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Home > Biology > Core > Maintaining a balance > Maintaining a balance: 2. A watery medium

9.2 Maintaining a balance: 2. A watery medium

Syllabus reference (October 2002 version)
2. Plants and animals transport dissolved nutrients and gases in a fluid medium	Students learn to: identify the form(s) in which each of the following is carried in mammalian blood: carbon dioxide oxygen water salts lipids nitrogenous waste other products of digestion explain the adaptive advantage of haemoglobin   compare the structure of arteries, capillaries and veins in relation to their function   describe the main changes in the chemical composition of the blood as it moves around the body and identify tissues in which these changes occur   outline the need for oxygen in living cells and explain why removal of carbon dioxide from cells is essential   describe current theories about processes responsible for the movement of materials through plants in xylem and phloem tissue	Students: perform a first-hand investigation to demonstrate the effect of dissolved carbon dioxide on the pH of water   perform a first-hand investigation using the light microscope and prepared slides to gather information to estimate the size of red and white blood cells and draw scaled diagrams of each   analyse information from secondary sources to identify current technologies that allow measurement of oxygen saturation and carbon dioxide concentrations in blood and describe and explain the conditions under which these technologies are used   analyse information from secondary sources to identify the products extracted from donated blood and discuss the uses of these products   analyse and present information from secondary sources to report on progress in the production of artificial blood and use available evidence to propose reasons why such research is needed   choose equipment or resources to perform a first-hand investigation to gather first-hand data to draw transverse and longitudinal sections of phloem and xylem tissue

Extract from Biology Stage 6 Syllabus (Amended October 2002) © Board of Studies, NSW
[Edit: 2 May 09]

Prior learning: Science Stages 4 - 5 syllabus: Outcomes 4.8 (content 4.8.4 b, c, d and e: multicellular organisms).

Preliminary course module 8.3 (sub sections 4, 5 and 6)

Background: Blood is the transport medium of mammals. It maintains the internal environment of all organs as it supplies material to every cell in the body and removes the unwanted substances that cannot be allowed to accumulate in cells. From the Preliminary course, recall that blood consists of 55% plasma, a straw-coloured liquid of which 90% is water. The other components of the blood are red and white blood cells and platelets.

Red blood cells are unique in that they do not contain a nucleus and have a biconcave shape. They are much smaller than white blood cells and more numerous. They contain the protein haemoglobin, a complex protein molecule consisting of four polypeptides, each containing an iron atom. The iron atom has an affinity for oxygen molecules. When haemoglobin is combined with an oxygen molecule, it is called oxyhaemoglobin.

Plants carry dissolved mineral nutrients in the xylem vessels and carry food (mostly glucose) in phloem tubes.

 

identify the form(s) in which each of the following is carried in mammalian blood:

• carbon dioxide
• oxygen
• water
• salts
• lipids
• nitrogenous waste
• other products of digestion

• Most carbon dioxide enters the red blood cells and is combined with water to form bicarbonate ions (HCO₃^-). Some is attached to haemoglobin molecules in red blood cells and a small percentage is transported in plasma as dissolved CO₂.
   
• Oxygen attaches itself to haemoglobin in the red blood cells, becoming a complex called oxyhaemoglobin (HbO₂).
   
• Liquid water is the solvent making up 90% of the plasma.
   
• Salts are carried as dissolved ions in the plasma.
   
• Lipids are carried with phospholipids and cholesterol in a protein coated package called a chylomicron.
   
• The nitrogenous wastes (urea, uric acid and creatinine) are dissolved in blood plasma.
   
• Other products of digestion, such as sugars, amino acids and various vitamins, are transported in the plasma.

perform a first-hand investigation to demonstrate the effect of dissolved carbon dioxide on the pH of water

• Perform your investigation, making sure you take readings of the initial pH of the distilled water.

perform a first-hand investigation using the light microscope and prepared slides to gather information to estimate the size of red and white blood cells and draw scaled diagrams of each

• To perform this investigation, you need to go through the steps for using a light microscope that you learned in the Preliminary course. Revise how to focus the microscope using low power (LP) first and then going to high power (HP).
   
• To gather information to estimate the size of blood cells, you will have to learn an appropriate technique for estimating. If your school has a grid slide, put it on the stage of the microscope and focus the microscope on low power. If you don't have a grid slide, use a plastic ruler so you can see the millimetre lines under the microscope.
   
