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Answer 1

Oxygen : when oxygen diffuses from the lungs respiratory surface into the blood, 98.5% of it is combined with haemoglobin. Red blood cells are ideally adapted to carrying oxygen, they have no nucleus (which allows for maximum transportation space), they contain haemoglobin (which has an affinity for oxygen, making it easier to transport), the shape of blood cells (biconcave) allows for maximum surface area to volume ratio, allowing for maximum diffusion. When blood combines with haemoglobin it forms oxyhaemoglobin, which has a bright red colour as opposed to the darker red of deoxygenated blood. Arteries carry bright red oxygenated blood and veins carry dark red deoxygenated blood.

Carbon Dioxide: most carbon dioxide is transported in the form of hydrogen carbonate ions, which are formed in red blood cells but carried in plasma. The remaining carbon dioxide is carried either as dissolved in the plasma or combined with haemoglobin. Carbon dioxide produced as a waste product, diffuses into the bloodstream where some of it dissolves into the plasma and the rest combines with water to form carbonic acid. It is not ideal for the carbon dioxide to dissolve into the plasma, as that would alter to pH of the blood as a result most of it enters the red blood cells where there are two possible scenarios.

1) Most of the carbon dioxide mixes with water to form carbonic acid which is them rapidly converted into hydrogen carbonate ions. These hydrogen carbonate ions move out of the red blood cells and into the plasma. This can be summarised as

Carbon dioxide + water  carbonic acid + hydrogen carbonate + buffered hydrogen ions

2) Some carbon dioxide binds to haemoglobin forming carbaminohaemoglobin. Unlike with oxygen, which bonds to the iron part of the haemoglobin, carbon dioxide bonds to the amino group - this reaction is also reversible.

Water and Salts : Water is the medium of transport for all substances in the body, it forms the basis of cytoplasm in all cells, along with the interstitial fluids (fluids between cells), blood and lymph (transport fluid in animals). Salts are carried in blood as ions dissolved in the plasma, for example sodium chloride is transported as sodium ions and chloride ions. Ionic solutions are known as electrolytes, as they can conduct electricity.

Lipids and other products of digestion: carbohydrates become glucose, proteins become amino acids, lipids become fatty acids and glycerol, and nucleic acids become nucleotides.

Glucose and amino acids are water soluble so they are transported in the bloodstream dissolved in the plasma, along with other soluble substances nitrogenous bases, vitamins and glycerol, absorbed from the digestive tract.

Lipids are harder to transport, as they are not water soluble and cannot be carried in the plasma. Most of the lipids are packaged into the lymphatic system and then into the blood stream. These are transported in small spherical particles called micelles, these are transported in a colloidal solution - not quite a true solution and a suspension. The lipids are absorbed into the lacteals inside of the villi of the small intestine, rather than directly into the bloodstream like water soluble substances. During absorption, they are processed to form micelles called chylomicrons, it is transported in this form. The lacteals, carrying the chylomicrons, are a part of the lymphatic circulation and eventually these join into the main blood supply.

Nitrogenous wastes: Nitrogenous wastes are harmful products formed in the body by the breakdown of proteins. These substances are transported in diluted form, from the cells to areas in which they can be excreted from the body. Nitrogenous wastes in the form of ammonia, urea, uric acid and creatinine are all carried dissolved in the blood plasma.

Haemoglobin is a protein made up of 4 polypeptide chains, each bonded is bonded to a haem (iron) group.

Haemoglobin is able to increase the oxygen-carrying capacity of blood. Due to the fact that each haemoglobin molecule contains 4 haem units, each molecule can bond to 4 oxygen molecules at once. Hence much more oxygen can be transported around the blood in haemoglobin, rather than being dissolved in plasma.

Another advantage is that once 1 oxygen molecule binds to the haemoglobin its ability to bind more oxygen molecules increases. The bonding of each oxygen molecule slightly alters the shape of the haemoglobin, making it easier for subsequent molecules to bind to it. This increases the rate and efficiency of oxygen uptake. As a result, a small increase in oxygen concentration at the lungs results in a large increase in oxygen saturation in the blood. For example during exercise.

Another advantage is that haemoglobin's capacity to release oxygen increases in the presence of carbon dioxide. Metabolizing cells produce carbon dioxide, which combines with water to form carbonic acid, which lowers the pH. Haemoglobin has a lower affinity for oxygen at a lower pH, as a result oxygen is released in areas where it is needed (metabolizing cells)

Once haemoglobin releases oxygen it has an increased ability to pick up carbon dioxide.

The fact that haemoglobin is enclosed inside red blood cells, means that it doesn't disturb the osmotic balance of the blood plasma.

The transport system within the body is involved in moving gases (carbon dioxide and oxygen), nutrients, wastes and hormones

The changing chemical composition of blood

The difference in the chemical concentration of blood entering or leaving an organ, depends on the function of the organ. External gaseous exchange occurs in the lungs (carbon dioxide is released from the blood and oxygen in picked up). Internal gaseous exchange occurs in all organs of the body and is the result of cellular respiration (oxygen combines with glucose to make energy, with carbon dioxide as a waste product). Absorption of nutrients into the bloodstream takes place in the digestive tract (particularly the small intestine). Nitrogenous waste is produced in the liver and is excreted by the kidneys. Hormones are secreted into the blood by glands and then travel to where they are required and used up by the target tissue.

Change in carbon dioxide and oxygen content of blood

The lungs are the organs of external gaseous exchange. Deoxygenated blood arrives at the lungs and it releases carbon dioxide and picks up oxygen. The haemoglobin binds with the oxygen, forming oxyhaemoglobin. Most oxygen (98.5%) travels as oxyhaemoglobin only 1.5% travels as dissolved in the plasma. The oxygenated blood is returned to the heart where it is pumped to other tissues of the body, where oxygen is released and used for cellular respiration.

