TRANSPORT OF MATERIALS IN LIVING THINGS.1

TRANSPORT OF MATERIALS IN LIVING THINGS.1

Introduction

The basic characteristics of all living things are nutrition, respiration, excretion, growth and development, movement, reproduction and sensitivity. In order for these life processes to take place, there must be transportation of materials. Materials are transported either from the environment into the organism or from one part of the organism to another. They can also be transported from the organism into the environment.

For example, during nutrition, organisms take in food substances that they need to provide them with energy. The food must also be transported to all parts of the organism. Respiration requires oxygen, which must be taken in from the environment. During excretion, waste materials from the organism are transported to the excretory organs and removed from the body. Growth requires the production and transportation of growth hormones to the growing parts of the organism. Movement and locomotion are made possible by the transportation of impulses to the relevant organs. Reproduction requires the movement of gametes (sex cells) or the transportation of genetic material. Sensitivity is made possible by the transportation of messages about the presence of a certain thing in the environment.

Transportation is therefore very important for the survival of living things.

Transportation is therefore very important for the survival of living things.

Ways of transportation of materials

Life processes in organisms take place at the cell level. Therefore, it is necessary for substances to move in and out of the cells. There are two ways through which substances can move across the cell membrane:

Passive transport; which occurs spontaneously without the need of energy to transport materials through the cell membrane.

Active transport; where the cell has to use energy to move materials across the cell membrane.

Processes like diffusion, osmosis and mass flow involve passive transport.

Diffusion

Diffusion is the movement of particles from an area of high concentration to one of low concentration.

A difference in the concentration of a substance between two regions is known as a concentration gradient. Diffusion causes particles to move from the area of high concentration to a low concentration area. This process continues until the particles are distributed evenly throughout the liquid. Figure below shows the diffusion of potassium permanganate in water.

       

FACTORS AFFECTING RATE OF DIFFUTION

• Concentration gradient: high diffusion rate with higher concentration and vice versa
• Surface area to volume ratio: the higher it faster the diffusion rate.
• Distance over which diffusion takes place: example a thin layer of cells increases diffusion rate

Osmosis

Osmosis is a form of passive transport considered as a special form of diffusion involves movement of water molecules through semi-permeable membrane.

Osmosis defined as the process by which water move from a weak solution into a strong through a semi-permeable membrane. The semi permeable membrane is only permeable to some solutes (dissolved substances).

For osmosis to take place there must be two separated solution by a semi-permeable membrane. One solution should have greater water and a lesser quantity of solute than other solution. This solution is hypotonic, it has a lower water potential. The second should have a lesser volume of water andvolume of solute than the other solution. This solution is hypertonic, meaning it has greater water potential.

Two solutions have the same water potential are said to be isotonic

Effects of osmosis in living organisms

Osmosis and animal cells

When an animal cell is put in a hypotonic solution, it absorbs water. If it remains in the solution for a long time, it absorbs excess amounts of water. A cell that does not have a mechanism for removing the excess water bursts due to the excessive internal pressure.

When an animal cell is placed in a hypertonic solution, it loses water. If it remains in the solution for a long time, it loses a lot of water, shrinks and shrivels.

These effects of osmosis on animal cells can be observed in red blood cells. Under normal conditions, the osmotic pressure of red blood cells is equal to that of the blood plasma, i.e. they are isotonic. Thus, there is equal movement of water in and out of the cells. This helps to maintain the disc shape of these cells.

When red blood cells are put in a hypotonic solution, they absorb water, causing the cell volume to increase. Excessive amounts of water cause haemolysis (bursting).

When red blood cells are put in a hypertonic solution, they lose water, leading to shriveling of the cell. This is referred tocrenation

Osmosis is important for the reabsorption water in the colon and the kidneys. This help to maintain the body's water balance.

Osmosis and plant cells

In an isotonic solution, plant cells neither lose nor gain water. In a hypotonic solution cells absorb water, causing the cell membrane to push against the cell wall. The cell is to be turgid. It does not burst because membrane exerts pressure on the cell wall restricts additional intake of water. Turgid plants to maintain their shape.

