topic 3C - circulatory system & mass transport in plants Flashcards
circulatory systems described as being one of two things…
open or closed
what occurs in a closed circulatory system?
blood is pumped around the body and is always contained within a network of blood vessels
which animals have closed circulatory systems?
all vertebrates and many invertebrates
what occurs in an open circulatory system?
blood is not contained within blood vessels but is pumped directly into body cavities
which organisms have an open circulatory system?
arthropods and molluscs
what system do humans have?
a closed double circulatory system
what is a closed double circulatory system?
in one complete circuit of the body, blood passes through the heart (the pump) twice
what does the right side of the heart do?
pulmonary circulatory system:
pumps deoxygenated blood to the lungs for gas exchange
what does the left side of the heart do?
systemic circulatory system:
blood then returns to the left side of the heart (from the lungs), so that oxygenated blood can be pumped efficiently (at high pressure) around the body
main circulatory system structures:
-heart
-arteries
-arterioles
-capillaries
-venules
-veins
what is the heart?
-a hollow, muscular organ located in the chest cavity which pumps blood
-cardiac muscle tissue is specialised for repeated involuntary contraction without rest.
what are the arteries?
Blood vessels which carry blood away from the heart
what are arterioles?
small arteries which branch from larger arteries and connect to capillaries.
what are capillaries?
-tiny blood vessels which connect arterioles and venules
what are venules?
small veins which join capillaries to larger veins
what are veins?
blood vessels which carry blood back towards the heart
what are the main blood vessels?
-pulmonary artery
-pulmonary vein
-coronary arteries
-aorta
-vena cava
-renal artery
-renal vein
what does the pulmonary artery do?
it carries deoxygenated blood away from the heart, towards the lungs
what does the pulmonary vein do?
carries oxygenated blood away from the lungs, towards the heart
what do coronary arteries do?
supply the heart with oxygenated blood
what does the aorta do?
it carries oxygenated blood out of the heart and to the rest of the body
what does the vena cava do?
carries deoxygenated blood into the heart
what does the renal artery do?
it supplies the kidneys with oxygenated blood
what does the renal vein do?
carries deoxygenated blood away from the kidneys, towards the heart
mass & size of the human heart:
-around 300g
-roughly the size of a closed fist
what is the heart protected by?
the pericardium:
a tough and fibrous sac
what is the heart divided into?
four chambers:
-two top chambers = atria
-bottom two chambers = ventricles
what are the left & right side of the heart separated by?
a wall of muscular tissue, called the septum
what is the part of the septum that separates the atria called?
interatrial septum
what is the part of the septum that separates the ventricles called?
interventricular septum
why is the septum important?
it ensures that blood doesn’t mix between the left and right sides of the heart
when do valves in the heart open?
when the pressure of blood behind them is greater than the pressure in front of them
when do valves in the heart close?
when the pressure of blood in front of them is greater than the pressure behind them
why are valves important?
-they keep blood flowing forward in the right direction and stopping it flowing backwards
-they maintain the correct pressure in the chambers of the heart
what are the right atrium & right ventricle separated by?
tricuspid (atrioventricular valve)
what are the right ventricle and pulmonary artery are separated by?
the pulmonary valve
what are the left atrium and left ventricle are separated by?
the bicuspid (mitral valve)
what are the left ventricle and aorta are separated by?
the aortic valve
which vessels are connected to the left atrium?
pulmonary vein (oxygenated blood enters here)
aorta (distributes oxygenated blood to the rest of the the body)
which vessels are connected to the right
atrium?
vena cava (brings deoxygenated blood to the right atrium)
pulmonary artery (carries deoxygenated blood to the lungs)
what is the heart and what does it therefore require?
a muscle:
it needs its own blood supply for aerobic respiration
where are the coronary arteries?
on the surface of the heart
how should the coronary arteries be kept?
-the arteries should remain clear of plaques
-this could lead to angina or a heart attack
what do coronary arteries do for cardiac muscle cells?
