MASS TRANSPORT

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PHLOEM

STRUCTURE OF PHLOEM.

  • sieve tube elements - long thin structures arranged end to end with perforated walls to form sieves
  • companion cells associated with sieve tube elements
  • transports sugars from sites of production, sources, to places where they are needed, sinks, in translocation

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MASS FLOW THEORY - STEP 1

TRANSLOCATION: process by which organic molecules/mineral ions are transported from one part of a plant to another.

1. sucrose manufactured from photosynthesis in chloroplasts

2. sucrose diffuses down conc gradient by facilitated diffusion from photosynthesising cells to companion cells

3. hydrogen ions actively transported from companion cells into spaces within cell walls using ATP

4. hydrogen ions diffuse down conc gradient through carrier proteins into sieve tube elements with sucrose molecules 

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MASS FLOW THEORY - STEP 2

1. sucrose has been actively transported into sieve tube elements

2. this lowers the water potential of the sieve tubes

3. the xylem has a higher water potential, so water moves into the xylem by osmosis creating high hydrostatic pressure

4. at the sinks, sucrose is used in respiration or stored as starch so the cells have low sucrose content, meaning sucrose is actively transported into them from the sieve tubes, lowering their water potential so water moves in by osmosis

5. low hydrostatic pressure of sieve tubes near sink

6. results in mass flow of sucrose solution down hydrostatic pressure gradient from source to sink

7. sucrose actively transported by companion cells out of sieve tube elements and into sink cells

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INVESTIGATING TRANSPORT IN PLANTS

RINGING EXPERIMENTS.

  • woody stems have outer layer of bark 
  • remove bark and phloem layer in a certain rgion
  • over time, the region immediately above the missing ring swells and is found to be rich in sugars and other organic substances - accumulation of sugars
  • region below withers and dies - interruption of flow of sugars
  • therefore, phloem is responsible for the transport of sugars

TRACER EXPERIMENTS.

  • use radioactive isotope 14C - grow plant in atmosphere containing 14CO2 as the 14C will be incorporated into sugars from photosynthesis
  • radioactive sugars can be traced as they move within the plant
  • e.g. thin cross sections of stem on x ray film, film turns black where exposed to 14C radiation and black regions correspond to location of phloem
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EVIDENCE FOR TRANSLOCATION

TRANSLOCATION OCCURS IN PHLOEM.

  • when phloem is cut, a solution of organic molecules (sap) flows out
  • plants provided with radioactive carbon dioxide show radioactive phloem after a short time
  • aphids - needle-like mouthparts penetrate phloem and can be used to extract contents of sieve tubes, showing different sucrose contents in the leaves followed by the same variations in the phloem
  • removal of phloem from the circumfurence of the stem leads to the accumulation of sugars above the ring and death of cells below it
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TRANSPORT OF WATER IN THE XYLEM

1. humidity of atmosphere is less than that of air next to stomata, therefore water potential gradient is creates so water vapour molecules diffuse out of oopen stomata, replaced by water from mesophyll cells

2. mesophyll cells lose water to air spaces, now have a lower water potential

3. water enters mesophyll cells from neighbouring cells by osmosis and they in turn do the same

4. water forms hdrogen bonds between one another, causing them to stick together - cohesion

5. water forms continuous, unbroken column across mesophyll cells and down the xylem

6. water drawn up from xylem into mesophyll cells as water evaporates from stomata causing colum of water to be pulled up the xylem - transpiration pull

7. transpiration puts xylem under tension - negative pressure - thus this is the cohesion-tension theory

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EVIDENCE FOR TRANSPIRATION PULL

  • change in diameter of tree trunks - during day, lots of transpiration = more tension = xylem vessels pulled inwards, decreasing the diameter + visa versa at night
  • if a xylem is broken and air enters, the tree can no longer draw up water as the continuous column of water is broken and molecules cannot stick together
  • water does not leak out when xylem is broken - air is drawn in due to the negative pressure
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HAEMOGLOBIN

  • protein molecules with a quaternary structure that makes it efficient at loading (associating) oxygen under certain conditions and efficient at unloading (dissociating) oxygen in other conditions.

PRIMARY: sequence of amino acids of 4 polypeptide chains

SECONDARY: each polypeptide chain coiled into helix

TERTIARY: each polypeptide folded into precise shape - important in ability to carry oxygen

QUATERNARY: all four polypeptides linked, each eassociated with a haem group containing an Fe2+ ion which can combine with one oxygen molecule

ROLES.

  • readily associate oxygen at the gas exchange surface
  • readily dissociate oxygen at the tissues requiring it
  • be able to change shape to change affinity for oxygen depending on conditions
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OXYGEN DISSOCIATION CURVES

  • graph of the relationship between saturation of haemoglobin with oxygen and partial pressure of oxygen.

