Smallest Units of Life, their Structure

1. All livings things, except for the very smallest, are made up of many units called cells. Although they basically contain the same structures, cells are very varied in shape, size and function. There are many millions of cells in every human being, all too small to be seen individually without microscope. Sperm cells are particularly small, about five-hundredths of a millimeter long. The very largest cells are birds eggs, in which most of the bulk is a food store called the yolk.

2. Each cell contains a nucleus, which looks like a dot under an ordinary microscope and is embedded in a jelly-like mass called the cytoplasm. Using an electron microscope, which gives a greater magnification, various structures (organelles) can be seen in the cytoplasm. Each of these organelles does a special job.

3. The nucleus controls the shape, size and functions of the cell and it contains the hereditary material. Around the outside of the cell is a very thin membrane called the plasma membrane, which is made of protein and fat. It acts as a sieve, allowing certain chemicals to pass through, while at the same time keeping others out. Within the cells is a complex series of channels bounded by membranes, the endoplasmic reticulum, which acts as a sieve and as a means of transport for substances in the cytoplasm. Another membrane system, the golgi apparatus, secretes substances made by the cell.

4. Among the most important cell structures in the cytoplasm are mitochondria and ribosomes. The mitochondria are organelles in which the complicated processes of respiration take place; they release the energy that keeps the cells alive.

Questions:

1. What are all living things made up of?

2. How many cells are there in every human being?

3. What cells are the smallest and what cells are the largest?

4. How does a nucleus look like?

5. What does the nucleus control?

6. Where is the endoplasmic reticulum?

7. How does the endoplasmic reticulum act?

8. What are the most important cell structures in the cytoplasm?