topic 3 Flashcards
What is the relationship between surface area and volume in organisms?
As organisms become larger, their volume increases at a faster rate than their surface area, leading to a decrease in the surface area to volume ratio.
What are adaptations to compensate for reducing SA:V ratio?
1) A flattened shape so that no cell is far from the surface.
2) Specialised exchange surfaces with large areas to increase surface areas.
Why does a large surface area increase metabolic rate?
More heat is lost rapidly to the surroundings, and more energy needs to be released to maintain body temperature.
What are the features of specialised exchange surfaces?
1) Large surface area to volume ratio (increases rate of exchange).
2) Very thin so diffusion distance is short (rapid exchange). 3) Selectively permeable (allows selected materials to cross). 4) Maintaining diffusion gradient (movement of environmental or internal medium).
How does gas exchange occur in single-celled organisms?
Substances diffuse directly through the membrane due to fast diffusion rates from short distances and large surface area.
How does gas exchange occur in insects?
Air passes into the trachea (microscopic air-filled pipes) through pores called spiracles. The trachea branches into smaller tracheoles with thin permeable walls allowing oxygen to diffuse directly into respiring cells.
How do gases move in and out of the tracheal system?
1) Down a diffusion gradient - oxygen concentration decreases at ends of tracheoles during respiration, and carbon dioxide concentration increases forming a gradient in the opposite direction.
2) Mass transport - contraction of muscles in insects can squeeze the trachea and force air in/out.
3) Ends of tracheoles are filled with water-
What are three ways the tracheal system is adapted to gas exchange?
1) Tracheoles have thin walls, resulting in a short diffusion distance.
2) Tracheoles are highly branched, providing a large surface area for gas exchange.
3) Body muscles can contract to move air and maintain gradients.
What is the importance of the ends of tracheoles being filled with water?
1) During periods of major activity, anaerobic respiration occurs and produces lactate, lowering water potential of cells.
2) This causes water from the tracheoles to move via osmosis into the cells, decreasing air pressure and forcing more air into the tracheoles.
3) Final diffusion pathway is in a gas rather than liquid, increasing the rate of exchange but also increasing water loss.
What are spiracles?
External openings to the respiratory system in insects, similar to stomata. When open, water can evaporate, so they are kept closed most of the time.
What are the limitations of the tracheal system?
1) Mostly relies on diffusion to exchange gases between the environment and the cells.
2) For diffusion to be effective, the diffusion pathway needs to be short, limiting the size that insects can attain.
Why do fish need a specialised gas exchange system?
Fish are relatively large, have a low surface area to volume ratio, and due to their waterproof outer covering, they are gas tight, making them inadequate for gas exchange.
What is the structure of the gills?
1) Gills composed of thousands of filaments.
2) Each filament is covered in lamellae perpendicular to themselves, increasing surface area.
3) Water is taken in through the mouth and forced over the gills and out through openings in the body.
4) Flow of water opposes flow of blood (countercurrent flow) maximising diffusion.
What are the reasons for countercurrent exchange in fish gills?
1) Blood that is already well loaded with oxygen meets the water with the highest oxygen concentration, so diffusion is still possible. 2) Blood with the least oxygen meets water with the least amount of oxygen, allowing diffusion to occur at all parts of the gill and lamelle.
3) A concentration gradient is maintained across the entire gill.
4) Blood and water flow in opposite directions.
What are adaptations for rapid diffusion of gases in plants?
1) Many small pores (stomata) so that no cell is far from a stomata, resulting in a short diffusion distance.
2) Lots of interconnecting air spaces in the mesophyll for air to readily contact mesophyll cells.
3) Large surface area of mesophyll cells for rapid diffusion.
What are the similarities between gas exchange in plants and insects?
1) Both can control the opening and closing of pores in their outer covering.
2) Both need to avoid excessive water loss.
3) Diffusion takes place in a gaseous state.
What are the differences between gas exchange in insects and plants?
1) Insects use mass transport of air using muscle contractions, while plants do not.
2) Insects have special structures called tracheae along which gases can diffuse.
How do insects limit water loss?
1) Small surface area to volume ratio.
2) Waterproof coverings.
3) Spiracles that can close when gas exchange is less important, e.g., when resting.
Why can’t plants have a small surface area to volume ratio?
