Gas Exchange and Transport (SCR)

Question 1

What is gas exchange?

Answer 1

All organisms absorb one gas from the environment and release another one. This is gas exchange. Plants absorb carbon dioxide for use in photosynthesis, and release oxygen produced in the process. Humans absorb oxygen for cell respiration and release the carbon dioxide produced.

Unicellular and other small organisms have a large surface area to volume ratio. They can therefore use their outer surface for gas exchange. In larger organisms, the surface area to volume ratio is smaller, so the outer surface of the organism cannot carry out gas exchange rapidly enough. A specialised gas exchange surface is required that is much larger than the outer surface, for example, alveoli in lungs or the spongy mesophyll in leaves.

Question 2

Features of gas exchange surfaces

Answer 2

Gas exchange happens at a surface where the cells of the organisms are exposed to the environment. In terrestrial organisms the gas exchange surface is where cells are exposed to air as in the lungs of a mammal. In aquatic organisms, it is where the cells are exposed to water as in the gills of a fish.

Gases are exchanged by diffusion across the surface. Because the molecules of oxygen and carbon dioxide move randomly, diffusion is a relatively slow process. To ensure that exchange is rapid enough for an organisms needs, its gas exchange surfaces must have these properties:

1. permeable- oxygen and carbon dioxide can diffuse across freely
2. large- the total surface are is large in relation to the volume of the organism
3. moist- the surface is covered by a film of moisture in terrestrial organisms so gases can dissolve
4. thin- the gases must diffuse only a short distance, in most cases though a single layer of cells

Question 3

The importance of concentration gradients in gas exchange

Answer 3

Gases and other substances diffuse is there is a concentration gradient. For example, carbon dioxide diffuses from the air into photosynthesising leaf cells because the carbon dioxide concentration of the cells is lower. Diffusion tends to reduce concentration gradients, which could decrease the rate and eventually stop gas exchange if the concentrations become equal. For gases to continue to diffuse across exchange surfaces, concentration gradients must be maintained. In small, aerobically respiring organisms, cell respiration maintains concentration gradients. Oxygen is continuously used and carbon dioxide is produced, so the oxygen concentration within the organism remains lower than outside and the carbon dioxide concentration remains higher.

Question 4

How do large multicellular animals maintain concentration gradients?

Answer 4

In large multicellular animals with a specialised organ for gas exchange (lungs or gills, pumping is required to maintain concentration gradients.

• blood is pumped through the dense capillary networks close to the gas exchange surface. Due to aerobic respiration in the animal, blood arriving at the surface has a low concentration of oxygen and a high concentration of carbon dioxide
• Air or water adjacent to the gas exchange surface is replaced by the process of ventilation. Mammals pump air in and out of the lungs to maintain high enough concentrations of carbon dioxide. Fish pump fresh water over their gills and then out through the gill slits. This one way flow of water combined with blood flow in the opposite direction ensures that the oxygen concentration in the water adjacent to the gills remains high and the carbon dioxide concentration remains low.

Question 5

Lung structures and ventilation in mammals

Answer 5

The lungs are in the thorax. Air can only get into or out of the thorax through the airways. The airways used to ventilate the lungs consist of the nose, mouth, trachea, bronchi and bronchioles.

If gas is free to move, it will always flow from regions of higher pressure to regions of lower pressure. During ventilation, muscle contractions cause pressure changes inside the thorax that pull extra air into the alveoli and then push it out again. The muscles causing this are:

• the diaphragm that divides the thorax and abdomen
• muscle in the front wall of the abdomen
• intercostal muscle between the ribs, in two layers (internal and external) that are antagonistic.

Question 6

How does inhalation (inspiration) work?

Answer 6

• the external intercostal muscles contract, moving the ribcage up and out
• the diaphragm contracts, becoming flatter and moving down
• these muscle movements increase the volume of the thorax
• the pressure inside the thorax therefore drops below atmospheric pressure
• air flows into the lungs from outside the body until the pressure inside the lungs rises to atmospheric pressure

Question 7

How does exhalation (expiration) work

Answer 7

• the internal intercostal muscles contract, moving the ribcage down and in
• the abdominal muscles contract, pushing the diaphragm up into a dome shape
• these muscle movements decrease the volume of the thorax
• the pressure inside the thorax therefore rises above atmospheric pressure
• air flows out from the lungs to outside the body until the pressure inside the lungs falls to atmospheric pressure

Question 8

How are the lungs adapted for efficient gas exchange?

