Topic 3: Organism Exchange substances with their Environment Flashcards
What ability do single-celled/ Small organisms have to ‘diffuse’ substances through their outer surface?
They have a Large Surface Area to Volume ratio, which provides them with a short diffusion pathway to all parts of the cell.
(As the size of an organism increases, the surface area to volume ratio decreases)
How do larger organisms perform gas exchange?
They have developed specialised exchange structures such as Lungs and Gills to maintain adequate rates of gaseous exchange.
How do larger organisms maintain a constant body temperature?
–>Changes in body shape/ specialised structures have evolved e.g. large ears in elephants to increase the surface area for heat loss.
+
–>The large amount of body cells release a lot of heat from metabolic processes (mainly cellular respiration)
How do smaller organisms maintain a constant body temperature?
Because smaller organisms have a large SA-to-volume ratio, they lose a large amount of heat (relative to their mass) due to this:
–>Have to respire at a high rate to help maintain a constant body temperature (respiration produces heat), therefore smaller organisms have a high metabolic rate
–>High oxygen uptake relative to their mass i.e. per gram of tissue.
–>Consume a lot of food, relative to their mass.
How do insects perform Gas exchange?
They perform gas exchange through a system of pipes called the ‘Tracheal System’:
1) Oxygen diffuses from the air, through the spiracles along the trachea and tracheoles to the cells.
–> Tracheoles are thin, providing a short diffusion + they are numerous, providing a large surface area.
2) They penetrate all body tissues and the oxygen then diffuses into the cells through the cell membrane of the tracheole (this is the gas exchange surface)
3) Carbon dioxide leaves the insect’s cells and travels to the atmosphere by the reverse pathway
Which factor determines the opening and closing of Spiracles?
Carbon dioxide levels:
-When carbon dioxide levels rise due to respiration, the spiracles open wider, increasing the rate of diffusion of gases in the tracheal system.
-During rest, the spiracles close to help reduce water loss via evaporation.
How do Insects increase the rate of diffusion?
1) (When flying)–> Muscles contract to compress the trachea, forcing air out of them.–>When the muscles relax, the trachea springs back into shape and fresh oxygen rich air is drawn into them
—>all of this in order to speed up the rate of diffusion.
2) Anaerobic Respiration–> lactic acid is produced which lowers the water potential of the cells and some of the water in the ends of the tracheoles moves into the cells by osmosis (enabling more air to move in)–> Therefore increasing the rate of diffusion.
How are gills adapted for Gaseous Exchange?
1) Large surface area–> provided by a large number of gill filaments.
–>These filaments possess lamellae (thin plates) increasing the SA further
2) Short Diffusion Pathway–> as blood and water are separated by a thin barrier of cells consisting of 2 layers:
-Epithelial Layer (of the gill lamellae)
-Endothelial layer of the blood capillaries.
3) Extensive Network Of Blood Capillaries–> Once oxygen diffuses into the blood it is carried away, maintaining a steep concentration gradient for oxygen .
4) The Counter-current System –> Blood and water flowing in opposite directions, which ensures that blood continually meets water with a higher oxygen concentration so that a high diffusion gradient is maintained along the whole length of the lamellae.–>
meaning around 80% of the oxygen in the water diffuses into the bloodstream (instead of 50%) as equilibrium between the water and the blood is never reached (because they flow in opposite directions)
When do Plants perform Respiration and Photosynthesis?
-Plants respire 24 hours a day but only photosynthesise during daylight.
(during daylight the rate of photosynthesis is greater than the rate of respiration so there is a net uptake of carbon dioxide and loss of oxygen)
How are Plants adapted for gas exchange?
–>Respiration and photosynthesis maintain diffusion gradients by using and producing oxygen and carbon dioxide.
–> Numerous mesophyll cells lining the inter-cellular air spaces in the leaf, providing a large surface area (for gaseous exchange).
–> Gases diffuse in through the stomata between the guard cells, down a concentration gradient rapidly through the intercellular air spaces.
–> The gases dissolve in the moist cell walls of the mesophyll cells. (short diffusion pathway)
–> Gases diffuse across the thin cell wall and cell membrane and of mesophyll cells.
