PLANT METABOLISM I and II
Objectives
1) Explain the purposes of and relationship between photosynthesis and respiration in plants.
2) Describe the sources and sinks involved in the acquisition and utilization of carbon in plant
systems.
3) Describe the means by which plants gain and lose mass.
4) Explain the significance of plant mass gain and loss to larger-scale ecosystem processes, such
as the global carbon cycle.
PLANT METABOLISM I: The Acquisition and Utilization of Energy and
Mass in Plants
Energy in Plants
If there was just one thing I could have more of, I would probably choose energy! We all know
what we mean when we use the term “energy”, but before you read on, see if you can attach a
definition to it. Merriam-Webster offers the following:
Main Entry: en·er·gy
Function: noun
Etymology: Late Latin energia, from Greek energeia activity, from energos active, from en in + ergon
work
Date: 1599
1 a : dynamic quality <narrative energy> b : the capacity of acting or being active <intellectual energy>
2 : vigorous exertion of power : EFFORT <investing time and energy>
3 : the capacity for doing work
4 : usable power (as heat or electricity); also : the resources for producing such power
Entries 3 and 4 are probably best suited for our interpretation of energy in biological systems.
We all need energy to do “work”, and “work” can mean a lot of different things: getting up from
the chair, training for a race, thinking about getting up from the chair to train for a race,
digesting a meal, pumping your blood, or making new skin cells to repair a paper cut. So, from
where do we get our “capacity to do work”? Food, right? If you're planning on running a
marathon (or cramming for finals) you know you can boost your energy level and performance by
eating the right “high-energy” foods.
What about plants? If you said “plants don't do work”, deduct one point from your final score!
They may not run races, or even think about them, but they do require energy for growth and
maintenance and to respond to environmental cues and stresses much in the same way you do.
So, from where do they get their “capacity to do work”? If you said “the Sun”, deduct another
point! You're on the right track, but plants get their energy from food just as you do. The
primary difference is that instead of hunting or gathering for their food, plants use energy
from the Sun in order to make their own.
In this lab, we will explore the concept of energy in plants by growing seedlings under a variety
of different environmental conditions. You will be challenged to explain how the environment
relates to the overall response of plants in terms of their ability to make and use food. Where
does a plant’s energy come from? What do plants need energy for? Where does the energy
ultimately go? If your plants gain mass during the experiment, where did that mass come from?
Can plants lose mass? How? Where does it go? And, can you describe the relationship between
mass and energy in plants? What about in you?
Throughout the lab (as well as the rest of the course), I urge you to revisit these questions in
the context of different labs, different plants, different environments, and the adaptations
and variations in plants that constrain or facilitate energy and mass acquisition and allocation.
Metabolism
“Metabolism” is often defined as “the sum of all the reactions of an organism”. When we speak
of our own metabolism, such as “this new diet is supposed to boost my metabolism!” we generally
are referring to the rate at which we burn substrates, such as fats and sugars, to fuel the
reactions necessary for our own growth and maintenance. But what does that really mean – to
burn fat? or burn calories? At the cellular level, burning a substrate for energy literally means
breaking the chemical bonds in a molecule, such as glucose, and converting the energy that was
released from the bond into a form that can be used by the organism. Most often, this means
transferring energy contained in the bonds of glucose to bonds in other molecules, such as ATP
(adenosine triphosphate).
You may recall from high school biology that ATP is the molecule responsible for transporting
chemical energy in living cells. ATP contains 3 phosphate groups linked together by high-energy
bonds. When one of the phosphate groups is released from ATP to form ADP (adenosine
diphosphate), energy that was stored in that bond is released and can be used to perform
cellular work. If you need an analogy, think of a rubber band stretched between two people.
The stretched rubber band represents a chemical bond in ATP with potential energy (stored
energy that could be used to do work). Now imagine a pair of scissors representing an enzyme
that breaks the ATP bond. As the rubber band is cut, the free end moves quickly through the
air to snap the fingers of the individual holding it - the potential energy has been converted to
kinetic energy (energy of movement) that can do work. Breaking the phosphate bond in ATP
liberates energy that had been stored in the bond so that it can be used by the cell to do work
(ATP --> ADP + P + energy).
ATP can be formed again if energy is added back to restore the bond between a free phosphate
and ADP. Food molecules (fats, sugars, proteins) are full of chemical bonds that store energy.
“Burning” glucose for energy quite literally means breaking bonds in the glucose molecule and
transferring some of that energy to the molecule ADP and a phosphate group in order to
restore ATP (ADP + P + energy (derived from bonds in glucose) --> ATP).
Respiration retrieves energy from food
All organisms get their metabolic energy by breaking down food substrates, primarily glucose,
via a set of reactions collectively referred to as “respiration”. Most, but not all respiration is
carried out in the presence of oxygen and is referred to as “aerobic respiration” (“aerobic”
from the root “aero”, meaning “in the presence of air”). The products resulting from the
breakdown of glucose in the presence of oxygen are: 1) water, 2) CO2 gas that is released into
the atmosphere, and 3) energy stored in the bonds of the molecule ATP.
