Edexcel IGCSE Biology · Spec 2.25
Vitamin A and Water in a Balanced Diet
Covers recall sources and functions of vitamin A, Explain why water is an essential dietary component and Link deficiency to biological effects.
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Vitamin A and Water in a Balanced Diet
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Explained
Vitamin A and water, the two components that are easy to overlook
Seven components make up a balanced diet: carbohydrates, proteins, lipids, vitamins, mineral ions, water and fibre. The first three are the ones that supply energy or building material, and they get most of the attention. The rest are needed in small amounts, or none at all in the case of water and fibre, and they are just as necessary.
Vitamin A
Sources are liver, dairy foods such as milk, cheese and butter, and orange or dark green vegetables, particularly carrots and spinach. The orange vegetables contain carotene, which the body converts into vitamin A.
It has two functions worth knowing. It is needed to make the light sensitive pigment in the retina, which is what allows vision in dim light. And it helps keep the surface of the eye and the linings of the airways and gut healthy.
Deficiency causes poor vision in dim light, called night blindness, and in severe and prolonged cases can lead to permanent damage to the eye. It is one of the commonest causes of preventable childhood blindness in parts of the world where the diet is dominated by a single staple crop.
That is the connection to the genetic modification topic, where rice has been engineered to contain carotene precisely because rice supplies almost no vitamin A and is eaten in enormous quantities.
Water
Water is not a nutrient in the ordinary sense. It supplies no energy and is not built into anything, and about two thirds of your body is made of it.
It acts as a solvent, so most of the chemical reactions in cells happen in solution and could not happen at all without it.
It is the main component of blood plasma, so it is what transports dissolved substances such as glucose, amino acids, urea, carbon dioxide, hormones and ions around the body.
It is needed for digestion, since the reactions that break large molecules down are hydrolysis reactions and add water to each bond broken.
It removes waste, in urine and in sweat, and it cools the body when sweat evaporates from the skin.
Losses through urine, sweat and breathing are continuous, so the intake has to be continuous too. Dehydration follows quickly, and its effects are on concentration and circulation before they are on thirst.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks about people who eat fewer vegetables, and credits a chain: that they eat fewer vegetables or dislike the taste, that they therefore get fewer vitamins, fewer minerals or less fibre, and that they may develop scurvy, night blindness or constipation.
Its guidance accepts other correctly named deficiency diseases, and accepts that vegetables contain vitamins, minerals and fibre. Then it adds one refusal: ignore nutrients.
Nutrients is a true and useless word here. Vegetables do contain nutrients, and so does everything else edible. The mark is for naming which class of nutrient is being missed, and the three named in the chain each lead to a different consequence: vitamins to a deficiency disease, minerals to their own problems, fibre to constipation.
Look at the pairing in the third point. Scurvy comes from a shortage of vitamin C, night blindness from vitamin A, constipation from a shortage of fibre. Three named conditions, each traceable to one named thing missing from the diet, which is what makes them creditable rather than vague.
It is worth setting that against a ruling on a different question in the same paper, about vitamin A in rice, which rejects scurvy, rickets and anaemia outright. Both mark schemes are asking the same thing: match the disease to the deficiency. One question is open about which deficiency, so several diseases are accepted. The other has specified vitamin A, so only its deficiency disease will do.
The deficiency diseases worth having straight
Vitamin A: night blindness, and damage to the eye.
Vitamin C: scurvy, causing bleeding gums, poor wound healing and joint pain, because vitamin C is needed to make collagen.
Vitamin D: rickets in children and osteomalacia in adults, because vitamin D is needed to absorb calcium.
Calcium: weak bones and teeth. Iron: anaemia, because iron is needed to make haemoglobin.
Each is one nutrient with one main function and one consequence when it is missing. Learning them as triplets rather than as a list means a question can ask you to enter at any point.
Spec 2.25
What you need to know
- Recall sources and functions of vitamin A
- Explain why water is an essential dietary component
- Link deficiency to biological effects
Active recall
Quick check
Answer each question before opening the answer.
Give one dietary source of vitamin A
Any one of liver, dairy foods, orange vegetables or dark-green vegetables
Why is water important in blood plasma?
It acts as a solvent and transports dissolved substances
What vision problem may result from vitamin A deficiency?
Poor vision in dim light, or night blindness
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