Organisms exchange substances with their environment Flashcards
What is the calculation for volume of a cylinder?
pi x radius^2 x height
How does body size affect heat exchange?
Larger organisms have relatively smaller surface areas. This makes it harder for it to lose heat from its body
If organism is small, its surface area is relatively large. So heat is lost more easily
This means smaller organisms need a relatively high metabolic rate in order to generate enough heat to stay warm
How does body shape affect heat exchange?
Animals of an size with a compact shape have a small surface area relative to their volume - minimising heat loss from their surface
Animals with a less compact shape have a larger surface area relative to their volume - this increases heat loss from their surface.
What two things that most gas exchange surfaces have that increase the rate of diffusion?
Large surface area
Thin (one layer of epithelial cells) - provides a short diffusion pathway across the gas exchange surface
What are the two gas exchange adaptations in fish?
Structure of gills
Counter-current system
How is the structure of a fishes’ gills adapted for gas exchange?
Water enters through mouth and passes out through gills
Each gill is made of lots of thin plates called gill filaments, which give a large surface area for exchange of gases (so increase rate of diffusion)
The gill filaments are covered in lots of tiny structures called lamellae, which increase the surface area even more
The lamellae have lots of blood capillaries and a thin surface layer of cells to speed up diffusion, between the water and the blood
How is the counter-current of a fish adapted for gas exchange?
In gills of fish, blood flows through the lamellae in one direction and water flows over them in the opposite direction
The counter-current system means that the water with a relatively high oxygen concentration always flows next to blood with lower concentration of oxygen
This in turn means that a steep concentration gradient is maintained between the water and the blood - so as much oxygen as possible diffuses from the water into the blood
What is the main gas exchange surface in dicotyledonous plants and how are they adapted?
Mesophyll cells have large surface area
What do gases in a plant move in and out of?
Special pores in the epidermis called stomata
How do stomata prevent too much water loss?
Can close
How do stomata let in gases?
Open
What controls the opening and closing of the stomata?
Guard cells
Describe gas exchange in insects
Tracheae - microscopic air filled pipes used for gas exchange
Air move into tracheae through pores in surface - spiracles
Oxygen travels down the conc. gradients towards cells
Tracheae branch off into smaller tracheoles which have thin, permeable walls and go to individual cells - the insect’s circulatory system doesn’t transport O2
CO2 from the cells moves down its own concentration gradient towards the spiracles to be released into the atmosphere
Insects use rhythmic abdominal movements to move air in and out of the spiracles
What are xerophytes?
Plants that are specially adapted for life in warm, dry or windy habitats, where water loss is a problem
Give five examples of xerophytic adaptations
-Stomata sunk in pits to trap water vapour, reducing the concentration gradient of water between leaf and air. This reduces evaporation of water from the lead
-Layer of hairs on the epidermis to trap water vapour round the stomata
-Curled leaves with the stomata inside, protecting them from wind (windy conditions increase the rate of diffusion and evaporation)
-Reduced number of stomata, so there are fewer places for water to escape
-Thicker waxy, waterproof cuticles on leaves and stems to reduce evaporation
Where does air travel when moving in?
Air enter trachea
Trachea splits into two bronchi - one bronchus leading to each lung
Each bronchus then branches off into smaller tubes called bronchioles
Bronchioles end in small air sacs - alveoli
What are intercostal muscles?
Found between ribs
Internal and external
What does ventilation consist of?
Inspiration
Expiration
Describe inspiration?
External intercostal muscles contract
Diaphragm contracts
Ribcage moves upwards and outwards and the diaphragm flattens
This increases volume of thoracic cavity
Lung pressure decreases below atmospheric pressure
Air flows in from area of higher pressure to area of lower pressure so air flows down trachea into lungs
Inspiration is an active process - requires energy
Describe expiration?
External intercostal muscles relax
Diaphragm muscles relax
Ribcage moves downwards and inwards
Diaphragm curves upwards again (dome)
Volume of thoracic cavity decreases
Air pressure increases above atmospheric pressure
Air is forced down pressure gradient and out of lungs
Passive process - no energy required
Describe forced expiration?
External intercostal muscles relax
Internal intercostal muscles contract, pulling ribcage further down and in
During this time, the movement of the two sets of intercostal muscles is antagonistic
What are alveoli surrounded by?
Network of capillaries
What is the structure of alveoli?
