Cells and microorganisms

Living systems contain cells, use energy in controlled chemical reactions, maintain internal conditions, respond to their surroundings and possess hereditary information. Populations of living organisms can reproduce and evolve. No single visible feature proves life: a crystal grows without cells, and a sterile animal remains alive. A dormant seed has living cells even though its metabolism is slow. Viruses depend on host cells for reproduction and lack independent cellular metabolism, so their status differs from that of cellular organisms.

Cell theory states that organisms consist of one or more cells, that the cell is the basic functional unit of life and that new cells arise from existing cells. Cells contain hereditary information copied and transmitted during division. A plasma membrane bounds each cell and separates its internal chemistry from the environment. This boundary is active in regulation rather than an impermeable wall.

Prokaryotic cells, including bacteria, have DNA without a membrane-bound nucleus. Their main chromosome is usually circular, and some also contain small plasmids. Eukaryotic cells have a nucleus containing linear chromosomes and membrane-bound organelles. Typical bacterial cells are smaller than typical eukaryotic cells, but size alone cannot establish identity. Both groups possess membranes, cytoplasm, nucleic acids, proteins and ribosomes. Ribosomes build proteins; they are not exclusive to eukaryotes.

Mitochondria support aerobic respiration in eukaryotic cells, while photosynthetic plant and algal cells contain chloroplasts. Fungi do not contain chloroplasts.

Before division, cellular DNA is copied. In bacterial binary fission the chromosome copies separate as the cell elongates, and the membrane and cell wall divide the cytoplasm. Represent the sequence as one cell → copied chromosomes → chromosome separation → two cells. In mitosis, replicated eukaryotic chromosomes condense, align, and their sister chromatids separate into two nuclei. Cytokinesis usually divides the cytoplasm afterwards. Represent this as DNA copying → mitosis → cytokinesis.

Both processes distribute hereditary information, but binary fission does not involve mitosis or nuclear division. DNA copying occurs before mitosis, not during chromosome separation.