Internal gaseous exchange occurs in the tissues of the body as a result of cellular respiration. Cells release carbon dioxide which diffuses into the capillaries in the tissues. When carbon dioxide enters the blood, some dissolves into the plasma, some is carried by haemoglobin, the rest is transported as bicarbonate ions. This forms the deoxygenated blood returning back to the lungs.

Changes in other chemicals in blood

- An increase in oxygen and a decrease in carbon dioxide is evident when the blood passes through the lungs

- A decrease in oxygen and increase in carbon dioxide is noted when the blood passes through any organ other than lungs

- An increase in digestive end products is evident in blood that passed through an organ involved in absorbing digested food (small intestines). These products travel into the bloodstream directly into the liver

- A decrease in digestive end products (glucose, fatty acids, amino acids) is evident as blood leaves the liver, as it is the center of food metabolism

- An increase in nitrogenous wastes as the blood leaves the liver, as it is the organ in which proteins are de-aminated

- A decrease in nitrogenous wastes as the blood passes through the kidneys, since they filter the wastes and excrete them

Oxygen is necessary for cellular respiration (energy is obtained from glucose). Energy is needed for life-sustaining processes such as growth, repair of tissues, movement, reproduction and excretion. Even though glucose is high in energy, it must be converted into a form which can be used by living cells. This process involves oxygen combining with the glucose, through a series of enzyme-controlled reactions, through which chemical energy is released as ATP. This is known as the oxidation of glucose.

Carbon dioxide is produced in all living cells as a waste product of chemical respiration. It must be removed from the cells, to prevent a change in the pH of cells, the bloodstream and the body. When carbon dioxide reacts with water, carbonic acid is produced. If carbonic acid is built up it is toxic and it can change the pH of cells and bloodstream - thus affecting the homeostatic balance within an organism. A low ph reduces enzyme efficiency, which affects cell functioning - hence the removal of carbon dioxide is essential as it affects the functioning of enzymes.

2 2 6 Describe Current Theories About Processes Responsible

The function of xylem and phloem in transport

Is mainly to carry materials for photosynthesis to the cells and move the products away from the cells to other parts of the plant. In small plants this may be achieved through diffusion and active transport however in larger plants specialized vascular tissue has developed to serve this function. The vascular system consists of xylem and phloem and the movement of materials from one part of the plant to another is known as translocation.

Xylem - transpiration stream theory

The transpiration stream in xylem occurs due to physical forces that result in water and ions being moved by passive transport. A column of water is sucked up by the stem, by the evaporation pull of transpiration - this is known as the transpiration stream. Once water has been absorbed into the roots (osmosis) along with mineral ions (diffusion and active transport), these substances move into the xylem. A small amount of root pressure results from the continual influx of ions and water - forcing the solution upwards, this is insufficient by itself. Most of the upward movement is a result of the transpiration stream - which is water is drawn up the xylem to replace water which is lost due to transpiration at the leaves.

Evidence for this theory :

- Xylem are hollow and narrow - very little resistance to the flow of water

- The physical properties of water contribute to a continuous stream. Adhesive forces (between water and xylem walls) lead to capillarity (water rises up the narrow bore of the xylem) and cohesive forces (between water molecules) form a continuous stream

- A concentration gradient exists. The surface of the leaf has high osmotic pressure (low water concentration) due to transpiration. The centre of the lead has a low osmotic pressure

Explanation

Water loss at the surface of the leaf results in the osmotic movement of water across from adjacent internal cells into those that have just lost some water. This osmotic flow continues across the leaf - until it reaches the xylem tissue. When water molecules leave the xylem and move along the concentration gradient, this creates a tension in the column of water rising up the xylem. Due to the properties of adhesion and cohesion the water column does not break and so the entire water column is not pulled upwards. The combination of adhesive and cohesive forces, together with the suction pull of the transpiration - creates the transpiration stream. Mineral ions dissolved in the water are carried along by the transpiration stream and can move out by active transport - to reach the tissues where they are needed.

Phloem - pressure flow theory

Translocation in phloem tissue moves products of photosynthesis by active transport. The flow of materials in phloem is an active process that requires energy. The mechanism of flow is driven by an osmotic pressure gradient, generated by difference in sugar and water concentrations. It involves the active loading of sugar into the phloem at one end (source)and then the active unloading from the phloem into surrounding tissues at the other end (sink). The loading of sugar into the phloem attracts water to flow in (due to the differences in osmotic pressure) and the offloading at the sink causes the water to flow out of the phloem.

Loading at the source

There are two theories for the loading of amino acids, sucrose and other minerals at the source.

- Symplastic loading - sugars and other nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through plasmondesmata (strands of cytoplasm that pass through pits into the cell walls)

- Apoplastic loading - sugars and other nutrients move along a pathway through the cell walls until they reach the sieve element. They then cross the cell membrane to enter the phloem tube. These sugars pass into the sieve cell by active transport

Offloading at the sink

Materials flow to the sink. At the sink (roots or flowers) sugars and material are removed from the phloem by active transport. As sugars move out of the phloem, they draw water out with them (osmosis). This results in a lower osmotic pressure (due to the higher water concentration) in the phloem at the sink

Pressure flow

This difference in osmotic pressure between the source and the sink in the phloem drives the phloem sap to flow. The direction of the flow depends on where the sink areas are in relation to the source. Water can move into the phloem by osmosis at any point along the gradient. The flow is continuous as sucrose is continually added at one end and removed at the other.