Mass Transport in Animals and Plants

Describe the structure of haemoglobin

• 4 polypeptide chains - quaternary structure

• Contains 4 haem groups with iron ions attached

• This is where oxygen binds to form oxyhaemoglobin

Explain the general shape of the oxyhaemoglobin dissociation curve

• Oxygen is loaded in areas of high pO2

• Oxygen is unloaded in areas of low pO2

• When one molecule of oxygen binds, the tertiary structure of the haemoglobin is changed

• More binding sites exposed so binding of oxygen is easier (steep curve gradient) ad Hb affinity for oxygen is higher

• After 4 molecules bind, the Hb is fully saturated and no more O2 can bind (curve levels)

Describe the Bohr Effect

• When respiration increase, more CO2 released into blood so blood pH decreases

• This causes the Hb to change tertiary structure slightly which makes it easier for oxygen to unload (affinity for oxygen decreases)

• More oxygen unloaded to site of respiring cells so aerobic respiration can continue

How its the haemoglobin of animals living in low pO2 environments adapted

• Have haemoglobin with a higher affinity for oxygen

• Oxygen loads more readily at low pO2

• Dissociation curve shifts to the left

How its the haemoglobin of small animals with high metabolism adapted

• Dissociation curve shifts to right despite normal CO2 levels

• Favours rapid oxygen unloading for respiring cells

• Hb has lower affinity for oxygen at high pO2

What is meant by a double circulatory system

The blood passes through the heart twice in each circuit. There is one circuit that delivers blood to the lungs and another circuit that delivers blood to the rest of the body

Why is it important for mammals to have a double circularity system

• Mammals require a double circulatory system to manage the pressure of blood flow

• The blood flows though the lungs at a lower pressure. This prevents damage to the capillaries and also reduces the speed at which the blood flows, enabling more time for gas exchange

• The oxygenated blood from the lungs can then be pumped at high pressure to the rest of the body to enable all respiring cells to be reached

Where do these blood vessels carry blood to and from

• Arteries

• Arterioles

• Capillaries

• Veins

• Arteries - carry blood away from the heart to the arterioles

• Arterioles - carry blood from arteries to the capillaries

• Capillaries - connect arterioles and veins

• Veins - carry blood back to the heart

How do the adaptations of arteries link to their function

• Thick middle layer rich in elastic tissue: to maintain blood pressure and smooth blood flow

• Smooth muscle: This can contract or relax to change the width of the artery, controlling blood flow

• Narrow opening compared to wall thickness: This high wall-to-opening ratio prevents the artery from bursting under pressure.

What is the structure and function of arterioles

• Branch of arteries and control blood flow

• Thick muscle layer which contracts to narrow the lumen (vasoconstriction) to reduce blood flow to the capillaries and relaxes to widen lumen (vasodilation) to increase blood flow to the tissues

What is the structure and function of veins

• Thin middle layer: Very little smooth muscle or elastic tissue, so veins can expand easily to hold more blood

• Wide opening: Creates very little resistance to blood flow

• One-way valves: Paired flaps of tissue prevent blood flowing backwards when pressure drops

• Muscle pumps: Your leg muscles and breathing movements squeeze veins, pushing blood towards the heart

• Surface location: Many veins run close to the surface where muscle movements can help pump blood, but this makes them more likely to collapse when empty

What is the structure and function of the capillaries

• Massive branching network: Creates an enormous surface area and slows blood flow right down, giving more time for exchange

• Ultra-thin walls: Often have tiny gaps in tissues like the intestine and kidney, allowing rapid movement of small molecules and water

• Narrow tubes: Red blood cells must travel in single file, bringing them very close to the wall and reducing the distance substances need to diffuse

What is the equation linking stroke volume, cardiac output and heart rate

CO = HR X SV

Describe the atrial systole

• The atrial muscles contract, causing atrial pressure to rise slightly above ventricular pressure.

• This pressure difference keeps the atrioventricular (AV) valves open.

• Blood flows into the ventricles. While most filling is passive, this phase provides the final "top-up" of blood.

• The semilunar valves (at the exits to the aorta and pulmonary artery) stay closed because the pressure in these arteries is still higher than in the ventricles.

Describe the ventricular systole

• The ventricles contract, causing the pressure inside them to rise rapidly.

• As soon as ventricular pressure exceeds atrial pressure, the AV valves close. This prevents blood from flowing back into the atria.

• For a very short moment, all valves are closed, so pressure rises sharply while the volume of blood remains constant.

