Clinical Applications of Human Anatomy and Physiology for Healthcare Professionals. Jassin M. Jouria

nucleolus is an organelle found within the nucleus. The nucleolus is responsible for the manufacture of ribosomes. Ribosomes are structures that produce protein. The nucleus of a cell may contain as many as four nucleoli, but every species has a predetermined set or fixed number of nucleoli.

      During cellular division, this nucleolus literally disappears.

      Endosomes are small areas found inside eukaryotic cells. They’re located in the cytoplasm. Endosomes are described as part of the pathway utilized in the rejuvenation or recycling of surface receptors. Endosomes can be characterized as a collection of organelles that serve in the function of identifying, sorting, and as a delivery vessel of materials from the cellular surface, and even as a transport vehicle from the Golgi apparatus to the lysosome.

      Endosomes play a role in the “recycling” of molecules from the plasma membrane through the endoplasmic pathway. This pathway is composed of unique compartments; each involved in the replenishment of degraded molecules. Three types of endosomes are typically defined as:

      •early endosome

      •recycling or sorting endosome

      •late endosome

      Molecules are transported along this pathway for degradation in the lysosome (think of a lysosome as a trash truck pulling up to the curb for garbage disposal). They are then recycled back into the plasma membrane.

      The process is known as endocytosis and follows the endocytic pathway. Endocytosis is a necessary component of cellular structures because the majority of substances that are vital for optimal function are large, polarized molecules that are unable to passively make their way through the plasma membrane.

       ■Characteristics of all Cells

      Cells share a number of common characteristics depending on various responses to their environment, among which is their ability to move and engage in metabolic processes. They have the ability to grow (anabolism)as well as literally self-destruct (catabolism). One of the most fascinating aspects of cellular characteristics is their ability to reproduce (mitosis.)

      Every cell has the potential to respond to their environment. They can literally be irritated into responses or stimulated by a number of factors. They have the potential to sense changes in their immediate environment and respond to those changes. This response to environment is achieved through their nuclear receptors, which can trigger a number of controlled responses.

      This type of “communication” between cells as well as their environment is known as homeostasis.

      Cells are capable of receiving and processing simultaneous signals. Cells don’t only receive signals, but can transmit them. For example, signaling molecules known as neurotransmitters can travel short distances between adjacent neurons or between a neuron and a muscle cell. Others can send signals much further. A prime example is FSH or follicle-stimulating hormone, which is sent by the hypothalamus to the female ovary, signaling (via FSH) the ovary to release an egg.

      In the skin, sensory cells respond to external cues such as touch. Cells inside the ear respond to sound waves. Cells found in blood vessels identify and respond to changes in blood pressure.

This is done through the presence of protein receptors. These receptors bind to “signaling” molecules to trigger physiological responses.8 Some receptors are found deep inside a cell while others are found on its surface, while still others are located in the nucleus.

      Motility

      How do cells move? They migrate. Cells utilize two basic methods of transportation: contractibility or self-propulsion. Contractibility is contraction. For example, think of contraction of a muscle cell or fibers in the act of bending the elbow and bringing the hand to the face. This contracting ability helps divide daughter cells during mitosis. Contraction occurs when molecular “motors” act on cytoskeletal filaments or microtubules, compelling them to draw toward one another.

      Cells can move in specific directions. This directional mobility is called chemotaxis and describes how cells move after triggering by an external signal or stimulus. In many cases, this external signal is caused by the influence of a short peptide or molecule (known as a chemoattractant). The cells automatically move toward the direction of the increased signal concentration. This type of movement is typical in wound-healing scenarios. A damaged or injured cell releases a chemoattractant. This signals the attraction of microphages and fibroblasts of the immune system.

For example, a chemical “scent” or trail is left by the movement of a damaged cell. Like bloodhounds following a scent, leukocytes, vital for defense, respond to the area ready to do battle. This is known as positive chemotaxis.9

      Another type of cellular movement in eukaryotic cells occurs via tiny appendages nicknamed “false feet”. Imagine a caterpillar’s tiny legs propelling it across the surface of a leaf or twig. Also known as pseudopods, these tiny appendages attach to a surface in the direction the cell is moving. During chemotaxis, pseudopods are extended in several directions, but they’ll only attach to surfaces that exhibit higher concentrations of chemical signals.

      On the other hand, bacterial cells facilitate movement through flagellum. Flagellum move almost like a boat propeller, urging the cell forward.

      Metabolism

      Metabolism is another common characteristic of all cells. Metabolism in cells occurs through a number of chemical processes.

      Cellular metabolism defines chemical changes in living cells through which a number of activities and processes are achieved. It’s a constant state of growth and destruction, which is how energy is produced and provided, as well as involves how new materials are assimilated by a structure.

      These biochemical reactions inside the cell can either accelerate or decelerate the functions of a cell based on need. A number of pathways are involved in these functions. Metabolism inside cells must be carefully balanced and coordinated.

      The importance of enzymes and their influence in these biochemical reactions is vital. Enzymes are protein-based catalysts or triggers that accelerate biochemical reactions. They do this by expediting rearrangement of molecules supporting cellular functions.

      Enzymes are defined as flexible proteins. They can change shape when binding to substrate molecules. This binding and “shape shifting” capability is how an enzyme can influence responses or reactions inside the cell.

      Specific enzymes can either activate or inhibit a molecule’s activity or function. Metabolism often occurs along metabolic pathways, which are simply defined as a coordinated process of chemical reactions that trigger a continual process from point A to point B. Enzymes are involved in every step of this reaction pathway and have the ability to transform molecules into different forms depending on the presence of certain enzymes. In such a way, these chemical processes are involved in biosynthesis or creation of new molecules. They can also be involved in catabolism or degradation of molecules.

      Some enzymes are involved in physically connected metabolic pathways. Chemical reactions in cellular structures are balanced through anabolic or catabolic pathways. Synthesis of new molecules requires the input of energy to become an anabolic pathway.

      The catabolic pathway triggers the breakdown of molecules and results in energy release. In this way, cells break down polymers that include polysaccharides and proteins, and in turn, sugars and amino acids.

      Molecules created by organic or enzymatic activities are known as metabolites. Metabolites ensure that energy is consistently available for both anabolic and catabolic pathway balances.

      This careful balance as well as maintenance of biochemical reactions of cells by enzymes is a vital component to cellular functions. Activity of enzymes enables cells to respond to ever-changing demands on their environment as well as in the regulation of metabolic pathways. Both are vital to the survival of every cell.

      Cellular mitosis

Mitosis