-supply them with nutrients
-remove waste products
EXAM TIP!! where will the left side of the heart appear in an exam?
on the right
muscular walls of the atria…
thinner than those of the ventricles
the role of atria walls
when the atria contract, the thin muscular walls do not generate much pressure, but enough to force blood down into the ventricles, through the atrioventricular valves
the walls of the ventricles…
thicker and more muscular
the role of the ventricular walls:
after the atria contract, the ventricles contract and squeeze blood inwards, increasing its pressure and pushing it out of the heart through right and left semilunar valves
which valves are the semilunar valves?
aortic &
which valves are the atrioventricular valves?
bicuspid & tricuspid
what is the difference between the left & right side of the heart?
the muscle of the left ventricle is much thicker than the right ventricle
why is the left ventricle thicker than the right?
-the blood leaving the right ventricle travels less distance (to the lungs) than blood leaving the left ventricle (to the rest of the body to deliver oxygen for respiration)
pressure of the left ventricle:
-to reach the rest of the body, the blood leaving the left ventricle must be under high pressure
-this is generated by the contraction of the muscular walls of the left ventricle
(the right ventricle generates less pressure from the contraction of its thinner walls, as blood only has to reach the lungs)
explain the differences in pressure between left atrium and ventricle
1) the walls of the left atrium are thin, so the pressure generated by their contraction is low
2) low pressure is sufficient because blood is forced only a short distance; from the left atrium down to the left ventricle
3) the muscular walls of the left ventricle are much thicker and generate much higher pressure
4) this is because much more pressure is generated when this thick muscle contracts and squeezes blood with enough force to reach the rest of the body
what is the cardiac cycle?
the series of events that take place in one heart beat, including muscle contraction and relaxation
what is contraction of the heart called?
systole
what is relaxation of the heart called?
diastole
how frequent are cardiac cycles?
one cardiac cycle is followed by another in a continuous process
what does a contraction in the heart cause?
a decrease in volume in the corresponding chamber of the heart, which then increases again when the muscle relaxes
what do volume changes in the heart lead to?
pressure changes
when volume decreases, pressure ______
increases
when volume increases, pressure ______
decreases
why to valves open & close throughout the cardiac cycle?
as a result of pressure changes in different regions of the heart
steps of atrial systole:
1) the walls of the atria contract
2) atrial volume decreases, atrial pressure increases
3) the pressure in the atria rises above that in the ventricles, forcing the atrioventricular valves open
5) blood is forced into the ventricles, there is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria
6) the ventricles are relaxed at this point; ventricular diastole coincides with atrial systole
steps of ventricular systole:
1) the walls of the ventricles contract
2) ventricular volume decreases, ventricular pressure increases
3) the pressure in the ventricles rises above that in the atria, this forces the AV valves to close, preventing back flow of blood
4) the pressure in the ventricles rises above that in the aorta and pulmonary artery
5) this forces the semilunar valves open so blood is forced into the arteries and out of the heart
6) during this period, the atria are relaxing; atrial diastole coincides with ventricular systole
7) the blood flow to the heart continues, so the relaxed atria begin to fill with blood again
steps of diastole:
1) the ventricles and atria are both relaxed
2) the pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close
3) the atria continue to fill with blood
(blood returns to the heart via the vena cava and pulmonary vein)
4) pressure in the atria rises above that in the ventricles, forcing the AV valves open
5) blod flows passively into the ventricles without need of atrial systole
6) the cycle then begins again with atrial systole
lines on the cardiac cycle graph
lines on the graph represent the pressure of the left atrium, aorta, and the left ventricle
points of crossing on the cardiac cycle graph:
indicate when valves open and close
point A - cardiac cycle graph (start)
(the end of diastole)
-the atrium has filled with blood during the preceding diastole
-pressure is higher in the atrium than in the ventricle, so the AV valve is open
point A - B: slight increase ⌒
(cardiac cycle graph)
(atrial systole)
-left atrium contracts, causing an increase in atrial pressure and forcing blood into the left ventricle
-ventricular pressure increases slightly as it fills with blood
-pressure is higher in the atrium than in the ventricle, so the AV valve is open
point B: slight dip after ⌒
(cardiac cycle graph)
(beginning of ventricular systole)
-left ventricle contracts causing the ventricular pressure to increase
