The Plant Cell

• Plasma membrane
• Nucleus and nucleolus
• Mitochondria
• Ribosomes
• Endoplasmic reticulum
• Golgi apparatus
• Peroxisomes (the crystal in the electron micrograph is enclosed within a peroxisome)
• Microtubules

• centrioles
• intermediate filaments

• plastids
• a cell wall
• large vacuoles

The electron micrograph shows cells from a sunflower leaf. It was supplied through the courtesy of H. J. Arnott and Kenneth M. Smith.

Plastids

Chloroplasts are the most familiar plastids. They are usually disk-shaped and about 5-8 µm in diameter and 2-4 µm thick. A typical plant cell has 20-40 of them.

Chloroplasts are green because they contain chlorophylls — the pigments that harvest the light used in photosynthesis.

Chloroplasts are probably the descendants of cyanobacteria that took up residence in the ancestor of the plants.

Plant cells that are not engaged in photosynthesis also have plastids that serve other functions, such as

• storing starch (when they are called amyloplasts) [View]
• storing the carotenoids that give flowers and fruits their color (when they are called chromoplasts).

The Cell Wall

The rigid cell wall of plants is made of fibrils of cellulose embedded in a matrix of several other kinds of polymers such as pectin and lignin.

The linear nature of cellulose molecules and the many opportunities for side-to-side intermolecular hydrogen bonding provide just what one would want to build long, stiff fibrils.

Primary cell walls

The cell walls of parenchyma and meristems are uniform in thickness and are primary cell walls.

Although each cell appears encased within a box, in fact primary cell walls are perforated permitting plasmodesmata to connect adjacent cells.

Secondary cell walls

• sclerenchyma
• collenchyma
• xylem

Vacuoles

• storing foods (e.g., proteins in seeds)
• storing wastes
• storing malic acid in CAM plants
• storing various ions (e.g., calcium, sodium, iron) which, among other functions, helps to
• maintain turgor in the cell.

Plant cells avoid bursting in hypotonic surroundings by their strong cell walls. These allow the build-up of turgor within the cell. Loss of turgor causes wilting.

Plasmolysis

When a freshwater (or terrestrial) plant is placed in sea water, its cells quickly lose turgor and the plant wilts.

The photomicrograph shows plasmolyzed cells in the freshwater plant Elodea which has been placed in sea water. Note how the cell walls now show clearly.