Wall of each alveolus is made from a single layer of thin, flat cells called alveolar epithelium
The walls of the capillaries are made from capillary endothelium
The walls of the alveoli contain a protein called elastin
This allows alveoli to return to normal shape after inhaling and exhaling air
Describe the movement of oxygen and carbon dioxide though the gas exchange system
Air moves down the trachea, bronchi and bronchioles into alveoli. This movement happens down a pressure gradient
Oxygen then moves into blood where it can be transported round the body -this movement happens down a diffusion gradient
CO2 moves down its own diffusion and pressure gradients, but in the opposite direction to O2, so that it can be breathed out
Oxygen diffuses out of alveoli, across alveolar epithelium and capillary endothelium into haemoglobin in blood
CO2 diffuses into alveoli from blood
What are two factors affecting rate of diffusion?
-Thin exchange surface - alveolar epithelium is only one cell thick. This means there’s a short diffusion pathway (which speeds up diffusion)
-Large surface area - millions of alveoli. Means there’s a large surface area for gas exchange
-Steep conc. gradient of O2 and CO2 between alveoli and capillaries
What is tidal volume?
Volume of air in each breath
What is ventilation rate?
Number of breaths per minute
What is forced expiratory volume1?
Max volume of air that can be breathed out in 1 second
What is forced vital capacity?
Max volume of air it is possible to breath forcefully out of lungs after really deep breath in
How do you calculate percentage change?
((final value - original value) original value) x 100
What are risk factors?
Factors that increase a person’s chance of getting a disease
What are ethical issues surrounding dissections of animals?
Morally wrong to kill animals just for dissections
Animals used for dissections are not always raised in a humane way - may be subject to overcrowding, extreme temperatures of lack of food. May not be killed humanely either
How can polymers be broken down into monomers?
Hydrolysis
What are hydrolysis reactions?
Break bonds by adding water
What are digestive enzymes used for?
Used to break down biological molecules in food
What does amylase catalyse?
Breakdown of starch
How does amylase catalyse the breakdown of starch?
Catalysing hydrolysis reactions that break the glycosidic bonds in starch to produce maltose
Where is amylase produced?
Salivary glands
Pancreas
What are membrane-bound disaccharides?
Enzymes that are attached to the cell membranes of epithelial cells lining the ileum
They help break down disaccharides into monosaccharides
Involves hydrolysis of glycosidic bonds
What is the disaccharidase of sucrose?
Sucrase
What is the disaccharidase of maltose?
Maltase
What is the disaccharidase of lactose?
Lactase
How are monosaccharides transported across the ileum?
Across epithelial cell membranes in the ileum via specific transporter proteins
What do lipase enzymes catalyse?
Breakdown of lipids into monoglycerides and fatty acids
How do lipase enzyme catalyse the breakdown of lipids?
Hydrolysis of ester bonds in lipids
Where are lipases made, secreted and where they act?
Made in pancreas
Secreted into small intestine
Act in small intestine
Where are bile salts produced?
Liver
What do bile salts do?
Emulsify lipids - causes them to form small droplets
Why are bile salts important in lipid digestion?
Several small lipid droplets have a bigger surface area than a single large droplet (for same volume of lipid)
So formation of small droplets greatly increases the surface area of lipid that’s available for lipases to work on
What happens to the lipid once it has been broken down by lipase?
Monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles
Micelles help the products of lipid digestion to be absorbed
What are endopeptidases?
Act to hydrolyse peptide bonds within a protein
What are exopeptidases?
Act to hydrolyse peptide bonds at the ends of protein molecules
They remove single amino acids from proteins
What are dipeptidases?
Exopeptidases that work specifically on dipeptides
They act to separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them
Where are dipeptidases usually located?
In cell-surface membrane of epithelial cells in the small intestine
How are monosaccharides absorbed across the ileum epithelium into the bloodstream?
Glucose absorbed by active transport with sodium ions via a co-transporter protein
Galactose absorbed in same way using the same co-transporter protein
Fructose absorbed via facilitated diffusion through a different transporter protein
How are monoglycerides and fatty acids absorbed across the ileum epithelium into the bloodstream?
Micelles help to move monoglycerides and fatty acids towards the epithelium
Because micelles constantly break up and reform they can release monoglycerides and fatty acids, allowing them to be absorbed
Whole micelles are not taken up across the epithelium
Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane
How are amino acids absorbed across the ileum epithelium into the bloodstream?
Sodium ions are actively transported out of the epithelial cells into the ileum itself
They then diffuse back into the cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids with them
Where is haemoglobin found?
Red blood cells
What is the role of haemoglobin?
