BIOLOGY Form 2 Topic 4

This is the movement of water molecules from a region of higher concentration to a region of lower concentration through a semi-permeable membrane. A partially-permeable membrane is a membrane that allows small particles such as water molecules to pass through it, but not larger particles such as sugar molecules and ions from salts. Examples of semi-permeable membranes are cell membranes and a pig’s bladder. These membranes allow transportation of water through them. In spite of the fact that they allow transportation of water through them, they do not permit the passage of sugar or salt molecules because they are solutes. Osmosis occurs when water moves down its concentration gradient across the semi-permeable membrane.

Diffusion and osmosis occurs very slowly and cover short distances. In animals and plants,materials are usually transported a long distance and in large quantities. For example, food nutrients from the small intestine have to be moved to cells in the extremities such as toes and fingers, where the nutrient materials have to be transported a long distance. Therefore, an efficient and fast mechanism is required to facilitate this movement. That is when mass flow comes in.

Mass flow is the movement of materials in large quantities and across a long distance in the body of an organism due to differences in pressure between the two regions. Materials in higher plants and animals are moved by the process of mass flow. For example, the manufactured food in plant leaves has to be moved to all plant parts, for use or storage, by mass flow.

Take a bottle of perfume and move to one corner of the classroom. Open a bottle and observe what happens. The result is, after a few seconds, the whole classroom is filled with a smell of the perfume. This means that the molecules of the perfume moves from the region of higher concentration (the bottle) to a region of lower concentration (air). That is why the smell is felt by a person standing several meters away from the source of the perfume.

Some important processes that involve diffusion are:

Mammals are the complex multicellular organisms whose bodies are made up of numerous cellsand tissues. In this case, diffusion alone is not enough to insure efficient carrying out of lifeprocess. Therefore mammals have an elaborate transport system that is made up of the heart,blood and blood vessel.

Blood vessels are intricate networks of hollow tubes that transport blood throughout the entirebody. This is an essential function as blood delivers valuable nutrients to and removes wastesfrom our cells. Blood vessels are constructed of layers of connective tissue and muscle. Theinner blood vessel layer is formed of endothelium. In capillaries and sinusoids, endotheliumcomprises the majority of the vessel. There are three types of blood vessels namely arteries,veins and capillaries. Each of these vessels has a different structure and function.

How different blood vessels are adapted for their function

Red blood cells (RBCs) have two main functions:

Erythrocytes transport oxygen in the blood through the red pigment called haemoglobin.Haemoglobin contains iron and proteins joined to greatly increase the oxygen carrying capacityof erythrocytes. The high surface area to volume ratio of erythrocytes allows oxygen to be easilytransferred into the cells in the lungs and out of the cells in the capillaries of the systemic tissues.Erythrocytes are produced inside red bone marrow from stem cells at the astonishing rate ofabout 2 million cells every second.

It is easy and inexpensive to determine an individual’s blood type from a few drops of blood.This is how blood typing/grouping it is done: A serum containing anti-A antibodies is mixedwith some of the blood. Another serum with anti-B antibodies is mixed with the remainingsample. Whether or not agglutination occurs in either sample indicates the blood type. Forinstance, if an individual’s blood sample is agglutinated by the anti-A antibody, but not the anti-Bantibody, it means that the A antigen is present but not the B antigen. Therefore, the blood typeis A.

Blood transfusion is the transfer of blood from one person (donor) to another person (recipient)through blood vessels. Transfusion is done to replace lost blood due to illness, accidents or bleeding. The donor is the person who gives blood while the recipient is the person who receives blood.

When performing blood transfusions it is important to avoid combining corresponding antigensand antibodies because they cause agglutination of red blood cells which may lead to death ofthe recipient. Agglutination is the clumping of red blood cells. Blood transfusion is only possibleif blood groups are compatible. Blood group compatibilities are as shown in the table below.

Note: a tick (√) means compatible and a cross (×) means incompatible.

Individuals with blood group AB can receive blood from individuals of all blood groups and areknown as universal recipients. Individuals with blood group O can donate blood to individualsof all blood groups and are known as universal donors.

Blood transfusion does so much for patients in need. The gift of life is donated, tested, processedand sent to hospitals’ transfusion service departments where more important work is done toensure it is compatible with the recipient.

Blood transfusion has a number of advantages. These are some of the benefits the donated bloodcan provide for patients in need:

Although blood transfusions can be life-saving, they are not without risks. The following aredisadvantages of blood transfusions:

Medical reactions:

Diseases:If proper screening of the donated blood is not observed, it can causetransmission of diseases from the donor to the recipient. Examples of such transmissiblediseases are HIV virus, hepatitis, and other infections.

Patients who are given too much blood can develop high blood pressure, a concern forpeople who have heart disease.

