3B More Exchange and Transport Systems Flashcards
How is food broken down in the body?
It is broken down into smaller molecules by digestion
Why do large biological molecules have to be broken down?
They’re too big to cross cell membranes, this means they can’t be absorbed from the gut into the blood
When large biological molecules are broken down what does this allow?
They are broken down into smaller molecules which can move across membranes
They can easily be transported from the blood to the gut & transported around the body for cell use
How are most large biological molecules broken down and how does this relate to their structure?
Hydrolysis reactions
Most large biological molecules are polymers therefore they can be broken down into monomers via hydrolysis reactions
What are carbohydrates broken down into using hydrolysis?
Disaccharides and then monosaccharides
What are fats broken down into using hydrolysis?
Fatty acids and monoglycerides
What are proteins broken down into using hydrolysis?
Amino acids
How is the variety of digestive enzymes produced?
Specialised cells in the digestive system produce the variety
What breaks down carbohydrates?
Amylase and Membrane-Bound Disaccharides
What is amylase?
A digestive enzyme that catalyses the conversion of starch into the smaller sugar (a disaccharide)
How does amylase break down carbohydrates?
The hydrolysis of the glycosidic bonds in starch
Where is amylase produced?
The salivary glands - release amylase in the mouth
The pancreas - releases amylase into the small intestine
What are membrane-bound disaccharides?
They’re enzymes that are attached to the cell membrane of epithelial cells lining the ileum
Where are membrane-bound disaccharides attached to?
The cell membranes of epithelial cells lining the ileum
What do membrane bound-disaccharides help to break down?
Disaccharides (i.e. maltose, sucrose and lactose) into monosaccharides (i.e. glucose, fructose and galactose)
How do membrane bound-disaccharides break down disaccharides?
The hydrolysis of glycosidic bonds
How are monosaccharides transported across the ileum?
relating to membrane-bound disaccharides
Transported across the cell membranes of the ileum epithelial cells via specific transported proteins
How are lipids broken down?
By lipase with the help of bile salts
What does lipase break lipids down into?
It catalyses the breakdown of lipids into monoglycerides & fatty acids
How does lipase break down lipids?
By the hydrolysis of the ester bonds in lipids
Where are lipases made?
In the pancreas
Where do lipases work?
In the small intestine
Where are bile salts produced?
In the liver
What is the role of bile salts?
They emulsify lipids - this means they can cause lipids to form small droplets
How do bile salts aid the process of lipid digestion?
Several small lipid droplets have a bigger SA than a single large droplet
Formation of droplets greatly increases the SA of lipid available for lipases to work on
What are micelles?
Once the lipid has been broken down, the monoglycerides & fatty acids stick with the bile salts - this forms the tiny structures called micelles
What is formed by the breakdown of small lipid droplets?
Micelles
What are proteins broken down by?
Peptidases (proteases)
What are the two types of peptidases called?
- Endopeptidases
- Exopeptidases
What do endopeptidases do?
Hydrolyse peptide bonds within a protein
Give 3 examples of endopeptidases
- Trypsin
- Chymotrypsin
- Pepsin
Where are Trypsin and Chymotrypsin produced and secreted?
Produced: In the pancreas
Secreted: Into the small intestine
Where is pepsin released?
Into the stomach by cells in the stomach lining
What conditions does pepsin work best in and how are these conditions provided?
Pepsin works best in acidic conditions - these conditions are provided by hydrochloric acid in the stomach
What do exopeptidases do?
Hydrolyse peptide bonds at the ends of protein molecules
They remove single amino acids from proteins
What are dipeptidases?
They are exopeptidases that work specifically on dipeptides
What do dipeptidases do?
They act to separate 2 amino acids that make up a dipeptide by hydrolysing the peptide bond between them
Where are dipeptidases often located?
In the cell surface membrane of epithelial cells in the small intestine
Where are the products of digestion absorbed?
Across the ileum epithelium into the bloodstream
How is glucose absorbed across the ileum?
By active transport with sodium ions via a co-transporter protein
How is galactose absorbed across the ileum?
By active transport with sodium ions via a co-transporter protein
How is fructose absorbed across the ileum?
Via facilitated diffusion through a different transporter protein
What do micelles do?
They help to move monoglycerides & fatty acids towards the epithelium
How do micelles “release” monoglycerides?
