3.3- Organisms exchange substances with their environment Flashcards
Why do small organisms have a good SA:V ratio?
They have a SA that is large enough compared with their volume to allow efficient exchange across their body surface.
What happens to SA:V ratio when organisms get bigger?
Their volume increases at a faster rate than their SA.
So SA:V ratio decreases.
Therefore, simple diffusion at the outer surface is not sufficient for activity levels/ to diffuse to inner layers.
How have organisms evolved to deal with their SA:V ratio changes?
- A flattened shape so no cell is far from the surface increases SA:V ( eg leaf)
- Specialised exchange surfaces with a large SA: increases internal SA:V ratio and maintains a conc gradient for diffusion eg by ventilation- ( eg lungs)
What is metabolic rate?
The amount of energy used up by an organism within a given period of time.
Explain the link between SA:V and metabolic rate.
As SA:V ratio increases in smaller organisms, metabolic rate increases as:
-Rate of heat loss per unit body mass increases so organisms need higher rate of respiration to release enough heat to maintain constant body temp.
How to calculate SA of a cube?
area of one side x number of sides
Work out the SA of a cube with length of one side 5cm?
5x5 = 25cm2
25 x 6 = 150cm2
How to calculate volume of a cube?
length x width x height
Work out the volume of a cube with length of one side 4cm?
l x w x h
4 x 4 x 4 = 64cm3
How to work out SA:V ratio?
SĄ of whole cube/ volume of cube
Work out SA:V ratio of a cube with SA 96 and V 64?
96/64 = 1.5:1
What are the specialised adaptations of exchange surfaces?
Large SA compared to V increasing rate of exchange.
Very thin so diffusion distance is short and materials cross exchange surface faster.
Selectively permeable.
Movement of environmental medium (air) to maintain a diffusion gradient.
Transport system to ensure movement of internal medium (eg blood) to maintain a diffusion gradient.
What is Ficks law regarding diffusion?
Diffusion = surface area x diff in concentration/ length of diffusion path
What are the adaptations of single- called organisms?
Small so have a large SA:V ratio. Oxygen absorbed by diffusion across their body surface, covered only by a cell- surface membrane.
Carbon dioxide from respiration diffuses out.
Cell walls don’t affect diffusion of gases.
Describe 3 sections of the tracheal system of an insect.
1) Spiracles- pores on surface that open/close to allow diffusion
2) Tracheae- large tubes of air that allow diffusion
3) Tracheoles- smaller branches form tracheae, permeable to allow gas exchange within cells.
What are the 3 ways respiratory gases move in and out the tracheal system?
-Along a diffusion gradient
-Mass transport
-By the ends of the tracheoles filling with water.
How is an insect’s tracheal system adapted for gas exchange?
1) Tracheoles have thin walls: short diffusion distance to cells.
2) High number of branched tracheoles so short diffusion distance to cells and large SA.
3) Tracheae provide tubes of air for fast diffusion.
4) Contraction of abdominal muscles changing pressure in body causing air to move in/out.
5) Fluid in end of tracheoles drawn into tissues by osmosis during exercise (lactate produced in anaerobic respiration lowers water potential of cells), diffusion faster through air than liquid.
Is diffusion more rapid in air or water?
Air as the particles in a gas are closer together so vibrate more= more kinetic energy so move faster.
4 key facts about fish?
-Waterproof, airtight external surface.
-Large so have a small SA:V ratio
-Body surface not sufficient to allow respiratory demands
-Internal gas exchange system (gills)
What is the function of gills?
Water enters fish’s mouth and is forced over gills, out through openings on each side.
Gills are the exchange surface for O2 into blood CO2 out of blood.
What is the structure/ adaptations of gills?
Made of many gill filaments covered with many lamellae which stack on top of eachother increasing SA for diffusion.
Thin lamellae wall/ epithelium so short diffusion distance between water and blood.
Lamellae have many capillaries to remove O2 and bring CO2 quickly for concentration gradient.
What is the counter current principle regarding fish?
Blood and water flow in opposite directions.
Blood always passes water with a higher oxygen concentration.
