Paper 1 and Paper 2
CB1 K ey Biological Concepts The bone-eating snot-flower worm (Osedax mucofloris) has no digestive system but still manages to feed on one of the hardest substances produced by vertebrate animals – their bones. These worms are a type of zombie worm, so-called because they have no eyes or mouth, and were discovered in the North Sea in 2005 feeding on a whale skeleton. Enzymes in the ‘foot’ of the worm cause the production of an acid, which attacks bone and releases lipids and proteins from inside the bone. Enzymes in bacteria on the foot of the worm then digest these large organic molecules into smaller molecules that the worms absorb (using processes such as diffusion). In this unit you will learn about some of the central ideas in biology, including ideas about cells, microscopy, enzymes, nutrition, diffusion, osmosis and active transport.
The learning journey Previously you will have learnt at KS3: •• how to use a microscope •• about the differences between cells from different organisms •• how some cells are specialised and adapted to their functions •• how enzymes help to digest food in the digestive system •• how substances can move by diffusion. In this unit you will learn: •• how developments in microscopy have allowed us to find out more about the sub-cellular structures found in plant, animal and bacterial cells The ‘foot’ of the worm is buried in the whale bone and contains many bacteria.
•• about the importance of enzymes in nutrition, growth and development •• how enzymes are affected by pH and temperature and why each enzyme only works on a certain type of molecule •• how substances are carried by diffusion, osmosis and active transport.
CB1a Microscopes Specification reference: B1.3; B1.4; B1.5
Progression questions •• What determines how good a microscope is at showing small details? •• What has the development of the electron microscope allowed us to do? •• What units are used for very small sizes? eyepiece lens
The most common microscope used today contains two lenses and was invented at the end of the 16th century. Robert Hooke (1635−1703) used a microscope like this to discover cells in 1665. Hooke’s microscope had a magnification of about ×30 (it made things appear about 30 times bigger). A person magnified 30 times would be roughly the size of the Statue of Liberty in New York.
focusing wheel – adjusts the focus to make the image clearer objective lens specimen holder
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1 a A photo of a water flea says it is magnified ×50. What does this mean?
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b On the photo, the flea is 5 cm long. Calculate the unmagnified length of the water flea.
To work out a microscope’s magnification, you multiply the magnifications of its two lenses together. So, the magnification of a microscope with a ×5 eyepiece lens and ×10 objective lens is: 5 × 10 = ×50
A Hooke’s microscope 7
2 A microscope has a ×5 eyepiece lens with ×5, ×15 and ×20 objective lenses. Calculate its three total magnifications.
Hooke’s microscope was not very powerful because the glass lenses were of poor quality. Antonie van Leeuwenhoek (1632–1723) found a way of making much better lenses, although they were very small. He used these to construct microscopes with single lenses, which had magnifications of up to ×270. In 1675, he examined a drop of rainwater and was surprised to find tiny organisms, which he called ‘animalcules’. Fascinated by his discovery, he searched for ‘animalcules’ in different places. 7
B r eplica of a van Leeuwenhoek
3 The top bacterium in photo C is 0.002 mm long in real life. At what magnification is the drawing?
microscope
Did you know? Van Leeuwenhoek examined his semen and discovered sperm cells.
C T hese are van Leeuwenhoek’s drawings of ‘animalcules’ found in scrapings from his teeth. We call them bacteria.
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Microscopes
The detail obtained by a microscope also depends on its resolution. This is the smallest distance between two points that can still be seen as two points. Van Leeuwenhoek’s best microscopes had a resolution of 0.0014 mm. Two points that were 0.0014 mm or further apart could be seen as two points, but two points closer together than this appeared as a single point. 5
4 Hooke’s microscope had a resolution of about 0.002 mm. What does this mean?
With the development of stains for specimens, and better lenses and light sources, today’s best light microscopes magnify up to ×1500 with resolutions down to about 0.0001 mm.
D T hese images of tiny beads have the same magnification but different resolutions.
