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Topic 3 - Biology > 3-4 Mass transport > Flashcards

3-4 Mass transport Flashcards

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1
Q

What is haemoglobin?

A
  • Haemoglobin is a water-soluble globular protein which consists of two beta polypeptide chains and two alpha helices.
  • Each molecule forms a complex containing a haem group.
  • It carries oxygen in the blood as oxygen can bind to the haem (Fe2+) group.
  • Each molecule can carry four oxygen molecules.
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2
Q

How does the affinity of oxygen for haemoglobin vary?

A
  • The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygen which is a measure of oxygen concentration.
  • The greater the concentration of dissolved oxygen in cells, the greater the partial pressure.
  • Therefore, as partial pressure increases, the affinity of haemoglobin for oxygen increases, that is, oxygen binds to haemoglobin tightly.
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3
Q

Where does this occur?

A
  • This occurs in the lungs in the process known as loading.
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4
Q

How does respiration affect the affinity of oxygen for haemoglobin?

A
  • During respiration, oxygen is used up and therefore the partial pressure decreases, thus decreasing the affinity of oxygen for haemoglobin.
  • As a result of that, oxygen is released in respiring tissues where it is needed.
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5
Q

What happens to haemoglobin after the unloading process?

A
  • Haemoglobin returns to the lungs where it binds to oxygen again.
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6
Q

What does dissociation curves show?

A
  • Dissociation curves illustrate the change in haemoglobin saturation as partial pressure changes.
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7
Q

What is the saturation of haemoglobin affected by?

A
  • The saturation of haemoglobin is affected by its affinity for oxygen, therefore in the case where partial pressure is high, haemoglobin has high affinity for oxygen and is therefore highly saturated, and vice versa.
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8
Q

How can saturation influence affinity?

A
  • Saturation can also influence affinity, as after binding to the first oxygen molecule, the affinity for oxygen increases due to a change in shape, thus making it easier for the other oxygen molecules to bind.
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9
Q

Describe and explain the shape of the oxygen dissociation curve?

A
  • Initially the curve is shallow because it is hard for the first oxygen molecule to bind.
  • Once it has bound though it changes the shape making it easier for oxygen molecules two and three to bind, hence the steep increase.
  • This is called positive cooperativity.
  • Finally, the gradient begins to flatten out because the likelihood of the fourth oxygen finding a binding site is low.
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10
Q

What is the difference between foetal haemoglobin and adult haemoglobin?

A
  • Foetal haemoglobin has a different affinity for oxygen compared to adult haemoglobin because by the time oxygen reaches the placenta, the oxygen saturation of the blood has decreased.
  • Therefore, foetal haemoglobin must have a higher affinity for oxygen for the fittest survive at low partial pressure.
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11
Q

What else is the affinity of haemoglobin for oxygen affected by?

A
  • The affinity of haemoglobin for oxygen as also affected by the partial pressure of carbon dioxide.
  • Carbon dioxide is released by respiring cells which require oxygen for the process to occur.
  • Therefore, in the presence of carbon dioxide, the affinity of haemoglobin and oxygen decreases, thus causing it to be released.
  • This is known as the Bohr effect.
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12
Q

How does the Bohr effect work?

A
  • It does this because carbon dioxide creates slightly acidic conditions which change the shape of the haemoglobin protein, thus making it easier for oxygen to be released.
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13
Q

Why are circulatory systems needed?

A
  • In large organisms the surface area to volume ratio is not large enough for diffusion alone to supply substances like oxygen, glucose, and other molecules to cells where they are needed.
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14
Q

What are the 4 common features of a circulatory system?

A
  1. Suitable medium, in mammals the transport medium is the blood, it is water based so substances can easily dissolve into it.
  2. Means of moving the medium, animals often have a pump known as the heart to maintain pressure differences around the body.
  3. Mechanism to control flow around the body, valves are used in veins to prevent any backflow.
  4. Close system of vessels, the circulatory system in most animals and plants is closed and is branched to deliver substances to all parts of the body.
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15
Q

What type of circulatory system is found in mammals?