What is ‘Transpiration’?
-The evaporation of water from a plant’s surface, particularly through the stomata when they open to allow the entry of carbon dioxide for photosynthesis
What are some environmental factors which affect the rate of transpiration?
Light:
during daylight, stomata open to allow carbon dioxide to enter for photosynthesis
–> This increases the rate of transpiration, as water evaporates from the mesophyll cells and diffuses out of the leaf.
Temperature:
–> increases the rate of transpiration by providing water molecules with more kinetic energy, allowing them to evaporate more easily.
Humidity:
–> Increases the water potential of the air, this will lead to a decrease in the rate of transpiration, as the water potential gradient for the diffusion of water decreases.
Air Movement:
–>Removes water vapour from the leaf surface, increasing the water potential gradient and the rate of transpiration
(In still air, water vapour builds up around the leaf, decreasing the water potential gradient and the rate of transpiration.
What are ‘Xherotypes’ and what adaptations do they have?
plants that possess a range of adaptations to limit water loss via transpiration:
-A thick cuticle–> provides a short diffusion pathway, reducing the rate of evaporation
-Hairs on the leaf surface–> trap a layer of still air, which becomes saturated with water vapour, reducing the water potential gradient for water loss
-Rolling up of leaves–>traps a layer of still air which becomes saturated with water vapour, reducing the water potential gradient for water loss, therefore reducing the rate of transpiration (e.g. marram grass)
-Reduced surface area to volume ratio of leaves (e.g. pine needles reduce the surface area for water loss)
-Stomata positioned in ‘epidermal pits/grooves’ beneath the leaf surface, reducing exposure to air currents. The trapped air becomes saturated with water vapour and reduces the water potential gradient for evaporation.
What does the gas exchange system in mammals consist of?
1) The trachea is supported by incomplete rings of cartilage that prevent it from collapsing during pressure changes that occur in ventilation.
2) The trachea divides into 2 bronchi that repeatedly divide into smaller tubes–> The bronchioles
3) The alveoli at the ends of the bronchioles provide a large surface area where gaseous exchange occurs.
How are Alveoli adapted as a gaseous exchange surface?
1) Shape and large number–>produces a large surface area
2) Fluid lining in the alveolus allows gases to dissolve and diffuse across
3) Two cell layer –> provides a short diffusion pathway:
1) Flattened epithelial cells (alveolar wall)
2) Endothelial cells (capillary wall)
4) An extensive network of blood capillaries surrounds each alveolus –> providing a large surface area for absorbing oxygen and releasing carbon dioxide
5) A circulatory system–> maintains a high concentration gradient for gaseous exchange by transporting deoxygenated blood to the lungs and removing oxygenated blood from the lungs
6) A ventilation system–> maintains a high concentration gradient for gaseous exchange by continually providing air with a high oxygen concentration and removing air with a high carbon dioxide concentration.
How does gas exchange occur in the alveoli?
1) Oxygen in the alveolar air space dissolves in the fluid lining in the epithelium of the alveolus
2) It then diffuses across from the flattened epithelial cells of the alveolus to the endothelial cells of the capillary wall
(Both layers are single-cell layers–> short diffusion pathway)
3) The oxygen combines with haemoglobin in the red blood cells to form oxyhaemoglobin, the oxygenated blood is then carried away from the lungs.
4) Carbon dioxide then diffuses from the blood in the opposite direction into the alveolar space
–>Blood flow ensures that deoxygenated blood continually reaches the alveoli SO that oxygenated blood is removed (Therefore maintaining a high concentration
gradient)
–>The ventilation mechanism continually replenishes the air in the alveolar space, ensuring it has a high concentration gradient and low carbon dioxide concentration (therefore maintaining a high concentration gradient)
What is Ventilation?
Ventilation is the method by which air (or water in fish) is passed over the respiratory surface to ensure a high concentration gradient is maintained
What is ‘Inspiration’ and how does it work?