For diffusion along whole length of lamellae.
Why is parallel flow worse for exchange in fish?
A diffusion gradient is only maintained half the distance of the lamellae as equilibrium would not be reached.
50% O2 diffuses into blood as equilibrium wouldn’t be reached.
Why is countercurrent flow better for exchange in fish?
A steep diffusion gradient is maintained all the way across the gill lamellae- almost all O2 diffuses into blood.
How may active, fast swimming fish have different gills?
Require more oxygen so may have more gill lamellae on each gill filament for increased SA for gas exchange.
Increased gill size as there would be less time the water would pass through the gills.
What are 3 key pieces of information regarding plants?
-Plants respire all the time.
-Plants photosynthesise when conditions are right.
-Volumes and types of gases that are being exchanged in a plant leaf change depending on balance of respiration and photosynthesis.
What are the adaptations of leaves of dicotyledonous plants for rapid diffusion?
Short diffusion path with many small pores (stomata).
Large surface area of mesophyll cells for rapid diffusion.
Air spaces interconnect throughout the mesophyll.
No specific transport system for gases, they move simply by diffusion.
Describe the cross-section of a leaf from top to bottom
Waxy cuticle
Upper epidermis
Palisade mesophyll
Spongy mesophyll
Lower epidermis
What are stomata?
Small holes/ pores on the underside of leaves, single hole= stoma.
Each stoma surrounded by 2 guard cells which control opening and closing of stoma so rate of gas exchange.
How do stomata control the rate of gas exchange?
When CO2 levels are low inside the plant, the guard cells gain water and become turgid- they curve out, opening the stoma allowing gases in/out. Water evaporates through stoma.
High CO2 levels in the plant cause the guard cells to lose water, closing the stoma.
How do insects limit water loss?
-Small SA:V ratio to minimise the area from which water can be lost from.
-Waterproof covering: rigid outer skeleton of chitin covered by waterproof coat.
-Spiracles: openings to the tracheae at the surface of the body which can close when insect is at rest, stopping water evaporating out.
How does a thick cuticle limit water loss in xerophytic plants (living in dry conditions)?
The waxy cuticle forms a waterproof barrier, the thicker the cuticle, the less water can escape.
How does rolling up of leaves limit water loss in xerophytic plants?
rolling of leaves in a way that protects lower epidermis from outside helps trap region of still air in rolled leaf that becomes saturated with water vapour so has very high water potential- no water potential gradient between inside and outside leaf= no water loss.
How do hairy leaves limit water loss in xerophytic plants?
Thick layer of hairs trap still moist air next to leaf surface: water potential between inside and outside is reduced= less water lost by evaporation.
How do stomata in pits/grooves limit water loss in xerophytic plants?
They trap still moist air next to the leaf surface and reduce the water potential gradient between the inside and outside.
How does reduced SA:V ratio of leaves limit water loss in plants?
Leaves that are small and roughly circular in cross-section rather than broad and flat have a rate of water loss that can be reduced.
Describe how carbon dioxide in the air outside the leaf reaches mesophyll cells inside the leaf.
Carbon dioxide enters via the stomata.
The stomata are opened by the guard cells.
CO2 diffuses through the air spaces down a diffusion gradient.
Why is gas exchange needed in the lungs? (CO2 and O2)
O2: aerobic respiration requires constant supply of O2 in order to release energy in the form of ATP.
CO2: respiration produces CO2 which needs to be removed from the lungs as a build up is toxic to the body and can change pH.
Why do we need a high volume of exchange regarding the lungs?
1) To maintain a high body temperature (37C) in order to have high metabolic and respiratory rates.
2) Relatively large volume of living cells.
Draw the structure of the lungs
Trachea- thin tube in the middle splits into 2 bronchi.
Bronchi split into bronchioles which have alveoli on the ends.
Intercostal muscles on sides of lungs, diaphragm at bottom.
Which part is the thorax and which is the abdomen?
The thorax includes the upper half, with the lungs.
Diaphragm splits thorax from abdomen.
The abdomen refers to the lower half.
Describe the structure of the lungs?
