A&P Lecture 1.8: Lysosomes

This is an in depth  explanation of Lysosomes as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.8

LYSOSOMES

After a phagocytic cell has engulfed the proteins, polysaccharides, and lipids present in a particle of “food” (such as a bacterium), these molecules are still kept isolated from the cytoplasm by the membranes surrounding the food vacuole. The large molecules of proteins, polysaccharides, and lipids must first be digested into their smaller subunits (including amino acids, monosaccharides, and fatty acids) before they can cross the vacuole membrane and enter the cytoplasm.

 The digestive enzymes of a cell are isolated from the cytoplasm and concentrated within membrane-bound organelles called lysosomes, which contain more than 60 different enzymes. A primary lysosome is one that contains only digestive enzymes (about 40 different types) within an environment that is more acidic than the surrounding cytoplasm. A primary lysosome may fuse with a food vacuole (or with another cellular organelle) to form a secondary lysosome in which worn-out organelles and the products of phagocytosis can be digested. Thus, a secondary lysosome contains partially digested remnants of other organelles and ingested organic material. A lysosome that contains undigested wastes is called a residual body. Residual bodies may eliminate their waste by exocytosis, or the wastes may accumulate within the cell as the cell ages.

Partly digested membranes of various organelles and other cellular debris are often observed within secondary lysosomes. This is a result of autophagy, a process that destroys worn-out organelles and proteins in the cytoplasm so that they can be continuously replaced. Lysosomes are thus aptly characterized as the “digestive system” of the cell.

Lysosomes have also been called “suicide bags” because a break in their membranes would release their digestive enzymes and thus destroy the cell. This happens normally in programmed cell death (or apoptosis ), . An example is the loss of tissues that must accompany embryonic development, when earlier structures (such as gill pouches) are remodeled or replaced as the embryo matures

SUMMARY

Lysosomes  are membrane-bound vesicles that pinch off from the Golgi apparatus. They contain a variety of hydrolytic enzymes that function as intracellular digestion systems. Vesicles taken into the cell fuse with the lysosomes to form one vesicle and to expose the endocytized materials to hydrolytic enzymes. Various enzymes within lysosomes digest nucleic acids, proteins, polysaccharides, and lipids. Certain white blood cells have large numbers of lysosomes that contain enzymes to digest phagocytized bacteria. Lysosomes also

digest the organelles of the cell that are no longer functional in a process called autophagia . In other

cells, the lysosomes move to the plasma membrane, and the enzymes are secreted by exocytosis. For example, the normal process of bone remodeling involves the breakdown of bone tissue by specialized bone cells. Enzymes responsible for that degradation are released into the extracellular fluid from lysosomes produced by those cells.

A&P Lecture 1.7: Secretory Vesicles

This is an in depth  explanation of the Secretory Vesicle as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.7

SECRETORY VESICLE

The membrane-bound secretory vesicles that pinch off from the Golgi apparatus move to the surface of the cell, their membranes fuse with the plasma membrane, and the contents of the vesicle are released to the exterior by exocytosis. The membranes of the vesicles are then incorporated into the plasma membrane.

Secretory vesicles accumulate in some cells, but their contents frequently are not released to the exterior until the cell receives a signal. For example, secretory vesicles that contain the hormone insulin do not release it until the concentration of glucose in the blood increases and acts as a signal for the secretion of insulin from the cells.

A&P Lecture 1.6: Golgi Complex

This is an in depth  explanation of the Golgi Complex as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.4

GOLGI COMPLEX

The Golgi complex, also called the Golgi apparatus, consists of a stack of several flattened sacs . This is something like a stack of pancakes, but the Golgi sac “pancakes” are hollow, with cavities called cisternae within each sac. One side of the stack faces the endoplasmic reticulum and serves as a site of entry for vesicles from the endoplasmic reticulum that contain cellular products. The other side of the stack faces the plasma membrane, and the cellular products somehow get transferred to that side. This may be because the products are passed from one sac to the next, probably in vesicles, until reaching the sac facing the plasma membrane. Alternatively, the sac that receives the products from the endoplasmic reticulum may move through the stack until reaching the other side. By whichever mechanism the cell product is moved through the Golgi complex, it becomes chemically modified and then, in the sac facing the plasma membrane, is packaged into vesicles that bud off the sac. Depending on the nature of the cell product, the vesicles that leave the Golgi complex may become lysosomes, or secretory vesicles (in which the product is released from the cell by exocytosis), or may serve other functions.

