Basic Functions & Metabolism Notes

Humans share a set of basic functional properties or capabilities essential for life, including organization, responsiveness, movement, development, growth, and reproduction.
Organization refers to the hierarchical structure of the body, with different levels of organization and organ systems that work together to perform specialized functions.
Responsiveness (responsiveness to change) is the ability to adjust to internal and external changes, previously touched on with the endocrine and nervous systems.
Movement encompasses the movement of the whole body and movement within and of individual cells, including moving toward or away from stimuli.
Development describes how humans change over time, including large morphological changes and cellular changes such as differentiation and maturation.
Growth refers to an increase in size or mass.
Reproduction is the ability to produce new organisms.
These properties are foundational and will be revisited throughout the course as they relate to various organ systems and functions.

Metabolism is the human ability to harness and use energy from the environment for all bodily functions.
To understand energy in biology, it is important to introduce the first and second laws of thermodynamics.
First law of thermodynamics: energy is neither created nor destroyed; energy can be transformed but the total amount remains constant. In notation: $\Delta E = 0.$ In words: energy is conserved.
Second law of thermodynamics: systems tend toward disorder (entropy); energy conversions occur with an increase in overall disorder. In notation: $\Delta S \ge 0$ for spontaneous processes.
These principles explain why living systems must continuously input energy to maintain their organized state and resist decay into a more disordered state.
Even with high levels of physical activity, the majority of calories burned in a day come from maintaining basic life processes (basal metabolic needs) rather than just exercise.
The main energy currency used by cells is ATP (adenosine triphosphate).
Energy from food and stored macromolecules is converted through metabolism into ATP, which powers cellular functions.
Metabolism comprises all the chemical reactions occurring in the body at any given time; the set of these reactions forms metabolic pathways.

ATP is the molecule that stores and provides energy for cellular work.
Energy release occurs when ATP is hydrolyzed: $\mathrm{ATP + H<em>2O \rightarrow ADP + P</em>i + \text{energy}}$
Cells synthesize ATP from ADP and inorganic phosphate (P_i) using energy from nutrients, via various metabolic pathways and reactions.
The energy stored in ATP is used to drive endergonic (energy-requiring) processes and to power physiological activities like respiration, movement, neural signaling, and growth.

Metabolic pathways are sequences of biochemical reactions that transform substrates into products within cells.
Catabolic pathways: large molecules are broken down into smaller molecules, releasing energy in the process. This is the “breakdown” side of metabolism, often releasing energy captured to form ATP.
Anabolic pathways: small molecules are assembled into larger, more complex molecules, which requires energy input.
Abbreviations and mnemonic: catabolic often aligns with consuming energy to break bonds (breakdown) and anabolic with building up (energy requiring).
Together, catabolic and anabolic pathways comprise metabolism and enable the conversion of dietary macromolecules into usable energy and biomass.

The body extracts energy from food by breaking down macromolecules into smaller units that feed into metabolic pathways.
Through a network of reactions, these smaller units are eventually used to synthesize ATP, which powers cellular functions.
These reactions occur in all cells at all times and collectively determine an organism’s energy balance and functional capacity.
Metabolic activity enables essential processes such as breathing, movement, speech, thought, reproduction, and growth.

If energy input is insufficient, the body cannot maintain its organized state and will begin to decay; this underscores the importance of nutrition.
People who track calories or biometrics typically observe that most daily energy expenditure is devoted to maintaining life-sustaining functions, not only to physical activity.
Understanding metabolism and energy flow helps explain why caloric balance influences body weight, health, and performance.

Much of what metabolism does is aimed at maintaining homeostasis, the stable internal conditions necessary for cells and organs to function properly.
The next topic will explore homeostasis in more depth, including how metabolic processes are regulated to keep internal environments within narrow limits.

The concepts tie back to energy conservation (First Law) and the drive toward increased entropy (Second Law) in biological systems.
Energy management through ATP links nutrition to physiology and to everyday activities such as exercise, sleep, and stress responses.
The organization and responsiveness themes connect structure to function across organ systems and developmental stages.

Metabolism: all chemical reactions happening inside the body at any given time, enabling energy production and use.
Catabolic pathway: a metabolic pathway that breaks down large molecules into smaller ones, releasing energy.
Anabolic pathway: a metabolic pathway that builds larger molecules from smaller ones, consuming energy.
ATP (adenosine triphosphate): the primary energy currency of the cell, produced from nutrients and used to power cellular work.
Homeostasis: the maintenance of a relatively stable internal environment; a topic to be discussed in the next section.

A simple reminder: even during rest, the body uses energy to maintain vital functions (breathing, circulation, neural activity, thermoregulation).
Distinct from exercise, basal metabolic rate (BMR) reflects the energy required to sustain life in the absence of activity.
When discussing energy, distinguish energy content in calories from energy expenditure; both concepts relate to metabolism and homeostasis.