-pressure in the left atrium drops as the muscle relaxes
-pressure in the ventricle exceeds pressure in the atrium, so the AV valve shuts
point C: steep increase /
(cardiac cycle graph)
(ventricular systole)
-the ventricle continues to contract
-pressure in the left ventricle exceeds that in the aorta
-aortic valve opens and blood is forced into the aorta
point D - peak and then down
(cardiac cycle graph)
(beginning of diastole)
-left ventricle has been emptied of blood
-muscles in the walls of the left ventricle relax and pressure falls below that in the newly filled aorta
-aortic valve closes
point D - E: very steep drop \
(cardiac cycle graph)
(early diastole)
-the ventricle remains relaxed and ventricular pressure continues to decrease
-in the meantime, blood is flowing into the relaxed atrium from the pulmonary vein, causing an increase in pressure
point E: similar to start
(cardiac cycle graph)
(diastole)
-the relaxed left atrium fills with blood, causing the pressure in the atrium to exceed that in the newly emptied ventricle
-AV valve opens
after point E: down and then upwards
(late diastole)
There is a short period of time during which the left ventricle expands due to relaxing muscles
This increases the internal volume of the left ventricle and decreases the ventricular pressure
-at the same time, blood is flowing slowly through the newly opened AV valve into the left ventricle, causing a brief decrease in pressure in the left atrium
-the pressure in both the atrium and ventricle then increases slowly as they continue to fill with blood
how to calculate heart rate with a cardiac cycle graph:
1) work out the length of one heart beat
2) calculate how many heart beats occur per second (1/length of heartbeat)
3) calculate how many heart beats occur per minute (x60)
how to calculate heart rate with a cardiac cycle graph:
1) work out the length of one heart beat
2) calculate how many heart beats occur per second (1/length of heartbeat)
3) calculate how many heart beats occur per minute (x60)
different types of blood vessels:
-arteries
-veins
-arterioles
what do arteries do?
transport blood away from the heart (usually at high pressure)
what do veins do?
transport blood to the heart (usually at low pressure)
what do arterioles do?
transport blood into capillaries
what part of a blood vessel does the blood flow through?
the lumen
size of lumen in arteries:
narrow
purpose of the artery:
must be able to withstand high pressures generated by the contracting heart, and maintain these pressures when the heart is relaxed
wall of the artery
(+ components)
relatively thick:
layers of collagen, smooth muscle and elastic fibres
purpose of elastic fibres in the artery
-they allow the artery wall to expand around blood surging through at high pressure when the heart contracts, -these fibres then recoil when the heart relaxes
purpose of artery structure in a nutshell:
maintains high pressure
size of lumen in veins:
wide
what blood do veins receive?
blood that has passed through capillary networks (low pressure & must be returned to the heart)
the wall of the vein
(+ components)
relatively thin:
thinner layers of collagen, smooth muscle and elastic fibres
what do veins contain?
valves that prevent the backflow of blood, helping return blood to the heart
role of arterioles
they contract and partially cut off blood flow to specific organs
example of when arterioles are used:
during exercise blood flow to the stomach and intestine is reduced which allows for more blood to reach the muscles
components of arteriole walls:
lower proportion of elastic fibres and a large number of muscle cells
why do arterioles need muscle cells?
allows them to contract and close their lumen to stop blood flow
what are capillaries?
another type of blood vessel present in the circulatory system
what can capillaries form?
networks called capillary beds which are very important exchange surfaces within the circulatory system
structures of the capillary:
-they have a very small diameter
-capillaries branch between cells
-wall is one cell thick
-think, leaky walls
-the cells of the wall have gaps called pores
structure of capillary (very small diameter) & purpose
forces the blood to travel slowly which provides more opportunity for diffusion to occur
structure of capillary (capillaries branch between cells) & purpose
substances can diffuse between the blood and cells quickly as there is a short diffusion distance
structure of capillary (wall is one cell thick) & purpose
wall of the capillary is made solely from a single layer of endothelial cells
↳ this reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body
structure of capillary (cells of the wall have pores) & purpose
allow blood plasma to leak out and form tissue fluid
structure of capillary (thin, leaky walls) & purpose
allow substances to leave the blood to reach the body’s tissues
what is plasma?
a straw-coloured liquid that amounts to around 55% of the blood
what is the plasma mainly comprised of?
water (95%)
what does this component do?
water is a good solvent, many substances can dissolve in it, allowing them to be transported around the body
what happens as blood passes through the capillary?
