Medical Physiology – Physical Structure of Cell

Medical Physiology – Physical Structure of Cell
The cell is not simply a container of fluid and chemicals; it also comprises highly structured physical structures known as organelles. The primary organelles of the cell include the cell membrane, nuclear membrane, endoplasmic reticulum (ER), Golgi apparatus, mitochondria, lysosomes, and centrioles.
The cell and its organelles are encased in membranes comprised of lipids and proteins. The membranes consist of the cell membrane, nuclear membrane, and the membranes of the endoplasmic reticulum, mitochondria, lysosomes, and Golgi apparatus. They create barriers that inhibit the unrestricted passage of water and water-soluble substances between cellular compartments.
Protein molecules within the membrane frequently traverse it, creating channels that facilitate the flow of certain substances across the membranes. The cell membrane consists of a lipid bilayer embedded with proteins. The lipid bilayer consists predominantly of phospholipids and cholesterol. Phospholipids possess a hydrophilic, water-soluble segment and a hydrophobic segment that is soluble exclusively in fats. The hydrophobic regions of the phospholipids orient towards one another, while the hydrophilic segments are directed towards the membrane surfaces in contact with the surrounding interstitial fluid and the cell cytoplasm. The lipid bilayer membrane exhibits significant permeability to lipid-soluble chemicals, including oxygen, carbon dioxide, and alcohol, while serving as a substantial barrier to water-soluble compounds, such as ions and glucose.

Proteins, predominantly glycoproteins (proteins conjugated with carbohydrates), are embedded within the lipid bilayer. There are two categories of membrane proteins: integral proteins, which extend across the membrane, together with peripheral proteins, which are affixed to the inner membrane surface and do not infiltrate.
A significant number of integral proteins provide structural channels (pores) that facilitate the diffusion of water-soluble molecules, particularly ions. Other integral proteins function as carrier proteins for the transfer of molecules, occasionally against their diffusion gradients. Integral proteins may function as receptors for substances, such peptide hormones, that cannot readily traverse the cell membrane.
Peripheral proteins are typically associated with integral proteins and generally act as enzymes that facilitate cellular chemical processes. Membrane carbohydrates mostly exist in conjunction with proteins and lipids as glycoproteins and glycolipids. The glyco components of these compounds typically extend outward from the cell. Numerous carbohydrate molecules, known as proteoglycans, consist mostly of carbohydrate components linked by tiny protein cores and are loosely affixed to the outside surface; hence, the entire outer surface of the cell frequently exhibits a loose carbohydrate layer referred to as the glycocalyx. The carbohydrates on the cell's exterior serve several functions: (1) they are frequently negatively charged, repelling other negatively charged molecules; (2) the glycocalyx facilitates cell adhesion; (3) certain carbohydrates function as receptors for hormone binding; and (4) some carbohydrate moieties participate in immune responses.

The endoplasmic reticulum synthesizes several substances within the cell. A vast network of tubules and vesicles, known as the endoplasmic reticulum (ER), permeates nearly all regions of the cytoplasm. The membrane of the endoplasmic reticulum offers a substantial surface area for the synthesis of various chemicals utilized within cells and secreted by certain cells. They encompass proteins, carbohydrates, lipids, and other structures such as lysosomes, peroxisomes, and secretory granules. Lipids are synthesized within the endoplasmic reticulum membrane. Ribosomes adhere to the external surface of the granular endoplasmic reticulum for protein synthesis. These operate in conjunction with messenger RNA to manufacture many proteins that then reach the Golgi apparatus, where the molecules undergo further modification prior to their release or utilization inside the cell. A section of the endoplasmic reticulum lacks associated ribosomes and is referred to as the agranular or smooth endoplasmic reticulum. The agranular endoplasmic reticulum Functions for the synthesis of lipid compounds and other cellular activities facilitated by intrareticular enzymes.

The Golgi apparatus operates in conjunction with the endoplasmic reticulum. The Golgi apparatus possesses membranes akin to those of the agranular endoplasmic reticulum, is prominent in secretory cells, and is situated on the side of the cell from which secretory chemicals are expelled. Small transport vesicles, known as ER vesicles, continuously bud out from the endoplasmic reticulum and then merge with the Golgi apparatus. Substances encapsulated in ER vesicles are conveyed from the endoplasmic reticulum to the Golgi apparatus. The chemicals are further processed in the Golgi apparatus to generate lysosomes, secretory vesicles, and other cytoplasmic components.
Lysosomes function as an intracellular digestive system.Lysosomes, abundant in numerous cells, are small spherical vesicles encased in a membrane that houses digestive enzymes. These enzymes enable lysosomes to decompose intracellular substances within structures, particularly damaged cellular components, ingested food particles, and extraneous materials such as bacteria. The membranes encasing the lysosomes typically inhibit the encased enzymes from interacting with other cellular components, so averting their digesting activity. When these membranes are compromised, the enzymes are unleashed and decompose the organic substances they encounter into highly diffusible compounds such as amino acids and glucose.

Mitochondria generate energy within the cell. A sufficient energy source is essential to drive the cell's chemical reactions. This is primarily supplied by the chemical reaction of oxygen with three categories of nutrients: glucose from carbs, fatty acids from fats, and amino acids from proteins. Upon entering the cell, nutrients are decomposed into smaller molecules that then reach the mitochondria, where further enzymes eliminate carbon dioxide and hydrogen ions in a process known as the citric acid cycle. An oxidative enzyme system located in the mitochondria facilitates the gradual oxidation of hydrogen atoms. The final products of mitochondrial processes are water and carbon dioxide. The energy released is utilized by the mitochondria to produce adenosine triphosphate (ATP).

This is a highly reactive molecule capable of diffusing throughout the cell to release energy when required for cellular activities. Mitochondria has the ability to self-replicate, allowing one mitochondrion to generate additional mitochondria as required by the cell for augmented ATP production.

Numerous cytoplasmic structures and organelles exist. The body contains numerous cell kinds, each with a distinct structure. Certain cells exhibit rigidity and include many filamentous or tubular structures formed of fibrillar proteins. A primary purpose of these tubular structures is to serve as a cytoskeleton, offering firm physical support for specific cellular regions. Certain tubular structures, known as microtubules, facilitate the transfer of chemicals within the cell. A crucial function of several cells is the secretion of specialized chemicals, including digestive enzymes. The majority of chemicals are synthesized by the ER-Golgi apparatus system and subsequently released into the cytoplasm within storage vesicles known as secretory vesicles. Following a duration of storage within the cell, they are ejected through the cell membrane for utilization in other regions of the body.

The nucleus serves as the cell's control center and houses substantial quantities of DNA, referred to as genes. The genes dictate the properties of cellular proteins, including cytoplasmic enzymes. They also regulate reproduction. They initially replicate via mitosis, resulting in two daughter cells, each inheriting one of the two sets of genes. The nuclear membrane, or nuclear envelope, delineates the nucleus from the cytoplasm. This structure consists of two membranes; the outer membrane is continuous with the endoplasmic reticulum, and the space between the two nuclear membranes is also continuous with the compartment within the endoplasmic reticulum. The membrane's two layers are perforated by several thousand nuclear pores, each over 100 nanometers in diameter. The nuclei of most cells contain one or more structures known as nucleoli, which, unlike many organelles, lack a surrounding membrane. The nucleoli contain substantial quantities of RNA and ribosomal proteins. The nucleolus enlarges during active protein synthesis within the cell. Ribosomal RNA is sequestered in the nucleolus and subsequently transferred via nuclear membrane pores to the cytoplasm, where it is used to produce mature ribosomes, which play an important role in the formation of the protein