The reverse of exocytosis is endocytosis, as previously described; the membranous vesicle formed by that process is an endosome. . This reverse pathway is called retrograde transport, because proteins within the extracellular fluid are brought into the cell and then taken to the Golgi apparatus and the endoplasmic reticulum. Some toxins, such as the cholera toxin, and proteins from viruses (including components of HIV) rely on retrograde transport for their ability to infect cells.

A&P Lecture 1.5: Endoplasmic Reticulum

This is an in depth  explanation of the Endoplasmic Reticulum as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.4

ENDOPLASMIC RETICULUM

The endoplasmic reticulum is a continuous a series of membranes distributed throughout the cytoplasm of

the cell located at the he outer membrane of the nuclear envelope. It is consists of broad, flattened, interconnecting sacs and tubules. The interior spaces of those sacs and tubules are called cisternae and are

isolated from the rest of the cytoplasm.

Most cells contain a system of membranes known as the endoplasmic reticulum, or ER. The ER may be either of two types: (1) a granular, or rough, endoplasmic reticulum or (2) an agranular, or smooth, endoplasmic reticulum. A granular endoplasmic reticulum bears ribosomes on its surface, whereas an agranular endoplasmic reticulum does not. The agranular endoplasmic reticulum serves a variety

of purposes in different cells; it provides a site for enzyme reactions in steroid hormone production and inactivation, for example, and a site for the storage of Ca 2 + in striated muscle cells. The granular endoplasmic reticulum is abundant in cells that are active in protein synthesis and secretion, such as

those of many exocrine and endocrine glands.

The rough endoplasmic reticulum is called “rough” because it has ribosomes attached to it. The ribosomes of the rough endoplasmic reticulum are sites where proteins are produced and modified for secretion and for internal use. The amount and configuration of the endoplasmic reticulum within the cytoplasm depend on the cell type and function. Cells with abundant rough endoplasmic reticulum synthesize large amounts of protein that are secreted for use outside the cell.

Smooth endoplasmic reticulum, which is endoplasmic reticulum without attached ribosomes, manufactures lipids, such as phospholipids, cholesterol, steroid hormones, and carbohydrates. Many phospholipids produced in the smooth endoplasmic reticulum help form vesicles within the cell and contribute to the plasma membrane. Cells that synthesize large amounts of lipid contain dense accumulations of smooth endoplasmic reticulum. Enzymes required for lipid synthesis are associated with the membranes of the smooth endoplasmic reticulum. Smooth endoplasmic reticulum also participates in the detoxification processes by which enzymes act on chemicals and drugs to change their structure and reduce their toxicity. The smooth endoplasmic reticulum of skeletal muscle stores calcium ions that function in muscle contraction.

A&P Lecture 1.4: Ribosome

This is an in depth  explanation of the Ribosome as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.4

RIBOSOME

Ribosomes  are the sites of protein synthesis. Each ribosome is composed of a large subunit and a smaller one. The ribosomal subunits, which consist of ribosomal RNA (rRNA) and proteins, are produced separately in the nucleolus of the nucleus. The ribosomal subunits then move through the nuclear pores into the cytoplasm, where the ribosomal subunits assemble with mRNA to form the functional ribosome during protein synthesis. Ribosomes can be found free in the cytoplasm or associated with an intracellular membrane complex called the endoplasmic reticulum. Free ribosomes primarily synthesize proteins used inside the cell, whereas ribosomes attached to the endoplasmic reticulum u

sually produce proteins that are secreted from the cell.

Ribosomes are often called the “protein factories” of the cell because it is here that proteins are produced according to the genetic information contained in messenger RNA. The ribosomes are quite tiny, about

25 nanometers in size, and can be found both free in the cytoplasm and located on the surface of an organelle called the endoplasmic reticulum.

Each ribosome consists of two subunits, which are designated 30S and 50S after their sedimentation rate in

a centrifuge (this is measured in Svedberg units, from which the “S” is derived). Each of the subunits is composed of both ribosomal RNA and proteins. Contrary to earlier expectations of most scientists, it now appears that the ribosomal RNA molecules serve as enzymes (called ribozymes ) for many of the reactions

in the ribosomes that are required for protein synthesis.