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Page Outline
- Introduction
- Leaves
- Chloroplasts
- Photosynthesis Occurs in Two Phases
- Quiz: The Two Phases of Photosynthesis
1. Introduction
In the previous tutorial, we described photosynthesis as an endergonic oxidation-reduction reaction that occurs in the chloroplasts of plant cells. If we view photosynthesis in the context of Earth’s history, however, that last part is only partially correct. From about 3.5 billion years ago until the origin of eukaryotic cells about 1.5 billion years ago, photosynthesis was carried out by cyanobacteria. These photosynthetic bacteria (also known as blue-green bacteria) are the organisms that put oxygen into our atmosphere. A modern example of cyanobacteria is shown on the left. Note that while the cells are combined into linear filaments, each cell is a completely autonomous individual: there’s no multicellularity in these prokaryotic organisms.
Through endosymbiosis, an ancient cyanobacterium took up residence inside another cell and evolved into the chloroplasts found today. You can find chloroplasts in unicellular protists (like the Euglena on the right, which you might have observed swimming in a drop of water under a microscope), algae (like seaweed), or plants. Let’s continue by putting chloroplasts into the context where terrestrial primates like us usually find them: in leaves.
2. Leaves
A leaf’s structure involves tradeoffs between maximizing surface area for absorption of light while minimizing water loss. To prevent water loss, a waxy cuticle (“1”) covers the upper (“2”) and lower (“5”) epidermis. The main function of the epidermis is protective, and much of the way that protection occurs is by secreting the waxy layer at “1.”
Within the leaf, most of the photosynthesis occurs in the middle, a section called the mesophyll (at “8”). The cells that make up the mesophyll are packed with chloroplasts (“3”).
Water comes into the leaf through bundles of vascular tissue (“4”), which we commonly refer to as veins. The veins also allow sugars to leave the leaf and move to other parts of the plant where they may, in a plant like a potato, get converted into polysaccharides like starch for long-term energy storage.
Carbon dioxide enters the plant through pores on the lower epidermis. These pores are called stomata (“7”), and they are formed by guard cells (“6”), which can change shape to adjust the size of the stomatal opening, even to the point of closing up altogether when the plant is experiencing a lack of water.
3. Chloroplasts
Looking at a chloroplast up close, you can see several vestiges of its bacterial origins. Like the cyanobacteria they evolved from, chloroplasts have a double membrane. The outer membrane is shown at “1.” The inner membrane is shown at “2.” For a very brief overview of chloroplast evolution, you can read this article on Wikipedia (the link opens in a new tab).
Chloroplasts, like mitochondria, have their own circular, bacteria-like chromosome (at “3”). Number 4 shows another vestige of the chloroplast’s once-independent existence. These are ribosomes, which have a bacterial (as opposed to a eukaryotic) molecular structure.
Chloroplasts are filled with membrane-bound sacs called thylakoids (“5”), which are organized into stacks (like a stack of pancakes). Outside of the thylakoid is a fluid called the stroma (“6”), which is analogous to the cytosol of a eukaryotic cell, or the matrix of a mitochondrion.
4. Within Chloroplasts, Photosynthesis Occurs in Two Phases
In the last tutorial, we looked at the overall equation for photosynthesis:
6CO2 + 6H2O + light energy –> C6H12O6 + 6O2
Within a chloroplast, photosynthesis occurs in two distinct phases, shown in roman numerals (“I” and “II”) in the diagrams below. Because of the complexity of the diagram, I’m providing you with both labeled and unlabeled versions.
Phase I (“I,” in the diagram below) makes up the light reactions. Driven by light energy (“1”), water gets oxidized: it’s split apart into high-energy electrons and protons (neither of which are shown below). These electrons and protons are used to reduce the oxidized electron carrier NADP+ (shown at “8”) into its reduced, high-energy form, NADPH (shown at “5”). Energy harvested during the light reactions is also used to convert low-energy ADP and inorganic phosphate (“9” and “10,” respectively) into high-energy ATP (“4”). The waste product (but one essential for aerobic organisms like us) is oxygen, shown at “3”. All of this occurs in the thylakoid sacs, with the outputs of light reactions (NADPH and ATP) moving into the stroma to support photosynthesis’ second phase.
