CELL METABOLISM
(a) Metabolic pathways
Key concepts
1.
Metabolic pathways involve biosynthetic processes (anabolism) and the breakdown of molecules (catabolism) to provide energy and building blocks.
2.
Synthetic pathways require the input of energy.
3.
Pathways that break down molecules usually release energy.
4.
Metabolic pathways can have reversible and irreversible steps .
5.
Alternative routes may exist that can bypass steps in a pathway.
New content areas
Students have previously only been asked to consider individual pathways.
This section requires students to understand that many reactions in the human body are linked together to form pathways. The words anabolism and catabolism may also be new to students.
Background information
Many students will associate metabolism with digestion as these are commonly linked in everyday life. It is important that in the introduction to this topic it is emphasised that this is a misconception and that metabolism refers to all the enzyme-controlled reactions within the human body and the pathways that they form.
Students should already be aware that reactions can involve the breakdown or synthesis of molecules to provide energy and building blocks to be used in other reactions. Depending on their level of chemistry knowledge they may or may not understand that synthetic reactions require energy and breakdown reactions release energy. Some time should also be spent discussing the reversible or irreversible nature of reaction s and that there may be alternative routes for pathways to follow. The coverage of these key points should form an introduction to the topic of metabolic pathways on which the role of enzymes can then be built. Specific examples of metabolic pathways will be covered later in this section.
CELL METABOLISM (H, HUMAN BIOLOGY) 1
© Learning and Teaching Scotland 2011

CELL METABOLISM
Further reading and other resources
Metabolic pathways
This PowerPoint is designed as a brief introduction to this section of work. It can be used to introduce the concept of metabolic pathways . Some slides pose a question and can be used to stimulat e class discussion.
2 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM
Key concepts
1.
Reactions are controlled by the presence or absence of particular enzymes.
2.
The rate of reaction of key enzymes in a pathway can be regulated.
3.
Regulation can be controlled by intra - or extracellular signal molecules.
4.
The induced fit concept of enzyme action .
5.
The active site orientates reactants, lowers the activation energy of the transition state and releases products with a low affinity for the active site.
6.
The effect of substrate and end product concentrations on the direction and rate of enzyme reactions.
7.
Enzymes often act in groups or as multi -enzyme complexes.
8.
Genes for some enzymes are continuously expressed .
9.
Non-competitive inhibition or stimulation of enzyme activity by the binding of molecules that change the shape of the active site .
10.
Competitive inhibitors compete for the active site and resemble the substrate.
11.
Competitive inhibition is reversed by increasing the substrate concentration.
12.
End products can inhibit an enzyme that c atalyses a reaction early in a pathway and therefore control the pathway.
Prerequisite knowledge
The properties and functions of catalysts: lower the energy input required for chemical reactions, speed up chemical reactions, take part in reactions but remain unchanged.
Enzymes are biological catalysts made by all living ce lls. They are made of protein. Named examples of enzymes involved in degradation and in synthesis reactions, including their substrates and products .
The characteristic shape of an enzyme molecule is complementary to its substrate. The presence of a specific active site as the location of substrate binding.
New content areas
Although enzymes were covered in the previous Higher Human Biology course, students are now required to know further details of their action.
CELL METABOLISM (H, HUMAN BIOLOGY) 3
© Learning and Teaching Scotland 2011

CELL METABOLISM

Metabolic pathways are controlled by the presence or absence of particular enzymes in the metabolic pathway and through the regulation of the rate of reaction of key enzymes within the pathway.

Regulation can be controlled by either intra- or extracellular signal molecules (inhibition was previously covered but no mention was made of the location of signal molecules).

The activity of enzymes depends on their flexible and dynamic shape.

The affinity of substrate molecules f or the active site of an enzyme and induced fit.

The role of the active site in orientating reactants, lowering the activation energy of the transition state and the release of products with low affinity for the active site.

The effects of substrate and end product concentration on the direction and rate of enzyme reactions.

Most metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of reactions in a particular direction.

Enzymes often act in groups or as multi-enzyme complexes.

Genes for some enzymes are continuously expressed. These enzymes are always present in the cell and they are controlled through the regulation of their rates of reaction.

