Plant cells are eukaryotic and have many of the structures found in animal cells.
- Plasma membrane
- Nucleus and nucleolus
- Endoplasmic reticulum
- Golgi apparatus
- Peroxisomes (the crystal in the electron micrograph is enclosed within a peroxisome)
Plant cells differ from animal cells as they lack centioles and intermediate filament; they also do not have plastids and a cell wall and large vacuoles.
Fig 3.16.1 Sunflower leaf micrograph courtesy of H. J. Arnott and Kenneth M. Smith
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 leucoplasts) and 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 and 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: The cells of sclerenchyma, collenchyma and xylem have secondary deposits of lignified cellulose which provide mechanical strength to the tissue.
Vacuoles are enclosed by a single membrane. Young plant cells often contain many small vacuoles, but as the cells mature, these unite to form a large central vacuole. Vacuoles serve several functions such as:
- 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.
When a freshwater (or terrestrial) plant is placed in sea water, its cells quickly lose turgor and the plant wilts. This is because sea water is hypertonic to the cytoplasm. As water diffuses from the cytoplasm into the sea water, the cells shrink — drawing their plasma membrane away from the cell wall.
Figure 3.16.2: Plasmolyzed cell of freshwater plant Elodea