A cell is a set of compartments with a selective border. This unit is about what each compartment does, why cells stay small, and how the membrane decides what gets in and out: the ideas behind every later unit on energy, signaling and genes.
Ribosomes make proteins in every cell. In eukaryotes the endomembrane system (ER, Golgi, lysosomes, vacuoles, vesicles, nuclear envelope, plasma membrane) builds, modifies and ships molecules. Mitochondria (folded inner membrane) make ATP; chloroplasts run photosynthesis; lysosomes digest; vacuoles store, and in plants keep cells firm.
Exchange happens across the surface, but demand comes from the volume. As a cell grows, its volume rises faster than its surface area, so the surface-area-to-volume ratio falls and exchange gets less efficient. That is why cells stay small, why some cells fold their membranes, and why small animals lose heat faster and run hotter metabolisms per gram.
The membrane is a fluid mosaic: a phospholipid bilayer with proteins, cholesterol, glycoproteins and glycolipids that can drift around. Its hydrophobic interior lets small nonpolar molecules (O₂, CO₂, N₂) through, lets small polar ones like water through only slowly, and blocks ions and large polar molecules, which need transport proteins. Cell walls add structure and protect against bursting.
Passive transport moves molecules down their concentration gradient with no energy input; facilitated diffusion is passive transport through channel or carrier proteins (ions, large polar molecules, water through aquaporins). Active transport uses energy and can move molecules up their gradient. Endocytosis and exocytosis move large amounts in vesicles and also use energy.
Water moves by osmosis from hypotonic to hypertonic: from lower solute concentration (higher water potential) to higher solute concentration (lower water potential). Animal cells swell or shrivel; plant cells are held firm by their walls when water comes in. Water potential Ψ = Ψp + Ψs, with solute potential Ψs = −iCRT.
Active transport uses metabolic energy, usually ATP, and membrane proteins to build and maintain electrochemical gradients. The Na⁺/K⁺ pump (an ATPase) pumps 3 Na⁺ out and 2 K⁺ in for each ATP, which helps maintain the membrane potential.
Internal membranes split eukaryotic cells into compartments, which keeps competing reactions apart and adds surface area for reactions. Mitochondria and chloroplasts evolved from free-living prokaryotes by endosymbiosis. Prokaryotes lack membrane-bound organelles but still have specialized regions.