Mass transport

What are haemoglobin molecules?

Protein molecules with a quaternary structure

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What is the primary structure of a haemoglobin molecule?

Sequence of amino acids in the four polypeptide chains

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What is the secondary structure of a haemoglobin molecule?

Each of the polypeptide chains is coiled into a helix.

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What is the tertiary structure of a haemoglobin molecule ?

Each has polypeptide chain is folded into a precise shape which is an important factor of the haemoglobin's ability to load oxygen

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What is the quaternary structure of a haemoglobin molecule?

All 4 polypeptide chains are linked together to form a spherical molecule.

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Each polypeptide is associated with a .... group?

Haem group

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What does the haem group contain?

A ferrous ion (Fe2+)

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What can a ferrous ion combine with?

A single oxygen molecule (O2)

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How many oxygen molecules can be carried by a single haemoglobin molecule ?

4

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The process by which haemoglobin binds with oxygenmix called what?

Loading or associating

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Where does loading of a haemoglobin molecule take place?

The lungs

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The process by which a haemoglobin molecule releases it's oxygen is called what?

Unloading or dissociation

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Where does unloading take place in a human?

Tissues

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What does it mean if haemoglobin has high affinity for oxygen?

It means the haemoglobin take up oxygen more easily but release it less easily

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What does it mean if haemoglobin has low affinity for oxygen?

It means the haemoglobin take up oxygen less easily but release it more easily

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What is the role of haemoglobin?

The transport oxygen

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What must be in place for the haemoglobin molecule to be efficient at transporting oxygen?

1) Readily associate with oxygen at the surface where gas exchange takes place 2) Readily dissociate from oxygen at those tissues requiring it

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Then devious cards answer seems to be contradictive, however it is achieved by a remarkable property of haemoglobin, which is...

It can change its affinity for oxygen under different conditions

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How can haemoglobin change its affinity under different conditions?

It's shape changes in the presence of certain substances such as carbon dioxide in the presence of carbon dioxide the new shape of haemoglobin molecule binds more loosely to oxygen. As a result haemoglobin releases it's oxygen)

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What is an oxygen dissociation curve?

The graph of a relationship between saturation of haemoglobin with oxygen and partial pressure of oxygen

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The shape of the haemoglobin molecule makes it difficult for the first oxygen moleate to bind to it. Why?

Because all the sites on its 4 polypeptide sub units are closely United. So at low oxygen concentrationso little oxygen will, bind to haemoglobin. (Making gradient curve shallow at first)

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Binding of the first oxygen molecule changes the quaternary structure of the haemoglobin molecule causing it to change shape. Why does this make bindingevery easier for the second oxygen molecule?

The binding of the first oxygen molecule induces the other sub units to bind to an oxygen molecule

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What is positive cooperativity?

This means, because of the first binding of an oxygen molecule and a polypeptide sub unit, it takes a smaller increase in partial pressure of oxygen to bind the second oxygen molecule than it did the first (the gradient curve steepens)

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After binding the third oxygen moleculemail it is harder to bind the final molecule. Why?

Due to probability, the majority of binding sites are occupied so it is less likely an oxygen molecule will find an empty site to bind to (the gradient of the curve reduces and graph flattens out)

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The further to the left the oxygen dissociation curve the...

Greater the affinity of haemoglobin for oxygen (loads readily unloads less easily)

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The further to the right the oxygen dissociation curve the...

Lower the affinity is of haemoglobin for oxygen (loads less readily unloads more easily)

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What happens to the haemoglobin's affinity for oxygen in the presence of carbon dioxide?

Affinity reduces

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The greater the concentration of carbon dioxide, will haemoglobin be more readily or less readily to release it's oxygen?

More readily ( Bohr effect)

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What is the bohr effect?

Haemoglobin's oxygen binding affinity is inversely related both to acidity and carbon dioxide concentration

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At the gas exchange surface the concentration of carbon dioxide is low so affinity for oxygen is increased meaning that oxygen will be readily loaded by haemoglobin. Will the oxygen dissociation curve move to the left or right?

