Photosynthesis - Light-dependent Stage - Post 16 Biology (A Level, Pre-U, IB, AP Bio)

Summary

Photosynthesis, 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

00:00

🌿 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.

05:00

💧 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.

Keywords

💡Photosynthesis

Photosynthesis is a two-stage process that plants use to convert light energy into chemical energy. It involves the light-dependent stage and the light-independent stage (Calvin Cycle). The video explains how chloroplasts utilize light to produce ATP and NADPH in the light-dependent stage, which are then used in the light-independent stage to synthesize glucose.

💡Light-dependent stage

The light-dependent stage of photosynthesis occurs in the thylakoid membranes of the chloroplasts, where light energy is absorbed by chlorophyll, leading to the production of ATP, NADPH, and oxygen as a waste product. This stage is crucial because it provides the energy and electron carriers needed for the light-independent stage of photosynthesis.

💡Light-independent stage

Also known as the Calvin Cycle, the light-independent stage does not require light and occurs in the stroma of the chloroplast. In this stage, the ATP and NADPH produced in the light-dependent stage are used to convert carbon dioxide into glucose, which serves as an energy source for the plant.

💡Chloroplast

Chloroplasts are double-membrane organelles within plant cells where photosynthesis takes place. They contain thylakoid membranes stacked into grana and surrounded by stroma. The structure of chloroplasts is critical because the thylakoid membranes are where the light-dependent reactions occur, while the stroma hosts the light-independent reactions.

💡Thylakoid membrane

The thylakoid membrane is the site of the light-dependent reactions of photosynthesis. It contains chlorophyll and other pigments that capture light energy, photosystems that initiate electron transport, and ATP synthase that produces ATP. The extensive folding of these membranes increases the surface area for light absorption and ATP production.

💡Grana

Grana are stacks of thylakoids found within chloroplasts. They play a key role in the light-dependent stage of photosynthesis by providing a large surface area for the chlorophyll molecules and photosystems involved in capturing light and producing ATP and NADPH.

💡Stroma

The stroma is the fluid-filled space surrounding the grana in chloroplasts where the light-independent stage of photosynthesis (Calvin Cycle) occurs. It contains enzymes that catalyze the reactions of the Calvin Cycle, using ATP and NADPH from the light-dependent stage to synthesize glucose from carbon dioxide.

💡Photosystem

Photosystems are protein complexes in the thylakoid membranes that play a key role in the light-dependent stage of photosynthesis. There are two types, Photosystem I and Photosystem II, which absorb light, energize electrons, and drive the production of ATP and NADPH. Photosystem II is involved first, despite its numerical designation, and is responsible for the initial splitting of water.

💡Electron transport chain

The electron transport chain is a series of proteins in the thylakoid membrane that transfer electrons from photosystem II to photosystem I, pumping hydrogen ions into the thylakoid lumen and generating an electrochemical gradient. This process is essential for the production of ATP and the overall energy conversion in the light-dependent stage.

💡Calvin Cycle

The Calvin Cycle, or the light-independent stage of photosynthesis, occurs in the stroma of the chloroplast. It uses ATP and NADPH produced in the light-dependent stage to fix carbon dioxide into organic molecules, ultimately producing glucose. The cycle is a crucial process that enables plants to synthesize their own food from inorganic carbon.

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