Micelles constantly break up and reform so they can “release” monoglycerides and fatty acids - this allows them to be absorbed
Can micelles be taken up across the epithelium?
Whole micelles cannot be taken up across the epithelium
How are monoglycerides and fatty acids transported across the epithelium?
They are lipid-soluble so they can diffuse across the epithelial cell membrane
How are amino acids absorbed across the ileum?
Via co-transport - similar to glucose and galactose
How is oxygen transported around the body?
By haemoglobin - contained in red blood cells
What is haemoglobin?
A large protein with a quaternary structure - made up of more than one polypeptide chain
What is contained in each chain in haemoglobin?
Each chain has a haem group - this contains an iron ion this gives haemoglobin its red colour
What does affinity mean?
Tendency to combine with
What is haemoglobins affinity to oxygen like?
It has a high affinity for oxygen
How many molecules of oxygen can each haemoglobin molecule carry?
4
What is formed by oxygen and haemoglobin joining?
Oxyhaemoglobin
Can the reaction to form oxyhaemoglobin be reversed?
Yes it is a reversible reaction - oxygen can dissociate from it and it will turn back to haemoglobin
Which animals is haemoglobin found in?
- All vertebrates
- Earthworms
- Starfish
- Some insects
- Some plants
- Some bacteria
What does haemoglobin saturation depend on?
The partial pressure of oxygen
What is the partial pressure of oxygen?
The measure of oxygen conc.
The greater the conc. of dissolved oxygen in calls, the higher the partial pressure
What is the partial pressure of carbon dioxide?
The measure of the conc. of CO2
What does haemoglobins affinity to oxygen vary with?
It depends on the partial pressure of oxygen
When there is a high pO2, what does oxygen do?
Oxygen loads onto haemoglobin to form oxyhaemoglobin when there’s a high pO2
What happens to oxygen when there is a low pO2?
Oxyhaemoglobin unloads its oxygen where there’s a lower pO2
Explain how oxygen is picked up and dropped off at cells
- Oxygen enters blood capillaries at the alveoli in the lungs
- Alveoli have a high pO2 so oxygen loads onto haemoglobin to form oxyhaemoglobin
- When cells respire, they use up oxygen - this lowers pO2
- RBCs deliver oxyhaemoglobin to respiring tissues where it unloads oxygen
- The haemoglobin then returns to the lungs to pick up more oxygen
What do oxygen dissociation curves show?
How affinity for oxygen varies
It shows how saturated the haemoglobin is with oxygen at any given partial pressure
What does 100% saturation on the oxygen dissociation curve mean?
Means every haemoglobin molecule is carrying the max of 4 molecules of oxygen
What does 0% saturation on the oxygen dissociation curve mean?
Means none of the haemoglobin molecules are carrying any oxygen
Why do RBCs drop off oxygen at cells?
When the conc. of oxygen is low, hemoglobin’s affinity for oxygen is low therefore the haemoglobin releases the oxygen rather than combining with it
Why is the oxygen dissociation curve ‘S-shaped’?
Because when haemoglobin (Hb) combines with the first O2 molecule, its shape alters in a way that makes it easier for other molecules to join
As the Hb becomes saturated it gets harder for more oxygen molecules to join
Due to this it has a steep curve in the middle and shallower bits at the start and end
What happens when the first oxygen molecule attaches to haemoglobin in a RBC?
It’s shape alters in a way that makes it easier for the other molecules to join
What happens after the third oxygen joins to the haemoglobin in a RBC?
The haemoglobin becomes saturated, this makes it harder for more oxygen molecules to join
How does CO2 affect haemoglobin carrying O2?
It affects the unloading of oxygen - the higher the partial pressure of CO2, the more readily hemoglobin gives up its O2
How do cells raise the conc. of CO2?
When cells respire they produce CO2, this raises the pCO2 (partial pressure of CO2)
How does increased pCO2 affect the oxygen dissociation curve?
Increased pCO2 increases the rate of oxygen unloading - so the dissociation curve shifts right
What is the Bohr effect?
The raised pCO2 increases the rate of oxygen unloading so the dissociation curve shifts right
The saturation of blood with O2 is lower for a given pO2, meaning that more oxygen is being released
Why do different animals have different types of haemoglobin?