Carry oxygen around the body
What structure does haemoglobin have?
Protein with a quaternary structure
How many polypeptide chains is haemoglobin made up of?
Four
What does each chain have?
A haem group that contains an iron ion
How many oxygen molecules can a molecule of haemoglobin carry?
Four
What is made when oxygen joins to haemoglobin?
Oxyhaemogloblin
What is it called when an oxygen molecule joins to haemoglobin?
Association or loading
What is it called when an oxygen leaves oxyhaemoglobin?
Dissociation
Unloading
What is affinity for oxygen?
The tendency a molecule has to bind with oxygen
What can affect affinity for oxygen?
Partial pressure of oxygen
What is pO2?
Measure of oxygen concentration
What makes a partial pressure higher?
The greater the concentration of dissolved oxygen in cells, the higher the partial pressure
What happens at high pO2?
Oxygen loads onto haemoglobin to form oxyhaemoglobin
How does a low pO2 occur?
When cells respire, the use up oxygen
Describe the conditions in the alveoli in lungs
HIGH oxygen concentration
HIGH pO2
HIGH affinity
Oxygen LOADS
Describe the conditions in the respiring tissue
LOW oxygen concentration
LOW pO2
LOW affinity
Oxygen UNLOADS
Describe an oxygen dissociation curve when pO2 is high
Haemoglobin has a high affinity for oxygen, so it has a high saturation of oxygen
Describe an oxygen dissociation curve when pO2 is low
Haemoglobin has a low affinity for oxygen, so it has low saturation of oxygen
Why is a dissociation curve s-shaped?
Saturation can affect affinity
When haemoglobin combines with the first O2 molecule, its shape alters in a way that makes it easier for other O2 molecules to join too
But as the haemoglobin starts to become saturated, it gets harder for more oxygen molecules to join
As a result, the curve has a steep bit in the middle where it’s really easy for oxygen molecules to join, and shallow bits at each end where it’s harder
When the curve is steep, a small change in pO2 causes a big change in the amount of oxygen carried by the haemoglobin
What is partial pressure of carbon dioxide?
A measure of the concentration of CO2 in a cell
How does pCO2 affect oxygen unloading
Haemoglobin gives up its oxygen more readily at a higher pCO2
What is the Bohr effect?
When cells respire they produce carbon dioxide, which raises the pCO2
This increases the rate of oxygen unloading - so the dissociation curve shifts right
The saturation of blood with oxygen is lower for a given pO2, meaning that more oxygen is being released
Describe haemoglobin in low oxygen environments
Organisms that live in environments with a low concentration of oxygen have haemoglobin with a higher affinity for oxygen than human haemoglobin with a higher affinity
Describe haemoglobin with high activity levels
Organisms that are very active and have a high oxygen demand have haemoglobin with a lower affinity for oxygen that human haemoglobin
This is because they need their haemoglobin to easily unload oxygen, so that it’s available for them to use
The dissociation curve of their haemoglobin is to the right of the human one
Describe haemoglobin with different sizes
Small mammals tend to have a higher surface area to volume ratio than larger mammals
This means they lose heat quickly, so they have a high metabolic rate to help keeps them warm - which means they have a high oxygen demand
Mammals that are smaller than humans have haemoglobin with a lower affinity for oxygen than human haemoglobin, because they need their haemoglobin to easily unload oxygen to meet their high oxygen demand
The dissociation curve of their haemoglobin is to the right of the human one
Where does the pulmonary artery carry blood from and to?
From heart
to lungs
Where does the pulmonary vein carry blood from and to?
From lungs to heart
Where does the aorta carry blood from and to?
From heart
to body
Where does the vena cava carry blood from and to?
From body
to heart
Where does the renal artery carry blood from and to?
From body
to kidneys
Where does the renal vein carry blood from and to?
From kidneys
to vena cava
Describe the structure of arteries
Arteries carry blood from the heart to the rest of the body
Their walls are thick and muscular and have elastic tissue to stretch and recoil as the heart beats, which helps maintain the high pressure
The endothelium is folded allowing artery to stretch - also helps is maintain high pressure
All arteries carry oxygenated blood except for the pulmonary arteries, which take deoxygenated blood to the lungs
Describe the structure of the arterioles
Divide into smaller vessels called arterioles
These form a network throughout the body
Blood is directed to different areas of demand in the body by muscles inside the arterioles which contract to restrict the blood flow or relax to allow full blood flow
Describe the structure of the veins
Veins take blood back to the heart under low pressure
They have a wider lumen than equivalent arteries, with very little elastic or muscle tissue
Veins contain valves to stop the blood flowing backwards
Blood flow through the veins is helped by contraction of the body muscles surrounding them
All veins carry deoxygenated blood, except for pulmonary veins which carry oxygenated blood to the heart from the lungs
Describe the structure of capillaries
Arterioles branch into capillaries, which are the smallest blood vessel
Substances, e.g. glucose and oxygen, are exchanged between cells and capillaries so they’re adapted for efficient diffusion, so there is a short diffusion pathway
Their walls are only one cell thick, which also shortens the diffusion pathway
There are a number of capillaries, to increase surface area for exchange
Networks of capillaries in tissue are called capillary beds
What is tissue fluid?
