Prokaryotic Photosynthesis: Videos & Practice Problems

Prokaryotic photosynthesis differs from eukaryotic processes, as prokaryotes lack chloroplasts. Instead, light reactions occur in the plasma membrane, while the Calvin cycle takes place in the cytoplasm. Oxygenic photosynthesis produces O₂ using water as an electron donor, utilizing chlorophyll a. In contrast, anoxygenic photosynthesis does not produce oxygen, using alternative electron donors like hydrogen sulfide and employing bacteriochlorophylls. Understanding these distinctions is crucial for grasping the diversity of photosynthetic mechanisms in microorganisms.

Prokaryotic Photosynthesis

In this video, we're going to briefly focus our attention on prokaryotic photosynthesis. And so while eukaryotes such as plants can perform photosynthesis, the majority of photosynthesis is actually done by prokaryotes. Now unlike eukaryotic plant cells, photosynthetic prokaryotes actually do not have chloroplast organelles and that's because recall that prokaryotes lack membrane-bound organelles. And so what this means is that the site of photosynthesis is going to be different in prokaryotes. And so instead of occurring in the thylakoids, the light reactions in prokaryotes occur in the plasma membrane or the cytoplasmic membrane. And instead of occurring in the stroma, the Calvin cycle in prokaryotes occurs in the cytoplasm. And so we'll be able to see this down below in our image.

Now notice that we have two images down below. We have the left image over here, which is focused on, the photosynthesis specifically in eukaryotes. And then we have the right image over here, which is focused on photosynthesis in prokaryotes. And so notice that the light reaction, specifically in eukaryotes occurs in the thylakoid membrane. And so notice that the light reactions highlighted here occur in the thylakoid membranes and, of course, the thylakoids are structures within the chloroplast. So this represents the chloroplast of a eukaryotic plant cell, for example. And notice that the Calvin cycle occurs in the stroma of the chloroplast, and so the chloroplast serves as the site of photosynthesis in eukaryotes. However, in prokaryotes over here on the right-hand side, notice that they lack chloroplast organelles, And so the cytophotosynthesis is going to be different. And notice that the light reactions here are going to occur in the cell's cytoplasmic membrane or plasma membrane. And the Calvin cycle, instead of occurring in the stroma, it's going to occur in the cell's cytoplasm. And so this is an important difference when it comes to the differences between eukaryotic and prokaryotic photosynthesis.

This here concludes our brief introduction to prokaryotic photosynthesis. As we move forward, we'll be able to learn a little bit more. So, I'll see you all in our next video.

The metabolic process photosynthesis occurs in which part of the bacterial cell?

• A. Gas vacuoles.
• B. Mitochondria.
• C. Chloroplasts.
• D. Cytoplasmic membrane.

Answer: (D)

Oxygenic vs. Anoxygenic Photosynthesis

In this video, we're going to begin our lesson on Oxygenic versus Anoxygenic Photosynthesis. And so oxygenic photosynthesis, as its name implies with the 'oxygenic' part here, is going to result in the production of oxygen gas, or O₂ for short. Now the reason that oxygenic photosynthesis results in the production of oxygen gas is because organisms that perform oxygenic photosynthesis will use water, or H₂O, as the primary electron donor. And so because it uses water as the primary electron donor, it is going to result in the production of oxygen gas. Oxygenic photosynthesis uses water as the electron donor and produces oxygen gas. Organisms that perform oxygenic photosynthesis are also going to use chlorophyll a as the primary reaction center pigment molecule in both photosystem 2, as well as in photosystem 1 as well.

Now on the other hand, anoxygenic photosynthesis, as its name implies with the 'an' root here in the 'oxygenic', does not produce oxygen gas. The reason that anoxygenic photosynthesis does not produce oxygen gas is because organisms that perform anoxygenic photosynthesis use other molecules other than water as the primary electron donor. For example, they could use hydrogen sulfide as the primary electron donor. And so because they're using other molecules other than water as the primary electron donor, they are not going to produce oxygen gas. Instead, they will produce other molecules. Organisms that perform anoxygenic photosynthesis are also going to use bacteriochlorophylls as their primary pigment molecules, instead of using chlorophyll a as the primary pigment molecule like what organisms that perform anoxygenic photosynthesis do. Also, organisms that perform anoxygenic photosynthesis, instead of using both photosystem 1 and photosystem 2, they are only going to use one or the other. So they will either only use photosystem 1 but not photosystem 2, or they will only use photosystem 2 but not photosystem 1.

And so those are some of the main differences between oxygenic and anoxygenic photosynthesis. If we take a look at our image down below, notice that the left-hand side of the image is focusing on oxygenic photosynthesis. Notice that organisms that perform oxygenic photosynthesis use water molecules as the primary electron donor, H₂O. This water molecule is going to be enzymatically split, and in the process, electrons are going to be taken away from it. As this water molecule splits, it is going to generate O₂, which is oxygen gas. Once again, oxygenic photosynthesis uses water as the electron donor and results in the production of oxygen gas.

