Biological molecules and food tests

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Notas de aula

Big idea: large molecules built from small units

Big idea (key concept): Systems. A living thing is a system of molecules. Carbohydrates, proteins, lipids and DNA each have a job, and the job depends on how the molecule is built.
Related concept: Structure. In this lesson you keep asking one question: how does the structure of this molecule help it to do its job?
Global context: Scientific and technical innovation. Food scientists, doctors and forensic teams use simple chemical tests on foods and body fluids. The same tests help to check food labels and to screen for diseases such as diabetes.
Many biological molecules are polymers: long chains made by joining many small units called monomers. A polymer can be very large but is built from only a few kinds of unit.
Carbohydrates contain carbon, hydrogen and oxygen. The simple sugar glucose is the monomer for the polysaccharides starch, glycogen and cellulose. All three are chains of glucose, but the chains are arranged differently.
Starch is the energy store in plants. It is insoluble, so it does not draw water into cells by osmosis. Glycogen is the energy store in animals, held mainly in the liver and muscles. Cellulose forms long straight chains that bundle into strong fibres in plant cell walls.

Starch, glycogen and cellulose are all chains of glucose

Proteins and lipids: structure decides function

Proteins contain carbon, hydrogen, oxygen and nitrogen (some also contain sulfur). They are polymers whose monomers are amino acids. There are about 20 different amino acids.
The order of amino acids in a chain decides how the chain folds into a particular 3D shape. The shape decides the function. This is why proteins can be enzymes (amylase), antibodies, hormones (insulin), transport molecules (haemoglobin) or structural fibres (collagen).
Denaturation: heat or extreme pH breaks the bonds that hold a protein in shape. The shape changes permanently and the protein stops working. This is why an enzyme stops working above its optimum temperature.
Lipids (fats and oils) contain carbon, hydrogen and oxygen. One lipid molecule is made of one glycerol and three fatty acids. Fats are solid at room temperature and oils are liquid. Lipids do not dissolve in water but do dissolve in ethanol.
Lipids are an energy store, they are used for insulation, they cushion organs, and they make up cell membranes. Gram for gram, fat releases about twice as much energy as carbohydrate (about 37 kJ per gram for fat, about 17 kJ per gram for carbohydrate and protein).
Worked example: a snack has 12 g of fat. Energy from the fat is 12 x 37 = 444 kJ. A meal with 60 g of carbohydrate gives 60 x 17 = 1020 kJ from carbohydrate.

Food tests: starch and reducing sugars

To test a solid food, crush it with a little water, shake it and filter or let it settle. Test the liquid. Use a fresh clean tube for each test.
Iodine test for starch: add a few drops of orange-brown iodine solution. If starch is present the colour changes to blue-black. If not, it stays orange-brown.
Benedict's test for reducing sugars (such as glucose and maltose): add Benedict's solution (blue) and heat in a hot water bath for about five minutes. Wear eye protection and use a holder for the tube.
Colour scale: blue (none) then green, yellow, orange and brick-red (a lot). The more reducing sugar there is, the further along the scale the colour goes, so the test is semi-quantitative.
Sucrose is a non-reducing sugar. Benedict's solution stays blue with sucrose, so a blue result does not prove that there is no sugar of any kind in the food.
Starch is not a reducing sugar, so a starchy food such as potato gives blue-black with iodine but stays blue with Benedict's solution. If amylase digests the starch, the mixture then gives a Benedict's colour change.

