Secondary Science Review

Staying dryInsects in the rain

Volume 23Number 3February 2013

Contents1 Speak to me, lichen Students of LSU School, London

4 Fracking: an energy revolution Thomas Lewton

6 Memory myths Lawrence Patihis, Ian W. Tingen, Elizabeth F. Loftus

9 Flying dry David Sang

13 Thermometry – a hot topic Mike Follows

16 Try this: Eating chocolate Vicky Wong

17 Uncovering the magical world of signalling Suzy Moody

20 Adventures in the Amazon Laura Plant

22 Forest fieldwork Laura Plant

Subscription informationCatalyst is published four times each academic year, in October, December, February and April. A free copy of each issue is available by request to individuals who are professionally involved in 14-19 science teaching in the UK and who are registered with the National STEM Centre. Teachers should visit www.nationalstemcentre.org.uk to find out how to register.

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David Sang PhysicsBrighton

Vicky Wong ChemistryDidcot

Gary Skinner BiologyHalifax

Editorial team

Editorial contact: 01273 562139 or catalyst@sep.org.uk

Fieldwork, labworkOn pages 1-3 of this issue of Catalyst, a group of students from La Sainte Union Catholic School in north London describe how they investigated the correlation between the lichen species growing on trees near their school and the level of atmospheric pollution. This study was part of the Silver Crest Award scheme, run by the British Science Association (http://www.britishscienceassociation.org/crest), and their work drew on the OPAL project (http://www.opalexplorenature.org).

Laura Plant, an older student, had the chance to botanical fieldwork in the Amazon rainforest of Peru. She describes her work and shares some of her photographs on pages 20-22.

While Biology provides plenty of opportunities for fieldwork, other secrets of nature have to be uncovered in the lab. On pages 9-12, we look at the work of a group of scientists and engineers who wanted to know just how mosquitoes manage to survive outside in the rain.

Published by the Gatsby Science Enhancement ProgrammeGatsby Technical Education ProjectsThe Peak5 Wilton RoadLondon SW1V 1AP

© 2013 Gatsby Technical Education ProjectsISSN 0958-3629 (print) ISSN 2047-7430 (electronic)

Design and Artwork: Pluma Design

The cover image shows a mosquito in an artificial rain shower. High-speed photography has revealed the strategies used by mosquitoes and other insects to survive when they are caught in a shower. See the article on pages 9-12. (Image courtesy of Tim Nowack.)

Volume 23 Number 3 February 2013

The Catalyst archiveOver 300 articles from past issues of Catalyst are freely available in pdf format from the National STEM Centre (www.nationalstemcentre.org.uk/catalyst).

Students: We have now created a website specially for you where you can browse hundreds of articles from past issues of Catalyst and find out how to subscribe.www.catalyststudent.org.uk

1Catalyst February 2013

Key words

air pollution

lichen

quadrat

identification

Speak to me, lichenHow clean is the air?

OS

U L

iche

n G

roup

.

Two lichen species: Parmelia sulcata is nitrogen-

sensitive, while Xanthoria parietina is nitrogen-tolerant.

W e are students at La Sainte Union Catholic School in north London. For our Silver CREST Project,

we explored the relationship between levels of the pollutant nitrogen dioxide (NO

2) and the

distribution of indicator lichens in the vicinity of our school. Lichens are indicators of changes in air quality; tolerant species replace those sensitive to a given pollutant. This effect is observed across urban and rural Britain, especially in regions where oxidised and reduced forms of nitrogen are present. NO

2 is the dominant air pollutant in

urban areas, due to pollution from road traffic. Therefore, measuring levels of NO

2 can indicate

the quality of our town air.

What lichens can tell usLichens are composed of two different organisms living symbiotically; fungus and alga. They absorb atmospheric moisture, rain water and minerals over their entire surface area. This makes them extremely sensitive to atmospheric pollution (such as NO2) and therefore very good biological indicators of levels of atmospheric pollution. Lichens can be placed into three categories:

•Nitrogen-sensitive; found in clean, non-polluted conditions

• Intermediate; found in clean AND polluted conditions

•Nitrogen-loving; found in conditions where levels of nitrogen dioxide are particularly high.

We monitored NO2 pollution by placing NO2-diffusion tubes at different sites (see below) and mapped the distribution of lichens on trees at different locations along and either side of Highgate Road.

Students of LSU School

London

2 Catalyst February 2013

Our hypotheses were:

•The concentration of pollutants would be highest along the main road (Highgate Road).

•There would be a correlation between data from the NO2 diffusion tubes and lichen distribution.

• Information from the Camden Air Monitoring website showed us that air quality is poor in our borough, Camden: in 90% of locations, NO2

levels exceed the Air Quality Standard.

MethodLearning to identify lichens: We started identifying lichens on twigs using keys. We then surveyed the trees on the Heath with the help of lichen experts Pat Wolseley and Holger Thüs of the Natural History Museum. We kept lichen samples that were authenticated by Holger as our ‘reference herbarium’.

Making a ladder quadrat: We cut out a strip of five 10 cm by 10 cm squares from thick plastic of a bin-liner (see photograph).

For a guide to

identifying lichens,

see Identification

on the OPAL

website: www.

OPALexplorenature.

org.

Monitoring NO2 using diffusion tubes: NO2-diffusion tubes (Gradko International) were placed into position with the open end at the bottom to prevent rainwater collection. NO2 in the air diffuses along the tube and is absorbed by a fluid on a grid in the lid. The concentration of NO2 was determined by Spectrophotometry. The tubes were replaced every three weeks and sent off to be analysed.

Surveying lichens in Hampstead Heath

2

Surveying lichens in Hampstead Heath – note the ladder quadrats

Then, for each tree:

• We recorded tree species and girth at 1.50 m above ground.

• We placed our plastic ladder quadrat on the north aspect of the trunk.

• We recorded the presence of species of foliose and/or fruticose lichens (‘Macrolichens’) in each square.

•We repeated on all compass points (N, E, S, W).

