A-Level Biology - 3.3.4 Mass transport

What are the haemoglobins?

A group of chemically similar molecules found in a wide variety of organisms

Describe the structure of haemoglobin

globular protein with quaternary structure

made of four polypeptide chains

each polypeptide chain is bound to a prosthetic protein called haem, which contains Fe2+

How much oxygen can each haemoglobin carry?

Each haemoglobin can carry four oxygen molecules (or eight oxygen atoms)

Give the equation for haemoglobin combining with oxygen

What is the role of haemoglobin in the transport of oxygen?

majority of the oxygen transported around the body is bound to the protein haemoglobin, found in red blood cells

each molecule of haemoglobin contains four haem groups, each able to bind with one molecule of oxygen

Recognise the oxyhaemoglobin dissociation curve

What does affinity of haemoglobin for oxygen mean?

how easily haemoglobin binds to and dissociates with oxygen

What does affinity of haemoglobin for oxygen depend on?

the partial pressure of oxygen

What does partial pressure of oxygen mean?

pressure exerted by oxygen within a mixture of gases

measures oxygen concentration

Explain the shape of the oxyhaemoglobin dissociation curve in terms of the affinity of haemoglobin for oxygen

at low pO2, haemoglobin has a low affinity for oxygen, leading to shallow curve at bottom left

once the first oxygen is bound, the affinity of haemoglobin for oxygen increases, leading to steeper gradient

at high pO2, most haem groups have already bound to an oxygen, so the chances of an oxygen molecule colliding with the fourth haem group is relatively low, leading to shallow curve in top right corner

Explain the shape of the oxyhaemoglobin dissociation curve in terms of the loading, transport and unloading of oxygen around the body

oxygen loads onto haemoglobin at high pO2, e.g. in alveoli, pO2 is high and saturation of haemoglobin is high

O2 unloads where pO2 is lower, e.g. body tissues since they are carrying out aerobic respiration

one oxygen molecule unloads, changing the shape of the haemoglobin and decreasing its affinity

in more active tissues, more oxygen molecules unload

for the last O2 molecule to unload, pO2 has to be very low, e.g. muscle cells during intense exercise

Describe the cooperative nature of oxygen binding

once the first oxygen molecule has bound, the haemoglobin changes shape

this makes the binding of further oxygens easier

Give three ways in which red blood cells are adapted for transporting oxygen

biconcave structure gives large surface area to volume ratio for rapid oxygen diffusion

contain a lot of haemoglobin to carry a lot of oxygen

no nucleus so can contain more haemoglobin

Show the effect of CO2 concentration on the shape of the oxyhaemoglobin dissociation curve (Bohr Shift)

What is the Bohr effect?

when CO2 concentration increases, haemoglobin's affinity for oxygen decreases

causes the oxyhaemoglobin dissociation curve to shift to the right

What effect does concentration of CO2 have on the dissociation of oxyhaemoglobin?

increased respiration leads to higher CO2 concentration in blood, lowering pH of blood

this decreases haemoglobin's affinity for oxygen

oxygen unloads more easily at respiring cells so haemoglobin less saturated with oxygen

allows aerobic respiration to continue

What is the advantage of the oxyhaemoglobin curve shifting to the right during vigorous exercise?

haemoglobin has lower affinity for oxygen, oxygen unloads more readily

to the cells

for rapid respiration

What is the advantage of the fetal oxyhaemoglobin dissociation curve being to the left of the mother's?

it has a higher affinity so loads more oxygen at the same partial pressure of O2

so oxygen moves from mother to fetus

Describe the haemoglobin of animals living in low oxygen environments

Less oxygen available

so haemoglobin has higher affinity and loads more oxygen at the same partial pressure of O2

to provide enough oxygen for respiration

Describe the haemoglobin of organisms with a high surface area to volume ratio/high metabolic rate

Small organisms lose heat more easily so need a higher metabolic rate to maintain body temperature

lower oxygen affinity so oxygen can easily unload at respiring cells to meet higher metabolic rate, allowing for faster rate of respiration

Describe the circulatory system of mammals

Closed double circulatory system

What is a single circulatory system?

Blood only passes through the heart once for each complete circuit of the body

What is a double circulatory system?

Blood passes through the heart twice for each complete circuit of the body

What are the advantages of a double circulatory system?

Blood flows through lungs at lower pressure, preventing damage to capillaries and providing more time for gas exchange

blood is pumped out at a higher pressure to the rest of the body, ensuring blood reaches all respiring cells

Describe the three main blood vessels connected to the heart

aorta:

carries oxygenated blood out of the heart to the rest of the body

vena cava:

carries deoxygenated blood into the heart

coronary arteries:

supply the heart with oxygenated blood

Describe the two main blood vessels that connect the heart to the lungs

Pulmonary artery:

carries deoxygenated blood away from the heart, towards the lungs

pulmonary vein:

carries oxygenated blood away from the lungs, towards the heart

Describe the two main blood vessels connected to the kidneys

Renal artery:

supplies the kidneys with oxygenated blood

renal vein:

carries deoxygenated blood away from the kidneys, towards the heart

Name four types of blood vessels in the mammalian circulatory system

arteries

arterioles

veins

capillaries

What are arteries?

