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Understand the Functions of Carbohydrates, Proteins and Lipids

In this worksheet, students will look at the structures and functions of carbohydrates, proteins and lipids, and will also learn about the chemical tests we use to detect them.

'Understand the Functions of Carbohydrates, Proteins and Lipids' worksheet

Key stage:  KS 4

GCSE Subjects:   Biology: Single Subject

GCSE Boards:   OCR 21st Century

Curriculum topic:   Living Together: Food and the Ecosystem

Curriculum subtopic:   How are Organisms in an Ecosystem Interdependent?

Difficulty level:  

Worksheet Overview

QUESTION 1 of 10

Have you ever heard the phrase, ‘You are what you eat’?

Well, it’s actually kind of true! Our body is a collection of the nutrients that we eat to sustain us.

 

Food is categorised into three main nutrients:

 

Carbohydrate food group        protein food group        Butter

CARBOHYDRATES                    PROTEINS                                 LIPIDS

 

In the body, nutrients are broken down for two reasons:

 Firstly, these molecules are often too big to be absorbed through the gut wall.

 Also, humans need their own versions of the nutrients so they need to be collapsed and rebuilt differently

 

 

Carbohydrates:

 

The chemical structure of carbohydrates

 

Carbohydrates give us energy and are actually long chains of single sugar molecules like glucose.

 

From plants, we digest starch which is the plant sugar storage, but humans store their glucose as glycogen, so starch needs to be broken down into glucose first, absorbed by the gut and can then be used for respiration, or restructured as glycogen storage.

 

 

Proteins:

 

The structure of proteins

 

They are important for growth and repair, and are actually large chains of 100s to 1000s of amino acids.


They are broken down after eating and then, once the amino acids have passed through the gut wall, they are reorganised into human proteins.

Unlike sugar, amino acids can’t be stored, so any that are leftover are broken down.

 

 

Lipids:

 

Lipids

 

They provide energy but also are the key part of cell membranes.

 

Lipids are fats and are made of two different units: fatty acids and glycerol, with every one glycerol bonding to three fatty acids which makes one lipid molecule.

Again, they must be digested first because they are too big to pass through the gut wall, after which they are reassembled.

 

 

Chemical Qualitative Tests:

 

We can use different qualitative tests (what it is, not how much there is) to detect whether we have a carbohydrate, protein or lipid.

 

Carbohydrate Tests Protein Tests Lipid Tests

Benedict's Test for sugars: 

 

This blue solution when heated creates a red/brown precipitate with glucose.

 

Benedicts Test  

 

Biuret Test:

 

Proteins turn a clear blue Biuret solution into a purple/mauve colour.

 

 

Emulsion Test:

 

1. Add ethanol to food.

2. Pour the liquid from this mix into another test tube with water.

3. A cloudy liquid shows lipids are present.

Iodine Test for starch:

 

Starch turns blue/black

Test for starch

Sudan III Test:

 

1. Equal amounts of water and test sample are put in a test tube.

2. Add drops of Sudan III into the mix and shake.

3. You should see a layer of red if you have lipids.

 

Let's have a go at some questions now.

The three types of nutrients are proteins, lipids and what?

What are the functions of the different nutrients we eat?

Column A

Column B

Energy for respiration
Proteins
Growth and repair
Carbohydrates
Energy and building cell membranes
Lipids

What are plant sugar stores called?

Column A

Column B

Energy for respiration
Proteins
Growth and repair
Carbohydrates
Energy and building cell membranes
Lipids

Nutrients are broken down in the body to their simplest units.

 

Match the units below to the correct nutrient.

Column A

Column B

Amino acids
Lipids
Fatty acids and glycerol
Carbohydrates
Glucose
Proteins

In a lipid molecule, how many fatty acids are bound to one glycerol unit?

One

Two

Three

Is it true that excess amino acids are stored in the body?

One

Two

Three

Why do we talk about breaking down nutrients when we eat them?

One

Two

Three

Fat is an example of which type of nutrient?

What does it mean when a chemical test is qualitative?

It tests how much of something we have

It tests what we have but not how much

It tests for impurities

Qualitative tests are useful when we have a food sample and want to find out which nutrient is present.

 

Match the nutrients below with their correct tests.

Column A

Column B

Positive Benedict's test: blue solution to red/bro...
Lipids
Positive emulsion test: cloudy mix forms
Protein
Positive Biuret's test: blue to mauve/purple
Glucose
Positive iodine test: black/blue
Starch
  • Question 1

The three types of nutrients are proteins, lipids and what?