-some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body
-this results in the formation of tissue fluid
composition of plasma vs tissue fluid:
virtually the same, although tissue fluid contains far fewer proteins
why does tissue fluid have less proteins?
proteins are too large to fit through gaps in the capillary walls and so remain in the blood
what bathes the cells outside of the circulatory system?
tissue fluid
exchange and tissue fluid
exchange of substances between cells and the blood occurs via the tissue fluid
example of exchange via tissue fluid:
(carbon dioxide)
-carbon dioxide produced in aerobic respiration will leave a cell, dissolve into the tissue fluid surrounding it, and then diffuse into the capillary
what does the amount of liquid that leaves the plasma to form tissue fluid depend on?
two opposing forces:
hydrostatic pressure
when does hydrostatic pressure occur?
when blood is at the arteriole end of a capillary, the hydrostatic pressure is great enough to push molecules out of the capillary
steps of tissue fluid formation:
1) high hydrostatic pressure is created because arteries narrow into arterioles
2) this pressure forces water & molecules (tissue fluid) out of the blood plasma
3) large proteins remain in the blood, creating a water potential difference between the capillary and the tissue fluid
4) this means there is low water potential in the capillary than in the tissue fluid
5) water re enters the capillary by osmosis (more fluid leaves the capillary than returns, leaving tissue fluid behind to bathe cells)
6) any remaining tissue fluid is is drained into the lymphatic system and is eventually returned to the blood (circulatory system)
what is odeoma?
-if blood pressure is high (hypertension) then the pressure at the arteriole end is even greater
-this pushes more fluid out of the capillary and fluid begins to accumulate around the tissues
which two destinations can the tissue fluids go to after formation?
1) re-enter the capillaries
2) enter the lymph capillaries
are lymph capillaries part of the circulatory system?
no, they are separate
structure of lymph capillaries:
have closed ends and large pores that allow large molecules to pass through
what happens to large molecules in lymph capillaries?
larger molecules that can’t pass through the capillary wall enter the lymphatic system as lymph
what is the entry point of the lymphatic system?
small valves in the vessel walls
how does liquid move along the lymphatic system?
-liquid moves along the larger vessels by compression caused by body movement
-backflow is prevented by valves
how does lymph re-enter the bloodstream?
through veins located close to the heart
lymph capillaries & proteins
-any plasma proteins that have escaped from the blood are returned to the blood via the lymph capillaries
-if plasma proteins weren’t removed from tissue fluid they could lower the water potential of the tissue fluid and prevent the reabsorption of water into the blood in the capillaries
lymph system & lipids
after digestion, lipids are transported from the intestines to the bloodstream by the lymph system
what is coronary heart disease (CHD)?
any condition that interferes with the coronary arteries
main risk factors for CHD:
-genetic factors
-age and sex
-high blood pressure
-smoking
-high concentrations of low-density lipoproteins (LDLs)
CHD risk factors: genetic factors
individuals can have a genetic predisposition that increases their chance of developing CHD
CHD risk factors: age & sex
the risk of CHD increases with age and is much more likely to affect men
CHD risk factors: high blood pressure
this can cause arteries to develop thicker walls, lumens to narrow, atheromas to develop
CHD risk factors: smoking
the chemical in tobacco can damage the heart and lungs and negatively impact blood pressure
CHD risk factors: high concentrations of low-density lipoproteins in the blood
these are the lipoproteins that cause atheromas to develop
what is a correlation?
an association or relationship between variables
what is causation?
when one variable has an influence or is influenced by, another
what are scatter diagrams used for?