They need to photosynthesise, which requires them to have a large leaf surface to catch light.
How do all plants limit water loss?
1) Waterproof covering over leaves.
2) Control over the opening and closing of stomata.
How do xerophytes limit water loss?
1) Thick waxy cuticle forms a waterproof barrier.
2) Rolled up leaves trap water vapour and reduce water vapour gradient.
3) Hairy leaves trap moist air, reducing gradient.
4) Stomata in pits/grooves trap moist air, reducing gradient. 5) Reduced surface area to volume ratio, e.g., needles instead of leaves, slows diffusion rate.
What are xerophytes?
Plants that are adapted to living in areas where water is in short supply.
Why do mammals exchange large volumes of gas?
A lot more is needed for respiration due to
1) large organisms with many living cells and
2) maintaining high body temperature, resulting in a high metabolic and respiratory rate.
Why are mammalian lungs located inside the body?
1) Air is not dense enough to support and protect these delicate structures.
2) The body would dehydrate and lose a lot of water.
What are the main parts of the human gas exchange system?
1) Lungs,
2) Trachea,
3) Bronchi,
4) Bronchioles,
5) Alveoli.
What are the lungs?
A pair of lobed structures made up of a series of highly branched tubules called bronchioles, which end in tiny air sacs called alveoli.
What is the trachea?
A flexible airway supported by rings of cartilage that prevents it from collapsing during air pressure fluctuations. It is made from muscle, lined with ciliated epithelium and goblet cells.
What are the bronchi?
Two divisions of the trachea, each leading to one lung, similar to the trachea in structure and also producing mucus to trap dirt for cilia to move towards the throat.
What are the bronchioles?
A series of branching subdivisions of the bronchi, with walls made of muscle lined with epithelial cells, allowing them to constrict and control the flow of air in and out of the alveoli.
What are the alveoli?
Minute air sacs at the end of bronchioles, with collagen and elastic fibres between them that allow them to stretch when breathing in and spring back to force air back out. The alveolar membrane is the gas exchange surface.
What is ventilation?
The constant movement of air in and out of the lungs to maintain gradients.
Which muscles are responsible for pressure changes in the lungs?
1) Diaphragm - a sheet of muscle separating the abdomen from the thorax.
2) Intercostal muscles (between ribs) - internal intercostal contract for expiration, external intercostal contract for inhalation.
What is the process of inspiration?
1) External intercostal muscles contract and internal relax. 2) This pulls the ribs upwards and outwards, increasing thorax volume.
3) Diaphragm muscles contract and flatten, further increasing thorax volume.
4) Increased volume leads to decreased pressure.
5) Atmospheric pressure is now greater than pulmonary pressure, so air is forced into the lungs down a pressure gradient.
What is the process of expiration?
1) Intercostal muscles contract and external relax
. 2) Ribs move downwards and inwards, decreasing thorax volume.
3) Diaphragm relaxes and is pushed back up by the contents of the abdomen compressed during inspiration, further decreasing thorax volume
. 4) Decreased volume leads to increased pressure.
5) Pulmonary pressure is greater than atmospheric, so air is forced out of the lungs down a pressure gradient.
Is inspiration an active or passive process?
Active process (requires energy).
Is expiration an active or passive process?
Passive process.
Where is the site of gas exchange in mammals?
The epithelium of the alveoli.
How is the alveoli adapted for gas exchange?
1) Thin, flat cells - epithelium is 1 cell thick.
2) Large surface area due to a large number.
3) Surrounded by a network of capillaries that are very narrow and only one cell thick, providing a good blood supply.
What is the significance of narrow pulmonary capillaries?
1) Red blood cells are slowed as they pass the alveoli, allowing more time for diffusion.
2) Red blood cells are flattened against capillary walls, decreasing diffusion distance.
Why is diffusion between alveoli and the blood so quick?
1) Red blood cells are slowed by the narrow capillaries, allowing a long time for diffusion.
2) Short diffusion distance as red blood cells are flattened against capillary walls.
3) Alveoli and capillary walls are only one cell thick for a short diffusion distance.
4) Extremely large surface area of both capillary and alveoli. 5) Breathing is a form of mass transport that constantly ventilates the lungs, maintaining the gradient.
6) Blood flow maintains the gradient.
What is meant by antagonistic interaction?