Answer 8

• airways for ventilation of each lung, consisting of branching bronchioles, ending in alveolar ducts, each of which leads to a group of five or six alveoli
• large surface area for gas exchange- provided by having about 300 million alveoli in a pair of adult lungs. One alveolus is only 0.2-0.5 mm in diameter so only provides a small surface area for gas exchange, but because there are so many of them, the total area is very large: about 40 times greater than the outer surface of the body.
• extensive capillary beds- the surface area of the basket-like networks of blood capillaries around the alveoli is almost as large as that of the alveoli
• short distance for diffusion- both the alveolus wall and adjacent capillary walls are single layers of extremely thin cells. Air and blood are therefore a very short distance apart. The distance apart. The distance for diffusion of O2 and CO2 is less than a micrometre
• moist surface with surfactant- a fluid is secreted by cells in the alveolus wall that keeps the lining of the alveolus moist, allowing oxygen to dissolve. The fluid contains a pulmonary surfactant, that reduces the surface tension and prevents the water from causing the sides of the alveoli to stick together when air is exhaled from the lungs. This helps prevent collapse of the lung`
  (Type II pneumocytes secrete surfactant)
• Type I pneumocytes allow O2 and CO2 exchange from alveolar space to the bloodq

Question 9

Measure of lung volumes by spirometry

Answer 9

Lung volumes are measured as part of tests for general health and to help diagnose conditions such as asthma, COPD or cystic fibrosis.

• ventilation rate is the number of times that air is drawn in or expelled per minute
• tidal volume is the volume of fresh air inhaled or the volume of stale air exhaled with each ventilation
• vital capacity is the total volume of air that can be exhaled after a maximum inhalation
• inspiratory reserve volume is the amount of air a person can inhale forcefully after normal tidal inhalation
• expiratory reserve volume is the amount of air a person can exhale forcefully after normal tidal exhalation

A spirometer is a device used to measure lung volumes. A simple version can be constructed using a bell jar and a tube. It is not safe to use this apparatus for repeatedly inhaling air as the carbon dioxide concentration will rise too high.

Doctors use specially designed electronic spirometers that measure flow rate into and out of the lungs and then use data logging software to deduce lung volumes. There are many different designs

Tidal volume is measured by breathing into the spirometer, three or more times, to check the readings are consistent

Vital capacity is measured by breathing in deeply and as fast as possible and then breathing out as fast and as forcefully as possible until the lungs are empty.

Question 10

How are leaves adapted for gas exchange?

Answer 10

Chloroplasts need a supply of carbon dioxide for photosynthesis. The oxygen produced during the process of photosynthesis must be removed. A large area of moist surface is required over which carbon dioxide can be absorbed and oxygen oxygen excreted, without excessive water loss.

The following leaf adaptations provide this moist surface area:

• Waxy cuticle- the upper and lower surface of leaves is covered in a layer of waterproof wax, secreted by the epidermis cells. It reduces water loss but also prevents movement of carbon dioxide and oxygen.
• Guard cells- there are pairs of guard cells in the epidermis, which can change their shape either to open up a pore or close it. The pore is called a stoma and it allows carbon dioxide and oxygen to pass through. The guard cells usually close the stomata at night when photosynthesis is not occuring and gas exchange is not required. Stomata also close during water stress when plants might die from dehydration.
• Air spaces- the stomata connect the air outside to a network of air spaces in the spongy mesophyll of the leaf. Carbon dioxide and oxygen can diffuse through these air spaces
• Spongy mesophyll- the inner tissue of the leaf with extensive air spaces. It provides a very large total surface area of permanently moist cell walls for gas exchange. Carbon dioxide in the air spaces dissolves and diffuses into the cells. Oxygen diffuses from the cells to the air. Photosynthesis maintains the concentration gradients
• Veins- inevitably, there is some loss of water by evaporation from the moist spongy mesophyll cell walls and diffusion out through the stomata. This is replaced by water supplied by the xylem vessels, located in the leaf veins

Question 11

What is transpiration?

Answer 11

Water molecules evaporate when hydrogen bonds between them break. The molecules separate from each other and become water vapour molecules in air. The opposite process is condensation, where water vapour molecules join others to become liquid water. If air is very humid and the number of water molecules evaporating is equal to the number condensing, the air is saturated with water vapour.

Air spaces inside the leaf are usually saturated (or close to). Water vapour molecules diffuse out of the leaf through the stomata unless the stomata are closed or the air outside the leaf is already saturated. This causes the humidity of the air spaces to drop below the saturation point, so more water evaporates from the permanently moist spongy mesophyll cell walls. Loss of water vapour from the leaves and stems is transpiration. Transpiration rates are affected by environmental factors.

