Photosynthesis: Light Reactions and the Calvin Cycle

Photosynthesis: Light Reactions and the Calvin Cycle


Professor Dave here, let’s learn about photosynthesis. We talked about how cellular respiration
transforms the food we eat into ATP, but what do plants do? They don’t really eat
in the normal sense of the word, apart from venus flytraps. As it turns out,
plants are getting their energy directly from the source, the sun. Plants use
sunlight as well as water and carbon dioxide, which are plentiful in our
earthly surroundings, to produce their own nourishment through a cellular
process called photosynthesis. This word makes sense, because photo comes from the
greek word for light, and synthesis means to build, so plants use light and small
molecules to build bigger molecules that they can then metabolize to produce
energy. Other organisms then eat the plants, and other organisms eat those
organisms, so plants really are the foundation of the entire food chain. Let’s learn about how photosynthesis
works. Photosynthesis consists of two stages. There are the so-called light
reactions, in which solar energy is converted into chemical energy, and then
there’s the Calvin Cycle, which uses that chemical energy to build G3P, the
precursor to glucose and other molecules. Both of these stages occur inside one of
the organelles found in plant cells which are called chloroplasts, and there
are around 30 to 40 of them per cell. A chloroplast has two membranes that
surround a fluid called stroma within which sacs called thylakoids are
suspended. These sacs are often stacked in columns called grana, and in the
membrane of these thylakoids sits a special pigment molecule called
chlorophyll. Chlorophyll can take either an A or a B form differing only in one
functional group inside the porphyrin ring, either a methyl or aldehyde group.
The porphyrin ring is what gives this molecule the capacity to absorb sunlight
in a unique way. If chlorophyll absorbs a photon of a particular energy, one of its
electrons jumps up to an excited state. These chlorophyll
molecules sit inside of a photosystem with many other chlorophylls, and this
excitation can get passed around from molecule to molecule, with one electron
relaxing back to the ground state emitting another photon that will excite
an electron in another molecule, and so forth, like a pinball machine. Eventually, a photon may strike the
reaction center complex, which also contains two molecules of chlorophyll.
When this happens, the high-energy excited electron, rather than relaxing
back down to the ground state, will instead be transferred to another
molecule called the primary electron acceptor, after which an enzyme supplies
the missing electron to the oxidized chlorophyll via a water molecule to get
it back to normal, which is how oxygen molecules are produced. This is the first
step of the light reactions. These are a series of redox reactions that occur
first in photosystem II and then in photosystem I. If they seem like they are
named backwards, it’s because they were named in the order that they were
discovered. In photosystem II, electrons flow beginning with the primary electron
acceptor through a series of compounds that are embedded in the thylakoid
membrane, sort of like the electron transport chain from cellular
respiration. ATP will be a byproduct of this process due to a proton gradient
and ATP synthase, also just like in cellular respiration. Now electrons will
move through photosystem I, another electron transport chain, but instead of
producing ATP, photosystem I will result in the conversion of nicotinamide
adenine dinucleotide phosphate, or NADP+ into NADPH. So the light reactions in the
thylakoid, which are summarized by the following equations, require sunlight and
water among the reactants, and generate oxygen, ATP, and NADPH as products. ATP and
NADPH are then used in the Calvin cycle which occurs in the stroma. This is where
all the synthesis occurs. Unlike the citric acid cycle, which is
catabolic, the Calvin cycle is anabolic, building organic molecules from smaller
components, requiring energy to do so. This happens in three phases. Phase one
is carbon fixation, which is catalyzed by an enzyme called RuBisCo. RuBisCo captures
CO2 from the atmosphere and attaches it to a five-carbon sugar called ribulose bisphosphate. The resulting six carbon molecule is unstable and splits
into two molecules of 3-phosphoglycerate. Phase two is reduction. Each of these
molecules receives a phosphate from ATP and is then reduced by NADPH, after which
it loses a phosphate to become glyceraldehyde-3-phosphate. Some of this
G3P is the output of the Calvin cycle going on to other pathways that generate
glucose and other organic compounds, and some goes back to generate more RuBP and start the cycle over again. To generate a net output of one G3P, the
cycle needs 9 ATPs and 6 NADPHs. So to summarize, photosynthesis occurs in two
phases. In the light reactions, light and water go in and oxygen comes out, as well
as the ATP and NADPH that are used in the Calvin cycle where carbon dioxide
comes in and organic products like sugars come out. In this way it’s almost
like the reverse of cellular respiration. They involve a series of redox reactions,
the electrons just flow in opposite directions. In one, a sugar is built, in the
other, sugar is degraded. And that’s how plants make their own food. Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always, feel free to email me:

46 thoughts on “Photosynthesis: Light Reactions and the Calvin Cycle”

  1. sir you are last minute saver today was my chemistry test and I revised it totally on your videos btw I love the jingle 😇😇

  2. how smart is this process, make me think of the guys who sorted all this out. If we guys have a bit of difficulty in understanding all of this I think we should spend a little time giving prase to all the people who discovered all of this,by checking out the history of these discoveriers.

  3. Professor Dave, at 2:14, from what part of chlorophyll does the excited electron come? Is it from the Mg2+ ion in the center, or from another part of the molecule?

  4. Professor, in your video you say that 1 electron is excited by the photon. However, 2 electrons from water are used to replenish chlorophyll, and it takes 2 electrons to reduce NADP+ to NADPH, so where does the other electron come from?

  5. I really love how you described the whole H+ gradient in the lumen and ATP synthesis from the flow of H+ through ATP Synthase it was really great! If you can't tell, I am being sarcastic. You literally didn't even mention the biggest part and main energy producer of the entire light reaction process. This is so misleading!!! DO NOT WATCH THIS VIDEO!!! Congrats on the great video. I'm trying to study for midterms and videos like these are the death of me.

  6. Man this deserves more views. Honestly this is the best video on photosynthesis I've found and I've been searching for 2 hours. This is even better than bozemanscience and khan academy
    THANK YOU I LOVE YOU

  7. what happens after plants produce GLUCOSE? will it converted into ATP's as happens in cellular respiration in animals?

  8. Been studying for about six hours now reviewing videos that my professor has provided and was lost…this just cleared it up in about six minutes. Thanks!

  9. I love these videos. Liked, and subscribed. Relating to photosynthesis, and chloroplasts, are cell walls, or anything specific to a plant cell required for the survival of chloroplasts?

  10. I was searching to what felt like an ETERNITY and I have finally found the right youtube video
    everyone subscribes to Professor Dave Explains
    and to pewdipie

  11. Great video! One minor correction: at 2:00 you refer to the ring structure of chlorophyll as porphyrin. It is in fact a chlorin ring, since it only has 20 π electrons and is therefore not fully aromatic like porphyrin (with 22 π electrons).

  12. Nice clip, thank you for sharing! After watching this clip, i realized Calvin Cycle, which helps me to understand photosynthesis.

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