3- Mass transport

Describe the role of red blood cells and haemoglobin in oxygen transport

• RBC contain lots of haemoglobin (Hb)= no nucleus, biconcave, high SA:V, short diffusion pathway

• Hb associates with/ binds/ loads O2 at gas exchange surfaces where partial pressure of O2 (pO2) is high

• This forms oxyharmoglobin which transports O2 (each can carry 4O2- one at each Haem group)

• Hb dissociates from/ unloads O2 near cells/ tissues where pO2 is low

Describe the structure of haemoglobin

• Protein with a quaternary structure

• Made of 4 polypeptide chains

• Each chain contains a Haem group containing an iron ion (Fe2+)

Describe the loading, transport, and unloading of oxygen in relation to the oxyhaemoglobin dissociation curve (area with low pO2= respiring tissue)

• Hb has a low affinity for O2

• so O2 readily unloads/ dissociates with Hb

• so % saturation is low

Describe the loading, transport, and unloading of oxygen in relation to the oxyhaemoglobin dissociation curve (area with high pO2= gas exchange surfaces)

• Hb has a high affinity for O2

• so O2 readily loads/ associates with Hb

• so % saturation is high

Explain how the cooperative nature of oxygen binding results in an S-shaped (sigmoid) oxyhaemoglobin dissociation curve

1. Binding of first oxygen changes tertiary/ quaternary structure of haemoglobin

2. This uncovers Haem group binding sites, making further binding of oxygens easier

Describe evidence for the cooperative nature of oxygen binding

• At low pO2, as oxygen increases there is little/ slow increase in % saturation of Hb with oxygen

  • when first oxygen is binding

• At higher pO2, as oxygen increases there is a big/ rapid increase in % saturation of Hb with oxygen

  • showing it has got easier for oxygens to bind

What is the Bohr effect?

Effect of CO2 conc on dissociation of oxyhaemoglobin= curve shifts to right

Explain effect of CO2 conc on the dissociation of oxyhaemoglobin

1. Increasing blood CO2 e.g. due to increased rate of respiration

2. Lowers blood pH (more acidic)

3. Reducing Hb’s affinity for oxygen as shape/ tertiary/ quaternary structure changes slightly

4. So more/ faster unloading of oxygen to respiring cells at a given pO2

Explain the advantage of the Bohr effect (e.g. during exercise)

more dissociation of oxygen= faster aerobic respiration/ less anaerobic respiration= more ATP produced

Explain why different types of haemoglobin can have different oxygen transport properties

• Different types of Hb are made of polypeptide chains with slightly different amino acid sequences

• Resulting in different tertiary/ quaternary structures/ shape= different affinities for oxygen

Explain how organisms can be adapted to their environment by having different types of haemoglobin with different oxygen transport properties (curve shift LEFT)

LEFT= Hb has higher affinity for O2

• More O2 associates with Hb more readily

• At gas exchange surfaces where pO2 is lower

• e.g. organisms in low O2 environments- high altitudes, underground, or foetuses

Explain how organisms can be adapted to their environment by having different types of haemoglobin with different oxygen transport properties (curve shift RIGHT)

RIGHT= Hb has lower affinity for O2

• More O2 dissociates from Hb more readily

• At respiring tissues where more O2 is needed

• e.g. organisms with high rates of respiration/ metabolic rate (may be small or active)

Describe the general pattern of blood circulation in a mammal

Closed double circulatory system= blood passes through heart twice for every circuit around body:

1. Deoxygenated blood in right side of heart pumped to lungs; oxygenated returns to left side

2. Oygenated blood in left side of heart pumped to rest of body; deoxygenated returns to right

Suggest the importance of a double circulatory system

• Prevents mixing of oxygenated/ deoxygenated blood

  • so blood pumped to body is fully saturated with oxygen for aerobic respiration

• Blood can be pumped to body at a higher pressure (after being lower from lungs)

  • substances taken to/ removed from body cells quicker/ more efficiently

Draw a diagram to show the general pattern of blood circulation in a mammal, including the names of key blood vessels