A
  • Closed double circulatory system.
  • The heart at the centre has two pumps.
  • One pumps bloods to the lungs to be oxygenated whilst the other is larger and stronger and pumps the oxygenated blood around the body to supply vital organs and tissues.
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16
Q

What are the two chambers found in each pump in the heart?

A
  • An atrium, thin walled and elastic, the atrium can stretch when filled with blood.
  • A ventricle, thick muscular wall to pump blood around the body or to the lungs.
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17
Q

Why are two separate pumps needed?

A
  • Two separate pumps are needed to maintain blood pressure around the whole body.
  • One pump would not be able to do this as the slow-down of the blood as it passes the lungs would cause it to lose all pressure.
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18
Q

What are the two valves?

A
  • The left atrioventricular valve, bicuspid valve.
  • The right atrioventricular valve, tricuspid valve.
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19
Q

What are the four main vessels connecting the heart?

A
  1. Aorta, connected to the left ventricle and carries oxygenated blood to all parts of the body except the lungs.
  2. Pulmonary Artery, connected. To the right ventricle and carries deoxygenated blood back to the lungs where it is oxygenated, and the carbon dioxide is removed.
  3. Pulmonary Vein, connected to the left atrium and bring oxygenated blood back from the lungs.
  4. Vena Cava, connected to the right atrium and brings deoxygenated blood back from the tissues except the lungs.
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20
Q

Why is the heart referred to as myogenic?

A
  • The heart is referred to as myogenic due to its ability to initiate its own contraction.
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21
Q

What initiates this contraction?

A
  • The region of specialised fibres in the right atrium called the sinoatrial node which is the pacemaker of the heart.
  • This initiates a wave of electrical stimulation which causes the atria to contract at roughly the same time.
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22
Q

Why do the ventricles not start contracting until the atria have finished?

A
  • The ventricles do not start contracting until the atria have finished due to the presence of tissue at the base of the atria which is unable to conduct the wave of excitation, known as the septum.
  • The electrical wave eventually reaches the atrioventricular node located between the two atria which passes on the excitation to ventricles, down the bundle of His to the apex of the heart.
  • The bundle of His branches into Purkyne fibres which carry the wave upwards.
  • This causes the ventricles to contract, thus emptying them.
  • The ventricles contract at the apex to force the most blood possible upwards out of the aorta and pulmonary artery.
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23
Q

What are the 3 stages of the cardiac cycle?

A
  1. Cardiac diastole.
  2. Atrial systole.
  3. Ventricular systole.
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24
Q

What happens in cardiac diastole?

A
  • Atria and ventricles relax.
  • Elastic recoil of the heart lowers the pressure inside the heart chambers and blood returns to the heart from the vena cava and the pulmonary vein and fills the atria.
  • Pressure increases in the atria until the atrioventricular valves open and blood flows into the ventricles.
  • The relaxed atria and ventricles mean that the semi-lunar valves are now closed.
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25
Q

What happens in atrial systole?

A
  • The atria then contract forcing any remaining blood into the ventricles.
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26
Q

What happens in ventricular systole?

A
  • Contraction of the ventricles causes the atrioventricular valves to close and semi-lunar valves to open thus allowing blood to leave the left ventricle through the aorta and the right ventricle through the pulmonary artery.
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27
Q

Describe the structure and function of arteries?

A
  • Adapted to carrying blood away from the heart to the rest of the body.
  • Thick walled to withstand high blood pressure.
  • Contain elastic tissue which allows them to stretch and recoil thus smoothing blood flow.
  • Contain smooth muscle which enables them to vary bloody flow.
  • Line with smooth endothelium to reduce friction and ease the flow of blood.
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28
Q

Describe the structure and function of arterioles?

A
  • Branch off arteries.
  • Have thinner and less muscular walls.
  • Their role is to feed blood into capillaries.
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29
Q

Describe the structure and function of capillaries?

A
  • Smallest blood vessels.
  • Site of metabolic exchange.
  • Only one cell thick for fast exchange of substances.
30
Q

Describe the structure and function of venules?

