- THE CELL
All living plants and animals are made up of a number of very small cells which are visible only under a microscope. The structure, function and reproduction of these cells vary considerably. The study of cells is called cytology.
Earlier in the Botany Course and again in the Animal Structure and Function Course, you have been told the different parts of a typical plant and animal cell and about some of the specialised cells that occur, particularly in mammals. In order to refresh your memory some examples of different cells are given below. It is important to remember that all young cells are usually more or less the same and the specialised cells become more complicated and specialised as they grow. The Figures on page 3 will help you to understand your revision.
|Lignin: a complex organic polymer deposited in the cell walls of many plants, making them rigid and woody.|
Some organisms consist of a single cell, examples being Bacteria, Viruses and Amoeba. Look at the first Figure on the next page, which is a typical plant cell, and let us do a little more revision. The outer layer of the cell is made up of a carbohydrate called Cellulose. Inside the cell wall is a layer of Cytoplasm, which is a semi-permeable membrane which allows water to flow into and out of the cell which happens in the process called Osmosis. Inside this layer of cytoplasm is a most important organ, a dense mass of protein called the Nucleus, which controls the activity of the cell, its growth and reproduction. The middle of the cell is called the vacuole and is filled with a sugary liquid called the cell sap. The cavity is filled with water by a process of osmosis and because plants do not have a rigid skeleton, they stand upright only because the cells are filled with liquid and in the condition known as turgid. If, for any reason, water flows out of the cell, the plant loses its rigidity, becomes limp and may collapse altogether. In this condition it is known as
As you know from the Animal Structure and Function and the Feeds and Feeding Courses, the ruminant mammal (cows, sheep, and goats) obtains both its Protein and Carbohydrate from plant cells. The cellulose in many cells is replaced by a much harder carbohydrate known as lignin, being actually wood. Ruminants can digest cellulose, but they cannot digest lignin, and this, together with the lack of protein, is why the veld offers such a poor feed for cattle during the Winter.
Figure 1: Typical plant cell
Figures 2 and 3: Mammary cells (left) and Motor nerve cell (right)
Figures 4 and 5: White blood cells (left) and Red blood cells (right)
Animal cells differ from Plant cells because the outer membrane of the cell is thin and difficult to see under the microscope. The cell is usually filled with cytoplasm and there may be vacuoles present if the cell is excreting waste products or producing hormones, enzymes, etc. The nucleus is large and easily seen. Simple animals made of single cells or a few cells have non-specialised cells within their
|Excreting: separate and expel as waste.|
bodies which can take in food, excrete waste, be sensitive, and reproduce. As animals become more complex and contain more and more cells, these cells lose the ability to perform basic tasks like excretion, sensitivity and reproduction. Special units of cells gather together to perform a function for the body such as excretory cells being massed into a unit called the kidney, and sensory cells form units called the eyes and ears. The ability to reproduce is taken over by specialised cells in the
Gonads (testes and ovaries), but there are many centres in the rest of the body where cells divide for growth and the replacement of worn-out tissue. For example, the bone marrow produces replacement red blood cells which deteriorate after about 3 months and have to be replaced.
- CELL DIVISION
It is important to understand the difference between mitosis and meiosis. This has already been explained earlier in the course, but is worth going over again. Both mitosis and meiosis take place in the nucleus of the cell, and are concerned with the division of the nucleus. In the case of mitosis, the nucleus and its cell divide into two halves which then grow and form two new cells, and the process is one of simple growth. Plants grow from small seedlings into mature plants by mitosis, dividing the cells usually at the root tips and the ends of the shoots – the apical meristem. Animals also develop from fertilised cells through the stage of the foetus and the young animal into the mature animal by mitosis. The process of the division of a cell nucleus by mitosis is shown in Figure 2 on the next page.
Figure 2: Mitosis
The nucleus of the cell not only controls the growth and activities of that cell, but also contains the “blueprint” or design for that cell and its functions. This “blueprint” is carried on the chromosomes of the nucleus. These consist of long threads of protein called chromatin woven into a dense mass inside the nucleus. If the chromosomes are examined under a very powerful microscope, it is possible to see that they are made up of sections called genes, and each gene controls some characteristic of the living plant or animal to which it belongs. In cattle, things like coat color and whether or not the animal has horns are controlled by the animals’ genes. Examples in man are
height and color of the hair and eyes. Each nucleus contains hundreds or thousands of genes, but the number of chromosomes is the same for all members of the same species, but varies from one species to another. Cattle have 60 chromosomes in each cell nucleus; chickens 78, sheep 54, pigs 38, goats 60, horses 64 and man 46.
At the beginning of the dividing of a cell by meiosis, the threads of Chromatin in the nucleus become thicker and divide up into individual Chromosomes, which then split up into separate parts, move to opposite ends of the nucleus and the nucleus and cell divide into two – See Figure 3 on the next page.
Figure 3: Meiosis
The important thing to realise is that the two cells resulting from this cell division contain only half the number of chromosomes of the original cell. The number in the original cell has been halved when the pairs of chromosomes separated and moved to opposite ends of the nucleus. These cells containing only half the correct number of chromosomes, are called gametes and in animals, are produced in the sex organs or Gonads, i.e. the testes or ovaries. Another important point to remember is that, while each of the two cells at the end of Meiosis contains half the Chromosomes of the original cell, the chromosomes in each cell are an exact replica of those in the other cell, because during Meiosis, the chromosomes have joined up into matching pairs and then separated. This is seen quite clearly from the diagram. So we have two new cells made from the original cell, each containing half the chromosomes of the original cell, but each half matching the other.
The reason for this is quite simple really, and can be easily explained when fertilisation takes place, for example in cattle. The sperm from the bull joins with the egg, or ovum, in the cow. The sperm and ovum are Gametes, each containing half the number of chromosomes of the parent cells, so that when they join together the number of chromosomes in the fertilized egg cell is brought back up to the normal number. This is shown in the diagram on the following page.
Two more terms which you should be familiar with are:
A Nucleus or cell, which contains the full number of chromosomes for the species, is called a diploid
nucleus or cell.
A Nucleus or cell which contains half the number of chromosomes for the species is called a haploid
nucleus or cell.
The two types of cell division discussed in this lecture, that these are called mitosis and meiosis.
Mitosis produces exact copies of the original cell, and is the way in which organisms grow by means of cell division.
Meiosis produces special cells called Gametes for reproduction and these have half the number of chromosomes of the original cell.
Figure 4: Summary of gamete formation in cattle and recombination on fertilization.