In this way, the presence of lichen species in 20 squares was recorded for each trunk. To check reliability we moved to a new tree every 15 minutes. Each group surveyed the same three trees and we then compared our results. When there was a discrepancy, we went back to the tree and rechecked.

Monitoring NO2: NO2-diffusion tubes (Gradko International) were placed into position with the open end at the bottom to prevent rainwater collection. NO2 in the air diffuses along the tube and is absorbed by a fluid on a grid in the lid. The tubes were replaced every three weeks and sent off to be analysed. The concentration of NO2 was determined by spectrophotometry.

What we foundThis graph shows the relationship of NO2

concentrations and the locations. Perhaps we should have used a bar chart because our independent variable is categoric, but line graphs made it easier to see trends.

The values for the front of the school (the main road) were highest.

The first set of data (10 March – 31 March 11) showed higher values than the rest. More people may have driven to work due to the cold weather, and there is less movement of air so the local concentrations of the NO2 would remain high.

This graph shows the nitrogen dioxide concentrations

at different locations from 10 March – 21 July 2011

(corrected for blank readings); ppb = parts per billion

Location of NO2 tubes and trees surveyed

3Catalyst February 2013

Macrolichen diversityThe table shows what we found when we looked for a correlation between the NO2 and lichen diversity. Nitrogen-loving lichens were found on all the trees sampled, but a greater diversity of macrolichens including more ‘intermediate’ lichens, were found on the Heath. We might have expected a wider diversity of lichens on the fruit trees in the Orchard as they are set back from the road.

However several factors may have affected lichen results, e.g:

•bark pH (related to tree species)

•age of tree

•proximity to sources of nitrogen (e.g. fertiliser)

•immediate surroundings of tree (e.g. hedge)

• shading of the orchard by the surrounding buildings.

We then calculated a pollution score based on the frequency of nitrogen-loving and nitrogen-sensitive lichens. Nitrogen-sensitive species counted +1 as they will not tolerate polluted air. Nitrogen-loving species counted -1 each as they grow in polluted air.

NO2

concentration

(ppb)

Average number

of different

lichens per tree

Hampstead Heath 11.1 9.7

Orchard 14.1 4.0

Highgate Road 22.0 4.5

Indicator lichens Total pollution score

nitrogen-loving species nitrogen-sensitive species

Location Xanthoria

ucrainica

Xanthoria

parietina

Xanthoria

polycarpa

Physcia

adscendens

Physcia

tenella

Evernia

prunastri

Flavoparmelia

caperata

Parmelia

sulcata

Orchard -1 -1 -1 -1 -1 -1 -4

Highgate Road -1 -1 -1 -1 -1 -5

Hampstead Heath -1 -1 -1 -1 -1 -1 -1 -3

The bar chart shows the correlation between the NO2 concentration and the pollution score (derived from the lichen data). The pollution score was minus 5 at the front of the school (on Highgate Road) where the NO2 levels were highest, while the pollution score was minus 3 on Hampstead Heath where the NO2 levels were the lowest.

Given more time, we could have measured NO2

levels and surveyed trees on the back roads and also deeper in Hampstead Heath away from the traffic. We could also have looked at lichens on twigs because this would have provided more recent history of lichen growth and air pollution.

Madeleine A., Mary D., Maureen L., Connie M., Isabel S., Linnet M., Hannah R., and Siobhan P. were Year 10-11 pupils at La Sainte Union Catholic School in Highgate, London, at the time of this research. Their project was exhibited at the Royal Society’s annual Summer Science Exhibition.

How can we improve air quality?Small changes in our everyday behaviour can make big differences to the quality of London’s environment.

•Turn down the central heating when possible.

•Install home energy efficiency measures e.g. loft, cavity wall insulation.

•Use public transport rather than the car wherever possible.

•Walk or cycle for short journeys, instead of using the car.

•Use eco-driving techniques to reduce your fuel use.

•Ensure your car is not wasting fuel, by regularly checking oil and tyre levels.

•Avoid burning garden or domestic waste, especially in urban areas.

What are lichens? Lichens consist of at least two organisms - a fungus and a photosynthesizing alga (a cyanobacterium) living together. In this amazing association both the fungus and the alga benefit.

•The alga provides food for itself and the fungus, using its chlorophyll for photosynthesis in the same way as green plants do. The fungus does not have any chlorophyll.

•The fungus plays a vital role in providing a physical structure to shelter the alga from excess sunlight and in particular, water loss. Also, the fungus absorbs water, nutrients and gases from the environment to share with the alga.

4 Catalyst February 20134

Thomas Lewton

Fracking: an energy revolution?

In spring 2011 two minor earthquakes were experienced in the Blackpool area of north England. The quakes had magnitudes 1.5

and 2.3, strong enough to be detected by humans and even to make a few houses shake. Such earthquakes are a regular occurrence in the UK, with roughly 20 detected by seismologists each year. But what makes the Blackpool earthquakes different is that they are very likely to have been caused by a manmade activity: fracking.

Hydraulic fracturing, commonly known as fracking, is the process of breaking up rocks deep underground using high-pressure water mixed with sand and chemicals. The process has been used for decades in the energy industry to free oil and gas trapped in rock formations. However, recently the technology has received a lot of attention for a new application in releasing natural gas from a type of sedimentary rock called shale.

The impact has been most dramatic in the United States. Over the last five years natural gas production in the US has increased by a quarter. As a result the price of natural gas in the US has halved in only three years, making it a cheaper way to produce electricity than coal.

Yet questions have been raised over the environmental impact and safety of fracking technology. As well as the potential to cause earthquakes, some people believe that fracking is

The UK’s first fracking

site near Singleton in

Lancashire.

contaminating drinking water. Dramatic images of tap water being set alight have given rise a large and vocal anti-fracking protest movement. All energy sources come with some risk to safety and to the environment, but do those of fracking outweigh the economic benefits?

Fracking technologyOil and gas are made from hydrocarbon molecules, chains or rings of carbon atoms with hydrogen atoms attached. When burnt in air the hydrocarbon reacts with oxygen to form carbon dioxide and water vapour. In the process a large amount of energy is released. We use this energy to power our cars, to produce electricity and to heat our homes.

