Photosynthesis - Light-dependent Stage - Post 16 Biology (A Level, Pre-U, IB, AP Bio)
Summary
TLDRPhotosynthesis, the process by which plants convert light energy into chemical energy, is a two-stage process: the light-dependent stage occurring on the thylakoid membranes of chloroplasts, and the light-independent stage taking place in the stroma. During the light-dependent stage, chlorophyll absorbs sunlight, splitting water to produce oxygen and generating ATP and NADPH. The light-independent stage, or the Calvin cycle, uses these products to fix carbon dioxide into glucose, without the need for light. The video script delves into the structural components of chloroplasts and the detailed mechanisms of both stages, including non-cyclic and cyclic photophosphorylation.
Takeaways
- 🔆 Photosynthesis is a two-stage process: light-dependent and light-independent stages.
- 🌿 Chloroplasts have a double membrane with thylakoid membranes stacked into grana, surrounded by stroma.
- 💡 The light-dependent stage occurs in the thylakoid membranes, while the light-independent stage occurs in the stroma.
- ⚡ In the light-dependent stage, sunlight hits chlorophyll, causing water to split and release oxygen as a waste product, while producing ATP and NADPH.
- 🌀 ATP and NADPH are used in the Calvin cycle during the light-independent stage to convert carbon dioxide into glucose.
- 🔋 Photosystems I and II, located in the thylakoid membrane, play key roles in the light-dependent stage by moving excited electrons through an electron transport chain.
- 💧 Water splitting (photolysis) replaces electrons lost in Photosystem II and generates oxygen as a byproduct.
- 🌱 ATP synthase helps convert ADP to ATP as hydrogen ions move down their electrochemical gradient.
- ♻ Cyclic photophosphorylation allows plants to produce more ATP without generating NADPH when there’s a shortage of NADP+.
- 🌳 The Calvin cycle uses ATP and NADPH to fix carbon dioxide into organic molecules, eventually producing glucose.
Q & A
What is the meaning of the word 'photosynthesis'?
-Photosynthesis is derived from the Greek words 'photo' meaning light and 'synthesis' meaning to make, thus it refers to the process by which plants make food using light energy.
How many stages are there in photosynthesis?
-Photosynthesis is a two-stage process, which includes the light-dependent stage and the light-independent stage.
What is the function of chloroplasts in photosynthesis?
-Chloroplasts are the organelles in plant cells where photosynthesis takes place. They contain thylakoids and stroma, which are essential for the light-dependent and light-independent stages of photosynthesis.
What is the role of thylakoids in the chloroplast?
-Thylakoids are the highly folded inner membrane structures within chloroplasts where the light-dependent reactions of photosynthesis occur.
What is the purpose of the stroma in chloroplasts?
-The stroma is the fluid surrounding the thylakoids in chloroplasts where the light-independent reactions, specifically the Calvin cycle, take place.
What happens during the light-dependent stage of photosynthesis?
-During the light-dependent stage, light energy is absorbed by chlorophyll, water is split to release oxygen and produce hydrogen ions, and ATP and NADPH are generated.
What are the two photosystems involved in the light-dependent stage?
-The two photosystems involved are Photosystem II and Photosystem I, where light energy is used to excite electrons and initiate the electron transport chain.
What is the significance of the electron transport chain in photosynthesis?
-The electron transport chain in photosynthesis is crucial for transferring energy from excited electrons to produce ATP and NADPH, which are used in the subsequent stages.
How does the Calvin cycle relate to the light-independent stage of photosynthesis?
-The Calvin cycle is the light-independent stage where CO2 is fixed into organic molecules using the energy from ATP and NADPH, ultimately producing glucose.
What is the difference between non-cyclic and cyclic photophosphorylation?
-Non-cyclic photophosphorylation produces both ATP and NADPH, while cyclic photophosphorylation only produces ATP, recycling electrons between Photosystem II and Photosystem I without producing NADPH or oxygen.
Why is water essential in the light-dependent stage of photosynthesis?
-Water is essential as it is split during photolysis to replace lost electrons in Photosystem II, also releasing oxygen as a byproduct and providing protons for ATP synthesis.