Lobed structures made of highly branched tubules called bronchioles that end in tiny air sacs called alveoli.
Describe the structure and function of the trachea?
Flexible airway surrounded by rings of cartilage that prevent trachea collapsing as the air pressure falls when breathing in.
Trachea walls are made of muscle lined with ciliated epithelial cells and goblet cells.
Describe the structure and function of the bronchi?
2 divisions of the trachea: each leading to a lung.
They secrete mucus (goblet) and have cilia to move mucus towards throat.
Larger bronchi have cartilage, reducing as bronchi get smaller.
Describe the structure and function of bronchioles?
Series of branching subdivisions of bronchi.
Walls are made of muscle lined epithelial cells to allow them to constrict to control the flow in/out the alveoli.
Describe the structure and function of the alveoli?
Minute air sacs (diameter 100-300 micrometres) found at end of bronchioles.
Between alveoli there is collagen and elastic fibres which allow alveoli to stretch as they fill with air and spring back to expel CO2 rich air.
What is the gas-exchange surface in the alveoli/lungs?
The alveolar membrane.
Explain features of alveolar epithelium that make it adapted for gas exchange.
1) Flattened cells (1 cell thick) so short diffusion distance.
2) Folded so large SA
3) Permeable to allow diffusion of 02/CO2
4) Moist so gases can dissolve for diffusion
5) Good blood supply from capillaries to maintain conc gradient.
How does gas exchange occur in the lungs?
O2 diffuses from alveolar air space into blood down its concentration gradient across alveolar epithelium then across capillary endothelium.
What is ventilation?
The constant movement of air into and out the lungs to maintain diffusion gradients to gas exchange.
Important as it brings in air with higher O2 conc and removes air with lower O2 conc maintaining gradient.
What are the 3 sets of muscles involved in ventilation?
Internal intercostal muscles.
External intercostal muscles.
Diaphragm.
Write the 7 steps to inhalation.
1) External intercostal muscles contract.
2) Internal intercostal muscles relax.
3) Ribs pulled up and out.
4) Diaphragm contracts/flattens.
5) Increases volume of thorax.
6) Decreases pressure in thorax.
7) Atmospheric pressure > pulmonary pressure= air forced in lungs down pressure gradient.
Write the 7 steps to exhalation.
1) Internal intercostal muscles relax.
2) External intercostal muscles contract.
3) Ribs move down and upwards.
4) Diaphragm relaxes/ curves up.
5) Decreases volume of thorax.
6) Increases pressure in thorax.
7) Pulmonary pressure > atmospheric pressure= air forced out lungs down pressure gradient.
Why is expiration normally passive at rest?
Internal intercostal muscles do not normally need to contract, expiration aided by elastic recoil in alveoli.
What is the pulmonary ventilation rate equation?
Pulmonary ventilation rate (dm3 min-1) = tidal volume (dm3) x breathing rate (min-1)
What is tidal volume?
Volume of air normally taken in at each breath when the body is at rest.
What is breathing rate?
Number of breaths taken in in 1 minute.
How do different lung diseases reduce the rate of gas exchange?
1) Thickened alveolar tissue (eg fibrosis) which increases the diffusion distance.
2) Alveolar wall breakdown which reduces surface area.
3) Reduced lung elasticity: lung expands/recoils less so reduced concentration gradients of O2/CO2.
How do different lung diseases affect ventilation?
1) Reduced lung elasticity (eg fibrosis- build up of scar tissue), lungs expand/recoil less reducing vol of air in each breath (tidal volume) and reducing max vol of air breathed out in one breath.
2) Narrow airways- reduced airflow in/ out of lungs (eg asthma- inflamed bronchi) reducing max vol of air breathed out in 1 second.
3) Reduced rate of gas exchange- increased ventilation rate due to reduced O2 in lungs.
What are the 5 risk factors for lung disease?
1) Smoking
2) Air pollution- increased likelihood of COPD eg heavy industry.
3) Genetics
4) Infections- frequent chest infections increase risk of lung disease.
5) Occupation- working with harmful chemicals, dust and gases
Describe the gross structure of the human gas exchange system and how we breathe in.