-Inspiration (breathing in) is an active process:
–> External intercostal muscles contract (pulling the ribs upwards and outwards), whilst the internal intercostal muscles relax
–>The diaphragm muscles contract, pulling it down so it flattens
-Thoratic cavity volume increases
-Pressure inside lungs decreases
How do the external and internal intercoastal muscles work?
They work antagonistically-when one of them contracts the other relaxes and vice versa.
What is ‘Expiration’ and how does it work?
-Expiration (breathing out) is mainly a passive process:
–> The external intercostal muscles relax
–> The diaphragm muscles relax and the diaphragm moves upwards to its dome shape
(These actions decrease the volume of the thoracic cavity)
–>The pressure inside the lungs increases above atmospheric pressure and air is forced out of the lungs
–>Elastic recoil of the lung tissue helps to force air out of the lungs during expiration
–> The internal intercostal muscles may also contract pulling the ribcage downwards and inwards.
What is Pulmonary Ventilation?
Pulmonary ventilation is the total volume of air that is moved into the lungs in one minute.
How do you calculate Pulmonary Ventilation?
Two factors are needed:
1) Tidal volume–> The volume of air normally taken in at each breath (about 0.5 dm cubed)
2) Ventilation rate–> The number of breaths per minute (normally around 12-30 bpm)
Pulmonary Ventilation =
Tidal Volume x Ventilation rate
What is a risk factor?
Anything which increases the chance of getting a disease (e.g. Diet/Smoking/Age/Occupation etc.)
(Occupation–> working with harmful chemicals/gases, or radiation)
What is ‘Correlation’?
Where a change in 1 of 2 variables is reflected by a change in the other variable.
–>(Some factors may show a correlation with a disease but there may be no actual evidence to prove it is the cause of the disease)
What is ‘Cause’?
A factor which is directly a cause of a disease
–>(This can only be established when scientists have produced compelling experimental evidence to show a particular factor is causing a disease)
What is ‘Relative Risk’?
Risk measured by comparing the likelihood of harm occurring in those exposed to a hazard with those who are not exposed to it
–>(e.g. smokers may be 15x more likely to develop lung cancer than non-smokers)
What is ‘Digestion’?
Digestion is the process in which large molecules are hydrolysed by enzymes to produce smaller molecules (that can be absorbed and assimilated)
How is the digestion of ‘Carbohydrates’ carried out?
1) In the mouth:
The sight/smell/taste of food all stimulate the secretion of saliva from the salivary glands, saliva contains–> Salivary amylase–> Amylase hydrolyses starch into maltose.
–>Only a small amount of starch is broken down (due to the short time food remains in the mouth)
–>Chewing breaks the food into smaller particles (increased surface area for hydrolysis by enzymes)
2) In the small intestine:
The pancreas releases pancreatic juice into the small intestine.
(Pancreatic juice is an alkaline fluid containing several different enzymes including Pancreatic Amylase)–>Hydrolysing the remaining starch into the disaccharide maltose.
3) The complete hydrolysis of starch into Alpha glucose occurs in the ileum by the disaccharidase enzyme maltase (part of the cell-surface membrane of the epithelial cells that line the ileum)–> Membrane-bound disaccharides.
–>The membrane-bound disaccharides include Maltase/Lactase/Sucrase
What are the advantages of having membrane-bound disaccharides?
-Enzymes are not lost in the food (as they stay in one place)
-Enzymes can be used repeatedly
-Products of hydrolysis are taken up immediately through the membrane
How is the digestion of ‘Lipids’ carried out?
1) Lipids are hydrolysed into glycerol, fatty acids, and monoglycerides (glycerol and one fatty acid) by the enzyme Lipase (which is secreted by the pancreas into the small intestines)
–>Lipase specifically hydrolyses the ester bonds of the triglycerides.
How is the rate of hydrolysis of lipids increased?
It is increased by bile salts which:
(1) emulsify lipids, causing them to form small droplets
(2) This increases the surface area of the lipids which speeds up the hydrolysis by Lipase
(Emulsification is a physical process and does not cause chemical breakdown)
How is the digestion of ‘Proteins’ carried out?