A
  • Larger than capillaries but smaller than veins.
31
Q

Describe the structure and function of veins?

A
  • Carry blood from the body to the heart.
  • Contain a wide lumen to maximise the volume of blood carried to the heart.
  • They are thin walled as blood is under low pressure.
  • Contain valves to prevent the backflow of blood.
  • A weak pulse of blood means there is little elastic tissue or smooth muscle as there is no need for stretching and recoiling.
32
Q

What is tissue fluid?

A
  • Tissue fluid is a liquid containing dissolved oxygen and nutrients which serves as a means of supplying the tissues with the essential solutes in exchange for waste products such as carbon dioxide.
33
Q

Describe how tissue fluid is formed?

A
  • Hydrostatic pressure is created when blood is pumped along the arteries, into arterioles and then capillaries.
  • This pressure forces blood fluid out of the capillaries.
  • Only substances which are small enough to escape through the gap in a capillary are components of the tissue fluid.
  • This includes dissolved nutrients such as amino acids, fatty acids, ions in solution, glucose, and oxygen.
34
Q

Why does some of the fluid get pushed back into the capillaries?

A
  • Hydrostatic pressure.
35
Q

What causes water to move down the water potential gradient from the tissue fluid to the blood by osmosis?

A
  • Both the tissue fluid and blood contain solutes, so they have a negative water potential.
  • The WP of the tissue fluid is less negative than the blood as the blood contains more solutes.
  • Therefore, the tissue fluid is positive in comparison to the blood.
  • This causes water to move down the water potential gradient from the tissue fluid to the blood by osmosis.
36
Q

What happens to the remaining tissue that is not pushed back into the capillaries?

A
  • The remaining tissue fluid that is not pushed back into the capillaries is carried back via the lymphatic system.
37
Q

What are the contents of the lymphatic system?

A
  • Lymph fluid, like tissue fluid but contains less oxygen and nutrients compared to tissue fluid as its main purpose is to carry waste products.
  • Lymph nodes, filter out bacteria and foreign material from the fluid with the help of lymphocytes which destroy pathogens as part of the immune system defences.
38
Q

Why do plants require a transport system?

A
  • Plants require a transport system to ensure that all the cells of a plant receive enough nutrients.
  • This is achieved through the combined action of xylem tissue which enables water as well as dissolved minerals to travel up the plant in the passive process of transpiration, and phloem tissue which enables sugars to reach all parts of the plant in the active process of translocation.
39
Q

What are the components of the vascular bundle?

A
  • Xylem
  • Phloem
40
Q

Describe the vascular bundle in the roots?

A
  • The xylem vessels are arranged in an X shape in the centre of the vascular bundle.
  • This enables the plant to withstand various mechanical forces such as pulling.
  • The X shape arrangement of xylem vessels is surrounded by an endodermis, which is an outer layer of cells which supply xylem vessels with water.
  • An inner layer of meristem cells known as the pericycle.
41
Q

Describe the vascular bundle in the stem?

A
  • The xylem is located on the inside in non-wooded plants to provide support and flexibility to the stem.
  • Phloem is found on the outside of the vascular bundle.
  • There is a layer of cambium in between the xylem and phloem, which are meristem cells involved in the production of new xylem and phloem tissue.
42
Q

Describe the vascular bundle in the leaf?

A
  • The vascular bundle forms the midrib and veins of a leaf.
  • Dicotyledonous leaves have a network of veins, starting at the midrib and spreading outwards which are involved in transport and support.
43
Q

Describe the features of the xylem?

A
  • They transport water and minerals and serve to provide structural support.
  • They are long cylinders made of dead tissue with open ends, therefore they can form a continuous column.
  • Xylem vessels also contain pits which enable water to move sideways between the vessels.
  • They are thickened with a tough substance called lignin which is deposited in spiral patterns to enable the plant to remain flexible.
44
Q

What is transpiration?

