Biology - Chapter 7: Mass Transport

Haemoglobins

A group of chemically similar molecules found in a wide variety of organisms. They are protein molecules with a quaternary structure.

What is the structure of haemoglobin (3)?

- 4 polypeptides linked together
- Forms an almost-spherical molecule
- Each polypeptide is associated with a haem group (contains Fe2+ ion)

How many molecules of oxygen can a single haemoglobin molecule carry and why?

4 O2 molecules - each polypeptide has an Fe2+ ion, which can combine with one O2 molecule each

Loading (and an alternate name for it)

The process by which haemoglobin binds with oxygen (aka associating)

Unloading (and an alternate name for it)

The process by which haemoglobin releases its oxygen (aka dissociating)

Where does loading and unloading take place?

Loading: in the lungs
Unloading: in the tissues

What does it mean if haemoglobins have a:
1. high affinity for oxygen?
2. low affinity for oxygen?
What is the effect on the oxygen-dissociation curve?

1. They associate with oxygen more easily, but dissociate with it less easily - graph is to the left
2. They associate with oxygen less easily, but dissociate with it more easily - graph is to the right

What is haemoglobin's role?

To transport oxygen

2 things necessary for haemoglobin to be efficient at transporting oxygen

1. It must readily associate with oxygen at the surface where gas exchange takes place
2. It must readily dissociate from oxygen at those tissues requiring it

Why do different species have different haemoglobins?

- Each species produces haemoglobin with slightly different amino acid sequence
- They each have slightly different tertiary and quaternary structures
- So they each have different affinities for oxygen

Oxygen dissociation curve

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

Why is the oxygen dissociation curve shaped like that? (3)

1. Initially, shallow gradient: shape of haemoglobin makes it difficult for first O2 molecule to bind to one of the sites as the 4 polypeptides are closely united, so at low O2 concentrations, little O2 binds
2. Then, gradient steepens: binding of first O2 molecule causes quaternary structure of haemoglobin to change, uncovering another binding site, so smaller increase in partial pressure of O2 is needed to bind to second O2 [positive cooperativity]
3. Gradient reduces, graph plateaus: after 3rd O2 molecule binds, majority of binding sites occupied (haemoglobin more saturated with oxygen) so less likely that O2 molecule will find empty space to bind to.

What does it mean if another curve is 1. further left and 2. further right of the original oxyhaemoglobin dissociation curve?

1. Further left of curve = greater affinity of haemoglobin for oxygen
2. Further right of curve = lower affinity of haemoglobin for oxygen

The Bohr effect and why does it happen?

The greater the concentration of carbon dioxide, the more readily haemoglobin releases its oxygen - occurs as dissolved CO2 is acidic so low pH causes haemoglobin to change shape

Effect of CO2 concentration on loading/unloading at the gas-exchange surface and at tissues

- At gas-exchange surface, low concentration of CO2 as it is excreted out of organism -> increased affinity of haemoglobin for O2 + high concentration of O2 at the surface so oxygen is more readily loaded (oxygen dissociation curve shifts leftwards)
- At rapidly-respiring tissues, high concentration of CO2 -> reduced affinity of haemoglobin for O2 + low concentration of O2 so oxygen is more readily unloaded (oxygen dissociation curve shifts rightwards)

Is dissolved CO2 acidic or alkaline?

Acidic

How does CO2 concentration affect the shape of haemoglobin at the gas-exchange surface and at the tissues? (4)

1. At gas-exchange surface, CO2 constantly removed and excreted - lower concentration so high pH (alkaline)
2. Haemoglobin shape changes due to pH, becomes one more readily able to load oxygen + increases affinity of haemoglobin for oxygen so it's not released while being transported in blood to tissues
3. At tissues, CO2 produced by respiring cells - higher concentration so lower pH (acidic)
4. Haemoglobin shape changes, has lower affinity for oxygen so it releases its oxygen into respiring tissues

Overall saturation of haemoglobin at atmospheric pressure

Around 97%

Why is it important that different species have different oxygen dissociation curves?