They have different types of haemoglobin with different oxygen transporting capacities
The different types of Hb are adapted to help the organism survive in particular environments
What is the haemoglobin like in animals who live in environments with a low conc. of oxygen?
They have haemoglobin with a higher affinity for oxygen than human haemoglobin - the curve shifts to the left
What is the haemoglobin like in animals who have a high demand for oxygen?
Animals who are very active or who have a high demand for oxygen
They haemoglobin has a lower affinity for oxygen than human haemoglobin - the curve shifts to the right
Why do we need the circulatory system?
Multicellular organisms like mammals have a low SA to volume ratio - they need a special transport system to carry and exchange raw materials to our body cells
What is the circulatory system made up of?
It is made up of the heart and blood vessels
What does blood transport?
Respiratory gases, products of digestion, metabolic wastes and hormones round the body
Where do the two loops of the respiratory system go?
- One takes one from the heart to the lungs and back to the heart
- The other loop takes blood around the rest of the body
How does the heart have its own bloody supply?
Through the left and right coronary arteries
Where does the renal artery go?
From the heart to the kidneys
Where does the renal vein go?
From the kidneys to the heart
Where does the hepatic vein go?
From the liver and the gut to heart
Where does the hepatic artery go?
From the heart to the liver
Where does the hepatic portal vein go?
From the heart to the gut
What are the artery walls like?
They are thick and muscular & have elastic tissue to stretch and recoil as the heart beats which helps maintain the high pressure
What helps the artery to stretch?
The inner lining (endothelium) is folded, allowing the artery to stretch
This also helps it to maintain high pressure
What sort of bloody do arteries carry?
They carry oxygenated blood - except for the pulmonary arteries which take deoxygenated blood to the lungs
What do arteries divide into?
Smaller vessels called arterioles - these form a network throughout the body
How does the body decide where the blood is distributed to?
Blood is directed to different areas of demand in the body by muscles inside the arterioles which contract and relax to control blood flow
What are the vein walls like?
They have a wider lumen than equivalent arteries, with very little elastic or muscle tissue - this creates low pressure
What do valves in veins do?
They stop backflow of blood
How does blood flow through veins?
Bloody flow through veins is helped by contraction of the body muscles surrounding them
What sort of blood do veins carry?
All veins carry deoxygenated blood, except from the pulmonary veins which carry oxygenated blood to the heart from the lungs
Where are substances exchanged between the blood and the body tissues?
The capillaries
What do arterioles branch into?
Capillaries
What are capillaries adapted for and why?
Efficient diffusion
As substances are exchanged between cells and capillaries
Where are capillaries found?
Near cells in exchange tissues - this creates a short diffusion pathway
What are the walls of capillaries like?
They’re one cell thick to shorten the diffusion pathway
What are capillary beds?
Networks of capillaries in a tissue
What are 3 adaptions that capillaries have to make gaseous exchange more effective?
- Near cells in exchange tissues
- One cell thick
- Capillary beds create a large surface area
How do large networks of capillaries help them with gaseous exchange?
The large number of capillaries increases the SA for exchange
Where is tissue fluid formed?
In the blood
What is tissue fluid?
The fluid that surrounds cells in tissues
What is tissue fluid made of?
Small molecules that leave the blood plasma
e.g. oxygen, water and nutrients
Why doesn’t tissue fluid contain red blood cells or big proteins?
They’re too large to be pushed through the capillary walls
How do cells use tissue fluid?
Cells take in oxygen & nutrients from the tissue fluid and release metabolic waste into it
In a capillary bed, how do substances move out of the capillaries and into the tissue fluid?
By pressure filtration
What are the stages in pressure filtration?
- At the start of capillary bed, nearest to arteries, the hydrostatic (liquid) pressure in capillaries is greater than hydrostatic pressure in tissue fluid
- Difference in hydrostatic pressure means overall outward pressure forces fluid out of the capillaries & into spaces around the cells, forming tissue fluid
- As fluid leaves, hydrostatic pressure reduces in capillaries - so hydrostatic pressure is much lower at venule end of capillary bed
- Due to fluid loss, and an increasing conc. of plasma proteins (which don’t leave the capillaries), water potential at the venule end of capillary bed is lower than water potential in tissue fluid
- This means that some water re-enters capillaries from tissue fluid at the venule end by osmosis
Where is excess tissue fluid drained to?
