The movement of food in phloem (or translocation) takes place by utilising energy. This happens as follows: The sugar (food) made in leaves is loaded into the sieve tubes of phloem tissue by using energy from ATP. Water now enters into sieve tubes containing sugar by the process of osmosis due to which the pressure in the phloem tissue rises.
This high pressure produced in the phloem tissue moves the food to all the parts of the plant having less pressure in their tissues. This allows the phloem to transport food according to the needs of the plant. For example, in spring, even the sugar stored in the root or stem tissue of a plan) would be transported through phloem to the buds which need energy to grow. Let us answer one question now.
Sample Problem:
The xylems in plants are responsible for:
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(a) transport of water (b) transport of food
(c) transport of amino acids (d) transport of oxygen
Answer:
(a) transport of water.
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Blood :
Blood is a red coloured liquid which circulates in our body. Blood is red because it contains a red pigment called haemoglobin in its red cells. Blood is a connective tissue. Blood consists of four thing plasma, red blood corpuscles, white blood corpuscles and platelets. Thus, the main components of blood are:
1. Plasma,
2. Red Blood Corpuscles (or Red Blood Cells),
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3. White Blood Corpuscles (or White Blood Cells), and
4. Platelets.
Plasma is a liquid (which is also called fluid matrix) and the three types of cells, red blood cells, white blood cells and platelets keep floating in it. We can now define blood as follows: Blood is a liquid (or fluid matrix) called plasma with red cells, white cells and platelets floating in it. We will now describe all the four components of blood in a little more detail.
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The liquid part (or fluid part) of blood is called plasma. Plasma is a colourless liquid which consists mainly of water with many substances dissolved in it. Plasma contains about 90 per cent water.
Plasma also contains dissolved substances such as proteins, digested food, common salt, waste products (like carbon dioxide and urea), and hormones. Plasma carries all these dissolved substances from one part to another part in the body. Red blood cells, white blood cells and platelets are immersed in this liquid called plasma.
Red blood cells are red in colour due to the presence of a red pigment called haemoglobin inside them. Red blood cells (RBC) are carriers of oxygen. Red blood cells carry oxygen from the lungs to all the cells of the body. It is actually the haemoglobin present in red blood cells which carries oxygen in the body.
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Haemoglobin performs a very important function of carrying oxygen from lungs to body tissues. Haemoglobin also carries some of the carbon dioxide from body tissues to the lungs (most of carbon dioxide is carried by plasma of blood in the dissolved form). Red blood cells are circular in shape.
Red blood cells do not have nuclei. Red blood cells have to be made quickly because they do not live for very long. Each red blood cell lives for about four months. One reason for the short life of red blood cells is that they do not have nuclei. It has been estimated that about three million red blood cells of the human blood die everyday but four times that number are made in the bone marrow everyday.
So, when we donate blood to save the life of a person, then the loss of blood from our body can be made up very quickly, within a day. This is because red blood cells are made very fast in our bone marrow. Please note that most of the cells in blood are red blood cells.
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White blood cells fight infection and protect us from diseases (see Figure 81). This is because white blood cells help to fight against germs and other foreign bodies which cause diseases. Some white blood cells can eat up the germs (like bacteria) which cause diseases. Other white blood cells make chemicals known as ‘antibodies’ to fight against infection.
In other words, white blood cells manufacture antibodies which are responsible for providing immunity in our body (due to which we are protected from disease and infection). In fact, white blood cells are called soldiers of the body. This is because they protect the body from the attack of disease-causing germs (pathogens) and other harmful foreign materials.
White blood cells are either spherical in shape or irregular in shape. All the white blood cells have a nucleus though the shape of nucleus is different in different types of white blood cells. White blood cells (WBC) in the blood are much smaller in number than red blood cells.
Platelets are the tiny fragments of special cells formed in the bone marrow. Platelets do not have nuclei. Platelets help in the coagulation of blood (or clotting of blood) in a cut or wound. For example, when a cut or wound starts bleeding, then platelets help clot the blood (make the blood semi-solid) due to which further bleeding stops. All the blood cells are made in the bone marrow from the cells called stem cells.