Cellular structures
Introduction
Cellular structures, also known as organelles, are specialized subunits within a cell that have a specific function, and are usually separately enclosed within their own lipid bilayers. The study of cellular structures is a fundamental aspect of cell biology, a discipline that seeks to understand the structure, function, and behavior of cells.
Overview of Cellular Structures
All cells, from simple prokaryotic organisms to complex eukaryotic organisms, contain cellular structures. These structures can be broadly divided into two categories: non-membranous and membranous organelles.
Non-membranous organelles are not enclosed by a lipid bilayer and are in direct contact with the cytosol. Examples of non-membranous organelles include ribosomes, the cytoskeleton, and centrioles.
Membranous organelles are enclosed within a lipid bilayer, separating their internal environment from the cytosol. Examples of membranous organelles include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes.
Non-Membranous Organelles
Non-membranous organelles are directly immersed in the cytosol and include structures such as ribosomes, the cytoskeleton, and centrioles.
Ribosomes
Ribosomes are the sites of protein synthesis in the cell. They are composed of rRNA and proteins, and can be found either free in the cytosol or attached to the endoplasmic reticulum.
Cytoskeleton
The cytoskeleton is a network of protein filaments and tubules that extends throughout the cytoplasm of the cell. It provides structural support, aids in cell movement, and plays a crucial role in cell division.
Centrioles
Centrioles are cylindrical structures composed of nine triplet microtubules. They play a key role in organizing the microtubules of the cytoskeleton during cell division.
Membranous Organelles
Membranous organelles are enclosed within a lipid bilayer, which separates their internal environment from the cytosol. These include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes.
Nucleus
The nucleus is the largest organelle in the cell and contains the cell's genetic material. It is enclosed by a double membrane known as the nuclear envelope, which separates the contents of the nucleus from the cytoplasm.
Mitochondria
Mitochondria are the powerhouses of the cell, responsible for the production of ATP, the cell's main source of energy. They have a double membrane structure, with the inner membrane being highly folded to increase surface area for ATP production.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a network of membranous tubules and sacs that plays a crucial role in protein and lipid synthesis. There are two types of ER: rough ER, which has ribosomes attached and is involved in protein synthesis, and smooth ER, which is involved in lipid synthesis.
Golgi Apparatus
The Golgi apparatus is a stack of flattened membranous sacs that modifies, sorts, and packages proteins and lipids for transport to their final destinations.
Lysosomes
Lysosomes are vesicles that contain digestive enzymes. They break down waste materials and cellular debris, and are involved in processes such as autophagy and apoptosis.