Trachea, bronchi, bronchioles, alveoli.
breathing in:
diaphragm contracts and external intercostal muscles contract, causing volume to increase and pressure to decrease in thoracic cavity, so air moves in.
breathing out:
diaphragm relaxes and internal intercostal muscles relax, causing volume to decrease and pressure to increase in thoracic cavity, so air moves out.
Explain the movement of oxygen into the gas exchange system of an insect when it is at rest.
Oxygen is used during aerobic respiration so there is an oxygen concentration gradient established so oxygen can diffuse in.
What is the difference between correlations and causal relationships?
Correlation means a change in one variable reflects a change in another whereas a causation means a change in one variable causes a change in another variable
What occurs in digestion?
Large insoluble biological molecules hydrolysed to smaller soluble molecules that are small enough to be absorbed across cell membranes into blood.
Describe the digestion of starch (carbohydrates) in mammals?
Amylase (produced by salivary glands/pancreas) hydrolyses alternate glycosidic bonds of starch to make maltose.
Membrane bound maltase (attached to cells lining ileum) hydrolyses maltose to glucose.
Describe the digestion of disaccharides in mammals?
Membrane bound disaccharides hydrolyse into 2 monosaccharides.
maltose—-> glucose + glucose (by maltase)
sucrose—-> fructose + glucose (by sucrase)
lactose —-> glucose + galactose (by lactase)
Describe the digestion of lipids in mammals?
Bile salts (produced by liver) emulsify lipids causing them to form smaller lipid droplets- micelles.
This increases SA of lipids for faster lipase activity.
Lipase (made in pancreas) hydrolyses lipids to produce a monoglyceride and 2 fatty acids or a glycerol and 3 fatty acid.
Describe the digestion of proteins by a mammal?
Endopeptidases hydrolyse internal peptide bonds within a polypeptide to form smaller peptides so more ends for exopeptidases.
Exopeptidases hydrolyse terminal peptide bonds at ends of polypeptide to form single amino acids.
Membrane bound dipeptidases hydrolyse peptide bond between a dipeptide forming a single amino acid.
Suggest why membrane-bound enzymes are important in digestion?
They are located on cell membranes of epithelial cells lining the ileum so by hydrolysing molecules at the site of absorption they maintain concentration gradients.
Describe the absorption of amino acids and monosaccharides in mammals (co transport)?
1) Na+ actively transported from epithelial cells lining ileum to blood (by Na+/K+ pump) establishing a conc gradient of Na+ higher in lumen than epithelial cell.
2) Na+ enters epithelial cell down its conc gradient with glucose against its conc gradient via a co-transporter protein.
3) Glucose moves down a conc gradient into blood by facilitated diffusion.
Describe the absorption of lipids by a mammal, including the role of micelles.
Micelles contain bile salts, monoglycerides and fatty acids.
They make monoglycerides and fatty acids more soluble in water, release them to lining of ileum, maintain high conc of fatty acids to lining.
Monoglycerides and fatty acids absorbed into epithelial cell by diffusion. Triglycerides reformed in epithelial cells.
Globules (aggregations of triglycerides) coated with proteins are packaged into vesicles which move to cell membrane and leave by exocytosis.
Lymphatic vessels enter and return to blood circulation.
What is the structure of haemoglobin?
Protein with a quaternary structure- made of 4 polypeptide chains (2 alpha, 2 beta).
Each chain contains a ‘haem’ group which contains an iron Fe2+ ion giving it its red colour.
What is the difference between dissociating and associating?
Associating/ loading is when haemoglobin binds with oxygen in the lungs whereas dissociating/ unloading is when haemoglobin releases with oxygen in the tissues.
What is affinity?
How easy it is for oxygen to load(associate) or unload (dissociate).
Haemoglobin can change its affinity for oxygen under different conditions by changing its shape.
What is the difference between high affinity haemoglobin and low affinity haemoglobin?
High affinity haemoglobin associates (takes up) with oxygen more readily and dissociates (releases) with oxygen less readily whereas low affinity haemoglobin associates with oxygen less readily but dissociates with oxygen more readily.