The lymphatic system
What is the lymphatic system?
A network of tubes that acts a bit like a drain
What does the lymphatic system do?
It transports excess fluid from the tissues & dumps it back in the circulatory system
How does high blood pressure affect the hydrostatic pressure?
High blood pressure means a high hydrostatic pressure in the capillaries, which can lead to an accumulation of tissue fluid in the tissues
How many muscular pumps does the heart consist of?
2
What are the 4 chambers in the heart called?
- Right atrium
- Left atrium
- Right ventricle
- Left ventricle
What are the veins connected to the heart called?
- Superior vena cava
- Inferior vena cava
- Pulmonary vein
What are the arteries connected to the heart called?
- Aorta
- Pulmonary artery
What are the valves in the heart called?
- Semi-lunar valves
- Right atrioventricular valve (tricuspid valve)
- Left atrioventricular valve (bicuspid valve)
What sort of blood does the right side of the heart pump?
Deoxygenated blood to the lungs
What sort of blood does the left side of the heart pump?
Oxygenated blood to the whole body
What is the left ventricle like compared to the right ventricle?
The left ventricle has thicker more muscular walls
Why does the the left ventricle have thicker and more muscular walls?
Because it needs to contract powerfully to pump blood all the way round the body
Why does the right ventricle have less muscular walls than the left ventricle?
The right ventricle has to get blood to the lungs which are nearby
Why do the ventricles have thicker walls than the atria?
Because they have to push blood out of the heart
Whereas the atria just need to push blood a short distance to the ventricles
What is the role of the atrioventricular valves?
They link the atria to the ventricles and stop the backflow of blood into the atria when the ventricles contract
What is the role of the semi-lunar valves?
They link the ventricles to the pulmonary artery and aorta and stop blood flowing back into the heart after the ventricles contract
Where are the cords in the heart?
They branch from the atrioventricular valves, creating a bishop’s hat shape which is why the left atrioventricular valve is also called the mitral valve
What is the role of cords in the heart?
The cords attach the atrioventricular valves to the ventricles to stop them being forced open when the ventricles contract
How can the valves in the heart open?
They can only open in one direction
What determines whether the chambers in the heart open?
Whether they’re open or closed depends on the relative pressure of the heart chambers
If there’s a higher pressure behind a valve, it’s forces open, but if pressure is higher in front of the valve, it’s forced shut.
This means blood flows in one direction through the heart
What is the cardiac cycle?
The ongoing sequence of contraction & relaxation of the atria & ventricles that keeps blood continuously circulating round the body
When does the volume of the atria and ventricles change?
As they contract and relax
Why do changes in pressure in the heart occur?
Due to the changes in the chamber volume
What are the 3 main stages in the cardiac cycle?
- Ventricles relax, atria contract
- Ventricles contract, atria relax
- Ventricles relax, atria relax
What happens in the first stage of the cardiac cycle (ventricles relax, atria contract)?
Ventricles are relaxed. Atria contract, decreasing the vol. of the chambers & increasing the pressure in them.
This pushes blood into the ventricles
Slight increase in ventricular pressure & chamber volume as the ventricles receive the ejected blood from the contracting atria
What happens in the second stage of the cardiac cycle (ventricles contract, atria relax)?
Atria relax. Ventricles contract decreasing vol. & increasing their pressure
Pressure becomes higher in the ventricles than the atria, this forces the AV valves shut to prevent back-flow
Pressure in ventricles is also higher than in the aorta & pulmonary artery –> forces open sl valves and blood is forced out into these arteries
What happens in the third stage of the cardiac cycle (ventricles and atria relax)?
Ventricles & atria both relax
Higher pressure in pulmonary artery & aorta closes the SL valves to prevent the back-flow into the ventricles
Blood returns to heart & the atria fill again due to higher pressure in vena cava & pulmonary vein
This increases pressure of atria, as ventricles continue to relax, their pressure falls below that of the atria so AV valves open
This allows blood to flow passively into the ventricles from the atria
Look at interpreting data on the cardiac cycle!!!!
:))))))))
Diseases associated with the heart & blood vessels are called what?
Cardiovascular diseases
What do most cardiovascular diseases start with?
