Transcript theoretical background - Pianeta Scuola Gallery

THEORETICAL
BACKGROUND
introduction
PHOTOSYNTHESI
S:A
LABORATORY
EXPERIMENT
EXPERIMENT
procedure
results
CONCLUSIONS
THEORETICAL BACKGROUND
NUTRITION IN LIVING ORGANISMS
Nutrition is the process of obtaining energy and matter in
order to drive metabolism in cells.
There are two types of nutrition:
1. Heterotrophic nutrition occurs when food is eaten by an organism
and is broken down to provide ready-made nutrients. Heterotrophic
means “other-feeding” and is found in all animals, humans included.
2. Autotrophic nutrition occurs when nutrients are made by the
organisms themselves from simple, inorganic chemicals and the
input of energy. Autotrophic means “ self-feeding”. Plants use this
method: photosynthesis.
THEORETICAL BACKGROUND
Photosynthesis is a chemical process that takes place in the chloroplasts
of green plants. Chloroplasts are organelles found in the cells of the green
parts of the plant: leaves and, sometimes, stems.
Photosynthesis takes place in daytime when it is light. Sunlight energy is
absorbed by green chlorophyll and used to turn carbon dioxide and water
into carbohydrate and oxygen gas. The carbon dioxide is absorbed by the
leaves from the outside air while water enters the plant through the roots
and travels upwards. This word equation sums up the whole process:
Sunlight + carbon dioxide + water =(chlorophyll)* glucose + oxygen
* chlorophyll acts as a catalyst for the process: so it helps photosynthesis
take place but is not itself permanently changed or used up.
THEORETICAL BACKGROUND
Photosynthesis is important because:
• the glucose formed is used for respiration and provides the plant
with energy.
• some glucose can be stored in the form of starch for later use.
• some glucose can be converted to cellulose and used for the
formation of cell walls for new cells.
• glucose, with the addition of minerals from the soil, can be
converted into pigments, vitamins, fats, oils and proteins, all of
which are required by the plant to grow and develop.
• some of the oxygen formed is used in plant respiration and is
released into the air.
• AND …
THEORETICAL BACKGROUND
Photosynthesis in plants provides food for all living organisms.
Plants are at the bottom of all food chains. Sunlight energy is passed to all
consumers through these chains.
Photosynthesis provides oxygen for all living organisms.
It is the only natural source of oxygen gas on the Earth.
Photosynthesis reduces the level of carbon dioxide in the
atmosphere. It is a vital part of the control of CO2 levels.
The destruction of young trees in rain forests has worsened the
accumulation of carbon dioxide in the atmosphere.
EXPERIMENT: INTRODUCTION
• Remember: photosynthesis is a chemical process through
which a plant produces an organic substance. Photosynthesis
is represented in this equation:
• 6 CO2 + 6 H2O + 686 KCAL (sunlight energy)
6O2 + C6 H12 O6
• Remember that the glucose C6 H12 O6 is a monosaccharide
sugar that usually polymerizes into complex sugars, such as
starch, that is stored in amyloplasts (leucoplasts) inside leaves
and in the plant’s roots.
EXPERIMENT: INTRODUCTION
• First prepare some geranium leaves by covering areas of some
leaves with dark cardboard so as to block out all light.
Photosynthesis does not take place when there is no light.
Expose the leaves to strong light for 48 hours.
• In order to see whether photosynthesis has taken place in the
areas exposed to light test for starch: starch is made during
photosynthesis. We expect there will be no starch in the parts of
the leaf that have not been exposed to light for 48 hours.
• To test for starch, boil the leaves in ethyl alcohol to remove some
chlorophyll. Then colour the leaf with Lugol’s reagent, a colouring
agent that reacts with complex sugars, i.e. with starch.
EXPERIMENT: AIM – RESOURCES – MATERIALS
AIM: test for starch in leaves (if photosynthesis has taken
place there will be starch in the leaves)
RESOURCES: test tube - Petri dish – pliers – bunsen burner
MATERIALS: geranium leaf / leaves
Lugol’s reagent
Ethyl alcohol
EXPERIMENT: PROCEDURE
PROCEDURE:
• Expose a plant to sunlight for 48 hours with some leaves partly
covered in black paperboard;
• Select a leaf and photograph it (to keep a record of chlorophyll
distribution);
• Lift the black covering, roll up the leaf and put it in a test tube
containing ethyl alcohol;
• Heat the test tube in a double boiler (in bain-marie), boiling the
alcohol will decolorize the leaf;
• Take the leaf out of the test tube and dip it in water to soften it;
• Place the leaf in a Petri dish;
• Flood the whole leaf with Lugol’s reagent drops;
• Rinse the leaf and observe the colour.
EXPERIMENT: RESULTS
RESULTS:
• Compare the photograph to the original, observe the
differences.
• Lugol (which reacts with starch) has reacted in the leaf
section exposed to sun (colour near violet), because
glucose is one of the products of photosynthesis:
glucose is a monosaccharide which easily polymerises
into starch.
• The covered part of the leaf has changed colour
because photosynthesis has not taken place so Lugol’s
reagent cannot detect starch.
EXPERIMENT: CONCLUSIONS
CONCLUSION:
What happened in the leaf?
The part of the leaf that was covered was not coloured by Lugol’s
reagent, it is lighter. This means that there were no carbohydrates,
starch, in these parts of the leaf for the Lugol’s reagent could react
with as Lugol’s reagent only changes colour in the presence of
starch.
Starch is not a product of the photosynthesis, but
when
photosynthesis takes place, most of the glucose formed is stored in
the leaf as starch.
Since Lugol’s reagent didn’t find any carbohydrate in the part
covered in cardboard, we can say that photosynthesis did not take
place there. Thus we can conclude that sunlight is essential for
photosynthesis, because without sun it does not take place.