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

      Figure 2-5 Golgi apparatus.

      Lysosomes

      Lysosomes are membranous organelles that roam the intracellular cytoplasm as “digestive sacs”. They are usually crafted and developed from the endoplasmic reticulum, where they bud off - circulating through the cytosol, seeking the digestion of cellular waste products and/or damaged cellular components.

      They contain acidic enzymes (which are constructed in the endoplasmic reticulum and modified in the Golgi apparatus) such as hydrolase, lipase, and amylase, among others, that can break down food particles, waste materials, and cellular debris. Lysosomes also aid in protection by engulfing and digesting microorganisms that have invaded the cell.

      Lysosomes play an essential role in cellular maintenance and recycling macromolecules – breaking them down and assimilating them to be reused by the cell in order to continue functioning. A crucial structure in the normal operation of lysosomes is their membrane, which allows for the potentially hazardous digestive enzymes to remain contained inside the lysosome and not leak out where they would be toxic to and destroy the cell in a process known as “auto-digestion”.

      Peroxisomes

      Peroxisomes are also membranous, or membrane-contained organelles that roam the interior of a cell, performing many different metabolic reactions at the cellular level. Structurally, they are similar to lysosomes except for one keynote difference; peroxisomes do not bud off of the endoplasmic reticulum. Instead, they are manufactured by free-floating ribosomes and released into the cytosol as completed polypeptide chains.

      The main function of peroxisomes – and most significant role – is in the production of hydrogen peroxide during oxidative reactions.

      Toxic to cells, hydrogen peroxide is contained by peroxisomes and kept in check with the enzyme catalase (also included in peroxisomes), which converts hydrogen peroxide into water or another harmless biological compound. Peroxisomes thereby contribute a major role in several metabolic pathways, including fatty-acid oxidation, lipid and bile-acid synthesis, and aiding in cholesterol production.

      Another vital function of lysosomes is the production of plasmalogen, the most prevalent phospholipid found in myelin sheaths – which is essential in carrying out the proper communication purposes of the central nervous system.

      Mitochondria

      Mitochondria is a unique structural component found within cells. Mitochondria (plural: mitochondrion) is a vital and crucial part of cellular structures, and therefore tissues and organs. Mitochondria take in food to provide energy for use by the cell.

      The mitochondria organelle found inside complex cellular structures is responsible for actually producing the energy required by that cell.

      The number of mitochondria found in cells differs: simple cells might contain one or perhaps two mitochondria. A cell that requires high levels of energy – like muscle – can contain thousands.

      The primary mechanism of action of mitochondria involves energy production. The molecule utilized for energy (adenosine triphosphate or ATP), is found inside the mitochondria.

      Mitochondria synthesizes energy through cellular respiration. This is achieved when the mitochondria consume food molecules such as carbohydrates and combines them with oxygen. This produces ATP. Protein enzymes are responsible for this action.

      Figure 2-6 Mitochondria.

      Mitochondria can take on different appearances depending on how the section of mitochondria is orientated within the cellular structure. Mitochondria are essential not only for energy production but also oxidation of nutrients.

      Mitochondria is composed of an inner and outer membrane. The outer membrane effectively controls what enters and exits the mitochondria, as well as substrate uptake and release of ATP.

      The structure of mitochondria is similar, although the shape of the mitochondria itself can differ:

      •Outer membrane – Often takes on different appearances. Mitochondria is not a consistently rigid shape, but is rather flexible and takes various shapes dependent upon its environment. It can be shaped similar to pill capsule or tablet, long and slender like a pencil, or rounded like a kidney bean.

      •Inner membrane – Differentiation of mitochondria organelles from others is due to the inner membrane. The inner membrane takes on a wrinkled appearance like cloth folded on top of itself. These folds are called cristae and are packed inside the mitochondria. These ‘folds’ increase the overall surface of the inner mitochondrial membrane.

      •Mitochondrial DNA is also found within the inner membrane, as are a number of other components including ribosomes and ATP synthase, an enzyme responsible for the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi).

      •Matrix – Located inside the inner membrane, the matrix is home to most of the proteins of mitochondria as well as mitochondrial DNA and ribosomes.

      The inner and outer membranes of the mitochondria are formed by the joining of components from two separate cellular compartments: the nucleus and the mitochondria.

      Mitochondria is responsible for a number of cellular functions including but not limited to:

      •Heat production (thermogenesis)/cellular metabolism

      •Maintenance of calcium concentrations

      •Citric acid cycle

      Mitochondria are able to alter their shape and relocate around cells when necessary. When a cellular structure requires more energy, the mitochondria reproduce, expand, and divide. In cases where a cellular structure requires less energy, the mitochondria either become inactive or die. In some ways, mitochondria are similar to some forms of bacteria.

      Different types of mitochondria produce different types of proteins; some are capable of producing hundreds of proteins used for a variety of body functions.

      The cytoskeleton supports the cellular structure. The cytoskeleton is a network of rod-like substances that pass through the cytoplasm. Proteins link these ‘rods’ to other cellular structures. Three major types of ‘rods’ are found in the cytoskeleton:

      •Microfilaments

      •Intermediate filaments

      •Microtubules

      Microfilaments are the thinnest, semi-flexible type of rod constructed of a special protein called actin. No two cells’ microfilament arrangements are identical. However, most do contain a web-like structure much like a net or web that attaches to the cytoplasmic side of the cell membrane.

      Figure 2-7 Interior cell components.

      Microfilaments serve to strengthen the surface of cells, resist crushing, and are capable of cellular movements that can influence shape.

      Intermediate filaments can be likened to rope in its structure – also constructed of protein fibers twisted together to enhance strength. They’re responsible for resisting external forces often placed on cells.

      Microtubules are literally hollow tubes much like a straw that are constructed from round protein subunits. These subunits are called tubulin. They extend from the center part of the centrosome. Microtubules are responsible for the overall shape of a cell and can change form; at times they can break apart and then form again, even at a different location inside the cell.

      Centrosomes

Centrosomes is a Latin word defining “center of the body” and is another organelle located close to the cell’s nucleus within the cytoplasm. The centrosome was discovered by Theodor Boveri5 (also spelled Boyeri) in 1888,