Left

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In respire tissue the concentration of carbon dioxide is high. Affinity of haemoglobin for oxygen is reduced which means that oxygen is readily unloaded into muscle cells. Will the oxygen dissociation curve move to the left or right

Right

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At the gas exchange surface carbon dioxide is constantly being removed. pH is slightly raised due to low concentration of carbon dioxide. What does this higher pH do to the haemoglobin molecule?

It changes it's shape into one that enable it to load oxygen readily also this shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood to the tissues

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In the tissues carbon dioxide is produced by respiring cells. Carbon dioxide is acidic in solution so pH of the blood within the tissues is lowered. The lower the pH does what to haemoglobin?

The shape of haemoglobin will change of that with a lower affinity for oxygen so haemoglobin releases it's oxygen into respiring tissues.

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The more active a tissue the more what will be unloaded?

Oxygen

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The higher the rate of respiration -> the more carbon d oxide is produced -> the lower the pH (finish this)

-> the greater the haemoglobin shape change -> the more readily oxygen is unloaded -> the more oxygen is available for respiration

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Species of animals that live in an environment with a lower partial pressure of oxygen have evolved haemoglobin that has a what type of affinty for oxygen?

Higher affinty

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Whether or not there is a specialised transport medium and whether or not it is circulated by a pump depends on 2 FACTORS:

1. The surface area to volume ratio 2. How active the organism is (the lower the surface area to volume ratio, and more active, the greater the need for a specialised transport system with a pump)

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What are some common features of a transport system? (Part1)

●a suitable medium in which to carry materials e.g blood. (usually liquid based on water because water readily dissolves substances and can be moved around easily or gas) ●a form of mass transport in which the transport medium is moved around in bulk

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What are some common features of a transport system? (Part2)

● a closed systemy of tubular vessels that contains the transport medium and forms a branching network to distribute to all parts of the organism●a mechanism for moving the transport medium within vessels. This requires a pressure difference

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Transport systems are achieved In 2 main ways: (animals and plants)

1. Animals use muscular contraction 2. Plants rely on natural passive processes such as evaporation

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Mammals have a closed double circulatory system. What does this mean?

*diagram* Blood is confined to vessels and passes twice through the heart for each complete circuit of the body.

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Why do mammals need a closed double circulatory system?

Because when blood is passed through the lungs it loses pressure which would cause circulation to be slow around the rest of the body so after it reaches the lungs it is returned back to the heart to replenish this pressure.

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Why do mammals need fast circulation of blood?

Because they have high body temperature so high rate of metabolism

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What are the 4 types of vessels?

Arteries, arterioles, veins and capillaries

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Diagram of the mammalian circulatory system

*

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The human heart is main up of two pumps, the left which deals with oxygenated blood from the lungs and the right which deals with deoxygenated blood from the rest of the body. Each pump has 2 chambers, what are they?

1. Atrium - thin walled and elastic streches to collect blood 2. Ventricle - thicker muscular walls as it contracts strongly to pump blood to some distance either the lungs or the rest of the body

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Where does the right ventricle pump blood to?

The lungs

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Where does the left ventricle pump blood to?

Rest of the body

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Which has a thicker muscular wall the right or left ventricle and why?

Left because it has to pump blood to the extremities of the body

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Both atrias contract together and both ventricle contract together. True or false and why?

True because this means they are pumping the same volume of blood

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Between each atrium and ventricle have valves. What is the purpose of the valves?

Prevent back flow of blood into atria when ventricles contract

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There are 2 types of valves. What are they?

1. Left atrioventricular (bicuspid) valve 2. Right atrioventricular (tricuspid) valve

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Each of the 4 chambers of the heart are connected to large vessles that carry blood towards or away from the heart. Do ventricles/atria pump blood away from or towards the heart and via what kind of vessel?

Ventricles pump blood away from the heart into arteries. Atria recieve blood from veins

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Vessels connecting the heart to lungs are called what?

Pulmonary vessels

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There are 4 vessels connected to the four chambers of the heart. What are they and what do they do and what chamber are they connceted to?