Question 12

What environmental factors affect transpiration rate?

Answer 12

• Temperature (positive correlation): at higher temperatures there is more energy available to break hydrogen bonds between water molecules , so the evaporation rate is higher and air hold more water vapour molecules before becoming saturated.
• Humidity (negative correlation): the higher the humidity of the air, the smaller the concentration gradient of water vapour between air spaces inside the leaf and the air outside, so the lower the rate of diffusion. There is no transpiration if the air outside the leaf is saturated with water vapour.
• Wind: in still conditions, transpiration is restricted by formation of pockets of saturated air near the stomata, even if the air further away is drier. Air movements prevent this, so increase transpiration, though stomata close in strong winds, so transpiration rate drops.
• Light intensity: when there is a higher light intensity, the rate of transpiration is higher, this is because more photosynthesis can occur, so more stomata are open, increasing the rate of transpiration

Question 13

Difference between adult and foetal blood?

Answer 13

Humans produce foetal haemoglobin before birth and adult haemoglobin afterwards. During pregnancy a foetus obtains oxygen via the placenta. Oxygen dissociates from haemoglobin in maternal blood in the placenta and binds to haemoglobin in foetal blood. This can only happen because foetal haemoglobin has a stronger affinity for oxygen than adult haemoglobin at any partial pressure of oxygen. This means foetal haemoglobin is more saturated with oxygen than adult haemoglobin. At birth, a baby still has red blood cells with foetal haemoglobin. It takes several months for all red blood cells carrying foetal haemoglobin to be replaced with cells carrying adult haemoglobin.

Question 14

What is Bohr shift?

Answer 14

Increases in carbon dioxide (as a result of it being produced at respiring tissues) concentration reduce the affinity of haemoglobin for oxygen by two mechanisms:

1. there is a positive correlation between pH and affinity of haemoglobin for oxygen. CO2 reduces pH (CO2 + H2O →H+ + HCO3-) so decreases affinity for oxygen, as a lower pH alters the tertiary structure of haemoglobin into one with a lower affinity for oxygen.
   Therefore haemoglobin dissociates oxygen into respiring tissues
2. CO2 binds reversibly to the polypeptides in haemoglobin, producing carbaminohaemoglobin, which has a lower affinity for oxygen than haemoglobin

Reduced affinity of haemoglobin for oxygen in high CO2 concentrations shifts the oxygen dissociation curve to the right
(Bohr shift)

The Bohr shift promotes release of oxygen in actively respiring tissues, such as contracting muscle, where high blood CO2 concentration causes low pH and haemoglobin to converted to carbaminohaemoglobin.

The Bohr shift allows blood to be fully oxygenated in the lungs where blood CO2 concentrations are low, so pH is high and carbohaemoglobin is converted back to haemoglobin

Question 15

What happens to haemoglobin in the lungs/ areas of low oxygen levels (sea or altitude)

Answer 15

There is an uptake of oxygen
- Haemoglobin needs to have a high affinity for oxygen
- Curve shifts left (Bohr shift)
- So haemoglobin can become saturated at lower partial pressures of oxygen
- So can get enough oxygen into blood and to cells/ tissues for respiration

Question 16

What happens to haemoglobin in respiring tissues?

Answer 16

Oxygen dissociates
- Haemoglobin needs to have a low affinity for oxygen
- Curve shifts right (Bohr shift)
- So haemoglobin can dissociate (unload) at higher partial pressures of oxygen
- So more oxygen is released faster to tissues that are respiring more

Question 17

How do oxygen dissociation curves work?

Answer 17

• each subunit in haemoglobin has a haem group to which one oxygen molecule can bind reversibly, so one haemoglobin molecule can transport up to four oxygens
• Binding is cooperative, because binding of oxygen to any haem group causes conformational changes that increase the affinity for oxygen in the other haem groups. The two most probable states for haemoglobin are with four oxygen molecules bound, or none.
• Blood in which all haemoglobin molecules are carrying four oxygens is 100% saturated. If no oxygen is bound to any of the haemoglobin molecules, it is 0% saturated. Any saturation level from 0→100% is possible
• Oxygen concentration is measured in partial pressures with kPa as the pressure units. There is a positive correlation between partial pressure of oxygen and % saturation of haemoglobin. In human adults, haemoglobin reaches 100% saturation when partial pressures of oxygen reaches 10kPa. This happens as blood flows through capillaries around the alveoli
• 100% oxygenated blood leaving the lungs is carried to all other organs of the body, where due to aerobic respiration the partial pressures of oxygen is below 10kPa, so oxygen dissociates from haemoglobin and diffuses into the tissues
• because of cooperative binding, oxygen saturation of haemoglobin is not directly proportional to oxygen concentration. Instead, it changes from fully saturated to unsaturated over a relatively narrow range of oxygen concentrations, ensuring rapid dissociation of oxygen tissues where it is needed for aerobic respiration

Question 18

How are capillaries adapted for exchange processes?