CORRECT ANSWER
Carbohydrates
EDDIE SAYS
From the food that we eat, the body needs and breaks down three nutrients called carbohydrates, proteins and lipids. These are the main nutrient groups so make sure you know them!
  • Question 2

What are the functions of the different nutrients we eat?

CORRECT ANSWER

Column A

Column B

Energy for respiration
Carbohydrates
Growth and repair
Proteins
Energy and building cell membrane...
Lipids
EDDIE SAYS
So we need to understand why our bodies crave what we eat. Carbohydrates provide our bodies with energy, especially for respiration, but also give us sugar stores to use for energy later. Proteins are vital for growth and repair, for example, maintaining our muscles. Lipids also provide energy but they are a vital component of all cell membranes and so are crucial for us to be able to function!
  • Question 3

What are plant sugar stores called?

CORRECT ANSWER
EDDIE SAYS
Did you feel confident about this question? Don't get muddled between plants and animals! Remember, starch is how plants store carbohydrate, while glycogen is how humans and animals store their excess glucose!
  • Question 4

Nutrients are broken down in the body to their simplest units.

 

Match the units below to the correct nutrient.

CORRECT ANSWER

Column A

Column B

Amino acids
Proteins
Fatty acids and glycerol
Lipids
Glucose
Carbohydrates
EDDIE SAYS
This question is fundamental to your understanding of this topic, so make sure you feel confident with these! Carbohydrates are broken down into simple sugars like glucose. Proteins are polymers of amino acids. Lipids are a combination of two units called fatty acids and glycerols - not to be confused with glucose!!!
  • Question 5

In a lipid molecule, how many fatty acids are bound to one glycerol unit?

CORRECT ANSWER
Three
EDDIE SAYS
As lipid molecules are made of two different types of unit, it's important to know how these are arranged. One glycerol unit bonds to three fatty acid chains to make one lipid molecule (refresh your memory with the diagram in the Introduction!)
  • Question 6

Is it true that excess amino acids are stored in the body?

CORRECT ANSWER
EDDIE SAYS
Did you get this one right? Unlike sugars, amino acids can't be stored, so once the body has used all it needs to make its own proteins, the rest is broken down by the liver and urinated away.
  • Question 7

Why do we talk about breaking down nutrients when we eat them?

CORRECT ANSWER
EDDIE SAYS
This is an overarching point for all the nutrients and one you need to feel comfortable explaining. There are two main reasons why our bodies use our teeth, stomach acid and enzymes to break down what we eat: 1. Nutrients are too big to be absorbed into the blood through our gut wall. 2. The form of nutrient that we eat isn't in the form that it exists in animals. For example, plant carbohydrates and human carbohydrates are both made of many glucose units, but they do not have the same structures! This means that they need to be broken down in order to be made into a form that our bodies can use.
  • Question 8

Fat is an example of which type of nutrient?

CORRECT ANSWER
Lipids
Lipid
EDDIE SAYS
Fats and oils are examples of lipids Common carbohydrates include bread, pasta and rice, and the protein group includes meat and eggs. We need to eat a selection from each of these groups to stay healthy.
  • Question 9

What does it mean when a chemical test is qualitative?

CORRECT ANSWER
It tests what we have but not how much
EDDIE SAYS
Don't worry if you've not come across this word before - it is pretty straightforward. There are two types of tests that we need to know about: Quantitative tests measure how much of something we have (its quantity!), but don't tell us about its identity. Qualitative tests tell us what the substance is, like telling us if a sample is a lipid, carbohydrate or protein, but it can't tell us how much of it we have!
  • Question 10

Qualitative tests are useful when we have a food sample and want to find out which nutrient is present.

 

Match the nutrients below with their correct tests.

CORRECT ANSWER

Column A

Column B

Positive Benedict's test: blue so...
Glucose
Positive emulsion test: cloudy mi...
Lipids
Positive Biuret's test: blue to m...
Protein
Positive iodine test: black/blue
Starch
EDDIE SAYS
This final question is a bit mean but it summarises all the information you need to know about chemical tests for nutrients. Carbohydrates have two tests: Benedict's test for reducing sugars like glucose, which makes a red/brown precipitate when heated, and the iodine test for starches, which will turn blue/black. Proteins have one test so, hopefully, that makes it a little easier to remember the Biuret's test in which the liquid turns from blue to mauve when protein is present. Finally, lipids have two tests but you were only asked about one of these here - the emulsion test, which is where lipids make everything cloudy. For a bonus pat on the back, do you know the other lipid test? It is called the Sudan III test and it produces a red layer on top of the mix when lipids are found. Well done on this activity about large biological molecules - there was a lot to remember! Don't forget to write down any useful explanations that have helped to improve your understanding of nutrients.
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