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Phase II of photosynthesis uses the products of Phase 1 as inputs (excepting oxygen, which bubbles into the atmosphere). Phase 2 is known as the Calvin cycle, after Berkeley Biochemist Melvin Calvin, who received the Nobel Prize for his co-discovery of the cycle in 1961 (also the year Mr. W was born). Because the Calvin cycle doesn’t depend directly on light, you’ll also see it referred to as the light-independent reactions (or even as the dark reactions).
During the Calvin Cycle, the key external input is carbon dioxide (“6”). Using energy from ATP (“4”), and electrons and hydrogen from the energy-rich, reduced electron carrier NADPH (“5”), carbon dioxide is reduced to become the three-carbon sugar glyceraldehyde phosphate, or G3P (“7”), which can be converted into glucose (C6H12O6 ) or anything else the plant needs.
5. Quiz: The Two Phases of Photosynthesis
In the next tutorials, we’ll dive into the details of the light reactions and the Calvin cycle. But let’s consolidate what we’ve learned above through the following quiz.
[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-PSN: The Two Phases of Photosynthesis”]
[h]The Two Phases of Photosynthesis
[i]
[q question_number=”1″ dataset_id=”SMV_PSN_Two_Phases_of_PSN|71727174b15e4″] The first organisms to perform photosynthesis were
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|716da41c2b1e4″ question_number=”2″] In the diagram below, which number shows the cells that secrete the upper waxy layer that helps prevent water loss?
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|71692145215e4″ question_number=”3″] In the diagram below, which number shows the organelle that’s performing photosynthesis?
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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|7164792d595e4″ question_number=”4″] Photosynthesis requires carbon dioxide as an input. Which part shows where carbon dioxide would enter the leaf?
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|715ff6564f9e4″ question_number=”5″] Photosynthesis requires water as an input. Which part shows where water would enter the leaf?
[textentry single_char=”true”]
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[q multiple_choice=”true” dataset_id=”SMV_PSN_Two_Phases_of_PSN|715b4e3e879e4″ question_number=”6″] The similarities between cyanobacteria and chloroplasts is evidence that chloroplasts arose through
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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|7156cb677dde4″ question_number=”7″] The light reactions occur in which part?
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|7152234fb5de4″ question_number=”8″] The Calvin Cycle occurs in which part of the chloroplast?
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[c]Ng ==[Qq]
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|714d7b37edde4″ question_number=”9″]One of the strongest pieces of evidence for the endosymbiotic origins of chloroplasts is the fact that they have DNA, which is shown at
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|7148f860e41e4″ question_number=”10″]If number 4 represents ATP, then which number would represent NADP+?
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[c]OA ==[Qq]
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|71447589da5e4″ question_number=”11″]What letter or number represents the stroma?
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|713fcd72125e4″ question_number=”12″]If 9 is ADP, then what letter or number represents the reduced molecule that’s an output of the light reactions and an input for the Calvin cycle?
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[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|713b255a4a5e4″ question_number=”13″]Which number or letter represents water?
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|71367d42825e4″ question_number=”14″]Which number or letter represents oxygen?
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[c]Mw ==[Qq]
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|71314027c15e4″ question_number=”15″]Which number or letter represents carbon dioxide?
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[c]Ng ==[Qq]
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[c]ICo=[Qq]
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Cg==[Qq]
[q dataset_id=”SMV_PSN_Two_Phases_of_PSN|712afe47cc9e4″ question_number=”16″]During photosynthesis, carbon dioxide gets reduced. What number or letter represents what carbon dioxide gets reduced into?
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