Apparatus for the Benedict's test

Food tests: protein, lipid and reading results

Biuret test for protein: add biuret reagent (or sodium hydroxide followed by a few drops of dilute copper sulfate). A blue mixture turning purple (violet or lilac) shows protein. It stays blue if protein is absent. Sodium hydroxide is corrosive, so wear eye protection.
Ethanol emulsion test for lipid: shake the food with ethanol so that any lipid dissolves, then pour the ethanol into a tube of water. A cloudy white emulsion shows lipid. The lipid is not soluble in water, so it forms tiny droplets. A clear liquid means no lipid.
Summary table. Starch: iodine, blue-black. Reducing sugar: Benedict's (heated), green to brick-red. Protein: biuret, purple. Lipid: ethanol emulsion, cloudy white.
Reading results. In one test a food gave blue-black with iodine, a green colour with Benedict's, purple with biuret and a cloudy white emulsion. It contains starch, some reducing sugar, protein and lipid, so it is a mixed food such as a biscuit.
What a result cannot show. A negative test means the nutrient was not detected, not that it is completely absent. Benedict's will not detect sucrose, and a very small amount of a nutrient may give no visible change.
Percentages: if 7.5 g of a 50 g sample is protein, the percentage is 7.5 / 50 x 100 = 15%. A 200 g portion that is 15% protein and 30% fat has 30 g of protein and 60 g of fat, giving 30 x 17 + 60 x 37 = 510 + 2220 = 2730 kJ.

Positive and negative biuret results

DNA: the molecule that stores instructions

DNA is a polymer made of units called nucleotides. Each nucleotide has three parts: a sugar, a phosphate group and one of four bases.
The four bases are A (adenine), T (thymine), C (cytosine) and G (guanine). Two strands twist together into a double helix.
The sugars and phosphates form the sugar-phosphate backbone on the outside. The bases point inwards and join in complementary pairs: A with T and C with G. Weak hydrogen bonds hold each pair together.
Using the rule: the strand TACGGA is paired with ATGCCT. If 28% of the bases in a sample of DNA are A, then 28% are T, so A + T = 56%. The other 44% are C and G, which are equal, so G = 22%.
A gene is a section of DNA, a sequence of bases, that codes for a protein. Each group of three bases codes for one amino acid. A gene of 600 bases can code for up to 200 amino acids. The order of the bases sets the order of the amino acids, which sets the protein's shape and function.
Size order: a base is part of a nucleotide, a gene is a sequence of many nucleotides, and a chromosome is a long molecule of DNA that carries many genes.

The DNA double helix and its base pairs

Think like a scientist: investigating a food sample

Question: how does the concentration of glucose affect how fast Benedict's solution changes colour? Make glucose solutions of 0.5, 1, 2 and 4 g per 100 cm³. Put 5 cm³ of each into a tube, add 5 cm³ of Benedict's solution and heat in a water bath at 80 °C. Time how long until the first colour change.
Variables: the independent variable is the glucose concentration. The dependent variable is the time to the first colour change. Control variables: the volume of sample, the volume of Benedict's solution, the water bath temperature and the size of the tube.
Results from one experiment: the times were 300, 190, 90 and 45 s. The time falls as the concentration rises. From 0.5 to 4 g per 100 cm³ the time fell by (300 - 45) / 300 x 100 = 85%.
Reliability: repeat each concentration three times and calculate a mean. For example, repeats of 88, 92 and 90 s give (88 + 92 + 90) / 3 = 90 s. A result far from the others is anomalous and is checked or left out.
Evaluating the method: judging the first colour change by eye is subjective. Better methods are to use a colorimeter, to compare against colour standards, or to filter the mixture and measure how blue the liquid is that remains.
Inquiry task: a student says that doubling the concentration always halves the time. Use the results to evaluate this claim, then design a follow-up that tests concentrations of 1, 1.5, 2, 2.5 and 3 g per 100 cm³.

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Questões de prática

Prévia grátis — 8 de 55 perguntas. Cadastre-se para ver todas.

