Cellular Metabolism: Definition, Process & the Role of ATP

Cells require energy for movement, division, multiplication and other processes. They spend a large portion of their lifetimes focused on obtaining and using this energy through metabolism.

Prokaryotic and eukaryotic cells depend on different metabolic pathways to survive.

Cellular Metabolism

Cell metabolism is the series of processes that take place in living organisms to sustain those organisms.

In cell biology and molecular biology, metabolism refers to the biochemical reactions that happen inside organisms to produce energy. The colloquial or nutritional use of metabolism refers to the chemical processes that happen in your body as you convert food into energy.

Although the terms have similarities, there are also differences. Metabolism is important for cells because the processes keep organisms alive and allow them to grow, reproduce or divide.

What Is the Cell Metabolism Process?

There are actually multiple metabolism processes. Cellular respiration is a type of metabolic pathway that breaks down glucose to make adenosine triphosphate, or ATP.

The main steps of cellular respiration in eukaryotes are:

  • Glycolysis
  • Pyruvate oxidation
  • Citric acid or Krebs cycle
  • Oxidative phosphorylation

The main reactants are glucose and oxygen, while the main products are carbon dioxide, water and ATP. Photosynthesis in cells is another type of metabolic pathway that organisms use to make sugar.

Plants, algae and cyanobacteria use photosynthesis. The main steps are the light-dependent reactions, and the Calvin cycle or light-independent reactions. The main reactants are light energy, carbon dioxide and water, while the main products are glucose and oxygen.

Metabolism in prokaryotes can vary. The basic types of metabolic pathways include heterotrophic, autotrophic, phototrophic and chemotrophic reactions. The type of metabolism that a prokaryote has can influence where it lives and how it interacts with the environment.

Their metabolic pathways also play a role in ecology, human health and diseases. For instance, there are prokaryotes that cannot tolerate oxygen, such as C. botulinum. This bacteria can cause botulism because it grows well in areas without any oxygen.

Enzymes: The Basics

Enzymes are substances that act as catalysts to speed up or bring about chemical reactions. Most biochemical reactions in living organisms rely on enzymes to work. They are important for cellular metabolism because they can affect many processes and help initiate them.

Glucose and light energy are the most common sources of fuel for cell metabolism. However, metabolic pathways would not work without enzymes. Most of the enzymes in cells are proteins and lower the activation energy for chemical processes to begin.

Since the majority of the reactions in a cell occur at room temperature, they are too slow without enzymes. For instance, during glycolysis in cellular respiration, the enzyme pyruvate kinase plays an important role by helping to transfer a phosphate group.

Cellular Respiration in Eukaryotes

Cellular respiration in eukaryotes occurs primarily in the mitochondria. Eukaryotic cells depend on cellular respiration to survive.

During glycolysis, the cell breaks down glucose in the cytoplasm with or without oxygen being present. It splits the six-carbon sugar molecule into two, three-carbon pyruvate molecules. In addition, glycolysis makes ATP and converts NAD+ into NADH. During pyruvate oxidation, the pyruvates enter the mitochondrial matrix and become coenzyme A or acetyl CoA. This releases carbon dioxide and makes more NADH.

During the citric acid or Krebs cycle, acetyl CoA combines with oxaloacetate to make citrate. Then, citrate goes through reactions to make carbon dioxide and NADH. The cycle also makes FADH2 and ATP.

During oxidative phosphorylation, the electron transport chain plays a crucial role. NADH and FADH2 give electrons to the electron transport chain and become NAD+ and FAD. The electrons move down this chain and make ATP. This process also produces water. The majority of the ATP production during the cellular respiration is in this last step.

Metabolism in Plants: Photosynthesis

Photosynthesis happens in plant cells, some algae and certain bacteria called cyanobacteria. This metabolic process occurs in chloroplasts thanks to chlorophyll, and it produces sugar along with oxygen. The light-dependent reactions, plus the Calvin cycle or light-independent reactions, are the main parts of photosynthesis. It is important for the overall health of the planet because living things rely on the oxygen plants make.

During the light-dependent reactions in the thylakoid membrane of the chloroplast, chlorophyll pigments absorb light energy. They make ATP, NADPH and water. During the Calvin cycle or light-independent reactions in the stroma, ATP and NADPH help make glyceraldehyde-3-phosphate, or G3P, which eventually becomes glucose.

Like cellular respiration, photosynthesis depends on redox reactions that involve electron transfers and the electron transport chain.

There are different types of chlorophyll, and the most common types are chlorophyll a, chlorophyll b and chlorophyll c. Most plants have chlorophyll a, which absorbs blue and red light wavelengths. Some plants and green algae use chlorophyll b. You can find chlorophyll c in dinoflagellates.

Metabolism in Prokaryotes

Unlike humans or animals, prokaryotes vary in their need for oxygen. Some prokaryotes can exist without it, while others depend on it. This means they may have aerobic (requiring oxygen) or anaerobic (not requiring oxygen) metabolism.

In addition, some prokaryotes can switch between the two types of metabolism depending on their circumstances or environment.

Prokaryotes that depend on oxygen for metabolism are obligate aerobes. On the other hand, prokaryotes that cannot exist in oxygen and do not need it are obligate anaerobes. Prokaryotes that can switch between aerobic and anaerobic metabolism depending on the presence of oxygen are facultative anaerobes.

Lactic Acid Fermentation

Lactic acid fermentation is a type of anaerobic reaction that produces energy for bacteria. Your muscle cells also have lactic acid fermentation. During this process, the cells make ATP without any oxygen through glycolysis. The process turns pyruvate into lactic acid and makes NAD+ and ATP.

There are many applications in industry for this process, such as yogurt and ethanol production. For example, the bacteria Lactobacillus bulgaricus help produce yogurt. The bacteria ferment lactose, the sugar in milk, to make lactic acid. This makes the milk clot and turns it into yogurt.

What Is Cell Metabolism Like in Different Types of Prokaryotes?

You can categorize prokaryotes into different groups based on their metabolism. The main types are heterotrophic, autotrophic, phototrophic and chemotrophic. However, all prokaryotes still need some type of energy or fuel to live.

Heterotrophic prokaryotes get organic compounds from other organisms to obtain carbon. Autotrophic prokaryotes use carbon dioxide as their source of carbon. Many are able to use photosynthesis to accomplish this. Phototrophic prokaryotes get their energy from light.

Chemotrophic prokaryotes get their energy from chemical compounds that they break down.

Anabolic vs. Catabolic

You can divide metabolic pathways into anabolic and catabolic categories. Anabolic means that they require energy and use it to build large molecules from small ones. Catabolic means that they release energy and break apart large molecules to make smaller ones. Photosynthesis is an anabolic process, while cellular respiration is a catabolic process.

Eukaryotes and prokaryotes depend on cellular metabolism to live and thrive. Although their processes are different, they both either use or create energy. Cellular respiration and photosynthesis are the most common pathways seen in cells. However, some prokaryotes have different metabolic pathways that are unique.

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