Water as a solvent in transport
This unit is the first half of Topic 1, and every section in it is examined on the Unit 1 paper, WBI11 (Molecules, Diet, Transport and Health).
Two of its practicals, the colour-standards core practical and the heart dissection, are also examined as methods on the practical paper, WBI13.
The unit starts with the molecule that makes transport possible at all: water.
- Each marking point earns one mark, shown as "(1)"; a header such as "three of the following points" caps the total. There are no letter codes.
- "/" separates alternative wordings; ACCEPT, IGNORE and DO NOT ACCEPT lines say what else is or is not credited, and "max" caps a group of points.
- Give exactly as many answers as asked, one per line: a wrong extra answer can cancel a right one.
- A calculation earns one mark for the key step and one for the answer in the form asked (s.f., d.p., standard form, ": 1"); a correct bald answer scores full marks, but working rescues part marks.
- Asterisked (*) 6-mark items are marked in three levels (1-2, 3-4, 5-6); each item has its own rule for the mark inside a level, and describing the stimulus alone stays in Level 1.
- Multiple-choice items (5 to 8 per paper, always one in Question 1) are answered with a cross in a box, never a tick.
The idea: a transport fluid must dissolve what it carries
Blood plasma is about 90% water.
Glucose from the gut, amino acids, mineral ions, urea from the liver and hormones from glands all travel in it, and they can only travel because they dissolve.
A substance carried as undissolved lumps would settle out or block capillaries.
So the first question about any transport system is: which substances will its fluid dissolve, and what happens to the ones it will not?
Water dissolves more substances than almost any other liquid.
The reason is its charge distribution.
Why water is a dipole
In a water molecule, one oxygen atom shares a pair of electrons with each of two hydrogen atoms (covalent bonds).
Oxygen attracts the shared electrons more strongly than hydrogen does, so the electrons spend more time near the oxygen.
The oxygen end becomes slightly negative (δ−) and each hydrogen becomes slightly positive (δ+).
The molecule is also bent, with an angle of about 105° between the two bonds.
If it were straight, the two δ+ ends would sit on opposite sides and their effects would cancel.
Because it is bent, the molecule has a δ− side and a δ+ side: it is a dipole, a polar molecule.
How the dipole dissolves things
Ions. When sodium chloride is added to water, water molecules cluster round each ion.
The δ− oxygen atoms face a positive sodium ion; the δ+ hydrogen atoms face a negative chloride ion.
These attractions pull the ions out of the crystal and keep them apart, each wrapped in a shell of water molecules.
The ions are now spread through the water and move with it.
Polar molecules. Glucose, amino acids and urea have polar groups such as –OH and –NH₂.
These form hydrogen bonds with water molecules: a δ+ hydrogen of one molecule is attracted to a δ− oxygen or nitrogen of another.
The solute molecules are surrounded by water and carried along in solution.
Non-polar molecules have no charged regions for water to attract.
Triglycerides, cholesterol and many fat-soluble vitamins do not dissolve; in water they clump together.
So blood carries them packaged with proteins, as lipoproteins (you meet HDLs and LDLs in unit 1b).
Oxygen is a small non-polar molecule and dissolves only slightly: plasma alone carries about 0.3 cm³ of oxygen per 100 cm³ of blood, against about 20 cm³ carried by haemoglobin.
That shortfall is why red blood cells exist (the haemoglobin sections at the end of this unit).
Substance
Polar or charged?
How blood carries it
sodium ions, chloride ions, hydrogencarbonate ions
charged
dissolved in plasma
glucose, amino acids, urea
polar
dissolved in plasma
oxygen
non-polar, slightly soluble
mostly bound to haemoglobin in red blood cells
triglycerides, cholesterol
non-polar
in lipoproteins
Hydrogen bonds between water molecules
Water molecules also hydrogen-bond to each other.
Each bond is weak, but there are very many of them.
This explains the other properties a multiple-choice item may list alongside the solvent property: water molecules stick together (cohesion, which matters for water moving up xylem in Unit 2), and a lot of energy is needed to raise water's temperature, so blood can carry heat round the body without its own temperature swinging.
These are background here: the statement you must be able to explain is water as a solvent in transport, from its dipole.
Where this leads
The same polar and non-polar logic runs through the rest of the course.
The phospholipid bilayer of a membrane (unit 2a) forms because phosphate heads are polar and fatty acid tails are not.
Osmosis (unit 2a) is the movement of free water molecules, and water molecules held in shells round solutes are not free.
In this unit, the insolubility of starch and glycogen (next section) is exactly what makes them good stores.
- Dipole (polar molecule): a molecule with an uneven distribution of charge, slightly negative (δ−) at one end and slightly positive (δ+) at the other.
- Hydrogen bond: a weak attraction between a slightly positive hydrogen atom of one molecule and a slightly negative atom, such as oxygen, of another molecule.
- Solvent: a liquid in which a solute dissolves to form a solution.
Learn these as written.
"Water is charged" or "water has positive and negative ions" loses the mark: the charges are partial (δ), and water is a covalent molecule, not an ionic one.
Explain how the structure of water allows potassium ions to be transported in blood plasma.