Certain precautions and guidelines must be adhered to in blood transfusion to ensure the safetyof the procedure. The precautions may include the following:

Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to theheart again. This is just one phase of the overall circulatory system. In this type of circulation,the blood flows from the right ventricle to the lungs and from the lungs to the left auricle. In thepulmonary circulation, the blood circulates to and from the lungs, to release the carbon dioxideand pick up new oxygen.

In the pulmonary circulation, blood from all body parts (except the lungs) enters the right auriclethrough vena cava. From the right auricle the blood descends into the right ventricle through thetricuspid valve. When the ventricle contracts, the blood is pushed into the pulmonary artery thatbranches into two main parts: one going to the left lung, and another to the right lung. The fresh,oxygenated blood returns to the left auricle of the heart through the pulmonary vein.

Systemic circulation is the flow of blood between the heart and the body parts. In this particularcirculation, the blood flows from the left ventricle to different parts of the body and from22different parts of the body to the right auricle. The systemic circulation supplies nourishment toall of the tissues located throughout your body, with the exception of the heart and lungs becausethey have their own systems. Systemic circulation is a major part of the overall circulatorysystem. In this circulation, the blood circulates into the body’s systems, bringing oxygen to all itsorgans, structures and tissues and collecting carbon dioxide waste.

The systemic cycle begins when the oxygenated blood coming from the lungs enters the leftauricle. As the chamber fills, it presses open the bicuspid valve and the blood flows down intothe left ventricle. When the ventricles contract during a heartbeat, the blood on the left side isforced into the aorta. This largest artery of the body is an inch wide. The blood leaving the aortabrings oxygen to all the body’s cells through the network of ever smaller arteries and capillaries.The used blood from the body returns to the heart through the network of veins. All of the bloodfrom the body is eventually collected into the two largest veins: the superior vena cava, whichreceives blood from the upper body, and the inferior vena cava, which receives blood from thelower body region. Both venae cavae empty the blood into the right auricle of the heart.

The process by which blood passes through the heart twice before it returns to the other parts ofthe body is called double circulation.

Blood circulation is essential for a healthy body. Blood circulation is important because itfacilitates the following processes to take place in the body:

The lymphatic system performs the following functions:

Lymphoedema is a chronic swelling of the limbs caused by the accumulation of lymph fluid thatoccurs if the lymphatic system is damaged or not functioning properly. While the limbs aretypically involved, the face, neck and abdomen may also be affected.

The lymphatic system consists of a series of lymph nodes (glands) connected by a network ofvessels, similar to blood vessels. Fluid surrounding body tissues usually drains into nearbylymph vessels so it can be transported back into the blood. However, if the lymph vessels areblocked, the fluid can’t be reabsorbed and will build up in the tissue.

Lymphatic filariasis, commonly known as elephantiasis, is a parasitic infection that causesextreme swelling in the arms and legs. It is a painful and profoundly disfiguring disease. Whilethe infection is usually acquired in childhood, its visible manifestations occur later in life,causing temporary or permanent disability.

The disease is caused by the filarial worm, which is transmitted form human to human via thefemale mosquito when it takes a blood meal. The parasite grows into an adult worm that lives inthe lymphatic system of humans.

Effective treatment and preventive efforts would include:

Materials to be transported across the plant body are water, minerals and food. Apart from thesenutrients, substances like the hormones also have to be transported. The transport of materialstakes place through a specialized tissue called the vascular tissue. The tissue is made up of xylemand phloem tissues. Xylem tissue transports water and mineral salts from the soil to all parts ofthe plant. Phloem tissue transports manufactured food from the sites of photosynthesis to allparts of the plant.

In between the xylem and phloem is the vascular cambium. The cells of cambium tissue divideto form a new xylem and phloem. As these cells divide and multiply, the plant increases its girth.The xylem grows inward from the vascular cambium while the phloem grows outward from thevascular cambium.

The arrangement of the vascular bundles in the stem, root and leaf of dicot and mocot plantsdiffers in a number of ways. The diagrams below show the manner they are arranged in therespective organs.

The vascular tissue in the root is arranged in the inner portion of the root, which is called thevascular cylinder. A layer of cells known as the endodermis separates the vascular tissue fromthe ground tissue in the outer portion of the root. The endodermis is exclusive to roots, andserves as a checkpoint for materials entering the root’s vascular system. A waxy substance calledsuberin is present on the walls of the endodermal cells. This waxy region, known as theCasparian strip, forces water and solutes to cross the plasma membranes of endodermal cellsinstead of slipping between the cells. This ensures that only materials required by the root passthrough the endodermis, while toxic substances and pathogens are generally excluded. Theoutermost cell layer of the root’s vascular tissue is the pericycle, an area that can give rise tolateral roots. In dicot roots, the xylem and phloem are arranged alternately in an X shape,whereas in monocot roots, the vascular tissue is arranged in a ring around the pith.In

In monocot stems, the vascular bundles are scattered throughout the stem as indicated in thefigure below.