Name the blood vessels entering and leaving the heart and lungs

• Vena cava= transports deoxygenated blood from respiring body tissues to heart

• Pulmonary artery= transports deoxygenated blood from heart to lungs

• Pulmonary vein= transports oxygenated blood from lungs to heart

• Aorta= transports oxygenated blood from heart to respiring body tissues

Name the blood vessels entering and leaving the kidneys

• Renal arteries= oxygenated blood to kidneys

• Renal veins= deoxygenated blood to vena cava from kidneys

Name the blood vessels that carry oxygenated blood to the heart muscle

coronary arteries= located on surface of the heart, branching from aorta

Label a diagram to show the gross structure of the human heart (inside)

Suggest why the wall of the left ventricle is thicker than that of the right

• Thicker muscle to contract with greater force

• to generate higher pressure to pump blood around entire body

Explain the pressure & volume changes and associated valve movements during the cardiac cycle that maintain an unidirectional flow of blood (Atrial systole)

• Atria contract= volume decreases, pressure increases

• Atrioventricular valves open when pressure in atria exceeds pressure in ventricles

• Semilunar valves remain shut as pressure in arteries exceeds pressure in ventricles

• so blood pushed into ventricles

Explain the pressure & volume changes and associated valve movements during the cardiac cycle that maintain an unidirectional flow of blood (Ventricular systole)

• Ventricles contract= volume decreases, pressure increases

• Atrioventricular valves shut when pressure in ventricles exceeds pressure in atria

• Semilunar valves open when pressure in ventricles exceeds pressure in arteries

• so blood pushed out of heart through arteries

Explain the pressure & volume changes and associated valve movements during the cardiac cycle that maintain an unidirectional flow of blood (Diastole)

• Atria & ventricles relax= volume increases, pressure decreases

• Semilunar valves shut when pressure in arteries exceeds pressure in ventricles

• Atrioventricular valves open when pressure in atria exceeds pressure in ventricles

• so blood fills atria via veins & flows passively to ventricles

Explain how graphs showing pressure or volume changes during the cardiac cycle can be interpreted, e.g. to identify when valves are open/ closed

SL valves closed:

• Pressure in … artery higher than in ventricle

• to prevent backflow of blood from artery to ventricles

SL valves open:

• When pressure in ventricle is higher than in … artery

• so blood flows from ventricle to artery

AV valves closed:

• Pressure in ventricle higher than atrium

• to prevent backflow of blood from ventricles to atrium

AV valves open:

• When pressure in atrium is higher than in ventricle

• so blood flows from atrium to ventricle

Describe the equation for cardiac output

Cardiac output (volume of blood pumped out of heart per min)= stroke volume (volume of blood pumped in each heart beat) x heart rate (number of beats per min)

How can heart rate be calculated from cardiac cycle data?

Heart rate (beats/ min)= 60 (seconds) / length of 1 cardiac cycle (seconds)

Explain how the structure of arteries relates to their function

Function= carry blood away from heart at high pressure

• Thick smooth muscle tissue= can contract and control/ maintain blood flow/ pressure

• Thick elastic tissue= can stretch as ventricles contract and recoil as ventricles relax, to reduce pressure surges/ even out blood pressure/ maintain high pressure

• Thick wall= withstand high pressure/ stop bursting

• Smooth/ folded endothelium= reduces friction/ can stretch

• Narrow lumen= increases/ maintains high pressure

Explain how the structure of arterioles relates to their function

Function= (division of arteries to smaller vessels which can) direct blood to different capillaries/ tissues

• Thicker smooth muscle layer than arteries

  • contracts= narrows lumen (vasoconstriction)= reduces blood flow to capillaries

  • relaxes= widens lumen (vasodilation)= increases blood flow to capillaries

• Thinner elastic layer= pressure surges are lower (as further from heart/ ventricles)

Explain how the structure of veins relates to their function

Function= carry blood back to heart at lower pressure

• Wider lumen than arteries= less resistance to blood flow

• Very little elastic and muscle tissue= blood pressure lower

• Valves= prevent backflow of blood

Explain how the structure of capillaries relates to their function

Function= allow efficient exchange of substances between blood and tissue fluid (exchange surface)