Atheroma formation
How is an atheroma formed?
The endothelium in an artery is usually smooth and unbroken. If damage occurs to this, then white blood cells & lipids from the blood clump together under the lining to form fatty streaks
Over time more white blood cells, lipids & connective tissue build up and harden to form a fibrous plaque
This fibrous plaque is called an atheroma
What is an atheroma?
It is a build up of lipids, white blood cells & connective tissue in the wall of the artey which forms a fibrous plaque
How do atheromas affect the blood flow in an artery?
The plaque partially blocks the lumen of the artery & restricts the blood flow, which causes blood pressure to increase
When does coronary heart disease (CHD) occur?
When the coronary arteries have lots of atheromas in them, which restricts blood flow to the heart muscle
It can lead to myocardial infarction
Which cardiovascular diseases do atheromas increase the risk of?
- Aneurysm
- Thrombosis
What is an aneurysm?
A balloon-like swelling of the artery
How is an aneurysm formed?
- Atheroma plaques damage & 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 a balloon like swelling
- This aneurysm may burst, causing a haemorrhage
What is a haemorrhage?
Bleeding from a blood vessel, likely caused by an aneurysm
What is thrombosis?
Formation of a blood clot
How does a thrombus form?
- An atheroma plaque can rupture the endothelium of an artery
- This damages the artery wall & leaves a rough surface
- Platelets & 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 & block a blood vessel elsewhere in the body
- Debris from the rupture can cause another blood clot to form further down the artery
What is a myocardial infarction caused by?
Interrupted blood flow to the heart
How is the heart muscles supplied with blood?
By the coronary arteries
Why is it important that the coronary arteries supply blood to the heart?
The coronary arteries carry blood that contain the oxygen that the heart muscles required to carry out respiration
What happens if a coronary artery becomes completely blocked?
An areas of the heart muscle will be totally cut off from its blood supply, receiving no oxygen
What is a myocardial infarction more commonly known as?
A heart attack
How does a myocardial infarction affect the heart muscle?
It can cause damage or death of the heart muscle
What are the symptoms of a myocardial infarction?
Symptoms include pain in the chest and upper body, shortness of breath and sweating
If large areas of the heart are affected by a myocardial infarction what could happen?
Complete heart failure can occur, which is often fatal
What are the 3 most common risk factors for cardiovascular disease?
- High blood cholesterol & poor diet
- Cigarette smoking
- High blood pressure
What is considered high blood cholesterol?
Above 240mg per 100cm^3
What happens if the blood cholesterol is high?
It increases the risk of cardiovascular disease is increased
Why does high blood cholesterol increase the risk of cardiovascular disease?
Cholesterol is one of the main constituents of the fatty deposits that form atheromas
How can high blood cholesterol lead to a myocardial infarction?
- Cholesterol is one of the main constituents to form atheromas
- Atheromas can lead to increased blood pressure and blood clots
- This could block the flow of blood to coronary arteries, which could cause a myocardial infarction
What sort of diet is associated with high blood cholesterol levels?
A diet high in saturated fat
What sort of diet increases the risk of high blood pressure?
A diet high in salt which increases the risk of cardiovascular disease
What are the two substances found in cigarette smoke that increase the risk of cardiovascular disease?
- Nicotine
- Carbon monoxide
How does nicotine increase the risk of cardiovascular disease?
It increases the risk of high blood pressure
How does carbon monoxide increase the risk of cardiovascular disease?
Carbon monoxide combines with haemoglobin & reduces the amount of oxygen transported in the blood, and so reduces the amount of oxygen available to the tissues
If the heart muscle doesn’t recieve enough it can lead to a heart attack
How does smoking affect the blood?
It reduces 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 is more likely
This can lead to an atheroma formation
What risk does high blood pressure pose?
An increased risk of damage to the artery walls
Damaged walls have an increased risk of atheroma formation - causing a further increase in blood pressure
How can high blood pressure pose a risk of death through an arethroma?
Arethromas can cause blood clots - blood clots could cause backflow of blood to the heart
This could result in a myocardial infarction
What are the two types of tissue involved in transport in plants?
Xylem and Phloem
What does the Xylem tissue tansport?
It transports water and mineral ions in solution
What direction do the substances in the xylem move?
