Review of Photosynthesis: Study with Video Lessons, Practice Problems & Examples

Photosynthesis occurs in chloroplasts and consists of two stages: light reactions and the Calvin cycle. Light reactions, taking place in thylakoids, utilize photons to split water, producing oxygen, ATP, and NADPH. The Calvin cycle, occurring in three phases, uses ATP, NADPH, and carbon dioxide to synthesize glucose. Under stress, plants may undergo photorespiration, with C4 and CAM plants adapting to minimize energy loss. Key components include rubisco for carbon fixation and the regeneration of RuBP, essential for sustaining the photosynthetic process.

Recap Map of Photosynthesis

In this video, we're going to do a review of our lesson on photosynthesis by recapping our map of the lesson on photosynthesis, which is down below right here. And so, really, in this video, there's going to be no new information covered. It's all going to be review information from our previous lesson videos. And so, if you're already feeling very good about photosynthesis, then you can feel free to skip this video if you'd like, because again, there's no new information in this video; it's only going to be review information. However, if you're a little bit hesitant about photosynthesis or our photosynthesis lesson, then feel free to stick around because this video could be very helpful for you. Now that being said, we're going to recap our map of the lesson on photosynthesis, which is down below right here. And of course, we know that photosynthesis is going to occur inside of the chloroplast of plants here. And so, this image that we have in the background represents the chloroplast. And we know that we've been following this map of the lesson on photosynthesis by following the leftmost branches first, and we talked about photosynthesis occurring in 2 stages. The first stage was the light reactions, and the second stage was the Calvin cycle. And so, we talked about photosynthesis occurring under the conditions where the stomata of the plant or the leaves were in an open position. And recall that the stomata are the openings or the pores or the holes that are found in the leaf that control gas exchange between the leaf itself and the outside environment. And so, here we have the stomata in an open position, so gas exchange is able to occur. Carbon dioxide gas can diffuse in, and oxygen gas can diffuse out. And also, water vapor can also diffuse out if the temperatures are too hot. Now, the light reactions are the first stage of photosynthesis, and the light reactions occur in the thylakoids of the chloroplast. And the thylakoids are those green pancake-looking structures that you can see here in the background, behind this image. And so, the light reactions, as their name implies, are going to use photons of light from our sun, and those photons of light are going to be absorbed by pigments that are found in photosystems. And recall there are 2 photosystems. There's photosystem II, followed by photosystem I. And ultimately, the light reactions are going to split water molecules. What you see here, it takes water molecules and it splits them to oxidize them to remove electrons from them, and those electrons are going to make their way through an electron transport chain and ultimately be used to generate NADPH, this electron taxi cab. And, as water molecules get split, it creates oxygen gas, and this oxygen gas is the oxygen that's associated with photosynthesis producing oxygen. And so, this oxygen here could either be used by the plant to drive aerobic cellular respiration, or this oxygen would just diffuse right out of the stomata and leave the plant into the outside environment. Now recall that when it comes to the light reactions, we had this interesting story that would help you guys remember the most important components and the most important steps of the light reactions in the correct order. And that was to remember, Luke and Ryan. And so, Luke and Ryan are supposed to represent the light reactions. Luke and Ryan, they wanted to play their PlayStation 2, their photosystem II, but