• Wall is a thin (one cell) layer of endothelial cells= reduces diffusion distance

• Capillary bed is a large network of branched capillaries= increases surface area for diffusion

• Small diameter/ narrow lumen= reduces blood flow rate so more time for diffusion

• Pores in walls between cells= allow larger substances through

Explain the formation of tissue fluid

At the arteriole end of capillaries:

1. Higher blood/ hydrostatic pressure inside capillaries (due to contraction of ventricles) than tissue fluid (so net outward force)

2. Forcing water (and dissolved substances) out of capillaries

3. Large plasma proteins remain in capillary

Explain the return of tissue fluid to the circulatory system

At the venule end of capillaries:

1. Hydrostatic pressure reduces as fluid leaves capillary (also due to friction)

2. (Due to water loss) an increasing concentration of plasma proteins lowers water potential in capillary below that of tissue fluid

3. Water enters capillaries from tissue fluid by osmosis down a water potential gradient

4. Excess water taken up by lymph capillaries and returned to circulatory system through veins

Suggest and explain causes of excess tissue fluid accumulation

• Low conc of protein in blood plasma

  • water potential in capillary not as low= water potential gradient is reduced

  • so more tissue fluid formed at arteriole end/ less water absorbed at venule end by osmosis

• High blood pressure (e.g. caused by high salt conc)= high hydrostatic pressure

  • increases outward pressure from arterial end AND reduces inward pressure at venule end

  • so more tissue fluid formed at arteriole end/ less water absorbed at venule end by osmosis

  • lymph system may not be able to drain excess fast enough

What is a risk factor? Give examples for cardiovascular disease

• An aspect of a person’s lifestyle or substances in a person’s body/ environment

• That have been shown to be linked to an increased rate of disease

• Examples- age, diet high in salt or saturated fat, smoking, lack of exercise, genes

RP5- What is RP5?

Dissection of animal or plant gas exchange system or mass transport system or of organ within such a system

RP5- Describe precautions that should be followed when performing a dissection

• Cover any cuts with a waterproof dressing

• When using a scalpel, cut away body onto a hard surface

• When using a scalpel, use a sharp blade

• When using a scalpel, carry with blade protected/ pointing down

• Wear disposable gloves and disinfect hands/ wash with soap

• Disinfect surfaces/ equipment

• Safe disposal- put gloves/ paper towels/ organ in a separate bag/ bin to dispose

• If poisonous chemicals/ toxins involved, work in a well ventilated environment

RP5- Suggest an ethical consideration when dissecting animals

• Morally wrong to kill animals just for dissection

• so use animals for dissection that have already been killed (humanely) for meat

RP5- Describe how you could prepare a temporary mount of a piece of plant tissue for observation with an optical microscope

1. Add a drop of water to glass slide

2. Obtain a thin section of specimen and place on a slide

3. Stain (e.g. with iodine/ potassium iodide to view starch)

4. Lower coverslip at angle using mounted needle without trapping air bubbles

Describe the function of xylem tissue

transports water (and mineral ions) through the stem, up the plant to leaves of plants

Suggest how xylem tissue is adapted for its function

• Cells joined with no end walls forming a long continuous tube= water flows as a continuous column

• Cells contain no cytoplasm/ nucleus= easier water flow/ no obstructions

• Thick cell walls with lignin= provides support/ withstand tension/ prevents water lots

• Pits in side walls= allow lateral water movements

Explain the cohesion- tension theory of water transport in the xylem

1- LEAF:

1. Water lost from leaf by transpiration- water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata

2. Reducing water potential of mesophyll cells

3. So water drawn out of xylem down a water potential gradient

2- XYLEM:

4. Creating tension (negative pressure/ pull) in xylem

5. Hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column

6. Water also adheres (sticks to) walls of xylem

3- ROOT:

7. Water enters roots via osmosis

Describe how to set up a potometer

1. Cut a shoot underwater at a slant= prevent air entering xylem

2. Assemble potometer with capillary tube end submerged in a beaker of water

3. Insert shoot underwater

4. Ensure apparatus is watertight/ airtight

5. Dry leaves and allow time for shoot to acclimatise

6. Shut tap to reservoir

7. Form an air bubble- quickly remove end of capillary tube from water

Describe how a potometer can be used to measure the rate of transpiration

Potometer estimates transpiration rate by measuring water uptake:

1. Record position of air bubble

2. Record distance moved in a certain amount of time (e.g. 1 min)

3. Calculate volume of water uptake in a given time:

   • Use radius of capillary tube to calculate cross-sectional area of water (pi r²)

   • Multiply this by distance moved by bubble

4. Calculate rate of water uptake- divide volume by time taken

Describe how a potometer can be used to investigate the effect of a named environmental variable on the rate of transpiration

• Carry out the above, change one variable at a time (wind, humidity, light, temp)

  • e.g. set up a fan OR spray water in a plastic bag and wrap around the plant OR change distance of a light source OR change temperature of room

• Keep all other variables constant

Suggest limitations in using a potometer to measure rate of transpiration

• Rate of water uptake might not be same as rate of transpiration

  • water used for support/ turgidity

  • water used in photosynthesis and produced during respiration

• Rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant

  • shoot in potometer has no roots whereas a plant does

  • xylem cells very narrow

Suggest how different environmental variables affect transpiration rate (light intensity)

Increases rate of transpiration:

• Stomata open in light to let in CO2 for photosynthesis

• allowing more water to evaporate faster

• Stomata close when it’s dark so there is a low transpiration rate

Suggest how different environmental variables affect transpiration rate (temperature)

Increases rate of transpiration:

• Water molecules gain kinetic energy as temperature increases

• so water evaporates faster

Suggest how different environmental variables affect transpiration rate (wind intensity)

Increases rate of transpiration:

• Wind blows away water molecules from around stomata

• decreasing water potential of air around stomata

• increasing water potential gradient so water evaporates faster

Suggest how different environmental variables affect transpiration rate (humidity)

Decreases rate of transpiration:

• More water in air so it has a higher water potential

• decreasing water potential gradient from leaf to air

• water evaporates slower

Describe the function of phloem tissue

transports organic substances e.g. sucrose in plants

Suggest how phloem tissue is adapted for its function

1. Sieve tube elements

   • no nucleus/ few organelles= maximise space for/ easier flow of organic substances

   • end walls between cells perforated (sieve plate)

2. Companion cells

   • many mitochondria= high rate of respiration to make ATP for active transport of solutes

What is translocation?

• Movement of assimilates/ solutes such as sucrose

• from source cells (where made e.g. leaves) to sink cells (where used/ stored e.g. roots) by mass flow

Explain the mass flow hypothesis for translocation in plants

1. At source, sucrose is actively transported into phloem sieve tubes/ cells

2. By companion cells

3. This lowers water potential in sieve tubes so water enters (from xylem) by osmosis

4. This increases hydrostatic pressure in sieve tubes (at source)/ creates a hydrostatic pressure gradient

5. So mass flow occurs- movement from source to sink

6. At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs

Describe the use of tracer experiments to investigate transport in plants

1. Leaf supplied with a radioactive tracer e.g. CO2 containing radioactive isotope 14C

2. Radioactive carbon incorporated into organic substances during photosynthesis

3. These move around plant by translocation

4. Movement tracked using autoradiography or a Geiger counter

Describe the use of ringing experiments to investigate transport in plants

1. Remove/ kill phloem e.g. remove a ring of bark

2. Bulge forms on source side of ring

3. Fluid from bulge has a higher conc of sugars than below- shows sugar is transported in phloem

4. Tissues below ring die as cannot get organic substances

Suggest some points to consider when interpreting evidence from tracer & ringing experiments and evaluating evidence for/ against the mass flow hypothesis

• is there evidence to suggest the phloem (as opposed to xylem) is involved?

• is there evidence to suggest respiration/ active transport is involved?

• is there evidence to show movement if from source to sink? what are these in the experiment?

• is there evidence to suggest movement is from high to low hydrostatic pressure?

• could movement be due to another factor e.g. gravity?