(3)
1. Water is a dipole / polar molecule, with δ− oxygen and δ+ hydrogen atoms (1)
2. the δ− oxygen atoms of water molecules are attracted to the positive potassium ions (1)
3. so potassium ions dissolve / are surrounded by water molecules and are carried in solution in the plasma (1)
Answer: three linked points: the dipole, the attraction to the ion with the correct end named, and the result for transport.
A weaker answer: "Water is a good solvent so potassium dissolves in it and is carried round the body."
This scores 1 at most.
It says what happens, not why: no dipole, no δ charges, no attraction.
In an Explain item each point needs its reason.
In a blood sample, the plasma above a layer of cells looks cloudy after a fatty meal.
Explain why triglycerides are not transported dissolved in the plasma.
(2)
1. Triglycerides are non-polar / have no charged groups (1)
2. so they do not form hydrogen bonds with / are not attracted to water molecules, so they do not dissolve (and form droplets or are carried in lipoproteins) (1)
Answer: non-polar, so no attraction to water.
A weaker answer: "Fat is insoluble."
This is a description of the fact the question gives you, so it earns nothing; the mark is for the reason.
A calcium ion (Ca²⁺) and a hydrogencarbonate ion (HCO₃⁻) are both dissolved in plasma.
Describe how water molecules are arranged round each ion.
(2)
1. Round Ca²⁺ (positive), water molecules turn their δ− oxygen atoms towards the ion (1)
2. Round HCO₃⁻ (negative), water molecules turn a δ+ hydrogen atom towards the ion (1)
Answer: opposite charges attract, so the end of the water molecule that faces the ion is the end with the opposite partial charge.
In a water item, name the atoms and their partial charges: "oxygen is δ− and hydrogen is δ+".
Then link the dipole to the solute in the question (an ion or a polar molecule) and to transport.
Write "sodium ions", not "sodium": the element and its ion are different things, and schemes credit the ion.
- "Water has positive and negative charges." It is tempting because the δ signs look like charges. They are partial charges on a neutral molecule; say "slightly negative" or δ−.
- Calling the bond inside a water molecule a hydrogen bond. The O–H bonds inside the molecule are covalent; hydrogen bonds are the weak attractions between molecules.
- Saying oxygen is carried dissolved in plasma. Only a tiny fraction is. Oxygen is non-polar and dissolves poorly, which is why haemoglobin is needed.
- Describing, not explaining. "It dissolves" repeats the stem. Add why: the dipole and which end attracts the solute.
Check your understanding
1. Which row describes how water molecules are arranged round a dissolved potassium ion? (1)
- A hydrogen atoms of water face the ion, held by covalent bonds
- B oxygen atoms of water face the ion, held by attraction between opposite charges
- C hydrogen atoms of water face the ion, held by attraction between opposite charges
- D oxygen atoms of water face the ion, held by covalent bonds Total [1]
2. Which substance is transported in human blood mainly dissolved in the plasma? (1)
- A oxygen
- B cholesterol
- C a triglyceride
- D urea Total [1]
3. A student writes: "Water is polar because its oxygen atom is negative and its hydrogen atoms are positive." Explain what the student has left out or got wrong. (2) Total [2]
4. A new painkiller is a non-polar molecule. When it is injected into a vein, most of it is found attached to a protein called albumin in the plasma.
(a) Explain why very little of the painkiller is carried dissolved in the plasma. (2)
(b) Suggest one advantage of the painkiller being carried attached to albumin. (1)
Total [3]
5. A sports drink contains glucose, sodium chloride and vitamin E. Vitamin E is a non-polar molecule.
(a) Explain how sodium chloride dissolves in the water of the drink. (3)
(b) Explain why glucose dissolves in water although it is not made of ions. (2)
(c) Suggest how vitamin E is carried in the blood after it is absorbed. (1)
Total [6]
Answers
1. B (1). A is incorrect because the hydrogen atoms (δ+) are repelled by a positive ion, and no covalent bond forms. C is incorrect because the δ+ hydrogen end would be repelled by the positive ion. D is incorrect because the attraction is between opposite partial and full charges, not a covalent bond.
2. D (1). A is incorrect because oxygen is non-polar and dissolves only slightly; most is bound to haemoglobin. B is incorrect because cholesterol is non-polar and is carried in lipoproteins. C is incorrect because triglycerides are non-polar and are carried in lipoproteins.
3. Any two from: the charges are partial / slight / δ, not full charges (1); the oxygen attracts the shared electrons more strongly (1); the molecule is bent / not linear, so the δ+ and δ− regions are on different sides (1). DO NOT ACCEPT "water contains ions".
4. (a) Non-polar / no charged groups (1); so it is not attracted to / does not form hydrogen bonds with water molecules (1). (b) Any one: allows more of the drug to be carried in the blood; stops it forming droplets that could block capillaries; releases it slowly (1).
5. (a) Water is a dipole, δ− oxygen and δ+ hydrogen (1); δ− oxygen atoms attracted to sodium ions and δ+ hydrogen atoms to chloride ions (1); ions separated / surrounded by water molecules so they dissolve (1). (b) Glucose has polar –OH groups (1); which form hydrogen bonds with water molecules (1). (c) In lipoproteins / attached to (plasma) proteins (1).