In a hypertonic solution, plant cells lose water this causes the vacuole to shrink and their cell membrane to pull away from wall, making the cell flaccid. Such a cell is to be plasmolyzed and the process plasmolysis.

If a plasmolyzed cell is placed in a hypotonic solution, it absorbs water and becomes turgid.

       

Osmosis is importantforthe absorption of water by plant roots. Opening and closing of stomata also depend on osmosis. When guard cells absorb water the stomata open and when they lose water the stomata close.

Osmosis and unicellular organisms

Unicellular organisms that live in fresh water, for example amoeba and euglena, are hypertonic to surrounding so water enters the organisms by osmosis. These organisms have a contractile vacuole. The contractile vacuole collects the excess water and removes it from the cell. This prevents the cells from bursting

     

Mass flow

Mass flow is the bulk movement of substances from one region to another due to the difference in pressure between the two regions. Mass flow occurs within a cell or along a vessel.

This mode of transport is important in large complex organisms where substances are required in large amounts and also have to be transported over large distances.

Examples of systems where mass flow occurs are:

• The circulatory system (flow of blood) in animals.
• The lymphatic system (flow of lymph) in animals.
• Transport of manufactured food material in plants from the site of manufacture (mostly leaves) to the point of use (all plant parts) through the phloem. This process is called translocation

Differences between diffusion, osmosis and mass flow

The following table gives a summary of the differences between diffusion, osmosis and mass flow.

Differences between diffusion, osmosis and mass flow

Characteristics	Diffusion	Osmosis	Mass flow
Substance transported	liquids and gases	Water molecules	Solids and liquids
Transportation	None structure	Semi permeable membrane	Cytoplasm and vessel
Causes of movement	Diffusion gradient	Osmotic pressure	Different in pressure

Chapter summary

1. Transport is necessary for the movement of substances within, into and out of cells so as to enable vital life processes to occur.
2. Transport can be carried out through diffusion, osmosis or mass flow.
3. Diffusion is the movement of particles from a region of high concentration to a region of low concentration.
4. Osmosis is the movement of water molecules from a weak solution to a strong solution through a semi-permeable membrane.
5. A hypotonic solution has a lower water potential.
6. A hypertonic solution has a higher potential.
7. A red blood cell haemolysis in a hypotonic solution and crenates in a hypertonic solution.
8. A plant cell becomes turgid in a hypotonic solution   and plasmolyzed in a hypertonic solution.
9. Mass flow is the bulk movement of substance due to pressure differences in two regions.

TRANSPORTATION IN MAMMALS

Introduction

Mammals are complex multicellular organisms. Their bodies are made up of numerous cells and tissues. Hence, diffusion alone is not enough to ensure efficient carrying out of life processes. Mammals therefore have an elaborate transport system called the circulatory system. The circulatory system is made up of the heart, the blood and the blood vessels.

The mammalian heart

An example of the mammalian heart is the human heart. The human heart is approximately the size of a clenched fist. It is located in the chest cavity between the two lungs.

The external structure of the mammalian heart

The mammalian heart is broader at the top and narrower at the bottom. It is enclosed by a double layer of tough inelastic membranes called the pericardium. The membranes prevent the heart from over-expanding when it is beating very fast. The pericardium also secretes a fluid called pericardial fluid. This fluid enables the membranes to move smoothly against each other

The wall of the heart is made up of the cardiac muscles. Cardiac muscle is never fatigued (tired). It works continuously as long as a person is alive. This type of muscle is found only in the heart.

The wall of the heart has three layers:

The epicardium is the outer protective layer.

The myocardium is the middle layer.

The endocardium is the inner most layer. This layer is continuous with the lining of the blood vessels attached to the heart.

The coronary artery supplies the heart with oxygenated blood. The coronary vein carries blood containing waste materials away from the heart.

The vena cava and pulmonary vein bring blood from the rest of the body to the heart. The aorta and pulmonary artery transport blood from the heart to the rest of the body.