What is the link between oxygen and partial pressure, carbon dioxide and partial pressure?
High concentration of oxygen= high partial pressure of oxygen (pO2)
High concentration of carbon dioxide= high partial pressure of carbon dioxide (pCO2)
What occurs during haemoglobin saturation?
When haemoglobin has loaded oxygen, the blood is saturated with oxygen.
It is loaded with oxygen at the lungs where there is a higher pO2 and lower pCO2 (high affinity for O2) and unloaded at the respiring tissues like muscles where there is a higher pCO2 and lower pO2 (low affinity for O2).
Describe the role of red blood cells and haemoglobin in oxygen transport.
1) Red blood cells contain haemoglobin (no nucleus, bioncave, high SA:V)
2) Haemoglobin associates with O2 at gas exchange surfaces where pO2 is high.
3) This forms oxyhaemoglobin which transports O2.
4) Haemoglobin dissociates from O2 near tissues where pO2 is low.
Draw an oxygen dissociation curve: O2 saturation of haemoglobin on y axis, pO2 on x axis.
Curve which is less steep at beginning, more steep in middle and less steep at the end (plateaus).
S shaped
Describe the 3 main parts of oxygen dissociation curve?
1) When first O2 molecule binds to haemoglobin, there is a change in its shape as it is difficult for O2 to bind making it easier for next molecule to bind. At low O2 concentrations, little O2 binds to Hb.
2) It takes a smaller increase in pO2 to bind the 2nd O2 molecule than the first = positive cooperativity due to change in quaternary structure of Hb. At higher PO2, as O2 increases there is rapid increase of % saturation of Hb with O2.
3) After binding of 3rd O2 molecule, although easier for Hb to bind, it is less probable of it doing so as most binding sites are full.
Explain the oxygen dissociation curve.
When there is a high pO2, Hb has high affinity for O2 so it will readily associate with it- % saturation is high.
When there is a low pO2, Hb has low affinity for O2 so will readily dissociate with it- % saturation is low.
What occurs when the oxygen dissociation curve moves to the right? Bohr effect (CO2)
Increasing blood CO2 due to increased respiration rate lowers the blood pH so haemoglobin has a lower affinity for oxygen so releases it more easily as its quaternary structure slightly changes.
Needs a higher change in pO2 to get same increase in saturation.
Good for very active organisms who need a lot of aerobic respiration, can drop off more O2 at respiring tissues.
Occurs with:
-Higher CO2
-Lower pH
-Higher temperature
What occurs when the oxygen dissociation curve moves to the left?
The haemoglobin has a higher affinity for oxygen so takes it up more easily.
Good when there is less oxygen/ lower pO2.
Smaller increase in PO2 generates the same increase in % saturation of Hb with O2.
Occurs with:
-Lower CO2
-Higher pH
-Lower temperature
What is the Bohr shift which occurs in respiring tissues?
Respiring tissues release CO2 reducing haemoglobin affinity, meaning it unloads more oxygen.
What is the effect of pH on ability of Hb to carry O2?
CO2 produced at actively respiring tissues, dissolves in solution increasing its acidity (producing H+ ions, decreasing pH)
Acidic conditions promote unloading of O2 from oxyhaemoglobin as H+ ions displace O2 from oxyhaemoglobin.
What factors cause a need for a transport system?
A small SA:V ratio
How active the organism is- very active needs more efficient transport system
Draw a diagram of a heart including: right+left atria, right+left ventricles, pulmonary vein and artery, semilunar and atrioventricular valves, aorta.
How does the circulatory system work?
Heart is organ that pumps blood around body, most of wall is made from muscle tissue.
2 separate circulations in the heart: LHS pumps oxygenated blood, RHS pumps deoxygenated blood.
Deoxygenated blood enters heart through vena cava, it is pumped from heart to the lungs by the pulmonary artery. Oxygenated blood enters the heart from lungs through pulmonary vein, it is pumped from heart to body by aorta.
What is the importance of a double circulatory system?
