Table of Contents
Metabolism and energy conversion are the core processes that keep organisms alive, growing, and able to respond to their environment. This chapter introduces how cells handle matter and energy in a coordinated way. Later chapters in this section will examine specific pathways and mechanisms (like ATP, enzymes, photosynthesis, and cellular respiration). Here, the focus is on the overarching ideas that connect them.
1. What Metabolism Is
All living organisms constantly exchange matter and energy with their surroundings. The entire set of chemical reactions in a cell or organism is called its metabolism.
Two broad directions can be distinguished:
- Anabolism (constructive metabolism)
Building up larger, more complex molecules from smaller ones.
Examples: synthesizing proteins from amino acids, building cell walls, storing energy as fats or starch. - Catabolism (degradative metabolism)
Breaking down complex molecules into smaller ones.
Examples: digesting carbohydrates to sugars, oxidizing glucose to carbon dioxide and water, mobilizing fat stores.
These processes:
- Are tightly interconnected (products of one are substrates for the other),
- Must be regulated so that supply and demand are balanced,
- Always involve energy transformations.
Metabolism is not a random collection of reactions; it is organized into pathways and networks that allow cells to use energy efficiently and adaptively.
2. Energy in Biology: Basic Concepts
Life depends on the ability to capture, transform, and use energy. From a physical perspective, organisms obey the same laws of thermodynamics as nonliving systems, but they use them in highly organized, regulated ways.
Key distinctions:
- Energy: the capacity to do work or cause change.
- Forms of energy relevant to cells:
- Chemical energy (in bonds of molecules like glucose, ATP),
- Electrical energy (membrane potentials),
- Mechanical energy (muscle contraction, movement of cilia and flagella),
- Light energy (captured by photosynthetic pigments),
- Thermal energy (heat).
Living organisms cannot create or destroy energy, but they transform it. For example:
- Plants transform light energy into chemical energy.
- Animals transform chemical energy in food into movement, heat, and electrical signals in nerves.
A central challenge for organisms: make enough “useful” energy available at the right time and place, and limit losses as unusable heat.
3. Open Systems Far from Equilibrium
A key feature that distinguishes living systems from most nonliving objects is how they maintain their internal order:
- Organisms are open systems:
- They take in energy and matter from the environment,
- Transform them,
- Release energy (often as heat) and waste products.
- Organisms maintain a highly ordered internal state (low entropy locally) by:
- Constantly replacing components,
- Repairing damage,
- Maintaining concentration gradients and membrane potentials.
- To maintain this order, organisms must remain far from thermodynamic equilibrium:
- If energy and matter exchange stop, metabolic processes cease,
- The system moves toward equilibrium and order breaks down (death).
Thus, continuous metabolism is required simply to keep structure and function intact, not just to grow or move.
4. Sources of Energy and Matter
Different organisms have evolved various strategies to obtain energy and carbon. The later chapters on anabolism and catabolism will detail major examples, but the general categories are important here.
4.1 Energy Sources
Organisms can be classified by where they get their primary energy input:
- Phototrophs:
- Use light as the energy source.
- Example: plants, algae, many bacteria.
- Capture light via pigments, use this energy to produce high-energy molecules.
- Chemotrophs:
- Use chemical energy stored in molecules.
- Chemoorganotrophs: use organic molecules (e.g., glucose, fatty acids) as energy sources — typical for animals, fungi, many bacteria.
- Chemolithotrophs: use inorganic molecules (e.g., hydrogen sulfide, ammonia, ferrous iron) — typical in certain bacteria and archaea.
4.2 Carbon Sources
Organisms also differ in where they obtain carbon for building biomass:
- Autotrophs:
- Use inorganic carbon, mainly carbon dioxide ($\mathrm{CO_2}$).
- Convert $\mathrm{CO_2}$ into organic molecules using energy from light (photoautotrophs) or inorganic chemicals (chemoautotrophs).
- Heterotrophs:
- Use organic carbon from other organisms.
- Depend on autotrophs (directly or indirectly) as ultimate carbon suppliers.
These categories combine. For example:
- Plants: photoautotrophs (light + $\mathrm{CO_2}$),
- Many bacteria at hydrothermal vents: chemoautotrophs,
- Animals: chemoheterotrophs (organic molecules for energy and carbon).
This classification underscores a fundamental ecological principle: autotrophs provide the organic matter and energy on which heterotrophs depend.
5. Oxidation, Reduction, and Energy Release
Most energy transformations in metabolism involve redox reactions:
- Oxidation: loss of electrons (often associated with loss of hydrogen or gain of oxygen).
- Reduction: gain of electrons (often associated with gain of hydrogen or loss of oxygen).