The control of metabolic pathways by feedback inhibition where an end product binds to an enzyme that catalyses a reaction early in the pathway.
Background information
Students are now looking beyond simple enzyme action towards the control of both enzymes and whole pathways. Pathway activity is controlled by both the presence and absence of a particular enzyme. The rate of enzyme reaction can also be regulated. This regulation is the result of intra - or extracellular signal molecules. A good example of this is lactose metabolism in E. coli .
The lac operon controls the expression of the gene for β -galactosidase, which is only expressed when lactose is present. In this case lactose is an example of an extracellular signal molecule. It should be noted that the lac operon is not specifically mentioned in the Arrangements document.
Enzyme activity depends on its flexible and dynamic shape. Until this point students have thought of enzymes as having a fixed shape, with an active site where the substrate binds. They must now move away from the ‘lock and ke y’ model of enzyme action towards the induced fit model. In the induced fit model the substrate has a high affinity for the active site. When the substrate initially interacts with the active site the enzyme changes shap e slightly to allow a stronger interaction or ‘better fit’ between the substrate and the active site. It might help for students to think about a hand closing around a door handle.
4 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM
When a substrate binds to the active site the interactions between the two pull the reactants into a more favourable orientation, eg certain chemical groups can be brought closer together to increase the chance of a reaction occurring.
This state is known as the transition state and due to its favourable orientation it has a low activation energy. Once the reaction has occurred the orientation of the products will change slightly, again reducing their affinity with the active site, therefore allowing them to be release d.
Students are likely to be aware that substrate concentration is one factor affecting an enzyme’s rate of reaction. They should now be introduced to the effects of the build-up of product and its likely inhibiting effect. Most metabolic reactions are reversible and the presence of a substrate or the removal of a product will drive a sequence of rea ctions in a particular direction.
Until this point students will probably have thought of enzymes as independent molecules but they must understand that enzymes often act in groups or as multi-enzyme complexes. The close proximity of the different enzymes increases the rate of reaction in a pathway. Many of the reactions involved in respiration are catalysed by multi -enzyme complexes, including the electron transport chain. DNA and RNA polymerases are also examples of multi-enzyme complexes.
The genes for some enzymes are continually expressed , leading to the permanent presence of these enzymes within cells. When enzymes are always present in a cell, their control involves the regulation of their rate of reaction.
Non-competitive inhibition or stimulation of an enzyme involves the binding of a molecule at an allosteric site on the enzyme away from the active site.
This binding causes a conformational change in the shape of the active site, either allowing or preventing the binding of the substrate. Heavy metals such as lead are non-competitive inhibitors.
Competitive inhibitors have a structure that resembles that of an enzyme’s substrate. This allows it the bind to the active site , preventing the binding of the substrate. This can be overcome by increasing the substrate concentration.
The end product of a pathway can act as an inhibitor to an enzyme earlier in the pathway. This is known as feedback inhibition. Once levels of the end product are reduced the inhibition is removed and the pathway can procee d again. This prevents a wasteful build-up of product occurring.
© Learning and Teaching Scotland 2011
CELL METABOLISM (H, HUMAN BIOLOGY) 5