They're adaptations to survive different conditions and environments - e.g. may have higher affinity for O2 if live in an environment with lower partial pressure of oxygen

x-axis and y-axis for oxygen dissociation curve?

x-axis: partial pressure of oxygen (kPa)
y-axis: saturation of haemoglobin with oxygen (%)

Partial pressure

The pressure exerted by an individual gas in a mixture (similar to concentration)

Why do large organisms need a transport system?

As they become larger, they have a smaller SA:V ratio so needs of organism can no longer be met by body surface alone - need specialist exchange surfaces

What factors determine whether an organism needs a specialist transport system and pump? (2)

SA:V ratio and how active the organism is

4 features of transport systems

1. Suitable medium to carry materials (normally liquid based on water - could be a gas)
2. Form of mass transport in which transport medium is moved in bulk over large distances (faster than diffusion)
3. Closed system of tubular vessels that contains transport medium + forms a branching network to distribute it to all parts of the organism
4. Mechanism for moving transport medium within vessels (requires a pressure difference between parts of the system)

How do animals achieve a pressure difference in their transport systems?

Use muscular contraction of either their body muscles or of a specialised pumping organ

What type of circulatory system do mammals have and why?

Closed, double circulatory system: when blood is passed through the lungs, has lower pressure - circulation would be too slow, so blood passes through heart again to increase pressure, allowing substances to be transported quickly

What blood vessels enter and leave the heart?

Enter: vena cava (from body), pulmonary vein (from lungs)
Leave: pulmonary artery (to lungs), aorta (to body)

What blood vessels enter and leave the lungs?

Enter: pulmonary artery (from heart)
Leave: pulmonary vein (to heart)

What blood vessels enter and leave the kidneys?

Enter: renal artery
Leave renal vein

Where is the heart located?

In the thoracic cavity, behind the sternum

Describe the structure of the heart

- Made of two pumps
- Each pump has two chambers: the atrium and the ventricle
- Right side pumps to lungs, left side pumps to body

Describe the atrium

Thin-walled and elastic and stretches as it collects blood (receives blood from veins)

Describe the ventricles and explain why they are like this.

Thick, muscular wall - has to contract strongly to create enough pressure to pump blood some distance (to lungs or rest of body)

Do the sides of the heart pump together?

Yes - both atria contract together and both ventricles contract together

What do valves do?

Prevent the backflow of blood

4 vessels connected to chambers of the heart

- Vena cava: connected to right atrium, brings deoxygenated blood from tissues of body to heart
- Pulmonary artery: connected to right ventricle, carries deoxygenated blood to lungs where O2 is added and CO2 is removed
- Pulmonary vein: connected to left atrium, carries oxygenated blood from lungs to heart
- Aorta: connected to left ventricle, carries oxygenated blood to body

Coronary arteries

Arteries that supply the heart muscle with oxygen

What happens if the coronary arteries are blocked?

Myocardial infarction: area of the heart is deprived of blood and so, O2 - muscle cells in this region are unable to respire so die

4 risk factors of cardiovascular disease

1. Smoking (CO means less O2 carried, heart pumps harder to supply enough, higher BP + coronary heat disease & nicotine results in adrenaline secreted, higher HR and BP + makes platelets more 'sticky')
2. High BP (heart must pump harder so more prone to failure + arteries become weaker, more likely to develop aneurysm and cause haemorrhage + artery walls become thicker and harder to withstand higher pressure)
3. Blood cholesterol (low-density lipoproteins, transports cholesterol from liver to tissues, infiltrate artery walls, may develop atheroma)
4. Diet (high levels of salt raise blood pressure + high levels of saturated fat increases LDL cholesterol levels - antioxidants reduce risk of cardiovascular disease)

Cardiac cycle

A sequence of events the heart undergoes - repeated around 70 times in humans at rest

2 phases to beating of the heart

Contraction (systole) and relaxation (diastole)

Describe the process of the relaxation of the heart (diastole) (5)

