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Feb 11, 2026
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Photosynthesis Light Reactions Explained Step-by-Step
Shea Swan
@iiamsheaaa
1 / 3
Photosynthesis: The Light Reactions Basics
Photosynthesis consists of two main parts: the Light Reactions and the Calvin Cycle. The Light Reactions occur only when sunlight is present and take place in the thylakoid membranes of plant cells.
During these reactions, plants use sunlight and water as reactants. They produce two essential energy carriers: ATP (energy currency) and NADPH (electron carrier). These molecules will later power sugar production in the Calvin Cycle.
The process begins when Photosystem II (PSII) absorbs light energy. The chlorophyll molecule (specifically Chlorophyll 680) at the center of PSII gets so excited by this energy that an electron pops off and moves to an electron acceptor. This creates an electron deficiency that needs to be filled.
💡 Think of chlorophyll like a solar panel that gets so charged up by sunlight that it releases electrons - these electrons then flow through the plant cell like electricity through wires!
To replace these lost electrons, plants perform water splitting - breaking water molecules apart. This process provides new electrons to PSII while releasing oxygen into the atmosphere as a byproduct. The hydrogen ions from water will be used later in the process.
The Electron Journey
After electrons leave Photosystem II, they enter the Electron Transport Chain (ETC) - a series of three proteins that pass electrons along like a bucket brigade. As these excited electrons move through the chain, they gradually lose energy, which is used to pump hydrogen ions across the thylakoid membrane.
This pumping creates a concentration gradient with many H+ ions inside the thylakoid membrane. This gradient stores potential energy, similar to water behind a dam.
The journey continues when light hits Photosystem I (PSI), exciting electrons in Chlorophyll 700. These newly excited electrons get picked up by another acceptor, while PSI replaces its lost electrons with those coming from the ETC.
🔋 The concentration gradient of H+ ions is like a biological battery that the cell can tap into when it needs energy!
The excited electrons from PSI are then passed to NADP+, which acts like an electron "taxi service." When NADP+ receives two electrons and a hydrogen ion , it transforms into NADPH. This molecule will carry this captured energy to the Calvin Cycle where sugar production happens.
Completing the Light Reactions
The formation of NADPH is a crucial step in the light reactions. NADP+ first accepts electrons to become NADP-, which then attracts hydrogen ions to form NADPH. This molecule is now charged with energy and electrons ready to fuel sugar production.
Meanwhile, the high concentration of H+ ions built up inside the thylakoid membrane creates pressure. These ions naturally want to diffuse back across the membrane (moving from high to low concentration). As they flow through special protein channels called ATP synthase, they provide the energy needed to combine ADP and phosphate into ATP.
This process, called photophosphorylation, is like a water wheel generating electricity as water flows through it. The flowing H+ ions power the ATP synthase "wheel" to produce ATP.
🌟 Every time you take a breath of oxygen, thank the light reactions! The oxygen we breathe is actually a byproduct of plants splitting water during photosynthesis.
In the final step, the newly created ATP and NADPH molecules move from the thylakoid membranes to the stroma . There, they'll deliver their captured energy to power the Calvin Cycle, where the plant will use carbon dioxide from the air to produce sugars.
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
Photosynthesis Light Reactions Explained Step-by-Step
Shea Swan
@iiamsheaaa
Photosynthesis powers life on Earth by converting sunlight into usable energy. The light reactions are the first stage of this process, capturing sunlight energy and transforming it into chemical energy that plants can use to build sugars. Understanding this process... Show more
Photosynthesis: The Light Reactions Basics
Photosynthesis consists of two main parts: the Light Reactions and the Calvin Cycle. The Light Reactions occur only when sunlight is present and take place in the thylakoid membranes of plant cells.
During these reactions, plants use sunlight and water as reactants. They produce two essential energy carriers: ATP (energy currency) and NADPH (electron carrier). These molecules will later power sugar production in the Calvin Cycle.
The process begins when Photosystem II (PSII) absorbs light energy. The chlorophyll molecule (specifically Chlorophyll 680) at the center of PSII gets so excited by this energy that an electron pops off and moves to an electron acceptor. This creates an electron deficiency that needs to be filled.
