Download Not only the chameleon changes colour

Authors:
“Not only the chameleon changes colour”
Courses
1º and 2º
Education
Compulsory
Secondary
Time
2 hours
Materiales
 Alcohol
 Concept cartoons
(Worksheets 1, 2, 3 y 4)
 Leaves: green, yellow and red
 Mortar
 Filter paper
 Whiteboard
 Petri dishes
 Markers
 Scissors
Skills
 Discussion
 Experimenting
 Making hypothesis
 Observation
 Thinking
 Team work
Overview:
Based on IBSE methodology (Inquiry Based
Science Education), students think about why
leaves change color in autumn. For this, they will
do a simple chromatography and learn the
different pigments using the resources offered by
the botanical garden (many species of plants with
lots of colors, scientific knowledge, etc.).
In addition, they will reflect, through discussion,
on the effect of climate change on plants and
photosynthesis in particular.
Aims:




Keywords: anthocyanins, climate change,
carotenoids, chlorophyll, chloroplast,
chromatography, carbon dioxide, water
availability, leaf, photosynthesis, pigment,
respiration, temperature, xhantophylls.
To understand the structure and function of
leaves.
To understand photosynthesis.
To study different pigments.
To reflect about the effects of climate
change on plants.
Curricular content: Science (1º and 2º Compulsory Secondary Education).
Previous knowledge: basic notions of the function and morphology of leaves and
the process of photosynthesis (learnt at school).
1
Teaching sequence
Divide the classroom into groups of 4 students. Consider group composition, try to mix
students with different abilities.
Review of morphology and function of leaves
1. Hand out Worksheet 1, they must match the words with the corresponding part of
the drawing.
Review of photosynthesis
2. To review the process of photosynthesis, ask the questions “How do plants feed?” and
“What is photosynthesis?” Then, hand out the concept cartoons (Worksheet 2), from
which, they will express and discuss their hypotheses. The spokesperson of each group
will shall state their hypothesis to the class while the teacher writes it down on the
whiteboard. Clarify any misunderstandings before moving to the next step.
Experimenting
3. Ask them “Why leaves change colour?” Explain them that they will work as scientists in the
lab to solve the mystery.
4. Deliver the material: Alcohol, leaves (green, yellow and red), mortar, filter paper, Petri
dishes, markers and scissors. If there is enough time before doing the activity, students
could walk through the garden looking for the leaves themselves.
5. They must crush each leave in the mortar adding a little of alcohol.
6. Then, they will filter the liquid and pour it into the Petri dishes. Be sure to mark each leaf
on the Petri dish.
7. Place a piece of folded filter paper (right angle into each dissolution and wait about 30
minutes.
2
8. After that time, they will observe that the liquid has risen, leaving coloured areas at
different heights. “What happened?” “Why are there different colours?” “Why are some
areas wider than others?”
9. Then, in order to summarize the process, distribute Worksheet 3 to fill the gaps. The
spokesman will read the results and the rest of the class will agree or disagree.
Discussion
10. Finally, deliver Worksheet 4. Students will have to solve the following questions:
“How plants will be affected by…
temperature increase?”
water supply: drought/flood?”
CO2 increase?”
other factors?”
Plenary
11. Whole class plenary to explain their decissions. At this point, the teacher will explain the
processes that are not fully understood and will ensure that there are no confusions.
Teacher´s notes
Chromatography let us observe the pigments in leaves. These substances have different
solubility. When they rise through the paper by capillarity, the most soluble ones will move
faster while the less soluble will move slower. The coloured areas are wider if the abundance
of the pigment in the solution is higher.
Leaves are green because the chlorophyll is the most abundant pigment. Chlorophyll absorbs
the wavelengths of blue and red and reflects the green. In addition, the leaves have other
pigments such as xanthophylls (yellow) and carotenoids (orange), but we do not perceive
them because they are in small quantities.
3
Furthermore, anthocyanins (red) are antioxidants that appear to protect the leaves from the
sun. They are responsible of giving the distinctive red colour to maples and beetroots. The
brown colour of oak leaves is due to the accumulation of waste products. Some pigments are
well observed in genres such as: Acer, Euonymus, Hedera, Prunus and Rhus.
When autumn arrives, there is less light and temperature decreases. In deciduous plants,
without water and nutrients, light destroys chlorophyll and it cannot be rebuilt. Then, the
green pigments disappear and others (xanthophylls and carotenes) give its yellow, red or
brown colour to the leaves. These plants do not die during the winter but they prepare
themselves by losing their leaves (dormancy). Evergreen plants do not lose their leaves and
continue to do photosynthesis.
How will affect climate change?
The consequences of climate change will depend on the adaptability of plants. Therefore,
some will be able to adapt by modifying their structures and they will survive. Some will move
to areas where the climate is more favourable. And some will disappear. Species react
differently to changes of environmental conditions, resulting in changes in species composition
and ecosystem structure.
Temperature






