- THE ANIMAL CELL
All living matter is composed of functional units known as cells. At one end of the scale in the animal kingdom, there are unicellular organisms composed of a single cell; an example is the amoeba, where all the vital processes of the animal take place inside a single cell. Cells are capable of digesting food, growing, respiring, excreting, secreting, reproducing and responding to stimuli. These things all happen in the single-celled animal. At the other end of the scale, there are the multicellular organisms, such as the higher animals and human beings. In these organisms the cells become specialised, and one or more of the above functions may be lost.
|Excreting: (of a living organism or cell) separate and expel as waste. Secreting: (of a cell, gland, or organ) produce and discharge.|
A diagram of a general animal cell is shown below. Remember that this is many times its normal size and that cells only can be seen under a microscope. The various parts of the cell and their functions are:
This is the outer layer of the cell which gives it its shape and holds the liquid inside the cell. This membrane is semi- permeable which allows certain things to pass into and out of the cell.
This also contains the nucleolus; that part of the cell which
houses the genetic material, chromosomes and chromatin which are concerned with the reproduction of the cell.
This is the fluid inside the cell which contains salts and sugars in solution.
This stores food inside the cell. Also note the fat droplets which float about in the cytoplasm are a means of storing food in the form of fats.
These are digestive centres in the cell that help to digest and break down food material.
This is also concerned with cell division, the reproduction of the cell, and the movement of the cell chromosomes.
These are concerned with the respiration of the cell, and as the end product of all respiration is energy, these are a source of energy.
This is the part where protein is manufactured and built up from nitrogen, containing compounds and amino acids.
These are contractile, concerned with nervous responses.
Although this type of cell is very small, you can see that it contains a number of different parts that can carry out a variety of different functions.
Animals found on a farm are multi-cellular organisms. The single cells, many of which are specialised so that they can perform a particular function, are grouped together to form tissues. These tissues, in turn, form special groups called organs. The organs make up a system, and the systems are integrated to form a living body.
- 2. ANIMAL TISSUES
There are five basic types of tissue found in animals, which are the following:
- Epithelial Tissues;
- Connective Tissues;
- Fluid Tissues;
- Muscle Tissues; and
- Nerve Tissues.
Figure 1: The General Animal Cell
This is formed from cells which join together to form covering layers, such as, the skin covering the body. This type of tissue also forms the covering layers of various organs in the body, the lining of the body cavities and the active parts of the glands of the body. Epithelial tissues are made up of specialised cells of various shapes and joined in different ways as shown below:
Figure 2: Epithelial Tissues
This is the tissue which joins other tissues together. Connective tissues give form and strength to many organs and often serve for protection and leverage. Examples are bones, tendons, ligaments, cartilage and fat. These will be covered in more detail later on in the course.
Figure 3: Internal Animal Structure
These tissues transport food nutrients and waste products around the body. Blood is a good example; it consists of plasma and a variety of cells.
There are three types of muscle tissue:
- Striated or voluntary muscle: which is the type found in your arms or legs, and which you can use or rest as you wish;
- Smooth involuntary muscle: works automatically and cannot be controlled by you; an example is the muscle in the intestine which moves the food along through the gut; and
- Cardiac muscle: is also involuntary and cannot be controlled by you; this is the muscle which makes up the heart.
Muscle tissue is made so that it can expand and contract; you contract the muscles of the arm when you pick up a brick, and expand them when you stretch your arm out. The involuntary muscles of the body expand and contract on their own, as in the beating of your heart. Striated muscle tissue makes up the flesh of an animal, and so forms the meat.
The nerve cells which make up this tissue are sensitive to stimuli, such as, heat and touch. They can link up charges and transmit impulses through the nervous system.
|Striated: striped or streaked.|
These are the different types of tissues, but remember, although they differ from each other because of the different functions they perform, they are all made up of cells. These cells are the basic units of life, and as such take in food or nutrients, utilise the nutrients and produce energy and waste products. That is the basic process of living; the breakdown of food into energy and waste products. It happens in plants and also in animals. In animals, blood is the carrier of nutrients and waste
products around the body. It carries the nutrients to the cells, and the waste products away from the cells, while the conversion of nutrients to waste products takes place inside the cells. The blood that carries the nutrients to the cells is called arterial blood because it travels through the arteries, and blood that carries the waste products away, is called venous blood because it travels through the veins.
