The Study of Life and the Chemistry of Life

This is Unit One of Biology, Part Two: what every living thing has in common. Part One was about method. This part is the first material the course commits to — a list of properties shared by everything alive, and then the structures that list is arranged into. One entry in the properties run is more easily misread than the rest. Order names structure, not activity. A thing can be highly ordered and doing nothing, and the property is about arrangement rather than about being busy. Then evolution, and the distinction the course insists on. Evolution is the process; natural selection is one mechanism by which it happens. The two are not synonyms, and the misconception entry between them corrects a third error — that organisms evolve because they need to. Variation is filtered, never summoned. The part closes on the levels of organization, a question the book deliberately leaves open about viruses, and two ways of grouping living things that are built from sequences rather than appearance. By the end you will be able to do five things. One: name the shared properties. Two: tell evolution from natural selection. Three: say what an adaptation is. Four: place the levels of organization. Five: say what the three domains rest on.

Seven properties, ordered so each hands to the next: structure, response, reproduction, the fit reproduction passes on, development, stability, and the energy that pays for it.

First property: structure.

Bio 0041 Order

Order is the highly organized structure living things are built from, atoms up through cells and tissues. A bacterium arranges its molecules no less than a frog does. Order describes structure, not activity.

Second: reacting to the world.

Bio 0043 Response to Stimuli

Organisms respond to stimuli, moving toward some and away from others. A Mimosa folds its leaves when touched; a houseplant bends toward a window. Toward and away are both responses.

Third: continuing the lineage.

Bio 0045 Reproduction

Organisms produce offspring, passing on genetic material that shapes the next generation; a single-celled organism duplicates its D N A and divides in two. This explains how traits pass on, not why.

Fourth: fit to the environment.

Bio 0047 Adaptation

An adaptation fits an organism to its environment, produced over generations by natural selection: a moth's tongue matched to one flower's depth. It fits present conditions, never a guaranteed future.

Fifth: from start to form.

Bio 0049 Growth and Development

Genetic instructions direct an organism from its start toward the form typical of its species. A kitten's genes aim it at its parents' shape and behavior, though no two kittens are identical.

Sixth: holding steady.

Bio 0051 Homeostasis

Homeostasis maintains a relatively stable internal environment despite change outside: a polar bear holds its temperature steady in freezing air. It keeps conditions within a narrow working range, not perfectly constant.

Seventh: paying for it.

Bio 0053 Energy Processing

Every organism captures or consumes energy to fuel its metabolic activity. A plant converts sunlight into chemical energy stored in sugar; an animal eats it instead. The source differs, not the requirement.

The eighth property, and the two entries after it. This course keeps evolution and natural selection separate: one names a change in a population, the other one mechanism producing it.

Eighth: change across generations.

Bio 0055 Evolution

In this course evolution names a change in a population's allele frequencies across generations: the process, not one mechanism driving it. The mix of gene variants can shift before any new trait appears.

The mechanism itself.

Bio 0056 Natural Selection

Individuals whose traits better fit their environment survive and reproduce at a higher rate, shifting allele frequencies over time. No single organism undergoes natural selection in its lifetime; only a reproducing population does.

Need, in the wrong place.

Bio 0057 "Organisms Evolve Because They Need To"

This claim reverses the mechanism: variation is filtered by survival and reproduction, not summoned by need. A bacterial population did not evolve resistance because it needed to; resistant individuals already present survived.

Seven entries to close: three build the hierarchy from atom to biosphere, one cuts across it at the cell, one marks the undecided place, and two sort life into lineages.

The ladder starts.

Bio 0060 Levels of Organization: Atom to Cell

Living structure builds upward from atoms through molecules, macromolecules and organelles to the cell. The cell is the smallest level here alive on its own; no organelle survives once removed from it.

The middle rungs.

Bio 0062 Levels of Organization: Tissue to Organism

Similar cells form tissues, tissues combine into organs, organs into organ systems, and organ systems into one organism. A single-celled organism skips every rung between cell and organism.

Beyond one organism.

Bio 0064 Levels of Organization: Population to Biosphere

Organisms of one species form a population, populations in an area a community, that community plus its surroundings an ecosystem, and ecosystems together the biosphere. A community holds living populations; an ecosystem adds soil and rain.

The cut at the cell.

Bio 0067 Prokaryote and Eukaryote

A prokaryote is a cell with no membrane-bound nucleus or organelles; a eukaryote has both. A bacterium's D N A floats free, an animal cell's sits inside a nucleus. No third category exists.

The open question.

Bio 0070 Is a Virus Alive?

Whether a virus counts as living is genuinely open, and this course gives both positions without a verdict: no cells and no reproduction outside a host, against the fact that it evolves. Both sides differ only over which facts define life.

Three lineages, by sequence.

Bio 0073 Three Domains of Life

Woese used gene sequences, not appearance, to sort life into Bacteria, Archaea and Eukarya. Archaea and Bacteria once looked alike; sequences show them as distinct from each other as from Eukarya. Placements can shift.

Drawing the relationships.

Bio 0076 Phylogenetic Tree

A phylogenetic tree diagrams evolutionary relationships, each branch point marking a common ancestor where one population diverged into two. Branch lengths, where shown, are distance or elapsed time, never a ranking of species.

Five things to carry out of Part Two. The shared properties of life include order, response to stimuli, reproduction, adaptation, growth and development, homeostasis and energy processing. Order is the one most often misread: it names structural arrangement, not activity, so something highly ordered may be doing nothing at all. Homeostasis names a working range rather than a fixed point. Evolution is the process; natural selection is one mechanism by which it occurs. Treating them as synonyms loses the fact that other mechanisms exist. And organisms do not evolve because they need to — variation arises first and is then filtered, never produced on demand. An adaptation is a trait that was favoured under past conditions and may still serve under present ones. It points backward and to the present, never forward: nothing is adapted in advance of a condition it has not met. The levels of organization run atom, molecule, organelle, cell, tissue, organ, organ system, organism, population, community, ecosystem, biosphere. The cell is the floor of life, and a single-celled organism occupies several rungs at once rather than skipping them. And the three domains are established from sequence data rather than from appearance, which is why they cut across older groupings. A phylogenetic tree shows relationship, not rank — no branch is higher than another. Part Three turns to the chemistry every one of these levels is built from.