1.Which simple sugar is the monomer of starch, glycogen and cellulose?
Easy
- ASucrose
- BAmino acid
- CFatty acid
- DGlucose
2.Match each carbohydrate to its job.
Easy
- Starch
- Glycogen
- Cellulose
- Glucose
- Sugar that cells use in respiration
- Energy store in plant cells
- Strengthens plant cell walls
- Energy store in the liver and muscles
3.Why is starch a good energy store inside plant cells?
Medium
- AIt is insoluble, so it does not affect the movement of water into the cell
- BIt dissolves easily, so it can move out of the cell quickly when needed
- CIt is made of amino acids, which the cell can use to make enzymes
- DIt releases more energy per gram than any other substance in the cell
4.Starch and cellulose are both made only from glucose. Which statement best explains why cellulose makes strong cell walls?
Medium
- ACellulose is made from a different sugar, which is stronger than glucose
- BCellulose dissolves in the cell sap, which pushes the wall outwards with force
- CCellulose chains are straight and lie side by side in tough fibres
- DStarch is made of amino acids, which are unable to form any fibres at all
5.Glycogen is the main carbohydrate energy store in animals.
Easy
True or false?
6.Complete the sentence about carbohydrates.
Easy
Large carbohydrates such as starch are made by joining many small ____ molecules together.
7.Proteins are polymers. What is the monomer of a protein?
Easy
- AAmino acid
- BGlucose
- CNucleotide
- DFatty acid
8.What decides the 3D shape of a protein molecule?
Medium
- AThe number of glucose units that are attached to the outside of the chain
- BThe sequence of its amino acids, which makes the chain fold
- CThe temperature of the room in which the protein is kept
- DThe colour that the protein gives when it is tested with biuret

Mais perguntas

9.Which of these are proteins? (select all that apply)
Medium
- AGlycogen
- BCellulose
- CAmylase (an enzyme)
- DHaemoglobin
- EAn antibody
10.Two proteins are built from the same 20 types of amino acid but do completely different jobs. What is the best explanation?
Hard
- AThey contain different elements, because each protein has its own set of atoms
- BOne of them is made of glucose, so it cannot fold into the same shape
- CTheir amino acids are in a different order, so they fold into different shapes
- DThey are digested by different enzymes, which changes their jobs in the cell
11.Proteins are made of fatty acids and glycerol.
Easy
True or false?
12.What is one molecule of a fat or oil made of?
Easy
- AOne glycerol and three fatty acids
- BThree glycerol and one fatty acid
- CMany amino acids joined in a chain
- DMany glucose units joined in a chain
13.Gram for gram, how does the energy released by fat compare with carbohydrate?
Medium
- AAbout twice as much
- BAbout half as much
- CAbout the same
- DAbout ten times as much
14.Lipids dissolve easily in water.
Medium
True or false?
15.A snack contains 12 g of fat. Fat releases 37 kJ per gram. Calculate the energy released from the fat.
Medium
16.A 240 g portion of a food is 15% fat by mass. Fat releases 37 kJ per gram. Calculate the energy released from the fat in the portion.
Hard
17.What colour change shows that starch is present in an iodine test?
Easy
- ABlue to brick-red after heating
- BBlue to purple after a minute
- CClear to cloudy white when shaken
- DOrange-brown to blue-black
18.What must be done to a mixture of food solution and Benedict's solution for the test to work?
Easy
- AIt must be shaken with ethanol
- BIt must be heated in a hot water bath
- CIt must be cooled in ice
- DIt must be mixed with iodine solution
19.Put the Benedict's test colours in order, from no reducing sugar to a large amount of reducing sugar.
Medium
- Green
- Blue
- Brick-red
- Yellow
- Orange
20.A solution of sucrose is heated with Benedict's solution and stays blue. Why?
Medium
- ASucrose is a protein, so it needs the biuret test instead
- BSucrose is a starch, so it needs iodine solution to react with it
- CSucrose was not heated for long enough to dissolve in the water bath
- DSucrose is a non-reducing sugar, so it does not react with it
21.What colour does biuret reagent turn when protein is present?
Easy
- ABrick-red
- BBlue-black
- CPurple
- DCloudy white
22.Biuret reagent is added to a food solution and it stays blue. What is the best conclusion?
Medium
- AThe food solution contains starch
- BThe food solution contains a reducing sugar
- CNo protein was detected in the food solution
- DThe food solution contains lipid
23.What is a positive result for the ethanol emulsion test?
Easy
- AA cloudy white emulsion forms in the water
- BThe ethanol turns blue-black when iodine is added to it
- CThe mixture turns purple when sodium hydroxide is added
- DThe mixture turns brick-red when it is heated
24.Why is ethanol used in the emulsion test for lipids?
Medium
- AEthanol turns the lipid into a protein, which then gives a purple colour change
- BEthanol breaks the lipid molecules apart into glucose and fatty acids in the tube
- CEthanol is boiled to make the lipid melt, and the lipid turns brick-red
- DLipids dissolve in ethanol but not water, so they form droplets when poured into water
25.Put the steps of the ethanol emulsion test in order.
Medium
- Pour the ethanol into a tube of water
- Look for a cloudy white emulsion
- Crush the food and shake it with ethanol
- Let any lipid dissolve in the ethanol
26.Match each food test to its positive result.
Easy
- Iodine solution
- Benedict's solution (heated)
- Biuret reagent
- Ethanol emulsion test
- Green to brick-red
- Purple
- Cloudy white liquid
- Blue-black

