Cellular Components and Their Functions

Posted on Mar 11, 2025 in Biology

Cytosol

The cytosol is the semi-solid fraction of the cell, primarily composed of water, salts, and both small and large water-soluble molecules. It includes supramolecular complexes such as the cytoskeleton, ribosomes, and centrioles. Additionally, it contains polysaccharide granules, lipid droplets, and pigments. The majority of cellular metabolism, including protein synthesis in ribosomes, occurs here.

Cytoskeleton

The cytoskeleton is a network of protein fibers that maintains cell structure and shape. It enables various types of cell movement and is responsible for all movements, including the transport of substances and organelles. It also controls the movement of cilia and flagella. The cytoskeleton consists of:

  • Microfilaments
  • Intermediate filaments
  • Microtubules

Ribosomes

Ribosomes are granules of RNA and protein. They have two subunits that bind to messenger RNA to synthesize proteins. They can be found attached to the endoplasmic reticulum or free in the cytoplasm.

Cytoplasmic Organelles

Cytoplasmic organelles are cell compartments surrounded by membranes, making up nearly half the cell volume. Their membranes are highly selective. Each organelle has distinct and unique features. These compartments allow for various chemical processes to occur simultaneously, controlled by enzymes. They provide large membrane areas for organizing enzymatic reactions, making them more efficient and allowing cells to achieve a larger size.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is part of the endomembrane system, with an area several times larger than the plasma membrane. It is a complex network of channels and cavities, serving as a center for synthesis, transport, and storage. There are two types:

  • Rough Endoplasmic Reticulum (RER)
  • Smooth Endoplasmic Reticulum (SER)

They form, along with the Golgi apparatus, lysosomes, and secretory vesicles.

Rough Endoplasmic Reticulum (RER)

The RER receives proteins synthesized by ribosomes. It chemically modifies these proteins and conducts them to the Golgi apparatus.

Smooth Endoplasmic Reticulum (SER)

The SER is responsible for the synthesis of lipids such as phospholipids, hormones, and cholesterol. It chemically modifies small molecules made by the cell and detoxifies substances to prevent damage to the body; this role is particularly important in liver cells. It is also the location of glycogen hydrolysis.

Golgi Apparatus

The Golgi apparatus is a group of flattened sacs, close to the nucleus and well-developed in secretory cells. Its functions include:

  • Receiving RER vesicles with proteins for final modifications.
  • Packaging proteins for exocytosis.
  • Signaling the destination of each protein via molecular signals.
  • Creating lysosomes.
  • Participating in cell wall synthesis.
  • Producing polysaccharides such as mucus.

Lysosomes

Lysosomes are vesicles formed in the Golgi apparatus, containing large amounts of enzymes. They isolate these enzymes to prevent them from acting upon the cytoplasm.

Mitochondria

Mitochondria are organelles formed by a double membrane: a smooth outer membrane and an inner membrane with folds called mitochondrial cristae. They degrade nutrients (usually glucose) through cellular respiration. They require O2 to store energy in ATP molecules. The number of mitochondria in a cell is related to cellular activity.

Cellular Respiration

Cellular respiration is a series of reactions that gradually degrade glucose. It involves a complex system of enzymes. The energy released is stored in ATP. For every molecule of glucose degraded, 38 ATP molecules are generated. The complete degradation of glucose is aerobic (involving O2). It is a universal process that occurs constantly in plants and animals.

Stages of Cellular Respiration

  • Glycolysis: Occurs in the cytoplasm; it is anaerobic, so energy efficiency is very low. Glucose is enzymatically degraded to pyruvic acid. It is one of the earliest mechanisms for energy production.
  • Krebs Cycle (Citric Acid Cycle): A set of chemical reactions in which acetyl-CoA reacts with oxaloacetate to form citric acid and regenerate oxaloacetate. Acetyl-CoA is oxidized to CO2, and H+ ions are stored as NADH and FADH2. It occurs in the mitochondrial matrix and is anaerobic.