The internal structure of the mammalianheart

Figure shows a longitudinal section of the mammalian heart

    

The heart has four chamber right auricle, right ventricle, left auricle and left ventricle. The auricles are also called atria (singular: atrium). The walls of the ventricles are thicker than those of the auricles. This is because the ventricles pump blood to a greater distance than the auricles. Auricles pump blood to the ventricles. Ventricles pump blood to all other parts of the body. The left ventricle is thicker than the right ventricle   because the right ventricle pumps blood to the lungs while the left ventricle pumps blood to the rest of the body.

The heart has several valves. Valves have flaps that ensure that blood flows in one direction only. The tricuspidvalve is found between the right auricle and right ventricle. The bicuspid valve is found between the left auricle and left ventricle. Semi lunar valves are located at the bases of the pulmonary artery and aorta to prevent blood from flowing back into the ventricles.

Valves close when blood tries to flow back.

The left and right sides of the heart are separated by the septum. The septum is a thick muscular wall that prevents mixing of oxygenated and deoxygenated blood.

The flow of blood through the heart;

The vena cava brings deoxygenated blood to the heart. Deoxygenated blood contains low amounts of oxygen.

The vena cava has two branches:

The superior vena cava which transports deoxygenated blood from the upper parts of the body such as head, neck and upper limbs.

The inferior   vena   cava   which   transports deoxygenated blood from the lower parts of body such as the lower limbs, kidney, liver, stomach and intestines.

The inferior vena cava and the superior vena cava unite to form the vena cava; the vena cava is connected to the right auricle.

When the right auricle relaxes, it fills up with deoxygenated blood from the vena cava. There is increased pressure in the right auricle when the muscles contract. This pushes the blood trough the tricuspid valve. The muscles of the

Right ventricles relax and it fills up with blood. The tricuspid valve closes to prevent blood from owing back into the right auricle. When the right ventricle is full, the increased pressure causes the muscles to contract and the Semi lunar valve in the pulmonary artery to open. The blood flows into lie pulmonary artery and the bicuspid valve closes prevent back flow of blood.

The pulmonary artery transports blood to the lungs. Blood absorbs more oxygen in the lungs, and thus becomes oxygenated.

Oxygenated blood flows to the heart through the pulmonary vein. This vein is connected to the left auricle. When the left auricle relaxes, the semi lunarvalve opens and blood from the pulmonary veinflows in. Pressure increases in the left auricle as itfills up with blood. The pressure causes the musclesof the auricle to contract and pump blood throughthe bicuspid valve into the left ventricle.

The muscles of the left ventricle contract, allowing blood to flow in. The bicuspid valve closes to prevent blood from flowing back into the left auricle. Pressure builds up in the left ventricle as blood flows in.

The muscles of the left ventricle contract, pumping blood through the semi lunar valve into the aorta. The aorta branches into smaller arteries that transport blood to all parts of the body. The heart beats in such a way that when the auricles contract, the ventricles relax and vice versa.

In the right atrium, there is a small patch of muscle called the sinoatrial node (SAN). This node acts as a pacemaker, setting the time and rate of cardiac muscle contraction.

Adaptations of the heart to its functions

Table below shows how the heart is adapted to its functions.

Adaptations of the heart

      

Adaptation	Function
Muscular walls	Contract to pump blood
Cardiac muscle	Contract and relax continuously without being fatigued. This ensures continuous pumping of blood
Valves	Ensure blood flows in only one direction
Septum	Separates oxygenated blood from deoxygenated blood
Connection to large blood vessels	Enables transportation of deoxygenated blood from all parts of the body to the heart and transportation of oxygenated blood from the heart to all parts of the body
Sinoatrial node	Sets time and rate of contraction of cardiac muscle
Coronary artery and coronary vein	The coronary artery nourishes the heart and supplies it with oxygen,     The coronary vein removes wastes which would harm the heart if left to accumulate

Blood vessels

Mammals have three types of blood vessels: arteries, veins and capillaries.

Arteries

Arteries are thick-walled, muscular and elastic vessels that transport blood from the heart to all parts of the body. All arteries transport oxygenated blood, except the pulmonary artery which transports deoxygenated blood from the heart to the lungs

       

The endothelium is the innermost layer of the artery. It has only one layer of cells. The endothelium surrounds the lumen (the central tube of the vessel). The lumen of an artery is narrow and smooth so that it can transport blood at high pressure.

