Mass Transport In Animals

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课程笔记

Mass Transport

Mass transport is the efficient movement of substances over large distances, usually via specialised transport systems.
It moves materials from exchange surfaces to the parts of an organism where they are needed, e.g. oxygen from alveoli to body cells, glucose from intestinal epithelium to body cells.
It also transports waste materials back to exchange surfaces, e.g. carbon dioxide from cells to alveoli, urea from cells to kidneys.
In animals, the circulatory system is a specialised transport system; in plants, it is the vascular tissue.
These systems use pressure changes to force substances to move in the required direction.
Specialised mass transport systems are needed in multicellular organisms because the distances between exchange surfaces and cells are very large.
Diffusion alone would be too slow to supply the cells with everything they need.
Do not confuse the need for a gas exchange system (due to low SA:V ratio) with the need for a mass transport system (due to large distances).

Haemoglobin Structure

Haemoglobin is a protein found in many different organisms; it has a quaternary structure consisting of four polypeptide chains.
It has two α–globins and two β–globins.
The four globin subunits are held together by disulphide bonds.
Hydrophobic R groups face inwards, helping to preserve the spherical tertiary structure.
Hydrophilic R groups face outwards, helping to maintain solubility.
Each chain contains a haem group which can reversibly combine with an oxygen molecule.
The haem group contains an iron II ion (Fe²⁺).
Each haemoglobin can therefore transport four oxygen molecules (O₂).

The Role of Haemoglobin

Red blood cells have a specialised structure to maximise oxygen transport: no nucleus to maximise space for haemoglobin.
They have a biconcave shape to maximise surface area for diffusion of oxygen.
They are highly flexible to pass through narrow capillaries.
Their diameter is approximately the same as that of capillaries, slowing down blood flow to maximise time for diffusion.
Haemoglobin is located within red blood cells and transports oxygen around the body.
When oxygen binds to haemoglobin, oxyhaemoglobin is formed: oxygen + haemoglobin ⇌ oxyhaemoglobin (4O₂ + Hb ⇌ Hb4O₂).
Each molecule of haemoglobin contains four haem groups, each able to bond with a single molecule of oxygen (O₂).
Each molecule of haemoglobin can carry four oxygen molecules, or eight oxygen atoms.

The Oxyhaemoglobin Dissociation Curve

The oxyhaemoglobin dissociation curve shows the percentage saturation of haemoglobin with oxygen at different oxygen concentrations.
Oxygen concentration is given as the partial pressure of oxygen (pO₂).
Haemoglobin is saturated when all of its oxygen binding sites are taken up with oxygen, i.e. when it contains four oxygen molecules.
The curve has a distinctive shape due to cooperative binding of oxygen.
Shallow curve at the bottom left: it is difficult for the first oxygen molecule to bind, so binding of the first oxygen molecule is slow.
Steep curve in the central region: after the first oxygen molecule binds, the haemoglobin protein changes conformation, making it easier for the next oxygen molecules to bind; this speeds up binding of the remaining oxygen molecules.
Levelling off in the top right: as the haemoglobin molecule approaches saturation, it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites.
At high pO₂, haemoglobin has a high affinity for oxygen; at low pO₂, haemoglobin has a low affinity for oxygen.
In the lungs, pO₂ is high, so haemoglobin binds oxygen easily; in muscles, pO₂ is relatively low due to high rates of respiration, so oxygen dissociates easily.
A steep curve indicates both increased binding of oxygen as pO₂ increases and increased dissociation of oxygen as pO₂ decreases.

The Bohr Effect

The Bohr effect (or Bohr shift) describes how the concentration of carbon dioxide in the blood influences the dissociation of oxyhaemoglobin.
Respiration produces carbon dioxide as a waste product, so the partial pressure of carbon dioxide (pCO₂) in the blood is high.
High pCO₂ reduces haemoglobin's affinity for oxygen.
This increases dissociation of oxyhaemoglobin, increasing the availability of oxygen.
The Bohr effect is greatest in actively respiring tissues, meaning haemoglobin gives up its oxygen more readily where it is needed.
At higher pCO₂, the oxyhaemoglobin dissociation curve shifts to the right.
This indicates that at any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher levels of CO₂.

