2.4 Energy and Metabolism

Energy in Biological Systems

Energy and metabolism underpin every biological process, from muscle contraction to DNA replication. Understanding how energy is captured, converted, and used by cells gives you the foundation for nearly everything else in anatomy and physiology.

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Forms of Energy

Energy is the capacity to do work or cause change in a system. It can be converted between forms but cannot be created or destroyed (the law of conservation of energy).

• Kinetic energy is the energy of motion. At the cellular level, this includes the movement of molecules across membranes or the beating of cilia. At the organism level, it's things like walking or blood flowing through vessels.
• Potential energy is stored energy with the capacity to do work. Chemical energy in the bonds of glucose or ATP is the most relevant form for this course.
• Thermal energy is the total kinetic energy of all molecules in a system, measured as temperature. Heat is the transfer of thermal energy between systems.
• Chemical energy is a form of potential energy stored in chemical bonds. When compounds like glucose are broken down during cellular respiration, that stored energy is released and captured as ATP.
• Mechanical energy is the sum of kinetic and potential energy in a physical system. Muscle contractions and the pumping of the heart are examples.
• Electromagnetic energy travels in waves. Visible light, which drives photosynthesis, is the most biologically important example.

Energy Transformations

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: Every energy transfer or transformation increases the overall entropy (disorder) of the universe. Some energy is always lost as heat.

In biological systems, cells couple exergonic (energy-releasing) reactions with endergonic (energy-requiring) reactions. ATP hydrolysis is the classic example: the energy released by breaking ATP's terminal phosphate bond drives otherwise unfavorable reactions, like building proteins or transporting ions against their concentration gradient.

Efficiency matters. The more efficiently an organism converts energy, the more it can devote to growth, reproduction, and maintaining homeostasis. Adaptations like countercurrent heat exchange in blood vessels and insulating fat layers help minimize energy loss.

Metabolism and Energy Transformations

Metabolic Pathways

Metabolism refers to the sum of all chemical reactions in a cell or organism. It has two major branches:

• Catabolism breaks down molecules to release energy (e.g., breaking glucose into pyruvate).
• Anabolism uses energy to build complex molecules the cell needs (e.g., synthesizing proteins from amino acids).

A metabolic pathway is a series of enzyme-catalyzed reactions that convert a starting molecule into a final product through intermediate steps. Glycolysis, for instance, converts glucose into pyruvate through ten sequential enzymatic reactions.

Enzymes are biological catalysts that lower the activation energy of reactions, allowing them to proceed faster and more efficiently. Enzymes are not consumed in the reaction and can be reused.

Coupled reactions link an exergonic reaction to an endergonic one so the overall process is thermodynamically favorable. For example, the energy from ATP hydrolysis (exergonic, releases about $-30.5 \text{ kJ/mol}$) can be used to drive the synthesis of macromolecules (endergonic).

Regulation of Metabolism

Cells must tightly regulate metabolic pathways to maintain homeostasis and respond to changing conditions. Several mechanisms make this possible:

• Feedback regulation uses products or intermediates of a pathway to control enzyme activity earlier in that pathway.
  • Negative feedback inhibits the pathway when product accumulates, preventing overproduction. This is the most common type.
  • Positive feedback accelerates the pathway when more product is needed (less common, but important in processes like blood clotting).
• Allosteric regulation occurs when a molecule binds to a site other than the enzyme's active site, changing the enzyme's shape and activity.
  • Allosteric activators increase activity; allosteric inhibitors decrease it.
• Compartmentalization separates metabolic processes into specific organelles. Mitochondria maintain the conditions needed for oxidative phosphorylation, while chloroplasts house photosynthesis. This allows different pH levels, ion concentrations, and reactant pools to coexist in the same cell.
• Hormonal regulation coordinates metabolism across tissues and organs. Insulin promotes glucose uptake and glycogen storage, while glucagon stimulates glycogen breakdown and glucose release from the liver. These two hormones work in opposition to keep blood glucose within a narrow range.

Cellular Respiration and Photosynthesis

Cellular Respiration

Cellular respiration is the process by which cells break down organic molecules (usually glucose) to produce ATP. The overall reaction:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

It occurs in three main stages:

1. Glycolysis takes place in the cytoplasm. Glucose (6 carbons) is split into two molecules of pyruvate (3 carbons each). This yields a net gain of 2 ATP and 2 NADH. Glycolysis does not require oxygen.