The fluid that surrounds cells in tissues
Made from small molecules that leave the blood plasma, e.g. oxygen, water, nutrients
Cells take in oxygen and nutrients from the tissue fluid, and release metabolic waste into it
In a capillary bed, substances move out of the capillaries, into the fluid, by pressure filtration
Describe the process of tissue fluid
At the start oft he capillary bed, nearest the arteries, the hydrostatic pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid
This differences means an overall outward pressure forces fluid out of the capillaries and into the spaces around the cells, forming tissue fluid
As fluid leaves, the hydrostatic pressure reduces in the capillaries - so the hydrostatic pressure is much lower at the venule end of the capillary bed
Due to fluid loss, and an increasing concentration of plasma proteins, the water potential at the venule end of the capillary bed is lower than the water potential in the tissue fluid
This means some water re-enters the capillaries from the tissue fluid at the venule end by osmosis
Any excess tissue fluid is drained into the lymphatic system, which transports this excess fluid from the tissues and passes it back into the circulatory system
What does a high blood pressure mean for tissue fluid?
High blood pressure means a high hydrostatic pressure int he capillaries, which can lead to an accumulation of tissue fluid in the tissues
Describe the left ventricle of the heart
Thicker, more muscular walls than the right ventricle - allows it to contract more powerfully and pump blood all the way around the body
The right side is less muscular so its contractions are only powerful enough to pump blood to the nearby lungs
Describe the ventricles of the heart
Thicker walls than atria therefore they can push blood out of the heart, whereas the atria just need to push blood for a short distance into the ventricles
Describe the atrioventricular valves of the heart
Link the atria to the ventricles and stop blood flowing back into the atria when the ventricles contract
Describe the semi-lunar valves of the heart
Link the ventricles to the pulmonary artery and aorta, and stop blood flowing back into the heart after the ventricles contract
Describe the cords of the heart
Attach the atrioventricular valves tot eh ventricles to stop them being forced up into the atria when the ventricles contract
Describe the movement of valves in the heart
The valves only open one way - whether they’re open or closed depends on the relative pressure of the heart chambers
If there’s high pressure behind the valve, it’s forced open, but if pressure is higher in front of the valve it’s forced shut
This means that the flow of blood is unidirectional - only flows in one direction
What is cardiac contraction called?
Systole
What is cardiac contraction called?
Diastole
Describe the first stage of the cardiac cycle
Ventricles are relaxed
Atria contract, decreasing the volume of the chambers and increasing pressure inside the chambers
This pushed the blood into the ventricles
There’s a slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracting atria
Describe the second stage of the cardiac cycle
Atria relax
Ventricles contract decreasing their volume and increasing their pressure
The pressure becomes higher in the ventricles than the atria, which forces the AV valves shut to prevent back-flow
The pressure in the ventricles is also higher than in the aorta and pulmonary artery, which forces open the SL valves and blood is forced out into these arteries
Describe the third stage of the cardiac cycle
Ventricles and atria both relax
Higher pressure in the pulmonary artery and aorta closes the SL valves to prevent back-flow into the ventricles
Blood returns to the heart and the atria fill again due to the higher pressure in the vena cava and pulmonary vein
In turn this starts to increase the pressure of the atria
As the ventricles continue to relax, their pressure falls below the pressure of the atria and so the AV valves open
This allows blood to flow passively into the ventricles from the atria
The atria contract, and the whole process begins again
What is the equation for cardiac output?
Stroke volume x heart rate
What does heart rate mean?
Number of beats per minute
What is stroke volume?
Volume of blood pumped during each heartbeat, measure in cm3
What is CVD?
CVD is a general term used to describe diseases associated with the heart and blood vessels
Include aneurysms, thrombosis and myocardial infarction
Most CVDs start with atheroma formation
What is CHD?