Respiration: Glucose + O2 --> CO2 + H20 + ATP energy
In order for us to carry out the above reaction, we must ingest substrates (food) that can be
converted or broken down to glucose that can be respired. Plants are no different. Well, they’re
only slightly different in that they do not need to consume food from the external environment
because they are able to make it themselves.
Plants make their own food by photosynthesis
At some point in your education, you learned that plants make food (glucose) by the process of
photosynthesis, represented by the equation below:
Photosynthesis: CO2 + H2O --(light energy)--> Glucose + O2
Note that light energy is not listed as a reactant, but is instead embedded in the reaction
arrow. This is because light energy is needed to power the reactions necessary to build a
molecule of glucose, but light is never converted to the material products of photosynthesis
(glucose, O2). CO2, H2O, Glucose, and O2 are molecules constructed of particles that have
mass - they are matter. Light is a form of energy. Matter and energy are not interchangeable in
biological systems! Energy can be transferred or converted from one form to another – for
example, potential energy in a stretched rubber band can be converted to kinetic energy, sound
energy, and heat energy. Matter can be reorganized or recycled, but is not fundamentally
changed. In other words, a carbon atom in carbon dioxide gas can be reorganized as a carbon
atom in a glucose molecule, but a carbon atom can never be converted to a different kind of
atom, such as oxygen or nitrogen, nor can a carbon atom ever be converted to any form of
energy.
The Calvin Cycle builds sugars using energy from the Light Reactions
Like respiration, “photosynthesis” is a complex assemblage of chemical reactions. The reactions
are typically divided into two phases - the Calvin Cycle and the Light Reactions. The Calvin
Cycle is the set of reactions responsible for adding carbon atoms from CO2 gas to other
molecules in the cycle (namely, the 5-carbon sugar, RuBP, or “ribulose bisphosphate”) that will
ultimately build the 6-carbon sugar, glucose .
It sounds simple enough, but it actually takes several cycles of CO2 addition and many
rearrangements of the carbon-based intermediates before actually getting to the final glucose
product. In other words, a great deal of work is involved (therefore a great deal of energy
required) just to build one molecule of glucose. To do the work of the Calvin Cycle, energy is
needed, and that energy is provided in the form of electrons transferred during the breaking of
bonds in ATP and a similar molecule called NADPH.
But where did that ATP and NADPH come from? If organisms need to burn sugars to get ATP,
but ATP (and NADPH) is needed to build sugars in the first place, how do plants get the initial
energy needed to build the sugar they will ultimately burn for energy? Confused? Don’t be.
Although the specifics are monstrously complicated, the essence of the process is fairly simple.
As stated above, photosynthesis consists of two subsets of reactions: the Calvin Cycle and the
Light Reactions. It is in the light reactions that electromagnetic energy from sunlight is used to
“energize” electrons that will be passed along a series of other “carrier” molecules. In doing so,
energy released from the electrons will be used to produce ATP and NADPH that will be used to
power the Calvin cycle.
So, how can the passing of electrons produce energy? Ever played “hot potato”? The first
person to get the hot potato gets burned – because the potato is really hot! But if you’re
number 10 in line, you’re much less likely to get burned. Heat is just one of many forms of
energy. As the potato gets passed from persons 1 through 9, some of the initial heat gets
transferred to the other players' hands and to the environment. By the time you (at position
#10) get the potato, much of the heat energy has already been dissipated to the other players
and you get a cooler potato.
This is a great analogy for the light reactions and the transfer of electrons along the
components in the electron transport chain (the sequence of carriers that receive and pass the
electrons). The electrons start out at a very high energy state – they’ve just been blasted by
photons from sunlight and they’re “hot”! But as they move along the sequence of carriers in the
electron transport chain, much of that energy gets dissipated – some is lost as heat, but some is
also used to power the production of ATP. The low-energy, or “spent” electrons then contribute
to bond formation in a new NADPH molecule. The ATP and NADPH generated in the light
reactions are then used to fuel the Calvin Cycle to make new sugars.
More of the specifics of photosynthesis and respiration will be examined in the lecture course,
but for now, focus on these key points:
1) Matter and energy are fundamentally different in biological systems.
2) In biological systems, energy may be contained IN matter in the form of chemical bonds
(shared electrons).
3) Respiration is the process that extracts bond energy from food and makes that energy
available to perform cellular work. Energy is released during respiration.
4) Photosynthesis is the process by which plants make their own food using simple molecules
(CO2 and water) containing few chemical bonds to construct complex molecules (glucose)
containing many chemical bonds. Energy is required for photosynthesis and is provided by
sunlight which energizes electrons.
METABOLISM I EXPERIMENT:
In this lab, you will set up an experiment in which you will grow plants under a variety of
environmental conditions, including different combinations of light and water. After 1 or 2
weeks of growth, you will harvest your experiment and analyze your results. You will also have
the opportunity to design your own experiment (Metabolism II) with your group to further
explore any aspect of plant metabolism that intrigues you. You will then present your
experimental results to your classmates in the form of an informal, oral poster session.
Note: keep careful notes of all your procedures and measurements in your lab notebook!!
