Microbiology, part 25: Metabolism - Biogeochemical Cycles

Hi, I'm Cathy with Level Up RN. In this video, I will be providing a high-level review of photosynthesis for your microbiology class and exams. At the end of the video, I'm going to give you guys a little quiz to test your understanding of some of the key facts I'll be covering, so definitely stay tuned for that. And if you have our Level Up RN Microbiology Flashcards, go ahead and pull out your flashcards on photosynthesis so you can follow along with me. And of note, our flashcards are also available in digital format via Flashables.

Photosynthesis is a process used by phototrophic organisms to convert sunlight into chemical energy, which is used to drive the production of sugar molecules. Photosynthesis occurs in two stages. The light-dependent reactions, followed by the light-independent reactions, which is also referred to as the Calvin cycle. In eukaryotic cells, photosynthesis takes place in a chloroplast. Each chloroplast contains stacks of grana, and each granum consists of sac-like membranes called thylakoid membranes. Photosynthetic pigments, such as chlorophyll, are organized into photosystems within the thylakoid membrane. And then the fluid surrounding the thylakoids is the stroma. So the light-dependent reactions take place in the thylakoid membrane, and the light-independent reactions take place in the stroma.

During the light-dependent reactions, energy from the sunlight is absorbed by pigments in the thylakoid membrane, which is then converted into high-energy electrons in the reaction center of the photosystem. These high-energy electrons are then delivered to the electron transport chain, which is ultimately used to generate ATP and NADPH through photophosphorylation. Photophosphorylation is similar to oxidative phosphorylation, except the energy source is different. In oxidative phosphorylation, energy is obtained from the breakdown of chemical bonds in biological molecules, whereas in photophosphorylation, the energy source is sunlight. During the light-dependent reactions, electrons lost from the reaction center to the electron transport chain must be replaced. With oxygenic photosynthesis, water replaces the electron in the reaction center, and splitting of the water molecule releases oxygen gas as a byproduct, which is why this process is referred to as oxygenic photosynthesis. Examples of organisms that use oxygenic photosynthesis include cyanobacteria, algae, and plants.

It's important to note that oxygenic photosynthesis is basically the opposite of aerobic respiration. With oxygenic photosynthesis, carbon dioxide and water react to produce glucose and oxygen. With aerobic respiration, it's the opposite. Glucose and oxygen react to produce carbon dioxide and water. With anoxygenic photosynthesis, a molecule other than water supplies the electron to the reaction center. For example, hydrogen sulfide can serve as an electron donor. So during this process, oxygen is not generated as a byproduct, which is why we refer to this process as anoxygenic. Examples of microbes that use anoxygenic photosynthesis include green and purple sulfur bacteria.

After the light-dependent reactions, we then move on to the light-independent reactions, which is referred to as the Calvin cycle. So it is my understanding that most microbiology professors do not expect you to understand the details of the Calvin cycle, which is hopefully the case for you as well. However, you should understand the overall purpose of the Calvin cycle, which is to use the energy produced during the light-dependent reactions to fix carbon dioxide. So when I say fix carbon dioxide, I mean that the carbon from carbon dioxide is used to build glucose molecules.

All right. It's quiz time, and I have three questions for you. Question number one. What are the two stages of photosynthesis? The answer is the light-dependent reactions and the light-independent reactions, which is also called the Calvin cycle. Number two. Chemical energy is used to fix carbon dioxide during which stage of photosynthesis? The answer is the light-independent reactions, or the Calvin cycle. Number three. During blank photosynthesis, water is the electron donor, and oxygen is a byproduct. The answer is oxygenic. All right. That's it for this video. I hope it was helpful. Take care, and good luck with studying.