The electron microscope was invented in the 1930s. Instead of light, beams of electrons pass through a specimen to build up an image. These microscopes can magnify up to ×2 000 000, with resolutions down to 0.0000002 mm. They allow us to see cells with great detail and clarity. 6
5 Explain why electron microscope images show more detail than light microscopes.
SI units The measurements on these pages are in millimetres. Adding the ‘milli’ prefix to a unit divides it by 1000. One metre (m) contains 1000 millimetres (mm). There are other prefixes that often make numbers easier to understand. Table E
× 1000 × 1000 × 1000
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Prefix
Effect on unit
Example
milli-
÷ 1000
millimetres (mm)
micro-
÷ 1 000 000
micrometres (µm)
nano-
÷ 1 000 000 000
nanometres (nm)
pico-
÷ 1 000 000 000 000
picometres (pm)
÷ 1000 ÷ 1000 ÷ 1000
6 Give the highest resolution of electron microscopes in micro-, nano- and picometres.
Checkpoint How confidently can you answer the questions at the top of the previous page? Strengthen
Exam-style question Exam-style questions will follow on publication of the sample assessment materials by Edexcel.
S1 Compare today’s light microscopes with Hooke’s. Extend E1 Diatoms are algae, 20–120 µm in length and with 1 µm diameter ‘pores’ in their outer coats. Van Leeuwenhoek described diatom shapes but not their pores. Explain why.
Please see www.edexcel.com/gcsesci16 for more details.
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CB1b Plant and animal cells Specification reference: B1.1; B1.4; B1.6
Progression questions •• How are animal cells different to plant cells? •• What do the sub-cellular structures in eukaryotic cells do? •• How can we estimate the sizes of cells and their parts? As microscopes improved, scientists saw more details inside cells. In 1828, Robert Brown (1773–1858) examined cells from the surface of a leaf and noticed that each cell contained a small, round blob. He called this the nucleus (meaning ‘inner part’ in Latin). 5
two guard cells (form a stoma in the surface of a leaf)
leaf surface cell
nucleus
A T his micrograph (‘microscope picture’) was taken using Brown’s original microscope, of the same cells in which he discovered nuclei (magnification ×67).
1 Photo A is at a magnification of 67. State what this means.
Brown wrote a scientific paper about his discovery. Matthias Schleiden (1804– 1881) read the paper and thought that the nucleus must be the most important part of a plant cell. He mentioned this idea to Theodor Schwann (1810–1882), who then wondered if he could find cells with nuclei in animals. He did. And so the idea of cells being the basic building blocks of all life was born. A cell with a nucleus is described as eukaryotic. We have now discovered many other sub-cellular (‘smaller than a cell’) structures in eukaryotic cells and worked out what they do.
The cell membrane is like a very thin bag. It controls what enters and leaves, and separates one cell from another.
The nucleus controls the cell and its activities. Inside it are chromosomes, which contain DNA. It is especially large in white blood cells.
The cytoplasm contains a watery jelly and is where most of the cell’s activities occur.
red blood cell
One of these blobs is a mitochondrion (see photo C). Mitochondria are jelly-bean shaped structures in which aerobic respiration occurs. Mitochondria are very difficult to see with a light microscope.
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The cytoplasm also contains tiny round structures called ribosomes. These make new proteins for a cell. It is impossible to see them with a light microscope.
2 Draw a table to show the parts of an animal cell and the function of each part.
B T he labelled central cell is a human white blood cell, which has been stained to
3 Estimate the diameter of the labelled red blood cell in photo B. Show your working.
The circular area you see in a light microscope is the field of view. If we know its diameter, we can estimate sizes. The diameter of the field of view in photo B is 36 µm. We can imagine that three white blood cells will roughly fit = 12 µm. across the field of view. So the cell’s diameter is about 36 3
make its features show up clearly (magnification ×1900).
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Plant and animal cells
Electron micrographs
4 µm
Photo C shows many parts inside a white blood cell that you cannot see with a light microscope. However, you still cannot see ribosomes because they are only about 25 nm in diameter. 5
4 a Look at photo C. What part has been coloured purple?
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5 State the diameter of a ribosome in micrometres.
b Use the magnification to estimate the width of the cell.