1. shape of haemoglobin makes it difficult to bind to one of the four sites as they are closely united - little partial pressure, little saturation, shallow gradient

2. first oxygen binding changes quaternary structure, changing its shape and making it easier for other oxygens to bind

3. only takes small increase in partial pressure to bind second and third oxygen - positive cooperativity

4. fourth oxygen is difficult to bind due to probability - only 1/4 chance of meeting an empty site

  • the further to the left the curve, the greater the affinity (loads readily, unloads less readily)
  • the further to the right the curve, the lower the affinity (unloads readily, loads less readily)
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CARBON DIOXIDE + OXYGEN DISSOCIATION

GAS EXCHANGE SURFACE.

  • concentration of CO2 is low - diffuses across the exchange surface in excretion
  • affinity of haemoglobin for oxygen increases + high concentration of oxygen at surface means oxygen is readily loaded
  • the reduced carbon dioxide concentration shifts the curve to the left

RESPIRING TISSUES.

  • concentration of CO2 is high
  • affinity of haemoglobin for oxygen decreases + low concentration of oxygen means oxygen is readily unloaded
  • the increased carbon dioxide concentration shifts the curve to the right
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HOW DOES CARBON DIOXIDE AFFECT HAEMOGLOBIN?

  • at the gas exchange surface, CO2 is constantly removed
  • pH is slightly raised due to low concentration of acidic CO2
  • increased pH changes shape of haemoglobin to one that enables it to load oxygen readily (high affinity)
  • also means that oxygen will not be unloaded as it is transported in the blood to the tissues
  • at the tissues, respiring cells produce CO2
  • pH is lowered due to high concentration of acidic CO2
  • lowered pH changes shape of haemoglobin to one with lower affinity for oxygen
  • haemoglobin unloads oxygen to respiring tissues
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OXYGEN DISSOCIATION + CONDITIONS

LUGWORMS.

  • lugworms live in u-shaped burrows on the shore
  • when lugworm is not covered by seawater, oxygen supply is reduced, cannot diffuse into blodo as readily
  • lugworm has to extract as much oxygen from the water
  • oxygen dissociation curve is thus shifted far to the left of that of a human
  • lugworm can readily associate oxygen even with very little available in the environment

ACTIVITY OF CELLS.

  • haemoglobin becomes saturated with oxygen as it passes the lungs - not all is loaded, usually 97% saturation at atmospheric pressure - TOP OF THE GRAPH
  • at resting cells (low respiratory rate), only one oxygen molecule is released - blood returning to lungs has 75% saturation
  • at highly active cells (high respiratory rate), three oxygen molecules are released - blood returning to lungs has 25% saturation
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TRANSPORT SYSTEMS

LARGE ANIMALS + TRANSPORT SYSTEMS.

  • large size = surface area to volume ratio decreases
  • needs of the organism cannot be met by body surface alone
  • transport system us required to transport materials between cells and exchange surfaces
  • high activity = high respiratory rate = more O2 and CO2 - need for specialised transport system with pump

MAMMAL CIRCULATORY SYSTEM.

  • closed, double circulatory system
  • blood is confined to vessels and passes twice through the heart before completing entire circuit of body
  • when blood passes lungs, pressure is reduced so movement would be very slow
  • returns to heart to increase pressure to ensure rapid delivery of substances
  • mammals have high body temp - high rate of metabolism
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FEATURES OF TRANSPORT SYSTEMS

 

  • suitable medium in which to carry materials - e.g. water based as it is a good solvent and moved around easily and can be a gas
  • form of mass transport - medium moved over large distances in bulk (more efficient than diffusion)
  • closed system of tubular vessels containing the transport medium in a branching structure to distribute materials to all parts of organism
  • mechanism for moving transport medium based on pressure differences - e.g. muscular contraction of the heart, evaporation of water
  • mechanism to maintain mass flow in one direction - e.g. valves
  • means of controlling flow of transport medium to suit changing needs of parts of organism
  • mechanism for mass flow of water/gases e.g. movement of intercostal muscles + diaphragm during ventilation
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STRUCTURE OF THE HEART

ATRIUM: thin walled and elastic as it stretches to fill with blood

VENTRICLE: thick muscular wall as it has to pump blood to the body or lungs

  • necessary to return blood to heart as it loses pressure in the capillaries

AORTA: connected to the left ventricle, carries oxytgenated blood to all parts of the body except the lungs

PULMONARY VEIN: connected to left atrium, brings oxygenated blood back from the lungs

VENA CAVA: connected to the right atririum, returns deoxygenated blood from all tissues except the lungs

PULMONARY ARTERY: connected to the right ventricle, carries deoxygenated blood to the lungs where its oxygen is replenished

CORONARY ARTERIES: branch off the aorta to supply the blood muscle with oxygen, blockage causes myocardial infarction (heart attack) where muscle is deprived of oxygen

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THE CARDIAC CYCLE

DIASTOLE - RELAXATION.

  • blood returns to the atria from the vena cava and pulmonary vein
  • the atria fill and the pressure within them rises
  • pressure exceeds that of the ventricles, atrioventricular valves open to allow blood to flow through (aided by gravity)
  • relaxation of ventricle walls calls them to recoll and reduce pressure within, ensuring that the pressure is lower than that of the aorta and pulmonary arter so the semi-lunar valves close

SYSTOLE - CONTRACTION.