27.

A student tested five foods (one set of results).

Food	Iodine	Benedict's (heated)	Biuret	Ethanol emulsion
W	blue-black	stays blue	stays blue	stays clear
X	orange-brown	brick-red	stays blue	stays clear
Y	orange-brown	stays blue	purple	stays clear
Z	orange-brown	stays blue	stays blue	cloudy white
V	blue-black	green	purple	cloudy white

Which food contains a reducing sugar and no starch?

Easy

28.

A student tested five foods (one set of results).

Food	Iodine	Benedict's (heated)	Biuret	Ethanol emulsion
W	blue-black	stays blue	stays blue	stays clear
X	orange-brown	brick-red	stays blue	stays clear
Y	orange-brown	stays blue	purple	stays clear
Z	orange-brown	stays blue	stays blue	cloudy white
V	blue-black	green	purple	cloudy white

Which food contains lipid but none of the other nutrients that were tested?

Medium

29.

A student tested five foods (one set of results).

Food	Iodine	Benedict's (heated)	Biuret	Ethanol emulsion
W	blue-black	stays blue	stays blue	stays clear
X	orange-brown	brick-red	stays blue	stays clear
Y	orange-brown	stays blue	purple	stays clear
Z	orange-brown	stays blue	stays blue	cloudy white
V	blue-black	green	purple	cloudy white

A student writes: 'Food W contains no sugar of any kind.' Which comment is best?

Hard

AThis is valid, because the Benedict's solution stayed blue after it was heated in the water bath
BNot valid, because Benedict's solution does not detect non-reducing sugars such as sucrose
CThis is valid, because W is positive for starch, which is a sugar
DThis is not valid, because the biuret test should have been used to find sugar

30.

A student tested five foods (one set of results).

Food	Iodine	Benedict's (heated)	Biuret	Ethanol emulsion
W	blue-black	stays blue	stays blue	stays clear
X	orange-brown	brick-red	stays blue	stays clear
Y	orange-brown	stays blue	purple	stays clear
Z	orange-brown	stays blue	stays blue	cloudy white
V	blue-black	green	purple	cloudy white

Food Y is egg white. Which conclusion is best supported by the results?

Hard

AEgg white contains protein, and no starch, reducing sugar or lipid was detected
BEgg white contains protein, starch and lipid, because it is a living tissue
CEgg white contains only protein, because the other three nutrients are completely absent
DEgg white contains protein and reducing sugar, because Benedict's solution is blue

