Download Leaf Physiology a Simulation

Leaf Physiology, a Simulation
Objectives & Goals
The purpose of this laboratory is to:
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Demonstrate how photosynthetic rates in different plants can change in response to
factors such as light intensity, light quality, CO2 concentration, and temperature.
Study the effects of light quantity and quality on photosynthetic rates.
Compare photosynthesis in sun and shade plants.
Introduction
In this laboratory, you will simulate photosynthesis in the leaves of different plants. By changing
environmental conditions such as light intensity, light quality, temperature, gas flow, and carbon
dioxide concentration, you will learn about the importance of each parameter. By measuring the
amount of carbon dioxide consumed by the plant cells as they undergo the reactions of
photosynthesis, you will observe how altering these conditions affects each plant type. From
these data you will then calculate and compare rates of photosynthesis.
Before You Begin: Prerequisites
Before beginning LeafLab you should review the following concepts:
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Energy
Conditions that influence enzyme function
Properties and function of a photosynthetic pigment
The photosynthetic pathway including the light reactions and the Calvin cycle.
Background Information
Less than 1% of the sun’s energy is captured and used by living organisms, yet without this
energy, life on Earth as we know it cannot exist. Plants capture the light energy from the sun and
convert it to carbohydrates in the process called photosynthesis. Because plants can produce
organic molecules to feed themselves and support their metabolism without eating other
organisms, they are referred to as autotrophs or producers. In addition to light energy,
autotrophs also rely on carbon dioxide, water, and soil nutrients to produce organic molecules by
photosynthesis. Below is the summary equation showing the yield of products created by
photosynthesis.
Light Energy + 6 CO2 + 12 H2O
C6H12O6 + 6 O2 + 6 H2O
Other autotrophs include algae, certain protists, such as Euglena, and some photosynthetic
bacteria. Plants are essential producers of energy for many animals, including humans. Members
of the Animal Kingdom are known as heterotrophs because they must obtain their energy by
eating other organisms. Plants are the primary source of carbohydrates entering the global food
web. Of equal importance, plants also provide the oxygen necessary for heterotrophs to convert
carbohydrates into ATP during the reactions of glycolysis and cell respiration. Remember that
photosynthesis in plants is not a replacement reaction for glycolysis and cell respiration. Plants
must still break down energy sources to produce ATP; however, photosynthesis provides plants
with their own source of carbohydrates. Approximately 50% of the carbohydrates produced by
most plant cells are used to produce ATP so the plant can survive.
The production of carbohydrates by photosynthesis can be grouped into two major metabolic
stages: (1) the light reactions and (2) the Calvin cycle. In plant cells, both sets of reactions
occur within chloroplasts.
The light reactions of photosynthesis occur in the stacked membrane-bound pancakes called
thylakoids inside the chloroplasts. These reactions produce the energy-storing molecules of
NADPH and ATP. Thylakoids absorb specific wavelengths of light because they contain a
number of photosynthetic pigments embedded in the membrane that are capable of absorbing
visible light. Chlorophyll a, which absorbs blue and red light, is the predominant pigment in the
thylakoids of most plants. In addition to chlorophyll a, other pigments -- including chlorophyll b
and a group of pigments known as the carotenoids -- are capable of absorbing other colors of
visible light. The overall color of most plant leaves is green because the chlorophyll a is the most
abundant and reflects green light.
The light reactions begin when light energy strikes pigment molecules in the thylakoid
membrane, resulting in photoexcitation of these pigments. Some of the electrons in these
molecules are elevated to higher electron shells, or excited, by the input of light energy. These
excited electrons are captured and used by the plant cell to drive the production of NADPH and
ATP. As electrons are transported along a chain of proteins in the membrane, an H+ gradient is
established inside the thylakoid disc.
The hydrogen gradient forms because as electrons are transferred along this chain and loose
energy, hydrogen ions are pumped into the thylakoid discs. This H+ gradient provides the energy
necessary for the enzyme ATP synthase. ATP synthase functions as an ion channel to allow H+
to flow down a gradient from inside the thylakoid into the chloroplast. When H+ flows through
ATP synthase, the enzyme synthesizes ATP from ADP and inorganic phosphate.
These light reactions supply ATP and NADPH to the reactions of the Calvin cycle. Because the
reactions of the Calvin cycle do not require light energy directly, these reactions are known as the
light-independent reactions of photosynthesis. The reactions of the Calvin cycle occur in the
stroma of the chloroplast. These reactions, which consume ATP and NADPH, produce
glyceraldehyde 3-phosphate (G3P), which is subsequently used to synthesize carbohydrates.
It has been estimated that worldwide production of carbohydrates by photosynthetic organisms is
approximately 160 million metric tons. Approximately 50% of the carbohydrates are consumed
by that cell, during glycolysis and respiration. The remaining carbohydrates may be stored as
starch in various parts of the plant or used to make other necessary molecules.
Many adaptations exist to allow plans to survive better under certain environmental conditions.
Consider the fact shade-tolerant plants (e.g., ferns) often grow in the dim sunlight of a forest floor
while sun plants (e.g., marigolds) prefer direct exposure to sun. Shade and sun plants have
developed differences in the photosynthetic enzymes, and differences in leaf structure that allow
each type to grow better under certain conditions. Because of these adaptations, photosynthetic
rate and other parameters of photosynthesis can differ even when these plants are exposed to the
same light intensity. Interestingly, for some plant species, both sun and shade leaves can be found
on the same plant.
Hopefully, the Leaflab simulations will provide you an introduction to the types of experiments
that current botanists use to study the factors that influence the rate of photosynthesis in plant
leaves.
Please Note: C4 plants have an extra set of chemical reactions that occur prior to the calvin cycle.
These reactions distinguish these plants from those that use only the Calvin cycle to convert CO2
to G3P.
References
1. Mauseth, J. D. Botany: An Introduction to Plant Biology, 2nd ed. Sudbury, MA: Jones
and Bartlett, 1998.
2. Raven, P. H., Evert, R. F., and Eichhorn, S. E. Biology of Plants, 6th ed. New York:
Freeman, 1999.
3. United States Department of Agriculture Natural Resources Conservation Service Web
Site. http://plants.usda.gov/
4. Audesirk, Audesirk, and Byers. Biology:Life on Earth (with Physiology) 8th ed.
Pearson/Prentice Hall 2008.