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BiologyDifficulty 1-3

Cell

The basic structural and functional unit of all known living organisms. Every living thing is made of one or more cells, each bounded by a membrane and containing the machinery needed to grow, respond to its environment, and reproduce. Cells fall into two broad types: prokaryotic cells (such as bacteria), which lack a nucleus, and eukaryotic cells (such as those in plants, animals and fungi), which organise their DNA inside a membrane-bound nucleus.

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Energy Metabolism and the Secretory Pathway

How Mitochondria Generate ATP

Mitochondria generate most of a eukaryotic cell's ATP through oxidative phosphorylation. Electrons harvested from glucose breakdown pass along an electron transport chain in the inner mitochondrial membrane; the energy released pumps protons across that membrane, building an electrochemical gradient. ATP synthase then uses the flow of protons back down that gradient to phosphorylate ADP into ATP.

How a Secreted Protein Moves Through the Cell

A protein destined for secretion is translated by a ribosome bound to the rough endoplasmic reticulum, folded and initially modified inside the ER lumen, then transported by vesicle to the Golgi apparatus for further modification and sorting, and finally packaged into a vesicle that fuses with the plasma membrane to release its contents outside the cell.

Info: This ER-to-Golgi-to-membrane route is the standard eukaryotic secretory pathway; the same organelles that appear in the basic organelle list are the successive stations along it, not separate systems.

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  • DNA — Has part

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Sources and methodology

  • All living organisms are composed of one or more cells, the cell is the basic structural and functional unit of life, and all cells arise from pre-existing cells (cell theory).
    • Molecular Biology of the Cell, 6th edition
  • A eukaryotic cell's nucleus is a membrane-bound organelle that houses the cell's DNA, while a prokaryotic cell such as a bacterium has no nucleus and keeps its DNA free in the cytoplasm.
    • Molecular Biology of the Cell, 6th edition
  • Mitochondria are the organelles that generate most of a eukaryotic cell's usable chemical energy, producing ATP through oxidative phosphorylation.
    • Molecular Biology of the Cell, 6th edition
  • The cell (plasma) membrane is selectively permeable, regulating which substances can move into and out of the cell.
    • Molecular Biology of the Cell, 6th edition

Content status: published, last reviewed 30 September 2026.