Oil and gas account for over half of the energy

consumed around the world.

oil

coal

gas

nuclear

hydro

other renewables

5Catalyst February 2013 5

Shale gas has been used in a small way since the 19th century. There are huge reserves across the world, yet most shale gas is trapped so tightly inside the rock that, for a long time, it was too costly to release it. George Mitchell, an American businessman, experimented with fracking technology when most believed shale gas to be a pipe dream.

Environmental friend or foe?Although economically beneficial, some argue that the development of shale gas may come at too high a cost to the environment and to our health. Fracking has probably caused minor earthquakes. The tremors are too small to be a danger to humans, but the same violent process of breaking apart rock formations could let fracking chemicals and natural gas seep into groundwater aquifers, from which we take our drinking water – see Box Chemicals for fracking.

Another problem arises from the large amount of water needed to fracture shale rock. If shale gas were produced in an area suffering a drought, it could take much needed water away from the already parched environment.

But shale gas isn’t all bad news for the environment: when burned, natural gas produces far less carbon dioxide than an equivalent amount of oil or coal. As a result of switching power station fuel from coal to natural gas, carbon dioxide emissions in the US have decreased to their lowest level in 20 years. In the short term, the development of shale gas could help to reduce global carbon dioxide emissions, and so reduce the risk of climate change.

Standard practice was to use high-pressure water to force open rock formations and release the natural gas. Adding sand to the mixture then kept the rock fractures open when the water pump was turned off. Mitchell’s innovation was to add chemicals to the water that enabled the fluid to be pumped into the well much faster and so fracture it more effectively. This refinement made it economical to extract natural gas from shale, providing a new source of cheap and abundant energy.

Chemicals for frackingThere are many different chemicals used in fracking. Some of the important types are:

•acids to clean and initiate fissures

•alcohols and guar gum to improve viscosity

•friction reducers, such as polyacrylamide.

In the UK, the Environment Agency must approve these chemicals before they are used. There have been protests that drinking water supplies have been contaminated by these chemicals. No one has yet provided conclusive evidence that this can happen, but the residents of fracking regions remain understandably concerned.

A global revolution?Estimates of natural gas reserves have doubled

because of the newly developed technologies that can extract shale gas. Some seven quadrillion (a seven with 15 zeros following it) cubic feet of shale gas is now classified as ‘technically recoverable’. Vast new reserves have been found across the world, from China to Argentina to Poland. Even the UK holds shale gas reserves worth around £1.5 trillion.

Before these reserves can be developed, the environmental and safety concerns of the public will have to be allayed. But whether these reserves are developed will depend on the tough decisions societies must make about how to support our future energy needs, and what the risks and rewards of each energy source are.

Thomas Lewton is a researcher on TV science programmes.

Well

Water tableGas flows out

Water, sand and chemicals injected into well

Shale

Hydraulic fracturing Fissures

Shale

Gas flows out

Fissures

Anti-fracking protesters fear that groundwater may become contaminated.

6 Catalyst February 2013

Lawrence Patihis

Ian W. Tingen

Elizabeth F. Loftus

Memory MythsKey words

memory

psychology

falsifiable theory

Memory is crucially important in everything we do. Without memory we would not know who we are,

whom we love, where we come from or even how to do simple tasks like get out of bed, brush our teeth or make tea. This is very clearly shown by the case of Clive Wearing who is unable to keep memories for more than thirty seconds. His diary, for example, consists of multiple entries where he records, time after time, having just woken up.

There are different kinds of memory and each is very important. Memory for facts is important for students taking GCSE exams when they must recall names, dates or equations. Memory for emotions reminds us of how we felt on our first day in a new school, or when we shared a special moment with our family. Memory for events allows us to picture the scene and recall the details of our last birthday party. Memory for movement and coordination allows us to walk, talk and ride a bicycle. While it is clear that memory is essential in so many ways, there are many myths about how memory works. In this article, we explore some of these fairytales and explain what modern psychological science has discovered about how memory really works.

Myth 1: Memory works like a video recorderThis idea would lead you to believe that all experience is recorded and we can then ‘play it back’ when we want to remember it. This idea became especially popular after the 1950s when brain surgeon Wilder Penfield was trying to treat epilepsy. In a quite shocking procedure, he electrically stimulated the brains of his patients before surgery and some reported vivid memory fragments coming back to

Clive Wearing suffered almost total loss of an ability to

make memories after having viral encephalitis in 1985.

He keeps a detailed diary of his thoughts. Wilder Penfield, Canadian neurosurgeon

7Catalyst February 2013

them. Penfield made the mistake of thinking that this meant all experience is stored in the brain in the form in which it is to be retrieved.

What the Science says: Psychology experiments have shown that we store very little of what we actually see, and these details fade with time. When we remember something, we put the memory together (reconstruct it) out of the fragments we did manage to store. Sometimes we fill in the gaps incorrectly - which leads us to the next myth.

Myth 2: Memory can not be changedMany people think that memory of past events does not change, and therefore it is very reliable. In a court of law, if a witness points a finger at a suspect and says, “That’s the thief, I remember that face!” most people assume the witness is correct. Similarly, there is a widely held belief that memory for our emotions remains the same, and is still completely accurate even many years later.

What the Science says: Memory for both past events and emotions can actually be changed. Many psychological studies show that misleading suggestions can completely change a memory. Misleading suggestions can also plant entire events into a person’s mind. If your mother incorrectly told you that, as a young child, you had caused a scene by spilling a drink all over a relative at a wedding, there is a real chance that you could develop a memory of this made-up event.

Similarly, memory for our past emotions can change over time. How we remember our feelings 10 years ago depends on how we are feeling today, and also on how we now think about that past situation.

Myth 3: Traumatic memories are blocked out

Over a century ago, the famous psychoanalyst Sigmund Freud suggested that traumatic experiences were repressed, which means they are blocked out immediately after the trauma so that they cannot be recalled at all. Freud thought traumatic memories were so painful that they are walled off from consciousness and that, years later, in a safe place, the blocked memories can be remembered in great detail.