Adaptations in Haemoglobin

Different organisms may have different types of haemoglobin that bind to and release oxygen in different environmental conditions; these differences arise through natural selection.
Organisms living in low pO₂ environments need haemoglobin with a higher affinity for oxygen, so it can pick up oxygen at a pO₂ at which adult human haemoglobin would release it.
Highly metabolically active organisms (e.g. small size, generating body heat) need haemoglobin with a lower affinity for oxygen, so it releases oxygen easily to respiring tissues.
At high altitude, pO₂ in the air is lower; species such as llamas have haemoglobin with a higher affinity for oxygen, allowing sufficient oxygen saturation in the blood.
Oxyhaemoglobin dissociation curves for haemoglobin with a higher affinity for oxygen shift to the left.
In the developing fetus, oxygen comes from the mother's blood, which has a low pO₂; fetal haemoglobin therefore has a higher affinity for oxygen than adult haemoglobin.
The dissociation curve for fetal haemoglobin shifts to the left of that for adult haemoglobin; at any given pO₂, fetal haemoglobin has a higher percentage saturation than adult haemoglobin.
After birth, a baby begins to produce adult haemoglobin which gradually replaces fetal haemoglobin; this is important for easy release of oxygen in the respiring tissues of a more metabolically active individual.
To identify which curve represents haemoglobin with the highest affinity, choose any pO₂ on the x-axis and read upwards; the line with the highest percentage saturation is for the haemoglobin with the highest oxygen affinity.

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练习题

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1.How many oxygen molecules can one molecule of haemoglobin transport?
Easy
- A1
- B2
- C4
- D8
2.Which of the following are adaptations of a red blood cell for oxygen transport? (select all that apply)
Medium
- ANo nucleus
- BBiconcave shape
- CHighly flexible cell surface membrane
- DContains many mitochondria
- ELarge number of lysosomes
3.Match each haemoglobin term to its correct description.
Medium
- Haem group
- Globin
- Oxyhaemoglobin
- Contains an iron(II) ion that reversibly binds one oxygen molecule
- Polypeptide chain; haemoglobin has two α and two β types
- The complex formed when oxygen binds to haemoglobin
4.Amino acid R groups that point outwards from haemoglobin are hydrophilic, which helps the protein remain soluble.
Easy
True or false?
5.What is the best definition of mass transport?
Medium
- AThe movement of individual molecules across a cell membrane by diffusion
- BThe efficient movement of substances over large distances, usually via specialised transport systems
- CThe breakdown of large molecules into smaller ones for absorption
- DThe random movement of particles from high to low concentration
6.A species of bird lives at high altitude where the partial pressure of oxygen is low. Compared with a low-altitude bird, which change to its haemoglobin would be most beneficial?
Medium
- AA lower affinity for oxygen, shifting the dissociation curve to the right
- BA higher affinity for oxygen, shifting the dissociation curve to the left
- CA higher affinity for carbon dioxide, shifting the curve to the right
- DNo change in affinity, but more haem groups per haemoglobin molecule
7.Oxygen binds to haemoglobin more easily after the first oxygen molecule has bound. What is this phenomenon called?
Hard
- ACompetitive binding
- BCooperative binding
- CThe Bohr effect
- DPositive hydrolysis
8.The dissociation curve for foetal haemoglobin lies to the right of the curve for adult haemoglobin.
Easy
True or false?