1. Blood returns to atria of heart through pulmonary vein and vena cava
2. Atria fills - pressure rises, eventually exceeding pressure in ventricles (pressure in atria > pressure in ventricles)
3. Atrioventricular valves open, blood passes into ventricles
4. Muscular walls of atria and ventricles are relaxed - relaxation of ventricle walls causes them to recoil, reducing pressure in the ventricle (pressure in ventricles < pressure in aorta/pulmonary artery)
5. Semi-lunar valves close

Describe the process of contraction of the atria (atrial systole) (3)

1. Atrial walls contract to push remaining blood into ventricles
2. Ventricle walls relaxed - recoil of relaxed ventricle walls forces remaining blood into ventricles from atria
3. Pressure in atria > pressure in ventricles so atrioventricular valves open more

Describe the contraction of the ventricles (ventricular systole) (4)

1. After ventricles fill with blood, their walls contract simultaneously
2. Blood pressure in them increases, (pressure in ventricles > pressure in atria), forcing atrioventricular valves shut to prevent flow of blood into atria
3. Pressure in ventricles increases further - when it exceeds pressure in aorta/pulmonary artery, blood is forced into those vessels (pressure in ventricles > pressure in aorta/pulmonary artery)
4. Ventricles have thick muscular walls, contract forcefully, creating high pressure necessary to pump blood around body

Why is the wall of the left ventricle thicker than the right wall?

The left side of the heart has to pump blood to the extremities of the body, while the thinner right side only has to pump blood to the lungs - left side needs muscle that can contract with enough force to pump blood

In the heart, how does blood flow in only one direction?

Due to pressure in chambers of the heart (will always flow from a region of higher pressure to a region of lower pressure) - valves close to further prevent backflow of blood

When do valves open?

When the difference in blood pressure either side of them favours movement in the required direction (i.e. when pressure before them > pressure after them)

3 types of valves

1. Atrioventricular valves: between atria and ventricles - prevents backflow of blood when pressure in ventricles > pressure in atria due to ventricular contraction
2. Semi-lunar valves: in aorta and pulmonary artery - prevents backflow of blood when pressure in aorta/pulmonary artery > pressure in ventricles
3. Pocket valves: in veins - when veins are squeezed (e.g. by contraction of muscles), blood flows towards heart rather than away

Cardiac output and units

The volume of blood pumped by one ventricle of the heart in one minute (dm^3 min^-1)

Cardiac output equation

Cardiac output = heart rate x stroke volume

Heart rate

The rate at which the heart beats

Stroke volume

Volume of blood pumped out at each beat

4 types of blood vessel and describe what they do

- Arteries (carry blood away from heart into arterioles)
- Arterioles (smaller arteries that control blood flow from arteries to capillaries)
- Capillaries (tiny vessels that link arterioles to veins)
- Veins (carry blood from capillaries back to heart)

5 structural features of arteries/arterioles/veins

1. Tough fibrous outer layer: resists pressure changes from inside and outside
2. Muscle layer: contracts to control the flow of blood
3. Elastic layer: helps to maintain blood pressure by stretching and recoiling
4. Endothelium (thin inner lining): smooth to reduce friction + thin to allow diffusion (short diffusion pathway)
5. Lumen: central cavity of the blood vessel through which the blood flows

4 structural features of arteries

1. Thicker muscle layer: smaller arteries can constrict and dilate to control volume of blood flowing
2. Thicker elastic layer: stretches and recoils to maintain a high blood pressure and to smooth pressure surges caused by heart beat
3. Overall thick wall: resists bursting under pressure
4. No valves: blood is constantly under high pressure due to heart pumping blood (so blood doesn't tend to flow backwards)

2 structural features of arterioles

1. Thicker muscle layer relative to arteries: muscles contract to constrict arteriole lumen to restrict flow of blood + blood's movement into capillaries
2. Thinner elastic layer relative to arteries: lower blood pressure

4 structural features of veins

1. Relatively thin muscle layer: veins carry blood away from tissues so constriction/dilation cannot control flow of blood to tissues
2. Relatively thin elastic layer: low blood pressure means they won't burst and pressure is too low to create a recoil action
3. Overall thin wall: too low pressure so no risk of bursting + can be flattened easily to aid flow of blood within them
4. Valves: prevent backflow of blood due to low blood pressure

What is the function of the arteries?