1. Aorta - connceted to left ventricle, carries oxygenated blood to all parts of the body except the lungs 2. Vena cava - connected to right atrium, brings deoxygenated blood back from the tissues

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There are 4 vessels connected to the four chambers of the heart. What are they and what do they do and what chamber are they connceted to? (part 2)

3. Pulmonary artery - connected to right ventricle, carries deoxygenated blood to the lungs (unusal for an artery to carry deoxygenated blood) 4. Pulmonary vein - cpnnected to left atria, brings oxygenated blood back from the lungs (unusal for a vein

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Diagram of a heart

*

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How is the heart muscle supplied with oxygen?

The heart muscle is supplied by its own blood vessels called the coronary artieres, which branch off the aorta shortly after they leave the heart

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Blockage of coronary arteries can lead to what?

Myocardial infarction (heart attack) because an area is deprieved of oxygen so cells can no longer repspire so they die

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What is the cardiac cycle?

The heart undergoes a sequence of events that is repeated in humans around 70 times a minute at rest

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There are 2 phases to the beating of the heart. What are they?

1. Contraction (systole) 2. Relaxation (diastole)

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Atria recieve blood through pulmonary vein (from lungs) and vena cava (from tissues). As the atria fill, pressure in them rises. What happens when this pressure exceeds the pressure of the ventricles?

Atrioventricular valves open allowing blood to pass into the ventricles.

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The passage of the blood is aided by what?

Gravity

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When blood passes from atria to ventricle via the atrioventricular valves, are chambers relaxed or contracting?

Relaxed because the flow of blood is aided by gravity (it is flowing down)

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When ventricle walls are relaxed this causes them to recoil and does what to the pressure in the ventricle?

Reduces it

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When ventricles pressure is now lower of that in the atria and pulmonary artery and so causes what?

Semi-lunar valves in aorta and pulmonary artery to close (dub sound of the heart)

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Cards 62 to 66 all describe what?

Relaxation of the heart (diastole)

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Diagram of diastole:

*

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What happens during contraction of the atria?

When atrial walls contract it forced the remaining blood into the ventricles (which are relaxed) from the atria.

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Diagram of contraction of atria:

*

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What happens during contraction of ventricles?

Ventricles fill with blood and so will contract simultaneously, which increases blood pressure within them which forces atrio ventricular valves preventing backflow to atria (lub sound) With the valves closed, the pressure in ventricles rises further

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What happens when pressure of ventricles exceeds that of the atria?

Blood is forced from ventricles into vessels

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Blood is kept flowing in one direction through the heart & around the body by the pressure created by what?

The heart muscle

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Blood will always move from a region of high pressure to low pressure, true or false?

True

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In some circumstances there might be blood flowing backwards due to pressure difference. What structure is in place to prevent this?

Valves

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Valves in cardiovascular system are designed so they open when and close when?

Open when difference in blood pressure on either side favours the movement of blood in the required direction. Close when these differnces are reversed, so blood would tend to flow in the opposite (undesirable) direction

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What are the 3 examples of valve, where are they located and what do each do? (part 1)

1. Atrioventricular valves - between left atrium and ventricle and right atrium and ventricle. Prevent backflow of blood when contraction of ventricles means the prssure in the ventricles will exceed that of the atria.

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What are the 3 examples of valve, where are they located and what do each do? (part 2)

Closure of the valves ensure, when ventricles contract, blood within them moves to aorta and pulmonary artery rather than back to atria. 2. Semi-lunar valves - in aorta & pulmonary artery. Prevent backflow of blood into ventricles when pressure

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What are the 3 examples of valve, where are they located and what do each do? (part 3)

in the aorta and pulmonary artery exceeds that in the ventricles. 3. Pocket valves - in veins, occur throughout the venous system. Ensure when veins are squeezed, blood flows back towards the heart rather than away from it

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What is the design of the valves? What happens when pressure is greater on each side of the valve?

Made up of flaps of tough, flexible, fibrous tissue which are cusp-shaped (bowls). When pressure is greater on the convex side of the cusp, they move apart to let blood pass between cusps. When pressure is greater on the concave side, blood collects

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What is the design of the valves? What happens when pressure is greater on each side of the valve? (part 2)

within the bowl. This pushes them togther to form a tight fit and prevents passage of blood

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Diagram of valve open:

*

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Diagram of valve closed:

*

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What is cardiac output?

Volume of blood pumped by one ventricle of the heart in 1 minute.

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What is cardiac output measured in?

dm3min-1

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Cardiac output depends on what 2 factors?

