Oct 14, 2020

Cellular Anatomy: Exploring the Intricacies of Cell Structures, Membranes, and Extracellular Environment

The cell, which is the fundamental structural and functional unit of the body, is the focus of cytology, a branch of science that studies cells.

The Cell Theory provides insights into the nature of cells, stating that:

a) All living organisms are made up of cells and cell products.

b) Cells are the basic units of structure and function in all living organisms.

c) Cells arise from the division of preexisting cells.

d) The collective activities and interactions of cells contribute to the understanding of the entire organism.

To gain a better understanding of cells, they can be divided into four main components:

Plasma (cell) membrane: This is the outer lining that separates the internal parts of the cell from the extracellular materials and the external environment.

Cytoplasm: This is the substance that surrounds organelles and is located between the nucleus and the plasma membrane.

Organelles: These are permanent structures with distinctive morphology that are highly specialized for specific cellular activities.

Inclusions: These are secretions and storage products of cells.

The extracellular materials, also known as the matrix, are substances found outside the cell surface.

The plasma membrane, a thin outer membrane, maintains the integrity of the cell by keeping its contents separate and distinct from the surrounding environment. It is a double layer composed of phospholipids, cholesterol, glycolipids, and carbohydrates (oligosaccharides) and is self-sealing, automatically sealing if punctured by a needle.

 

Functions of the plasma membrane include

Separating the cytoplasm inside the cell from the extracellular fluid.

Separating cells from one another.

Providing an abundant surface for chemical reactions to occur.

Regulating the passage of materials into and out of the cell, exhibiting selective permeability.

Movement across the cell membrane occurs in two ways: passive and active movements. Passive movement does not require energy, while active movement consumes ATP (adenosine triphosphate) energy.

Passive movement includes:

a) Simple diffusion: the random movement of molecules from areas of high concentration to areas of low concentration. An example is the exchange of gases in the alveoli of the lungs.

b) Facilitated diffusion: larger molecules that are not soluble in lipids require protein channels to pass through the plasma membrane. No direct energy is needed. An example is the transport of amino acids across the cell membrane.

c) Osmosis: a special type of diffusion referring to the passage of water through a selectively permeable membrane from an area of high water concentration to an area of low water concentration.

d) Filtration: small molecules pass through a selectively permeable membrane in response to pressure. An example is the filtration process in the kidneys during urine formation.

Active movements across membranes involve substances moving from areas of low concentration on one side of the membrane to areas of higher concentration on the other side. This movement occurs against the concentration gradient and requires energy.

Active transport occurs when equilibrium is reached and more molecules are needed. Substances are pumped through the membrane against the concentration gradient, requiring the use of ATP. Examples of active transport processes include the sodium-potassium pump and the calcium pump. In these processes, molecules bind to carrier proteins, and the molecule-carrier complex passes through the membrane, assisted by an enzyme and ATP. The carrier protein then returns to its original shape and repeats the process.

Endocytosis, which involves pocketing of the plasma membrane, includes:

a) Pinocytosis: cell drinking.

b) Receptor-mediated endocytosis: endocytosis facilitated by receptors.

c) Phagocytosis: cell eating.

Exocytosis is the opposite of endocytosis and involves the removal of undigested particles."

 

Cytoplasm

Cytoplasm is a matrix or ground substance in which various cellular components are found. It is a thick, semi-transparent, elastic fluid containing suspended particles and a series of minute tubules and filaments that form the cytoskeleton. Water constitutes 75-90% of the cytoplasm. It also contains solid components such as proteins, carbohydrates, lipids, and inorganic substances. The inorganic components exist as solutions because they are soluble in water. The majority of organic substances, however, are found as colloids, which are particles that remain suspended in the surrounding medium.

 

Organelles

Organelles are specialized portions of the cell with characteristic shapes that assume specific roles in growth, maintenance, repair, and control.