31.A 50 g sample of a food contains 7.5 g of protein. Calculate the percentage of the sample that is protein.
Medium
32.A 200 g portion of food is 15% protein and 30% fat by mass. Protein releases 17 kJ per gram and fat releases 37 kJ per gram. Calculate the total energy released by the protein and fat in the portion.
Hard
33.A student investigates how glucose concentration affects the time for Benedict's solution to change colour. What is the independent variable?
Easy
- AThe time to the first colour change
- BThe temperature of the water bath
- CThe volume of Benedict's solution
- DThe concentration of the glucose solution
34.Which design gives a fair test of how glucose concentration affects the time for Benedict's solution to change colour?
Medium
- AUse a different water bath temperature for each concentration, so the results differ
- BKeep the volumes, temperature and tubes the same, and change only the concentration
- CUse a larger volume of reagent for each higher concentration, for a clearer colour
- DTest each concentration once and use a different type of sugar for each test
35.In the glucose investigation the time to first colour change was 300 s at 0.5 g per 100 cm³ and 45 s at 4 g per 100 cm³. Calculate the percentage decrease in time.
Medium
36.
A student tested glucose solutions and recorded these times (one experiment).
Glucose concentration (g per 100 cm³) Time to first colour change (s)
0.5 300
1 190
2 90
4 45
The student claims that doubling the concentration always halves the time. Which statement is best supported by the data?
Hard
- AThe claim is fully supported, because the time falls every time the concentration doubles
- BThe claim is fully supported, because 300 s is more than twice 90 s, so the pattern is clear
- CTime falls with concentration, but 300 s to 190 s is not a halving, so the claim is partly supported
- DThe claim cannot be tested, because the concentrations are not the same as the times in the table
37.A student judges the Benedict's colour 'by eye' to compare the sugar in three drinks. Which change makes the results more reliable and more quantitative?
Hard
- AUse a different volume of each drink, so that more sugar is seen in the biggest sample
- BCompare with colour standards or use a colorimeter, and repeat each test three times
- CHeat the first drink for longer than the others, so that its colour is clearer
- DAsk three people to decide which colour looks the most orange
38.Which description of DNA is correct?
Easy
- AA polymer of amino acids that speeds up chemical reactions
- BA polymer of glucose that stores energy in animals
- CA lipid that insulates the body and stores energy
- DA polymer of nucleotides that carries the genetic instructions
39.Which pairs of bases join together in DNA?
Easy
- AA with T, and C with G
- BA with G, and C with T
- CA with C, and G with T
- DA with A, and C with C
40.Which part of the DNA molecule forms the two sides of the ladder?
Medium
- AThe pairs of bases in the middle
- BThe hydrogen bonds between strands
- CThe sugar-phosphate backbone
- DThe amino acids in the strand
41.What holds the two strands of DNA together?
Medium
- AHydrogen bonds between complementary bases
- BCovalent bonds between the sugar-phosphate backbones
- CPeptide bonds between the amino acids
- DStarch molecules between the bases
42.Complete the sentence about base pairing.
Easy
In DNA, adenine always pairs with ____.
43.One strand of DNA has the base sequence TACGGA. What is the sequence of the complementary strand?
Medium
- ATACGGA
- BATGCCA
- CGCATTG
- DATGCCT
44.In a sample of DNA, 28% of the bases are adenine (A). Calculate the percentage of guanine (G).
Hard
45.A gene has 600 bases in the strand that is read. Assume that each group of three bases codes for one amino acid. Calculate the number of amino acids.
Hard
46.Put these in order from the smallest to the largest.
Medium
- A nucleotide
- A base
- A chromosome
- A gene
47.A mutation changes one base in a gene. Why might this change the protein that is made?
Hard
- AThe mutation changes the sugar in the backbone, so the protein is made of glucose
- BThe base order sets the amino acid order, so the protein may fold differently
- CThe mutation changes the temperature of the cell, which denatures the DNA
- DThe mutation turns the protein into a lipid, so it dissolves in water
48.Which statements describe the structure of DNA? (select all that apply)
Medium
- AThe bases join in the pairs A with G and C with T
- BIt is a double helix
- CThe bases pair as A with T and C with G
- DIt has a sugar-phosphate backbone
- EIt is a single straight strand of amino acids
49.In any sample of double-stranded DNA the percentage of adenine always equals the percentage of guanine.
Hard
True or false?
50.Describe how you would test a food sample for starch.
Medium
51.Explain why fat is a better long-term energy store than carbohydrate for an animal.
Medium
52.Explain why a protein stops working if it is heated strongly.
Hard
53.Compare the structure and the function of starch and cellulose.
Hard
54.A student heats food with Benedict's solution in a flame and writes 'it went a bit orange'. Suggest two improvements to the method.
Hard
55.A gene contains the base sequence TAC GGA. Explain how the order of bases in a gene affects the protein that is made.
Hard