Scale bars are often shown on micrographs and these are also used to estimate sizes. The scale bar on photo C shows how long 4 µm is at this magnification. About three of these bars could fit across the cell at its widest point; the cell is about 3 × 4 = 12 µm wide. Plant cells may have some additional structures compared with animal cells, as shown in diagram D. nucleus
The cell wall is made of cellulose and supports and protects the cell.
cytoplasm
cell membrane
mitochondria
small, temporary vacuoles
C e lectron micrograph of a white blood cell (magnification ×3800)
Did you know? The pigment in human skin is made in sub-cellular structures called melanosomes. 6 Use the scale bar on photo C to estimate the:
Plant cells have a large, The chloroplasts contain chlorophyll, permanent vacuole which which traps energy transferred from stores cell sap and helps to the Sun. The energy is used for keep the cell firm and rigid. photosynthesis.
X
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a width of the nucleus at its widest point
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b length of the longest mitochondrion (coloured red).
Checkpoint 5 μm
D a cell from inside a plant leaf
How confidently can you answer the questions at the top of the previous page? Strengthen
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7 Look at diagram D. What is part X?
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8 Cells on leaf surfaces contain vacuoles and carry out aerobic respiration but are not green. Suggest what part they lack. Explain your reasoning.
S1 Draw a plant cell and label its parts, describing what each part does. Extend
Exam-style question Exam-style questions will follow on publication of the sample assessment materials by Edexcel. Please see www.edexcel.com/gcsesci16 for more details. 5
E1 An ‘organelle’ is a structure inside a cell with a specific function. Compare the organelles found in plant and animal cells.
CB1c Specialised cells Specification reference: B1.2; B1.4; B1.6
Progression questions •• How are some specialised cells adapted to their functions? •• What is the function of a gamete? •• What is the function of cilia?
Did you know? Human nerve cells (neurones) carry information very quickly. Many are adapted by being extremely long, with some reaching lengths of about 1.4 m.
Specialised cells have a specific function (job). There are about 200 different types of specialised cells in humans. All human cells have the same basic design, but their sizes, shapes and sub-cellular structures can be different so that specialised cells are adapted to their functions. 5
1 List three specialised human cells and state their functions.
Specialised cells for digestion The cells that line the small intestine absorb small food molecules produced by digestion. They are adapted by having membranes with many tiny folds (called microvilli). These adaptations increase the surface area of the cell. The more area for molecules to be absorbed, the faster absorption happens.
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2 a Draw a small intestine cell and label its parts.
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b These cells are 20 µm long. Add a 10 µm scale bar to your drawing.
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c Explain why a cell with microvilli absorbs substances more quickly than one without.
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3 Cells called hepatocytes make a lot of a substance called serum albumin. These cells contain many ribosomes. Suggest what type of substance serum albumin is. Explain your reasoning.
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4 Nerve cells require a lot of energy. Suggest the adaptation that allows them to get enough energy.
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5 a State whether a sperm cell is haploid or diploid.
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b Explain why it needs to be like this.
outer membrane folded to form microvilli
food substances
A small intestine cells Cells in an organ called the pancreas make enzymes needed to digest certain foods in the small intestine. The enzymes are proteins and so these cells are adapted by having a lot of ribosomes. The wall of the small intestine has muscles to squeeze food along. The muscle cells require a lot of energy and are adapted by having many mitochondria.
Specialised cells for reproduction During sexual reproduction, two specialised cells (gametes) fuse to create a cell that develops into an embryo. Human gametes are the egg cell and the sperm cell. Most human cell nuclei contain two copies of the 23 different types of chromosome. Gametes contain just one copy of each. This means that the cell produced by fertilisation has two copies. Cells with two sets of chromosomes are diploid and those with one copy of each chromosome are haploid. 6
Specialised cells
The cell membrane fuses with the sperm cell membrane. After fertilisation, the cell membrane becomes hard to stop other sperm cells entering.
The jelly coat protects the egg cell. It also hardens after fertilisation, to ensure that only one sperm cell enters the egg cell.
The cytoplasm is packed with nutrients, to supply the fertilised egg cell with energy and raw materials for the growth and development of the embryo.
haploid nucleus
B adaptations of a human female gamete streamlined shape
nucleus
The tip of the head contains a small vacuole called the acrosome. It contains enzymes that break down the substances in the egg cell’s jelly coat. This allows the sperm cell to burrow inside.