As one muscle relaxes, the other contracts.
What is digestion?
The process in which large biological molecules are hydrolysed to smaller molecules that can be absorbed across a cell membrane.
What are the major parts of the digestive system?
Oesophagus, Stomach, Ileum, Large intestine, Rectum, Salivary glands, Pancreas.
What is the function of the oesophagus?
A muscular tube that carries food from the mouth to the stomach.
What is the function of the stomach?
A muscular sac with an inner layer that produces enzymes, storing and digesting foods.
What is the function of the ileum?
A long muscular tube where food is further digested by enzymes and products of digestion are absorbed into the bloodstream.
What is the function of the large intestine?
Absorbs water from the remaining indigestible food matter and transmits the useless waste material from the body.
What is the function of the rectum?
Faeces are stored here before being removed in egestion.
What is the function of salivary glands?
Secretion of lubricating fluid containing enzymes that break down carbohydrates.
What is the function of the pancreas?
Produces pancreatic juice, which contains digestive enzymes, e.g., proteases, lipases, and carbohydrases.
What are the two stages of digestion?
Physical breakdown and chemical digestion.
What is physical breakdown?
Large food is broken into smaller pieces for ingestion, creating a large surface area for chemical digestion.
What is chemical digestion?
Hydrolysis of large insoluble molecules into smaller soluble molecules by enzymes.
What is the hydrolysis of carbohydrates?
Produces monosaccharides.
What is the hydrolysis of lipids?
Produces fatty acids and glycerol.
What is the hydrolysis of proteins?
Produces amino acids.
What is meant by a membrane-bound enzyme?
An enzyme that isn’t released into the ileum lumen but is part of the cell membrane of epithelial cells that line the ileum.
What is the carbohydrate digestion process?
1) Starch (polysaccharide) is hydrolysed into maltose (disaccharide) by salivary amylase.
2) Continued in ileum by pancreatic amylase.
3) Maltose (disaccharide) is then hydrolysed to form glucose (monosaccharide) by the membrane-bound enzyme maltase.
Where is amylase produced?
In the salivary glands and pancreas.
Where is lipase produced?
In the pancreas.
Where are proteases produced?
In the stomach, small intestine, and pancreas.
What is the process of lipid digestion?
1) Lipids are emulsified by bile salts to form micelles, increasing surface area for lipase action in the ileum called emulsification (physical)
2) Lipids are hydrolysed to form fatty acids and monoglycerides by lipase hydrolysing ester bonds (chemical)
Where is bile produced and stored?
Produced in the liver and stored in the gallbladder.
What is the function of bile?
Emulsification and neutralisation.
What are the types of peptidase?
1) Endopeptidase.
2) Exopeptidase
3) Dipeptidase.
What is the function of endopeptidases?
Hydrolyse peptide bonds between amino acids in the central region of polypeptides, forming smaller polypeptide chains (more terminal ends for exopeptidase).
What is the function of exopeptidases?
Hydrolyse peptide bonds on the terminal amino acids formed by endopeptidases, progressively releasing single amino acids and dipeptides.
What is the function of dipeptidase?
A membrane-bound enzyme in epithelial celles lining the illeum that hydrolyses peptide bonds in between 2 amino acids
How is the ileum adapted for absorption?
1)It has villi - finger-like projections that increase surface area.
How do villi increase the efficiency of absorption?
1) They increase the surface area for diffusion.
2) They are very thin-walled, reducing the distance over which diffusion takes place.
3) They contain muscles and can move, helping maintain diffusion gradients and mixing the contents of the ileum.
4) They have a rich blood supply to maintain concentration gradient.
5)They possess microvilli, further increasing the surface area for absorption.
How are amino acids and monosaccharides absorbed?
Via diffusion and co-transport.
How are triglycerides absorbed into epithelial cells?
Lipid droplets are emulsified into monoglycerides and fatty acids combined with bile salts (micelles). Micelles come into contact with epithelial cells, where they break down, releasing monoglycerides and fatty acids, which are non-polar and easily diffuse across the cell surface membrane into the epithelial cells.
What happens to triglycerides once in epithelial cells?