What the Science says: It is possible for people to not think about a past event for a while and be reminded about it later. However, there is no credible scientific evidence that traumatic memories can be completely walled away for many years and then can come back in great detail much later on. In fact, traumatic memories are usually remembered all too well. For example, people who have gone through terrifying events, such as war, often remember the trauma even when they do not want to. Highly emotional memories tend to be well remembered, but are not always remembered completely accurately. Distressing memories do fade with time, although often more slowly than non-emotional events.

You might ask: why would the brain evolve so that memory is faulty? Memory errors are the result of a flexible memory system, and that flexibility is usually beneficial. Changing past memories may improve future behavior and problem solving. It also makes sense in terms of evolutionary adaptation that the essence of traumatic events are well remembered to avoid situations like that in the future.

You are the witnessTest yourself. This image was shown in a classic misinformation study in the1970s. It shows a car which, moments later, was involved in an accident. Study the photograph and remember the details. Your memory will be tested later in the article.

8 Catalyst February 2013

Just as important, police should be careful not to change the memory of a criminal suspect as this has led to false confessions in the past. Examples illustrating these problems can be found at the website of the Innocence Project (InnocenceProject.org), which documents heartbreaking stories of wrongful imprisonment. Knowing how traumatic memory really works can improve the treatment for those traumatized by war or abuse. Knowing that memory can change affects the fundamental way we all look back at our own lives. Think about it.

Calvin Johnson’s book describes how he was wrongly

identified as a rapist and spent 16 years in jail. DNA

evidence eventually showed that his accuser’s

memory was unreliable.

How do we know the old myths are wrong? Over the last 100 years, the field of psychology has gradually become more scientific as it has moved towards theories that can actually be tested (these are known as falsifiable theories). This approach has contributed, in part, to faster scientific progress in recent decades.

Lawrence Patihis, Ian W. Tingen and Elizabeth F. Loftus are psychology researchers at the University of California, Irvine.

In the witness boxThink back to the old photo of the car on page 7. Don’t look back! An accident occurred just a few seconds later, and the police ask you this question:

Can you remember if there was another car passing along the road as the car waited at the give-way sign?

Think carefully. Was there another car? Think back and recall as much detail as you can of the picture on page 7. Do you remember a stop sign or a give-way sign?

If you pictured a give-way sign then your memory has been changed by suggestion contained in the question. The image is taken from a classic misinformation experiment performed by Loftus et al. (1978). For a detailed account of the research see also the book Eyewitness Testimony (Loftus, 1979).

Look here!60 Minutes episode Eyewitness: www.cbsnews.com/video/watch/?id=5153451n

A BBC interview with Elizabeth Loftus: http://www.bbc.co.uk/programmes/b00yhv36

A photo identity lineup (or parade) where all six faces

are shown all at once. Not only is it important that the

police do not use leading questions; research also

suggests that memory is more accurate if the witness

is shown the series of photos one at a time rather than

all at once. (Photos: Lifespan Database of Adult Facial

Stimuli; Minear & Park, 2004).

Why memory is importantAt the beginning of this article, we pointed out just how important memory is in everything we do. We know that memory does not record everything like a video recorder, that memory can be changed, and that terrifying events are usually remembered all too well. In light of this knowledge what could this mean for society? It is important in the legal system, the treatment of traumatized people and the way we all think about our lives. For example, this knowledge affects how we judge someone accused of a crime. If the police know how changeable memory can be, they can avoid giving leading suggestions to witnesses during questioning or when viewing an identity parade or lineup.

1 2

3 4

5 6

9Catalyst February 2013

Go out in the rain and you get wet. It’s unpleasant, but you survive. Individual raindrops are small compared to a

human being so they can do little damage.But imagine that you are a small, flying insect

such as a mosquito. A raindrop is much bigger than you. How can you avoid being flattened? Some clever photography shows how it’s done.

Physics for fliesPicture a mosquito. Its mass is only 2 or 3 μg (micrograms). It likes living in damp places so it’s likely to be out in the rain.

Now picture a raindrop. Its mass may be as much as 100 μg, 50 times that of the mosquito. It may be falling at 9 m/s. When a raindrop hits a mosquito, it’s like a double-decker bus hitting a human at top speed.

If the mosquito is sitting on the ground, it’s likely to be crushed as the raindrop breaks up. But, if the mosquito is flying, the result is different.

Flying dry How mosquitoes survive the rain

Photographing fliesTo find out how mosquitoes survive impacts with raindrops, a group of engineers from Georgia Institute of Technology (USA) developed a system which allowed them to photograph collisions between water droplets and flying mosquitoes. Initially, they experimented with fake flies described as ‘mimics’.

Figure 2 shows the apparatus. Water from jet A passes through an infra-red beam (a light gate) B. This triggers the controller C and power supply D which operates the solenoid E which pulls downwards, releasing the insect mimic F (the green ball) made of expanded polystyrene (Styrofoam).

A high-speed camera films the impact. The photographs on pages 10-11 show what happens when the apparatus was adapted so that a water drop hit a flying mosquito.

Figure 1 a As it falls, a raindrop soon reaches terminal

velocity, a bit less than 10 m/s. This is when its weight

is balanced by the force of air resistance. b When a

large mass collides with a smaller, stationary mass,

the large mass will slow very slightly. Some of its

momentum has been transferred to the smaller mass.

weight

air resistance

V1

a b

V2

Figure 2 The apparatus used to film insect mimics as

they are hit by water drops.

Key to pages 10-11

a A mosquito tips its wings to escape from under a raindrop. b A direct hit spells danger. c The raindrop breaks over the mosquito. d A wet mosquito.

a

bc

d

David Sang

10 Catalyst February 2013

Tim

Now

ack/

Tim

Now

ackP

hoto

grap

hy.c

om

A mosquito struck by a raindrop must avoid breaking the drop.

www.catalyststudent.org.uk

12 Catalyst February 2013

Under impactThe experimenters found two possible outcomes.

• A fly hit off-centre is tipped sideways but manages to shake off the drop. It does this within about one-hundredth of a second, before recovering its flight.