Outlines
🌿 Understanding Photosynthesis: The Two-Stage Process
Photosynthesis is a two-stage process involving the light-dependent and light-independent stages. The light-dependent stage occurs on the thylakoid membranes of chloroplasts, where sunlight energy is used to split water, releasing oxygen and producing hydrogen ions that contribute to ATP and NADPH synthesis. The light-independent stage, or the Calvin cycle, takes place in the stroma and uses ATP and NADPH to fix carbon dioxide into glucose. The script discusses the structure of chloroplasts, including the envelope, thylakoids, grana, and stroma, and explains the role of photosystems and electron transport chains in the light-dependent reactions.
💧 Photolysis and ATP Production in Photosynthesis
The script delves into the photolysis of water, which is a key part of the light-dependent stage, replacing electrons lost in photosystem 2 and generating oxygen as a waste product. It also details how the electrochemical gradient created by hydrogen ions in the thylakoid lumen drives ATP synthesis. Two pathways are discussed: non-cyclic photophosphorylation, which produces both ATP and NADPH, and cyclic photophosphorylation, which generates ATP without producing NADPH or consuming water. The latter process is an alternative when plants have a surplus of sunlight but are low on NADP+. The Calvin cycle, which uses the products of the light-dependent reactions to fix carbon dioxide into glucose, is also introduced.
Mindmap
Keywords
💡Photosynthesis
💡Light-dependent stage
💡Light-independent stage
💡Chloroplast
💡Thylakoid membrane
💡Grana
💡Stroma
💡Photosystem
💡Electron transport chain
💡Calvin Cycle
Highlights
Photosynthesis is a two-stage process: the light-dependent and light-independent stages.
The term 'photo' means light and 'synthesis' means to make, referring to how photosynthesis creates energy.
The light-dependent stage takes place in the thylakoid membranes of the chloroplast.
The light-independent stage occurs in the stroma of the chloroplast.
Chloroplasts have an inner and outer membrane, known as the envelope.
Grana are stacks of thylakoid membranes within chloroplasts.
In the light-dependent stage, sunlight energizes chlorophyll, which splits water molecules, releasing oxygen and producing ATP and NADPH.
Photosystems I and II are essential for capturing light energy in the chloroplast.
Photosystem II absorbs light first, followed by Photosystem I.
Excited electrons move through an electron transport chain, pumping hydrogen ions to generate ATP.
Water splitting, or photolysis, replaces lost electrons in Photosystem II and releases oxygen as a waste product.
ATP synthesis occurs when hydrogen ions move down an electrochemical gradient through ATP synthase.
The Calvin cycle in the light-independent stage uses ATP and NADPH to convert carbon dioxide into glucose.
Cyclic photophosphorylation allows the plant to generate ATP when NADP+ is low, without producing NADPH.
Non-cyclic photophosphorylation produces both ATP and NADPH, and requires water for photolysis.
Transcripts
[Music]
so photosynthesis is actually really a
two-stage process and the clue is in the
word photo meaning light and synthesis
meaning make so the first part of photos
synthesis is something called what we
call the light dependent stage and the
second part of photosynthesis is
something called the light independent
stage
now in order to understand
photosynthesis we've really got to learn
or recap a little bit about the
structure of chloroplasts they are
double membrane organel which means they
have an inner and an outer uh membrane
which together are referred to as the
envelope the inner membrane is highly
folded into these things called thids
and they are stacked up into things
called
Grana and then surrounding all these
thids is a fluid called the
stroma and you should be able to not
only label a diagram of a chloroplast
but also an electron micrograph of a
chloroplast so let's go back to this
twostep process the light dependent
stage of photosynthesis takes place on
these thid membranes and that's why
there's so many of them packed inside a
chloroplast and the light independent
stage happens in the stroma so let's
look at this overall summary of the
process before we get into the
nitty-gritty of of these two stages of
photosynthesis what happens in the light
dependent stage is that energy from the
Sun hits chlorophyll in the thilo covid
membranes um water is split uh which
releases oxygen as a waste gas but it
also provides hydron which are used to
make ATP and
nadph these can then pass through to the
second part of photosynthesis the light
independent stage it doesn't require any
light for this stage uh to happen the
ATP and the nadph are using something
called the Calvin cycle Now The Calvin
Cycle takes carbon dioxide from the
atmosphere it uses the energy from ATP
and nadph to produce the glucose that we
need from this whole