Monoglycerides and fatty acids are transported to the ER, recombined to form triglycerides. In the ER and Golgi apparatus, triglycerides associate with cholesterol and lipoproteins to form chylomicrons. Chylomicrons are adapted for lipid transport, moving out of epithelial cells by exocytosis, entering lymphatic capillaries (lacteals), and passing via lymphatic vessels into the bloodstream. Triglycerides in chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries, from where they diffuse into cells.
What are haemoglobins?
A group of chemically similar molecules found in a wide variety of organisms, each adapted to efficiently associate and dissociate with O2 in different environments.
What is the structure of haemoglobin?
A protein with a quaternary structure consisting of 4 polypeptide chains, each associated with a haem group.
What is the role of haemoglobin?
1) Readily associates with O2 at gas exchange surfaces.
2) Readily dissociates with O2 at tissues requiring it.
How are there different types of haemoglobin?
Each species produces haemoglobin with a slightly different amino acid sequence, resulting in different tertiary and quaternary structures.
Explain the shape of the oxygen dissociation curve.
1) Initially shallow as the closely united quaternary structure shape makes it difficult for O2 to bind.
2) Binding of the first O2 molecules changes the quaternary structure, making it easier for the next O2 molecule to bind. 3) Positive cooperativity - it becomes easier for the third one to bind, resulting in a steep curve.
4) Majority of binding sites are now occupied, making it harder for the fourth O2 molecule to bind due to lower probability.
What does a right shift in the oxygen dissociation curve indicate?
A lower affinity for oxygen.
What does a left shift in the oxygen dissociation curve indicate?
A greater affinity for oxygen.
What is the oxygen dissociation curve?
1) Initially shallow as closely united quaternary structure shape makes it difficult for O2 to bind to.
2) Binding of first O2 molecules changes quaternary structure and shape so it’s easier for next O2 molecule to bind.
3) Positive cooperativity - easy for third one to bind therefore steep curve.
4) Majority of binding sites now occupied so it’s harder for fourth O2 molecule to bind due to lower probability.
What does a right shift in the oxygen dissociation curve indicate?
Lower affinity for oxygen.
What does a left shift in the oxygen dissociation curve indicate?
Greater affinity for oxygen.
How does CO2 concentration affect the oxygen dissociation curve?
The greater the concentration of CO2, the more readily the haemoglobin releases O2.
In lungs, low CO2 high O2 so high affinity for O2 and oxygen is loaded. In rapidly respiring tissues, high CO2 low O2 so low affinity for O2 and oxygen is unloaded.
How does CO2 concentration affect oxygen unloading?
CO2 is acidic and the low pH causes the haemoglobin to change shape to increase O2 unloading.
What is the process of loading oxygen molecules onto haemoglobin?
1) At gas exchange surfaces, CO2 is constantly removed.
2) Lack of CO2 means higher pH which changes the shape of haemoglobin.
3) Haemoglobin has a shape that more readily binds to O2 and has a higher affinity so doesn’t release oxygen in blood.
What is the process of unloading oxygen molecules from haemoglobin?
1) At respiring tissues, CO2 is constantly produced.
2) This lowers pH and so changes haemoglobin shape to have a lower affinity.
3) Haemoglobin releases its oxygen.
What are the features of transport systems?
1) A suitable medium in which to carry materials (e.g., blood).
2) A form of mass transport in which transport medium is moved around in bulk over large distances (more rapid diffusion).
3) Closed system of tubular vessels that contains transport medium and forms a branching network to distribute it to all parts of the organism.
4) A mechanism for moving transport medium within vessels (requires pressure difference between two parts of the system).
5) A mechanism to maintain mass transport movement in one direction.
6) A means of controlling the flow of transport medium to suit the changing needs of different parts of the organism. 7) A mechanism for mass flow of water or gases.
What type of circulatory system do mammals have?
Closed double circulatory system.
What factors determine if an organism needs a specialised transport medium and pump?
1) Surface area to volume ratio.
2) How active the organism is.
Why do we have a double circulatory system?
If the blood was carried straight from the lungs it would be at a much lower pressure but we need the blood to be transported quickly due to our high metabolism.
What is the vena cava?
Carries deoxygenated blood from the body to the heart.
What is the aorta?
Carries oxygenated blood from the left ventricle to the rest of the body.
What is the pulmonary artery?
Carries deoxygenated blood from the heart to the lungs.
What is the pulmonary vein?
Carries oxygenated blood from the lungs to the heart.