• A direct hit causes the fly to fall with the drop, which remains intact. It may fall about 20 cm, so it is not advisable for flies to stay close to the ground.

A mosquito flies

among artificial

raindrops.

Staying dryIf a raindrop hits a solid object, it usually splatters. What prevents this when it hits a mosquito?

• Firstly, a mosquito’s mass is very small compared to that of a raindrop. The force of the raindrop on the mosquito is small, about the weight of a small feather. The mosquito is pushed so that it moves with the raindrop.

• Secondly, a raindrop is held together by the force of surface tension. This is a result of the attractive forces which act between water molecules and which pull the drop into a roughly spherical shape. The force of the impact is not enough to break the drop.

• Thirdly, a mosquito is covered in water-repellent hairs. These probably help it to pull itself free of a raindrop as they fall together.

• A mosquito’s body is flexible with a tough exoskeleton. This allows it to survive the impact and cope with the sudden acceleration it experiences – up to 300 times the acceleration due to gravity.

So, provided a mosquito can avoid breaking the surface of a raindrop and thereby getting wet, it can survive an impact. It must twist its body and wings so that it slips out from under the drop.

Look here!This work was done by Andrew K. Dickerson, Peter G. Shankles, Nihar M. Madhavan and David L. Hu. Follow the link to see their films of mosquitoes in the rain and read their published paper: http://dickerson.gatech.edu/file/Mosquitoes_in_Rain.html

David Sang is Physics editor of Catalyst

Scanning electron microscope images of water-

repellent hairs on the wing of a mosquito.

Tim

Now

ack/

Tim

Now

ackP

hoto

grap

hy.c

om

13Catalyst February 2013 13

Mike Follows

Temperature and how we measure it is one of the most important and interesting areas of physics. This is reflected in the huge number and variety of thermometers that have been developed. In this article, Mike Follows describes the surprising range of thermometers available to scientists today.

Many physical properties of materials depend on temperature. Our biochemical reactions work best at 37°C and we are

in serious danger if our body temperature strays more than a couple of degrees either way. Being able to record the global mean surface temperature of the Earth is important in order to establish the magnitude of global warming. We have even found ways of working out how the temperature of the Earth has changed over the last half a million years as well as the temperature of distant stars and of Outer Space itself.

Some definitionsThe temperature of a substance is a measure of the average kinetic energy of the constituent molecules – the faster the molecules are moving or vibrating, the hotter the body will feel. Temperature also tells you the direction that heat or thermal energy will flow; it flows down a thermal gradient from a hotter body to a colder body. In the process, the hot body will lose thermal energy to the cold body.

Thermometry ... a hot topic

Key words

temperature

thermometer

Kelvin scale

ideal gas

Thermometer historyThe thermometer has been more of a development than a single invention. Philo of Byzantium (280 - 220 BC) was aware that air expands and contracts with changes in temperature and described a demonstration that was developed by Galileo to become his air thermometer or thermoscope in around 1600. This consisted of a glass bulb containing air with a long tube extending downward into a container of wine or other coloured liquid (see figure 2). Engraving a scale on the tube converted the thermoscope into the first thermometer.

Figure 1 A mercury-in-glass thermometer

– as the mercury gets warmer, it expands

and rises up the tube.

Figure 2 A thermoscope – as the air is

heated, it expands and pushes the liquid

down the tube. On cooling, the air contracts

and atmospheric pressure pushes the liquid

back up the tube.

The Galilean thermometer (Figure 3) was developed after Galileo’s death, based on principles that he developed. Each glass bulb is partially filled with a coloured liquid and has a metal disc, engraved with the temperature, suspended from it. The bulbs are adjusted by varying the mass of each metal disc so that they all have slightly different densities. When they are immersed in the column of liquid paraffin, they will float if they are less dense than the paraffin, and sink if they are more dense.

Figure 3 A Galilean thermometer shows

the temperature of its surroundings.

What is a thermometer?The word thermometer comes from the Greek thermos (meaning ‘hot’) and metron (‘measure’). Figure 1 shows a mercury-in-glass thermometer. As with most thermometers, it comprises a thermometric substance that changes in response to temperature (mercury expands on being heated) as well as a means of converting this physical change into a numerical value (the visible scale marked on the glass).

Air

Wine

14 Catalyst February 2013

Figure 4 Measurements of the pressure of a gas show

that pressure increases linearly with temperature.

Notice that the graph in figure 4 can be extrapolated to zero pressure. The graph intersects the horizontal temperature axis close to minus 273.15 °C. This is absolute zero and so the absolute temperature scale was created. Sometimes called the Kelvin scale, it is regarded as the fundamental measure of temperature. The Celsius and Kelvin scales are related by the equation:

K = °C + 273.15

Because the temperature can be calculated without any unknown quantities, thermometers based on an ideal gas are known as primary thermometers.

Going electricalGas thermometers are not very convenient or easy to use so, in 1871, Sir William Siemens introduced the Platinum Resistance Thermometer. This is now widely used as a thermometer and covers the

temperature range from about minus 260 °C to 1000 °C. Driven by the voltage between the ends of the metallic resistor, the free electrons drift along. However, increasing the temperature of the resistor increases the vibration of the metal atoms. In turn, this increases the number of collisions between the atoms and the electrons, slowing them down. This reduces the current – the resistance has increased. Though the platinum resistance thermometer is fairly reproducible, it is regarded as a secondary thermometer as it needs to be calibrated against a primary thermometer.

GlowingHot objects glow – they radiate light. We can use this to find out the temperatures of hot objects, even distant stars and deepest space.

When heated, the colour of metals pass through red, orange and yellow. Eventually, they become white hot when all the colours of the visible spectrum are emitted. They also glow more brightly because they are hotter and more thermal energy is emitted.

Figure 5 shows how the spectrum of a hot object changes as it is heated. In this graph:

•the x-axis shows the wavelength of the light

•the y-axis shows the intensity of the light.

You can see that, as the temperature increases, the peak in the graph gets higher – more energy is being radiated. At the same time, the peak moves to the left, to lower wavelengths which are more energetic.