A large number of mitochondria are arranged in a spiral around the top of the tail, to release lots of energy to power the tail. cell surface membrane
The tail waves from side to side, allowing the sperm cell to swim.
6 a Make a drawing of a human egg cell and label its parts.
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b Describe how an egg cell is adapted to prevent more than one sperm cell entering.
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c A human egg cell has a diameter of 0.1Â mm. Calculate the magnification of your drawing.
10 Âľm
C adaptations of a human male gamete Fertilisation occurs in the oviducts of the female reproductive system. Cells in the lining of the oviduct transport egg cells (or the developing embryos after fertilisation) towards the uterus. The oviduct cells are adapted for this function by having hair-like cilia. These are like short sperm cell tails and wave from side to side to sweep substances along. Cells that line structures in the body are called epithelial cells, and epithelial cells with cilia are called ciliated epithelial cells.
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egg cell Cilia are covered in cell membrane and contain strands of a substance that can contract and cause wavy movement.
Checkpoint How confidently can you answer the questions at the top of the previous page? Strengthen D adaptations of oviduct lining cells
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7 Compare and contrast microvilli and cilia.
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8 Explain why an egg cell does not need a tail but a sperm cell does.
S1 List the steps that occur between an egg cell entering an oviduct and it becoming an embryo, and explain how adaptations of specialised cells help each step. Extend
Exam-style question Exam-style questions will follow on publication of the sample assessment materials by Edexcel. Please see www.edexcel.com/gcsesci16 for more details. 7
E1 Explain how both human gametes are adapted to ensure that the cell produced by fertilisation can grow and develop.
CB1d Inside bacteria Specification reference: B1.1; B1.5
Progression questions •• What are the functions of the sub-cellular structures in bacteria? •• What are the differences between prokaryotic and eukaryotic cells? •• How do we change numbers to and from standard form? Bacteria are difficult to see with light microscopes because they are very small and mostly colourless. Stains are often used to make them show up.
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1 a Estimate the size of one bacterium in photo A. Explain your reasoning.
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b The length of the bacterium in photo B is 3.8 cm (without its tail). Calculate its size in real life. Show all your working.
A l ight micrograph of Vibrio cholerae bacteria stained with safranin (field of view = 20 µm)
B e lectron micrograph of Vibrio cholerae bacterium, with colours added by a computer (magnification ×12 600)
The extra magnification and resolution of an electron microscope allow scientists to see bacteria in more detail. Photo B shows that this bacterium has a flagellum, which spins round like a propeller so the bacterium can move. The yellow colour shows its DNA. Bacteria are prokaryotic, which means that their cells do not have nuclei or chromosomes. Instead, the cytoplasm contains one large loop of chromosomal DNA, which controls most of the cell’s activities. There are also smaller loops of DNA, called plasmids (shown in photo C). Plasmid DNA controls a few of the cell’s activities. Prokaryotic cells do not have mitochondria or chloroplasts.
100 nm
C e lectron micrograph of plasmids from a bacterium (magnification ×50 000)
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2 a Describe the location of the plasmid in photo B.
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b Give the name of the substance it is made from.
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c Where is this substance mainly found in a eukaryotic cell?
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3 Describe the function of a bacterium’s flagellum.
Information from microscope images and other work has allowed scientists to discover more about the parts of bacterial cells and their functions, as shown in diagram D. 8
Inside bacteria
flagellum (is not covered in a membrane and not all bacteria have them, but some have many flagella) plasmids chromosomal DNA
slime coat (for protection – not all bacteria have this) flexible cell wall (for support – not made out of cellulose)
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4 Draw a table to show the functions of the different parts of a bacterial cell.
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5 Estimate the diameter of one plasmid shown in photo C.
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6 Suggest why ribosomes are not shown on diagram D.
cell membrane cytoplasm (contains ribosomes, which are smaller than eukaryotic ribosomes)
DD ifferent bacteria are different shapes and sizes but usually have these parts.
Did you know?
Standard form
Your body contains more bacterial cells than human cells. Most of these are found in the digestive system.