-Proteins are hydrolysed by: Endopeptidases, Exopeptidases, and Dipeptidases
1) Endopeptidases break bonds in the middle of the polypeptide chain to produce shorter polypeptide chains. (they do this by hydrolysing the internal peptide bonds between the amino acids of proteins)–>(they do this to produce ‘more ends’ for exopeptidases to act on to hydrolyse the terminal.)
–>The endopeptidase in the stomach of humans is ‘pepsin’ and has an optimum PH of 1-2
2) The Pancreas secretes Exopeptidases which hydrolyse the peptide bonds at either end of a polypeptide chain to produce dipeptides or individual amino acids.
3) The hydrolysis of the dipeptides into 2 single amino acids by Dipeptidase enzymes.
(Dipeptides —(hydrolysis by dipeptidases)–> Amino Acids
–>Dipeptidases are membrane-bound enzymes in the folded cell-surface membrane (microvilli) of the epithelial cells of the ileum.
How is the ‘Ileum’ adapted for absorption of digested products?
-Large Surface Area–> due to its long length + the presence of villi and microvilli (folds of the cell membrane)
-Villi contain blood capillaries that absorb monosaccharides and amino acids –> maintaining a high diffusion gradient for further absorption
-The wall of each villus consists of a single layer of epithelial cells–> providing a short diffusion pathway (for absorption)
-Large number of mitochondria present to supply ATP (for Active Transport)
-Carrier and Channel Proteins in the cell-surface membrane for–> absorption of specific molecules by Active Transport (carrier proteins) + Facilitated Diffusion (carrier & channel proteins)
What are the steps in the ‘Co-transport of Monosaccharides and Amino Acids’?
1) Sodium ions are actively transported out of the epithelial cells and into the blood (–> this occurs via specific Carrier proteins + Hydrolysis of ATP)
—> Carrier protein is used, so sodium ion binds to the protein, causing it to change shape and move the sodium ion across the membrane
–> step 1 ensures that a low concentration of sodium ions is maintained within the epithelial cell
–>A higher concentration of sodium ions will be present in the lumen
2) Therefore, there is a concentration gradient for sodium ions to diffuse into the cell–> they diffuse into the epithelial cells from the lumen of the intestines through a Co-transport Protein
–>Glucose also moves through these co-transport proteins with the sodium ions
3) The Glucose molecules then move out of the cell towards the blood capillary by Facilitated Diffusion through another specific channel or carrier protein.
–>(The same mechanism applies to the uptake of other monosaccharides and amino acids)
What is the Counter-Current System?
Blood and water flowing in opposite directions, which ensures that blood continually meets water with a higher oxygen concentration so that a high diffusion gradient is maintained along the whole length of the lamellae.
What is ‘Haemoglobin’?
-Haemoglobin is an iron-containing pigment, which loosely and reversibly combines with oxygen to form oxyhaemoglobin
–> Each haemoglobin molecule consists of 4 haem units and 4 polypeptide chains
–> Each haem unit can combine with one oxygen molecule so that one haemoglobin molecule can transport 4 oxygen molecules
How does ‘Haemoglobin’ work?
1) Deoxygenated blood enters enters lung capillaries
2) Alveoli contain a high concentration of oxygen, giving a concentration gradient for the diffusion of oxygen through the epithelium of the alveolus and the endothelium of the capillary
3) Oxygen enters red blood cells and combines with haemoglobin to form oxyhaemoglobin
-(Oxygen is then carried away by the blood, maintaining a concentration gradient)
–> In the body tissues respiration uses oxygen, and oxyhaemoglobin releases oxygen which diffuses into respiring cells
What is the relationship between the concentration of oxygen carried by haemoglobin and the partial pressure of oxygen in the surrounding environment?
At High Partial Pressure of Oxygen (PO2):
–> Has a very high affinity (attraction) for oxygen and quickly loads (associates with) Oxygen (becomes 96% saturated) in the lungs (where partial pressure is high)
At Low Partial Pressure of Oxygen (PO2):
–> In the tissues, haemoglobin unloads (dissociates) some of its O2 to the respiring cells
Why does the Oxyhaemoglobin dissociation curve have a characteristic ‘S’ (sigmoid) shape?