CELL METABOLISM
Further reading and other resources
Science and Plants for Schools (SAPS) have developed a variety of practicals that would be suitable for this section of the course. http://www-saps.plantsci.cam.ac.uk/prac_enzymes.htm
1.
Phosphatase enzymes in plants.
2.
Microscale investigations with catalase .
3. The inhibition of catechol oxidase by lead .
4. The effect of competitive and non -competitive inhibitors on the enzyme
β-galactosidase.
5. The effect of the end product, phosphate, on the enzyme phosphatase .
6.
Induction of the lac operon in E. coli .
This experiment can be used to highlight the ability of cells to switch genes on and off to control the presence or absence of enzymes at any given time.
http://programs.northlandcollege.edu/biology/Bio logy1111/animations/e nzyme.html
A good, simple animation showing inhibition of enzymes. http://www.lpscience.fatcow.com/jwanamaker/animations/Enzyme% 20a ctivity.html
Possibly best for student/student lead revision of enzymes. May be good for any student doing a crash higher.
6 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM
1. How can the rate of metabolic pathways be controlled?
This activity involves carrying out a simple pract ical to illustrate the breakdown of hydrogen peroxide using catalase. Students should be able to explain what is happening from prior knowledge. Most schools will already have risk assessments in place for this experiment as it is commonly carried out in several current courses.
2. Enzymes prior knowledge quiz
Students can use this to identify areas that they may need to revise. Can be done as a straightforward quiz or students could traffic light their answers to highlight areas where they feel they need revision.
3. Induced fit
The website www.chem.ucsb.edu/~molvisual/ABLE/induced_fit/index.html has diagrams and descriptions of induced fit in the enzyme hexokinase. If computer access is available the site can be worked through individually by students. If this is not possible it could be talked through as a class. Students are then asked to complete a poster comparing and contrasting the lock and key model with the induced fit model of enzyme action. This could be carried out as a co-operative learning exercise. The third part of this activity asks students to consider why an enzyme binding increases the rate of reaction. This lends itself to think, pair, share before feeding back in a whole-class discussion.
4. The effect of increasing substrate concentration on reaction rates
Students should work in groups to design an experiment to investigate the effect of increasing substrate concentration on the rate of breakdown of hydrogen peroxide by catalase. Most students should be familiar with this experiment from previous courses and vegetable s containing the enzyme are cheap and widely available. It is also easy to dilute hydrogen peroxide to obtain different concentrations.
The second part of this activity is a paper-based exercise looking in more detail at what is happening.
5. Inhibition
This activity is based on an experiment looking at the inhibition of βgalactosidase by galactose. Students could carry out this experiment and use their own results in the analysis, but the PowerPoint does include exemplar results. Alternatively, the slides could be printed out and students asked to work through them. The experimental procedure can
CELL METABOLISM (H, HUMAN BIOLOGY) 7
© Learning and Teaching Scotland 2011

CELL METABOLISM be found at the effect of competitive and non -competitive inhibitors on the enzyme β-galactosidase.
Hyperlinks to animations showing each of the three types of inhibition are also included in the PowerPoint.
6. Metabolic reactions card sort
Students work in groups to discuss each card and sort them into piles of true or false.
8 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM
Key concepts

The metabolic pathways of cellular respiration are central to metabolism as they yield energy and are linked to many other pathways .

Glucose is broken down in a series of enzyme -controlled steps.

Hydrogen and high-energy electrons are removed by dehydrogenase enzymes.

Adenosine triphosphate (ATP) is used to transfer the energy released from cellular respiration.

ATP is broken down into ADP and phosphate , releasing energy.

ATP is regenerated from ADP and phosphate using t he energy released from cellular respiration.

Molecules can be phosphorylated to alter their reactivity.

ATP is generated by using high-energy electrons to pump H ions across a highly selective membrane.

The return flow of the H ions rotates part of the membrane protein ATP synthase, catalysing the synthesis of ATP.

Glycolysis is the break down of glucose to pyruvate.

Energy is put into the pathway initially by phosphorylation of intermediates (investment phase).

The direct generation of ATP is an energy pa y-off stage.

The first phosphorylation leads to a product that can continue to a number of pathways.

The second phosphorylation is catalysed by phosphofructokinase and is an irriversable reaction leading only to the glycolytic pathway.

Pyruvate progresses to the citric acid cycle only if oxygen is available .

Pyruvate is broken down to an acetyl compound that combines with coenzyme A, forming acetyl coenzyme A.

Acetyl coenzyme A combines with oxaloacetate to form citrate.

A further series of enzyme-controlled reaction, called the citric acid cycle, reforms oxaloacetate.

During this series of reactions some ATP is generated and carbon dioxide is released.

Also during glycolysis and the citric acid cycle dehydrogenase enzymes transfer hydrogen ions with the associated high-energy electrons to carriers NAD and FAD, forming NADH and FADH
2
.

These carriers release the electrons to the electron transport chain and release energy in a cascade reaction.

The electron transport chain is a collection of proteins attached to a membrane.

The energy is used to pump H ions across the inner membrane of the mitochondrion.

Their return flow drives ATP synthase and results in the production of ATP.
CELL METABOLISM (H, HUMAN BIOLOGY) 9
© Learning and Teaching Scotland 2011

CELL METABOLISM

The electrons finally combine with oxygen and hydrogen ions to form water.

Starch and glycogen can be broken down to glucose for respiration.

Other sugar molecules can be converted to glucose or glycolosis intermediates.

Fats are reduced to intermediates of glycolysis and the citric acid cycle.