更多题目

9.Place the following events of the Bohr effect in the correct order.
Medium
- Respiration in active tissues produces carbon dioxide
- Partial pressure of carbon dioxide in the blood increases
- Haemoglobin's affinity for oxygen is reduced
- Oxyhaemoglobin dissociates, releasing oxygen
10.What is the advantage of a red blood cell having a diameter approximately the same as a capillary?
Medium
- AIt increases the surface area for oxygen diffusion
- BIt slows blood flow to maximise time for diffusion
- CIt allows the cell to carry more haemoglobin
- DIt prevents the cell from being damaged by high blood pressure
11.Which of the following statements about the oxyhaemoglobin dissociation curve are correct? (select all that apply)
Medium
- AAt high partial pressure of oxygen, haemoglobin has a high affinity for oxygen
- BThe curve is S-shaped due to cooperative binding
- CAt low partial pressure of oxygen, haemoglobin has a high affinity for oxygen
- DThe curve levels off at high partial pressure because binding sites become saturated
- EA higher partial pressure of carbon dioxide shifts the curve to the left
12.Which of the following are functions of mass transport systems in multicellular organisms? (select all that apply)
Medium
- ATransporting oxygen from the alveoli to body cells
- BTransporting glucose from the intestinal epithelium to body cells
- CTransporting carbon dioxide from cells back to the alveoli
- DTransporting urea from the kidneys to the cells
- EIncreasing the rate of diffusion across exchange surfaces
13.Match each component of haemoglobin structure with its correct description.
Medium
- Quaternary structure
- α-globins and β-globins
- Disulphide bonds
- Haem group
- Four polypeptide chains held together
- Two of each type of polypeptide subunit
- Hold the globin subunits together
- Contains an iron II ion that reversibly binds oxygen
14.Place the following steps of the Bohr effect in the correct order.
Medium
- Respiration produces carbon dioxide as a waste product
- Partial pressure of carbon dioxide in the blood is high
- Haemoglobin's affinity for oxygen is reduced
- Dissociation of oxyhaemoglobin increases
- Availability of oxygen increases
15.What is the advantage to a mammal of haemoglobin having a lower affinity for oxygen?
Medium
- AOxygen binds more readily in the lungs.
- BOxygen is released more easily to respiring tissues.
- CHaemoglobin becomes fully saturated at lower partial pressures.
- DCarbon dioxide binds to haemoglobin instead of oxygen.
16.At high partial pressures of oxygen, haemoglobin has a high affinity for oxygen.
Easy
True or false?
17.Which of the following best explains why the oxyhaemoglobin dissociation curve for foetal haemoglobin is to the left of that for adult haemoglobin?
Medium
- AFoetal haemoglobin has a lower affinity for oxygen so it can release oxygen to the mother.
- BFoetal haemoglobin has a higher affinity for oxygen so it can take oxygen from the mother's blood at the placenta.
- CFoetal haemoglobin can carry more oxygen molecules per molecule than adult haemoglobin.
- DFoetal haemoglobin is not affected by the partial pressure of carbon dioxide.
18.A species of bird lives at high altitude where the partial pressure of oxygen is low. Which of the following adaptations would be beneficial? (select all that apply)
Hard
- AHaemoglobin with a higher affinity for oxygen
- BA dissociation curve shifted to the left
- CA dissociation curve shifted to the right
- DHaemoglobin that binds oxygen more readily
- EHaemoglobin with a lower affinity for oxygen
19.
A red blood cell is placed in a solution and its volume is measured over time. The table shows the results.
Time / s Cell volume / μm³
0 90
10 90
20 90
30 90
What can be concluded about the solution?
Medium
- AIt is isotonic to the cytoplasm of the red blood cell