-A molecule of oxygen binds to one of the 4 haem units
-This causes a slight change in the tertiary structures of the other haem units/quaternary structure of haemoglobin
-This increases their affinity for oxygen, allowing the second oxygen molecule to bind more easily, and the third and fourth even more easily
–> (This is known as Cooperative binding)
–>(By the same process, the ability for haemoglobin to lose oxygen increases as fewer oxygen molecules are bound)
What is the ‘Bohr Effect’?
(During exercise, muscle cells respire more rapidly, producing larger concentrations of CO2)
1) Due to the decrease in PH produced as CO2 dissolves in the blood plasma to form an acid
2) The change in pH in the tissues changes the quaternary structure of haemoglobin, lowering its affinity for oxygen
3) Thus, the Oxygen dissociation curve moves to the right, so Haemoglobin releases more oxygen to respiring tissues
Note: An increase in temperature will have the same results because more heat is released when respiration increases so more oxygen is released to the cells
What advantage does the ‘Bohr Effect’ have to the body?
The Bohr Effect has a physiological advantage for the body as it ensures that haemoglobin releases more oxygen to the most active cells for the increase in respiration taking place.
What type of Haemoglobin do organisms who live in low oxygen concentration environments possess?
–>Possess haemoglobin with a higher affinity for oxygen (meaning the oxygen dissociation curve is further to the left)
–>This is so that they can load more oxygen in the lungs
For example, organisms who live:
-In the mud
-At high altitudes
-At the bottom of polluted ponds
How is ‘Fetal Haemoglobin’ different to maternal Haemoglobin?
Fetal haemoglobin has a higher affinity for oxygen, enabling oxygen to be transferred from maternal haemoglobin to fetal haemoglobin at the low partial pressures of oxygen found in the placenta.
What is ‘Myoglobin’?
–> Myoglobin is a muscle protein made up of 1 polypeptide chain and 1 haem group (it has a tertiary structure).
–> It acts as a ‘store’ of oxygen in muscle cells
What does ‘Myoglobin’ do?
Myoglobin has a much higher affinity for oxygen than haemoglobin
–> It releases its oxygen only when the partial pressure of oxygen in the tissues is very low, and when haemoglobin has already released almost all of the oxygen it carries
Where is ‘Myoglobin’ present?
Myoglobin is present in high concentrations in diving mammals such as whales and seals
What is meant by a ‘Double Circulatory System’?
-Blood is pumped from the heart (right ventricle) to the lungs and returns to the heart (pulmonary circulatory system) before being pumped (left ventricle) to the rest of the body (systemic circulatory system)
What is the role of the ‘Coronary Arteries’?
The coronary arteries branch off from the aorta and supply oxygen and glucose to the heart muscle
Describe ‘The Cardiac Cycle’
1) Blood flows in through the Vena Cava and Pulmonary Vein (causing the pressure in the atria to rise)
2) At a certain point, the pressure in the atria is greater than the pressure in the ventricles, causing the Atrioventricular (AV) valves to open (blood flows down from the atria into the ventricles)
4) Then the Atria contract, pushing the remaining blood from the atria down to the ventricles
5) Then the Ventricles contract (Ventricular Systole)–> ventricular pressure is now greater than the atrial pressure so the AV valves close (preventing any blood from moving back into the atria)
6) The Semi-lunar valves in the pulmonary artery and aorta also open (blood is pumped from the ventricles out of the heart)
7) Then the Ventricles relax (Ventricular diastole)–> ventricular pressure is now lower than the pressure in the aorta and pulmonary artery
8) This causes the Semi-lunar valves to shut, preventing blood from flowing back into the ventricles
Why does the left ventricle have a much thicker muscular wall compared to the right ventricle?
Because when it contracts, it produces a greater pressure, because it has to pump blood a greater distance i.e. to all part of the body
What prevents the AV valves from turning inside out?
Heart strings (tendinous cords) prevent the AV valves inverting as the ventricular pressure increases above that in the atria
What is ‘Cardiac Output’?