to identify correlations between two variables to determine if a factor does increase the risk of developing a disease
what can make it hard to determine some causal relationships?
the interaction between risk factors in studies and investigations
mineral ions and organic compounds within plants:
both transported while dissolved in water:
-the dissolved mineral ions are transported in the xylem tissue
-the dissolved organic compounds are transported in the phloem tissue
what are plant roots responsible for?
the uptake of water and mineral ions
what do plant roots have and what do they do?
they have root hairs
↳ increase the surface area for absorption of the substances
what process is used to uptake water?
osmosis
what process is used to uptake minerals?
diffusion or active transport
plants must take in a constant supply of water and dissolved minerals to…
to compensate for the continuous loss of water via transpiration in the leaves
so that they can photosynthesise and produce proteins
what are two pathways that water (and the dissolved solutes) can take to move across the cortex?
-apoplast/apoplastic
-symplast/symplastic
which pathway does most water travel via?
the apoplast pathway
what is the apoplast pathway?
the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
what process does water move by during the apoplast pathway?
by diffusion (as it is not crossing a partially permeable membrane)
where does water move when moving through the apoplast pathway?
from cell wall to cell wall directly or through the spaces between cells
which pathway is quicker?
the movement of water through the apoplast pathway occurs more rapidly than the symplast pathway
what happens when the water reaches the endodermis? (apoplast pathway)
the casparian strip and forms an impassable barrier for the water
what is the casparian strip?
a thick, waterproof, waxy band of suberin within the cell wall
what must water and dissolved minerals do when they reach the casparian strip?
they must take the symplast pathway
what parts are included in the symplast pathway?
cytoplasm, plasmodesmata, and vacuole of the cells
how does water move in the symplast pathway?
1) the water moves by osmosis into the cell (across the partially permeable cell surface membrane)
2) moves into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata
why does the movement of water through a plant’s xylem occur?
due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules
how is the water potential gradient between the roots and the leaves?
plants are constantly taking water in at their roots and losing water via the stomata (in the leaves)
what is the water potential gradient the driving force behind?
the movement of water from the soil (high water potential) to the atmosphere (low water potential), via the plant’s cells
what happens to around 99 % of the water absorbed?
it is lost through evaporation from the plant’s stem and leaves through transpiration
what is transpiration?
the loss of water vapour via the stomata by diffusion
what is the transpiration stream?
the movement of water from the roots to the leaves
why is transpiration important to the plant?
-it provides a means of cooling the plant (evaporative cooling)
-the transpiration stream is helpful in the uptake of mineral ions
-the turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants
what does transpiration result in in the leaves?
lower water potential in the leaves:
-this creates a concentration gradient between the roots and leaves
-this causes water to move upwards
environmental conditions and the leaves:
-certain environmental conditions can cause a water potential gradient between the air inside the leaves (higher water potential) and the air outside (lower water potential) which results in water vapour diffusing out of the leaves through the stomata (transpiration)
what does the water vapour lost by transpiration do to the spongy mesophyll?
-it lowers the water potential in the air spaces surrounding the mesophyll cells
-the water within the mesophyll cell walls evaporates into these air spaces resulting in a transpiration pull
what does transpirational pull result in?
1) water moves through the mesophyll cell walls (apoplast) or out of the mesophyll cytoplasm (symplast)
2) the pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits
3) water then moves up the xylem vessels to replace this lost water (due to the cohesive and adhesive properties of the water (transpiration stream)
4)
what happens when transpiration rates are high?
the walls of the xylem are pulled inwards by the faster flow of water
what is transpiration controlled by?
the pairs of guard cells that surround stomata
movement of stomata coordinated by guard cells:
-guard cells open the stomata when they are turgid
-guard close the stomata when they lose water
transpiration & gas exchange when guard cells are open vs closed:
stomata open = greater rate of transpiration and of gaseous exchange
stomata open = greater rate of transpiration and of gaseous exchange
when are stomata open generally?
stomata allow gaseous exchange (CO2 in and O2 out), they are generally open during the day
water moving through a plant from the top
water evaporates from the mesophyll cells into the air spaces in the leaf and then water vapour diffuses through the stomata
which factors affect transpiration rates?