  • atria contract which, combined with the recoil of ventricular walls, forces remaining blood into the ventricles from the atria
  • after the ventricles fill with blood, their walls contract simultaneously, increasing the pressure within them, forcing the atrioventricular valves shut
  • ventricular pressure exceeds that of aorta and pulmonary artery, opening the semi-lunar valves
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VALVES

  • blood moves from region of higher pressure to lower pressure
  • some situations mean that pressure differences result in blood flowing in undesirable direction
  • valves used to prevent backflow - open when pressure difference favours the correct movement of blood

ATRIOVENTRICULAR VALVES: between the left and right atria and ventricles, preventing the backflow of blood when ventricular pressure exceeds that of the atria

SEMI-LUNAR VALVES: between the ventricles and aorta/pulmonary artery, preventing the backflow of blood when the pressure in vessels exceeds that of the ventricles caused by the recoil of the elastic walls in the vessels and the relaxation of the ventricles

POCKET VALVES: occur throughout the venous system, ensuring tht when veins are squeezed (e.g. when skeletal muscles contract) blood flows back towards the heart, not away from it

  • all valves made up of flaps of fibrous tissue in cusp shaps that move apart when pressure is greature on the convex side and push together when pressure is greater on the concave side so that blood collects in the 'bowl''

CARDIAC OUTPUT (volume of blood pumped by one ventricle in one minute) = stroke volume (volume of blood pumped with each beat) x heart rate (rate at which heart beats)

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PRESSURE CHANGES IN THE HEART

(http://mgh-images.s3.amazonaws.com/9780321901583/508920-18-1ISR21.png)

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BLOOD VESSELS

ARTERIES - carry blood away from the heart towards arterioles

ARTERIOLES - smaller arteries that control blood flow from arteries to capillaries

CAPILLARIES - tiny blood vessels that link arterioles to veins

VEINS - carry blood from capillaries to heart

LAYERS FROM OUTSIDE INWARDS.

  • tough fibrous outer layer - resists internal and external pressure changes
  • muscle layer - contracts to control flow of blood
  • elastic layer - maintains blood pressure through stretch and recoil
  • thin inner lining - smooth to reduce friction and thin to allow diffusion
  • llumen - central cavity through which blood flows
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ARTERIES + ARTERIOLES

ARTERY STRUCTURE + FUNCTION.

  • thick muscle layer compared to veins - allows vasoconstriction and dilation to control blood flow
  • elastic layer thick compared to veins - allows stretch at systole and recoil at diastole to maintain high blood pressure and smooth pressure surges
  • great overall thickness - resists bursting under pressure
  • no valves - high pressure ensures blood flows in one direction

ARTERIOLE STRUCTURE + FUNCTION.

  • muscle layer thicker than in artieries - allows vasoconstriction + dilation to control volume of blood entering capilaries
  • thinner elastic layer than in arteries - blood is at lower pressure
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VEINS + CAPILLARIES

VEIN STRUCTURE + FUNCTION.

  • thin muscle layer compared to arteries - blood is carried away from tissues, no need to control flow
  • thin elastic layer compared to artieries - low pressure so no risk of bursting or enough pressure to produce recoil action
  • low overall thickness - no need due to low pressure and allows easy flattening to aid blood flow
  • valves at intervals throughout - ensures blood does not flow backwards when body muscles contract

CAPILLARY STRUCTURE + FUNCTION.

  • walls consist mostly of lining layer - thin to ensure short diffusion pathway
  • numerous and highly branched - high surface area
  • narrow diameter - permeate tissues, short diffusion pathway to cells
  • narrow lumen - red blood cells squeezed flat to reduce diffusion pathway
  • spaces between endothelial cell lining - allows white blood cells to escape
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TISSUE FLUID

  • watery liquid containing glucose, amino acids, fatty acids, ions and oxygen to supply to tissues + receives carbon dioxide and other wastes from the tissues
  • immediate environment of cells - bathes the tissues

FORMATION - ULTRAFILTRATION.

  • heart pumped, producing high hydrostatic pressure at the arterial end of capillaries
  • causes tissue fluid to move out of blood plasma
  • outward pressure opposed by hydrostatic pressure of tissue fluid outside capillaries + lower water poential of blood due to plasma proteins
  • creates overall force that pushes tissue fluid out at arterial end - water + small molecules

RETURN.

  • loss of tissue fluid from capillaries lowers hydrostatic pressure at venous end
  • tissue fluid forced back into capillaries due to higher hydrostatic pressure
  • plasma has also lost water but still contains plasma proteins - low water potential so water leaves tissue by osmosis
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THE LYMPHATIC SYSTEM

  • drains excess tissue fluid that HASN'T returned to the capillaries
  • carries tissue fluid back through system of vessels, eventually leading to two ducts joining veins close to the heart
  • movement of contents relies on hydrostatic pressure of tissue fluid that has left the capillaries and the contraction of body muscles that squeeze the lymph vessels 
  • lymph vessels also have valves to ensure fluid moves away from tissues
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georgersmith

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this is really good , it really helped me , great detail too!

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