2. The citric acid cycle (Krebs cycle) takes place in the mitochondrial matrix. Pyruvate is first converted to acetyl-CoA (releasing one $CO_2$), which then enters the cycle. Each turn of the cycle produces $CO_2$, NADH, $FADH_2$, and a small amount of ATP (as GTP). Two turns occur per glucose molecule.

3. Oxidative phosphorylation occurs at the inner mitochondrial membrane and has two components:

   • The electron transport chain (ETC) passes electrons from NADH and $FADH_2$ through a series of protein complexes, using the released energy to pump $H^+$ ions into the intermembrane space.
   • Chemiosmosis uses the resulting proton gradient to drive $H^+$ back through ATP synthase, which generates the bulk of ATP (approximately 30-32 ATP per glucose molecule total from all stages combined).

Photosynthesis

Photosynthesis is the process by which plants, algae, and certain bacteria convert light energy, $CO_2$, and $H_2O$ into glucose and oxygen. The overall reaction:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

It occurs in two stages:

1. Light-dependent reactions take place in the thylakoid membranes of chloroplasts.

   • Light energy excites electrons in chlorophyll pigments.
   • These electrons pass through an electron transport chain, generating ATP and NADPH.
   • Water is split (photolysis) to replace the lost electrons, releasing $O_2$ as a byproduct.

2. Light-independent reactions (Calvin cycle) occur in the stroma of chloroplasts.

   • ATP and NADPH from the light-dependent reactions power the fixation of $CO_2$ into three-carbon molecules (G3P).
   • G3P molecules are then used to build glucose and other organic compounds.

Photosynthesis provides the organic molecules and oxygen that nearly all ecosystems depend on. It also plays a role in regulating atmospheric $CO_2$ levels.

Energy Input vs. Output in Organisms

Energy Balance and Metabolic Rate

Energy balance is the relationship between energy intake and energy expenditure:

• Positive energy balance (intake > expenditure) leads to energy storage, often as glycogen or fat.
• Negative energy balance (expenditure > intake) forces the body to break down stored reserves, leading to weight loss. Prolonged negative balance can compromise physiological function.

Metabolic rate is the total energy an organism expends per unit time. It's influenced by body size, age, sex, physical activity level, and environmental temperature.

Basal metabolic rate (BMR) is the minimum energy needed to sustain basic physiological functions at rest (breathing, circulation, cell maintenance). BMR accounts for the majority of daily energy expenditure in most people and is influenced by body size, lean muscle mass, and thyroid hormone levels. More muscle mass means a higher BMR because muscle tissue is metabolically active even at rest.

Thermoregulation and Torpor

Thermoregulation is how organisms maintain a relatively stable internal body temperature despite changes in the environment.

• Endotherms (mammals and birds) generate heat internally through metabolic activity. This requires high energy intake but allows them to remain active across a wide range of environmental temperatures. Mechanisms include shivering (rapid muscle contractions that generate heat) and vasodilation/vasoconstriction to control heat loss through the skin.
• Ectotherms (reptiles, amphibians, most fish) rely on external heat sources to regulate body temperature. They generally have lower metabolic rates and lower energy requirements than endotherms, but their activity levels are more dependent on environmental conditions.

Torpor is a state of reduced physiological activity marked by decreased body temperature and metabolic rate. It allows animals to conserve energy when food is scarce or conditions are harsh.

• Hibernation is prolonged torpor lasting days to months, typically during winter. Body temperature and heart rate drop dramatically. Examples include ground squirrels and some bat species. (Bears enter a milder form sometimes called winter dormancy, where their temperature drops less than in true hibernators.)

Energy Transfer in Ecosystems

Energy flows through ecosystems but is lost at every step. Only about 10% of the energy at one trophic level is transferred to the next. The rest is lost as heat or used for the organism's own metabolic processes.

• Primary producers (autotrophs) form the base of food webs by converting light or chemical energy into organic compounds. Plants, algae, and photosynthetic bacteria are the most common examples.
• Consumers (heterotrophs) obtain energy by eating other organisms:
  • Primary consumers (herbivores) eat producers.
  • Secondary consumers (carnivores) eat primary consumers.
  • Tertiary consumers eat secondary consumers.
• Decomposers (fungi, bacteria, certain invertebrates) break down dead organic matter, recycling nutrients back into the ecosystem. Without decomposers, nutrients would remain locked in dead tissue and unavailable to producers.

This inefficiency of energy transfer explains why ecosystems support far fewer top predators than primary producers, and why food chains rarely extend beyond four or five trophic levels.