Plant Anatomy and Physiology (Bellairs)

Plants have eukaryote cells. Compared to prokaryote cells, plant cells are larger and they have organelles. They have their DNA contained in a nucleus and photosynthesis occurs in chloroplasts. Like plant cells, animal cells are also eukaryote cells. Unlike animal cells, plant cells have a cell wall. The cell wall is made of cellulose but may be thickened and strengthened in some cells.

A eukaryotic plant cell differs considerably from a prokaryotic cell of a bacteria or archaea. These are much simpler and smaller. Their DNA is found in a single chromosome and is not bound by a membrane. Similarly photosynthetic cyanobacteria do not have chloroplasts but rather photosynthesis occurs within the general cavity of the cell.

The cell wall is initially deposited on the surface of the middle lamella. This primary cell wall occurs on the surface of all plant cells. It is substantially composed of cellulose molecules bundled together to form fibrils. The primary cell wall is the only cell wall present in some cells. In other cells a secondary cell wall is deposited inside the primary cell wall. This secondary cell wall may contain lignin. Lignin makes the cell wall rigid and stronger. A cell membrane lies immediately adjacent to the cell wall, on the interior surface, and surrounds the contents of the cell.

Immediately inside the cell wall there is a cell membrane surrounding the contents of the cell. It is made up of a phospholipid bilayer with proteins. The outer surface of the phospholipid layer is attracted by water (hydrophilic) whereas the tail of the phospholipid molecule is repelled by water (hydrophobic). The proteins may be embedded in the membrane or just attached to the surface. Some of the embedded protein molecules pass right through the membrane and are important for transport of substances through the membrane.

Cell Membrane functioning

The cell membrane is important for compartmentalizing different parts of the cell to allow metabolic functioning to occur and to control substances in the cell. It allows control of substances entering the cell and it allows the cell to compartmentalize waste into the vacuole. Oil droplets, vesicles from the golgi apparatus and vacuoles just have a single bilipid membrane around them. The nucleus, chloroplasts and mitochondria have a double bilipid membrane around them.