Answer 18

1. Large surface area:
   Capillaries are the narrowest blood vessels with a diameter of around 10 micrometres, so blood cells have to pass through in single file, this means that the flow of blood slows and so there is more time for diffusion (and a short distance between RBC’s and cells, so faster diffusion). Capillaries branch and rejoin repeatedly to form a capillary network with a huge total length. This gives a very large total surface area for exchange processes
2. Thin walls with poresThe capillary wall consists of one layer of endothelium cells which are a very thin and permeable layer. The layer of cells is supported by a coating of extracellular fibrous proteins. The basement membrane acts as a filter that allows small or medium sized particles to pass through but not macromolecules (such as proteins). Fluid leaks out of capillaries through the basement membranes because blood pressure is higher than pressure in the surrounding tissue and because there are pores between epithelium cells. The fluid passing out (tissue fluid) contains oxygen, glucose and other substances in blood plasma, but not plasma proteins. The tissue fluid flows between cells, allowing them to absorb useful substances and excrete waste products. The fluid flows between cells, allowing them to absorb useful substances and excrete waste products. The fluid then re-enters capillaries where pressure inside them has dropped, near where blood is transported out of the tissue in veins (venous end).
3. FenestrationsIn some tissues, there are many particularly large pores (fenestrations) in the capillary walls. Fenestrated capillaries allow larger volumes of tissue fluid to be produced , which speeds up exchange between the tissue cells and the blood. Fenestrated capillaries in the kidney allow production of large volumes of filtrate in the first stage of urine production.

Question 19

Arteries

Answer 19

Arteries carry pulses of high-pressure blood away from the heart to every organ of the body
- thick wall
- narrow lumen
- circular in section
- inner surface corrugated
- fibres visible in the wall
- thick elastic tissue allowing them to stretch and recoil

Question 20

Veins

Answer 20

Veins carry a stream of low pressure blood from the organs back to the heart.
- thin wall
- wide lumen
- circular/ flattened in section
- inner surface smooth
- no or few fibres visible

Question 21

How are arteries adapted to carry blood away from the heart?

Answer 21

Collagen fires are tough rope-like proteins with high tensile strength. They make arteries strong enough to withstand high and variable blood pressures without bulging outwards or bursting. Each time the ventricles of the heart pump (systole), a burst of blood under high pressure enters the arteries and flows along them. The wall of the artery expands due to the high blood pressure and the elastic fibres in the wall stretch and store potential energy. When the ventricles stop pumping (diastole) and blood pressure declines, the elastic fibres recoil, applying pressure on the lumen, which helps pump blood on along the artery, and makes flow more even. Artery walls also contain smooth muscle cells. They are circular (rather than radial or longitudinal), so they make the lumen narrower when they contract (vasoconstriction) and wider when they relax (vasodilation). There is a high density of smooth muscle fibres in branches of arteries (arterioles), so flow rate of blood to tissues in each organ can be adjusted, depending on the availability and need.

Question 22

How are veins adapted to return blood to the heart?

Answer 22

Veins collect blood from all organs of the body and return it to the heart. Blood drains out of capillaries into veins continuously, so there is no pulse and blood pressure is low.

Pressure in veins can drop so low that blood flow stops and there is a risk of backflow towards the capillaries. Valves in main veins prevent this. Each valve consists of three pocket-shaped flaps of tissue. If blood starts to flow backwards, it gets caught in the flaps, filling them and closing the valve. Blood flowing towards the heart pushes the valve open.

Venous blood flow is aided by skeletal muscles adjacent to veins that become wider when they contract, pressing the vein flatter and raising blood pressure. Valves ensure that the resulting flow of blood is towards the heart.

Systole / contraction of heart pumps blood (through arteries) into veins / residual arterial pressure / negative pressure in chest due to inspiration;

Recoil of heart muscle during diastole / after contraction;
Draws blood from veins into atria;

Wide lumen little resistance / friction

Question 23

How does transpiration occur?

Answer 23

Plants transport large volumes of water from roots, where it is absorbed, to leaves. In flowering plants, xylem vessels transport most of the water, as part of the xylem sap which has low concentrations of K+ and other dissolved ions.