-air movement
-humidity
-temperature
-light intensity
how does air movement affect the rate of transpiration?
high air movement increases transpiration:
-good airflow removes water vapour from the air surrounding the leaf
-this sets up a concentration gradient between the leaf and the air, increasing water loss
how does humidity affect the rate of transpiration?
more humidity decreases transpiration rate:
-when the air is saturated with water vapour the concentration gradient is weaker so less water is lost
what is humidity?
a measure of moisture (water vapour) in the air
how does light intensity affect the transpiration rate?
an increase in light intensity increases transpiration rate:
-guard cells are responsive to light intensity
-when it is high they are turgid and the stomata open allowing water to be lost
how does temperature affect the transpiration rate?
high temperature increases transpiration rate:
-at higher temperatures, particles have more kinetic energy so transpiration occurs at a faster rate
-water molecules evaporate from the mesophyll and diffuse away faster than at lower temperatures
what can a potometer be used for?
to investigate the effect of environmental factors on the rate of transpiration
setting up a potometer:
1) cut a shoot underwater to prevent air from entering the xylem
2) place the shoot in the tube
3) set up the apparatus as shown in the diagram
4) the junction between the shoot and potometer is sealed (usually with petroleum jelly) to prevent any air leaks
5) dry the leaves of the shoot, any moisture present on the leaves will affect the rate of transpiration
6) remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water
7) set up the environmental factor you are investigating
8) allow the plant to adapt to the new environment for 5 minutes
method of using a potometer to investigate transpiration rate:
1) record the starting location of the air bubble
2) leave for a set period of time
3) record the end location of the air bubble
4) change factor being investigated so that a new DV can be looked at
5) reset the bubble by opening the tap below the reservoir
6) repeat the experiment
7) the further the bubble travels in the same time period, the faster transpiration is occurring and vice versa
how to investigate airflow with a potometer:
set up a fan or hairdryer
how to investigate humidity with a potometer:
spray water in a plastic bag and wrap around the plant
how to investigate light intensity with a potometer:
change the distance of a light source from the plant
how to investigate temperature with a potometer:
temperature of room (cold room or warm room)
what is translocation?
transport of substances (assimilates) in the phloem
what are assimilates?
substances that will become incorporated into biological tissue (e.g. sucrose in the phloem tissue)
translocation in the phloem definition:
the transport of assimilates from source to sink
what does translocation require?
the input of metabolic energy (ATP)
what is the liquid that is being transported in the phloem?
phloem sap (found within phloem sieve tubes)
what does phloem sap consist of?
-sugars (mainly sucrose)
-water
-other dissolved substances such as amino acids, hormones and minerals
what can the source of assimilates be?
-green leaves and green stem
-storage organs
-food stores in seeds
sources: green leaves and green stem
-photosynthesis produces glucose which is transported as sucrose
-sucrose has less of an osmotic effect than glucose
sources: storage organs
tubers and tap roots (unloading their stored substances at the beginning of a growth period)
sources: food stores in seeds
germinating seeds
what are sinks?
where the assimilates are required
examples of sinks:
-meristems
-roots
-storage areas
sinks: meristems
are actively dividing
sinks: roots
growing and / or actively absorbing mineral ions
sinks: storage areas
-part of the plant where the assimilates are being stored
(eg. developing seeds, fruits or storage organs)
which process is used during translocation?
the loading and unloading of the sucrose from the source to the phloem, and from the phloem to the sink is an active process
what can translocation be inhibited by?
can be slowed down or even stopped at high temperatures or by respiratory inhibitors
what direction is translocation?
up or down in the phloem sieve tubes
how are carbohydrates generally transported in plants?
in the form of sucrose
why is it good to transport carbohydrates as sucrose?
- it allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy)
-it’s less reactive than glucose as it is a non-reducing sugar and no reactions occur as it is being transported
which living cells does the phloem consist of?