process so looking at the light
dependent stage in more detail um we
need to look at the Philo covid membrane
now if we zoom in on the Philo covid
membrane we can see that are some
structures there are some things called
photosystems photos system one and
photos system 2 which is where we find
the chlorophyll and there are also some
electron uh
carriers uh and this is an electron
transport chain just like you will see
in respiration um these can pick up
electrons and they can be passed along
down energy levels there is also an ATP
synthes just like what we saw again in
um chem osmosis in respiration so light
is constantly traveling into the plant
into their cells and hitting photosystem
one and photo system 2 now you may be
wondering why photos system 2 is
actually the first one in this chain
here but it's because photos system one
was discovered first and then they
discovered photos system 2 so they've
kept it in the order um that they were
discovered that's how they're named but
it is actually where we're going we're
going to start our journey at photos
system 2
so a photon of light actually hits the
chlorophyll uh inside photos system 2
and the chlorophyll starts vibrating and
it if enough um if it starts doing this
enough then that energy will be passed
into its electron and that electron will
be free will become free from the
chlorophyll molecule and that excited
electron moves up to this higher energy
State um it then gets passed down an
electron transport chain losing energy
as it goes and this energy is used to
pump hydrogen ions uh across from the
stroma into the actual Lumen of the
thids in photos system one another
Photon needs to re excite that electron
it's lost a lot of its energy now and so
we need more light um to hit and re
excite that electron back up again to a
higher energy state it then gets
eventually pass along down another sort
of electron transport chain until it
combines with something called
nadp+ this is uh um an electron acceptor
and it's going to become reduced nadp or
nadph as it's also known and we are
going to need that in the second part of
photosynthesis now we have now lost an
electron and it's very important that
that electron is replaced and the way we
get that electron is to use use the
water this is where the water comes in
that light is actually used to split
water um which is called
photolysis and that replaces the
electrons into photosystem 2 that were
lost and that is where our oxygen waste
gas comes from which is going to uh
diffuse out of the cell it also adds
some extra hydrons into the Lumen as
well now these hydrogens have built up
uh a lot in the Lumen and we have an
electrochemical gradient and so they're
going to want to pass down the
electrical chemical gradient and the way
they do that is through ATP synthes and
as they do that they can turn ADP and
phosphates into
ATP so what we've done in the light
dependent stage is that we have produced
nadph and we've produced ATP and they
are both needed in the light independent
stage later on in photosynthesis now the
the process that we've just talked
through is more specifically known as
non-cyclic photo
phosphorilation and it requires ADP and
nadp plus for it to
work but at the end of a long day of
sunlight the plant can actually become
short in
nadp+ uh and there might still be a lot
of sunlight coming in and in this case
it can actually switch to an alternative
um light dependent stage called cyclic
photoos phosphorilation now cyclic photo
phosphorilation actually starts just
like non-cyclic Photo phosphorilation
did Photon of light comes in hits
photosystem 2 excites the chlorophyll
electron moves up to a higher energy
level gets passed down um the electron
transport chain pumping hydrogens into
the Lumen but when it gets to
photosystem One Things become a little
bit
different because there isn't any NAD p+
there um for uh that electron to be
accepted so that electron actually gets
passed all the way back into photosystem
2 it can get re excited and can get
passed back along generate more hydrogen
ions into the Lumen which can go P go
through the ATP syntheses and generate
more ATP so through cyclic photo
phosphorilation we can continue to
generate ATP in this way we're not not
going to make NA any more
nadp uh H but we're going to make lots
more
ATP we also don't need water for this
we're not going to split water because
we don't need to replace that electron
in photos system 2 because we're just
recycling the same electron back from
photo system one to photo system 2 over
and over again so what's going to happen
next is the light independent stage of
photosynthesis and that ATP and that
nadph can now go into something called
the Calvin cycle Now The Calvin Cycle
takes carbon
dioxide uh and it fixes that carbon
dioxide um into or organic molecules
which will eventually make glucose
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