1. Heart rate (rate at which the heart beats 2. stroke volume (volume of blood pumped out at each beat)

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What is the equation for working out cardiac output?

cardiac output = stroke volume x heart rate

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What is an electrodiagram?

The heart undergoes a series of electrical current changes, which are related to waves of electrical activity created by the sinoatrial node & the heart's response to these. If displayed on a cathode ray oscilloscope these changes produce a trace.

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What is an ECG?

An electrocardiograph, where there is a pattern of large peaks and small troughs that repeat at idnetical intervals.

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What would and ECG look like during a heart attack?

Less pronounced peaks and learger troughs that are repeated similarily but not identical.

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What happens during a condition called fibrillation on an ECG?

The heart muscle contracts in a disorganised way which is reflected on the ECG.

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What are the four different types of blood vessels and what do they do?

1.Arteries-carry blood away from heart into arterioles 2.Arterioles-smaller arteries, control blood flow from arteries to capillaries 3.Capillaries-tiny vessels, link arterioles to veins 4.Veins-carry blood from capillaries back to heart.

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Arteries, arterioles and veins all have the same basic laered structure. What are the layers and what do they do? (from outisde inwards) (part 1)

1.Tough fibrous outer layer-resists pressure changes from within & outisde 2. Muscle layer - contract to control blood flow 3.Elastic layer - helps maintain blood pressure by stretching & recoiling

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Arteries, arterioles and veins all have the same basic laered structure. What are the layers and what do they do? (from outisde inwards) (part 2)

4. Thin inner lining (endothelium) - smooth to reduce friction and thin to allow diffusion 5. Lumen - central cavity (not actually a layer) through which the blood flows

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What differs between the structure of each type of blood vessel?

The proportion of each layer is different for the different types of blood vessels

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Diagram of artery structure

*

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Diagram of vein structure

*

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Digram of capillary structure

*

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What is the function of the artieries?

Transport blood rapidily under high pressure from the heart to the tissues.

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Why do arteries have a muscle layer that is thicker than that of veins?

Because by having this, this means smaller arteries can be constricted and dilated in order to control the volume of blood passing through them

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Why do arteries have an elastic layer which is thicker than that of veins?

Because it is important to keep blood pressure high to reach the extremities of the body. The streching and recoiling of the elastic layer means high and smooth pressure is maintained

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Why do arteries have a thick wall?

Resist the vessel bursting under pressure

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Why are there no valves in arteries? (except in arteries leaving the heart)

Because blood is under high pressure constantly due to the heart pumping blood into arteries so it tends not to backflow

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What is the function of the arterioles?

Carry blood, under lower pressure than that of the artieries, from arteries to capillaries. They control the blood flow between the two.

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Why do arterioles have a muscle layer thicker than that of the arteries?

The contraction of this muscle layer allows constriction of the lumen of the arteriole which restricts blood flow so controlling movement into capillaries that supply tissues with blood

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Why do arterioles have an elastic layer thinner than that of the arteries?

Because blood pressure is lower in the arterioles

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What is the function of the veins?

Transport blood slowly, under low pressure from the capillaries in tissues to the heart.

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Why do veins have a muscle layer thinner than that of the arteries?

Because veins carry blood away from tissues so constriction & dilation cannot control the blood flow to the tissues.

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Why do veins have an elastic layer thinner than that of the arteries?

Because the low pressure of the blood within the veins will not cause them to burst and pressure is too low to create a recoil action

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Why do veins have a thin wall?

Because there is no need for a thicker wall as pressure within the wall is too low to cause the vessel to burst, it also allows them to be flattened easily aiding the flow of blood within them

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Why are there valves at intervals through out the veins?

To ensure does not flow backwards which it might do because pressure is low. When body muscles contract, veins are compressed, pressuring the blood within them, the valves ensure this pressure directs the blood in 1 direction towards the heart

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Diagram of action of valves in veins

*

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What is the function of the capillaries?

To exchange metabolic materials between blood and cells.

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Why is flow of blood much slower in the capilarries than any other vessel?

As this allows more time for exchange of materials

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Why are the walls of a capillary mostly consistant of the lining layer?