In transpiring leaves, water evaporates from walls of spongy mesophyll cells and the water vapour molecules then diffuse out through stomata. This is transpiration. Cell walls contain a mesh of cellulose molecules which are hydrophilic and form hydrogen bonds with water. Any water lost by evaporation from the surfaces of leaves is replaced by water drawn through pores between cellulose molecules in leaf cell walls, due to adhesion of water to cellulose and cohesion between water molecules. This is a type of capillary action.

The water drawn through interconnected leaf cell walls comes from xylem vessels in a leaf vein. Tensions (pulling forces) are generated inside xylem vessels when water is drawn out of them

Because of cohesion between water molecules, tension generated in the leaf is transmitted down the continuous columns of water in the xylem vessels to the roots. This is transpiration pull. It is remarkable because it can transport water passively against the force of gravity to the top of the tallest tree. The energy that drives it comes from the heat used in transpiration and the rope like resistance breaking the column of water due to hydrogen bonding.

Question 24

How are xylem vessels adapted for transport of water?

Answer 24

Xylem vessels are the structures in xylem tissue that are adapted for transport of water inside plants. They develop from columns of cells, arranged end to end.

• Wall thickening and lignification

Side walls of xylem vessels are thickened and the thickenings are impregnated with a polymer called lignin. This prevents the vessels from collapsing when pressure inside is very low because the plant is transpiring

• Lack of end walls and cell contents

End walls between adjacent cells are removed during development of a xylem vessel and the plasma membranes and contents of the cells break down. This creates long continuous tubes, in which flow of xylem sap is unimpeded. When mature, xylem vessels are non-living, so the flow of water through them must be a passive process

• Pits for entry and exit of water

Lignified wall thickenings are impermeable to water but there are always gaps in the thickening through which water can enter and exit. In the xylem vessels formed by young plants, the wall thickenings are in rings of helices with large gaps for water passage. In older plants, the wall thickenings are more extensive, with holes called pits through which water can pass

Question 25

what is the cohesion tension theory?

Answer 25

a. water is polar/a dipole/oxygen slightly negative and hydrogen slightly positive

b. polarity results in hydrogen bonds/attraction between water molecules

c. hydrogen bonding/polarity causes cohesion of water «molecules»

d. cohesion/hydrogen bonding allows water to withstand tension/withstand low pressure/be pulled «upwards»/moved against gravity

e. cohesion/hydrogen bonding prevents column of water «in xylem» from breaking/column of water is maintained

f. adhesion of water to xylem/vessel walls «due to hydrogen bonds»

Question 26

Distribution of tissues in stems of dicots

Answer 26

There are many variations in stem structure, Dicotyledonous (dicots) are plants with two embryo leaves in their seeds. Their stems are composed of epidermis, cortex and pith, with a ring of vascular bundles containing xylem and phloem. Some stems have a hollow centre.

• epidermis; single layer of cells with waxy cuticle on the outside to reduce water loss
• pith; large thin walled cells that fill the centre of the stem
• cortex; medium sized thin-walled cells that strengthen the stem when turgid
• phloem; thin-walled cells, some small and some very small that transport sugars and other foods
• cambium; small cells with thin walls that divide by mitosis
• xylem; wide tubular structures with thick walls, round in cross section that transport water and mineral ions
  (xylem on the inside, phloem on the outside)

Question 27

Distribution of tissues in roots of dicots

Answer 27

Vascular tissue is grouped in the centre of dicot roots, with xylem in a star shaped area and phloem between the points of the star. The outer layer of cells is epidermis, with small cells that may have root hairs protruding. Between the vascular tissue and the epidermis there is cortex, with relatively large and thin-walled cells

• epidermis; absorbs water and mineral ions from the soil using root hair cells
• cortex; bulks out the root to strengthen it and increase the surface area
• phloem; transports sugars and other foods from the leaves to the roots
• xylem; transports water and mineral ions up to the stem and leaves

Question 28

What is tissue fluid?

Answer 28

Oxygenated and nutrient rich blood enters capillary networks from arterioles (branches of arteries). Capillary walls are permeable and the blood is initially at high pressure, so some of the blood plasma leaks out, forming tissue fluid. The process that produces tissue fluid is pressure filtration. At any time, there are about 14 litres of tissue fluid in the tissues of a 70kg human, so it constitutes about 20% body mass.

Blood drains out of capillary networks into venules (vessels that unite to form veins). This happens because of low blood pressure in venules. Drainage of blood from capillaries into venules lowers blood pressure in capillaries, which allows tissue fluid that is nutrient-depleted and deoxygenated to re-enter the capillaries.

Question 29

How is tissue fluid formed?