The muscular layer is made of smooth muscle and elastic fibres. Smooth muscle is arranged in circles round the endothelium. This layer makes it possible for the artery to contract and relax for the efficient movement of blood.

The outermost layer is the fibrous layer made of connective tissues such as collagen. The fibres are arranged parallel to the length of the vessel. They enable the artery to withstand the pressure caused by the blood coming from the heart.

When the ventricles contract, the arteries relax allowing blood from the heart to flow into them. When the ventricles relax, the arteries contract, forcing the blood forward. This contraction and relaxation of arteries is felt as a pulse.

Pulse rate is the number of pulses per minute. The pulse rate reflects the heartbeat. An adult human’s heart beats at an average of 72 times a minute. However, this can increase or decrease due to physical activity, emotional state or health factors

Arteries branch to form arterioles. Arterioles in turn branch to form capillaries. Capillaries are joined at the other end by venules which join to form veins.

Veins

Veins are vessels that transport blood to the heart from all parts of the body. All veins transport deoxygenated blood except the pulmonary vein. The pulmonary vein transports oxygenated blood from the lungs to the heart

      

Veins have a larger lumen and less muscular walls compared to arteries. This is because the blood in the veins flows at low pressure.

Vein have valves at regular intervals. The valves prevent the back flow of blood.

                  

The muscles next to the veins squeeze the veins and help to force blood to flow towards the heart. The contraction of the ribs during breathing also helps to squeeze some veins and keep blood flowing.

Capillaries

Capillaries are the smallest blood vessels. They are narrow and have walls that are one cell thick

Capillaries are in direct contact with the tissues of the body. They form a network for the efficient diffusion of substances. Their thin walls maximize the rate of diffusion.

The thin walls of the capillaries enable oxygen and nutrients to diffuse from the blood to the cells, carbon dioxide and other waste products to diffuse from the cells into the blood and white blood cells to reach sites of infection.

Capillaries join to form venules (small veins) which join to form veins.

Differences between arteries, veins and capillaries

Table below gives a summary of the structural and functional differences between arteries, veins and capillaries.

Differences between arteries, veins and capillaries

Arteries	vein	Capillaries
Have narrow smooth lumens	Have wide irregular lumens	Have narrow smooth lumens
Have thick muscular walls	Have thin, less muscular walls	Have one cell ' thick walls
Lack valves except where they   are connected to the heart	Have valves at regular intervals	Lack valves
Transport blood at high pressure	Transport blood at low pressure	Transport blood at low pressure
Transport blood away from the heart	Transport blood towards the heart	Transport blood within the tissues
Transport oxygenated blood, except the pulmonary artery	Transport deoxygenated blood, except the pulmonary vein	Transport either oxygenated or deoxygenated blood
Contract and relax to create a pulse	Blood flows smoothly	Blood flows smoothly

Blood

Blood is a fluid tissue. It consists of cells (red blood cells and white blood cells) and platelets (fragments of cells) suspended in a fluid called plasma. An adult human has 4 to 6 liters of blood. The pH of blood is 7.4.

Plasma

Plasma is a pale-yellow fluid. Approximately 55% of the blood is plasma. Plasma is mostly made up of water but it also has dissolved substances such as food nutrients, metabolic wastes, oxygen, proteins and mineral ions. These solutes make up 8% of the plasma while water makes up 92%.

The major functions of plasma are the transportation of:

1. nutrients from the digestive system to the whole body
2. red blood cells containing oxygen to the tissues
3. wastes such as carbon dioxide and urea to the excretory organs
4. white blood cells and antibodies to sites of infection
5. hormones to the target organs
6. mineral ions such as sodium, potassium and chlorides
7. Platelets to sites of bleeding.

Plasma is also important for distributing heat to all parts of the body, regulating the pH of body fluids and it is where the exchange of nutrients and waste products takes place in the body.