-sieve tubes
-companion cells
description of sieve tubes:
specialised for transport:
-have no nuclei
-each has a perforated end so its cytoplasm connects one cell to the next
-sucrose and amino acids are translocated in living cytoplasm of the sieve tubes
description of companion cells:
-transport of substances in the phloem needs energy
-1 + compells attach to each sieve tube to provide this energy
-a sieve tube is dependent on its companion cell
what is transported in the phloem vs xylem?
xylem:
water
phloem:
organic substances
which pathways could sucrose molecules use to travel to the sieve tubes?
(not fully understood)
symplast or apoplast
what happens if sodium molecules take the apoplast pathway? (steps)
1) modified companion cells (transfer cells) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls. this is an active process and therefore requires ATP as an energy source
2) the large concentration of hydrogen ions in the cell wall of the companion cell results in the hydrogen ions moving down the concentration gradient back to the cytoplasm of the companion cell
3) the hydrogen ions move through a cotransporter protein. while transporting the hydrogen ions this protein also carries sucrose molecules into the companion cell against the concentration gradient for sucrose
4) the sucrose molecules then move into the sieve tubes via the plasmodesmata from the companion cells
feature & function of companion cells:
have infoldings in their cell surface membrane:
-increase the available surface area for the active transport of solutes
many mitochondria
-provide the energy for the proton pump
effects of the mechanism of the loading of assimilates:
some plants can build up the sucrose in the phloem to much higher than the concentration in the mesophyll
what does a high concentration of sucrose do?
-it decreases the water potential in the phloem, water enters by osmosis
-the entry of water results in a high pressure (pressure gradient) which enables the mass flow of sugars towards sink tissues
-at the sink tissues, sugars are unloaded
what happens at the sinks?
unloading of assimilates
the unloading of sucrose at the sinks:
1) sucrose is actively transported out of the companion cells
2) it then moves out of the phloem tissue via apoplastic or symplastic pathways
how is a concentration gradient maintained in sink tissues? (sucrose)
-sucrose is converted into other storage molecules such as starch
-this is a metabolic reaction so requires enzymes
what is the mass flow hypothesis?
the model initially used to explain the movement of assimilates in the phloem tissue
who made the first mass flow hypothesis model and when?
ernst münch in 1930
what did the mass flow hypothesis model originally consist of?
-two partially permeable membranes containing solutions with different concentrations of ions (one dilute the other concentrated)
-these two membranes were placed into two chambers containing water and were connected via a passageway
-the two membranes were joined via a tube
-as the membranes were surrounded by water, the water moved by osmosis across the membrane containing the more concentrated solution which forced the solution towards the membrane containing the more dilute solution (where water was being forced out of due to hydrostatic pressure)
do scientist still support the mass flow hypothesis?
no, they now support a modified version of this hypothesis – the pressure flow gradient
how does phloem sap move in a plant?
it moves by mass flow up and down the plant
what is the advantage of mass flow?
it moves the organic solutes faster than diffusion
what causes mass flow?
a pressure difference in xylem tissue
why does the concentration gradient that causes mass flow occur?
water potential gradient between the soil and leaf (no energy needed)
how is mass flow created in the phloem?
in phloem tissue energy is required to create pressure differences for the mass flow of the organic solutes
what is the pressure difference in the phloem generated by?
actively loading sucrose into the sieve elements at the source, this lowers the water potential in the sap
what does a water gradient in the phloem lead to?
results in water moving into the sieve elements as it travels down the water potential gradient by osmosis
steps of mass flow in the phloem:
1) the presence of water within the sieve elements increases the hydrostatic pressure at the source and as solutes (eg. sucrose) are removed / unloaded from the sieve elements causing water to follow by osmosis at the sink (creating a low hydrostatic pressure), a hydrostatic pressure gradient occurs
2) the pressure difference between the source and the sink results in the mass flow of water from the high hydrostatic pressure area to the low hydrostatic pressure area
3) the mass flow of organic solutes within the phloem tissue occurs above and below the sources (which is typically photosynthesising leaves). therefore sap flows upwards and downwards within a plant
what have scientists debated about phloem transport?
the mechanism of transport that occurs within the phloem
what does the mass flow hypothesis suggest about the phloem?
