The Light Dependent Reaction: Photophosphorylation and Chemiosmosis

We’ve previously seen that there are two stages of photosynthesis: the light-dependent reaction and the light-independent reaction.

And, for the light-independent reaction to take place, two products of the light-dependent reaction are required.

Now, the light-dependent reaction takes place in the thylakoids

Specifically, this reaction takes place in the thylakoid membrane, which sits between the thylakoid space…

And the stroma.

The light-dependent reaction requires an enzyme in the membrane, which catalyses the production of ATP…

And ATP is the first important molecule produced in the light-dependent reaction.

However, in order for this enzyme to function, a range of other proteins and molecules are needed.

So, in this video, we’ll focus on the enzyme responsible for production of ATP…

And then, in subsequent videos, we’ll look at how other proteins and molecules contribute to the process.

So, to produce ATP, an enzyme in the thylakoid membrane is needed

This enzyme catalyses the reaction between ADP and a free inorganic phosphate group…

Producing ATP

And since a fancy term for producing is synthesising…

Biologists have given this enzyme a pretty unimaginative name – ATP synthase 

Now, ATP synthase requires energy to combine ADP and a phosphate group

And in this case, the energy is provided by H+ ions, which are also called protons

But how does this absolutely tiny ion transfer any useful amount of energy to ATP synthase?

Well, first, there’s a higher concentration of protons in the thylakoid space…

Compared to the stroma. 

So, there’s a kind of concentration gradient between the thylakoid space and the stroma

And since this concentration gradient involves protons, biologists call it a proton gradient 

Next, protons diffuse down the proton gradient through ATP synthase

So, the protons move…

The protons move from the thylakoid space to the stroma 

This movement of ions across a partially permeable membrane, from an area of high concentration to an area of low concentration is known as chemiosmosis 

And so, which of the following is an example of chemiosmosis? 

Here, the ions move across a partially permeable membrane, from an area of a high concentration to an area of low concentration

So, this is an example of chemiosmosis. 

Next, it’s the chemiosmosis of protons through ATP synthase that provides energy for ATP synthase to catalyse the production of ATP

And while the diffusion of protons through ATP synthase might seem similar to the action of a transport protein 

Biologists don’t class ATP synthase as a transport protein, just as an enzyme that catalyses the production of ATP!

Next, this method of producing ATP is known as photophosphorylation

Where photo is the Greek word for light, since this reaction takes place in the light-dependent reaction…

And this method involves the phosphorylation of ADP!

Finally, in other resources, you might also see the proton gradient called an electrochemical gradient

An electrochemical gradient is just a concentration gradient that involves a charged particle, like a proton

So, a proton gradient is a specific type of electrochemical gradient…

And we’ll stick with calling it a proton gradient here on out.

To sum up, the light-dependent reaction takes place in the…

The light-dependent reaction takes place in the thylakoid membrane.

The first important molecule produced in the light-dependent reaction is ATP

And to produce ATP, an enzyme is needed called…

This enzyme is called ATP synthase

And to catalyse the production of ATP, ATP synthase needs energy, which is supplied to ATP synthase through the diffusion of protons down a proton gradient 

And the movement of these ions across a partially permeable membrane, from an area of a high concentration to an area of low concentration is known as…

And the movement of ions across a partially permeable membrane, from an area of a high concentration to an area of low concentration is known as chemiosmosis