BIO 311C Lecture Notes - Lecture 10: Mitochondrial Matrix, Accessory Pigment, Photosystem I
Chapter 10: Photosynthesis
1. The Process That Feeds the Biosphere
a. Plants and other photosynthetic organisms contain organelles called chloroplasts
b. Photosynthesis is the process that converts solar energy into chemical energy within chloroplasts
c. Directly or indirectly, photosynthesis nourishes almost the entire living world
d. Autotrophs are “self-feeders” that sustain themselves without eating anything derived from other organisms
i. Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other
inorganic molecules
e. Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules
i. Photosynthesis occurs in plants, algae, certain other unicellular eukaryotes, and some prokaryotes
f. These organisms feed not only themselves but also most of the living world
g. Heterotrophs obtain organic material from other organisms
i. Heterotrophs are the consumers of the biosphere
ii. Some eat other living organisms; others, called decomposers, consume dead organic material or feces
h. Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
i. Earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of
years ago
j. Fossil fuels are being consumed faster then they are being replenished
k. Researchers are exploring methods of using the photosynthetic process to produce alternative fuel
2. Concept 10.1: Photosynthesis converts light energy to the chemical energy of food
a. Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
b. The structural organization of these organelles allows for the chemical reactions of photosynthesis
c. Chloroplasts: The Sites of Photosynthesis in Plants
i. Leaves are the major locations of photosynthesis in plants
ii. Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf
iii. Each mesophyll cell contains 30–40 chloroplasts
iv. CO2 enters and O2 exits the leaf through microscopic pores called stomata
v. A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma
vi. Thylakoids are connected sacs in the chloroplast which compose a third membrane system
1. Thylakoids may be stacked in columns called grana
vii. Chlorophyll, the pigment which gives leaves their green color, resides in the thylakoid membranes
d. Tracking Atoms Through Photosynthesis: Scientific Inquiry
i. Photosynthesis is a complex series of reactions that can be summarized as the following equation:
1. 6 CO2 12 H2O Light energy → C6H12O6 6 O2 6 H2O
ii. The overall chemical change during photosynthesis is the reverse of the one that occurs during cellular
respiration
e. The Splitting of Water
i. Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar
molecules and releasing oxygen as a by-product
f. Photosynthesis as a Redox Process
i. Photosynthesis reverses the direction of electron flow compared to respiration
ii. Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced
iii. Photosynthesis is an endergonic process; the energy boost is provided by light
g. The Two Stages of Photosynthesis: A Preview
i. Photosynthesis consists of the light reactions
(the photo part) and Calvin cycle (the synthesis part)
ii. The light reactions (in the thylakoids)
1. Split H2O
2. Release O2
3. Reduce the electron acceptor NADP
to NADPH
4. Generate ATP from ADP by photophosphorylation
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iii. The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
iv. The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
3. Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH
a. Chloroplasts are solar-powered chemical factories
b. Their thylakoids transform light energy into the chemical energy of ATP and NADPH
c. The Nature of Sunlight
i. Light is electromagnetic energy, also called electromagnetic radiation
ii. Electromagnetic energy travels in rhythmic waves
iii. Wavelength is the distance between crests
of electromagnetic waves
iv. Wavelength determines the type of electromagnetic energy
v. The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
vi. Visible light consists of wavelengths (380 nm to
750 nm) that produce colors we can see
vii. Visible light also includes the wavelengths that drive photosynthesis
viii. Light also behaves as though it consists of discrete particles, called photons
d. Photosynthetic Pigments: The Light Receptors
i. Pigments are substances that absorb visible light
ii. Different pigments absorb different wavelengths
iii. Wavelengths that are not absorbed are reflected or transmitted
iv. Leaves appear green because chlorophyll reflects and transmits green light
v. A spectrophotometer measures a pigment’s ability to absorb various wavelengths
1. This machine sends light through pigments and measures the fraction of light transmitted at each
wavelength
vi. An absorption spectrum is a graph plotting a pigment’s light absorption versus waveleng
vii. There are three types of pigments in chloroplasts:
1. Chlorophyll a, the key light-capturing pigment
2. Chlorophyll b, an accessory pigment
3. Carotenoids, a separate group of accessory pigment
viii. The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for
photosynthesis
ix. An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a
process
1. The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann
2. In his experiment, he exposed different segments of a filamentous alga to different wavelengths
3. Areas receiving wavelengths favorable to photosynthesis produced excess O2
4. He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production
x. The action spectrum for photosynthesis is broader than the absorption spectrum of chlorophyll
xi. Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
xii. The difference in the absorption spectrum between chlorophyll a and b is due to a slight structural
difference between the pigment molecules
xiii. Accessory pigments called carotenoids may broaden the spectrum of colors that drive photosynthesis
xiv. Some carotenoids function in photoprotection; they absorb excessive light that would damage chlorophyll
or react with oxygen
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