G. The experiment done by Mittler to prove the role of sieve tubes in translocation of high-energy food.

H. The experiment done by Rapeden and Bohr to prove that the high-energy food is translocated out of the leaves to various parts of the plant.

9. Mention the disadvantages of the following theories:

A. Root pressure theory.

B. Capillarity theory.

C. Imbibition theory.

Transport in animals.

Human transport system.

Transport in Humans is a process that takes place through 2 closely related systems:

I. Blood vascular system. (Circulatory system)

II. Lymphatic system.

I. The circulatory system:

It consists of the heart and the blood vessels through which the blood passes. These vessels form a complete circuit. (A closed circulatory system)

1. The heart:

The heart is a hollow muscular organ which lies in the middle of the chest cavity. It is enclosed in the pericardium that protects the heart and facilitates its pumping action.

The heart is divided into four chambers:

- The 2 atria (auricles): The upper two chambers with thin walls. Those receive blood from veins.

- The 2 ventricles: The lower two chambers with thick muscular walls. Those pump blood through arteries.

The heart is divided longitudinally into two sides by means of muscular walls. Each atrium is connected to its own ventricle through an opening which is guarded by means of a valve. Each valve consists of thin flaps. In order for these flaps not to turn inside out, the free edges of these flaps are attached to the ventricle wall by means of tendons. Thus blood is permitted to flow only from the atrium to the ventricle not in the reverse direction. The right valve (The tricuspid valve) is made up of three flaps, while the left valve (the bicuspid valve or the mitral valve) is similar in structure and action, except it is formed up of 2 flaps. There are also semi-lunar valve at the connection of the heart with both Aorta and pulmonary artery. The heart beats regularly throughout the whole life time.

2. The blood vessels:

A. Arteries:

Arteries are wide vessels that carry blood from the heart to other organs of the body. The wall of an artery is build up of three layers of tissues:

- The outer layer: A coat of connective tissue.

- The middle layer: Is relatively thick and consists of involuntary muscles which contract and relax under the control of nerve fibers.

- The inner layer: The endothelium, which consists of one row of tiny epithelial cells followed by elastic fibers that give the elasticity of the artery.

Arteries are usually buried among the body muscles. They carry oxygenated blood except the pulmonary artery which comes out of the right ventricle to the lungs (that carries deoxygenated blood).

B. Veins:

Veins are the vessels that carry blood to the heart. Walls of veins are composed of the same three layers of that of arteries with the following modifications:

- There are less elastic fibers.

- The middle layer is less thick.

Accordingly, The wall of the vein is thinner than that of the artery, and it doesn’t pulsate. Veins carry deoxygenated blood except the pulmonary veins that open in the left atrium (that carries oxygenated blood).

A number of veins possess a system of internal valves along their length to prevent the backflow of blood, and allowing it to pass only in the direction of the heart. Sites of these valves can be observed in the arm veins. (when the arm is tied tightly with a bandage above the elbow). This was done by William Harvey the English doctor, who discovered the blood circulation in the 17^th. Century.

C. The Capillaries:

Capillaries are tiny, microscopic vessels which connect the arterioles with the venules. This fact was discovered by Malpighi, the Italian scientist at the end of the 17^th. Century, thus he completed the work of Harvey.

The average diameter of capillaries ranges between 7 – 10 microns. Their walls are very thin and consist of one row of thin epithelial cells with tiny pores between them. The wall of the capillary is about 0.1 micron thick, which facilitates quick exchange of substances between the blood and the tissue cells. Capillaries spread in the spaces between cells all over the body tissues. Capillaries reach all the body cells and supply them with their requirements. If all capillaries in the Human body were put end to end, their overall length would be about 80000 kilometers.

3. Blood:

Blood is a liquid tissue. It contains red blood cells, white blood cells, and blood platelets. The fluid part of this liquid tissue is the blood plasma. Blood is the principal medium in the process of transport. It is a viscous red liquid. Blood is weakly alkaline. (PH = 7.4). The Human body contains 5 to 6 liters of blood in average.

Components of blood:

a. Plasma:

Plasma is about 54% of blood volume. It contains:

- 90% water.

- 1% inorganic salts as Ca++, Na+, (HCO3)-

- 7% proteins as albumin, globulin, and fibrinogen.

- 2% other components as:

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