Figure 5 The spectrum of light radiated by a hot object

depends on its temperature. The peak wavelength

λmax (in m) is related to temperature T (in K) by the

equation λmax × T = 0.0029.

If the temperature rises, the paraffin expands and becomes less dense. One or more bulbs will now be too dense to float, and will sink. (The density of each bulb is fixed because neither its mass nor volume changes with temperature.) The temperature is then shown by the metal disc on a bulb which is free-floating in the gap.

Galileo’s thermometer has the advantage that it is not affected by changes in air pressure in the way that his thermoscope was. The Galilean thermometer uses paraffin because the density of water changes very little in response to changes in temperature.

Ideal gasesMany different thermometers followed but, in 1780, Jacques Charles returned to the gas thermometer. He showed that, for the same increase in temperature, all gases exhibit the same increase in volume. In a similar way, the pressure of a gas increases as it is heated if its volume is fixed – see figure 4.

We can use this to find the temperature of a distant object such as a star. A telescope can be linked to a spectroscope that measures radiation intensity across the electromagnetic spectrum. The star’s temperature can be deduced from the peak wavelength.

15Catalyst February 2013

Proxies We can work out the Earth’s past temperature and climate using proxy thermometers. Dendrochronology is probably the best known technique and uses the width of tree rings to infer past climate. Wide tree rings correspond to conditions that favour growth.

We can go back almost half a million years using the ice cores that are being drilled out of the Antarctic ice at Lake Vostok. Apart from the measuring the concentration of greenhouse gases like methane, isotopes of oxygen can also be analysed. There are two important isotopes of oxygen, 16O and the heavier 18O. Water molecules with 16O atoms are lighter and evaporate more easily. Water with 18O atoms is heavier and is rained out more easily when the water vapour condenses. In a colder world, more of the heavier water is rained out before it reaches the poles so that polar ice has a smaller fraction of the 18O isotope. This can be used to infer past temperature – see figure 7.

Mike Follows teaches Physics

The cricket as a thermometerTo many, the sound of chirping crickets is synonymous with summer. Only the males stridulate, which is the scientific term for chirping, and they do this to attract a female.

Amos Dolbear noticed that the frequency of chirping of the narrow-winged tree cricket Oecanthus niveus depended on the prevailing air temperature and, in 1897, he published his law relating the temperature T to the number of chirps per minute N:

T = 10 + (N - 40)/7

This is the equation of a straight-line graph. Why does the chirping frequency increase with temperature? Well, crickets are cold-blooded. As the temperature rises, it becomes easier to reach the activation energy required for the chemical reactions that drive the muscle contractions used to produce chirping, so they happen more often.

TJ W

alke

r

A male snowy tree cricket

This graph shows that the rate of chirping of

the snowy tree cricket Oecanthus fultoni shows

the same pattern as Dolbear’s crickets as the

temperature increases. Published by Thomas J.

Walker in the Annals of the Entomological Society

of America in 1962.

Figure 7 Past temperature variations deduced from

oxygen-isotope measurements of an Antarctic ice core.

Ice formed 450 000 years ago was found at a depth of

over 3 km.

Figure 6 The spectrum of cosmic background

radiation, as measured by the COBE satellite. The line

shows that it corresponds to a temperature of 2.728 K.

© Nick Strobel www.astronomynotes.com

In 1964, Arno Penzias and Robert Wilson, two radio astronomers, accidentally discovered the cosmic background radiation, the afterglow of the Big Bang. From its spectrum, we now know its temperature to be 2.73 K – see figure 6.

16 Catalyst February 2013

TryThis

Chocolate is very popular. But what happens if you melt and re-harden it?

You will need:

•2 identical bars of chocolate – dairy milk works well. •A warm place•A fridge

What you doLeave both bars of chocolate in their wrapping.

Keep one of the bars of chocolate at room temperature. Melt the other one by putting it into a warm place such as on a radiator or in the hot sun. When it has fully melted, put it into the fridge to re-set. We will call this the melted chocolate although it has re-hardened.

Remove it an hour or so before you want to do the experiment and allow it to come up to room temperature.

Un-wrap both bars of chocolate. Try snapping each bar and then try eating chocolate from each. What do you notice? How are they different? It is probably best not to read ‘What you may find’ until you have tried tasting it.

You may need to repeat the taste tests a few times to make sure!

Cocoa pods growing on a tree in India

Cocoa butter is a fat, extracted from seeds in the

cocoa bean.

What is going on?You added nothing to the chocolate so exactly the same atoms are present as were there at the start. The way the atoms are arranged and the structure of the components have changed. In particular, one of the ingredients of chocolate, cocoa butter, can have many different structural forms which can easily be converted from one to another. The forms have different melting points.

When the chocolate is melted and put in the fridge it usually changes to a form with a lower melting point than the original chocolate. This causes a change in the properties. In the original chocolate, the melting point is about the same as body temperature, 37°C. Melting is an exothermic process so tends to cool the mouth. The melted chocolate has a lower boiling point so no longer has the cooling effect. It melts more quickly, however, which causes the flavour to be released faster.

Vicky Wong is Chemistry editor of Catalyst.

Eatingchocolate

What you may findYou may notice that the normal chocolate makes a distinctive noise when you break it. This is called ‘snap’. The melted chocolate does not normally do this – it is much softer. Sometimes the melted chocolate seems to have a more intense flavour. The normal chocolate seems to cool the mouth. The melted chocolate often seems ‘grainier’ and less smooth than the normal chocolate.

Try This

17Catalyst February 2013

All these things we tend to take for granted in life: heart beats; sleep; being able to see; being able to move at will. Yet all these functions of

Uncovering the magical world of signalling

The Nobel Prize for Chemistry 2012

Suzy Moody

Key words

hormone

receptor

crystallography

Nobel prize

Imagine you are lying in bed. Your heart is beating. Gradually you wake, open your eyes and realise it is light outside. You get

up and start the day.

the human body require signals to be released, detected by a receptor in the right place and an appropriate response to be mounted. For example, hormones can be released into the blood stream but, if there is no receptor to detect the hormone level changing, there will be no response.

