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Flashcards in Mass transport Deck (13)
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Structure of haemoglobin

Haemoglobin is a protein with a quaternary structure of 4 polypeptide chains and 4 haem groups. Each haem group can carry one oxygen molecule.


Affinity of haemoglobin for oxygen

In places with a high concentration of oxygen and a low concentration of carbon dioxide, for example the lungs, haemoglobin has a high affinity for oxygen. This means it can readily associate with oxygen at the right gas exchange surface.

In places with a low concentration of oxygen and a high concentration of carbon dioxide, for example respiring tissues, haemoglobin has a low affinity for oxygen. This means it can readily dissociate with oxygen at the right gas exchange surface.


How can you tell from an oxygen dissociation curve how much affinity a haemoglobin has for oxygen?

The further to the left the curve, the greater the affinity of that haemoglobin.


What is oxygen concentration measured in?

Partial pressure (kPa)


What would the affinity of llama haemoglobin be like compared to that of a human and why?

Llama haemoglobin has a high affinity for oxygen because it lives at high altitudes where there is a lower partial pressure of oxygen so it needs to get as much of it as it can.


Structure of the heart

There are two atria, from which blood enters, on top of two ventricles, from which blood leaves. The left atria and ventricles are usually depicted on the right whereas the right atria and ventricles are depicted on the left. The atria are thin-walled and elastic as they need to stretch, but the ventricles have much thicker muscular walls to pump blood to the body. In particular, the left ventricle has the thickest walls as it pumps blood into the aorta.


Arteries, veins and valves of the heart

The aorta is the biggest artery that takes blood to the rest of the body and is connected to the left ventricle.
The pulmonary artery takes deoxygenated blood to the lungs and is connected to the right ventricle.
The vena cava is the biggest vein and connects to the right atrium.
The pulmonary vein brings blood back from the lungs and connects to the left atrium.
The atrioventricular valves separate the atria and ventricles from each other.
The semi-lunar valves separate the ventricles and arteries from each other.


The cardiac cycle

Blood enters the atria of the heart. As the atria fill, the pressure increases.

Atrial systole
When the pressure of the atria exceeds that of the ventricles, the atrioventricular valves open and blood rushes into the ventricles. The atria contract to push blood into the ventricles.

Ventricular systole
With the rising pressure of the ventricles, the ventricular walls contract, forcing shut the atrioventricular valves and further increasing the pressure of the ventricles. When the ventricular pressure exceeds that of the arteries, the semi-lunar valves open and blood is forced into the arteries.


Structure of blood vessels

Arteries have a thick muscular layer to pump blood around the body and a thick elastic layer to withstand high pressures.
Arterioles have a relatively thicker muscular layer (despite a smaller lumen) than arteries as they need to control the flow of blood into arteries and a relatively thinner elastic layer than arteries as the blood pressure is lower.
Veins have a thin muscular layer and a thin elastic layer as blood pressure is low. They have wider lumens than arteries and valves to prevent the backflow of blood.
Capillaries have the smallest lumens and only have the epithelial layer (which all other blood vessels also have) which provide a short diffusion pathway.


Tissue fluid

Tissue fluid is a watery liquid formed from blood plasma. It bathes all the cells of the body. It supplies glucose, amino acids, fatty acids, ions and oxygen to tissues, and receives carbon dioxide and waste materials from them.

Tissue fluid, along with smaller molecules, is forced out of capillaries near the arterial end due to the high hydrostatic pressure caused by the pumping of the blood. However, this movement of substances into the tissue fluid outside of the capillaries increases the hydrostatic pressure of the tissue fluid outside of the capillaries. It also decreases the water potential of the blood, as larger proteins stay in the blood but water moves out. These two forces force tissue fluid back into the capillaries. Not all the tissue fluid returns to the capillaries; the remainder enters the lymphatic system which returns the tissue fluid back to the circulatory system via veins near the heart.


Transport of water in the xylem

Water evaporates from mesophyll cells, leading to transpiration.
Water molecules from hydrogen bond with one another - known as cohesion.
Water forms a continuous, unbroken column down the xylem
As water evaporates from mesophyll cells it pulls a column of water up the xylem - known as the transpiration pull.
Transpiration pull puts the xylem under tension, hence the name cohesion-tension.


Transport of organic substances in the phloem

Mass flow theory:
Sucrose moves from sources to companion cells through facilitated diffusion.
Sucrose travels with diffusing hydrogen ions into the sieve tube elements through co-transport.
This lowers the water potential of the sieve tube elements so water moves in from the xylem by osmosis.
This creates a high hydrostatic pressure in the sieve tubes.
Sucrose is actively transported into sinks, lowering their water potential, so water from the sieve tubes move in by osmosis.
The hydrostatic pressure of the sieve tubes in that region is lowered.
The high pressure near the source and low pressure near the sink creates a mass flow of sucrose solution down the pressure gradient.


Investigating transport in plants

A ring of phloem and bark is removed from a tree.
The region immediately above the ring swells.
This is because of the sugars of the phloem accumulating above the ring.
Some non-photosynthetic tissues below the ring die.
This is because they can't get the sugars they need to respire.
Conclusion: Phloem, rather than xylem, is responsible for translocating sugars in plants.

Radioactive isotopes can be used to trace the movement of substances in plants. For example, a plant can be grown in an atmosphere with radioactive carbon dioxide. The radioactive carbon isotope will be incorporated into the sugars produced during photosynthesis. The radioactive sugars can be traced using autoradiography - cutting thin cross-sections of the plant and putting them on X-ray film; the film blackens where it has been exposed to radiation produced by the radioactive sugars. The blackened regions are found to correspond to where phloem tissue is in the stem, but other tissues don't blacken the film.