• Note what size each grid is and when you focus the slide, count the number of grids across the diameter of the field of view. If there is part of one grid, estimate what fraction it is. If the grids are one millimetre apart, you might estimate that the diameter of the LP field of view is 1.5 mm, which is 1500 μm.
   
• When you go from LP to HP, you see less in the field of view. The diameters of the low and high power fields are inversely proportional to their magnification. If your LP magnifies 100X and the HP magnifies 400X, you will see one quarter of the field that you saw in HP than you saw in LP, so if you saw twenty cells across the diameter under LP, you would see approximately five of the same cells under HP.
   
• Once you have focused on some red blood cells under LP, estimate the size using the above method, then turn to HP and see if you agree with the first estimate. Draw several cells and draw a scale bar.
   
• Once you have drawn a few red blood cells, browse over the field looking for some white blood cells. Look for larger cells with clearly defined and stained nuclei and draw several. Draw a scale bar to indicate the size of the cells.

  Red blood cells in a blood vessel Department of Biomedical Engineering and Computational Science (BECS) Centre of Excellence in Computational Complex Systems Research, Helsinki, Finland

  Blood cells Wadsworth Center, New York State Department of Health
  

explain the adaptive advantage of haemoglobin

• Oxygen is not very soluble in water and so cannot be carried efficiently dissolved in the blood plasma. Most of the oxygen is carried by haemoglobin in the red blood cells. Thus, the presence of haemoglobin in red blood cells in blood increases the blood's capacity to carry oxygen. Organisms with blood (containing haemoglobin) are able to deliver oxygen to cells more efficiently than other organisms with blood that has no haemoglobin. The net effect is that these organisms are more effective operators in a given environment than their competitors.
   
• At high altitudes, blood is not able to absorb as much oxygen as at sea level. The human body adapts to what is effectively oxygen deprivation by initially increasing heart rate, breathing rate, then the number of red blood cells (more haemoglobin), then density of capillaries.

analyse information from secondary sources to identify the products extracted from donated blood and discuss the uses of these products

A good source of information on the components of blood is the Australian Red Cross, Blood Bank Service web site:

Different Donation Types Australia

Uses of blood can be found at Science clarified , USA 2007. Scroll down to Separation of blood components.
(These website were last accessed 2 June 2009).

• Analyse the information so that it fits under the headings Products of blood and Use of the product.

analyse information from secondary sources to identify current technologies that allow measurement of oxygen saturation and carbon dioxide concentrations in blood and describe and explain the conditions under which these technologies are used.

The Internet is more likely to carry information about current technologies than reference books. Here is a place to begin.

Pulse oximetry Nuffield Department of Anaesthetists, University of Oxford, UK (This site last accessed 12 June 2008)

Assess the reliability by comparing with information from different sources. Information from an organisation, like the World Federation of Societies of Anaesthesiologists, is likely to be more reliable than information from an individual who is not affiliated to any organisation.

• Analyse the information to make a generalisation about current technologies being used. You will also need to describe and explain the conditions under which the different technologies are used. The information from the Internet will come from different countries. Some countries may be ahead of other countries so making generalisations will avoid the problem of some information contradicting other information. However, you will need to keep in mind the above point about reliability.

analyse and present information from secondary sources to report on progress in the production of artificial blood and use available evidence to propose reasons why such research is needed

An Internet search on artificial blood will provide links, which include the history, current research and uses of artificial blood substitutes in blood transfusions. Some sites you could start with are:

Perflurochemicals McGill Faculty of Medicine, Montreal, Quebec, Canada

Modified haemoglobin McGill Faculty of Medicine, Montreal, Quebec, Canada (Last accessed 22 December 2005)

• Analyse the information you have gathered to identify examples of the interconnectedness of ideas concerning artificial blood. In your analysis, consider the two types of artificial blood, one based on chemically modified haemoglobin and the other based on perfluoroorganic compounds.
   
• You could present the information as a speech to fellow students.
   
• Use the available evidence to propose reasons why research on artificial blood is needed.

compare the structure of arteries, capillaries and veins in relation to their function

• Arteries carry blood away from the heart under high pressure and so must have a structure that can withstand the pressure. They have thick, but elastic walls, made up of three tissue layers: endothelium as a lining, smooth muscle to contract the vessel and connective tissue to allow for expansion. Arteries do not pump blood.

  Veins carry blood back toward the heart. They carry the same quantity of blood as the arteries but not at the same high pressures. Veins have the same three layers as the arteries: endothelium, smooth muscle and connective tissue. However, the layers are not as thick. The veins also contain valves that prevent the backflow of blood.