Scientists have discovered many different signalling molecules, among them names may be familiar such as adrenaline, dopamine and

18 Catalyst February 201318

Sps

mile

r

need. Yet the receptor that adrenaline binds to was unknown for many years.Mental illnesses such as manic depression and schizophrenia are caused by problems in signalling in the brain. As mentioned above, dopamine is a neurotransmitter signal that passes between brain cells. When dopamine activity is excessive, the brain cells are over-stimulated and the person is unable to think coherently or behave rationally. These debilitating conditions can be treated with drugs. Dopamine as a signal must have a receptor on the surface of brain cells that it binds to. Anti-psychotic drugs bind to the same receptor, and block dopamine binding. The amazing thing is that the drugs were found to be effective before we knew what the dopamine receptor looked like.

So what has all of this got to do with the Nobel Prize for Chemistry? The 2012 prize has recently been awarded to two American scientists, Robert Lefkowitz and Brian Kobilka.

The prize was awarded for ‘studies of G protein-coupled receptors’ or GPCRs. These two scientists have led the way to finding those elusive receptors, and trying to characterise them as much as possible, to advance our understanding of how our bodies work and how we can medicate them when things go wrong.

In the 1980s, Lefkowitz started work on receptors which he describes as ‘gateways’ to the cells for all the hormones and signal transmitters. He radiolabelled hormones so he could visualise how they get through the cell membrane. It became apparent that the hormone would bind to a specific receptor (GPCR) in the membrane. The GPCR then activates a specific protein inside the cell, and a response to the signal is made. Over 1000 different receptors have now been identified. They are known to mediate the senses (sight, taste and smell), pain tolerance, glucose metabolism and a huge variety of other physiological responses.

The first receptor to be characterised by Lefkowitz and his colleagues was an adrenergic receptor – the receptor responsible for detecting adrenaline and noradrenaline. They rapidly found several other receptors, and it became clear that they all shared great similarity. The receptors make up a protein family, with specific amino acid sequences common to all. Over time, the GPCR family have been described in more detail. They are large proteins which sit in the membrane (thereby having contact with the outside and inside of the cell simultaneously). The proteins all have a long stretch of amino acids that winds back and forward through the membrane seven times.

Robert Lefkowitz and Brian Kobilka, winners of the

Nobel Prize for Chemistry 2012

Radiolabelling uses molecules containing radioactive isotopes. These can be detected and followed as their radiation makes them stand out from other molecules.

Nobel Prize

Patients in intensive care are often given adrenaline.

serotonin. Adrenaline is part of the complex system that moderates your heart beat and blood pressure. When you are scared or nervous, you feel your heart pounding. That is the body sensing and responding to an increase in adrenaline. Dopamine is a neurotransmitter, a signal that is detected by receptors on brain cells. Problems with regulating dopamine can lead to Parkinson’s Disease and are also thought to be the underlying cause of many mental illnesses.

Drug designWhat is interesting about these examples (and many others) is that we have used our knowledge of the signal and what it does to create drugs that mimic or inhibit their action, thereby offering treatment for many different conditions. We have done this without actually knowing the nature of the receptor that the signal or the drug will bind to. To put that in context, it is thought that over half of the medicines we use bind to these receptors that we assume are present but which nobody has yet found.

For example, in every Intensive Care Unit in every hospital in Britain, many patients’ survival depends on being given infusions of adrenaline. This is because when the body is extremely sick or injured it is likely to have a very low blood pressure. This is dangerous as it stops oxygen getting to all the essential parts of the body like the brain, heart and kidneys. Giving artificial adrenaline can help the body maintain a good blood pressure and ensure that all the vital organs get as much oxygen as they

19Catalyst February 2013

Protein structuresThese properties of GPCRs make them very difficult to work with. Crystallisation is the best way to characterise the structure of any protein, but this requires the protein to be isolated in high quantity and purity. GPCRs rely on the membrane of the cell to keep all the 7-fold structure intact, and removing them from the membrane can easily result in the protein falling apart. GPCRs, like many membrane proteins, were found to be unstable in solution. This again makes any manipulation or characterisation very difficult.

One of the junior scientists who worked with Lefkowitz during the 1980s was Brian Kobilka. He was involved in the work on adrenergic receptors, and when he set up his own lab in 1989 he continued working on GPCRs. Kobilka was particularly interested in producing GPCRs in sufficient quantity and purity that he could use them for X-ray crystallography to determine their structure. As mentioned above, these processes were fraught with difficulty.Kobilka used DNA sequencing (devised by fellow Nobel prize winner Fred Sanger) to identify and clone the genes coding for GPCRs. This allowed the protein to be produced in insect cells, using the cell membranes to keep the protein intact until it could be coaxed out without becoming unstable. It took over 20 years of lab work for him to have big enough crystals of pure GPCR protein that crystallography could be used to solve the 3D structure of the receptor. This work has improved understanding of how signals and receptors work and means drugs can now be designed to target specific receptors.

Kobilka and Lefkowitz fully deserve their Nobel Prize for Chemistry. They recognised the need for identifying and understanding these receptors and the vital role they play in maintaining homeostasis. When asked what kept him going on such a difficult challenge, Kobilka’s response is typical of many passionate scientists. ‘I just wanted to know how they work.’

This is the kind of model that

can be generated from X-ray

crystallography. The blue is

the protein, the curly tubes

are common structures (called

motifs) and give the protein its

3D shape. This is a protein that

complexes with haem groups;

this is shown in red. The crystal

of this protein was grown

by the author, and the X-ray

crystallography was by Bin

Zhao at Vanderbilt University,

USA. It was published this year

in Int J Mol Sci.

How X-ray crystallography works: the beam of X-rays is

diffracted by the regular array of protein molecules in

the crystal; the crystal structure can be deduced from

the pattern of the diffracted beams.

Look here!The official Nobel Prize website description of Lefkowitz and Kobilka’s work: http://tinyurl.com/8q7psvo

Find out about the work of two important British crystallographers: Rosalind Franklin and Dorothy Hodgkin.

