CELLS - GENERAL
Every cell, plant or animal, has a cell membrane around it. Inside the cell is a jelly-like substance called cytoplasm , in which are many small structures called organelles . The most obvious of these organelles is usually called the nucleus . The whole content of the cell is called protoplasm .
All plant cells are surrounded by a cell wall made of cellulose . Animal cells NEVER have cells walls made of cellulose . Cellulose belongs to a group of substances called polysaccharides . This substance forms fibres which give protection and support to the cell.
The cell wall is a non-living layer of cellulose fibres. It is freely (fully) permeable to aqueous solutions and maintains the shape of the plant cell.
All cells have a membrane surrounding the cell, called the cell or the plasma membrane. In a plant cell, it is very difficult to see because it is right against the cell wall. It is composed of a very thin layer of protein and fat .
The cell membrane (plasmalemma) is selectively permeable and involved in osmosis and active uptake of ions and molecules.
Cytoplasm is a clear jelly. It is nearly all water; about 70% is water in many cells. It contains many substances dissolved in it, especially proteins.
A vacuole is a space in a cell, surrounded by a membrane and containing a solution. Plant cells have very large vacuoles, and contain a solution of sugars and other substances called cell sap. Animal cells have much smaller vacuoles, which may contain food or water.
Endoplastic reticulum is a membrane network in the cytoplasm, which make fats and proteins out of smaller molecules in the cell. This substance is found in all cells.
The nuclear membrane is a double unit membrane continuous with the endoplastic reticulum.
Pores in the nuclear membrane allow exchanges of materials between nucleus and cytoplasm.
Choroplasts are NEVER found in animal cells, but most of the cells in the green parts of plants have them. They contain the green pigment called chlorophyll. Chlorophyll absorbs sunlight, and the energy of sunlight is then used for making food for the plant by photosynthesis .