Red blood cells

Another name for the red blood cells is erythrocytes. They are red, round biconcave cells with no nucleus. One milliliter of blood has approximately 5 to 6 million red blood cells

                     

Red blood cells are formed in the bone marrow. Their lifespan is about 120 days. The liver and the spleen destroy old red blood cells and release haemoglobin for the formation of new cells.

Haemoglobin is the red pigment in erythrocytes. It has a high affinity for oxygen.

The function of red blood cells is to transport oxygen and carbon dioxide. The adaptation red blood cells that make them suited forthis function are the presence of haemoglobin, their large numbers, biconcave shape and the lack of nucleus which increases the total surface area of gaseous exchange.

Transport of oxygen                                                                    

In the lungs (where there is a high concentration of oxygen), haemoglobin combines with oxygen to form oxyhaemoglobin. This is an unstable compound which releases oxygen when it reaches tissues that have a low concentration of oxygen. The formation of oxyhaemoglobin and release oxygen and haemoglobin can be shown using the following equation.

Haemoglobin + oxygen = Oxyhaemoglobin

Oxygen diffuses out of the red blood cells, through the capillary walls to the tissues.

Transport of carbon dioxide

In the red blood cells, carbon dioxide combines with haemoglobin to form carbominohaemoglobin. This compound is transported to the lungs where carbon dioxide is released and expelled from body.

White blood cells

Another   name   for   the   white   blood   cells is leucocytes. These cells have irregular shapes; milliliter of blood has approximately 5000 to 10 white blood cells.

       

White blood cells are produced in the bone marrow and in the lymph nodes.

The function of white blood cells is to protect body against infection. They perform this function by:

                            

                             Phagocytosis in a white blood cell

1. Engulfing and destroying pathogens (a process called phagocytosis).                                                                                                     
2. Producing substances that neutralize toxins produced by pathogens.
3. Causing   clumping   together   of   foreign materials in the body.
4. Killing infected body cells.
5. Preventing clotting in damaged tissues.

The effect of HIV on white blood cells

The Human Immunodeficiency Virus (HIV) attacks a type of white blood cells called helper-T cells. These cells are essential for body immunity. When they encounter an antigen, the helper-T cells divide themselves to form new cells. This increases the number of cells available to fight the infection. After the infection, some cells remain as memory cells to activate an immune response if the infection happens again, in addition helper-T cells activate other cells in the immune system.

HIV has a protein envelope that can only bind to its receptor called CD4 found on the cell membrane of the helper-T cell. When it enters the human body, HIV fuses its protein envelope with the CD4 then enters the cell. Once inside the cell, the virus becomes part of the helper-T cell and replicates together with it as it undergoes division. This increases the amount of HIV in the blood. The HIV destroys helper-T cells resulting in the reduction of the number of helper-T cells and reducing the CD4 count.

Diagram HIV attacking T-helper

HIV destroys helper-T cells in the following ways:

1. It reproduces inside the helper-T cell, and then ruptures the cell's membrane and the new viruses are released.
2. It alters the helper T-cells so that when it responds to an infection, it kills itself instead of dividing to form new cells.
3. It   marks helper-T cells as targets for destruction by other cells in the immune system.
4. It causes the fusion of many helper-T cells to form a giant' cell. Such a cell can survive but it cannot perform normal helper-T cell functions.

Thus, HIV lowers the body's immunity significantly making it vulnerable to opportunistic infections.

Platelets

Platelets are also called thrombocytes. They are fragments of cells produced in the bone marrow. One milliliter of blood contains about 250 000 to 400 000 platelets.They play an important role in the clotting process.

            
          

The clotting process

Platelets at the site of an injury produce thromboplastin which starts off the clotting process. Thromboplastin, with the help of vitamin K and calcium neutralizes heparin, an anticoagulant in blood.

Heparin converts prothrombin (which is an inactive plasma protein) to thrombin (an active plasma protein).

Thrombin catalyzes the conversion of soluble fibrinogen to insoluble fibrin. Fibrin forms a network of fibres that traps debris and blood cells. The result is a clot at the site of the wound preventing further loss of blood.

         

                                          Blood clot