METHODS:
1. Prepare Containers
Label 4 glass containers with your group name, section number, and one of the 4 following
treatments:
1) light + water,
2) light + dry
3) dark + water
4) dark + dry.
2. Prepare Seeds
Use tared weighing boats to measure out 5 batches of seeds of equal weight (as close as you
can). Record the exact weights used for each treatment. Place 4 of the batches of weighed
seeds into the labeled containers and cover with cheesecloth and a band.
Place the 5th batch in a labeled envelope (group name, section #) and place them in the drying
oven. This batch of seeds will be thoroughly dried to remove any traces of water that might
exist in the viable seeds so that you can calculate the true dry weight (biomass) of your seeds
in the other treatments. Because water is highly variable in plants, and because it does not
count as “tissue mass”, it must be removed for experimental analysis. For your analysis, you will
use the dry mass of this batch to adjust the initial weight of your seeds for the other
treatments – be sure to keep track of both the “fresh mass” and “dry biomass” in your notebook
for all treatments.
For the wet treatments (1 and 3), water the seeds thoroughly and allow them to soak a few
minutes to absorb (imbibe) as much water as they can. Drain off excess water after the seeds
have imbibed, but allow the seeds to remain saturated to partially submerged.
Place the containers in the proper areas designated by your instructor in the teaching
greenhouse. Treatments 1 and 2 will remain in a lighted area; treatments 3 and 4 will be placed
in the dark.
3.Maintain Your Plants!
Plants should be checked daily to track progress and assure that wet treatments remain moist
throughout the experiment. You can access the greenhouse through the exterior door 8am –
7pm Mon - Fri, and 10am – 5pm on weekends. Work with your group to devise a schedule that
distributes the responsibility among members.
4. Harvest Your Plants 1-2 Weeks After Planting
We will provide paper weighing bags for you to put your harvested plant material in. Label 4
bags with your group name, section number, and the 4 treatments (light+water, light+dry,
dark+water, dark+dry).
Carefully transfer all your fresh plant material into the bags – check to be sure there are no
seeds or seedlings clinging to the sides of the jar or in the cheesecloth. Place your sample bags
in the drying oven and allow them to dry completely (at least 48 hours).
5. Analyze Your Results
Weigh your dried samples in tared weigh boats and record your data.
Calculate the percent mass change for each treatment. Remember to use your water-adjusted
values (true biomass) for your initial seed mass.
(Biomass final) – (Biomass initial) = % Change Biomass
Biomass initial
Plot your results as histograms in Excel with biomass on the y-axis and treatment on the x-axis.
Discuss your results with your group members. Make sure you all understand and agree on your
interpretation.
METABOLISM II EXPERIMENT:
Design and conduct an experiment with your group that builds upon the concepts you are
learning about in Met I – particularly, factors that influence plant growth. Dependent variables
you might consider measuring include biomass, plant height, number of leaves, etc. . .
Discuss your ideas with your instructor and try to set your experiment up at the same time as
Met I. You may set it up later during the week or during the next lab if you choose, but be
aware of the materials and time you will need to complete the project on schedule.
(35 pts) Met II Presentation
With your group, design the PowerPoint presentation you will give to the class at the specified
date about your Met II results. Your presentation should consist of the following:
a) a brief statement about the development of your testable question
b) a clear and succinct statement of your hypothesis
c) a BRIEF description of your experimental methods
d) a graphical display of your results
e) your interpretation of the outcome
Presentations should be no more that 10 minutes per group and ALL members are required to
speak!
POSTLAB QUESTIONS: Metabolism I Pre-Harvest
1. Which of the treatments do you expect to weigh the MOST at the end of the experiment?
Why?
2. Which of the treatments do you expect to weigh the LEAST at the end of the experiment?
Why?
3. Do you expect any of the treatments to lose mass during the experiment? Explain.
4. What do plants use the sugars for that are made during photosynthesis?
5. In order to better approximate “truth” in science, scientists will often conduct the same
experiment several times or perform the same measure of a variable several times before
settling on an answer. The data that is collected from each iteration can then be averaged
across all the repeats (or “replicates”) of that measure in order to reach an estimate that is
likely to be closer to reality than any single measurement. If other groups in class performed
this same experiment in the same way with the same species you did, we can think of their data
as “replicates” of yours.
What is the advantage of using replication in an experimental design?
6. Draw a histogram of the data as you expect them to appear at the end of the experiment.
Label the axes, including units, but don’t worry about specific numerical values – we’re looking
for patterns or trends in the data.
POSTLAB QUESTIONS: Metabolism I and II Post-Harvest
1. Which of the 4 treatments in Met I weighed the most and the least in your experiment?
Were your outcomes consistent with your predictions? Did any of the treatments lose mass
during the experiment? Explain your results in terms of what you understand about plant
growth and biomass acquisition and loss.
2. Plot your data for Metabolism I and II in Excel (as histograms) and paste into your notebook.
Don’t forget to label the axes, including units.
3. As a group, write the Methods, Results, and Discussion sections of the paper that would
result from your Metabolism II experiment.