In general, higher temperatures speed up growth when other factors are not limiting. As
temperature rises, an optimum is reached followed by a sharp decline, where damage to
plant tissue leads to cessation of growth and ultimate death of the plant.
Rising temperatures plus higher precipitation will increase the productivity. However, the
drought in Europe in 2003 combined unusually high temperatures with water stress and
reduced primary productivity by 30%. If temperature increases too much, faster
respiration may tip the balance towards plants becoming a CO2 source.
In the same way, the closing of the stomata will decrease transpiration and thus also the
cooling effect around the plant.
There are widespread examples of an extended growing season due to temperature rise
and concomitant changes in key biological processes, including earlier budburst, delayed
autumn leaf fall, and extended flowering. This leads to imbalances in the plant-pollinator
network.
Dormancy is a period of limited to no growth which enables plants to survive temporary
climatic extremes, such as sub-zero winter conditions or prolonged drought. It has
evolved to ensure that plants have no soft growing tissues that could be damaged by
prevailing seasonal weather. Some species use temperature cues as a sign that it is safe to
break dormancy in order to maximize growth during favourable climatic conditions.
Where temperature changes, ability of plant species to successfully predict appropriate
times for growth is altered. Development is impaired, resulting in the delay, abnormality
or failure of flowers and fruits.
It is not just the magnitude of the change but the unpredictability of the change. Early
onset of growth in response to mild weather combined with unexpected frosts is likely to
cause significant damage to plants.
4
Water availability





If water is in short supply in the soil because of drought, or does not fall during periods
when plants most need it, plants will suffer from water stress. To deal with evaporative
loss from prolonged water stress, a plant may limit leaf production and leaf surface area
to reduce water loss, or close its stomata. Each response decreases the ability of the plant
to carry out photosynthesis, with clear implications on net primary productivity and
carbon storage, and can ultimately lead to plant death. Other indirect effect includes
altered flowering times as stressed plants flower and set seed rapidly before dying.
Some studies show that the stomatal density decreases with increasing CO2, either
because water conservation is more important than taking advantage of increased CO2 or
because they need fewer stomata to capture the equivalent amount of CO2. Therefore,
the scarcity of water would be limit stomatal opening and carbon trapping despite having
a high concentration of atmospheric CO2.
More adaptations are required for longer periods: reduction of the vegetative apparatus,
larger and deeper roots, reduced size of the stomata, thicker leaves, hairs or scales on the
surface, secretion of resins, etc.
With increased precipitation and flooding, as predicted for parts of the northern
hemisphere, it is likely that some species of plants will be affected by waterlogging. When
soil becomes saturated with water leaving no air spaces in the soil and depriving plant
roots of oxygen, as well as preventing CO2 being diffused away, plants are unable to draw
up soil moisture, leaves will wilt and roots will rot, leading to plant mortality.
Water vapour is the most abundant greenhouse gas in the atmosphere. It is increasing in
the atmosphere, not as a direct result of industrialisation (as in the case of CO2), but as
the result of feedbacks associated with climate change, such as plant transpiration. In
general, rising temperatures mean more water is evaporated from the Earth’s surface,
increasing humidity. Since water vapour is a GHG, the more of it that is held in the
atmosphere the more heat the atmosphere retains, thus warming. The warmer it gets,
the more water vapour the atmosphere holds, and so on. As such, water vapour is a
positive feedback to anthropogenic GHG emissions. Eventually however, atmospheric
water vapour will condense into clouds. These reflect incoming solar radiation and may
reduce warming. Though critically important, this aspect of climate change is still fairly
poorly measured and understood.
CO2