What is the renal vein?
The vein that brings blood from the kidney back to the heart.
What is the renal artery?
The artery that brings blood from the heart to the kidney.
What is the valve between the left atrium and left ventricle?
Left atrioventricular valve (bicuspid).
What is the valve between the right atrium and right ventricle?
Right atrioventricular valve (tricuspid).
What are coronary arteries?
Arteries that branch off the aorta & surround the heart, ensuring that it gets all the oxygenated blood it needs.
Blockage leads to cell death and a heart attack.
What are the valves between ventricles and arteries?
Semi-lunar valves.
What are the two phases of the beating of the heart?
Contraction (systole) and relaxation (diastole).
What are the three stages of the cardiac cycle?
- Diastole. 2. Atrial Systole. 3. Ventricular Systole.
What happens in the diastole stage?
1) Blood returns to the atria from pulmonary vein and vena cava.
2) Pressure increases in atria and when this exceeds that of the ventricles, the atrioventricular valves open. 3) Blood moves from atria to ventricles aided by gravity. 4) All muscular walls relaxed and therefore recoil, reducing pressure in ventricle so semi-lunar valves remain closed.
What happens in the atrial systole stage?
Contraction of atrial walls along with the recoil of the relaxed ventricle walls forces the remaining blood from atria into ventricles.
What happens in the ventricular systole stage?
1) Both ventricles contract and the atrio-ventricular valves are pushed shut as ventricular pressure is greater than the atrial pressure
. 2) The semilunar valves in the aorta and pulmonary artery are pushed open as pressure exceeds that of the arteries. 3) Blood flows from the ventricles into the arteries.
What are the functions of valves?
Prevent backflow of blood so same direction regardless of pressure changes.
What are the valves within veins called?
Pocket valves.
How do valves work?
When pressure is greater within the ‘bowl’ of the cups, the pressure forces the valves shut and vice versa.
What is the equation for cardiac output?
Stroke volume x heart rate.
What is the equation for pulmonary ventilation rate?
Tidal volume x breathing rate.
What are the types of blood vessel?
- Arteries. - Arterioles. - Capillaries. - Veins.
What is the function of arteries?
Carry blood away from the heart.
What is the function of arterioles?
Smaller arteries that control blood flow from arteries to capillaries.
What is the function of capillaries?
Serve as exchange vessels for nutrients, wastes, fluids and link arteries to veins.
What is the function of veins?
Carry blood back to the heart.
What is the basic structure of a blood vessel (not capillary)?
- Tough fibrous outer layer. - Muscle layer. - Elastic layer. - Endothelium. - Lumen.
What is the function of the tough fibrous outer layer in blood vessels?
Resists pressure changes from both within and outside.
What is the function of the muscle layer in blood vessels?
Can contract and so control blood flow.
What is the function of the elastic layer in blood vessels?
Helps maintain blood pressure by stretching and recoiling.
What is the function of the endothelium in blood vessels?
Thin inner lining that is smooth to reduce friction and allow diffusion.
What is the function of the lumen in blood vessels?
Central cavity where the blood flows.
What is the difference between arteries and arterioles?
Arterioles have a much smaller diameter and a larger muscle layer and lumen.
How is artery structure related to function?
- Thick muscle layer which means they can constrict and dilate to control the volume of blood passing through them. - Elastic layer is thick to maintain high pressure and smooth pressure by stretching and recoiling with the heartbeat. - Thick wall stops the vessel from bursting under pressure. - No valves because blood is under constant high pressure.
How is arteriole structure related to function?
- Thicker muscle layer than arteries to control flow of blood into capillaries. - Thinner elastic layer than arteries because blood pressure is lower.
How is vein structure related to function?
- Thin muscle layer as they carry blood away from tissues so no constriction or dilation is needed. - Thin elastic layer because the low pressure of blood will not cause them to burst. - Thinner wall as the pressure is too low for them to burst, also allows them to flatten easily. - Valves at intervals throughout to prevent back flow of blood.
How is capillary structure related to function?
- Walls consist mostly of endothelial (lining layer) = makes them very thin, so short diffusion pathway, allows rapid diffusion of materials between blood and cells to take place. - Many of them, they are highly branched = provides large surface area for exchange/diffusion. - Narrow diameter = no cell is far from a capillary and there is short diffusion pathway. - Lumen is narrow = red blood cells are squeezed flat against side of capillary, bringing them even closer to cells to which they supply oxygen, reducing diffusion distance. - Spaces between lining (endothelial) cells = allow white blood cells to escape in order to deal with infections within tissues.