A prokaryotic ribosome is 20 nm in diameter and a football is 0.22 m in diameter. It is hard to compare these sizes because they have different units. 1 m is 1 000 000 000 nm, so a football is 220 000 000 nm in diameter. The units are now the same but figures with so many zeros can be difficult to read and use. To solve this problem, we can show figures in the form of a number between 1 and 10 multiplied by a power of 10.
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7 Make a copy of table E from CB1a, adding another column to show in standard form the effect of adding each prefix. For example, ‘milli-’ divides a number by a thousand, which in standard form is the same as multiplying by 10–3.
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8 Write the diameters of a ribosome and a football in metres in standard form.
A × 10n where A is between 1 and 10 and n is the power of 10; n is also called the index number. This is standard form. The index number tells you how many place values to move the digit. For numbers greater than 0, count how many times you need to move the unit to the right until you form a number between 1 and 10. 1 2 3 4 5 6
Write this number as the power of 10, insert the decimal point and remove the zeros.
1150000 = 1.15 × 10
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For numbers less than 0, count how many times you need to move the unit to the left until you form a number between 1 and 10. 8
This becomes a negative power.
Checkpoint
7 6 5 4 3 2 1
How confidently can you answer the questions at the top of the previous page?
0.00000007 = 7 × 10
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Ew riting numbers in standard form
Make sure you know how to input numbers in standard form on your calculator.
Exam-style question Exam-style questions will follow on publication of the sample assessment materials by Edexcel.
Strengthen S1 Draw a bacterium and label its parts, describing what each part does. Extend E1 Compare eukaryotic and prokaryotic cells.
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CB1e Enzymes and nutrition Specification reference: B1.12
Progression questions •• What are enzymes made out of? •• What do enzymes do? •• Why are enzymes important for life? Most animals get substances for energy, growth and development by digesting food inside their bodies. Bacteria, on the other hand, release digestive enzymes into their environments and then absorb digested food into their cells. Starfish use a similar trick for large items of food. In humans, digestive enzymes turn the large molcules in our food into the smaller subunits they are made of. The digested molecules are then small enough to be absorbed by the small intenstine. A To eat large items of food, a starfish pushes its stomach out of its mouth and into the food. The stomach surface releases enzymes to break down the food, which can then be absorbed. part of protein molecule
amino acids breakdown (digestion) synthesis
1 Which small molecules make up the following large molecules? 6
a carbohydrates
6
b proteins
6
c lipids
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2 When you chew a piece of starchy bread for a while it starts to taste sweet. Suggest a reason for this. 3 Which monomers make up:
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a proteins
6
b carbohydrates?
part of starch molecule
lipid molecule
glucose molecules
fatty acids
glycerol
B Large molecules such as carbohydrates, proteins and lipids (fats and oils) are built from smaller molecules.
Once the small molecules are absorbed into the body, they can be used to build the larger molecules that are needed in cells and tissues. Building larger molecules from smaller subunits is known as synthesis. Carbohydrates and proteins are both polymers because they are made up of many similar small molecules, or monomers, joined in a chain. 10
Enzymes and nutrition
The breakdown of large molecules happens incredibly slowly and only if the bonds between the smaller subunits have enough energy to break. Synthesis also happens very slowly, since the subunits rarely collide with enough force or in the right orientation to form a bond. These reactions happen much too slowly to supply all that the body needs to stay alive and be active. Many reactions can be speeded up using a catalyst. In living organisms, the catalysts that speed up breakdown (e.g. digestion) and synthesis reactions are enzymes. So enzymes are biological catalysts that increase the rate of reactions. Enzymes are a special group of proteins that are found throughout the body. The substances that enzymes work on are called substrates, and the substances that are produced are called products. 6
4 Define the term 'biological catalyst'.
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5 a Which type of smaller molecule are enzymes built from?
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b Explain your answer.
Enzyme name
Where found
Reaction catalysed
amylase
saliva and small intestine
breaking down starch to small sugars, such as maltose
catalase
most cells, but especially liver cells
breaking down hydrogen peroxide made in many cell reactions to water and oxygen
starch synthase
plant
synthesis of starch from glucose
DNA polymerase
nucleus
synthesis of DNA from its monomers
C examples of enzymes, where they are found and what they do 6
6 Name the substrate of amylase, and the products of the reaction it catalyses.
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7 Give two examples of processes that are controlled by enzymes in the human body.
