Overview of Photosynthesis - Online Tutor, Practice Problems & Exam Prep

Photosynthesis is the process by which plants convert light energy into ATP and sugars, involving two main reactions: light-dependent and light-independent (Calvin cycle). Light-dependent reactions generate ATP and NADPH through photophosphorylation, utilizing chlorophyll to absorb light. The Calvin cycle uses ATP and NADPH to fix CO₂ into a 3-carbon sugar, which can be further synthesized into glucose. This process releases O₂, essential for respiration. Key components include the electron transport chain and the role of NADP⁺ in photoreduction.

Overview

Hi. In this video, I'm going to be talking about an overview of photosynthesis. So, photosynthesis is just the process of converting light into ATP and sugars. And, you know this; you've probably talked about photosynthesis since, I don't know, elementary school. But in Cell Biology, we are going to go over a lot more details of photosynthesis than you would ever care to know about. It is composed of two different types of reactions. There are the light-dependent reactions, which are responsible for creating ATP and NADPH. Do you remember, notice this one is different because it has this special NADPH, and so that's going to be specific to photosynthesis, electron carrier in plants. In a light-dependent reaction, the cells undergo photophosphorylation, which is just a fancy term that means it uses an electron transport chain to drive ATP synthesis, and its starting point is light, because it is photo. This is very similar to oxidative phosphorylation. I mean, it is essentially the exact same definition as oxidative phosphorylation. But, the difference is that the final electron donation is actually not given to oxygen. We'll talk more about that and the exact steps, which I'm sure you're thrilled about.

So, how this happens is light energy comes in, it's absorbed by this pigment chlorophyll, which we all know about, and that light energy is then used by the cell to drive ATP synthesis. And photoreduction happens, which is the process through which the final electron, remember, is not given to O₂ and instead is given to this electron carrier NAD⁺ which creates NADPH. Now, you're saying, "Oh my gosh, this is so detailed. Why are you even telling me about this?" Well, don't worry about it right this second. In future videos, we're going to go over every single step and everything that's important about every single step. But now just know there's this light-dependent reaction and a light-independent reaction, which you may see is called carbon fixation. And these reactions use ATP and NADPH that was created in a light-dependent reaction to create sugar from CO₂. So, plants take in CO₂, we know this. So, the product of this reaction is going to be a 3-carbon sugar, which then the cell can use to create much larger sugars. For instance, if it needs to create glucose, which is a very common thing it creates, it's going to need 18 ATP, 12 NADPHs and converts 12 waters into oxygen. And then we use that oxygen to breathe. I don't know why I said 'need', but yay. Photosynthesis.

So just an overview of what happens. Light comes in. The light reactions happen. They create all these fun things: ATP, NADPH. This then goes into the Calvin cycle, or the light-independent or carbon fixation. You'll see it as so many different words because for some reason we can't name anything one term. And it uses CO₂ to create sugar and releases O₂. So this is the overview of photosynthesis and we're going to get into much more detail in future videos, so be excited for that. So now, let's turn the page.

Which of the following is not a part of photosynthesis?

• A. Light dependent reactions
• B. Photoreduction
• C. Light independent reactions
• D. Photocycle

The 3-carbon sugar created during photosynthesis is generated in which of the following processes?

• A. Light independent reaction
• B. Photophosphorylation
• C. Photoreduction
• D. Light dependent reaction

Here’s what students ask on this topic:

What are the main stages of photosynthesis?

Photosynthesis consists of two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). The light-dependent reactions occur in the thylakoid membranes of the chloroplasts, where light energy is absorbed by chlorophyll and converted into chemical energy in the form of ATP and NADPH. These reactions also produce oxygen as a byproduct. The Calvin cycle, which takes place in the stroma of the chloroplasts, uses the ATP and NADPH generated in the light-dependent reactions to fix carbon dioxide (CO₂) into a 3-carbon sugar. This sugar can then be used to form glucose and other carbohydrates.

How do light-dependent reactions generate ATP and NADPH?

In light-dependent reactions, light energy is absorbed by chlorophyll, exciting electrons to a higher energy state. These high-energy electrons travel through the electron transport chain (ETC) in the thylakoid membrane. As electrons move through the ETC, they drive the pumping of protons (H⁺) into the thylakoid lumen, creating a proton gradient. This gradient powers ATP synthase to produce ATP from ADP and inorganic phosphate (Pi) in a process called photophosphorylation. Simultaneously, the electrons reduce NADP⁺ to NADPH, which is used in the Calvin cycle for carbon fixation.

What is the role of chlorophyll in photosynthesis?

Chlorophyll is a pigment located in the thylakoid membranes of chloroplasts that plays a crucial role in photosynthesis. It absorbs light energy, primarily in the blue and red wavelengths, and converts it into chemical energy. When chlorophyll absorbs light, its electrons become excited and are transferred to the electron transport chain, initiating the light-dependent reactions. This process ultimately leads to the production of ATP and NADPH, which are essential for the Calvin cycle to synthesize sugars from CO₂.

What is the Calvin cycle and how does it function in photosynthesis?

The Calvin cycle, also known as the light-independent reactions or carbon fixation, occurs in the stroma of chloroplasts. It uses ATP and NADPH produced in the light-dependent reactions to convert CO₂ into a 3-carbon sugar, glyceraldehyde-3-phosphate (G3P). The cycle involves three main phases: carbon fixation, reduction, and regeneration of the starting molecule, ribulose-1,5-bisphosphate (RuBP). For every three molecules of CO₂ fixed, the cycle produces one G3P, which can be used to form glucose and other carbohydrates.

How is oxygen produced during photosynthesis?

Oxygen is produced during the light-dependent reactions of photosynthesis. When chlorophyll absorbs light energy, it excites electrons that are then transferred to the electron transport chain. To replace these lost electrons, water molecules (H₂O) are split in a process called photolysis. This splitting of water releases electrons, protons (H⁺), and oxygen (O₂). The oxygen is released as a byproduct and diffuses out of the chloroplast, eventually exiting the plant through the stomata.