Making them extremely thin so distance for which diffusion has to cover is small allowing for rapid diffusion

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Why are capillaries in numerous amounts and highly branched?

Because this provides a large surface area for exchange

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Why are lumen of capillaries so narrow?

So red blood cells are squeezed flat agaisnt the side of the capillary so they are even closer to cells so reducing diffusion distance

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Why are capillaries narrow in diameter?

So they can permeate tissues, meaning that no cell is far from a capillary and there is a short diffusion pathway

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Why are there spaces between the lining (endothelium) in capillaries?

To allow white blood cells to escape in order to deal with infections within tissues

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Although capillaries are small, they cannot serve every single cell directly. So the final journey of materials is made in a liquid solution that bathes tissues. What is this liquid called?

Tissue fluid

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What does tissue fluid contain?

It is wa tery liquid that contains glucose, amino acids, fatty acids, ions and oxygen

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Tissue fluid supplies all tissues with the materials but what does it recieve from the tissues?

Carbon dioxide and other waste materials.

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What is tissue fluid formed from?

Blood plasma

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What is the compositon of blood plasma controlled by?

Homeostatic systems.

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Pumping of the heart creates what type of pressure?

Hydrostatic pressure

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Where is hydrostatic pressure found?

Arterial end of capillaries

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Diagram of capillary network:

*

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What does hydrostatic pressure cause the tissue fluid to do in the blood plasma?

Move out of the blood plasma

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What 2 factors can oppose the tissue fluid moving out of the blood plasma?

1. Hydrostatic pressure of the tissue fluid outside the capillaries 2. Lower water potential of the blood, due to plasma proteins, that causes water to move back into the blood within the capillaries.

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An overall pressure pushes tissue fluid out of capillaries at the arterial end. This pressure is only enough to dio what?

Force small molecules out of capillaries, leaving cells and prteins that are too large to get through the membrane .

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What is ultrafiltration?

Filtration under pressure where larger molecules cannot pass through the membrane.

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Once tissue fluid has exchanged metabolic materials, it is returned to the circulatory system via what?

Capillaries

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How does tissue fluid return via the capillaries? (part 1)

1. The loss of tissue fluid from capillaries reduces hydrosattic pressure inside them 2. As a result, by the time the blood has reached the venous end of the capillary network its hydrostatic pressure is lower than that of the tissue fluid around it

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How does tissue fluid return via the capillaries? (part 2)

3. Therefore tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outisde them 4. In addition, the plasma has lost water and still contains proteins. It therefore has a lower water potential than the tissue fluid

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How does tissue fluid return via the capillaries? (part 3)

5. As a result, water leaves the tissue by osmosis down a water potential gradient

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Diagram of formation and return of tissue fluid:

*

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Not all tissue fluid can return via the capillaries, so remainder is carried back via?

Lymphatic system

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What is the lymph?

A vessel that begin in tissue, resemble capillaries except they have dead ends, they gradually merge into larger vessels that form a network. These networks drain their contents back into the bloodstream via 2 ducts that join veins close to the heart

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The contents of the lymphatic system are not moved by the pumping of the heart . Instead they are moved by 2 things:

1. Hydrostatic pressue of tissue that has left capillaries 2. Contraction of body muscles that squeeze the lymp vessels - calves in lymph vessels ensure that the fluid inside them moves away from tissues in the direction of the heart

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Summary of formation and return of tissue fluid to the blood stream

*

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What is the lymph?

A vessel that begin in tissue, resemble capillaries except they have dead ends, they gradually merge into larger vessels that form a network. These networks drain their contents back into the bloodstream via 2 ducts that join veins close to the heart

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The contents of the lymphatic system are not moved by the pumping of the heart . Instead they are moved by 2 things:

1. Hydrostatic pressue of tissue that has left capillaries 2. Contraction of body muscles that squeeze the lymp vessels - calves in lymph vessels ensure that the fluid inside them moves away from tissues in the direction of the heart

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Summary of formation and return of tissue fluid to the blood stream

*

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In plants water is absorbed by roots through root hair cells. In flowering plants, water is transported through hollow thick walled tubes called?

Xylem vessels

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What is the main force that pulls water through the xylem up the stem?

Transpiration

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What is transpiration?

