Biology AS 4.1 Plants

Plant structure

Typical Plant cell structure

Plant cell wall

Plant cells tend to be more regular in their shape than animal cells. A cell wall gives a plant strength and support. It is made up of insoluble celluose. This is long chains of glucose joined by glycosidic bonds.

Cellulose

Beta glucose in cellulose means hydrogen bonds can be foremd through a condensation reaction where water is lost. Cross linking occurs, holding nighbouring chains together. Hydrogen bonds and crosslinking make cellulose a very strong material. Cellulose molecules do not spiral or coil, they remain as long straight chains.Most animals cannot digest the 1,4 glycosidic bonds so cellulose is used as roughage. This is important for digestion.

Starch

In contrast to cellulose, starch forms compact globular molecules useful for storage as it has 1,4 and 1,6 glycosidic bonds between alpha molecules.Starch is an important source of energy in many animals.

Microfibrils

Microfibrils are formed by groups of 10 to 100000 cellulose molecules. Fibrils are laid down in layers held together by a matrix of hemicelluloses and other short chain carbohydrates which act as glue. Examples of sugars are mannose, xylose and arabinose. The combination of fibrils in the matrix makes a composite material. Cells are turgid unless the plant wilts from short water supply and the cells become flaccid. Individidual celluose chains do not spiral but the microfibrils do. The more vertical the spiral and the closer together it turns, the stronger the cell is vertically.

Suberin and Lignin

In normal circumstances the cell wall is freely permeable to anything that dissolves in water. However in cork tissue the cell wall has suberin and in wood the cell walls are lignified. this reduces permeability.

The Middle Lamella

the middle lamella is the first layer to form when a plant cell divides. It is made largely of pectin. Pectin acts like a glue. Negatively charged carboxyl groups combine with positive charced calcium ions to form calcium pectate. This binds to cellulose on either side. The cellulose microfibrils and matrix biuld up on either side of the middle lamella. To start, the cell is felxible, with fibrils all orientated in a similar direction. this is the primary cell wall. However as the plant ages a secondary cell wall may build up, with the fibrils laid densley at different agles to each other, making the composite material more rigid. Hemilcelluloses harden it further. Sometimes lignin is added to produce wood which is even more rigid. Long lignified cells with cellulose walls are called fibres.

Plasmodemata

Plasmodemata are special cytoplasmic bridges that allow intercellular exchanges. They are produces as cells divide and don't separate completely. Some threads of cytoplasm remain. The interconnected cytoplasm of the cells is known as the symplast.

Typical Plant Cell Organelles

Vacuole

A vacuole is any fluid filled space inside the cytoplasm surrounded by a membrane. They are permanent structures in plant cells. The surrounding membrane is called a tonoplast. Plant vacuoles are filled with cell sap, a solution of various substances in water…