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8 Suggest what will happen in the cells of someone who does not make phenylalanine hydroxylase. Explain your answer.
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9 Sketch a diagram or flowchart to explain how the starfish in photo A absorbs food molecules into its body.
Exam-style question Exam-style questions will follow on publication of the sample assessment materials by Edexcel. Please see www.edexcel.com/gcsesci16 for more details. 11
Did you know? The heel prick test takes a small amount of blood to test for several factors, including the enzyme phenylalanine hydroxylase. This enzyme catalyses the breakdown of an amino acid called phenylalanine. A few babies are born without the ability to make the enzyme, which can result in nerve and brain damage as they grow older.
D Babies are given the heel prick test before they are a week old.
Checkpoint How confidently can you answer the questions at the top of the previous page? Strengthen S1 Draw a concept map that includes all the important points on these pages. Link words to show how they are related. Extend E1 Many bacteria have flexible cell walls made by linking together chains of a polymer. The links are formed in reactions catalysed by an enzyme. Penicillin stops this enzyme from working. Explain how penicillin causes bacteria to be weakened.
CB1f Enzyme action Specification reference: B1.7; B1.8
Progression questions •• What is the function of the active site of an enzyme? •• Why do enzymes only work on specific substrates? •• How are enzymes denatured?
Did you know? There are about 3000 different enzymes in the human body, catalysing reactions that would otherwise not occur. For example, an enzyme called OMP decarboxylase helps to produce a substance used to make DNA in 18 milliseconds. Without the enzyme, this reaction would take 78 million years!
A The bombardier beetle repels attackers by releasing a very hot, foul liquid. The liquid is made by enzymes that rapidly break down substances (including hydrogen peroxide) in a reaction chamber at the end of the beetle’s body. A protein is a large three-dimensional (3D) molecule formed from a chain of amino acids. The 3D shape is caused by folding of the chain, which depends on the sequence of the amino acids in the chain. The 3D shape of enzymes is important in how they work, because within that shape is a small pocket called the active site. The active site is where the substrate of the enzyme fits at the start of a reaction. Different substrates have different 3D shapes, and different enzymes have active sites of different shapes. This explains why every enzyme can only work with specific substrates that fit the active site. B Glucose is the substrate for the enzyme hexokinase (blue). The substrate (yellow) fits neatly within the enzyme's active site.
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1 What is the active site of an enzyme?
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2 Why is the active site a different shape in different enzymes?
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3 What is meant by 'enzyme specificity'? 12
Enzyme action
One model of how enzymes work is called the lock-and-key model, because of how the enzyme and substrate fit together. enzyme active site two different substrate molecules
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4 Explain why amylase does not break down proteins.
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5 Use the lock-and-key model to suggest how an amylase enzyme catalyses the breakdown of starch to small sugar molecules.
normal conditions The active site holds the substrate molecules tightly in the right position for bonds to form between them and make a product molecule. enzyme molecule
one product molecule
substrate molecule
extreme conditions shape of active site has changed substrate will no longer fit
The product molecule is a slightly different shape to the substrate molecules so it no longer fits tightly in the active site and is released.
denatured enzyme
C When substrate molecules are held in the right position in the active site, the
D Changes in an enzyme's environment
bonds between them are more easily broken and formed.
can change the shape of the active site.
Changes in pH or temperature can affect how the protein folds up, and so can affect the shape of the active site. If the shape of the active site changes too much, the substrate will no longer fit neatly in it. If the active site changes shape too much, the enzyme will no longer catalyse the reaction. We say that the enzyme has been denatured. 8
6 Explain what denaturing of an enzyme means.
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7 Systems in humans keep our body temperatures constant. Using your knowledge of enzymes, explain why this is important.
Checkpoint How confidently can you answer the questions at the top of the previous page? Strengthen S1 Sketch one flowchart to show how an enzyme normally works, and another to show what happens when the enzyme is denatured. Extend E1 Sketch labelled diagrams to show the following:
Exam-style question
a why enzymes have a particular shape
Exam-style questions will follow on publication of the sample assessment materials by Edexcel.
b why enzymes are specific to a particular substrate
Please see www.edexcel.com/gcsesci16 for more details.
c what happens when an enzyme is denatured. 13
CB1g Enzyme activity Specification reference: B1.9; B1.10; B1.11
Progression questions •• How is enzyme activity affected by temperature, pH and substrate concentration? •• How do you calculate the rate of enzyme activity?