• Once ventricular pressure rises higher than the pressure in the arteries (aorta and pulmonary artery), the semilunar valves are forced open.

• Blood is ejected into the arteries. The volume of blood pumped out during this contraction is called the stroke volume.

Describe diastole

• The ventricular muscles relax and pressure inside the ventricles drops rapidly.

• Once ventricular pressure falls below the pressure in the arteries, the blood tries to flow back, causing the semilunar valves to close.

• For a brief period, pressure continues to fall with all valves closed.

• When ventricular pressure drops below atrial pressure, the AV valves open.

• Blood flows passively from the atria into the ventricles (filling phase).

• Both atrial and ventricular pressures remain low during this filling stage.

Describe how tissue fluid is formed and reabsorbed

• Tissue fluid forms as follows:

  • at the arterial end of a capillary the hydrostatic pressure is greater than the osmotic pull

  • water and small molecules are forced out of the capillary down a hydrostatic pressure gradient, forming tissue fluid

    • Large molecules, e.g. large plasma proteins, remain in the blood as they are too large to pass out of the capillaries

• Tissue fluid returns to the capillaries as follows:

  • at the venous end the osmotic pull is now higher than the hydrostatic pressure

    • Hydrostatic pressure in the capillary has decreased due to loss of plasma volume and flow resistance in the narrow capillary

  • dissolved proteins in the blood lower the water potential and create a water potential gradient between the capillary and the tissue fluid

  • fluid is drawn back into the capillary down its water potential gradient

How is excess tissue fluid reabsorbed

• Reabsorbed into the circulatory system via the lymphatic system

• Excess water of the tissue fluid enters the lymphatic system via the lymphatic capillaries#

Define transpiration

The passive process which involve the loss of water vapour from the leaves of plants via evaporation

Describe how water is transported in the xylem by the cohesion tension theory

1. Water lost from leaf because of transpiration / evaporation of water (molecules) / diffusion from mesophyll / leaf cells;

OR

Transpiration / evaporation / diffusion of water (molecules) through stomata / from leaves;

2. Lowers water potential of mesophyll / leaf cells;

3. Water pulled up xylem (creating tension);

4. Water molecules cohere / ‘stick’ together by hydrogen bonds;

5. (forming continuous) water column;

6. Adhesion of water (molecules) to walls of xylem which creates tension, and pushes water column up xylem

What are the four factors impacting transpiration

• Temperature

increasing temperature increases transpiration;​
more kinetic energy (of water molecules) therefore faster diffusion ;​
faster evaporation of water (due to more latent heat available);

• Light intensity

causes stomatal opening in morning;​
increasing light increases transpiration;​
because stomatal opening increases;​
no light causes stomatal closure, reducing transpiration;

• Wind

removes water vapour from around leaf;​
increases water vapour / humidity gradient so increases transpiration;​
lack of wind can reduce transpiration;​
no increase in transpiration if humidity is 100 %;

• Humidity

high humidity lowers transpiration rate;​
high humidity reduces water vapour gradient so lowers transpiration;​
lowering humidity can increase transpiration rate ​
at very low humidity stomata may shut down;

Describe the process of translocation

1. Sucrose actively transported into phloem (cell); OR Sucrose is co-transported/moved with H+ into phloem (cell); Accept sieve (element/tube/cell) for phloem (cell)

2. (By) companion/transfer cells;

3. Lowers water potential (in phloem) and water enters (from xylem) by osmosis;

4. (Produces) high(er) (hydrostatic) pressure; OR (Produces hydrostatic) pressure gradient; Accept description of gradient, eg higher WP

5. Mass flow to respiring cells OR Mass flow to storage tissue/organ; Accept transport OR movement for flow Accept buds/young leaves/fruit/seeds/shoot tip/root tip/ meristems/root

6. Unloaded/removed (from phloem) by active transport;

How can radioactive carbon be used to show translocation

Scientists use radioactive isotopes like to track how carbon moves through plants. They give a leaf radioactive carbon dioxide, which gets built into radioactive sucrose during photosynthesis. They can then track this radioactive sugar using autoradiography - a technique where radioactive areas show up as dark spots on photographic film.

How can tree ringing be used to show translocation

Ringing experiments provide clear evidence that organic substances travel in phloem, not xylem. A ring of bark (which contains the phloem) is carefully removed from a tree trunk, leaving the inner xylem intact.

Results after a few days:

• Above the ring: sugars accumulate and the stem swells

• Below the ring: tissues begin to die from lack of carbohydrates

This shows that organic substances normally flow down through the phloem in the bark, not through the xylem.