The picture shows a thin slice cut vertically through the tip of a plant stem as seen under the microscope. The slice is so thin that light can pass through it. You can see that the stem, leaves and buds are made up of a large number of tiny, box-like structures. These are called cells. The magnification on this page is about x50
Sections of plant tissue can be cut by hand, using a razor blade. This should not be attempted without instruction and supervision. Soft tissues, particularly animal tissues, need to be embedded in wax or plastic and the sections cut by special apparatus. Extremely thin sections can be cut by this method.
All cells consist of cytoplasm, nucleus and a cell membrane. The next three slides explain the function of each of these
Under the ordinary microscope, the cytoplasm appears to be a semi-liquid gel. In some plant cells it can be seen streaming around the cell. Small particles and granules can be observed in the cytoplasm but, under higher magnification, a complex arrangement of microscopic tubules and vacuoles is revealed. These structures enable the chemical reactions, which make the cell alive, to take place in an orderly and controlled manner
The nucleus contains a substance called DNA. This substance controls what kind of a cell is produced, such as a liver cell, muscle cell or blood cell. Chemicals produced by the DNA escape from the nucleus and regulate all the chemical reactions in the cytoplasm. Cells without a nucleus cannot divide.
A cell needs to take up oxygen and food substances to keep it alive. The cell membrane allows these substances to pass into the cytoplasm but keeps unwanted, and possibly harmful, substances out.
These cells have been scraped from the inside of the cheek. They are not .stained but viewed by a special type of lighting. The magnification on the print-out is about x 950
The cell membrane is pressed up against the inside of the cell wall by the pressure of the vacuole and is difficult to see.. The cell wall is made of cellulose . It is permeable , that is, it lets water and dissolved substances pass in and out freely. The vacuole contains a solution of chemicals such as salts, sugars and, sometimes, coloured substances (pigments). This solution is called ‘cell sap’. Its outward pressure on the cell wall keeps the cell firm.
These cells are from a moss leaf. The magnification on this page is about x400. Chloroplasts contain a green pigment, chlorophyll , which absorbs sunlight and helps the plant to make its food
The diagram shows the cells cut open to allow the positions of the cytoplasm and vacuole to be seen
In the diagram, a small piece of the cell wall has been cut away to show the position of the cell membrane
There are exceptions to these general statements. Red blood cells, for example, do not have a nucleus. Animal cells often have vacuoles but these are small and scattered throughout the cytoplasm.
Very few tissues consist of a single cell type. There are usually nerve fibres and blood vessels supplying the tissue
Cutting sections <ul><li>The slice of tissue must be only 0.01-0.1 mm thick if cells are to be seen clearly </li></ul><ul><li>Thin slices of plant tissue can be made by hand using a sharp razor blade </li></ul><ul><li>The slices are called sections </li></ul><ul><li>Special apparatus and techniques are needed to make very thin sections </li></ul><ul><li>The next slide shows the difference between a transverse and a longitudinal section </li></ul>2
A group of cells from animal tissue cytoplasm cell membrane nucleus 4
Cytoplasm 5 The cytoplasm is a living, semi-liquid material Although it looks structureless it contains many structures which are not visible at this magnification All the processes which keep the cell alive, take place in the cytoplasm or nucleus cytoplasm
Nucleus 6 The nucleus is a specialised region of cytoplasm which regulates all the chemical changes which take place in the cytoplasm It sends chemical signals to the cytoplasm which start or stop the chemical processes going on in it The nucleus also controls cell division nucleus
Cell membrane 7 The cell membrane encloses the cytoplasm and stops it flowing out It also controls which substances can get into and out of the cell It is only about 0.00001mm thick If it is damaged, the cell will die cell membrane
Cells are 3-dimensional Cells seen in sections look flat, 2-dimensional. This is misleading and the picture shows how three plant cells might look if you could see them in 3 dimensions cell wall cytoplasm nucleus 11
Structure of a leaf cell cell wall cell membrane cytoplasm Longitudinal section Transverse section vacuole nucleus The diagram shows how a 3-dimensional leaf cell would appear in transverse and longitudinal section 12
Plant and animal cells <ul><li>All cells have c ytoplasm , a nucleus and a cell membrane . </li></ul><ul><li>Plant cells have also a cell wall , a central vacuole and in many cases, chloroplasts . </li></ul><ul><li>Animal cells do not have these structures </li></ul>13
Organs, cells and tissues The stomach is an organ , part of the digestive system The stomach wall contains muscle tissue The muscle tissue consists of muscle cells 15
Self test <ul><li>The following slides contain questions with alternative answers, only one of which is correct. Click on the button next to your answer to see if it is correct. </li></ul>16
Question 1 <ul><li>Which of these structures controls cell division? </li></ul><ul><li>Cytoplasm </li></ul><ul><li>Nucleus </li></ul><ul><li>Cell membrane </li></ul><ul><li>Cell wall </li></ul>17
Question 2 <ul><li>In a plant cell, which of these describes the vacuole? </li></ul><ul><li>A semi-liquid living material </li></ul><ul><li>A specialized region of cytoplasm </li></ul><ul><li>A region containing chloroplasts </li></ul><ul><li>A solution of salts and sugars </li></ul>18
Question 3 <ul><li>The cell structure which controls the entry or exit of substances is </li></ul><ul><li>the nucleus </li></ul><ul><li>The cell wall </li></ul><ul><li>The cell membrane </li></ul><ul><li>The cytoplasm </li></ul>19
Question 4 <ul><li>Plant cells do not have cell walls </li></ul><ul><li>Plant cells have no vacuoles </li></ul><ul><li>Plant cells have nuclei </li></ul><ul><li>All plant cells have chloroplasts </li></ul>Which one of these statements is correct? 20