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.
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