The cell, the fundamental unit of all living beings, is a complex structure composed of different elements working in harmony. The plasma membrane: the protective envelope The plasma membrane is a semi-permeable barrier that delimits the cell and separates it from its environment. It consists of a double layer of phospholipids, in which various proteins are integrated. These proteins play a crucial role in the transport of substances across the membrane, cell communication and cell recognition. The cytoplasm: the internal environment The cytoplasm is the aqueous medium inside the cell. It contains many organelles, inclusions, and the cytosol. The cytosol is the liquid part of the cytoplasm, rich in enzymes and molecules necessary for the chemical reactions of the cell. The Core: The Control Center The nucleus is a characteristic organelle of eukaryotic cells. It contains the genetic information of the cell in the form of DNA, organized into chromosomes. The nucleus is surrounded by a nuclear envelope pierced by nuclear pores that allow the exchange of molecules between the nucleus and the cytoplasm. Organelles: specialized factories Organelles are specialized structures present in the cytoplasm. Each of them fulfills a specific function: Mitochondria: Nicknamed the "power plants" of the cell, they are the seat of cellular respiration, a process that produces the energy needed by the cell. The endoplasmic reticulum: A network of membranes involved in the synthesis of proteins and lipids. The Golgi apparatus: A set of flattened sacs that modify, package, and distribute proteins and lipids synthesized by the endoplasmic reticulum. Lysosomes: Small vesicles containing digestive enzymes that break down unnecessary or damaged molecules. The cytoskeleton: the skeleton of the cell The cytoskeleton is a network of protein filaments that gives the cell its shape, allows it to move, and organizes its organelles. It is made up of three types of filaments: microfilaments, intermediate filaments, and microtubules. The differences between prokaryotic and eukaryotic cells There are two main types of cells: prokaryotic cells (bacteria) and eukaryotic cells (animals, plants, fungi). Prokaryotic cells are simpler and do not have a nucleus or membrane-bound organelles. Eukaryotic cells, on the other hand, are more complex and have a nucleus and many organelles.

Structure of the plasma membrane The plasma membrane is mainly composed of: Phospholipids: These amphipathic molecules (having both a hydrophilic and a hydrophobic part) are organized into a double lipid layer. This structure gives the membrane its semi-permeable barrier property. Proteins: They are integrated into the lipid bilayer and perform various functions: transport of molecules, reception of signals, cellular recognition, etc. Carbohydrates: Linked to lipids (glycolipids) or proteins (glycoproteins), they play an important role in cell recognition and cellular interactions. Function of the plasma membrane Selective barrier: The plasma membrane allows the selective passage of certain molecules while preventing others. Transport: Membrane proteins provide passive (simple or facilitated diffusion) or active (pumps) transport of molecules across the membrane. Receiving signals: Membrane receptors allow the cell to detect and respond to signals from its environment. Cell recognition: Molecules present on the surface of the membrane allow cells to recognize and interact with each other. Maintenance of homeostasis: The plasma membrane helps maintain a stable intracellular environment by regulating exchanges between the cell and its environment. Properties of the plasma membrane Fluidity: The plasma membrane is not a rigid structure but rather fluid, which allows it to deform and change shape. Asymmetry: The two faces of the plasma membrane have different compositions, which gives the membrane polarity. Plasma membrane models Fluid mosaic model: This model, the most accepted, describes the membrane as a constantly changing mosaic of lipids and proteins.