Search the Catalyst archive for earlier articles about Nobel prize winners in the Sciences: www.catalyststudent.org.uk

Suzy Moody is a microbiologist who investigates signalling in bacteria.

haem

20 Catalyst February 201320

Laura Plant describes the time she spent in the Amazon rainforest in northern Peru on a project researching the impacts of

forestry on the plants and animals that live there.

Timber has many uses – in building, paper-making and as an energy resource. Bbut demand for wood is increasingly rapidly. How can we ensure that this resource is available for long into the future? This is the principle of sustainable use, and it is vital in conserving forests and their plant and animal communities for future generations.

Forest managementIn tropical regions, large areas of rainforests have been untouched for centuries. This means that many rare and unique plants and animals have become specialized to live only in these habitats, making rainforests highly biodiverse. This biodiversity is threatened when rainforests are cut down to supply wood. For example, mahogany has become very rare due to over-harvesting.

The practice of Sustainable Forest Management (SFM) aims to harvest wood from forests in ways that cause minimal damage to the surrounding area and allows sufficient time for the trees to re-grow before they are harvested again. This involves: only harvesting on average one tree per hectare, a long rotation time (around 20 years) before re-

harvesting, the cutting of lianas that are attached to multiple trees to prevent other trees being pulled down, only cutting down trees that are bigger than a certain size, and the creation of conservation and regeneration areas. A company that does all these things can be certified by the Forest Stewardship Council (FSC).

Adventures in the

AmazonKey words

rainforest

forestry

sustainability

conservation

The Amazon flows

from Peru into Brazil.

Cut timber is marked with the logo of the Forest

Stewardship Council to show that it has been

harvested responsibly.

However, any type of logging disturbance could have a lasting negative impact on a forest. The sustainable forestry company GreenGold Forestry (GGF) wanted to find out how big this impact is in their concession in the Peruvian Amazon and they asked me to help them find out.

Laura Plant

21Catalyst February 2013 21

Assessing the impactOur project had two aspects. Firstly, we wanted to know how the felling of a single large tree would affect the growth of the surrounding trees and vegetation. Would the large space that has been created in the canopy allow more light into the usually shaded undergrowth and cause faster growth? Or would damage caused by tractors and logging machinery to the area slow the growth of the surrounding vegetation? To assess this we decided to establish permanent 20 m x 20 m plots in the rainforest containing a tree of commercial interest. Half of the plots will have the tree harvested, the other half will be kept as controls with no harvesting and all will be monitored over the next 5 years. All the trees in the plots were tagged with metal discs and their size and species recorded so they can be re-measured in years to come.

Our second aspect was to look at the how animals are affected by the disturbance to their habitat. How long does it take for them to return to the logged area or do they never return at all?

Finding mammals in the rainforest is no easy task and sightings are usually rare; however, where there are mammals, there is mammal dung and where there is dung, there will be dung beetles. This meant that we could use the diversity of dung beetles as a measure of the diversity of mammals in the area.

We needed bait for this and the obvious bait is dung. Luckily, we found a small zoo in Iquitos where, after much laughter, the zoo-keeper allowed us to remove dung from the cages of a variety of jungle mammals including monkeys, pumas and jaguars.

Carrying out the projectArmed with thick wellies, mosquito repellent and machetes to cut a path through the dense rainforest, we set off. A four-day boat ride up the Amazon river took us to the area of rainforest owned by the logging company where we camped for the next three weeks.

Days were spent carrying equipment, setting up plots and collecting data. The near 100% humidity makes it so warm that even the ‘shower’- a plastic tub to pour water over yourself while perched on a plank of wood across a stream, felt like luxury! The wildlife was truly incredible. We saw many

You can see some of Laura’s pictures on the back page.

Collecting jaguar dung at the zoo

species of hummingbirds and monkeys, as well as a herd of stampeding huangana (wild boar). The plants were formidable, with their dense canopy effectively blocking out the sky from our view, and their aggressive or prickly defence systems making them a danger to anyone who tyripped and grabbed them for support.

Potential resultsSince rainforest trees grow slowly, our research will need a few more years of data collection before growth rates can be obtained and a comparison made between our logged and unlogged plots. Results from similar studies have shown that an intermediate amount of disturbance can increase growth rate and diversity of vegetation. This is thought to be due to the following reasons:

1. When there is little disturbance, one or two well-adapted tree species tend to dominate the area and the canopy is so thick that little light gets through to allow regeneration on the forest floor.

2. When there is too much disturbance (as in clear-felling), few trees are able to grow at all and important seed-dispersing animals have been lost from the area. After the land has been abandoned, a few colonizer species will be able to start the process of secondary succession.

3. At an intermediate level of disturbance, caused naturally by heavy storms, a lot of different niches are opened up that a wide diversity of species can occupy. For example, light gaps in the canopy or the exposure of new soil.

Logs can be stored in the river to prevent insect damage.

Sustainable logging practices attempt to have an intermediate disturbance effect on the forest, avoiding irreversible damage to the forest and perhaps actually increasing biodiversity.

This project was an amazing opportunity for me to see how data is collected in the field and the challenges faced by scientists as they try to understand how a complicated system like a rainforest works. I hope that more research in this area will show people how vulnerable the rainforest is to human influence and how important it is that we harvest resources from this beautiful ecosystem in a sustainable way.

Laura Plant is a graduate student at the University of Cambridge studying for an MPhil in Environmental Policy.

Bar

un P

atro

Forest fieldwork

Buttress roots make measuring trees difficult.

Laura Plant spent three weeks in the Amazon rainforest of Peru. Her photos reveal some of the difficulties of working in this environment.

A forestry worker with

a baby huangana or

white-lipped peccary Birds such as the scarlet

macaw are important seed

dispersers in the rainforest.You wouldn’t want to hug this tree!

A dung beetle

trap with bait

– in one trap

we caught a

tarantula!

Laura working in the rainforest

The Amazon rainforest covers a

large fraction of the landmass of South

America.