Time taken for complete digestion of 100 g of starch (minutes)
•• Why is enzyme activity affected by temperature, pH and substrate concentration? Enzymes are affected by the conditions in their surroundings. The results of a series of experiments that measure the time taken for an enzyme to complete the breakdown of a substrate at different temperatures can be combined to produce a graph.
Time taken for amylase to digest starch depends on temperature 30 20
6
10 0
0
10
20
30
40
50
60 7
combined on to one graph
35 30 25
c 50 °C.
2 Suggest an explanation for the difference in reaction rates at 40 °C and 50 °C.
100 = 20 g/min 5 Converting the values in graph A in this way gives graph B.
20 15 10
Why does graph B have this shape?
5 0
b 40 °C
Graph A can be converted to a graph showing the rate of reaction by calculating the amount of substrate broken down or product formed in a given time. For example, graph A shows that, at 30 °C, 100 g of starch was broken down in 5 min. The mean rate of reaction was:
How the rate of starch breakdown using amylase depends on temperature Rate of reaction (amount of starch digested) (g/min)
a 10 °C
Temperature (°C)
A results from experiments at different temperatures
1 Use graph A to identify how long it took for the complete breakdown of starch at the following temperatures:
0
10
20
30
40
50
Temperature (°C)
B the data in graph A shown as a rate of reaction graph
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3 a Identify the optimum temperature for the enzyme shown in graph B.
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b Explain your answer. 4 Explain why enzymes work more slowly when the temperature is:
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a below the optimum
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b above the optimum.
60
•• As the temperature increases, molecules move faster. Higher speeds increase the chance of substrate molecules bumping into enzyme molecules and slotting into the active site. •• However, when the temperature gets too high, the shape of the enzyme molecule starts to change. The amount of change increases as the temperature increases. So it becomes more and more difficult for a substrate molecule to fit into the active site. The temperature at which an enzyme works fastest is called its optimum temperature.
Did you know? Many human enzymes have an optimum temperature of around 37 °C. 14
Enzyme activity
Enzymes are now used in many processes in industry. Some industrial processes take place at high temperatures, so the search for new enzymes that have a high optimum temperature is important. Some other factors that affect the rate of an enzyme-controlled reaction are pH and the concentration of the substrate, as shown in graphs D and E. How the rate of an enzyme-controlled reaction depends on pH optimum pH Rate of reaction
C The bacterium Thermus aquaticus was At pHs below and above the optimum, the shape of the active site is affected and so the enzyme does not work so well.
discovered growing in this hotspring pool, which is at about 70 °C.
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5 a Sketch a graph to show the effect of pH on the enzyme pepsin, which has an optimum pH of 2.
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b Annotate your sketch to explain the shape.
pH
D the effect of pH on the rate of an enzyme-controlled reaction
Rate of reaction
How the rate of an enzyme-controlled reaction depends on substrate concentration
Checkpoint
At high concentrations, most enzyme active sites contain substrate molecules, and the rate of reaction is as fast as it can be.
How confidently can you answer the questions at the top of the previous page?
At low concentrations, many enzyme molecules have empty active sites so the rate of reaction is slow.
Strengthen
Substrate concentration
E the effect of substrate concentration on the rate of an enzyme-controlled reaction 9
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6 Explain the effect of substrate concentration on the rate of an enzyme-controlled reaction. 7 The pH in the stomach is about 2, but in the small intestine it is about 6. Explain why different protease enzymes are found in the two digestive organs.
Exam-style question Exam-style questions will follow on publication of the sample assessment materials by Edexcel. Please see www.edexcel.com/gcsesci16 for more details. 15
S1 Close the book, then sketch and annotate a graph to show how temperature affects the rate of an enzyme-controlled reaction. Extend E1 A manufacturer is testing several high-temperature cellulase enzymes to break down plant cell walls in plant waste used for making biofuels. Suggest how the manufacturer might carry out the test and how they would decide which is the best enzyme for this process.