Blood vessels that carry blood away from the heart (at high pressure)

What are arterioles?

smaller blood vessels than arteries that connect to capillaries

What are veins?

Blood vessels that transport blood to the heart (at low pressure)

How does the structure of arteries relate to their function?

narrower lumen and thicker elastic layer than veins to help maintain blood pressure - the walls can stretch and recoil in response to heartbeat

thicker muscle layer than veins so that constriction and dilation can occur to control blood volume

thicker walls and veins to help prevent the vessels bursting due to high pressure

no valves

How is the structure of the aorta related to its function?

Thick elastic tissue to allow stretching and recoil

Elastic tissue stretches when ventricles contract and recoils when ventricle relaxes

Muscle for vasoconstriction

Thick wall withstands pressure and stops bursting

Smooth endothelium reduces friction

Aortic/semi-lunar valve prevents backflow

How does the structure of arterioles relate to their function?

Thicker muscle layer than arteries to help restrict blood flow into the capillaries

thinner elastic layer and walls than arteries since blood pressure is lower

no valves

How does the structure of veins relate to their function?

Relatively thin muscle layer so can't control volume of blood

relatively thin elastic layer and wall since blood pressure is lower and there is a lower risk of bursting

valves to prevent backflow of blood

How does the structure of capillaries relate to their function?

Narrow diameter forces blood to travel slowly, so more time for diffusion to occur

capillaries branch between cells to reduce diffusion distance

epithelium is one cell thick to reduce diffusion distance

the walls have pores, allowing plasma to leak out

Describe the three main layers in the walls of blood vessels

Smooth muscle layer:

contracts to control the flow of blood in arteries, arterioles and veins

elastic layer:

allows the vessel to stretch and recoil in arteries, arterioles and veins

endothelium:

thin inner lining which is smooth to reduce friction

What are capillary beds?

network of capillaries running through tissues

They are important exchange surfaces where substances diffuse between the blood and the cells

What is tissue fluid?

Plasma leaks out of capillaries to surround cells

exchange of substances between cells and the blood occurs via tissue fluid

How does tissue fluid form?

blood enters capillaries from arterioles

smaller diameter results in higher hydrostatic pressure, which is great enough to push small molecules out of capillaries through pores

cells are surrounded by tissue fluid containing required substances (O2, glucose, water, ions etc)

large molecules like proteins remain in the blood, creating low water potential

How does tissue fluid return to the circulatory system?

towards end of capillaries, hydrostatic pressure is lowered due to loss of liquid, but water potential is very low

water re-enters capillaries by osmosis

equilibrium of liquid will be absorbed

remaining tissue fluid absorbed into lymphatic system, which eventually drains back into the bloodstream near the heart

Describe the structure of the human heart

1) Vena Cava:

- vein returning blood from the body

2) Right Atrium:

- only pump blood to ventricles (thin muscular walls)

3) Right Ventricle:

- only pumps blood to lungs (thinner muscular wall)

- closer to lungs so need blood to flow slower

4) Pulmonary Artery:

- carries deoxygenated blood to lungs

- has unidirectional semilunar valves

- open if pressure in ventricles > than in blood vessels

5) Pulmonary Vein:

- carries oxygenated blood from lungs

6) Left Atrium:

- only pumps blood to ventricles (thinner muscular wall)

- has unidirectional atrioventricular valves

- open when pressure in atria > than in ventricle

7) Left Ventricle:

- thick muscular wall, contracts with more force to pump blood at higher pressure all around the body

8) Aorta:

- biggest artery

- carries oxygenated blood to body

- has unidirectional semilunar valves

- open when pressure in ventricles > than in aorta

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

left ventricle needs to pump blood around the whole body, so blood must be at higher pressure

thick muscular wall can contract with more force to generate a higher pressure

What valves can be found in the human heart?

atrioventricular (tricuspid) valve separates right atrium and right ventricle

atrioventricular (bicuspid) valve separates left atrium and left ventricle

semilunar (pulmonary) valve separates right ventricle and pulmonary artery

semilunar (aortic) valve separates left ventricle and aorta

Draw a diagram of the structure of the human heart

What is the function of valves in the human heart?

valves are unidirectional to prevent backflow of blood

also help maintain correct pressure in chamber of the heart

Give the equation to calculate cardiac output (CO)

CO = stroke volume x heart rate

What causes valves to open and close?