  Capillaries have walls that are only one endothelium cell thick, as they have to allow diffusion of materials through their wall to reach the cells found in the tissues in which the capillary is located.

describe the main changes in the chemical composition of the blood as it moves around the body and identify tissues in which these changes occur

• The blood circulates through two systems in the body: the pulmonary system and the systemic system.
   
• In the pulmonary system, blood flows from the heart to the lungs and then back to the heart. Blood travels in the pulmonary artery from the right ventricle to the lungs where carbon dioxide is released into the alveoli of the lungs. This is then ultimately released out of the body. Oxygen is picked up from the alveoli and diffused into the red blood cells to then be taken back to the heart. So via the pulmonary system, carbon dioxide is decreased and oxygen levels increased.
   
• In the systemic system, blood flows from the heart to the rest of the body, except the lungs, and then returns. The left ventricle pumps oxygenated blood to the rest of the body, and as this blood circulates in capillaries, oxygen is delivered to the cells and carbon dioxide is picked up. Other waste products, such as urea, are also picked up from the liver and transported in the blood to the kidneys. Blood flowing to the small intestines collects the products of digestion and transports them to the liver. Glucose is circulated in the blood stream to all cells in the body for respiration. Deoxygenated blood returns to the heart via the inferior and superior vena cava.

outline the need for oxygen in living cells and explain why removal of carbon dioxide from cells is essential

• Cells require oxygen in the process of respiration: Glucose + oxygen carbon dioxide + water + energy (in the form of ATP)
   
• Carbon dioxide is a waste product and must be removed to maintain the normal pH balance of the blood. By removing excess carbon dioxide, it prevents a build up of carbonic acid, which causes the lowering of the pH, and therefore increasing breathing rate and depth. Carbonic acid forms when carbon dioxide dissolves in water. At normal levels, (after excess removal of carbon dioxide) the carbon dioxide - bicarbonate ion (HCO₃^-) equilibrium is an important mechanism for buffering the blood to maintain a constant pH.

choose equipment or resources to perform a first-hand investigation to gather first-hand data to draw transverse and longitudinal sections of phloem and xylem tissue

• Talk to your teacher to help you decide which are the best plants to use for this activity. Obtain the plants by students bringing them from home or if necessary obtain them from a nursery.
   
• Choose appropriate equipment, which should include sharp instruments so you can do a clean cut, without tearing the plant material. The instruments you choose will depend on their availability at school.
   
• Perform the investigation by doing transverse and longitudinal cuts on stems of the chosen plants. One way of doing this is to embed the plant in material like a carrot or potato or if available in wax.
   
• Put the cut pieces in water, then mount them on a slide. You may choose to stain the material. If so, decide what is an appropriate stain. Cover the slide with a cover slip.
   
• Gather first-hand data by drawing the samples, while looking under the LP magnification of your microscope. It is better to draw just a few cells and do them as accurately as possible than draw many cells that don't show clearly the differences between xylem and phloem vessels.

describe current theories about processes responsible for the movement of materials through plants in xylem and phloem tissue

• Xylem: The transpiration-cohesion-tension mechanism is currently the theory that accounts for the ascent of xylem sap. This sap is mainly pulled by transpiration rather than pushed by root pressure. Cohesion is the “sticking” together of water molecules so that they form a continuous stream of molecules extending from the leaves down to the roots. Water molecules also adhere to the cellulose molecules in the walls of the xylem. As water molecules are removed by transpiration in the leaf, the next molecule moves upwards to take its place, pulling the stream of molecules continuously along. This is passive transport.
   
• Phloem: The pressure-flow mechanism (or Source to Sink) is a model for phloem transport now widely accepted.

  The model has the following steps.

  Step 1: Sugar is loaded into the phloem tube from the sugar source, e.g. the leaf (active transport)

  Step 2: Water enters by osmosis due to a high solute concentration in the phloem tube. Water pressure is now raised at this end of the tube.

  Step 3: At the sugar sink, where sugar is taken to be used or stored, it leaves the phloem tube. Water follows the sugar, leaving by osmosis and thus the water pressure in the tube drops.

  The building up of pressure at the source end, and the reduction of pressure at the sink end, causes water to flow from source to sink. As sugar is dissolved in the water, it flows at the same rate as the water. Sieve tubes between phloem cells allow the movement of the phloem sap to continue relatively unimpeded.