Proteins digested to amino acids are converted to intermediates of glycolysis and the citric acid cycle.

Phosphofructokinase activity can be inhibited by ATP and citric acid.

This allows the cell to conserve its resources and only produc e ATP when required.
Prerequisite knowledge
Students should know the general equation for respiration. They should also know what is meant by a metabolic pathway and the role enzymes play in their control, with particular reference to end product inhibition. Students also require to know a little about mitochondri a. This may have to be covered if cell ultrastructure is new.
New content areas
Cellular respiration was covered in the previous Higher Human Biology course therefore there are only a few details that are new to this course.
ATP

The metabolic pathways of cellular respiration are of central importance to cells. They yield energy and are connected to many other pathways.

Glucose is broken down in a series of enzyme -controlled steps. Hydrogen and high-energy electrons are removed by dehydrogenase enzymes an d used to yield ATP.

To synthesise the bulk of its ATP requirements, a cell uses a source of high-energy electrons to pump H ions across a membrane. The return flow of these ions rotates part of the membrane protein ATP synthase, catalysing the synthesis of ATP.
Glycolysis

The breakdown of glucose to pyruvate during glycolysis. The phosphorylation of intermediates in glycolysis in an energy investment phase and the direct generation of ATP in an energy pay -off stage.

The first phosphorylation leads to a product that can continue to a number of pathways and the second phosphorylation, catalysed by phosphofructokinase, is an irreversible reaction leading only to the glycolytic pathway.
10 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM
Citric acid cycle

Acetyl coenzyme A combines with oxaloacetate to form citrate, followed by the enzyme-mediated steps of the citric acid cycle , with some generation of ATP, the release of carbon dioxide and the regeneration of oxaloacetate.
Electron transport chain

The electron transport chain is a collection of proteins attached to a membrane. At certain steps in the glycolytic and citric acid pathways, dehydrogenase enzymes remove hydrogen ions from the substrate along with associated high-energy electrons. These hydrogen ions and high energy electrons are passed to the coenzymes NAD or FAD, forming
NADH or FADH
2
.

NADH and FADH
2
release the high-energy electrons to the electron transport chain, where they cascade down the chain, releasing energy. The energy is used to pump H ions across the inner mitochondrial membra ne.
The return flow of H ions drives ATP synthase and produces the bulk of the ATP generated by cellular respiration. The final electron acceptor is oxygen, which then combines with hydrogen ions and electrons to form water.
Regulation

Phosphofructokinase activity can be inhibited by ATP and citric acid.
These feedback mechanisms help to synchronise the activity of glycolysis and the citric acid cycle to ensure the cell conserves its resources by only producing ATP from cellular respiration when it is req uired.
Background information
Cellular respiration has previously been covered in both the Higher Biology and Higher Human Biology courses, and so there is a wealth of resources available to cover this topic. It should be noted that the level of detail students are required to cover has increased and so many of the current resources may have to be altered.
It should be emphasised that cellular respiration is central to metabolism and is linked to many other pathways, including the creatine phosphate syst em and lactic acid metabolism. Both of these will be covered later in the unit.
When glucose is broken down, hydrogen and high-energy electrons are removed by dehydrogenase enzymes and used to yield ATP.
ATP
ATP is used to transfer the energy released fro m cellular respiration to synthetic pathways and other cellular processes where energy is required.
When ATP is broken down into ADP and phosphate energy is released. ATP
CELL METABOLISM (H, HUMAN BIOLOGY) 11
© Learning and Teaching Scotland 2011