Now on the right-hand side over here, we're focusing on anoxygenic photosynthesis. Once again, organisms that perform anoxygenic photosynthesis will be using other molecules other than water as the primary electron donor. Here we're showing you hydrogen sulfide, H₂S, as the primary electron donor. And so it is going to be used to donate electrons to just one photosystem, either photosystem 1 or photosystem 2. Again, because a molecule other than water is serving as the electron donor, oxygen gas is not going to be generated in anoxygenic photosynthesis. Here what we're seeing is that some molecule that is going to contain sulfur will be generated.

Now some classic examples of bacteria that perform anoxygenic photosynthesis include green bacteria, as you see here in this image, as well as purple bacteria. And cyanobacteria is a classic example of bacteria that perform oxygenic photosynthesis. This here concludes our brief introduction to oxygenic versus anoxygenic photosynthesis, and we'll be able to get some practice applying some of these concepts as we move forward. So I'll see you all in our next video.

Choose the correct definition of oxygenic photosynthesis:

• A. It uses oxygen gas as the primary electron donor to produce water.
• B. It uses glucose as the electron primary donor to produce oxygen gas.
• C. It uses water as the electron primary donor to produce NAD⁺.
• D. It uses water as the electron primary donor to produce oxygen gas.

Answer: (D)

Oxygen produced during oxygenic photosynthesis comes directly from:

• A. Light energy.
• B. CO₂.
• C. H₂O.
• D. Glucose.
• E. None of the above.

Answer: (C)

Which of these answers illustrates a major difference between oxygenic and anoxygenic photosynthesis?

• A. Oxygenic photosynthesis uses H₂O as the electron donor, while anoxygenic photosynthesis uses H₂S.
• B. Oxygenic photosynthesis produces O₂ gas, while anoxygenic photosynthesis produces elemental sulfur.
• C. Oxygenic photosynthesis uses PSI & PSII, while anoxygenic photosynthesis uses PSI or PSII.
• D. Oxygenic photosynthesis uses chlorophyll-a to absorb light, anoxygenic photosynthesis uses bacterio-chlorophylls.
• E. All of the above are major differences between oxygenic and anoxygenic photosynthesis.

Answer: (E)

Here’s what students ask on this topic:

What is the main difference between prokaryotic and eukaryotic photosynthesis?

The main difference between prokaryotic and eukaryotic photosynthesis lies in the cellular structures involved. Prokaryotes lack membrane-bound organelles, including chloroplasts. Therefore, in prokaryotes, the light reactions occur in the plasma membrane, and the Calvin cycle takes place in the cytoplasm. In contrast, eukaryotic photosynthesis occurs within chloroplasts, with light reactions happening in the thylakoid membranes and the Calvin cycle in the stroma. This structural difference is crucial for understanding how photosynthesis is adapted to different cellular architectures.

What are the key differences between oxygenic and anoxygenic photosynthesis?

Oxygenic photosynthesis uses water (H₂O) as the primary electron donor, resulting in the production of oxygen gas (O₂). It employs chlorophyll a as the primary pigment in both photosystem I and II. In contrast, anoxygenic photosynthesis does not produce oxygen because it uses alternative electron donors like hydrogen sulfide (H₂S). Organisms performing anoxygenic photosynthesis use bacteriochlorophylls and only one photosystem, either photosystem I or II, but not both. These differences highlight the diversity in photosynthetic mechanisms among microorganisms.

Where do the light reactions and the Calvin cycle occur in prokaryotic photosynthesis?

In prokaryotic photosynthesis, the light reactions occur in the plasma membrane or cytoplasmic membrane. The Calvin cycle, on the other hand, takes place in the cytoplasm. This is different from eukaryotic photosynthesis, where the light reactions occur in the thylakoid membranes of chloroplasts, and the Calvin cycle occurs in the stroma of the chloroplasts. Understanding these locations is essential for grasping how prokaryotic cells adapt to their lack of membrane-bound organelles.

Why does anoxygenic photosynthesis not produce oxygen gas?

Anoxygenic photosynthesis does not produce oxygen gas because it uses electron donors other than water (H₂O). Common alternative electron donors include hydrogen sulfide (H₂S). Since water is not used, oxygen gas (O₂) is not generated as a byproduct. Instead, other molecules, often containing sulfur, are produced. This process also involves bacteriochlorophylls and only one photosystem, either photosystem I or II, but not both. This distinction is crucial for understanding the metabolic diversity among photosynthetic microorganisms.

What types of organisms perform oxygenic photosynthesis?

Organisms that perform oxygenic photosynthesis include cyanobacteria, algae, and plants. These organisms use water (H₂O) as the primary electron donor, resulting in the production of oxygen gas (O₂). They utilize chlorophyll a as the primary pigment in both photosystem I and II. Cyanobacteria are a classic example of prokaryotes that perform oxygenic photosynthesis, while algae and plants are eukaryotic examples. Understanding these organisms helps in studying the ecological and evolutionary significance of oxygenic photosynthesis.