Cytology - Part VII


Organelles of Synthesis, Storage, and Cytoplasmic Transport

Endoplasmic Reticulum

A complex and extensive system of convoluted membrane(s) taking up a sizable portion of the cytoplasm is known as the endoplasmic reticulum.

The ER is a dynamic ever-changing structure; a continuous membrane which forms a closed sac. Its internal space or lumen, may account for 10% of cytoplasmic volume.

The ER forms the outer membrane of the nuclear envelope so actually the nuclear material is separated from the ER lumen by a single membrane.

The endoplasmic reticulum appears in two main forms smooth and rough.

Note: In the cartoon of rough endoplasmic reticulum below many of the ribosomes (illustrated as dark spots) are much too regularly spaced.

The rough endoplasmic reticulum receives its name from the appearance of ribosomes, dense granules that stick temporarily to the outer sides of the endoplasmic reticulum.

RER (rough endoplasmic reticulum) occurs in cells that are actively making proteins destined for secretion outside the cell -- examples of such proteins are antibodies, digestive enzymes, and certain hormones. The diagram below outlines the major steps in this process.


Ribosomes

Ribosomes (2) are sites of protein synthesis. They are much too large and complex to be considered simple molecules.

A ribosome is actually made of two subunits each composed of rRNA (r = ribosomal) and many kinds of proteins which come together during polypeptide synthesis.

The ribosomal subunits in bacteria are smaller and differ somewhat chemically from those of eukaryotes. It is this difference that makes many antibiotics at least temperarily effective against bacterial infection.

Polyribosomes (or polysomes) are groups of free ribosomes held together like beads on a string by a molecule of RNA during protein synthesis. 

 

 


Smooth endoplasmic reticulum

The smooth ER is an intricate network of tubes and sacs found in most cells but is abundant in those cells involved in the synthesis, secretion, and storage of non-protein products such as phospholipids, cholesterol, and other lipids.

Cells involved in lipid metabolism contain lots of SER. For example, steroid hormones are produced in the endocrine glands of the testis, and oils are produced by exocrine glands found in skin.

It may be that SER in small intestine cells plays an important role in the way animals absorb and store the products of lipid digestion.

The smooth ER in the liver has two functions:

  • it contains oxidizing enzymes that detoxify certain chemicals
  • it is active in breaking down glycogen to glucose


Golgi apparatus

Modern electron microscope studies of the Golgi complex show it to be a series of compressed baglike sacs or cisternae, generally found near the nucleus, and always associated with endoplasmic reticulum.

A stack of Golgi bodies in plants may be referred to as a dictyostome.

The Golgi complex is one of the cell’s more dynamic organelles, constantly changing and busy. Golgi cisternae (protein filled sacs) actually originate from portions of the nearby endoplasmic reticulum known as cis vesicles

The dynamics of the Golgi complex involves:

  • vesicles originating from nearby endoplasmic reticulum merge with the cis face of the Golgi apparatus
  • the cisterna buds off vesicles at either end which then migrate to the next cisterna in the stack.
  • eventually vesicles containing the finished cellular product bud from the trans face of the Golgi apparatus, and migrate through the cytoplasm to their final destination (usually the plasma membrane for exocytosis.)

A cartoon of the Golgi complex illustrates the dynamics described above.

 


Lysosomes

Spherical bags of hydrolytic enzymes, synthesized in the RER and which mature in the Golgi complex are called lysosomes.

The lysosome is able to make its own acidic condition by actively pumping H+ ions from the cytoplasm to its interior.

The 40 or so hydrolytic enzymes so far discovered in lysosomes are most effective in an acidic environment. This is an important safeguard for most cells which have lysosomes otherwise the cell containing them would be a risk of self destruction.

Christian de Duve, the cell biologist who predicted the existence of lysosomes from chemical evidence, eventually found what he was looking for by using the electron microscope.

Some lysosomes act as "suicide bags" because they are known to engage in autophagy. Apoptosis or programmed cell death, is now recognized as an essential part of multicellular development.

Lysosomes are optically dense bodies -- they form continuously from the trans face of the Golgi complex, emerging as primary lysosomes. Visit the University of Virginia to view a detailed cartoon of lysosome formation and function.

Three functions of lysosomes are:

  1. digestion of large food particles (such as invading bacteria)
  2. modification and storage of molecular materials obtained through endocytosis
  3. destruction of aging or damaged cellular organelles

 Visit this following site for more information about lysosomes and other microsomes.


Microbodies

Minor organelles, primarily sacs used to store or process various molecules include peroxisomes, glyoxysomes, and various vacuoles.

  1. peroxisomes are found in most eukaryotes. They probably have a protective function and may have been another organelle derived from the endosymbiosis with an ancient free living prokaryote. peroxisomes found in the liver contain the enzyme catalase which neutralizes dangerous peroxides such as H2O2.
  2. caveolae or "little caves" play a significant role in intercellular communication.
  3. glyoxysomes are microbodies found in seeds and posses enzymes capable of mobilizing lipids, converting them into sugars to supply energy needed during germination.
  4. vacuoles are general all purpose storage containers. The large central vacuole of plants stores a watery solution which also regulates turgor pressure, the force plants use to stay turgid and enlarge developing cells.


Endomembrane System

The endomembrane system is a network of shared and transfered membranes including the following components:

  • the outer membrane of the nuclear envelope
  • smooth and rough endoplasmic reticulum
  • Golgi apparatus and associated vesicles and the
  • plasma membrane.

The endomembranal system is necessary to keep the products and reactants of metabolism separate and compartmentalized for appropriate disposal and to satisfy energy requirements. Make sure you observe the animated diagram at the bottom of this page


Modified July 9, 2005