CELL METABOLISM can be regenerated using the energy released from aerobic respiration. To synthesis the bulk of its ATP a cell uses a source of high -energy electrons to pump hydrogen ions across a membrane. The return flow of these ions rotates part of the membrane protein ATP synthase, catalysing the synthesis of ATP.
Glycolysis
Glucose is broken down into pyruvate. This involves two phosphorylation reactions. The first leads to an intermediate molecule that can continue on to a number of pathways. The second is an irreversible reaction leading only to the glycolytic pathway and the production of pyruv ate. This second phosphorylation is catalysed by phosphofructokinase. The first phosphorylation requires energy ( two ATP) and so is an energy investment phase. The second reaction produces four ATP in an energy pay-off stage.
This means there is a net gain of two ATP. Pyruvate will progress to the citric acid cycle if oxygen is available.
Citric acid cycle
Pyruvate is broken down to an acetyl group tha t combines with coenzyme A to be transferred to the citric acid cycle as acetyl coenzyme A. Acetyl coenzyme A combines with oxaloacetate to form citrate. This is followed by the enzyme-mediated steps of the citric acid cycle , with some generation of
ATP, the release of two carbon dioxide molecules and the regeneration of oxaloacetate. Oxaloacetate is the only molecule in the cycle that is specifically named in the Arrangements so the details of intermediates do not need to be learned. It should also be noted that students are not required to know the number of carbons in the main compounds but th is knowledge may help them to follow the steps in the process.
The proper names for the cyclic oxidation of substrates in the mitochondria matrix are citric acid cycle or tricarboxylic acid cycle . However, many people refer to the process as the Krebs cycle in recogniti on of the contribution of Hans Krebs to the discovery. Krebs, a German biochemist, first postulated the mechanism in 1937, under the name citric acid cycle .
Electron transport chain
This is a collection of proteins attached to a membrane. At certain steps in the glycolytic and citric acid pathways (the students do not need to specify where) dehydrogenase enzymes remove hydrogen ions from the substrate along with associated high-energy electrons. These hydrogen ions and high energy electrons are passed to the coenzymes NAD or FAD, forming NADH or FADH
2
. NADH and FADH
2
release the high-energy electrons to the electron transport chain, where they cascade down the chain, releasing energy. The energy is used to pump H ions across the inner mitochondrial membrane. The return flow of hydrogen ions drives ATP synthase and produces the bulk of the ATP generated by cellular respiration. The final
12 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM electron acceptor is oxygen, which combines with hydrogen ions and electrons to form water.
Substrates for respiration
When glucose is in short supply other molecules can be used to provide energy. Starch and glycogen are broken down to glucose for use as a respiratory substrate. Other sugar molecules can be converted to glucose or glycolysis intermediates for use as respi ratory substrates. Proteins can be broken down to amino acids and converted to intermediates of glycolysis and the citric acid cycle for use as respiratory substrates. Fats can also be broken down to intermediates of glycolysis and the citric acid cycle.
Regulation
Phosphofructokinase activity can be inhibited by ATP and citric acid. These feedback mechanisms help to synchronise the activity of glycolysis and the citric acid cycle to ensure the cell conserves its resources by only producing
ATP from cellular respiration when required.
Areas of difficulty
The structure of the mitochondrion may need to be revised. The action of ATP synthase can be demonstrated by use of computer graphics.
Students may need extra help with the concept of hydrogen carrier mo lecules.
Further reading and other resources
ATP http://kentsimmons.uwinnipeg.ca/cm1504/atp.htm
Simple details with diagrams. http://www.youtube.com/watch?v=TgJt4KgKQJI&feature=related
Useful video animation of a mitochondrion , showing its role in ATP synthesis.
Respiration
Krebs’ own account of the history of the discovery of the cycle can be found in his article, ‘The history of the tricarboxylic acid cycle’, Perspect. Biol.
Med.
, 14 : 154–170 (1970). http://www.learnerstv.com/animation/biology/tca.swf
Good animation with some interactive activities. Some sections are a bit complicated. http://www.wiley.com/college/pratt/0471393878/student/animations/citric_aci d_cycle/index.html
A good animation but quite complicated. http://www.sp.uconn.edu/~terry/images/anim/ETS.html
Animation of electron transport system
CELL METABOLISM (H, HUMAN BIOLOGY) 13
© Learning and Teaching Scotland 2011