Prevents mixing of oxygenated/ deoxygenated blood so blood pumped to body is fully saturated with O2 for aerobic respiration.
Blood can be pumped at higher pressure after being lower from lungs, substances taken to/removed from body cells quicker.
What is the role of the vena cava?
Transports deoxygenated blood from respiring body tissues to the right atrium.
What is the role of the pulmonary artery?
Transports deoxygenated blood from right ventricle to the lungs.
What is the role of the pulmonary vein?
Transports oxygenated blood to the left atrium from the lungs.
What is the role of the aorta?
Transports oxygenated blood from the left ventricle to respiring body tissues.
Why is the wall of the left ventricle thicker than the right?
Thicker muscle to contract with greater force to generate higher pressure to pump blood around the entire body.
Difference between systole and diastole?
Systole= contraction
eg ventricle systole
Diastole= relaxed
eg heart diastole when all muscles of the heart are relaxed
Difference in blood pressure in diastole and systole?
Blood pressure is highest when ventricles contract (systolic pressure) and lowest when they are relaxed and filling with blood (diastolic pressure).
What are the 3 stages of the cardiac cycle?
Relaxation of the heart, contraction of atria (atrial systole), contraction of ventricles (ventricular systole).
What occurs when ventricles relax (and atria contract)?
ATRIAL SYSTOLE: BLOOD DOWN.
L+R atria contract: volume inside these chambers decreases but pressure increases.
Atrioventricular valves open when pressure in atria exceeds pressure in ventricles.
Semi-lunar valves remain shut as pressure in arteries exceeds pressure in ventricles, so blood pushed INTO L+R VENTRICLES.
What occurs when the ventricles contract (and atria relax)?
VENTRICULAR SYSTOLE: BLOOD OUT
L+ R ventricles contract, volume inside these chambers decreases but pressure increases.
Atrioventricular valves shut when pressure in ventricles exceeds pressure in atria to prevent backflow.
Semilunar valves open when pressure in ventricles exceeds pressure in arteries, so BLOOD PUSHED OUT OF HEART THROUGH ARTERIES.
What occurs when ventricles and atria are relaxed?
HEART DIASTOLE: BLOOD FILLS
Atria and ventricles relax: volume inside these chambers increases but pressure decreases.
Semilunar valves shut when pressure in arteries exceeds pressure in ventricles to prevent the backflow of blood from arteries into ventricles.
Atrioventricular valves open when pressure in atria exceeds pressure in ventricles, so blood fills atria via veins and flows passively to ventricles. Atria contract and process repeats.
How to remember where the valves are and if open or closed on cardiac cycle graph?
SL valves are higher on heart diagram, so 2 higher dips on graph.
AV valves lower so 2 lower dips.
COCO perfume- first dip= closed, second=open, third=closed, fourth= open.
Describe the 4 main sections of cardiac cycle graph + events?
1- AV valves close when pressure in ventricle exceeds pressure in atrium to prevent backflow of blood from ventricles to atria.
2- SL valves open when pressure in ventricle is higher than in artery so blood flows from ventricle to artery.
3- SL valves close when pressure in artery is higher than in ventricle to prevent backflow of blood from artery to ventricles.
4- AV valves open when pressure atrium is higher than in ventricle so blood flows from atrium to ventricle.
What is the equation for cardiac output?
Cardiac output (vol of blood pumped out of heart per min)
= stroke volume (vol of blood pumped each heart beat) x heart rate (n of beats per min)
How do calculate heart rate from cardiac cycle data?
Heart rate (bpm) = 60 seconds/ length of one cardiac cycle (s)
Explain how the structure of arteries relates to their function?
Thick smooth muscle tissue- can contract and maintain blood flow.
Thick elastic tissue- can stretch and recoil as ventricles move maintaining a high pressure.
Thick wall withstands pressure.
Smooth endothelium reduces friction.
Narrow lumen increases high pressure.
Explain how the structure of arterioles relates to their function?
Thicker smooth muscle layers than arteries- contracts to narrow lumen and reduce blood flow to capillaries/ vice versa.
Thinner elastic layer so pressure surges lower.
Explain how the structure of veins relates to their function?