Cardiac Output is the volume of blood pumped out of one ventricle per minute
Cardiac Output=
Stroke Volume (cm3) x Heart Rate (min -1)
What is ‘Stroke Volume’?
Stroke Volume is the volume of blood (cm3) expelled from the left ventricle of the heart per contraption (beat)
What is ‘Heart Rate’?
Heart Rate is the number of contractions per minute
What does regular exercise do for the heart?
Increased blood flow to the heart, causing the cardiac muscle to contract more strongly, pumping out an increased volume of blood per beat (Stroke volume increases)
Where is the SAN (Sinoatrial Node) found?
The Sinoatrial node is a group of cells located in the right atrium
–>(pacemaker cells which generate electrical impulses for heart contraction)
How does blood flow in the ‘Arteries’?
-Arteries carry blood away from the heart at high blood pressure
1)–>The aorta has a large amount of elastic tissue which allows it to stretch when the left ventricle contracts
2)–>When the left ventricle relaxes, the artery wall recoils due to its elasticity and forces blood to the body tissues
(Blood flows at high velocity under high pressure–> because the S.A. of the arteries is higher so there is less friction)
How are ‘Arteries’ adapted for blood flow?
Generally arteries have:
–>Generally arteries have a thicker wall and a smaller lumen than veins + contain more elastic fibres and smooth muscle fibres
(to withstand the high blood pressure and to help smooth out blood flow)
–>Do not possess valves except for the aorta and pulmonary artery (because the pressure is so high, blood can only flow in one direction)
–>Transport blood at high pressure
–>Carry oxygenated blood (except for the pulmonary artery)
What are ‘Arterioles’ and what do they do?
Arterioles are small blood vessels which branch out from arteries.
–>They control the flow of blood to different tissues and organs by contraction or relaxation of the smooth muscle in their wall
(Contraption of the smooth muscle causes vasoconstriction of the arteriole, reducing blood flow to the capillaries)
(the opposite is also true)
How are ‘Veins’ adapted for blood flow?
-Vein carry blood under low pressure towards the heart
–>The walls of veins are thinner than arteries and contain less elastic fibres and smooth muscle
–>The lumen is larger than in arteries-(so that even at low pressure, blood flows back at the heart at the same rate that it leaves along the arteries)
What is ‘Venous return’ and how is it achieved?
Venous return is done in order for the blood in the veins to return to the heart despite the low pressure and the force of gravity
–>It is achieved via the contraction of the ‘Skeletal muscles’ which squash the vein and push blood along them
How are ‘Capillaries’ adapted for blood flow?
–>Capillary walls are one endothelial cell thick (Providing a short-diffusion pathway for the exchange of substances with the tissues)
–>Gaps between the endothelial cells increase the permeability of the capillary
–>Large number of capillaries and they are highly branched, (giving a large surface area for exchange with the tissues)
How does blood flow in the ‘Capillaries’?
–>High S.A, so a large frictional resistance, reducing the rate of blood flow, therefore allowing time for the exchange of substances
How are substances transported in the ‘Blood Plasma’?
1) At the arteriole end of a capillary the hydrostatic (blood) pressure is high (due to the contraction of the left ventricle)
2) This high blood pressure (which forces water out) is greater than the water potential of the blood (which forces water in). As a result, filtration of blood plasma occurs
3) Smaller molecules such as water/glucose/amino acids/ions pass through the permeable capillary wall
(whilst the blood cells and large plasma proteins remain in the capillary)
(The filtered plasma forms tissue fluid which surrounds the body cells)
4) The large plasma proteins which remain in the blood reduce the water potential of the blood plasma, causing the osmotic uptake of water in the capillaries (i.e. some of the water is reabsorbed by osmosis)
What is the ‘Cohesion Tension Theory’?
1) Solar heat energy causes the evaporation (or transpiration) of water from leaves
2) Water moves from cell to cell across the leaf by osmosis down a water potential gradient
3) Water is drawn from the Xylem, creating negative pressure in the Xylem vessels, ‘pulling up’ the water and dissolved ions
4) The water column is maintained in the Xylem by ‘Cohesive forces’ (water molecules being attracted to each other via hydrogen bonding)
+ ‘Adhesive Forces’ (water molecules being attracted to the Xylem walls)
5) Transpiration maintains a water potential gradient across the root cells, providing the mechanism for the water uptake from the soil via osmosis (as the soil has a higher water potential than the root hair cell)
What are some pieces of evidence for the Cohesion Tension Theory?
