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Nutrition: Nutrients, Food Tests and Energy in Food — Answers

Answer sheet with model answers and success criteria for the 8th Grade the human body worksheet.

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Success TutoringWorksheetsAnswer sheet8th Grade · Science · The Human Body

Nutrition: Nutrients, Food Tests and Energy in Food — Answers

Part A: Nutrients

Food provides nutrients: substances the body needs for energy, growth, repair and staying healthy. The main nutrients are carbohydrates, proteins, fats, vitamins, minerals, fiber and water.

A balanced diet contains the right amounts of each nutrient. Eating too little of a nutrient can cause a deficiency disease, while too much energy-rich food over time can lead to weight gain.

1.Match each nutrient to its main job in the body.
  • Carbohydrates
  • Proteins
  • Fats
  • Fiber
  • Calcium (a mineral)
  • Vitamin C
  • Strong bones and teeth
  • Healthy skin and gums, and helps wounds heal
  • Main source of energy
  • Growth and repair of cells and tissues
  • Helps food move through the intestines
  • Stored energy, insulation and making cell membranes

Answer: Carbohydrates → Main source of energy; Proteins → Growth and repair of cells and tissues; Fats → Stored energy, insulation and making cell membranes; Fiber → Helps food move through the intestines; Calcium (a mineral) → Strong bones and teeth; Vitamin C → Healthy skin and gums, and helps wounds heal

2.Which food is the best source of protein?
  1. ARice
  2. BEggs
  3. CButter
  4. DApples

Answer: Eggs

Eggs, meat, fish, beans and lentils are rich in protein. Rice is mainly carbohydrate, butter is mainly fat and apples are mainly sugar, water and fiber.

3.Water is not a nutrient because it contains no energy.

Answer: False

Water provides no energy, but it is still an essential nutrient: about 60% of the body is water and it is needed for chemical reactions, transport and temperature control.

4.Scurvy causes bleeding gums and wounds that heal slowly. Which nutrient is lacking, and name one food that would help prevent it.

Answer: Vitamin C. Foods rich in vitamin C include oranges, lemons, kiwifruit, strawberries, broccoli and tomatoes.

Part B: Food tests

Simple chemical tests show which nutrients a food contains. Iodine (starch): orange-brown → blue-black. Benedict's solution (sugar, needs heating): blue → green, yellow or brick red. Biuret solution (protein): blue → purple. Ethanol emulsion (fat): a cloudy white layer forms in water.

These tests are done in the school laboratory with eye protection.

5.A food sample turns blue-black when iodine solution is added. What does it contain?
  1. AProtein
  2. BStarch
  3. CFat
  4. DSugar

Answer: Starch

Iodine solution changes from orange-brown to blue-black when starch is present.

6.A food is heated with Benedict's solution and turns brick red. What does it contain?
  1. AStarch
  2. BProtein
  3. CSugar (glucose)
  4. DFat

Answer: Sugar (glucose)

Benedict's solution turns from blue to green, yellow, orange or brick red (with heating) when sugars such as glucose are present.

7.Biuret solution is added to a sample of milk and turns from blue to purple. What does this show?

Answer: The milk contains protein.

8.Food is shaken with ethanol and then poured into water, and a cloudy white layer forms. What nutrient is present?

Answer: Fat (lipid).

9.Why must Benedict's test be done with safety glasses and a water bath rather than an open flame?

Answer: The test needs heating, and a water bath heats the test tube gently and evenly; safety glasses protect the eyes in case the hot liquid spits.

Part C: Energy in food

The energy in food is measured in kilojoules (kJ). Each gram of carbohydrate releases about 17 kJ, each gram of protein about 17 kJ and each gram of fat about 37 kJ.

10.How much energy is released by 10 g of fat?

Answer: 370 kJ

  1. 10 × 37 = 370 kJ
11.A snack contains 20 g of carbohydrate, 5 g of protein and 8 g of fat. Calculate its total energy content.

Answer: 721 kJ

  1. Carbohydrate: 20 × 17 = 340 kJ
  2. Protein: 5 × 17 = 85 kJ
  3. Fat: 8 × 37 = 296 kJ
  4. Total = 340 + 85 + 296 = 721 kJ
12.A breakfast cereal contains 1600 kJ per 100 g. How much energy is in a 40 g serving?

Answer: 640 kJ

  1. 1600 ÷ 100 = 16 kJ per gram
  2. 16 × 40 = 640 kJ
13.A 40 g serving of the cereal contains 12 g of sugar. What percentage of the serving is sugar?

Answer: 30%

  1. 12 ÷ 40 × 100 = 30%
14.A teenager needs about 9000 kJ of energy a day. One meal provides 1500 kJ. How many of these meals would supply the whole day's energy?

Answer: 6 meals

  1. 9000 ÷ 1500 = 6

Part D: Evaluate a claim

15.A student says: "Fat is bad for you, so the healthiest diet has no fat at all." Explain whether you agree, using what you know about the functions of nutrients and energy in food.[4 marks]

Model answer: I disagree. The body needs some fat: it is used to make cell membranes, it stores energy, insulates the body and protects organs, and some vitamins (such as A and D) dissolve in fat, so fat is needed to absorb them. However, fat contains about 37 kJ per gram, more than twice as much as carbohydrate or protein (17 kJ per gram), so eating too much fatty food can supply more energy than the body uses, and the extra is stored, leading to weight gain. A healthy diet contains fat in moderate amounts, as part of a balance of all the nutrients.

Success criteria:

  • States a clear position
  • Gives at least two functions of fat
  • Uses the energy values to explain the risk of eating too much fat
  • Concludes that balance is needed
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