Resumos de revisão

## The big idea - **Systems** and **structure**: a living thing is a system of molecules, and each molecule's structure suits its job. ## Polymers - Large molecules are **polymers** built from small **monomers**. - **Carbohydrates:** glucose makes starch (plant store), glycogen (animal store) and cellulose (cell walls). - **Proteins:** amino acids; the sequence sets the shape and the function. Heat **denatures** them. - **Lipids:** glycerol and three fatty acids; about 37 kJ per gram, twice carbohydrate. ## Food tests - Starch: iodine, orange-brown to blue-black. - Reducing sugar: Benedict's, heated, blue to green, yellow, orange, brick-red. Sucrose is not detected. - Protein: biuret, blue to purple. - Lipid: ethanol then water, cloudy white emulsion. - Negative means not detected. ## DNA - Polymer of nucleotides (sugar, phosphate, base) in a double helix. - A-T and C-G by hydrogen bonds. Three bases code for one amino acid. ## Think like a scientist - Change one variable, control the rest, repeat and calculate a mean, and evaluate the method.

Polymer: A large molecule made of many small units (monomers) joined in a chain.
Monomer: A small unit that joins with others to make a polymer, such as glucose or an amino acid.
Glucose: A simple sugar that is the monomer of starch, glycogen and cellulose.
Starch: A polymer of glucose that stores energy in plants.
Glycogen: A branched polymer of glucose that stores energy in animals, mainly in the liver and muscles.
Cellulose: A polymer of glucose in straight chains that forms strong fibres in plant cell walls.
Amino acid: The monomer of a protein; there are about 20 types.
Denaturation: A permanent change in the shape of a protein caused by heat or extreme pH.
Lipid: A fat or oil made of one glycerol and three fatty acids.
Reducing sugar: A sugar such as glucose that gives a colour change with heated Benedict's solution.
Biuret test: A test in which blue reagent turns purple when protein is present.
Emulsion test: A test in which a lipid dissolved in ethanol gives a cloudy white emulsion in water.
Nucleotide: The monomer of DNA: a sugar, a phosphate and a base.
Complementary base pairing: The rule that A pairs with T and C pairs with G in DNA.
Gene: A section of DNA, a sequence of bases, that codes for a protein.

Flashcards

What is a polymer?

A large molecule made of many small units (monomers) joined in a chain.

Which molecule is the monomer of starch, glycogen and cellulose?

Glucose.

What is the job of starch, glycogen and cellulose?

Starch stores energy in plants, glycogen stores energy in animals, and cellulose strengthens plant cell walls.

Why is starch good for storage?

It is insoluble, so it does not affect the water balance of the cell.

What is the monomer of a protein?

An amino acid.

What decides what a protein can do?

The sequence of amino acids sets the folded shape, and the shape sets the function.

What is a lipid made of?

One glycerol and three fatty acids.

How much energy per gram do fat and carbohydrate release?

About 37 kJ per gram for fat and about 17 kJ per gram for carbohydrate.

What is the positive result of the iodine test?

Orange-brown turns blue-black: starch is present.

How is the Benedict's test done and what does it show?

Heat with Benedict's solution in a hot water bath. Blue to green, yellow, orange or brick-red shows a reducing sugar.

Why does Benedict's solution stay blue with sucrose?

Sucrose is a non-reducing sugar.

What is the positive result of the biuret test?

Blue turns purple: protein is present.

How is the ethanol emulsion test done?

Shake the food with ethanol, then pour the ethanol into water. A cloudy white emulsion shows lipid.

What does a negative test result mean?

That the nutrient was not detected, not that it is completely absent.

What are the parts of a nucleotide?

A sugar, a phosphate group and a base.

Which bases pair in DNA?

A with T, and C with G, held by hydrogen bonds.

If 28% of the bases in DNA are A, what percentage are G?

22%. T is 28%, so C + G = 44%, and C = G = 22%.

How many amino acids does a 600-base gene code for?

Up to 200, because three bases code for one amino acid.

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Em breve

Glucose concentration (g per 100 cm³)	Time to first colour change (s)
0.5	300
1	190
2	90
4	45