then they realized that they couldn't find their electronic controllers, and so, the electrons make their way through an electron transport chain. And so then they decided to play their PlayStation I or their photosystem I, and then ultimately, they realized that their mom had reduced the number of games that they had for their PlayStation I, and that represents the reduction of NADP⁺ into NADPH. And so then Luke and Ryan decided to play, to just study chemistry, and the chemi in chemistry represents the chemiosmosis. And so, chemiosmosis ends up producing the ATP that's associated with the light reactions. And so, ultimately, oxygen gas, as well as NADPH and ATP. And this NADPH and ATP are going to be the energy that's used to power the Calvin cycle, which recall the Calvin cycle is the second stage of photosynthesis. And the Calvin cycle occurs in 3 phases. There is the carbon fixation phase, which uses rubisco, the enzyme that is going to affix carbon dioxide to the starting molecule, RuBP. And ultimately, the first stable three-carbon molecule, that's generated is going to be PGA, which is not shown here in this image. But ultimately, PGA is going to be converted into G3P in the second stage of the Calvin cycle, G3P synthesis. And G3P is the precursor that's needed to create glucose, which is a sugar that we can see down below right here. And then ultimately, the remainder of the G3P is going to be utilized in this final stage of the Calvin cycle, RuBP regeneration, to regenerate the original molecule, RuBP. And so one way to help you remember the Calvin cycle was to remember Calvin's can of sugar. And so, the can of sugar, the CAN in can were reminding you of the reactants. The C in CAN reminds you of the C in carbon dioxide. The A in CAN reminds you of the A in ATP, and the N in CAN reminds you of the N in NADPH. And so ATP, NADPH, and carbon dioxide are all used as reactants, and ultimately, the product is going to be sugar. And so, you think of Calvin's can of sugar, and that can help you remember the Calvin cycle. And of course, the Calvin cycle, when it uses the ATP and NADPH, those high-energy forms, it will convert them into their lower energy forms, ADP and NADP+, which are needed by the light reactions so that they can form the higher energy version forms. And so, this here concludes this entire left-hand side of the branch. And so after we had talked about the normal conditions for photosynthesis where the stomata is open, then we decided to talk about what happens if the temperatures are too hot, and the stomata are closed to prevent dehydration. Well, under those conditions, we had talked about photorespiration occurring, and how photorespiration occurs mainly in C3 plants. But some plants have been able to evolve solutions to this photorespiration problem, which wastes energy in the form of ATP and NADPH to make carbon dioxide. And so, the plants that have evolved the ability to avoid photorespiration are the C4 plants and the CAM plants, and these are the same images that we had utilized in our previous lesson videos to discuss these. And so, recall that the C4 plants kind of sound like a C4 explosive, and the C4 explosives are going to break things into pieces. And so the C4 plants have 2 pieces, they have the mesophyll cell and then they have the bundle sheath cell, where they separate the light reactions in the Calvin cycle. And ultimately, the CAM plants, you think of a camel wearing pajamas here, and that can remind you that the CAM plants are going to be found in really really hot and dry environments like deserts. And that, because the camel is wearing pajamas, that carbon fixation is going to occur at times of the day. We've got carbon fixation occurring at night and carbon fixation occurring during the day as well. And again, this is just a recap of what we've already covered in our previous lesson videos. And so, this here concludes our recap of the map on photosynthesis and we'll be able to get some practice applying the concepts that we've reviewed here as we move forward. So, I'll see you all in our next video.