MOVEMENT OF NUTRIENTS AND WATSE
Figure 4: The Passing of Nutrients and Waste through the Capillary System
We will now consider how the nutrients pass from the arteries into the cells, and the waste products from the cells into the veins.
Both arteries and veins begin at the heart and spread throughout the body. To begin with, they are large tubes with thick walls about the size of one’s little finger. As they spread out they divide into branches, and get progressively smaller and narrower until finally, they become very fine, thread-like tubes called capillaries. The arterial system and the venous system connect up with each other through these capillaries (see figure 4 above). Between the capillaries and the cells to which they carry nutrients, there is a liquid called interstitial fluid, and this surrounds the cells and the capillaries, acting as a connecting link.
The nutrients carried by the arteries pass through the walls of the capillaries into the interstitial fluid and then through the walls of the cells. The waste products pass in the opposite direction, through the cell walls into the interstitial fluid, through the capillary walls and into the venous system. This movement takes place and is controlled in a number of ways.
OSMOSIS AND FILTRATION
Cell walls are what are known as semi-permeable membranes. This means that although they enclose the cell and contain the liquid and other parts of the cell, they do allow water and other items in solution (i.e. dissolved in the water) to pass through. Osmosis is the process whereby water can pass through a semi-permeable membrane from a weak solution to a stronger solution.
Figure 5: Osmosis
Look at figure 5 above. A and B are two liquids separated by a semi-permeable membrane such as a cell wall (parchment, which is dried skin is also semi-permeable membrane and is usually used to demonstrate osmosis in a laboratory). Liquid A is a solution of sugar and contains dissolved sugar particles or molecules (very small particles). Liquid B is pure water, which is called distilled water, and this does not contain any dissolved matter at all.
The movement which takes place is of water from liquid B, through the semi-permeable membrane, to solution A, causing solution A to increase in volume and solution B to decrease. The pressure exerted by this movement is called osmotic pressure, and as solution B becomes less and solution A increases, the osmotic pressure will increase until water starts forcing its way back through the membrane from solution A to solution B. This counter pressure is called the filtration pressure.
The movement with osmosis is always of water, from a weak solution to a stronger solution. If solution A were a strong sugar solution and solution B a weak sugar solution, the water movement would go on until both solutions were of equal strength before stopping. The object of osmosis is to have solutions of equal strength on each side of the semi-permeable membrane.
This is the movement of very small particles of matter or molecules across a semi-permeable membrane where the pressure on one side of the membrane is much greater than the pressure on the other side. A good example of this is the blood pressure of the body caused by the heart pumping blood around the system. This causes the pressure inside the capillaries to be higher than that in the surrounding fluid, and forces particles through the wall of the capillary.
This is the movement of particles, or molecules, across a semi-permeable membrane against the force exerted by osmotic pressure or hydrostatic pressure. This movement requires energy before it can take place.
This is the action of a cell when it reaches out, engulfs a molecule into the cell and digests it.
Figure 6: Phagocytosis
The molecule attaches itself to the cell wall, and is drawn into the body of the cell, still surrounded by part of the cell wall. The gap is repaired by the cell wall growing together, as shown in the diagrams below:
Figure 7: Pinocytosis
Some substances when dissolved in water form ions (see Soil Science Lecture 4). For instance, common salt is called sodium chloride, and the chemical formula is NaCl. When dissolved in water, the salt dissolves very easily, and forms sodium Ions, Na+ carrying a positive electrical charge, and chlorine Ions, Cr a negative charge. When two solutions on either side of a semi-permeable membrane contain an unequal number of ions, the ions will pass through the membrane until each solution has the same number of positive and negative ions.
Figure 8: Electro-chemical Gradient