- BIt is hypotonic to the cytoplasm of the red blood cell
- CIt is hypertonic to the cytoplasm of the red blood cell
- DIt contains no water
20.A student investigates the effect of carbon dioxide concentration on the oxygen saturation of haemoglobin. They bubble carbon dioxide through a sample of blood at a constant partial pressure of oxygen of 5 kPa. The percentage saturation of haemoglobin decreases. Which of the following best explains this result?
Hard
- ACarbon dioxide binds to haemoglobin instead of oxygen
- BCarbon dioxide increases the affinity of haemoglobin for oxygen
- CCarbon dioxide decreases the affinity of haemoglobin for oxygen, causing more oxygen to dissociate
- DCarbon dioxide increases the partial pressure of oxygen in the blood
21.
A scientist measures the volume of blood in the left ventricle of a healthy adult at rest over one second. The results are shown in the table.
Time / s Volume / cm³
0.0 115
0.1 125
0.2 98
0.3 68
0.4 53
0.5 58
0.6 85
0.7 93
0.8 103
0.9 115
1.0 119
Calculate the heart rate in bpm.
Medium
- A60 bpm
- B75 bpm
- C80 bpm
- D100 bpm
22.Which of the following statements about the structure of haemoglobin are correct? (select all that apply)
Medium
- AIt is a quaternary protein with four polypeptide chains
- BIt contains two α and two β globin subunits
- CEach haemoglobin molecule can transport four oxygen molecules
- DThe haem group contains an iron(III) ion
- EThe polypeptide chains are held together by hydrogen bonds only
23.A species of bird lives at high altitude where the partial pressure of oxygen is low. Compared with a low-altitude bird, which of the following adaptations would be beneficial? (select all that apply)
Hard
- AHaemoglobin with a higher affinity for oxygen
- BA dissociation curve shifted to the left
- CA dissociation curve shifted to the right
- DA higher percentage saturation of haemoglobin at any given partial pressure of oxygen
- EHaemoglobin that releases oxygen more readily in the lungs
24.At a high partial pressure of oxygen, haemoglobin has a high affinity for oxygen.
Easy
True or false?
25.Match each term about haemoglobin to its correct description.
Medium
- Cooperative binding
- Bohr effect
- Saturated haemoglobin
- Partial pressure of oxygen
- The shape change after the first oxygen binds, making further binding easier
- A reduction in haemoglobin's affinity for oxygen caused by a high pCO₂
- Haemoglobin with all four oxygen-binding sites occupied
- A measure of oxygen concentration, shown on the x-axis of the dissociation curve
26.A scientist takes an electron micrograph of a red blood cell. The actual diameter of the red blood cell is 7 μm, but in the micrograph image it measures 5 cm. Calculate the magnification used. Give your answer to the nearest 100.
Medium
- A×7,100
- B×71,000
- C×710
- D×1,400
27.Which of the following are valid experimental controls when investigating the effect of carbon dioxide concentration on the oxygen saturation of haemoglobin in vitro? (select all that apply)
Medium
- AUsing the same volume of blood sample for each carbon dioxide concentration
- BMaintaining a constant temperature throughout the experiment
- CUsing different types of haemoglobin for each trial
- DMeasuring the oxygen saturation using the same method for all samples
- EAllowing the blood to equilibrate with each gas mixture for the same time
28.Match each term related to haemoglobin to its correct description.
Medium
- Cooperative binding
- Bohr effect
- Saturation
- When binding of the first oxygen molecule makes it easier for subsequent oxygen molecules to bind
- When increased carbon dioxide concentration reduces haemoglobin's affinity for oxygen
- When all oxygen binding sites on haemoglobin are occupied by oxygen