What happens because capillaries cannot meet every cell?
The final journey of metabolic materials is made in a solution that bathes the tissues called tissue fluid.
What is tissue fluid?
A watery substance containing glucose, amino acids, oxygen, and other nutrients. It supplies these to the cells, while also removing any waste materials, providing a constant environment for the cells it surrounds.
What is the tissue fluid formation process?
- At the arterial end of capillaries, the blood pumped by the heart creates a high hydrostatic pressure.
- This outward pressure forces tissue fluid to move out of the blood plasma and force small molecules out of capillaries (ultrafiltration).
What is ultrafiltration?
Filtration of the blood at a molecular level under pressure.
What stays in the blood during ultrafiltration?
Cells and proteins.
What is the return of tissue fluid to the circulatory system?
- Loss of tissue fluid from the capillaries reduces the hydrostatic pressure at the venous end. - Tissue fluid outside the capillaries has a higher hydrostatic pressure so it is forced back in. - Aided by the decreased water potential of the plasma (as it has lost water but still contains the same amount of proteins), so water also leaves the tissue by osmosis down the water potential gradient. - Not all of the tissue fluid can return to the capillaries, the rest is carried back via the lymphatic system (system of vessels that drain their contents back into the bloodstream through a vein near the heart).
What is lymph?
Contents of the lymphatic system.
How are the contents of the lymphatic system moved?
- Hydrostatic pressure of the tissue fluid that has left the capillaries. - Contraction of body muscles that squeeze the lymph vessels aided by valves to maintain direction.
What is the function of xylem vessels?
Tissue that transports water in the stem and tissue.
What is transpiration?
Evaporation of water from plant leaves.
How does water evaporate out of stomata?
- Humidity of atmosphere is less than that of air spaces next to stomata. - This establishes a water potential gradient. - Therefore, provided the stomata are open, water can diffuse out of air spaces into atmosphere. - The water that diffuses out is replaced by water evaporating from mesophyll cell walls.
How do plants control water loss?
Plants’ stomata are surrounded by guard cells. If the plant is dehydrated, guard cells lose water and become flaccid which closes the pore.
How is water lost from mesophyll cells?
By evaporation from their cell walls to the air spaces of the leaf.
Is transpiration an active or passive process?
Passive.
What is the movement of water across the cells of a leaf?
1) Mesophyll cells lose water to the air spaces due to evaporation. 2) Cells now have lower water potential and so water enters by osmosis from neighbouring cells. 3) The neighbouring cells now have lower water potential. 4) These cells then take water from their neighbouring cells due to osmosis.
What are the routes water moves from xylem to mesophyll cells?
- Cell wall route. - Cytoplasmic route.
What is the main factor that means water can move up the plant?
Cohesion-tension.
What is the movement of water up the stem in the xylem?
1) Water evaporates from mesophyll cells in transpiration due to sun heat. 2) Water molecules form hydrogen bonds, sticking together AKA cohesion. 3) Water forms a continuous, unbroken column across the mesophyll cells and down xylem. 4) As water evaporates, more molecules of water are drawn up behind it due to cohesion. 5) This creates a column of water being pulled up the plant (transpiration pull). 6) Aided as water molecules adhere to walls of xylem (capillarity) and root pressure due to osmosis in roots.
What is the evidence for cohesion-tension theory?
1) Trees shrink in diameter when transpiration is at its highest because of the higher tension in the xylem, pulling the xylem walls inwards due to adhesion. 2) Broken xylem vessels take up air rather than letting water out due to negative pressure. 3) Broken xylem vessels can’t move water because the continuous stream has broken and water isn’t sticking together.
What are the features of xylem vessels?
- Lignin to strengthen walls. - No end walls to allow continuous column. - Cells are dead so waterproof and little resistance.
What is translocation?
The process by which organic molecules and some mineral ions are transported from one part of a plant to another.
What vessel transports biological molecules in plants?
Phloem.
What are the three parts of the cohesion tension theory?
1) Cohesion due to H bonds. 2) Adhesion with xylem wall caused capillarity. 3) Root pressure as water moves into roots by osmosis forces water upwards.
