Photophosphorylation 1 - Online Tutor, Practice Problems & Exam Prep

Photophosphorylation is the process of synthesizing ATP during photosynthesis, primarily using chlorophyll a, which absorbs light at 680 nm. Autotrophs convert sunlight into carbohydrates and oxygen, while heterotrophs produce carbon dioxide and water through oxidative phosphorylation. Accessory pigments like chlorophyll b and carotenoids, such as beta carotene, enhance light absorption. The action spectrum illustrates the correlation between light wavelength and photosynthetic activity, highlighting the efficiency of these pigments in capturing light energy for metabolic processes.

Photophosphorylation 1

Before diving into the details of photophosphorylation, I just want to talk about something that's really cool in terms of the evolution of these biochemical processes that we've been talking about and that we're about to talk about. And that's that autotrophs, which get their energy from the sun as a product of their metabolic reactions, produce carbohydrates and oxygen whereas heterotrophs like us, through oxidative phosphorylation processes we were just talking about, produce carbon dioxide, right, from the citric acid cycle, and water from oxygen receiving electrons in the electron transport chain. So, just really cool how there's this reciprocity between the inputs and outputs of these metabolic pathways.

Now, let's actually jump in and talk a little bit about photophosphorylation, which is the synthesis of ATP from photosynthesis and it's going to require these photopigments which are molecules that can absorb sunlight energy in the form of photons and the main photosynthetic pigment is chlorophyll a. This molecule right here is chlorophyll A. You can see it contains a porphyrin ring right here. In the center of that, unlike the porphyrin rings we saw in cytochromes, this has a magnesium. And this molecule is going to absorb light maximally at a wavelength of 680 nanometers. Now, photosynthetic organisms also use accessory pigments to broaden the range of light that they're able to use for photosynthetic processes. So, they will use molecules like chlorophyll b and the carotenoids, and if you notice the word carrot in there, that's because this molecule right here, beta carotene which is a carotenoid, is in high concentration in carrots. Carrots have a ton of beta carotene in them. It's actually what gives them their orange color. Now, you might also notice that beta-carotene has these isoprene units and that is characteristic of the carotenoids. They all have isoprenes. These other molecules that are used to absorb light called phycobilins. Here's one right here. You can see that they have a tetrapyrrole, right? 1, 2, 3, and 4 And that's just like chlorophyll right here, right? Look at the porphyrin ring in chlorophyll. It also has 4 pyrroles in it, right? Except it's in a ring form whereas in phycobilins, they're linear. So, these molecules absorb light and we call the spectrum of light that they're able to absorb the absorption spectrum. And you can see their absorption spectra of chlorophyll a and chlorophyll b right here. And notice how they kind of complement each other. Chlorophyll. Now, the plot of photosynthetic activity against the wavelength of light is called an action spectrum. That's what this is over here. This is an action spectrum. So you can see that on the Y-axis we have the rate of photosynthesis. And as you can see, there's a correlation between the rate of photosynthesis, right? And the absorption ranges of the photopigments. So hopefully, you can get why the rate of photosynthesis will be highest in the ranges where powering photosynthesis essentially.

With that, let's flip the page.

Here’s what students ask on this topic:

What is photophosphorylation and how does it differ from oxidative phosphorylation?

Photophosphorylation is the process of synthesizing ATP during photosynthesis, primarily using chlorophyll a, which absorbs light at 680 nm. It occurs in autotrophs, which convert sunlight into carbohydrates and oxygen. In contrast, oxidative phosphorylation occurs in heterotrophs, where ATP is produced from the oxidation of nutrients, resulting in the production of carbon dioxide and water. The key difference lies in the energy source: photophosphorylation uses light energy, while oxidative phosphorylation relies on chemical energy derived from nutrient oxidation.

What are the main pigments involved in photophosphorylation?

The main pigment involved in photophosphorylation is chlorophyll a, which absorbs light maximally at 680 nm. Additionally, accessory pigments like chlorophyll b and carotenoids, such as beta carotene, enhance light absorption by broadening the range of light wavelengths that can be utilized. Phycobilins are another group of pigments that absorb light and contribute to the process. These pigments collectively capture light energy and convert it into chemical energy during photosynthesis.

How do chlorophyll a and chlorophyll b differ in their absorption spectra?

Chlorophyll a and chlorophyll b have different absorption spectra, which means they absorb light at different wavelengths. Chlorophyll a absorbs light maximally at around 680 nm, while chlorophyll b absorbs light at slightly different wavelengths, typically around 450-500 nm and 640-660 nm. This complementary absorption allows plants to utilize a broader range of the light spectrum for photosynthesis, enhancing their efficiency in capturing light energy.

What is the action spectrum and how does it relate to photosynthetic activity?

The action spectrum is a plot that shows the rate of photosynthesis against the wavelength of light. It illustrates the correlation between the light wavelengths absorbed by photosynthetic pigments and the resulting photosynthetic activity. The action spectrum highlights the efficiency of different wavelengths in driving photosynthesis, with peaks corresponding to the absorption maxima of key pigments like chlorophyll a and b. This relationship helps us understand which wavelengths are most effective in powering photosynthesis.

What role do accessory pigments play in photophosphorylation?

Accessory pigments, such as chlorophyll b and carotenoids, play a crucial role in photophosphorylation by broadening the range of light wavelengths that can be absorbed. These pigments capture light energy at wavelengths that chlorophyll a cannot absorb efficiently, thereby enhancing the overall light absorption capacity of the photosynthetic apparatus. This increased range of light absorption allows plants to maximize their photosynthetic efficiency and adapt to varying light conditions.