29.A student investigates the effect of carbon dioxide concentration on the oxygen saturation of haemoglobin. They bubble carbon dioxide through a sample of blood and measure the percentage saturation at a constant partial pressure of oxygen. Which of the following would they observe?
Hard
- AThe percentage saturation of haemoglobin increases.
- BThe percentage saturation of haemoglobin decreases.
- CThe percentage saturation of haemoglobin remains unchanged.
- DThe percentage saturation of haemoglobin first increases then decreases.
30.Which term describes the form of haemoglobin that is produced when oxygen binds to it?
Easy
- AOxyhaemoglobin
- BCarbaminohaemoglobin
- CHaem group
- DGlobin
31.Which ion is contained within each haem group of a haemoglobin molecule?
Easy
- AIron(II) ion
- BIron(III) ion
- CMagnesium ion
- DCalcium ion
32.Which type of protein structure does haemoglobin have?
Easy
- AQuaternary
- BPrimary
- CSecondary
- DTertiary
33.A haemoglobin molecule is fully saturated. How many oxygen atoms is it carrying?
Medium
- A8
- B4
- C2
- D16
34.Which statement correctly describes the affinity of haemoglobin for oxygen at a low partial pressure of oxygen?
Medium
- AHaemoglobin has a low affinity for oxygen, so oxygen dissociates easily
- BHaemoglobin has a high affinity for oxygen, so oxygen binds easily
- CHaemoglobin has a high affinity for oxygen, so oxygen dissociates easily
- DHaemoglobin has a low affinity for oxygen, so oxygen binds easily
35.Which of the following is the best description of the Bohr effect?
Easy
- AA high partial pressure of carbon dioxide reduces haemoglobin's affinity for oxygen, increasing the dissociation of oxyhaemoglobin
- BA high partial pressure of oxygen reduces haemoglobin's affinity for carbon dioxide, increasing the dissociation of oxyhaemoglobin
- CA high partial pressure of carbon dioxide increases haemoglobin's affinity for oxygen, reducing the dissociation of oxyhaemoglobin
- DA low partial pressure of carbon dioxide reduces haemoglobin's affinity for oxygen, increasing the dissociation of oxyhaemoglobin
36.A mammal species lives at high altitude where the partial pressure of oxygen in the air is low. Which property of its haemoglobin would be beneficial?
Medium
- AA higher affinity for oxygen than low-altitude haemoglobin
- BA lower affinity for oxygen than low-altitude haemoglobin
- CA greater number of haem groups per molecule than low-altitude haemoglobin
- DA smaller number of polypeptide chains than low-altitude haemoglobin
37.Which of the following statements about haemoglobin are correct? (select all that apply)
Medium
- AIt is made of two α-globins and two β-globins
- BThe four globin subunits are held together by disulphide bonds
- CHydrophobic R groups face inwards, helping to preserve the spherical tertiary structure
- DHydrophilic R groups face inwards, helping to maintain solubility
- EEach subunit contains a haem group that can reversibly combine with an oxygen molecule
38.Which of the following are adaptations of red blood cells that maximise their ability to transport oxygen? (select all that apply)
Medium
- AThey have no nucleus
- BThey have a biconcave shape
- CThey are highly flexible
- DThey have a large number of mitochondria
- ETheir diameter is approximately the same as that of the capillaries
39.Haemoglobin is saturated when all of its oxygen binding sites are taken up with oxygen, i.e. when it contains four oxygen molecules.
Easy
True or false?
40.At higher partial pressures of carbon dioxide, the oxyhaemoglobin dissociation curve shifts to the left.
Easy
True or false?
41.Name the effect that describes how an increased partial pressure of carbon dioxide reduces haemoglobin's affinity for oxygen and shifts the dissociation curve to the right.
Easy
42.Complete the sentences about red blood cell structure using the correct terms.
Easy
Red blood cells have no ____ to maximise the space available for haemoglobin, and their ____ shape increases the surface area for oxygen diffusion.