In dicot stems, the vascular bundles are arranged in a ring around the pith.

The arrangement of vascular bundles in the leaves of dicots and monocots differs. The diagramsbelow show the differences in arrangement of the bundles. Can you notice the differences? Thexylem and phloem vessels are enclosed in a bundle sheath.

It is the vascular tissue that transports water across the plant body. Xylem is made up of fourdifferent types of cells. They are tracheids, vessels, xylem fibres and xylem parenchyma. Ofthese only tracheids and vessels are involved in the transport of water and minerals.

The xylem functions in transport (translocation) of manufactured food from the leaves to the cells of the plant, storage organs, fruits, etc.

Plants absorb water from the soil through the root and transport it to the stem, leaves andflowers. Roots have root hairs that are unicellular, thin-walled outgrowths of the epiblema (skinof the root).

The root hairs are in close contact with the thin film of water surrounding the soil particles.There are mineral salts such as nitrates, chlorides, sulphates, phosphates, etc., dissolved in thiswater.

Water is absorbed by osmosis, while the minerals are absorbed as ions by active transport(transport against the law of diffusion, by spending cellular energy). The cell membrane hastransport proteins that allow the ions to cross the membrane. The ions then move upward throughthe xylem, to the leaves and other aerial parts of the plant.

The cell wall of each root hair is permeable to water and minerals, but its cell membrane and themembrane around the vacuole are semi permeable membranes. The root hair cells take upmineral ions by active transport.

This creates a concentration difference of these ions between the root and the soil. Now, the soilsolution has higher water content than the cell sap of the root hair. Hence, water from the soildiffuses into the root hair. The root hair cells now become turgid, while the adjacent cells of thecortex have lower water content.

This results in the diffusion of water from the root hairs into the cortical cells (see figure below).After passing through the cortical cells by osmosis, the water reaches the endodermis (tissueseparating the cortex from the vascular tissues). The endodermis forces water into the xylemtubes through passage cells.

The pressure with which water is pushed into the xylem tubes of the root is called root pressure.The water moving upwards forms a column, which is maintained up to a certain height due toroot pressure.

There are several processes that enable the water to move up a plant. These processes includeroot pressure, transpiration pull, cohesion, adhesion and capillarity.

Transpiration is the loss of water through the leaves and other parts of the plant. Mosttranspiration occurs through openings, called stomata, on the underside of the leaves. Astranspiration occurs, water is lost. This water is replaced by water in the xylem vessels. Thiscauses an upward pull (transpiration pull or transpiration stream) on the water in the vessels.Thus, water is pulled up through the plant, and more enters by the roots to replace it.

The leaves absorb the radiant energy. Some of the light energy is utilized in photosynthesis. Therest is converted into heat energy resulting in an increase in leaf temperature. However, rapidloss of water in the form of water vapour from the aerial parts of the plant through transpirationbrings down their temperature. Transpiration thus provides a significant cooling effect whichkeeps the plant from being overheated.

Mineral salts remain dissolved in the soil water and are absorbed by the roots. Minerals that arcabsorbed and accumulated in the xylem duct of the root move up and are distributed in the plantby the transpiration stream.

The absorbed water is transported from roots to leaves through the xylem vessels. This is greatlyinfluenced by transpiration pull. Water loss due to transpiration results in the development of lowwater potential in the leaf tissues. Thus, water moves from the xylem vessels to the leaf cells.

Greater amount of transpiration helps in the development of mechanical tissues in plants. Theplants become healthier and more compact, the cell walls become thick and cutinized and theplants are able to resist the attack of fungi and bacteria.

Transpiration maintains an optimum degree of turgidity in cells. Under favourable conditions,plants absorb excess amount of water, which is given off by transpiration to maintain theoptimum turgidity for better growth.

The solutes inside the cell become more concentrated when transpiration is rapid. Consequently,the concentration of sugar solution in the cells of fruits increases and fruits taste sweeter.

When the rate of evaporation is higher than that of absorption of water from the soil, as it occursduring drought conditions, the plant wilts. Wilting is beneficial when a plant cannot obtainenough water to replace that lost by the plant through transpiration because it causes the closureof the stomata (singular: stoma). Thus, the rate of evaporation is greatly reduced.

These plant parameters help plants control rates of transpiration by serving as forms of resistanceto water movement out of the plant. They include the following:-

Some environmental conditions create the driving force for movement of water out of the plant.Others alter the plant’s ability to control water loss.