Answer 29

• Blood flows into the capillary at the arterial end under high hydrostatic pressure
• Molecules such as water, glucose and hormones are forced out of the blood plasma in the capillary due to the high pressure (ultrafiltration) and go into the space in between the surrounding cells. Tissue fluid is formed
• Molecules such as water, ions, glucose and hormones then move from the tissue fluid and into the cells in the surrounding tissue
• Waste products, such as carbon dioxide, move from the tissue cells into the tissue fluid
• These molecules then moved from the tissue fluid into the plasma in the capillary. This is due to their being a much lower hydrostatic pressure a the venule end as the plasma has lost a lot of its fluid. The substances also move from a high concentration to a low concentration
• The majority of the tissue fluid is returned back into the capillary and their circulatory system
• 10% of the tissue fluid drains into the lymph capillaries
• The lymph is then taken to the lymph nodes and into the lymphatic system

Question 30

Substances exchanged between tissue fluid and cells

Answer 30

Tissue fluid contains oxygen, glucose and other substances in blood plasma apart from large plasma proteins. As the fluid drains through the intercellular space in a tissue, there is an exchange of substances. Cells absorb oxygen, glucose and other useful substances from the tissue fluid around them and release carbon dioxide and other waste products of metabolism. Tissue re-entering capillaries is therefore depleted in oxygen and other useful substances and has raised concentrations of waste products. The capillaries merge to form venules, which carry the waste products out of the tissue. Carbon dioxide is excreted by the lungs and other waste products are detoxified by the liver or excreted by the kidneys.

Question 31

What happens to excess tissue fluid?

Answer 31

Most of the tissue fluid released by capillaries returns to them, but some does not. Of the 20 litres of tissue fluid produced per day in an average adults body, 17 litres return to the capillaries. If the other 3 litres of fluid stayed in tissues it would cause swelling, called oedema. This is prevented by the drainage of tissue fluid into vessels of the lymphatic system.

In all tissues, there are now blind-ended lymphatic vessels with permeable walls through which tissue fluid can pass. After entering the lymphatic vessels, the fluid is known as lymph rather than just tissue fluid. The narrow vessels join up repeatedly to form wider lymphatic vessels, with valves to prevent backflow. All lymph from the left and right sides of the body is returned to the blood circulation through two large lymphatic ducts (left and right) which merge with the left and right subclavian veins. Blood in the subclavian veins flows into the vena cava and onto the right side of the heart.

90% of the tissue fluid drains to the blood circulation. The remaining 10% goes from lymph vessels to lymph nodes to the neck where lymph is then returned to the blood circulation.

Question 32

How do single circulation systems work?

Answer 32

In fish, the heart only has one ventricle (pumping chamber), which pumps deoxygenated blood to the gills. The blood can be pumped at high pressure through the gills, because the surrounding water provides support and prevents the capillaries bursting. After flowing through the gills, the blood is oxygenated and still has enough pressure to flow directly to another organ of the body. While passing through capillaries in an organ, the blood becomes deoxygenated and its pressure falls, so it must return to the heart to be repumped to the gills. Fish thus have a single circulation.

Question 33

How do double circulation systems work?

Answer 33

In mammals, the heart has two sides, right and left, each with a ventricle. The right ventricle pumps deoxygenated blood to the lungs via the pulmonary arteries. The blood must be at relatively low pressure to prevent capillaries in the alveoli from bursting.

After flowing through the alveolar capillaries, the pressure is too low for the blood to flow on to another organ of the body, so it returns to the left side of the heart via the pulmonary veins to be repumped. The left ventricle pumps oxygenated blood via the aorta to all organs of the body apart from the lungs. This requires relatively high blood pressure. The kidneys in particularly carry pressure filtration of blood, so need much higher pressure than the lungs. Oxygenated blood pumped by the left ventricle flows through capillaries in one organ of the body and is then deoxygenated and low pressure. It returns to the right side of the heart via the vena cavae for repumping to the lungs. Mammals thus have a double circulation, with the blood passing twice though the heart to make a full circuit. The heart is a double pump, delivering blood under different pressures to different organs of the body. The two circulations are known as the pulmonary and systemic systems.

