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

At the arterial end of capillaries, hydrostatic pressure is high. This pressure is greater than the opposing force created by the water potential gradient, so fluid filters out carrying oxygen, glucose, and other nutrients (ultrafiltration).

At the venous end, hydrostatic pressure has dropped due to friction. However, the plasma proteins are still present, so the water potential of the blood remains low. The force pulling water back in (due to the water potential gradient) is now greater than the hydrostatic pressure pushing it out, so water and waste products like carbon dioxide are reabsorbed by osmosis.

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 evaporates from leaves - Water on the surface of leaf cells evaporates into the air spaces inside the leaf, then diffuses out through the stomata

2. This creates tension - As water evaporates, it creates a 'pull' or negative pressure in the leaf's xylem vessels

3. The pull travels down - This tension is transmitted down the continuous water column, like pulling on a rope

4. Water is pulled up from roots - The pull draws more water up from the root xylem to replace what was lost

5. Cohesion keeps it together - Hydrogen bonds between water molecules (cohesion) and attraction between water and vessel walls (adhesion) prevent the water column from breaking

What are the four factors impacting transpiration

• Temperature - Warmer air holds more water vapor, so it increases evaporation from leaves and creates stronger pull

• Humidity - High humidity reduces evaporation (like how you sweat less on humid days), weakening the transpiration pull

• Wind - Air movement removes water vapor from around leaves, maintaining a steep concentration gradient for evaporation

• Light intensity - Bright light causes stomata to open wider, allowing more water loss

Describe the process of translation

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 translation

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 translation

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.