Cellular Biology: Structure, Functions, and Organelles

The Cell: A Fundamental Unit of Life

The cell is the structural unit of all organisms, both animals and plants, as they are composed of cells and cellular products. It is also the functional unit, since in all cell types, chemical composition and metabolic processes are similar. The cell can develop its activity individually (single-celled organisms) or associate with other cells, interacting and complementing each other to build multicellular organisms. It is the unit of origin because every cell originates from another cell. Finally, it is the biochemistry unit, for the cell has the biochemical machinery that allows it to copy itself and its genetic information. Through this, it controls the biosynthesis of macromolecules involved in its structure and membrane proteins.

The Plasma Membrane: A Selective Barrier

The plasma membrane has different functions: to be enzyme systems, acting as receptor molecules and transmitting messages inside the cell, enabling the transfer of materials through the membrane. Carbohydrates are found on the outer surface of the membrane, contributing to the reception of messages and the recognition of similar cells to form tissues.

In summary, the plasma membrane functions may include:

1. Participating in molecular recognition processes.
2. Participating in determining cell shape.
3. Receiving external information and transmitting it to the cell.
4. Regulating the movement of materials between intra and extracellular media and maintaining the optimal concentration to carry out the processes of the cell.

Membrane phospholipids restrict the entry and exit of polar substances (i.e., with one side positive and negative), allowing the movement of some substances and preventing others. This is why it is called a semipermeable membrane.

The Nucleus: Control Center of the Cell

The nucleus has two main functions:

1. Controlling heredity and cell reproduction.
2. Acting as a regulator of most of the cytoplasmic activity.

The nuclear envelope or karyotheca is a double-membraned pore that separates the cytoplasmic material from the chromosomal material. It is part of the endomembrane system, showing continuity with the endoplasmic reticulum.

The Endoplasmic Reticulum: An Intracellular Transport System

The Endoplasmic Reticulum (ER) is an organelle composed of a system of interconnected tubules and vesicles that connects intermittently with the plasma and nuclear membranes. It functions as an intracellular transport system of materials.

Rough Endoplasmic Reticulum (RER)

Rough Endoplasmic Reticulum (RER) has membranes with ribosomes on its outer surface. Since the function of ribosomes is the synthesis of proteins, the primary function of RER is to store proteins made by ribosomes.

Smooth Endoplasmic Reticulum (SER)

Smooth Endoplasmic Reticulum (SER) lacks ribosomes on its surface. It participates in functions such as lipid synthesis (steroids, phospholipids, and triglycerides) and detoxification of harmful materials entering the cells, especially in the liver.

The Golgi Complex: Processing and Packaging Center

The Golgi complex:

1. Facilitates intracellular movement of materials synthesized in the smooth and rough portions of the reticulum.
2. Synthesizes molecules that cover part of the cell or cell membrane (glycolipids and glycoproteins).
3. Participates in the formation of lysosomes containing digestive enzymes and the acrosome, a sperm structure that allows its penetration into the egg.

Ribosomes: Protein Synthesis Machinery

Ribosomes are corpuscular organelles related to protein synthesis. They can be found free in the cytosol or attached to the surface of the RER. Free ribosomes synthesize proteins for internal use, while ribosomes attached to the reticular membranes synthesize lysosomal proteins (enzymes) and secretion proteins.

Lysosomes: Cellular Digestion and Recycling

Lysosomes:

1. Digest food and other materials incorporated into the cell. This allows certain types of cells that feed on bacteria (white blood cells) to play an important role in organic defense.
2. Digest remains of cell membranes through a process called autophagy. This feature allows the renovation and replacement of cellular components in injured or aging cells.
3. Digest extracellular material by releasing enzymes into the surrounding environment, such as in the digestion of food in the digestive tract, the remodeling of bone, and the penetration of sperm during fertilization.

Chemical Composition of the Membrane

The chemical composition of the membrane is 40% lipids, 50% proteins, and 10% carbohydrates. The three eukaryotic membrane lipids are phospholipids, glycolipids, and cholesterol. They have a hydrophobic part and a hydrophilic part. When housed in an aqueous medium, they are oriented to form a lipid bilayer. Fluidity is one of the most important characteristics of the membranes. It depends on factors such as temperature; fluidity increases with increasing temperature. The nature of lipids, the presence of unsaturated lipids and short-chain lipids favor the increase of fluidity. The presence of cholesterol hardens the membranes, reducing fluidity and permeability. Proteins can be integral transmembrane proteins or peripheral proteins.