What are the four factors that increase transpiration?
- Higher temperature. - Higher light intensity. - Higher wind speed. - Lower humidity.
What is the structure of the phloem?
Made up of sieve tube elements end to end, with their end walls perforated to form sieve plates. These are associated with companion cells.
Is xylem transport single directional?
Yes.
Is phloem transport single directional?
No.
What are examples of what phloem transports?
Amino acids, sucrose, and inorganic ions.
What theory describes phloem transport?
Mass flow theory.
What are the three phases of mass flow theory?
1) Transfer sucrose into sieve elements from photosynthesising tissue. 2) Mass flow of sucrose through sieve tube elements. 3) Transfer of sucrose from sieve tube elements into sink cells or storage.
What is the process of transferring sucrose into sieve tube elements from photosynthesising tissue?
1) Sucrose manufactured in chloroplasts and diffuses by facilitated diffusion into companion cells. 2) H ions are actively transported from companion cells into spaces in their cell wall. 3) These then diffuse through a co-transport protein into sieve tube elements - carrying sucrose molecules with them.
What is the process of mass flow of sucrose through sieve tube elements?
1) Sieve tubes have a low water potential and so water moves into sieve tubes from xylem via osmosis - creating hydrostatic pressure. 2) At respiring tissues, sucrose is actively transported out of sieve tubes for respiration or storage. 3) This lowers the water potential at the sink and so water moves out of phloem into respiring cells. 4) So high hydrostatic pressure at source, low at sink, therefore mass flow of sucrose down hydrostatic gradient.
How is sucrose transported out of sieve tubes to sink?
Diffusion.
What is the structure of sieve tube elements?
Many cells joined end to end to form a hollow structure with no nucleus and few organelles.
What is the function of companion cells?
- Sieve tube elements don’t contain nucleus and other organelles, so they can’t survive on their own. 2. Companion cells carry out the living functions for themselves and the sieve tube elements e.g., they provide the energy for the active transport of solutes.
What is the evidence supporting mass flow hypothesis?
- There is a pressure within sieve tubes, as shown by sap being released when they are cut. - The concentration of sucrose is higher in leaves (source) than in roots (sink). - Downward flow in the phloem occurs in daylight, but ceases when leaves are shaded, or at night. - Increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later. - Metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem. - Companion cells possess many mitochondria and readily produce ATP.
What is the evidence questioning the mass flow hypothesis?
- Function of sieve plates unclear and would hinder the process. - Not all solutes move at the same speed but they should if it’s mass flow. - Sucrose is delivered to regions at the same rate not dependent on sink sucrose concentrations.
Is mass flow active or passive?
Passive; however, it depends on the active transport of sucrose therefore is active as a whole and will be impacted by metabolic poisons.
What is meant by mass flow?
Bulk movement of a substance through a given channel in a specified time.
What is a ringing experiment?
A method used to study the transport of nutrients in plants by removing a ring of bark from a tree, which leads to the accumulation of sugars above the ring.
What is mass flow?
Bulk movement of a substance through a given channel in a specified time.
Is mass flow active or passive?
Passive; however, it depends on the active transport of sucrose, therefore is active as a whole and will be impacted by metabolic poisons.
What is a ringing experiment?
Outer bark tissue is removed (phloem), sucrose accumulates above the ring (showing phloem transports) and none below (used by tissues but not replaced).
This shows phloem is responsible for sugar transport.
What are tracer experiments?
Plants are grown in an environment that contains radioactively labeled carbon dioxide (14CO2). This carbon is used to make the plants’ sugars, and the movement of these sugars can now be traced through the plant using autoradiography.
Areas exposed to radiation from 14C in the sugars will appear black, indicating where the phloem is and suggesting this is where sugars are transported.
What evidence questions the mass flow hypothesis?
- Function of sieve plates unclear and would hinder the process.
- Not all solutes move at the same speed, but they should if it’s mass flow.
- Sucrose is delivered to regions at the same rate, not dependent on sink sucrose concentrations.
What evidence supports that translocation of organic molecules occurs in phloem?
- When cut, organic molecule solution flows out.
- Aphids can be used to extract sugars from phloem, and the content of leaves changes daily, mirrored by an identical change in the phloem.