💡 Think of chlorophyll like a solar panel that gets so charged up by sunlight that it releases electrons - these electrons then flow through the plant cell like electricity through wires!
To replace these lost electrons, plants perform water splitting - breaking water molecules apart. This process provides new electrons to PSII while releasing oxygen into the atmosphere as a byproduct. The hydrogen ions from water will be used later in the process.
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The Electron Journey
After electrons leave Photosystem II, they enter the Electron Transport Chain (ETC) - a series of three proteins that pass electrons along like a bucket brigade. As these excited electrons move through the chain, they gradually lose energy, which is used to pump hydrogen ions across the thylakoid membrane.
This pumping creates a concentration gradient with many H+ ions inside the thylakoid membrane. This gradient stores potential energy, similar to water behind a dam.
The journey continues when light hits Photosystem I (PSI), exciting electrons in Chlorophyll 700. These newly excited electrons get picked up by another acceptor, while PSI replaces its lost electrons with those coming from the ETC.
🔋 The concentration gradient of H+ ions is like a biological battery that the cell can tap into when it needs energy!
The excited electrons from PSI are then passed to NADP+, which acts like an electron "taxi service." When NADP+ receives two electrons and a hydrogen ion , it transforms into NADPH. This molecule will carry this captured energy to the Calvin Cycle where sugar production happens.
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Completing the Light Reactions
The formation of NADPH is a crucial step in the light reactions. NADP+ first accepts electrons to become NADP-, which then attracts hydrogen ions to form NADPH. This molecule is now charged with energy and electrons ready to fuel sugar production.
Meanwhile, the high concentration of H+ ions built up inside the thylakoid membrane creates pressure. These ions naturally want to diffuse back across the membrane (moving from high to low concentration). As they flow through special protein channels called ATP synthase, they provide the energy needed to combine ADP and phosphate into ATP.
This process, called photophosphorylation, is like a water wheel generating electricity as water flows through it. The flowing H+ ions power the ATP synthase "wheel" to produce ATP.
🌟 Every time you take a breath of oxygen, thank the light reactions! The oxygen we breathe is actually a byproduct of plants splitting water during photosynthesis.
In the final step, the newly created ATP and NADPH molecules move from the thylakoid membranes to the stroma . There, they'll deliver their captured energy to power the Calvin Cycle, where the plant will use carbon dioxide from the air to produce sugars.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan S
iOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha Klich
Android user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
Anna
iOS user
I think it’s very much worth it and you’ll end up using it a lot once you get the hang of it and even after looking at others notes you can still ask your Artificial intelligence buddy the question and ask to simplify it if you still don’t get it!!! In the end I think it’s worth it 😊👍 ⚠️Also DID I MENTION ITS FREEE YOU DON’T HAVE TO PAY FOR ANYTHING AND STILL GET YOUR GRADES IN PERFECTLY❗️❗️⚠️
Thomas R
iOS user
Knowunity is the BEST app I’ve used in a minute. This is not an ai review or anything this is genuinely coming from a 7th grade student (I know 2011 im young) but dude this app is a 10/10 i have maintained a 3.8 gpa and have plenty of time for gaming. I love it and my mom is just happy I got good grades
Brad T
Android user
Not only did it help me find the answer but it also showed me alternative ways to solve it. I was horrible in math and science but now I have an a in both subjects. Thanks for the help🤍🤍
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
I found this app a couple years ago and it has only gotten better since then. I really love it because it can help with written questions and photo questions. Also, it can find study guides that other people have made as well as flashcard sets and practice tests. The free version is also amazing for students who might not be able to afford it. Would 100% recommend
Aubrey
iOS user
Best app if you're in Highschool or Junior high. I have been using this app for 2 school years and it's the best, it's good if you don't have anyone to help you with school work.😋🩷🎀
Marco B
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This app is phenomenal down to the correct info and the various topics you can study! I greatly recommend it for people who struggle with procrastination and those who need homework help. It has been perfectly accurate for world 1 history as far as I’ve seen! Geometry too!
Paul T
iOS user