Carbon gets in and out of the atmosphere in a complex biogeochemical cycle, which is
summarized in terms of "sources" and "sinks". Natural vegetation, specifically forests,
absorbs CO2 from the atmosphere through the photosynthesis process which combined
with water produces glucose by the reaction:
6CO2 + 6H2O → C6H12O6 + 6O2

Human beings not only add CO2 to the atmosphere by burning fossil fuels, but also
contribute to that gain through changes on landscape. That is to say, the destruction of
forests and other plant communities reduces CO2 absorption capacity increasing the CO2
net amount in the atmosphere.
5


The CO2 concentration growth creates an increase in photosynthetic rate and biomass,
but only during the first days of exposure. Then, the photosynthetic rate decreases
because the sink effect of the plants is limited.
Increased CO2 levels in the atmosphere may increase plant productivity when other
factors are no limiting (such as water). However, it is likely to be a temporary effect until
the plants are acclimatised to the change. The increased levels of CO2 might enable plants
to be more efficient, using less water to achieve the same productivity.
Light


Ultraviolet radiation of 280-300 nm. wavelength has harmful effects on organisms: photooxidation of chloroplast pigments and reduction of photosynthetic rates.
Plants have several defense mechanisms as cutin layers or wax coatings.
Other limiting factors


Plants need nutrients to grow such as the nitrogen as nitrates. Therefore, the availability
of nitrogen limits growth and thus the ability to absorb carbon from the CO2 increase.
The increase of atmospheric CO2, water vapour and temperature simulates an artificial
greenhouse, where growth and biomass production exceed normal values. However, for
this to happen in a natural environment would require that plants are provided with other
essential nutrients: N, P and K. Fertilized with nitrogen soils lead to problems such as
eutrophication and acidification. In addition, they emit nitrous oxide (N2O) which is a 200
times more potent greenhouse gas than CO2.
Assessment



When the students complete Worksheet 1, you will notice if they need further
explanations or if they have worked long enough.
Concept cartoons (Worksheet 2) and the exercise in Worksheet 3 will be used to assess
prior knowledge, the ability to develop hypothesis and how students relate concepts,
teamwork, the ability to listen to their peers, etc.
The results of Worksheet 4 will give you a general idea of which concepts and processes
have been understood or not.
6
Bibliography
Arellano, J.B., De Las Rivas J. (2006). Plantas y cambio climático. Investigación y Ciencia. Marzo
2006: pp. 42-50.
Ekici, F., Ekici, E., Aydin, F. (2007). Utility of Concept Cartoons in Diagnosing and Overcoming
Misconceptions Related to Photosynthesis. International Journal of Environmental & Science
Education, 2 (4): 111-124.
Izco, J., Barreno, E., Brugués, M., Costa, M., Devesa, J. A., Fernández-González, F., Gallardo, T.,
Llimona, X., Prada, C., Talavera, S., Valdés, B. (2004). Botánica. 2ª Edición. McGraw Hill.
Hawkins, B., Sharrock, S., Havens, K. (2008).The physiological responses of plants to climate
change. Plants and Climate Change : which future? Botanic Garden Conservation International,
pp. 17-25.
Van Dyke, F. (2008). Biodiversity Conservation and Climate Change. Conservation Biology.
Foundations, Concepts, Applications, pp. 121-152.
Velázquez de Castro, F. (2005). 25 Preguntas sobre el cambio climático. Conceptos básicos del
efecto invernadero y del cambio climático, pp. 105-114.
http://www.conceptcartoons.com
http://www.euita.upv.es/VARIOS/BIOLOGIA/Temas/tema_11.htm#Desarrollo%20histórico
http://www.sciencemadesimple.com/leaves.html
7