A group of epidermis cells taken from
the inside of an onion - stained with iodine
The nucleus can be clearly seen inside each of these cells.
The cell walls (which are actually double as two cells are touching) give each cell a clear outline.
The cytoplasm forms a very thin layer, only seen in some of the cells (near the nuclei).
What cannot be easily seen is the cell membrane - pushed against the cell wall - although it is peeling away in some places, leaving a small gap.
I usually introduce my classes to plant cell structure using these specimens
The main extra feature that plant cells have is a cell wall
on the outside. This is mostly made of cellulose, and it gives a tough covering on the outside of the cell membrane. It therefore gives plant cells a distinctive polygonal outline
The cell wall also prevents the plant cell from swelling or shrinking as much as an animal cell would if placed in different liquids.
Surprisingly, the cell wall does not usually prevent much passing through it. Water can usually pass through the cell wall, as it is actually made of a mesh of cellulose fibres running in different directions, rather like a basket.
Cellulose is actually a polysaccharide carbohydrate, but it is difficult to break down
. It forms most of the fibre in our diets.
Most plant cells also have a vacuole, filled with a liquid called cell sap, which is mostly water with some substances dissolved in it. It forms a relatively large space within the cytoplasm, which is usually pushed up against the cell wall in a thin layer. The cell membrane is held closely against the cell wall and often cannot be easily seen. The nucleus is also usually fairly to the edge of the cell. Although it is said that plant cells have permanent vacuoles, they may vary in size or disappear altogether if water is removed from the cell.
Some (but not all) plant cells - mostly those in the middle of leaves - have chloroplasts - green structures which contain the pigment chlorophyll. This absorbs light energy which is then used in photosynthesis to make sugars. These sugars are used by the plant as food for several purposes: to provide energy from respiration within plant cells and for growth.
A group of leaf cells from the pondweed Elodea
- not stained
The cell walls are quite noticeable in this image - and they do not have the green colouration.
The green dots are chloroplasts.
Notice the colourless space - the vacuole - in the middle of the cells.
The nucleus appears to be hidden (between the chloroplasts?).