Type of CVD
Occurs when the coronary arteries have lots of atheromas in them, which restricts blood flow to the heart muscle
Can lead to myocardial infarction
Describe atheroma formation
Wall of artery is made of several layers
Endothelium is usually smooth and unbroken
If damage occurs to the endothelium, white blood cells and lipids from the blood, clump together under the lining to form fatty streaks
Over time, more white blood cells, lipids and connective tissue build up and harden to form a fibrous plaque called an atheroma
This plaque partially blocks the lumen of the artery and restricts blood flow, which causes blood pressure to increase
Describe aneurysm formation
A balloon-like swelling of the artery
It starts with the formation of atheromas
Atheroma plaques damage and weaken arteries
They also narrow arteries, increasing blood pressure
When blood travels through a weakened artery at high pressure, it may push the inner layers of the artery through the outer elastic layer to form an aneurysm
This aneurysm may burst, causing a haemorrhage (bleeding)
Describe thrombosis formation
Formation of a blood clot
It also starts with the formation of atheromas
An atheroma plaque can rupture the endothelium of an artery
This damages the artery wall and leaves a rough surface
Platelets and fibrin accumulate at the site of damage and form a blood clot
This blood clot can cause a complete blockage of the artery, or it can become dislodged and block a blood vessels elsewhere in the body
Debris from the rupture can cause another blood clot to form further down the artery
Describe myocardial infarction
The heart muscle is supplied with blood by the coronary arteries
This blood contains the oxygen needed by the heart muscle cells to carry out respiration
If a coronary artery becomes completely blocked an area of the heart muscles will be totally cut off from its blood supply, receiving no oxygen
This causes a myocardial infarction
A heart attack can cause damage and death of the heart muscle
Symptoms include pain in the chest and upper body, shortness of breath and sweating
If large area of the heart muscle are affected complete heart failure can occur, which is often fatal
What are the risk factors for CVD?
High blood pressure
High blood cholesterol and poor diet
Cigarette smoking
Describe how high blood pressure is a risk factor for CHD
Increases risk of damage to artery walls
Damaged walls have an increased risk of atheroma formation, causing a further increase in blood pressure
Atheromas can also cause blood clots to form
A blood clot could block flow of blood to the heart muscle, possibly resulting in myocardial infarction
Anything increasing blood pressure also increases the risk of CVD e.g. overweight, not exercising, excessive alcohol consumption
Describe high blood cholesterol and poor diet as risk factors for CVDs
If levels are high then the risk of CVD increases
Cholesterol is one of the main constituents of the fatty deposits that form atheromas
Atheromas can lead to increased blood pressure and blood clots, which could cause a myocardial infarction
A diet high in saturated fat is associated with high blood cholesterol levels
A diet high in salt also increases the risk of CVDs because it increases the risk of high blood pressure
Describe how cigarette smoking is a risk factor for CHD
Both CO and nicotine, found in cigarette smoke, increase the risk of CVDs and myocardial infarction
CO combines with haemoglobin and reduces the amount of oxygen available to tissues
If the heart muscle doesn’t receive enough oxygen it can lead to a heart attack
Smoking also decreases the amount of antioxidants in the blood - these are important for protecting cells from damage
Fewer antioxidants means cell damage in the coronary artery walls are more likely, and this can lead to atheroma formation
What is the xylem?
Xylem tissue transports water and mineral ions in solution
These substances move up the plant from the roots to the leaves
What is the phloem?
Phloem tissue transports organic substances like sugars both up and down the plant
What type of systems are the xylem and phloem?
Mass transport systems
Describe how water moves up the plant through cohesion and tension
- Water evaporates from the leaves at the ‘top’ of the xylem. This is transpiration
- This creates tension, which pulls more water into the leaf
- Water molecules are cohesive so when some are pulled into the leaf other follow. This means the whole column of water in the xylem, from the leaves down the the roots, moves upwards
- Water then enter the stem through the roots
What is transpiration?
Evaporation of water from a plant’s surface, especially the leaves
Water evaporates from the moist cell walls and accumulates in the spaces between the cells in the leaf
When the stomata open, it moves out of the leaf down the water potential gradient
What four factors affect transpiration rate?
Light intensity
Temperature
Humidity
Wind
How does light intensity affect transpiration rate?
The lighter it is the faster transpiration rate
Because stomata open when it gets light to let in CO2 for photosynthesis
When it’s dark the stomata are usually closed, so there’s little transpiration
How does temperature affect transpiration rate?