Wider lumen than arteries so less resistant to blood flow.
Little elastic and muscle tissue so blood pressure lower.
Valves prevent backflow.
Explain how the structure of capillaries relates to their function?
Wall is one cell thick reducing diffusion distance.
Large network of branched capillaries increases SA.
Narrow lumen reduces blood flow rate.
Pores in walls between cells.
Explain the formation of tissue fluid (ARTERIOLE end)
At the arteriole end of capillaries there is a higher hydrostatic pressure inside the capillaries than the tissue fluid so net outward force.
This forces water and dissolved substances out capillaries.
Large plasma proteins remain in capillary.
Explain the return of tissue fluid to the circulatory system (VENULE end)
At the venule end of capillaries the hydrostatic pressure reduces as fluid leaves capillary.
An increasing concentration of plasma proteins lowers water potential in capillary below it of tissue fluid.
Water enters capillaries from tissue fluid by osmosis down a water potential gradient.
Excess water taken up by lymph capillaries and returned by circulatory system through veins.
Explain the causes of excess tissue fluid accumulation.
1) Low concentration of protein in blood plasma= water potential in capillary not as low so water potential gradient reduced, more tissue fluid formed at arteriole end.
2) High blood pressure leading to a high hydrostatic pressure, increases outward pressure from arteriole end so more tissue fluid formed at arteriole end, lymph system may not be able to drain excess fast enough.
What is the function of xylem tissue?
Transports water and mineral ions through the stem, up the plant to leaves of plants.
How is xylem tissue adapted for its function?
Cells joined with no end walls forming a long continuous tube.
Cells contain no cytoplasm/ nucleus for easier water flow.
Thick cell walls with lignin to provide support and withstand tension.
Pits in side walls which allow lateral water movements.
Explain the cohesion-tension theory of water transport in the xylem (including the leaf, xylem and root).
1) Leaf:
water lost from leaf by transpiration, evaporates from mesophyll cells into air spaces, water vapour diffuses through stomata reducing water potential of mesophyll cells. Water drawn out of xylem down water potential gradient.
2) Xylem:
tension is created in xylem, hydrogen bonds result in cohesion between water molecules so water pulled up as continuous column, sticks to walls of xylem.
3) Root:
water enters roots via osmosis.
What is the difference between transpiration and the transpiration stream?
Transpiration is the loss of water vapour from the leaves or stem whereas the transpiration stream is the movement of water through the xylem tissue and the mesophyll cells.
How does light intensity affect transpiration rate?
Increases rate of transpiration as stomata open in light to allow CO2 in, allowing more water to evaporate faster.
Stomata close when dark so there is a low transpiration rate.
How does temperature affect transpiration rate?
Increases rate of transpiration as water molecules gain kinetic energy as temperature increases so water evaporates faster.
How does wind intensity affect transpiration rate?
Increases rate of transpiration as wind blows away water molecules from around stomata, decreasing water potential of air around stomata, increasing water potential gradient so water evaporates faster.
How does humidity affect transpiration rate?
Decreases rate of transpiration as more water in air so it has a higher water potential, decreasing the water potential gradient from leaf to air so water evaporates slower.
What is the function of phloem tissue?
Transports organic substances- eg sucrose in plants.
How is phloem tissue adapted for its function?
Sieve tube elements
No nucleus and few organelles to maximise space for organic substances.
End walls between cells have holes (sieve plate)
Companion cells with mitochondria for high rate of respiration to make ATP for active transport of solutes.
What is translocation?
The movement of assimilates/ solutes such as sucrose from source cells to sink cells by mass flow.
Explain the mass flow hypothesis for translocation in plants (PHLOEM CELLS AND SUCROSE)
1) At the source, sucrose is actively transported into phloem cells by companion cells.
2) This lowers water potential in sieve tubes so water enters from xylem by osmosis.
3) This increases hydrostatic pressure in sieve tubes at source, creating hydrostatic pressure gradient.
4) Mass flow occurs- movement from source to sink.
5) At sink, sucrose removed by active transport to be used in respiring cells/ stored in storage organs.
