Cell Structures: Golgi, Lysosomes, Vacuoles, Ribosomes, Mitochondria

Posted on Mar 21, 2025 in Biology

Golgi Apparatus

The Golgi apparatus is composed of flattened, sac-like units (cisternae) formed by smooth membranes. These cisternae are typically clustered in groups of 5 to 8, forming a stack called a dictyosome. At the edges of the cisternae, vesicles bud off, containing various cellular products. Dictyosomes have two faces: a forming face (cis) and a maturing face (trans), which leads to the corresponding vesicles.

Lysosomes

Lysosomes are characteristic of animal cells. They are membrane-bound, smooth organelles containing hydrolytic enzymes (approximately 40 types) for digesting substances. The general reaction is: AB + H₂O + Enzyme → AH + BOH.

The interior of a lysosome maintains a pH of approximately 5, which contrasts with the cell cytosol’s pH of 7.2. Lysosomes are formed from invaginations of the membranes of the Golgi apparatus. There are two types:

• Primary Lysosomes: Vesicles loaded with enzymes before they begin their role in the cell.
• Secondary Lysosomes: Smooth structures attached to active parts being attacked and digested by their enzymes. If they are digesting external particles, they are called phagolysosomes (formed by phagosomes and lysosomes). If they are digesting internal components, they are called autophagolysosomes (formed by autophagosomes and lysosomes).

Function: Intracellular digestion of external or internal particles. Autophagy is very useful to provide nutrients under fasting conditions.

Vacuoles

Vacuoles are large, membrane-bound sacs (the membrane is called the tonoplast) that store various products and liquids. They are generally well-developed in plants (occupying 50-95% of the cell volume and regulating water content, maintaining cell volume and turgor pressure). In animals and fungi, they are less prominent. Vacuoles arise from dilations of the smooth endoplasmic reticulum (SER) due to the accumulation of substances, or they may be derived from the Golgi apparatus. They can store:

• Nutrient reserves (starch, proteins, fats)
• Mineral salt crystals, such as calcium oxalate
• Autocyanin pigments (responsible for the color of flowers)
• Water
• Essential oils
• Toxic substances that could be dangerous to the cell, including alkaloids (atropine and nicotine) and glycosides (digitalis, amygdalin, and opium).

Ribosomes

Ribosomes are non-membrane-bound particles with an elliptical or spherical shape, existing in all cells, except for very few in red blood cells and none in mature spermatozoa. They can be found:

• Free in the cytoplasm (isolated or interconnected to form polysomes).
• Attached to the outside of the rough endoplasmic reticulum (RER) or the cytoplasmic face of the external nuclear membrane.
• Free in the matrix of mitochondria and chloroplasts.

Structure: Ribosomes are composed of two subunits, separated by a transverse slit. The two subunits are formed in the nucleolus, where their two components, rRNA and ribosomal proteins, join. In prokaryotic cells, they are approximately 29nm long and 21nm wide (70S). In eukaryotic cells, they are approximately 32nm long and 22nm wide (80S).

Function: Involved in protein synthesis, linking amino acids in a specific order. Proteins synthesized by free ribosomes remain in the cytosol, while those synthesized by attached ribosomes pass through the lumen of the reticulum to join other intracellular organelles or be secreted outside the cell.

Mitochondria

Mitochondria are organelles present in all eukaryotic and aerobic cells. They are large and spherical. The collection of mitochondria in a cell (up to 2000) is called the chondriome.

Structure: Mitochondria have two membranes: a smooth external membrane and an internal membrane with infoldings (cristae). The internal cavity, called the stroma, contains:

• Mitoribosomes (70S ribosomes)
• Circular DNA molecules
• Enzymes for the respiratory chain, the Krebs cycle, and the oxidation of fatty acids, as well as DNA replication.

The inner membrane contains tripartite structures, each consisting of enzymes called ATP synthase (which catalyzes the phosphorylation of ADP to form ATP).

Mitochondrial Function: Cellular respiration takes place in mitochondria (C₆H₁₂O₆ + 6O₂ → [lower E → ATP] → 6CO₂ + H₂O). The organic molecules metabolized in the mitochondria are internal fatty acids and pyruvate, which become acetyl-CoA, and this is oxidized to CO₂ via the Krebs cycle. All these oxidations are associated with the formation of NADH + H⁺ and FADH₂, which pass to the respiratory chain, releasing H⁺ and electrons, passing through the tripartite units, which form ATP. (These are energy-producing organelles).