43.There is around 150 g of haemoglobin in 1 dm3 of blood. In a healthy adult at room temperature, 1 g of haemoglobin can combine with 1.4 cm3 of oxygen. Calculate how much oxygen can be carried in 1 dm3 of blood, in cm3.
Medium
44.Complete the equation for the reversible binding of oxygen to haemoglobin.
Medium
4O2 + Hb ⇌ ____
45.A red blood cell has an actual diameter of 7 μm. In an electron micrograph it measures 5 cm. Calculate the magnification used. Give your answer to the nearest 100.
Medium
46.Complete the equation for the reversible binding of oxygen to haemoglobin using the correct terms.
Easy
oxygen + ____ ⇌ ____
47.What is the name of the process by which the binding of the first oxygen molecule to haemoglobin makes it easier for the next oxygen molecules to bind?
Easy
48.There is around 150 g of haemoglobin in 1 dm³ of blood. In a healthy adult at room temperature, 1 g of haemoglobin can combine with 1.4 cm³ of oxygen. Calculate how much oxygen can be carried in 1 dm³ of blood.
Medium
49.Complete the sentences about the oxyhaemoglobin dissociation curve using the correct terms.
Medium
At low partial pressures of oxygen, haemoglobin has a ____ affinity for oxygen. At high partial pressures of oxygen, haemoglobin has a ____ affinity for oxygen.
50.Calculate the magnification used by the scientist. Give your answer to the nearest 100.
Medium
51.There is around 150 g of haemoglobin in 1 dm³ of blood. In a healthy adult at room temperature, 1 g of haemoglobin can combine with 1.4 cm³ of oxygen. Calculate how much oxygen can be carried in 1 dm³ of blood. Give your answer in cm³.
Medium
52.A man's left ventricle has a maximum volume of 125 cm³ and a minimum volume of 53 cm³ during one cardiac cycle. His heart rate is 75 bpm. Calculate his cardiac output in cm³ min⁻¹.
Medium
53.Describe the role of haemoglobin within the human body.
Hard
54.Explain why a higher amount of oxygen is offloaded in an active muscle.
Hard
55.An athlete has discovered that he is anaemic. A person suffering from anaemia has a decreased concentration of haemoglobin in their blood. Suggest and explain how being anaemic may affect the athlete's performance.
Hard
56.Describe and explain two ways in which red blood cells are adapted to oxygen transport.
Hard
57.State and explain two ways in which the structure of a red blood cell means that they are adapted to oxygen transport.
Medium
58.There is around 150 g of haemoglobin in 1 dm3 of blood. In a healthy adult at room temperature, 1 g of haemoglobin can combine with 1.4 cm3 of oxygen. Calculate how much oxygen can be carried in 1 dm3 of blood, in cm3.
Medium
- A189 cm3
- B210 cm3
- C252 cm3
- D231 cm3
59.A red blood cell has an actual diameter of 7 μm. In an electron micrograph it measures 5 cm. Calculate the magnification used. Give your answer to the nearest 100.
Medium
- A8520
- B7810
- C6390
- D7100
60.There is around 150 g of haemoglobin in 1 dm³ of blood. In a healthy adult at room temperature, 1 g of haemoglobin can combine with 1.4 cm³ of oxygen. Calculate how much oxygen can be carried in 1 dm³ of blood.
Medium
- A189 cm³
- B231 cm³
- C210 cm³
- D252 cm³
61.Calculate the magnification used by the scientist. Give your answer to the nearest 100.
Medium
- A7810
- B8520
- C6390
- D7100
62.There is around 150 g of haemoglobin in 1 dm³ of blood. In a healthy adult at room temperature, 1 g of haemoglobin can combine with 1.4 cm³ of oxygen. Calculate how much oxygen can be carried in 1 dm³ of blood. Give your answer in cm³.
Medium
- A210 cm³
- B252 cm³
- C189 cm³
- D231 cm³
63.A man's left ventricle has a maximum volume of 125 cm³ and a minimum volume of 53 cm³ during one cardiac cycle. His heart rate is 75 bpm. Calculate his cardiac output in cm³ min⁻¹.
Medium
- A5940 cm³ min⁻¹
- B4860 cm³ min⁻¹
- C5400 cm³ min⁻¹
- D6480 cm³ min⁻¹

