Introduction:
In this lab we will examine photosynthesis, one of the two major processes by which most organisms
convert energy from an unusable form to a usable chemical form (photosynthesis), and then convert
that chemical energy into ATP, the energy currency of the cell (cellular respiration). We begin with a
definition of each process:
Photosynthesis: Conversion of unusable sunlight energy into the energy locked inside the covalent
bonds of glucose (a simple sugar you recall from previous labs). Carbon dioxide (CO2) and water
(H2O) serve as the raw materials that the cells assembles into sugar (C6H12O6), with dioxygen (O2)
given off (as a by-product).
Cellular Respiration: Transfer of the energy locked inside glucose (and sometimes other molecules)
into the most readily usable energy for cells, Adenosine Triphosphate (ATP). Sugar and dioxygen are
the starting products that produce carbon dioxide and water plus heat and ATP (“recharged” from the
lower energy form, ADP, Adenosine Diphosphate).
Objectives:
1. Learn about the requirements for the photosynthesis process to occur
2. Conduct a series of
guided investigations in a virtual science lab
Think about it:
1.Which of the Five Kingdoms of Life contain organisms that carry out photosynthesis?
2.Which of the Five Kingdoms of Life contain organisms that carry out cellular respiration?
Recall the discussion of the characteristics of life at the beginning of the class. What characteristic of
life does this lab deal with?
The Nature Of Light
White light is separated into the different colors (=wavelengths) of light by passing it through a prism.
The order of colors is determined by the wavelength of light. Visible light is one small part of the
electromagnetic spectrum as shown in below. The longer the wavelength of visible light, the more red
the color. Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths
longer than red are referred to as infrared, while those shorter than violet are ultraviolet.
The electromagnetic spectrum. Note the relatively narrow range of the visible light wavelengths
Think about it : What is your familiarity with the terms infrared and ultraviolet? What uses and/or
risks do they pose to humans?
Plant pigments
A pigment is any substance that absorbs light. The color of the pigment comes from the wavelengths
of light reflected (in other words, those not absorbed). Chlorophyll, the green pigment common to all
photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be
detected by our eyes. Black pigments absorb all of the wavelengths that strike them. White
pigments/lighter colors reflect all or almost all of the energy striking them. Pigments have their own
characteristic absorption spectrum, the absorption pattern of a given pigment.
Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other
photosynthetic organisms. All photosynthetic organisms (plants, certain protistans, and cyanobacteria)
have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory
pigments include chlorophyll b, xanthophylls, and carotenoids (such as beta-carotene). Chlorophyll a
absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the
intermediate (Green-Yellow-Orange) wavelengths.
The site of photosynthesis
Eukaryotic cells have numerous membrane-bound organelles in which specific processes necessary
for cell function take place. Prokaryotic cells lack such compartments, instead having the various
enzymes involved in the process either bound to the inside of the prokaryote’s cell membrane or loose
in the cytoplasm. Photosynthesis occurs in both prokaryotic and eukaryotic creatures.
Plants are a eukaryotic group, and their cells (as I hope you recall) are characterized by green
(usually) organelles known as chloroplast. Using electron microscopes, scientists have discovered the
compartments that occur in chloroplasts: flattened membrane sacs known as thylakoids, surrounded
by a fluid stroma.
The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes
have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have
chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known
collectively as grana. The areas between grana are referred to as stroma.
Photosynthesis: Two Steps
Photosynthesis is a two-stage process. The first process is the Light Dependent Process (Light
Reactions), requires the direct energy of light to make energy carrier molecules that are used in the
second process. The Light Independent Process (or Dark Reactions) occurs when the products of the
Light Reaction are used to form C-C covalent bonds of carbohydrates (mostly sugars). The Dark
Reactions can usually occur in the dark, if the energy carriers from the light process are present. The
Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the
chloroplasts.
In the Light Dependent Processes (Light Reactions) a series of reactions convert unusable sunlight
energy into usable energy stored in ATP and NADPH. Water is split in the process, releasing dioxygen
(O2) as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the Light
Independent Process (Dark Reactions).
In the Light Independent Process (Dark Reactions), carbon dioxide from the atmosphere (or water for
aquatic/marine organisms) is captured and modified by the addition of hydrogen to form
carbohydrates. The incorporation of carbon dioxide into organic compounds is known as carbon
fixation. The energy for this comes from the energy carriers produced by the light reactions.
What Photosynthesis Accomplishes
Photosynthesis is among the most important chemical processes on the planet. Oxygen produced by
photosynthesis has transformed our ocean and atmosphere. The vast majority of living things depend
either directly or indirectly on the energy-rich carbohydrates made by photosynthetic creatures.