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Explain the Structures and Roles of Carbohydrates, Proteins and Lipids

In this worksheet, students will explain the structures and functions of carbohydrates, proteins and lipids, and will also explore the chemical tests we use to detect them.

'Explain the Structures and Roles 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 we eat to sustain us.

 

Food is categorised into three main types of nutrients:

 

Carbohydrates        Proteins        Olive oil

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 into the blood.

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

 

Let’s work through each nutrient:

 

 

Carbohydrates:

 

The structure of carbohydrates

 

Carbohydrate sources include bread, rice and pasta, and they give us energy.

Carbohydrates are actually long polymer chains of single sugar molecules like glucose.

 

From plants, we digest starch which is plant sugar storage and cellulose from plant cell walls.

However, humans and animals store sugar as glycogen (not starch), so starch needs to be broken down into glucose first, absorbed by the gut and can then be used for respiration or reassembled as glycogen storage.

However, cellulose is too difficult to break down further and so it’s egested from the gut as the body can’t store sugar in this form.

 

 

Proteins:

 

The structure of proteins

 

They come from sources like meat and eggs and are important for growth and repair.

Proteins are polymers of 100s to 1000s of amino acids that are linked in a specific sequence.


They are broken down after eating and reassembled into human proteins in protein synthesis.

Unlike sugar, amino acids can’t be stored, so any that are leftover are broken down by the liver and urinated out.

 

 

Lipids:

 

The structure of lipids

 

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

Lipids are fats (like butter) and oils and are made of two different units: fatty acids and glycerol.

 

The arrangement is three fatty acids attached to one glycerol molecule, altogether making one lipid molecule which a chemist would call an ester.

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 to detect whether we have a carbohydrate, protein or lipid.

 

Test tube

 

Carbohydrate Tests Protein Tests Lipid Tests

Benedict's Test for reducing sugars: 

 

This blue solution creates a red/brown precipitate with reducing sugars like glucose and heat.

 

If there is only some reducing sugar present, the solution turns green/yellow.

 

Test tubes

 

Biuret Test:

 

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

 

 

A biuret test

Emulsion test:

 

1. Add ethanol to the food.

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

3. A cloudy liquid or emulsion shows lipids are present.

Iodine test for starch:

 

Starch turns blue/black when it comes into contact with dark brown iodine solution

Test for iodine

Sudan III test:

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

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

3. If a layer of red appears on top of the mix, lipids are present.

 

Now it's time for some questions.

What are the three types of nutrient?

Proteins

Lipids

Sugars

Carbohydrates

Fats

Oils and fats are examples of which nutrient?

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

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

 

Match the units below to the correct nutrient.

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

What are the functions of the different nutrients that we eat?

Column A

Column B

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

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

Column A

Column B

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

Animals and plants store sugars as different complex carbohydrates.

 

State the names of the different sugar stores in plants and animals.

Column A

Column B

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

What problem do humans have with cellulose?

It's not possible for humans to break it down

The human body is allergic to it

When broken down, its units can't be rebuilt into glycogen

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 for reducing sugars
Cloudy mix forms
Positive emulsion test for lipids
Brown solution turns black/blue
Positive Biuret's test for proteins
Blue turns to mauve/purple
Positive iodine test for starch
Red layer forms on top of the water
Positive Sudan III test for lipids
Blue solution to red/brown precipitate
  • Question 1

What are the three types of nutrient?

CORRECT ANSWER
Proteins
Lipids
Carbohydrates
EDDIE SAYS
From the food we eat, the body needs and breaks down three nutrients called carbohydrates, proteins and lipids. The health of our bodies depends upon consuming a good mixture of these three food groups, so make sure you know what they are!
  • Question 2

Oils and fats are examples of which nutrient?

CORRECT ANSWER
Lipids
EDDIE SAYS
Did you feel confident with this question? Lipids are fats and oils like butter and olive oil - think about what you put in frying pans when you cook!
  • Question 3

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

CORRECT ANSWER
Three
3
EDDIE SAYS
As lipid molecules are made of two different types of units, it's important to know how these are arranged. One glycerol molecule bonds to three fatty acid chains to make one lipid molecule (refresh your memory with the diagram in the Introduction!)
  • Question 4

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

 

Match the units below to the correct nutrient.

CORRECT ANSWER
EDDIE SAYS
This question is fundamental to your understanding of this topic! 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

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 6

What are the functions of the different nutrients that we eat?

CORRECT ANSWER

Column A

Column B

Energy for respiration
Carbohydrates
Growth and repair
Proteins
Energy and building cell membrane...
Lipids
EDDIE SAYS
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 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

Animals and plants store sugars as different complex carbohydrates.

 

State the names of the different sugar stores in plants and animals.

CORRECT ANSWER
EDDIE SAYS
So, as we've mentioned, animals and plants don't store sugars the same way, although both carbohydrates are made up of glucose units. Plants use starch as their storage medium, while animals create a structure called glycogen which is broken down when blood glucose levels are low and need topping up.
  • Question 9

What problem do humans have with cellulose?

CORRECT ANSWER
It's not possible for humans to break it down
EDDIE SAYS
Plant carbohydrates include cellulose from cell walls as well as starch from cell sugar storage. However, while we can break down starch into the glucose we need, cellulose is just too tough to take apart and so it's egested out of the body.
  • 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 for redu...
Blue solution to red/brown precip...
Positive emulsion test for lipids
Cloudy mix forms
Positive Biuret's test for protei...
Blue turns to mauve/purple
Positive iodine test for starch
Brown solution turns black/blue
Positive Sudan III test for lipid...
Red layer forms on top of the wat...
EDDIE SAYS
This final question is a bit mean but it summarises all the information you need to know about chemical tests for nutrients - so study it well!! Carbohydrates have two tests: Benedict's test for reducing sugars like glucose, which makes a red/brown precipitate on heating, and the iodine test for starches which turn blue/black. Proteins have one test so, hopefully, that makes it a little easier to remember the Biuret's test that turns from blue to mauve in the presence of protein. Finally, lipids have two tests. These are the emulsion test where lipids make everything cloudy, and the Sudan III test which 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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