A
  • Transpiration is the process where plants absorb water through the roots, which then moves up through the plant and is released into the atmosphere as water vapour through pores in the leaves.
  • Carbon dioxide enters, while water and oxygen exit through a leaf’s stomata.
45
Q

What is the transpiration stream?

A
  • Movement of water up the stem.
46
Q

What does the transpiration stream enable?

A
  • Enables processes such as photosynthesis, growth, and elongation as it supplies the plant with water which is a necessary component of all these processes.
  • The transpiration stream supplies the plant with the required minerals
  • Enables it to control its temperature via evaporation.
47
Q

What three processes does transpiration involve?

A
  • Osmosis, water from xylem to mesophyll cells.
  • Evaporation, surface of mesophyll cells into intercellular spaces.
  • Diffusion, water vapour down a water vapour potential gradient out of the stomata.
48
Q

How can the rate of transpiration be measured?

A
  • Potometer, where water lost by the leaf is replaced by water in a capillary tube.
  • Therefore, measuring the movement of the meniscus or a bubble can be used to determine the rate of transpiration.
49
Q

What are the factors which affect the rate of transpiration?

A
  • Number of leaves.
  • Number of stomata.
  • Size of stomata.
  • Position of stomata.
  • Presence of waxy cuticle.
  • Light intensity.
  • Temperature.
  • Humidity.
  • Air movement.
  • Water availability.
50
Q

What are xerophytes?

A
  • Xerophytes are plants adapted to living in dry conditions.
  • Able to survive in such conditions due to adaptations which minimise water loss.
51
Q

What are the adaptations of xerophytes that allow them to live in dry conditions?

A
  • Smaller leaves to reduce the SA for water loss.
  • Densely packed mesophyll and thick waxy cuticle to prevent water loss via evaporation.
  • Ability to close stomata to prevent water loss.
  • Hairs and pits which trap moist air, thus reducing the water vapour potential gradient.
  • Ability to roll leaves to reduce the exposure of the lower epidermis to the air, trapping air that is moist.
52
Q

Describe the movement of water in the root?

A
  • Water enters trough root hair cells and moves into the xylem tissue located in the centre of the root.
  • This movement occurs because of a water potential gradient, as the water potential is higher inside the soil than inside the root hair cells, due to the dissolved substances in the cell sap.
53
Q

What is the purpose of root hairs?

A
  • To provide a large SA for the movement of water to occur.
  • Minerals are absorbed through the root hair cells by active transport, as they need to be pumped against the concentration gradient.
54
Q

What are the two ways that water is taken up by root hair cells?

A
  • Symplast pathway, where water enters the cytoplasm through the plasma membrane and passes from one cell to the next through plasmodesmata, the channels which connect the cytoplasm of one cell to the next.
  • Apoplast pathway, where the water moves through the water filled spaces between cellulose molecules in the cell walls, in this pathway, water doesn’t pass through any plasma membranes, therefore, it can carry dissolved mineral ions and salts.
55
Q

What happens when the water reaches the endodermis?

A
  • When water reaches a part of the root called the endodermis, it encounters a layer of suberin which is known as the Casparian strip, which cannot be penetrated by water.
56
Q

How does water cross the endodermis?

A
  • Therefore, for the water to cross the endodermis, the water that has been moving through the cell walls must now enter the Symplast pathway.
57
Q

What does water do once it has crossed the endodermis?

A
  • Once it has moved across the endodermis, the water continues down the water potential gradient from cell to cell until it reaches a pit in the xylem vessel which is the entry point of water.
58
Q

Describe the movement of water up the xylem in the stem?

A
  • Water is removed from the top of the xylem vessels into the mesophyll cells down the water potential gradient.
  • The push of water upwards is aided by the root pressure which is where the action of the endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis, thus pulling water up.
59
Q

How is the flow of water maintained?

A
  • Surface tension
  • Cohesion.
60
Q

What is this known as?

A
  • Cohesion-tension theory.
61
Q

What also helps the flow of water?

A
  • Capillary action, where the forces involved in cohesion cause the water molecule to adhere to the walls of xylem, thus puling water up.
62
Q

What vessel is used in translocation?

