Compartments That Were Once Cells
Mitochondria and plastids share an origin and a design: a double membrane, their own circular DNA, 70S ribosomes, and division by fission independent of the host.
A mitochondrion has an outer membrane and a heavily folded inner one. The folds, cristae, carry the electron transport chain, and folding is the point: it multiplies the area available for ATP synthesis within a fixed volume. Cells with high energy demand have both more mitochondria and more densely folded cristae, and heart muscle is the extreme case.
Plastids are a family. Chloroplasts photosynthesise, with an internal thylakoid membrane system stacked into grana, suspended in the stroma. Chromoplasts store the pigments that colour fruits and flowers. Amyloplasts store starch and, in roots, also sense gravity. All three interconvert, a tomato ripening from green to red is chloroplasts becoming chromoplasts.
Peroxisomes handle oxidative reactions producing hydrogen peroxide and immediately destroy it with catalase, one of the fastest enzymes known. In plant seeds, specialised glyoxysomes convert stored fat into sugar so a seedling can grow before it can photosynthesise.
Two structures make a plant cell recognisable. The central vacuole occupies up to 90 % of the cell volume, and it does far more than store: it maintains turgor pressure against the wall, which is what holds a non-woody plant upright. A wilting plant is one whose vacuoles have lost water.
The cell wall is outside the membrane, built of cellulose microfibrils in a matrix. The primary wall is thin and extensible, allowing growth; the secondary wall, deposited inside it once growth stops, is thicker and often lignified. Walls are perforated by plasmodesmata, channels connecting adjacent cytoplasms so that a plant tissue is partly one continuous compartment.
Common pitfall: describing the cell wall as simply "for protection". It also determines cell shape, limits how far the cell can expand, and bears the turgor pressure that supports the whole plant. A cell with a wall grows by loosening it, growth is a controlled failure of the wall, not an inflation against it.