compounds
  • Examples include plants, algae, and some bacteria
  • Form the base of most food webs and ecosystems
• Consumers (heterotrophs) obtain energy by eating other organisms
  • Primary consumers (herbivores) eat primary producers
  • Secondary consumers (carnivores) eat primary consumers
  • Tertiary consumers (higher-level carnivores) eat secondary consumers
• Decomposers (detritivores) break down dead organic matter, releasing nutrients back into the ecosystem
  • Examples include fungi, bacteria, and some invertebrates
  • Play a crucial role in nutrient cycling and energy flow in ecosystems