1) As the rate of transpiration increases, the diameter of the tree trunk and branches decreases
2) Evaporation of water from the leaves draws water from the xylem via osmosis, water is pulled up the xylem creating a tension
3) The tension pulls in the walls of the vessels so the overall diameter of the trunk and branches decreases
Why is using a Potometer to measure the rate of transpiration not a valid method?
This method assumes the rate of water uptake is the same as the rate of transpiration however, this is not true as some water taken up by the plant is used to maintain turgidity (for support) and a small percentage is used for photosynthesis
What measurements need to be taken to calculate the rate of transpiration?
–>The distance (d) the air bubble moves (cm)
–>The time taken for the air bubble to move that distance
–>The radius (r) of the lumen of the capillary tubing
How can you show evidence for the movement of ions in the Xylem?
1) The Xylem and Phloem are separated (in a section of the stem) using a Wax Cylinder to prevent lateral transport
2) The roots of the plant are supplied with radioactively labelled potassium ions
3) The plant is left for a few hours and then the amount of radioactivity is compared
4) It is much greater in the Xylem, indicating that transport of the potassium ions occurs in this tissue
–>(the relatively small amount of radioactivity in the phloem tissue is due to lateral transport from the Xylem in the region where the wax cylinder is not present)
How does ‘Translocation’ occur in the Xylem?
1) Companion cell Actively Transports sucrose from the leaf to the Sieve Tube Element
2) The Sieve Tube Element (or Phloem cell) is loaded up with a high concentration of sucrose
3) This decreases the water potential of the Phloem cell
4) Water then enters from the Xylem into the Phloem cell via osmosis
5) Hydrostatic Pressure and volume in Phloem cell increases
6) This pressure pushes sap towards the ‘sink’ (up or down) down a pressure gradient
How can you show evidence for the transport of photosynthetic products in the Phloem?
1) Ringing:
–> Remove a complete ring of phloem from the trunk of a tree
–> A few months later, a slight swelling develops above the ring
–> This swelling is linked to the build up of photosynthetic products from the leaves, which were prevented from being transported past the ring due to the removal of the phloem
2) Use of radioactive isotopes as tracers:
–> Obtain 2 plants of the same species at similar stages of growth
–> The stem of 1 plant (A) is ringed, whereas the other plant (B) is left intact (it is the control)
–> A leaf below the ring (girdle) in plant (A) and at a similar position in (B), is supplied with either radioactive CO2 or injected with radioactive glucose
–> After a few hours, the transport of the radioactively labelled photosynthetic products can be detected using a process called ‘Autoradiography’
How is ‘Autoradiography’ carried out?
1) Place each plant between X-ray film, then expose the film to radioactively labelled compounds, showing their location
2) The control plant (B) shows transport of photosynthetic products throughout the plant, particularly to the growing regions (shoot tip, root tip, young leaves, and storage areas)
3) Photosynthetic products cannot be transported in the phloem past the ring and therefore remain below this point
4) Sometimes, a very small amount of these products does appear above the ring due to transport in the Xylem
–> Due to Lateral Transport between the Phloem and Xylem tissue below the ring
What is the ‘P Value’?
It represents the probability that the observed results (or results more extreme) occurred by chance
What is some evidence supporting the ‘Mass Flow’ hypothesis?
–> Cutting the stem of a plant results in Phloem sap being released, indicating hydrostatic pressure in the sieve tubes
–> Lowering the temperature or use of respiratory inhibitors reduces the rate of translocation, indicating an active transport mechanism is involved
What is some evidence against the ‘Mass Flow’ hypothesis?
–> The specific structure of sieve tubes and sieve plates is not required for mass flow
–> In young Phloem tissue, substances have been observed moving in opposite directions in the same sieve tube.