In biological systems, redox reactions are typically linked to electron carrier molecules (such as NAD⁺/NADH, FAD/FADH₂), which will be discussed in detail later. Important points here:
- When an organic molecule is oxidized (for example, glucose during cellular respiration), energy is released.
- This energy is not allowed to dissipate as heat all at once; it is captured stepwise in forms the cell can use (like ATP and reduced electron carriers).
- Conversely, in anabolism (e.g., synthesizing glucose in photosynthesis), energy must be input to reduce $\mathrm{CO_2}$ to organic carbon.
Thus, metabolism can be viewed as a guided flow of electrons from high-energy donors to lower-energy acceptors, with the cell capturing part of the energy released along the way.
6. Energy Coupling and ATP as a Central Concept
Cells must repeatedly perform energy-requiring (endergonic) reactions: synthesis, active transport, movement. These do not proceed spontaneously. To drive them, cells couple them to energy-releasing (exergonic) reactions.
The detailed treatment of ATP comes later, but at the conceptual level:
- Energy from catabolic reactions (e.g., oxidizing nutrients) is used to produce a small, versatile “energy currency” molecule.
- That currency is then spent to power anabolic and other energy-requiring processes.
- This energy coupling ensures that energy released from breakdown reactions is captured, stored briefly in a usable form, and directed into exactly those processes the cell needs at that time.
Without such coupling, catabolism would simply release heat, and anabolism would not proceed.
7. Metabolic Organization and Regulation
Metabolic reactions are arranged into pathways and networks, not isolated steps. Even without going into specific pathways, several general features are important.
7.1 Pathways and Compartments
- Metabolic pathways consist of a series of enzyme-catalyzed steps.
- Each step produces an intermediate that serves as the substrate for the next.
- In eukaryotic cells, many pathways are compartmentalized:
- Some reactions occur in the cytosol, others in mitochondria, chloroplasts, peroxisomes, etc.
- Compartmentalization allows conditions (pH, concentrations, redox state) to be controlled locally and pathways to be separated or linked as needed.
7.2 Irreversible Steps and Directionality
- Some steps in metabolic pathways are effectively irreversible under cellular conditions (strongly exergonic).
- These steps often act as control points: turning them up or down affects the flow through the entire pathway.
- Directionality of pathways (e.g., net breakdown vs. net synthesis) often depends on:
- Which enzymes are present or active,
- How energy carriers (like ATP, NADH) are used.
7.3 Regulation and Homeostasis
To maintain internal stability (homeostasis), cells must adjust metabolism continuously:
- Short-term regulation:
- Adjusting enzyme activities (e.g., via allosteric regulation, reversible phosphorylation).
- Changing transport of substrates across membranes.
- Long-term regulation:
- Altering which enzymes are synthesized and in what amounts (gene expression).
- Remodeling tissues (e.g., building more mitochondria with training).
Regulation allows organisms to:
- Switch between energy sources,
- Decide whether to store or use excess nutrients,
- Adapt to stress (heat, starvation, toxins),
- Coordinate metabolism among tissues and organs (in multicellular organisms).
8. Metabolism and the Whole Organism
While metabolic reactions occur at the cellular level, their consequences are evident at the scale of the whole organism and even ecosystems.
8.1 Metabolic Rate and Energy Needs
- The metabolic rate is the overall rate of energy use by an organism.
- It depends on:
- Body size and composition,
- Activity level,
- Temperature (especially in ectotherms),
- Life stage (growth, reproduction), among others.
- Organisms balance:
- Energy intake (food, light),
- Energy expenditure (maintenance, activity, thermoregulation, growth, reproduction),
- Energy storage (fat, starch, glycogen).
Later subsections will cover how metabolic rate is measured and interpreted.
8.2 Metabolism, Ecology, and Evolution
Metabolism shapes how organisms fit into ecosystems:
- Producers (mainly autotrophs) convert inorganic substances into organic matter, forming the base of food webs.
- Consumers and decomposers rely on this organic matter, transforming and returning elements like carbon and nitrogen to the environment.
- Metabolic strategies (e.g., endothermy vs. ectothermy, anaerobic vs. aerobic metabolism) influence:
- Where organisms can live,
- How active they can be,
- How quickly they grow and reproduce.
Over evolutionary time, innovations in metabolism (e.g., oxygenic photosynthesis, efficient aerobic respiration) have reshaped Earth’s atmosphere and biosphere.
This chapter has outlined how metabolism and energy conversion provide the fundamental framework for all life processes: the organized and regulated flow of matter and energy that keeps organisms far from equilibrium. The following chapters will delve into specific pathways, molecular mechanisms, and quantitative aspects of how this is achieved in different organisms and conditions.