复习笔记

## Mass Transport - **Definition:** the efficient movement of substances over large distances, usually via specialised transport systems. - Moves materials from exchange surfaces to cells (e.g. oxygen from alveoli to body cells; glucose from intestinal epithelium to body cells) and waste back (e.g. carbon dioxide to alveoli; urea to kidneys). - Specialised systems (circulatory system in animals, vascular tissue in plants) use **pressure changes** to force substances in the required direction. - Needed in multicellular organisms because distances between exchange surfaces and cells are very large; **diffusion alone would be too slow**. - Don't confuse the need for a gas exchange system (low SA:V ratio) with the need for a mass transport system (large distances). ## Haemoglobin Structure - **Quaternary structure:** four polypeptide chains — two α–globins and two β–globins. - Held together by **disulphide bonds**. - Hydrophobic R groups face inwards (preserve spherical tertiary structure); hydrophilic R groups face outwards (maintain solubility). - Each subunit contains a **haem group** with an **iron II ion (Fe²⁺)** that reversibly combines with one oxygen molecule. - Each haemoglobin transports **four oxygen molecules (O₂)**. ## The Role of Haemoglobin - Red blood cells are adapted for oxygen transport: - no nucleus — maximises space for haemoglobin - biconcave shape — maximises surface area for diffusion - highly flexible — pass through narrow capillaries - diameter ≈ capillary diameter — slows blood flow, maximising time for diffusion - **oxygen + haemoglobin ⇌ oxyhaemoglobin** - **4O₂ + Hb ⇌ Hb4O₂** - Each haemoglobin carries four oxygen molecules, or eight oxygen atoms. ## The Oxyhaemoglobin Dissociation Curve - Shows **percentage saturation of haemoglobin** against **partial pressure of oxygen (pO₂)**. - Saturated = all oxygen binding sites occupied (four oxygen molecules). - **Cooperative binding** explains the curve shape: - shallow bottom left — first oxygen binds slowly - steep central region — after first oxygen binds, haemoglobin changes conformation, making further binding easier - levelling off top right — approaching saturation, fourth oxygen binds more slowly due to shortage of binding sites - **Affinity:** high pO₂ → high affinity; low pO₂ → low affinity. - Lungs: high pO₂ → oxygen binds easily. Muscles: low pO₂ (high respiration) → oxygen dissociates easily. ## The Bohr Effect - High carbon dioxide concentration reduces haemoglobin's affinity for oxygen → more dissociation of oxyhaemoglobin → more oxygen available. - Sequence: respiration produces CO₂ → pCO₂ high → affinity reduced → dissociation increases → oxygen availability increases. - Greatest in actively respiring tissues. - At higher pCO₂ the curve **shifts to the right**: at any given pO₂, percentage saturation is lower. ## Adaptations in Haemoglobin - Different haemoglobins arise by **natural selection**. - **Low pO₂ environments (e.g. high altitude):** higher affinity for oxygen; curve shifts **left**. Example: llamas bind oxygen more readily. - **Highly metabolically active organisms:** lower affinity for oxygen, so oxygen releases easily to respiring tissues. - **Fetus:** gains oxygen from mother's blood at low pO₂; fetal haemoglobin has **higher affinity** than adult haemoglobin; curve shifts **left**. At any given pO₂, fetal haemoglobin has a higher percentage saturation. - After birth, adult haemoglobin gradually replaces fetal haemoglobin, allowing easy oxygen release in the more metabolically active individual. - To identify highest affinity from a graph: choose any pO₂, read upwards; the line with the highest percentage saturation has the highest affinity.

Mass transport: The efficient movement of substances over large distances, usually via specialised transport systems.
Haemoglobin: A quaternary protein with four polypeptide chains (two α–globins and two β–globins), each containing a haem group with an iron II ion (Fe²⁺) that reversibly combines with one oxygen molecule.
Oxyhaemoglobin: The complex formed when oxygen binds to haemoglobin: oxygen + haemoglobin ⇌ oxyhaemoglobin; 4O₂ + Hb ⇌ Hb4O₂.
Partial pressure of oxygen (pO₂): The concentration of oxygen given on the x-axis of the oxyhaemoglobin dissociation curve.
Oxyhaemoglobin dissociation curve: A graph showing the percentage saturation of haemoglobin with oxygen at different partial pressures of oxygen.
Cooperative binding: The shape change of haemoglobin after the first oxygen molecule binds, making it easier for the next oxygen molecules to bind.
Affinity for oxygen: The tendency of haemoglobin to bind oxygen; high at high pO₂ and low at low pO₂.
Bohr effect (Bohr shift): The change in haemoglobin's oxygen dissociation caused by carbon dioxide: high pCO₂ reduces affinity for oxygen, increasing dissociation and shifting the curve to the right.
Saturated haemoglobin: Haemoglobin with all oxygen binding sites occupied, i.e. containing four oxygen molecules.
Fetal haemoglobin: Haemoglobin with a higher affinity for oxygen than adult haemoglobin, so its dissociation curve is shifted to the left; allows the fetus to obtain oxygen from the mother's blood at the placenta.

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Time / s	Volume / cm³
0.0	115
0.1	125
0.2	98
0.3	68
0.4	53
0.5	58
0.6	85
0.7	93
0.8	103
0.9	115
1.0	119

Time / s	Cell volume / μm³
0	90
10	90
20	90
30	90

Time / s	Volume / cm³
0.0	115
0.1	125
0.2	98
0.3	68
0.4	53
0.5	58
0.6	85
0.7	93
0.8	103
0.9	115
1.0	119

Time / s	Volume / cm³
0.0	115
0.1	125
0.2	98
0.3	68
0.4	53
0.5	58
0.6	85
0.7	93
0.8	103
0.9	115
1.0	119