CB1h Transporting substances Specification reference: B1.15; B1.17
Progression questions •• What is the difference between diffusion and osmosis? •• How do cells move substances against a concentration gradient? •• How do you calculate a percentage change in mass? Bacteria living on your body cause body odour. The smelly substances they produce are released into the air and reach our noses. Smells spread by diffusion. Particles in gases and liquids are constantly moving past each other in random directions. This causes an overall movement of particles from where there are more of them (a higher concentration) to where there are fewer (a lower concentration). A difference between two concentrations forms a concentration gradient. Particles diffuse down a concentration gradient. The bigger the difference between concentrations, the steeper the concentration gradient and the faster diffusion occurs. The number of particles decreases as you go down a concentration gradient.
diffusion
A an experiment to assess body odour 6
1 Explain why smells spread.
7
2 a A dish of perfume is put at the front of a lab. Describe the perfume’s concentration gradient after 5 minutes.
B diffusion occurs down a concentration gradient
7
b Describe the overall movement of the perfume molecules.
A membrane that allows some molecules through and not others is semi‑permeable.
7
3 Muscle cells in the leg use up oxygen but are surrounded by a fluid containing a lot of oxygen. Explain why oxygen moves into the cells.
higher concentration
lower concentration
Diffusion allows small molecules (such as oxygen and carbon dioxide) to move into and out of cells.
Osmosis
Cell membranes are semi-permeable and trap large soluble molecules inside cells, but water molecules can diffuse through the membrane. If there are more water molecules in a certain volume on one side of the membrane than the other, there will be an overall movement of water molecules from the side where there are more water molecules (a more dilute solute concentration) to the side where there are fewer water molecules (a more concentrated solution of solute). This diffusion of small molecules of a solvent, such as water, through a semi-permeable membrane is called osmosis. The overall movement of solvent molecules will stop when the concentration of solutes is the same on both sides of a membrane. 16
Transporting substances
7
4 a In diagram C, in which direction will water flow, X to Y or Y to X?
9
b Explain why this flow occurs.
9
5 Red blood cells contain many solute molecules. Explain why red blood cells burst if put in pure water.
soluble molecule that is too large to pass through the membrane (e.g. sucrose)
partially permeable membrane allows molecules to pass through if they are small enough
Osmosis can cause tissues to gain or lose mass. To calculate the mass change: •• work out the difference between the mass of tissue at the start and at the end (start mass – final mass) •• divide this difference by the start mass
water molecule X
Y
•• multiply by 100. So, percentage change in mass = (start mass – final mass) × 100 start mass A negative answer is a percentage loss in mass. 9
6 An 8 g piece of potato is left in water for an hour. Its mass becomes 8.5 g. Calculate the percentage change in mass.
Active transport Cells may need to transport molecules against a concentration gradient or transport molecules that are too big to diffuse through the cell membrane. They can do this using active transport. This process is carried out by transport proteins in cell membranes. The transport proteins capture certain molecules and carry them across the cell membrane. This is an active process and so requires energy. Osmosis and diffusion are passive processes, so do not require an input of energy.
concentration gradient
cell membrane
C In osmosis, a solvent flows from a dilute solution of a solute to a more concentrated one. 7
7 Look at diagram D. Explain why active transport is needed to move the molecules.
Checkpoint How confidently can you answer the questions at the top of the previous page?
transporter protein
Strengthen A molecule sticks to the transporter protein.
energy The transporter protein changes shape and carries the molecule across the cell membrane.
D active transport 7
more more dilute concentrated solution solution
8 Explain how cells that carry out a lot of active transport would be adapted to their function.
Exam-style question Exam-style questions will follow on publication of the sample assessment materials by Edexcel. Please see www.edexcel.com/gcsesci16 for more details. 17
S1 A small number of sugar molecules are in your small intestine. Describe how they will be absorbed into cells in the small intestine and why they need to be absorbed in this way. Extend E1 Sorbitol is a sweet-tasting substance that is not broken down or absorbed by the body. It is used in some sugar-free sweets. Explain why eating too many of these sweets can cause diarrhoea.