The evaporation of water from leaves

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Where is energy for transpiration supplied from?

The sun

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The humidity of the atmosphere is usually less than that of the air spaces next to the stomata. What is the result of this?

There is a water potential difference from air spaces through stomata and the atmosphere.

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What will happen if there is a water potential difference between air spaces through stomata and the air?

Provided the stomata are open, water vapour molecules will diffuse out of air spaces into air.

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How is water lost by diffusion replenished?

By water evaporating from the cell walls of surrounding mesophyll cells

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How can plants control the rate of transpiration?

By changing the size of stomatal pores

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How is water lost from mesophyll cells?

Evaporation. from their cell walls to the air spaces of leaf

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How is water replaced back into mesophyll cells?

By the xylem via cell walls or via cytoplasm

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In the case of the cytoplasmic rate, how does water movement occur?

1. Mesophyll cells lose water to air spaces by evaporation due to heat from the sun 2. These mesophyll cells now have a lower water potential gradient so water enters by osmosis from neighboring cells

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In the case of the cytoplasmic rate, how does water movement occur? (PT2)

3. loss of water from these neighbouring cells lowers their water potential 4. they take in water from their neighbours from osmosis

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Diagram of movement across a leaf

*

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What is the main factor responsible or the movement of water up the xylem from roots to leaves?

Cohesion-tension theory

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What is cohesion-tension theory? (pt1)

1.Water evaporates from mesophyll cells due to heat from sun leading to transpiration 2.water molecules form hydrogen bonds between 1 another so stick together-cohesion 3.Water form a continuous unbroken column across the mesophyll cells & down xylem

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What is cohesion-tension theory? (pt2)

4.as water evaporates from mesophyll cells into air space beneath the stomata, more molecules are drawn up behind it as a result of cohesion 5. a column of water is therefore pulled up the xylem as a result of transpiration - transpiration pull.

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What is cohesion-tension theory? (pt3)

6.transpiration pull puts tension on the xylem

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What is some evidence to support cohesion-tension theory?

Change in the diameter of tree trunks according to the rate of transpiration. During the day when transpiration is high so there is more tension in the xylem. This pulls the walls of xylem vessels inwards & causes the trunk to shrink in diameter.

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Xylem vessels are dead. True or false?

True

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Xylem have no end walls so they just form continuous tubes. Energy is needed to drive the process of transpiration to create the passive process of transpiration pull. What does passive mean?

No need for energy

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Summary of water transport through plant:

*

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What is translocation?

Process by which organic molecules and some mineral ions are transported from one part of a plant to another

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In flowering plants what part transport biological molecules?

Phloem

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What is phloem made up of?

Sieve tube elements

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Structure of phloem:

* sieve tube elements arranged end to end, end walls are perforated to form sieve plates

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What are next to sieve tube elements?

Companion cells

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What are sources and sinks?

1. Plant transports sugars, that were produced during photosynthesis, from sites of production - sources 2. To places where they will be used or stored - sinks

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Sinks can be anywhere, either above or below a source true or false?

True

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Mass flow theory is used to explain the mechanism by which translocation is achieved. What are the 3 phases of mass flow theory?

1. Transfer of 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 storage or other sink cells.

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What is stage 1 of mass flow theory? (pt1)

1. Sucrose is manufactured from products of photosynthesis in cells and chloroplasts 2. Sucrose diffuses down a concentration gradient by facilitated diffusion from photosynthesising cells into companion cells.

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What is stage 1 of mass flow theory? (pt2)

3. Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP. 4. These hydrogen ions then diffuse down a concentration through carrier proteins into the sieve tube element.

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What is stage 1 of mass flow theory? (pt3)

5. Sucrose molecules are transported along with the hydrogen ions in a process known as co-transport. The protein carriers are also known as co-transport proteins

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What is stage 2 of mass flow theory? (pt1)

Mass flow of sucrose thorugh sieve tube elements:1. the sucrose produced by photsyntheisisng cells is activly transported into the sieve tube through a given channel in a specified time. 2. This causes the sieve tube elements to have a lower water

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What is stage 2 of mass flow theory? (pt2)

potential. 3. As the xylem have a much higher water potential water moves from xylem into sieve tube elements by osmosis, creating a high hydrostatic pressure within them 4.At the respiring cells, sucrose is either used up during respiration or

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What is stage 2 of mass flow theory? (pt3)

converted to starch as storage. 5. So respiring cells have low sucrose content so sucrose is activly transported into them from sieve tubes lowering their water potential. 6. Due to lowered water potential, water also moves into these respiring cells

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What is stage 2 of mass flow theory? (pt4)

from sieve tubes by osmosis. 7. The hydrostatic pressure of the sieve tubesin lowered 8. As a result of water entering the sieve tube elements at the source and leaving at thie sink, there is high hydrostatic pressure at the source and low at sink.