CELL METABOLISM
1. Making models of ATP and ADP
This simple activity allows students to visualise ATP. They can use polystyrene balls to make models of ATP and demonstrate its conversion to ADP + Pi. Two sizes of ball are needed and can be held together with cocktail sticks. If polystyrene balls are not available plasticine works just as well.
2. Cellular respiration
This worksheet allows students to look at the chemistry of respiration.
3. Respiration summary activity
Students can use labels to complete a diagram of cellular respiration.
This could be completed independently or as a group to assess understanding. It may be beneficial to print this diagram out on A3 paper. This activity also provides an opportunity to self - or peer-assess understanding if the answer sheet is made available.
4. The citric acid cycle
Experiments can be carried out on the inhibition of the citric acid cycle with malic acid and Dichlorophenolindophenol (DCPIP )as an indicator of dehydrogenase activity. This experiment replicates that of Krebs , who also looked at what he described as the effe cts of malonate poisoning.
5. Dehydrogenase experiments
The electron transport chain can be illustrated by carrying out experiments with yeast dehydrogenase. Yeast dehydrogenase catalyses the removal of hydrogen from glucose. This can be visualised using resazurin. When resazurin is reduced it changes colour from blue to colourless. A protocol for this experiment can be found in the previous
Higher Human Biology support materials: http://www.ltscotland.org.uk/Images/3644biol2_tcm4 -124455.pdf This experiment is similar to those carried out during the original discovery of the respiration pathway.
6. Substrates for respiration
The above practical can be developed to allow students to investigate substrates for respiration. The time taken for resazurin to become colourless using different sugars as respiratory substr ates can be measured. This indicates the different rates of reaction occurring.
7. Who am I?
This consolidation exercise can be carried out as a whole class or in small groups. The cards are distributed amongst the students. One
14 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM person begins by reading the statement at the bottom of their card. The person who has the card with the correct answer on it reads out the answer and then their statement and so on until all the cards have been read.
8. Familiarisation of biochemical vocabulary
Students can use classroom resources or the internet to create a glossary of new vocabulary. This will be specific to each individual , depending on the route they have taken to the Higher Human Biology course.
Bingo cards can be used as part of a game to test the students’ vocabulary knowledge. Examples can be found at http://www.bingocardcreator.com/bingo -cards/biology/parts-of-a-cell.
© Learning and Teaching Scotland 2011
CELL METABOLISM (H, HUMAN BIOLOGY) 15

CELL METABOLISM
Key concepts
1.
During strenuous activity muscle cells break down ATP, releasing ADP, phosphate and energy.
2.
Creatine phosphate breaks down to provide energy and phosphate to convert ADP to ATP by phosphorylation.
3.
This process sustains maximal muscle contraction for a short period of time.
4.
When demand for energy in muscle is low, ATP acts as a source of phosphate for the phosphorylation of phosphate into creatine phosphate.
5.
Creatine phosphate acts as a store of energy for the muscle tissue.
Prerequisite knowledge
Students require to understand the structure and function of ATP/ADP and its link with energy. They also should have covered a erobic respiration.
New content areas
This section is entirely new to students.
Background information
When an athlete runs a 100-metre sprint he or she requires maximal muscle contraction for a short period of time. During strenuous activity such as this muscle cells break down ATP, releasing ADP and phosphate, along with energy. ATP must be replaced quickly for further release of energy to occur. It is this process that involves creatine phosphate. Creatine phosphate is found in muscle cells and is broken down to provide energy and phosphate to convert ADP to ATP by phosphorylation.
When demand for energy in the muscles is low, ATP produced by cellular respiration acts as a source of phosphate for the phosphorylation of creatine into creatine phosphate. The creatine phosphate created acts as a store of energy for the muscle tissue.
16 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM
Further reading and other resources http://www.pponline.co.uk/encyc/creatine -supplements-can-boost-yourendurance-training-without-encouraging-weight-gain-143
Information ion creatine in sport . http://www.rice.edu/~jenky/sports/creatine.html
Creatine supplementation in athletes: a review.
1. The use of creatine suppliments
This PowerPoint can be used to facilitate either group or whole -class discussion of the creatine phosphate system. The student’s understanding can then be assessed by the production of a magazine article.
© Learning and Teaching Scotland 2011
CELL METABOLISM (H, HUMAN BIOLOGY) 17