To transport blood rapidly under high pressure from the heart to the tissues

What is the function of the arterioles?

To carry blood, under lower pressure than arteries, from arteries to capillaries + to control the flow of blood between the two

What is the function of the veins?

To transport blood slowly, under low pressure, from the capillaries in tissues to the heart

What is the function of the capillaries?

To exchange metabolic materials (e.g. oxygen, carbon dioxide, glucose) between the blood and the cells of the body

Why is the flow of blood in the capillaries much slower?

To allow more time for the exchange of materials

5 structural features of capillaries

1. Walls are mostly endothelium: extremely thin so short diffusion pathway
2. Numerous and highly branched: large surface area
3. Narrow diameter: can permeate tissues so no cell is far from a capillary - short diffusion pathway
4. Narrow lumen: RBC are squeezed flat against the side of a capillary so shorter diffusion pathway
5. Spaces between endothelial cells - fenestrations: allow WBC to escape in order to deal with infections between tissues

What does tissue fluid contain?

Glucose, amino acids, fatty acids, ions in solution, oxygen + receives carbon dioxide and other waste materials

Tissue fluid

The environment that surrounds the cells of multicellular organisms - it is the means by which materials are exchanged between blood and cells

What is tissue fluid formed from?

Blood plasma

Hydrostatic pressure and where is it created at in the capillaries?

Pressure created by the heart when it pumps - it is created at the arterial end of the capillaries

What inward pressures oppose the outward hydrostatic pressure exerted on tissue fluid? (2)

1. The hydrostatic pressure of tissue fluid outside the capillaries (resists outward movement of liquid)
2. The lower water potential of the blood (due to plasma proteins), causing water to move back into the blood in capillaries

Ultrafiltration

Filtration assisted by blood pressure - the process where small molecules are forced out of the capillaries, leaving all cells and proteins in the blood as they are too large to cross the CSM

How does tissue fluid form and move out of the capillaries?

Contraction of ventricles creates high hydrostatic pressure forces water out, so large proteins remain in the capillaries

Does all the tissue fluid return to the capillaries?

No

What happens to the remaining tissue fluid?

It is carried back via the lymphatic system (system of vessels that begins in tissues then gradually merge into larger vessels that forms network throughout body - drains contents into blood stream via two ducts that join veins close to the heart)

Xylem tissue

The tissue that transports water in the stem and leaves of plants

Transpiration

The evaporation of water from the leaves of a plant

Describe how water moves out through the stomata (3)

- Lower water potential in air compared to stomata (atmosphere is less humid than air spaces next to stomata)
- When stomata are open, water vapour diffuses out of the air spaces into the surrounding air
- This water is then replaced by water evaporating from cell walls of nearby mesophyll cells

Describe how water moves across the cells of a leaf (3)

- Water is lost from mesophyll cells by evaporation from cell walls to air spaces of leaves
- Replaced by water reaching mesophyll cells via cell wall or via cytoplasm (cells that lose water due to evaporation have lower WP, water enters by osmosis from neighbouring cells, those cells have lower WP etc)
- Establishes WP gradient that pulls water from xylem, across leaf mesophyll, out into atmosphere

Describe how water moves up the stem in the xylem - cohesion-tension theory (5)

1. Water is lost from the leaves due to transpiration or evaporation of water molecules
2. This lowers the water potential of mesophyll cells
3. So water is pulled up the xylem, creating tension (or negative pressure)
4. Cohesion - water molecules form hydrogen bonds with each other to produce a continuous column of water
5. Water molecules also adhere to the walls of the xylem

Is transpiration a passive process and why?

Yes - it doesn't use metabolic energy, although it does use energy from the sun to drive the process by evaporating water from the leaves

What are 3 pieces of evidence for cohesion-tension theory?