Question 34

Adaptations of the mammalian heart

Answer 34

• Atria- Collecting chambers with relatively thin muscular walls, which gradually fill with blood returning in veins to the heart, and then pump the blood into ventricles
• Ventricles- Pumping chambers with thick muscular walls that pump blood out into the arteries (left side is thicker as it needs to pump around the whole body)
• Septum- The wall between the left and the right sides of the heart that ensures the heart acts as a double pump and that deoxygenated and oxygenated blood do not mix.
• Valves- Atrioventricular valves between the atria and ventricles and semilunar valves between the ventricles and the arteries ensure that blood circulates by preventing backflow
• Cardiac muscle- Muscle tissue with the special property of contracting on its own without being stimulated by a nerve
• Coronary vessels- The many capillaries in the muscular wall of the heart supply oxygen and glucose for aerobic respiration and remove waste products. The blood running through these capillaries is supplied by the coronary arteries and removed by coronary veins
• Pacemaker- A region of specialised cardiac muscle cells in the wall of the right atrium that initiates each contraction. This region is also known as the Sinoatrial node (SAN)

Question 35

what are the three main phases of the cardiac cycle?

Answer 35

1. The walls of the atria contract, pushing blood from the atria into the ventricles through the atrioventricular valves, which are open. The semilunar valves are closed, so the ventricles fill with blood.
2. The walls of the ventricles contract powerfully and the blood pressure rapidly rises inside them. This first causes the atrioventricular valves to close, preventing back-flow to the atria and then causes the semilunar valves to open, allowing blood to be pumped out into the arteries. At the same time the atria start to refill by collecting blood from the veins.
3. The ventricles stop contracting, so pressure falls inside them. The semilunar valves close, preventing back-flow from the arteries. When the ventricular pressure drops below the atrial pressure, the atrioventricular valves open. Blood entering the atrium from the veins then flows on to start filling the ventricles. The next cardiac cycle begins when the walls of the atria contract again.

Question 36

what is the pacemaker (SAN/ sinoatrial node)

Answer 36

Each heartbeat is initiated by the sinoatrial node (SAN or pacemaker) which sends an electrical impulse that spreads out in all directions through the walls of the atria, causing atrial contraction. The impulses are prevented from spreading directly into the walls of the ventricles by a layer of fibrous tissue. Instead, they travel to the ventricles via a second node and a bundle of conducting fibres that lead down to the base of the ventricles. This gives the atria time to pump blood into the ventricles before the ventricles contract. Impulses spread out through the walls of the ventricles from the base upwards, causing contraction and pumping of blood into the arteries.

Question 37

what is the process of the cardiac cycle?

Answer 37

1. The atria and ventricles are relaxed. The atria start to fill with blood. This is diastole
2. The electrical impulse is generated in the sinoatrial node
3. The electrical impulses spread across both atria, causing atria to contract. This is called atrial systole
4. Blood is forced through the atrioventricular valves from the atria and into the ventricles
5. There is a layer of non-conductive tissue that means the impulse cannot pass to the ventricles
6. The electrical impulse then stimulates the atrioventricular node
7. There is a slight delay, and then the atrioventricular node sends impulses down the bundle of His
8. The impulse conducts down the septum to the Purkinje fibres at the base of the ventricles
9. The walls of the ventricles then contract from the apex upwards. This is ventricular systole
10. The increased pressure forces the atrioventricular valves to close and semilunar to open (lub sound)
11. Blood flows through semilunar valves to arteries
12. Pressure in arteries is now higher than in the ventricles, so the semilunar valves close (dub sound)

Question 38

What are blood pressure measurements

Answer 38

Blood pressure measurements comprise of two numbers. The systolic pressure (the pressure exerted on the aorta when the left ventricle contracts) and the diastolic pressure value (the pressure on the arterial wall when the ventricle relaxes). The units for blood pressure are mmHg.

Question 39

How is heart and ventilation rate controlled (when there is high blood pressure and decrease in blood CO2)

Answer 39

Baroreceptors (in the aortic arch and carotid sinus) detect an increase in blood pressure as they are stretched.

Chemoreceptors (in the carotid bodies and aortic arch) detect a decrease in CO2 or increase in pH.

There is an increase in frequency of impulses sent to the cardiovascular centre of the medulla in the brain

The parasympathetic nervous system is activated, (Acetylcholine released, and sinoatrial node frequency is decreased)

Heart rate and the stroke volume are decreased and there is vasodilation of the blood vessels

The blood pressure decreases back to normal levels. Nerve impulses sent to the diaphragm and intercostal muscles to contract and relax less in order to decrease the ventilation rate

Question 40

How is heart and ventilation rate controlled (when there is low blood pressure and increase in blood CO2)

Answer 40

Baroreceptors (in the aortic arch and carotid sinus) detect an decrease in blood pressure as they aren’t stretched.

Chemoreceptors (in the carotid bodies and aortic arch) detect a increase in CO2 or decrease in pH.