 

Nucleus

The nucleus is oval in shape and is the largest structure in the cell. It contains the hereditary factors in the cell; hence it controls cell activity and structure. Most cells contain a single nucleus, but some, like matured red blood cells, do not contain a nucleus. However, muscle cells contain several nuclei. The nucleus is separated from other cell structures by a double membrane called the nuclear membrane. Pores in the nuclear membrane allow the nucleus to communicate with the cytoplasm. The nucleus contains a jelly-like fluid that fills the nucleus called karyolymph (nucleoplasm), which contains the genetic material called chromosomes. The nucleus also contains a dark, somewhat spherical, non-membrane-bound mass called the nucleolus, which contains DNA, RNA, and protein that assist in the construction of ribosomes.

 

Ribosomes

Ribosomes are tiny granules composed of Ribosomal RNA (rRNA). They are the site of protein synthesis in the cell, where they use information from the DNA in the nucleus to synthesize proteins. Ribosomes can be found free in the cytoplasm or attached to the endoplasmic reticulum.

 

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a double-membrane channel system that is involved in protein synthesis, lipid metabolism, and detoxification of drugs and toxins. It can be classified into two types: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). RER has ribosomes attached to its surface, giving it a rough appearance, and is involved in protein synthesis and protein folding. SER lacks ribosomes and is involved in lipid metabolism, detoxification, and calcium storage.

 

Golgi Apparatus

The Golgi apparatus, also known as the Golgi complex or Golgi body, is a stack of flattened sacs that processes, modifies, and packages proteins and lipids synthesized in the ER for transport to their final destinations within or outside the cell. It plays a crucial role in intracellular transportation and secretion.

 

Mitochondria

Mitochondria are double-membraned organelles often referred to as the "powerhouses of the cell" because they are responsible for generating energy in the form of ATP through cellular respiration. They contain their own DNA and can self-replicate, suggesting that they originated from a symbiotic relationship between early eukaryotic cells and bacteria.

 

Lysosomes

Lysosomes are membrane-bound organelles that contain enzymes capable of breaking down cellular waste materials, cellular debris, and macromolecules. They play a crucial role in cellular waste disposal, recycling of cellular components, and the immune response.

 

Inclusions

Inclusions are secretions and storage products of cells. They can include pigments, crystals, and other substances that are not bound by a membrane. Inclusions are important for various cellular functions such as energy storage, pigment production, and detoxification.

 

Extracellular

Extracellular refers to the space outside the cell. It includes the interstitial fluid, which is the fluid that surrounds and bathes the cells in tissues, as well as other extracellular fluids such as blood plasma and lymph. The extracellular environment plays a critical role in cellular communication, nutrient exchange, waste elimination, and overall tissue function.

 

Cell Membrane

The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that separates the interior of the cell from the extracellular environment. It consists of a phospholipid bilayer embedded with proteins and other molecules. The cell membrane controls the passage of substances in and out of the cell, allowing for the exchange of nutrients, gases, and waste products, as well as communication with neighboring cells and the external environment.

 

Cell Junctions

Cell junctions are specialized structures that connect cells together and play a role in tissue integrity and communication. There are several types of cell junctions, including tight junctions, desmosomes, and gap junctions. Tight junctions are found in epithelial tissues and form a seal between adjacent cells, preventing the passage of substances between them. Desmosomes are spot-like adhesions that anchor cells together, providing mechanical stability to tissues subjected to mechanical stress. Gap junctions are channels that allow for direct communication between adjacent cells, allowing for the exchange of ions and small molecules.

 

Extracellular Matrix

The extracellular matrix (ECM) is a complex network of proteins and carbohydrates that surrounds cells in tissues and provides structural support, mechanical strength, and signaling cues for cellular functions. The ECM is composed of various components, including fibrous proteins such as collagen and elastin, proteoglycans, glycoproteins, and other molecules. The ECM plays a crucial role in tissue development, maintenance, and repair, as well as cell behavior and function, including cell adhesion, migration, proliferation, and differentiation. 

 

The Cell


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