1. Change in diameter of tree trunks according to rate of transpiration: bigger diameter when low transpiration as less tension in xylem (nighttime) and vice-versa
2. Tree can no longer draw up water if a xylem vessel is broken: continuous column of water is broken so water molecules can no longer stick together
3. Water doesn't leak out when xylem vessel is broken: air is drawn in, suggesting it is under tension

Transpiration stream

The movement of water through the plant

Describe 4 features of the xylem

1. Xylem vessels are long and joined end to end with no end walls (hollow): allows continuous column of water
2. The cells are thickened with lignin: provides strength to resist tension due to column of water
3. No cytoplasm or organelles: allows easier water flow (won't obstruct flow)
4. Pits in walls: allow lateral movement + get around blocked vessels

Translocation

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

Phloem tissue

The tissue that transports biological molecules in flowering plants

Sources

The sites where photosynthesis occurs and sugars are produced

Sinks

The sites where the sugars produced by photosynthesis are used directly or stored for future use

Does transpiration occur in any direction?

No - it only occurs from roots to leaves

Does translocation occur in any direction?

Yes - sinks (where biological molecules need to be transported to) can be in any place in the plant

What does the phloem transport?

Organic molecules (e.g. sucrose, amino acids) and inorganic ions (K+, Cl-, PO4 3-, Mg2+)

What is the mechanism for translocation called?

The mass-flow theory

Mass flow

The bulk movement of a substance through a given channel or area in a specified time

Describe the mass flow of sucrose through sieve tube elements to the sink (6)

AT SOURCE:
- After sucrose is actively transported into sieve tubes sieves have lower WP
- Xylem has much higher WP so water moves from xylem to sieve tubes by osmosis, creating a high hydrostatic pressure

AT SINKS:
- Sucrose is used up in respiration/stored as starch
- Those cells have high sucrose content so sucrose is transported to them from the sieve tubes by active transport, lowering WP
- This means water moves into these respiring cells from sieve tubes by osmosis, lowering hydrostatic pressure of sieve tubes

- Causes high hydrostatic pressure at sources, forming mass flow of sucrose solution down the hydrostatic pressure gradient in the sieve tubes

Is mass flow a passive process?

No - while mass flow itself is passive, it occurs as a result of the active transport of sugars

Give 6 pieces of evidence for the mass transport theory

1. Pressure within sieve tubes (saps released when they're cut)
2. Concentration of sucrose is higher in leaves (source) than in roots (sink)
3. Downward flow in phloem occurs in daylight, stops at night /when leaves are shaded
4. Increasing sucrose levels in leaves is followed by later similar increases in phloem
5. Metabolic poisons or lack of oxygen inhibits translocation of sucrose in the phloem
6. Companion cells possess many mitochondria and readily produce ATP

Give 3 pieces of evidence against the mass-flow hypothesis

1. Function of sieve plates is unclear as they seem to hinder mass flow (potentially have structural function)
2. Not all solutes move at same speed, which they should do if movement is by mass-flow
3. Sucrose is delivered at same rate to all regions instead of moving quicker to regions with lowest sucrose concentrations (as mass-flow theory suggests)

Describe how sucrose is transferred from sieve tube elements to storage or other sink cells

Sucrose is actively transported by companion cells out of sieve tube elements and into sink cells

2 experiments used to investigate transport in plants

Ringing experiments and tracer experiments

How does a potometer work (12)?

1. Cut shoot at a slant under water
2. Use waterproof jelly to seal all joints
3. Check apparatus is full of water and air bubble free
4. Insert shoot into apparatus under water
5. Remove potometer from water and ensure it's airtight
6. Dry leaves
7. Keep environmental factors constant (e.g. humidity, wind)
8. Allow time for shoot to acclimatise (time 5 mins)
9. Shut screw clip, open tap to let in an air bubble
10. Keep ruler fixed and record position of air bubble on scale, then start timing
11. After 10 minutes, measure distance the air bubble moved and calculate distance moved per unit of time
12. To repeat, open tap of reservoir to move air bubble to starting position

With a potometer, why do you cut the shoot under water?

To prevent air from being drawn in, which would stop the continuous column of water from forming

With a potometer, why do you cut the shoot at a slant?

To increase surface area

With a potometer, why do you seal all joints with waterproof jelly?

To prevent water from leaking out - would cause an inaccurate reading