Pressure changes

Valves open when pressure of blood behind > pressure of blood in front

Valves close when pressure of blood in front > pressure of blood behind

Name the three main stages in the cardiac cycle

1) Diastole

2) Atrial Systole

3) Ventricular Systole

What happens during diastole in the cardiac cycle?

atria and ventricles are relaxed

pressure in ventricles < pressure in aorta and pulmonary artery, so SL valves close

atria fill with blood from the vena cava/pulmonary vein

pressure in atria > pressure in ventricles, so AV valves open

What happens during atrial systole in the cardiac cycle?

atria contract, ventricles relax

- atrial volume decreases

- atrial pressure increases

pressure in atria > pressure in ventricles, so AV valves are open and blood forced into ventricles

What happens during ventricular systole in the cardiac cycle?

ventricles contract, atria relax

- ventricular volume decreases

- ventricular pressure increases

pressure in ventricles > pressure in atria so AV valves closed to prevent backflow

pressure in ventricles > pressure in aorta and pulmonary artery, so SL valves open

blood forced into aorta/pulmonary artery and out of the heart

Draw a graph for the cardiac cycle

What causes a myocardial infarction? (heart attack)

Buildup of fatty plaque on inside of coronary arteries

Coronary arteries supply cardiac muscle with oxygenated blood, so blockages prevent oxygen supply to cardiac muscle

Lead to anaerobic respiration, and eventually the cardiac muscle won't be able to contract

What is the xylem?

tissue that transports water and dissolved mineral ions in the stem and leaves of plants

Describe the structure of the xylem

hollow tube of dead cells joined end to end

no end walls between cells

thick walls strengthened with lignin to withstand pressure of flowing water

What is cohesion in the xylem?

attraction between molecules of water

What is adhesion in the xylem?

attraction between molecules of water and walls of the xylem vessels

What is the transpiration stream?

The movement of water from the roots through the xylem and out of the leaves

Explain the cohesion-tension theory

Water is absorbed by osmosis via root hair cells (which are adapted to maximise osmosis by having thin walls and a large surface area)

Transpiration, the evaporation of water from leaves via the stomata, creates a low pressure at the top of the xylem

Remaining water pulled up against xylem by adhesion

Creates tension due to cohesion of water molecules

Water molecules are pulled upwards towards the leaves in a transpiration stream

Name four factors that affect rate of transpiration

1) Increased light increases transpiration.

2) Increased temperature increases transpiration.

3) Increased humidity decreases transpiration.

4) Increased air movement increases transpiration.

How does light intensity affect transpiration rate?

Brighter light increases rate of photosynthesis and transpiration

Causes stomata to open to let carbon dioxide in, but this allows water to diffuse out

How does temperature affect transpiration rate?

Higher temperature = higher transpiration rate

Warmer water molecules have more kinetic energy so they evaporate and diffuse out of the leaf faster

How does air movement affect transpiration rate?

increased air movement = increased transpiration rate

layer of water vapour surrounding leaf is removed, leading to steeper water potential gradient

How does humidity affect transpiration rate?

lower humidity = faster the transpiration rate

If air around the leaf is dry, water potential gradient between the leaf and the air is steeper, which increases transpiration rate.

What equipment can be used to investigate the effect of a named environmental variable on the rate of transpiration?

potometer

Give a method for investigating the effect of a named environmental variable on the rate of transpiration

1) Sample:

- must be cut from a plant underwater to prevent any air entering the xylem and breaking the water column

2) Potometer setup:

- potometer filled with water and air bubbles removed

- sample attached to potometer using rubber seals and petroleum jelly to make equipment airtight

- one bubble introduced; record distance of bubble at start, leave for set amount of time, then record the distance it moved

3) Calculation:

- distance that bubble moved can be used to work out volume of water in tube that evaporated

- volume of water divided by time it took to lose that volume gives the rate of transpiration

- apparatus reset and conditions changed (eg move lamp) to investigate the effect on the rate

Draw a diagram showing how potometer should be set up

What is the phloem?

the tissue that transports organic substances in plants (eg glucose, amino acids etc)

Describe the structure of the phloem

Made up of living cells

Contains sieve tube cells, which are long and thin, and arranged as a column

Have perforated end walls called sieve plates

Have few organelles, no mitochondria and are hollow

Companion cells are adjacent to sieve tube elements, supporting the sieve tube cells by providing ATP for active transport

Describe the mass flow hypothesis for the mechanism of translocation in plants

1) High concentration of solute at source, eg sucrose made in leaf cell

2) Active transport of solute from companion cell to sieve tube cell (or co-transport with H+ ions)

3) Lowers water potential in sieve tube cells, so water enters by osmosis from companion cells and xylem

4) Creates high hydrostatic pressure in phloem, creating a pressure gradient in sieve tubes (high at source, low at sink), which pulls solutes along by mass transport

5) At sink, solute is broken down or converted into another substance

6) This increases water potential of sieve tube cell at sink, so water moves out by osmosis back into xylem, decreasing hydrostatic pressure

How are tracers used to investigate transport in plants?

Use radioactive C-14 grow plants in C-14 atmosphere (in ¹⁴CO₂)

sucrose formed from photosynthesis will be radioactive so its movement around the plant via translocation can be traced

amounts of radioactive carbon present in different parts of the plant can be detected

What observation in a tracer experiment would provide evidence to support the mass flow hypothesis?

bulk flow of phloem sap should be in one direction (from source to sink) and occur at the same rate in any sieve tube at the same time

How is ringing used to investigate transport in plants?

Remove bark in a ring from the trunk (the ring contains phloem but not xylem)

Solutes can't move up or down

Bulge forms above the ring

Fluid above the ring has more solutes than below, which is evidence that most solutes are moving down (from source to sink)