Each cell has a limiting boundary, the cell membrane, plasma membrane or plasmalemma. It is a living membrane, outermost in animal cells but internal to cell wall in plant cells.
It is flexible and can fold in or fold out. The plasma membrane is made of proteins and lipids.
- The plasma membrane encloses the cell contents.
- It provides cell shape in animal cells.
- It allows transport of certain substances into and out of the cell but not all substances. So, it is termed as selectively permeable.
Transport of Small Molecules
Transport of small molecules such as glucose, amino acids, water, mineral ions, etc. can be transported across the plasma membrane by any one of the following three methods:
Diffusion: Molecules of substances move from their region of higher concentration to the regions of lower concentration. This does not require energy. Example: absorption of glucose in a cell.
Osmosis: Movement of water molecules from the region of their higher concentration to the region of their lower concentration through a semipermeable membrane. There is no expenditure of energy in osmosis. This kind of movement is along concentration gradient.
Active Transport: When the direction of movement of a certain molecule is opposite to that of diffusion (from region of their lower concentration towards the region of their higher concentration), it would require an active effort by the cell for which energy is needed. This energy is provided by ATP (adenosine triphosphate). The active transport may also be through a carrier molecule.
Transport of Large Molecules (Bulk Transport)
During bulk transport the membrane changes its form and shape. It occurs in two ways:
- Endocytosis (taking the substance in)
- Exocytosis (passing the substance out)
Cell membrane regulates movement of substance into and out of the cell. If the cell membrane fails to function normally, the cell dies.
In bacteria and plant cells the outermost cell cover, present outside the plasma membrane is the cell wall. Bacterial cell wall is made up of peptidoglycan.
Structure of Plant Cell Wall
- Outermost non-living layer present in all plant cells.
- Secreted by the cell itself.
- In most plants, it is chiefly made up of cellulose but may also contain other chemical substances such as pectin and lignin.
- The substance constituting the cell wall is not simply homogeneous but it consists of fine threads or fibres called microfibrils.
- It may be thin (1 micron) and transparent as in the cells of onion peel. In some cases it is very thick as in the cells of wood.
Functions of Plant Cell Wall
- The cell wall protects the delicate inner parts of the cell.
- Being rigid, it gives shape to the cell.
- As it is rigid, it does not allow distension of the cell, thus leading to turgidity of the cell that is useful in many ways.
- It freely allows the passage of water and other chemicals into and out of the cells.
- There are breaks in the primary wall of the adjacent cells through which cytoplasm of one cell remains connected with the other. These cytoplasmic strands which connect one cell to the other one are known as plasmodesmata.
- Walls of two adjacent cells are firmly joined by a cementing material called middle lamella made of calcium pectinate.