Review of Photosynthesis Example 1

Alright. So here we have an example problem that wants us to complete the following diagram that's down below by filling in all of the blanks. And so notice that this diagram has the light reactions over here occurring inside of the thylakoids, and then it has the Calvin cycle over here occurring in the stroma. And so really what we need to recall from our previous lesson videos is that the light reactions is of course going to use light as one of the reactants. And not only does it use light, but it also uses water as one of the reactants. And so ultimately, in terms of the products of the light reactions, it ends up producing oxygen gas or O2. And then it also ends up producing chemical energy in the form of ATP and NADPH. And this ATP and NADPH is going to be utilized by the Calvin cycle as an energy source to drive the reactions. And so the Calvin cycle over here, which is occurring in the stroma of the chloroplast, is going to utilize carbon dioxide gas as a reactant or CO2 as a reactant. And ultimately, it's going to utilize the ATP and NADPH along with the CO2, and it's going to be able to produce glucose as one of the products here. And, of course, as it consumes the ATP and NADPH, it's going to convert them into their lower energy forms. So that would be ADP and NADP+. And so ultimately, this is filling out the entire diagram here to review the 2 major stages of photosynthesis: the light reactions and the Calvin cycle. And so this here concludes our example problem, and I'll see you all in our next video.

All of these are similarities between the light reactions in photosynthesis and the electron transport chain/chemiosmosis in cellular respiration EXCEPT which of these answers?

• A. Both create a H⁺ concentration gradient.
• B. Both possess ATP synthase which uses the potential energy from the H+ concentration gradient to create ATP.
• C. Both reduce electron carriers.
• D. Both have the goal of creating energy storing molecules such as ATP.

A key difference between aerobic cellular respiration and the light reactions of photosynthesis is (are):

• A. The light reactions of photosynthesis generate ATP, but aerobic cellular respiration consumes ATP.
• B. In aerobic cellular respiration, ATP is produced through chemiosmosis, but in photosynthesis, ATP is produced through substrate level phosphorylation
• C. Aerobic cellular respiration consumes reduced electron carriers (ex: NADH) to make ATP, but the light reactions synthesize reduced electron carriers (ex: NADPH) while also synthesizing ATP.
• D. The electron transport chain of aerobic cellular respiration ends with the reduction of NAD⁺, while the electron transport chain of the light reactions starts with the reduction of NADP⁺.

Here’s what students ask on this topic:

What are the two main stages of photosynthesis and where do they occur?

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle. The light reactions occur in the thylakoids of the chloroplasts. During this stage, photons from sunlight are absorbed by pigments in photosystems, leading to the splitting of water molecules and the production of oxygen, ATP, and NADPH. The Calvin cycle takes place in the stroma of the chloroplasts. It uses ATP and NADPH produced in the light reactions, along with carbon dioxide, to synthesize glucose through a series of reactions involving carbon fixation, reduction, and regeneration of RuBP.

How do C4 and CAM plants adapt to hot and dry environments?

C4 and CAM plants have evolved adaptations to mitigate the effects of photorespiration in hot and dry environments. C4 plants, like maize, separate the light reactions and the Calvin cycle into different cells: the mesophyll and bundle sheath cells. This spatial separation helps to concentrate CO2 around rubisco, reducing photorespiration. CAM plants, such as cacti, temporally separate these processes. They open their stomata at night to fix CO2 into organic acids, which are stored until daylight. During the day, the stomata close to conserve water, and the stored CO2 is released for use in the Calvin cycle.

What role do stomata play in photosynthesis?

Stomata are small openings on the surface of leaves that play a crucial role in gas exchange during photosynthesis. They allow carbon dioxide (CO2) to enter the leaf, which is essential for the Calvin cycle to synthesize glucose. Simultaneously, oxygen (O2), a byproduct of the light reactions, exits the leaf through the stomata. Stomata also facilitate the release of water vapor in a process called transpiration. The opening and closing of stomata are regulated to balance the plant's need for CO2 with the risk of water loss, especially under hot and dry conditions.

What is the function of rubisco in the Calvin cycle?

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is a critical enzyme in the Calvin cycle of photosynthesis. It catalyzes the first major step of carbon fixation, where it attaches carbon dioxide (CO2) to ribulose-1,5-bisphosphate (RuBP). This reaction produces two molecules of 3-phosphoglycerate (3-PGA), which are then used in subsequent steps of the Calvin cycle to eventually form glucose. Despite its importance, rubisco is relatively slow and can also catalyze a wasteful reaction with oxygen, leading to photorespiration. This dual activity makes rubisco a key focus in studies of photosynthetic efficiency.

How do light reactions contribute to the Calvin cycle?

The light reactions of photosynthesis generate ATP and NADPH, which are essential for the Calvin cycle. During the light reactions, photons from sunlight are absorbed by chlorophyll in the thylakoid membranes, leading to the splitting of water molecules and the release of oxygen. The energy from this process is used to produce ATP through chemiosmosis and to reduce NADP+ to NADPH. Both ATP and NADPH provide the energy and reducing power needed for the Calvin cycle to convert carbon dioxide into glucose. Without the products of the light reactions, the Calvin cycle cannot proceed.