A
  • Phloem
63
Q

Describe the features of phloem?

A
  • They’re tubes made of living cells involved in translocation of nutrients to storage organs and growing parts of the plant.
  • Consist of sieve tube elements and companion cells.
  • Sieve tube elements form a tube to transport sugars such as sucrose, in the dissolved form of sap.
  • Companion cells are involved in ATP production for active processes such as loading sucrose into sieve tubes.
  • The cytoplasm of the sieve tube elements and companion cells are linked through structures known as plasmodesmata which are gaps between cell walls which allow communication and flow of substances such as minerals between the cells.
64
Q

What is translocation?

A
  • Translocation is an energy requiring process which serves as a means of transporting assimilates such as sucrose in the phloem between sources which release sucrose such as leaves and sinks, e.g., roots and meristem which remove sucrose from the phloem.
65
Q

Describe the process of translocation?

A
  1. Sucrose enters the phloem in a process known as active loading where companion cells use ATP to transport hydrogen ions into the surrounding tissue, thus creating a diffusion gradient, which causes the H+ ions to bring sucrose molecules into the companion cells.
  2. Facilitated diffusion involving co-transporter proteins allows the returning H+ ions to bring sucrose molecules into the companion cells, thus causing the concentration of sucrose in the companion cells to increase.
  3. As a result, the sucrose diffuses out of the companion cells down the concentration gradient into the sieve tube elements through links know as plasmodesmata.
  4. As sucrose enters the sieve tube elements, the water potential inside the tube is reduced, therefore causing water to enter via osmosis from the xylem, increasing the hydrostatic pressure of the sieve tube element.
  5. As a result, water moves down the sieve tube from an area of high hydrostatic pressure to an area of low hydrostatic pressure.
  6. Eventually, sucrose is removed from the sieve tube elements by diffusion or active transport into the surrounding cells, thus increasing the water potential in the sieve tube. This in turn means that water leaves the sieve tube by osmosis back into the xylem and as a result, reduces the pressure in the phloem at the sink.
66
Q

Summarise the purpose of the process?

A
  • Therefore, in summary the mass flow of water from the source to the sink down the hydrostatic pressure gradient is a means of supplying assimilates such as sucrose to where they are needed.
67
Q

What is the evidence for mass transport?

A
  • There is pressure in the sieve tube elements, as shown by sap being released when the stem of a plant is cut.
  • The concentration of sucrose is higher in the leaves (source) of plants than in roots (sink).
  • Increases in sucrose levels in the leaves are followed by a similar increase in sucrose concentration in the phloem.
  • Metabolic poisons or a lack of oxygen inhibit translocation of sucrose in the phloem.
68
Q

What is the evidence against mass transport?

A
  • The function of the sieve plates is unclear as they would appear to hinder mass flow.
  • Not all solutes move at the same speed, they should do if it is mass flow.
  • Sucrose is delivered at the same rate to all regions, rather than going quicker in the ones with the lowest sucrose concentration, which the mass flow theory would suggest.
69
Q

What is the ringing experiment?

A
  • To investigate if the phloem is responsible for mass flow a ringing experiment can be used.
  • In this, the bark and phloem of a tree are removed leaving just the xylem in the centre.
  • Overtime the tissues above the missing ring swell with sucrose solution and the tissue below dies.
  • This shows that sucrose is transported in the phloem.
70
Q

What is the tracer experiment?

A
  • Tracer experiments can also be used to investigate the transport of sucrose in plants.
  • Plants are grown in an environment that contains radioactively labelled carbon dioxide (14CO2).
  • The presence of this means that they are incorporated into the sugar produced in photosynthesis.
  • The movement of these sugars can now be traced through the plant using autoradiography.
  • Those areas that have now been exposed to the radiation produced by the 14CO2 in the sugars will appear black.
  • It follows that these regions correspond to the area where the phloem is and therefore suggests that this is where the sugars are transported.

Topic 3 - Biology (4 decks)

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