Up the plant, from the roots to the leaves
What does the phloem tissue transport?
Organic substances like sugars (in solution like the xylem)
What direction do the substances in the phloem move?
Both up and down the plant
Why are the phloem and xylem considered mass transport systems?
They move substances over large distances in plants
What are xylem vessels like?
They are very long, tube like structures formed from dead cells joined end to end
There are no end walls on the cells, making an uninterrupted tube that allows water to pass through easily
What direction does water move in?
Moves up the plant against the force of gravity
What helps water move up the plant?
Cohesion and tension
How is water moved up the plant (using cohesion & tension)?
1) Water evaporates from leaves at the ‘top’ of the xylem
2) Creates tension (suction) - pulls more water into the leaf
3) Water molecules are cohesive so when some are pulled others follow - this means whole column of water is pulled up the xylem
4) Water enters stem through roots
What is cohesion?
Water molecules sticking together
What is tension?
The water gripping to the the side of the plant
What is transpiration?
The process of water being lost from the plants surface
Occurs especially in the leaves
What is the process of transpiration?
1 - Water evaporates from the moist cell walls & accumulates in the spaces between cells in leaf
2 - When the stomata open, it moves out of the leaf down the conc. gradient (more water in leaf than in the air)
What are the 4 main factors that affect transpiration rates?
- Light
- Temp
- Humidity
- Wind
How can light affect transpiration rates?
The lighter it is, the faster the transpiration rate
This is because stomata open when it gets light to let in CO2 for photosynthesis
When it’s dark stomata are usually closed, so there is little transpiration
How does temperature affect transpiration rates?
The higher the temp the faster the transpiration rate
Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster
This increases conc. gradient between inside & outside of leaf, making water diffuse out of the leaf faster
How does humidity affect transpiration rates?
Lower humidity = faster transpiration rate
If the air around the plant is dry, the conc. gradient between leaf & air is increased, which increases transpiration
How does wind affect transpiration rates?
The windier it is, the faster the transpiration rate
Lots of air movement blows away water molecules from around the stomata
This increases the conc. gradient, which increases the rate of transpiration
What is a potometer used for?
It can be used to estimate transpiration rate
Also to estimate how different factors affect the transpiration rate
What is a potometer and what does it measure?
A piece of apparatus used to estimate transpiration rates
It measures water uptake by a plant, it is assumed that water uptake by the plant is directly related to water loss by the leaves
What are the stages for testing transpiration rates using a potometer?
Refer to textobook diagram
1 - Cut shoot underwater to prevent air from entering xylem, cut it at a slant to increase SA avialble for water uptake
2 - Assemble potometer in water & insert shoot underwater, so no air can enter
3 - Remove apparatus from water, keeping end of capillary tube submerged in beaker of water
4 - Check apparatus is water tight
5 - Dry leaves, allow shoot to acclimatise & shut the tap
6 - Remove end of capillary tube from beaker until 1 air bubble forms & put end of tube back in water
7 - Record starting position of bubble
8 - Start stopwatch & record distance moved by bubble per unit time –> rate of air bubble movement is an estimate of transpiration rate
9 - Only change one variable at a time! e.g. temp
Why would you have to dissect a plant?
To inspect a sample of a tissue under a microscpe
How would you dissect a plant?
1) Use scalpel to cut cross-section of the stem. Cut sections as thinly as possible - thin sections are better for viewing under a microscope
2) Use tweezers to gently place cut sections, put in water until you come to use them - stops them from drying out
3) Transfer each section to a dish containing a stain e.g. toluidine blue O & leave for 1 min - TBO stains lignin in walls of xylem vessels blue-green. Will let you see position of the xylem vessels & examine thier structure
4) Rinse off sections in water & mount each one onto a slide
What is the dye used when dissecting a plant and what does it do?
Toluidine blue O (TBO)
It stains lignin in walls of xylem vessels blue-green. Will let you see position of the xylem vessels & examine thier structure
What is the role of the phloem?
It is adapted for transporting solutes
What are solutes?
Dissolves substances - transported by the phloem round plants (mainly sugars like sucrose)
How are the xylem and phloem similar?
The cells are arranged in tubes
Are sieve tubes elements and companion cells found in the phloem or xylem?
Phloes
What are sieve tube elements?