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What is stage 2 of mass flow theory? (pt5)

9. There is therefore a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes

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What is stage 3 of mass flow theory ?

Sucrose is activly transported by companion cells out of sieve tubes into sink cells

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Mass flow is a passive process but occurs from active transport so is the whole process active or passive?

Active

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What is some evidence suppporting mass flow hypothesis?

1. there is pressure within sieve tubes, as shown by sap released when they are cut. 2.concentration of sucrose is higher in leaves (source) than in roots (sink) 3.companion cells possess many mitochondria and readily produce ATP

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What is some evidence questioning mass flow theory?

1.Function of sieve plates is unclear as they would seem to hinder mass flow 2. not all solutes move at the same speed - they should do so if movement is by mass flow 3.sucrose is delievered at more or less the same rate to all regions rather than

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What is some evidence questioning mass flow theory?

going to regions with less sucrose concentrations as mass flow would suggest

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Movement of sucrose from source to sink through the phloem of plant diagram

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What are ringing experiments?

investigating that woody stems have an outer protective layer of bark on the inside of which is a layer of phloem that extends all round the stem. Inside the phloem layer is xylem

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What is the process of ringing experiments? (pt1)

1. a section of outer layers is removed around the complete circumference of woody stem while it is still attached to the rest of the plant 2. After a period of time, the region above the cut seems to swell 3. samples of the liquid that has

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What is the process of ringing experiments? (pt2)

accumulated in the wollen part are found to be rich in sugars and other dissolved substances 4. The region below the cut seems to wither and die

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What do the observations found in ringing experiments suggest?

1. sugars of the phloem accumulating above the ring, leading to swelling 2. interruption of flow of sugars to the region below the ring and death to tissues

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What conclusion can be made from ringing experiments?

Phloem rather than xylem is the tissue responsible for translocating sugars in plants. As the ring of tissues removed had not extended into xylem, its continuity is not broken. If xylem were the tissue responsible you would have not expected sugars

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What conclusion can be made from ringing experiments?

to accumulate above the ring nor tissues below it to die.

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What are tracer experiments?

Where radioactive isotopes are used to trace movement of substances in plants.

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What is the process of tracer experiments? (pt1)

1. isotope 14C can be used to make radioactive carbon dioxide. 2. A plant is then grown in an evironment with this radioactive carbon dioxide then the isotope will be incoprated into the sugars produced during photosynthesis. 3. sugars can be traced

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What is the process of tracer experiments? (pt2)

using an autoradiography. 4. Take thin cross section of plant stem and place on x ray film. 5. film becomes blackened where it has been exposed to radtion from the isotope. 6. blackened regions correspond to where phloem tissue is in stem

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What conclusion can be made from tracer experiments?

Other tissues do not blackened the film so they wont carry sugars and that phloem alone is responsible for their translocation

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What is some evidence that translocation of organic molecules occurs in phloem? (pt 1)

1. When phloem is cut, a solution of organic molecules flow out 2. Plants provided with radioactive carbon dioxide can be shown to have radioactivly labelled carbon in phloem

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what is some evidence that translocation of organic molecules occurs in phloem? (pt 2)

3. Aphids are atype of insect that feed on plants and have needle like mouthparts which penetrate phloem so can be used to extract contents of sieve tube elements. Contents show daily variations in sucrose content of leaves that are mirrored a later

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what is some evidence that translocation of organic molecules occurs in phloem? (pt 3)

by identical changes in sucrose content of phloem 4. The removal of a ring of phloem from around the whole circumference of a stem leads to accumulation of sugars above ring and dissapperance below it

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Mass transport

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