CELL METABOLISM
Key concepts
1.
During vigorous exercise the muscles do not get sufficient oxygen to support the electron transport chain.
2.
Under these conditions pyruvate is converted to lactic acid.
3.
This conversion involves the transfer of hydrogen from the NADH produced during glycolysis to pyruvic acid , to produce lactic acid.
4.
This regenerates the NAD needed to maintain ATP production through glycolysis.
5.
Lactic acid accumulates in the muscles, causing fatigue.
6.
Oxygen debt is repaid when exercise is complete and respiration can provide the energy to convert lactic acid back to pyruvic acid and glucose.
7.
This occurs in the liver.
Prerequisite knowledge
Students require an understanding of the process of aerobic respiration.
New content areas
Although students will probably be familiar with the production of lactic acid during anaerobic respiration the chemical processes involved in its production and removal will be new.
Background information
During vigorous exercise, the muscle cells do not get sufficient oxygen to support the electron transport chain. Under these conditions, pyruvate is converted into lactic acid. The muscles are described as respiring anaerobically. The conversion of pyruvate to lactic acid involves the transfer of hydrogen from the NADH produced during glycolysis to pyruvic acid to produce lactic acid. This regenerates the NAD needed to maintain ATP production through glycolysis.
When muscles respire anaerobically lactic acid accumulates in muscles, causing fatigue. This manifests itself as a reduction in the muscle’s contractile ability. This oxygen debt is repayed when exercise is complete as aerobic respiration provides the energy to convert lactic acid back to pyruvic acid and glucose in the liver.
18 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM
Further reading and other resources http://www2.estrellamountain.edu/faculty/farabee/biobk/biobooktoc.html
Section 12 has a nice diagram showing lactic acid metabolism and the cycling of NAD.
1. Lactic acid metabolism
This activity is a straightforward worksheet in which students link lactic acid metabolism into their knowledge of respiratio n.
© Learning and Teaching Scotland 2011
CELL METABOLISM (H, HUMAN BIOLOGY) 19

CELL METABOLISM
Key concepts
1.
Skeletal muscles consist of slow twitch and fast twitch muscle fibres.
2.
Slow twitch muscle fibres contract more slowly but can sustain contractions for longer.
3.
These are good for endurance activities like long-distance running, cycling or cross-country ski-ing.
4.
They rely on aerobic respiration to generate ATP.
5.
They have many mitochondria, a large blood supply and a high concentration of myoglobin.
6.
The major storage fuel of these muscle fibres is fats .
7.
Fast twitch muscle fibres contract quickly and cannot sustain contractions.
8.
These are good for activities like sprinting or weightlifting.
9.
They generate ATP through glycolysis only.
10.
They have few mitochondria and a low blood supply.
11.
The major storage fuels of these muscle fibres are glycogen and creatine phosphate.
Prerequisite knowledge
Students require to have an understanding of the steps in aerobic respiration and its role in energy production. They should also understand the creatine phosphate system.
New content areas
The details of slow and fast twitch muscle fibres are entirely new to the
Higher Human Biology course.
Background information
Most human muscles contain a mixture of both slow (type 1) and fast (type 2) twitch muscle fibres. Slow twitch muscle fibres contact more slowly, but can sustain contractions for longer. These muscle fibres are good for endurance activities like long-distance running, cycling or cross-country ski-ing. Slow twitch muscle fibres rely on aerobic respiration to g enerate ATP and have many mitochondria, a large blood supply and a high concentration of the oxygen-storing protein myoglobin. The major storage fuel of slow twitch muscle fibres is fats.
Fast twitch muscle fibres contract more quickly, but cannot sustain contractions for as long as slow twitch muscle fibres. These muscle fibres are
20 CELL METABOLISM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011

CELL METABOLISM good for activities like sprinting and weightlifting. Fast twitch muscle fibres can generate ATP through glycolysis only and have few mitochondria and a lower blood supply than slow twitch muscle fibres. The major storage fuels of fast twitch muscle fibres are glycogen and creatine phosphate.
Athletes show distinct patterns of muscle fibres that reflect their sporting activities. Sprinters have a much higher proportion of fas t twitch muscle fibres and endurance athletes have a much higher proportion of slow twitch muscle fibres. It is generally accepted that the distribution of muscle fibre type cannot be changed through training so although not a guarantee of success in a particular sport, it is certainly a contributing factor. It is also accepted that the number of fast twitch fibres in a muscle decreases with age.
Further reading and other resources http://www.bbc.co.uk/science/humanbody/body/factfiles/fastandslowtwitch/so leus.shtml
Good general information on muscle fibre types. http://run-down.com/guests/je_black_athletes_p2.php
An article summarising the success of black athletes and discussing some of the science involved.
1. Comparison of muscle composition in elite athletes
This activity allows students to investigate muscle fibres using data.
They have to collate information from different sources to gain a full understanding of the role of muscle fibres in sport.
© Learning and Teaching Scotland 2011
CELL METABOLISM (H, HUMAN BIOLOGY) 21