Living cells that form the tube for transporting solutes, they have no nucleus & few organelles
What are companion cells?
They aid sieve tube elements as they have no nucleus and few organelles
Companion cells carry out living functions for sieve cells
e.g. prividing the energy needed for the active transport of solutes
What is translocation?
The movement of solutes (e.g. sugars like sucrose & amino acids) to where they’re needed in the plant
What can solutes sometimes be called?
Assimilates
Where does translocation take place and does it require energy?
It happens in the phloes & is an energy requiring process
Where does translocation move molecules to/from?
It moves solutes from ‘sources’ to ‘sinks’
From an area of hihg conc. to lower conc.
What is a source (concerning translocation)?
The source of a solutes is where it’s made (so there is a high conc. here)
What is a sink (concerning translocation)?
The area where solutes are used up (so there is a lower conc. here)
What helps to maintain a conc. gradient from source to sink in translocation?
Enzymes - they maintain this conc. gradient by changing the solutes at the sink (e.g. by breaking them down etc)
This makes sure there’s always a lower conc. at the sink than the source
What theory best explains phloem transport?
The mass flow hypothesis
Scientists aren’t certian exactly how the solutes are transported but this is the best supported theory
What happens in part 1 of the mass flow hypothesis (translocation)?
1 - Active transport used to actively load solutes from companion cells into sieve tubes of phloem at source
2 - Lowers water potential inside sieve tubes, so water enters tubes by osmosis from xylem & companion cells
3 - Creates high pressure in sieve tubes at the source end of phloem
What happens in part 2 of the mass flow hypothesis (translocation)?
1 - At sink end, solutes are removed from phloem to be used up
2 - This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis
3 - This lowers the pressure inside the sieve tubes
What happens in part 3 of the mass flow hypothesis (translocation)?
1 - Result is a pressure gradient from the source end to the sink end
2 - This gradient pushes solutes along the sieve tubes towards the sink
3 - When they reach the sink, the solutes will be used (e.g. in respiration) or stored (e.g. in starch)
What is the evidence for mass transport being true?
- If a ring of bark is removed from a bulge forms above the ring
- Radioactive tracer
- Aphids
- Metabolic inhibitor
What is the evidence against mass transport being true?
- Sugar travels to many different sinks
- Sieve plates
How are sive plates evidence against the theory of mass transport being true?
They would create a barrier to mass flow
A lot of pressure would be needed for the solutes to get through at a reasonable rate
How is sugar travels to many different sinks evidence against the theory of mass transport being true?
They travel to many different sinks, not jsut the one witht he highest water potential - as the model would suggest
How are rings in trees evidence for the theory of mass transport being true?
When a ring is removed (that inc. the ploem but not the xylem) a bulge forms above the ring
Fluid from the bulge has a higher conc. of sugars than the fluid from below the ring
This is evidence there’s a downward flow of sugars
How are radioactive tracers evidence for the theory of mass transport being true?
They can be used to track the movement of organic substances in a plant
Radioactive carbon (14C) can be used
How are aphids evidence for the theory of mass transport being true?
Pressure in phloem can be investigated using aphids - they peirce phloem, their bodies are removed leaving the mouthparts behind, allowing sap to flow out
Sap flows out quicker nearer leaves than further down the stem - this is evidence there’s a pressure gradient
How are metabolic inhibitors evidence for the theory of mass transport being true?
If a metabolic inhibitor (stops production of ATP) is put in phloem, then translocation stops
This is evidence that active transport in involved
How can translocation of solutes in plants be modelled in an experiment?
Using radioactive tracers
What is the process of modelling traslocation in plants using radioactive tracers?
1 - Supply part of plant (often leaf) with an organic substance that has a radioactive label e.g. CO2 has radioactive isotope 14^C - will be inserted into leaf
2 - Radioactive carbon will then be incorporated into organic substances produced by leaf (e.g. sugars), which will be moved around the plant by transloaction
3 - Movement of substances can be tracked using a technique called autoradiography - reveals where radioactive tracer has spread in a plant, it is killed (e.g. freezing it in liquid nitrogen) then whole plant is placed on photographic film –> radioactive substance is present wherever film turns black
4 - Results demonstrate translocation of substances from source to sink over time –> e.g. audiographs of plants killed at different times show overall movement of solutes from leaves to roots