The higher the temperature the faster the transpiration rate
Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster
This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster
How does humidity affect transpiration rate?
The lower the humidity, the faster the transpiration
If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration rate
How does wind affect transpiration rate?
The windier it is, the faster the transpiration rate
Lots of air movement blows away water molecules from around the stomata
This increases the water potential gradient, which increases the rate of transpiration
What is a potometer?
Used to measure transpiration rate
Measure water uptake by a plant, but it’s assumed that water uptake by the plant is directly related to water loss by the leaves
You can use it to estimate how different factors affect the transpiration rate
Explain how a potometer is used
- Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the surface area available for water uptake
- Assemble the potometer under the water and insert the shoot with the apparatus still under the water, so no air can enter
- Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water
- Check that the apparatus is water tight and air tight
- Dry the leaves, allowing time for the shoot to acclimatise and then shut the tap
- Remove the end of the capillary tube from the beaker of water once an air bubble has formed, then put the end of the tube back into the water
- Record the starting position of the air bubble
- Start a stopwatch and record the distance moved by the bubble per unit time, e.g. per hour. The rate of air bubble movement is an estimate of the transpiration rate
- Only change one variable at a time. All other conditions must be kept constant
Describe the structure and function of the phloem
Phloem tissue transports organic solutes round plants
Like xylem, phloem is formed from cells arranged in tube
Sieve tube elements and companion cells are important cell types in phloem tissue
What are sieve tube elements and companion cells?
Living cells that form the tube for transporting solutes
They have no nucleus and few organelles so there’s a companion cell for each sieve tube element
They carry out living function for sieve cells, e.g. providing the energy needed for the active transport of solutes
What is translocation?
Movement of solutes to where they’re needed in a plant
Energy-requiring process that happens in the phloem
What are assimilates?
Solutes
Where does translocation move solutes to and from?
Sources to sinks
What is the source?
Where assimilates are produced
What is the sink?
Where assimilates are used up
How do enzymes maintain a concentration gradient form the source to the sink?
Changing the solutes at the sink
Makes sure there’s always a lower concentration at the sink than at the source
What happens at the source in the mass flow hypothesis?
Active transport is used to actively load solutes from companion cells into the sieve tubes of the phloem at the source
This lowers the water potential inside the sieve tubes, so water enter the tubes by osmosis from the xylem and companion cells
This creates a high pressure inside the sieve tubes at the source end of the phloem
What happens at the sink in the mass flow hypothesis?
At the sink end, solutes are removed from the phloem to be used up
This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis
This lowers the pressure inside the sieve tubes
What happens to the flow in the mass flow hypothesis?
The result is a pressure gradient from the source end to the sink end
This gradient pushed solutes along the sieve tubes towards the sink
When they reach the sink the solutes will be used or stored
The higher the concentration of sucrose at the source, the higher the rate of translocation
Describe supporting evidence of the mass flow hypothesis - ring of bark
If a ring of bark is removed from a woody stem, a hug bulge forms above the ring. The fluid from the bulge has a higher concentration of sugars than the fluid from below the ring. This is because the sugars can’t move past the area where the bark has been removed - this is evidence that there can be a downward flow of sugars
Describe supporting evidence of the mass flow hypothesis - aphids
Pressure in the phloem can be investigated using aphids (they pierce the phloem, then their bodies are removed leaving the mouthparts behind, which allows the sap to flow out)
The sap flows out quicker nearer the leaves than further down the stem - the is evidence that there is a pressure gradient
Describe supporting evidence of the mass flow hypothesis - radioactive tracer
A radioactive tracer such as radioactive carbon 14C can be used to track the movement of organic substances in a plant.
CO2 containing radioactive isotope 14C is used as a radioactive tracker
This radioactively-labelled CO2 can be supplied to a single leaf by being pumped into a container which completely surrounds the leaf. The radioactive carbon will then be incorporated into organic substances produced by the leaf which will be moved around the plant by translocation.
The movement of these substances can be tracked by autoradiography. To reveal where the radioactive tracer has spread to in a plant, it is killed and then the whole plant is placed onto photographic film - wherever the film turns black is where the radioactive substance is present;
Describe supporting evidence of the mass flow hypothesis - metabolic inhibitor
If a metabolic inhibitor which stops ATP production is put into the phloem, then translocation stops - this is evidence that active transport is involved
Describe an objection of the mass flow hypothesis - different sinks
Sugar travels to many different sinks, not just to one with the highest water potential
Describe an objection of the mass flow hypothesis - sieve plate barriers
The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate
