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The Plant Cell

Plant cells are eukaryotic and have many of the structures found in animal cells. Link to pages describing these. Plant cells differ from animal cells in lacking: and having:

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.

Link to page on chloroplast structure.

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

Link to Chlorophyll
Links to Photosynthesis

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

Link to discussion of the endosymbiotic origin of chloroplasts.

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

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.

Link to a picture showing how fibrils of cellulose are deposited in the cell wall.

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

The cells of have secondary deposits of lignified cellulose which provide mechanical strength to the tissue.

Vacuoles

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

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.

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.

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.

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28 February 2011