-translocation of sucrose and other sugars within the phloem occurs via a continuous unidirectional flow of water and dissolved nutrients
-the direction of flow is from the source to the sink
what should happen if the mass flow hypothesis is correct?
in any one sieve tube there should be a bulk flow of phloem sap in one direction, travelling at the same rate
what should happen if the mass flow hypothesis is correct?
in any one sieve tube there should be a bulk flow of phloem sap in one direction, travelling at the same rate
evidence supporting the mass flow hypothesis:
-when the phloem sieve tube is punctured phloem sap oozes out, this suggests that it is under pressure
-phloem sap taken from near a source has a higher sucrose concentration than sap taken from near a sink (this suggests that different water potentials would result in osmosis into/out of the sieve tubes at those two locations)
evidence supporting the mass flow hypothesis (plant virus)
-when a plant virus is applied to leaves that are well-lit the virus can be observed moving down the phloem towards the roots, this demonstrates the bulk flow of substances in one direction
-when the virus is applied in the dark it is not transported, this suggests that for translocation to occur photosynthesis and the production of sucrose is required in the source tissue
evidence contradicting the mass flow hypothesis:
-the rate of translocation of different organic substances was measured and the results showed that amino acids appeared to travel more slowly than sucrose (the mass flow hypothesis states that substances should be flowing at the same rate)
-some scientists have conducted experiments that detected different substances (within the same sieve element) moving in opposite directions
-it has been suggested that some sieve tubes translocate at different times
(the mass flow hypothesis states that nearly all sieve tubes should be involved in translocation at the same time as they are all connected to the same leaves)
which experiments have been used to investigate mass transport in plants?
tracer and ringing experiments
what is a ringing experiment?
it involves the removal of a ring of surface tissues from the stem of the plant while leaving the stem core intact
what is included in the ring that is removed?
the ring removes the phloem only with the xylem remaining intact
what happens after ringing has been done?
the plant can then be exposed to a radioactive tracer so that the direction and rate of translocation can be investigated
what is usually used for experiments investigating mass transport?
-14CO2 is commonly used for these experiments as it is readily absorbed by the leaves and used in photosynthesis to produce sucrose
-the sucrose formed will be radioactive so its subsequent movement around the plant via translocation can be traced
what is detected from a ringing experiment?
the amounts of radioactive carbon present in different parts of the plant can be detected
what should the results of a ringing experiment be if the mass flow hypothesis is right?
the bulk flow of phloem sap should be in one direction (from source to sink) and occur at the same rate in any sieve tube at the same time
complications of the ringing experiment:
if the xylem is damaged during the ringing process the plant will not have an adequate supply of water and will wilt
steps of a ringing experiment with radioactive carbon dioxide:
1) an experiment was carried out when different plants were ringed at different locations on the stem
2) the plants were then supplied with radioactive carbon dioxide
3) after a period of time the levels of radioactive carbon in the different parts of the plant were measured
(there is a control plant used in this experiment to illustrate where sucrose is translocated when a plant is intact)
the results of a ringing experiment with radioactive carbon dioxide:
the phloem is involved in the transport of sucrose:
-there is no radioactive sucrose detected past the ringing point on the stems (where the phloem has been removed)
in the phloem the transport of sucrose occurs both upwards and downwards:
-sucrose is translocated from the source tissues in the leaves to the sink tissues above and below
other uses of radioactive tracers:
-they can also be used to show where substances are transported using autoradiographs
-after a plant has taken up radioactive carbon dioxide and carried out photosynthesis and translocation, it is pressed against photographic paper (in the dark) for several hours
-the radioactive material present in the plant tissues creates an image to show here it is present
other uses of radioactive tracers:
-they can also be used to show where substances are transported using autoradiographs
-after a plant has taken up radioactive carbon dioxide and carried out photosynthesis and translocation, it is pressed against photographic paper (in the dark) for several hours
-the radioactive material present in the plant tissues creates an image to show here it is present