There is a decrease in frequency of impulses sent to the cardiovascular centre of the medulla in the brain

The sympathetic nervous system is activated (noradrenaline is released, and sinoatrial node increases nerve impulse frequency.

Heart rate and the stroke volume are increased and there is vasoconstriction of the blood vessels

The heart rate increases back to normal levels. Nerve impulses are sent to the diaphragm and intercostal muscles to contract and relax more in order to increase the ventilation rate to remove the CO2.

Question 41

How is root pressure generated in xylem vessels?

Answer 41

Root cells absorb water from the soil. The water can pass by capillary action through the walls of root cells until it has nearly reached the xylem in the centre of the root. It then enters an endodermis cell by osmosis- the endodermis cells have a higher solute potential and therefore a lower water potential than the water in the soil. Water then passes from the endodermis cell into an adjacent xylem vessel- the water potential is lower than in the xylem vessel than in the endodermis cell.

When a plant is transpiring, water xylem vessels are filled by sap under tension, which has a very low water potential. The tension draws water into xylem vessels even though the solute concentration is low. A different mechanism- root pressure- is used when a plant is not transpiring, but needs xylem sap to flow upwards. Endodermis cells load mineral ions into the adjacent xylem vessels by active transport, making the xylem sap hypertonic, so water moves from the endodermis cells to the xylem vessels by osmosis. This raises the pressure inside the vessels and pushes the sap upwards, against the force of gravity. Unlike pumps that cause fluids to rise by creating a vacuum above them, root pressure is not dependent on atmospheric pressure so there is no limit to the height to which xylem sap can rise. Root pressure is used in spring to refill xylem vessels that have been air-filled in deciduous plants that become leafless in winter. It is also used when the atmosphere is saturated with water vapour, so transpiration isn’t occurring, but a plant needs to transport minerals up to the leaves.

Question 42

what is translocation?

Answer 42

Plants use phloem tissue to translocate carbon compounds, such as sucrose, from sources to sinks. Sources are parts of the plant where photosynthesis is occurring (stems and leaves) and storage organs where the stores are being mobilised. Sinks are parts of a plant which need a supply of carbon compounds, such as roots and storage organs that are accumulating stores.

Question 43

How is the phloem adapted for translocation?

Answer 43

Translocation from source to sink happens in phloem sieve tubes. Sucrose is loaded into the sieve tubes by active transport, using pump proteins in the plasma membrane. This increases the solute concentration inside the sieve tube and lowers the water potential. The hydrostatic pressure of sap in the sieve tube therefore increases. These processes are reversed in sieve tubes in the sink, reducing the pressure. There is therefore a pressure gradient which causes phloem sap containing sucrose to flow from source to sink.

Phloem sieve tubes develop from columns of cells that break down their nuclei and almost all of their cytoplasmic organelles, but remain alive, with a plasma membrane that can pump substances by active transport. Large pores develop in the end walls between adjacent cells, creating the sieve plates that allow sap to flow. By not breaking down the end walls completely, as happens in xylem vessels, the sieve tube is better able to resist high pressures, with the sieve plates acting as cross braces.

Companion cells are small cells adjacent to sieve tubes that help with loading. They have many pump proteins in their plasma membranes for pumping sucrose into their cytoplasm. They also have many mitochondria to produce the ATP needed for active transport. Plamsodesmata (cytoplasmic connections) allow sucrose rich sap to flow from the companion cell to flow from the companion cell to the adjacent sieve tube.

Question 44

Describe the process of transpiration

Answer 44

a. water moved/transported in xylem vessels;

b. transported under tension/suction/pulled up (in xylem vessels);

c. transpiration/loss of water (vapour) generates pulling forces/low pressure/tension;

d. tension/pull generated when water evaporates from cell walls (in mesophyll);

e. transpiration is loss of water vapour from leaf (surface)/stomata;

f. cohesivity/cohesion in water due to hydrogen bonding/attractions between water molecules;

g. cohesion/WTTE so chain/column of water (molecules) doesn’t break/remains continuous;

h. transpiration stream is a column of/flow of water in xylem from roots to leaves;

Question 45

Describe how plants transport organic compounds from where they are made to where they are stored (translocation)

Answer 45

(overall) process is translocation / bidirectional / movement from source to sink;

sugars/sucrose/organic compounds produced in leaves;

(loaded by) active transport / passage by apoplast route;

loaded into companion cells / transported in phloem / sieve tubes;

high concentrations of solutes at the source cause uptake of water (by osmosis);

water provides hydrostatic pressure for transport (from source to sink);

unloaded / stored / used at sink;

lowers pressure at sink / creates pressure differential / water re-entry to xylem;