Micro Chapter 4

Eukaryotic Cells Endomembrane

All eukaryotic cells have within them a functionally interrelated membrane system—the endomembrane system—consisting of the nuclear envelope, ER and Golgi apparatus, vesicles and other organelles derived from them, and the plasma membrane. Many materials are moved around the cell by the endomembrane system, including some proteins.

Features of Prokaryotic Cells

Prokaryotes, which include all bacteria and archaea, are the simplest cellular organisms. Prokaryotic cells are fundamentally different in their internal organization from eukaryotic cells. Notably, prokaryotic cells lack a nucleus and membrane-bound organelles.

All cells, whether they are prokaryotic or eukaryotic, have some common features. The common features of prokaryotic and eukaryotic cells are:

1) DNA, the genetic material, contained in one or more chromosomes and located in a non-membrane bound nucleoid region in prokaryotes and a membrane-bound nucleus in eukaryotes 2) Plasma membrane, a phospholipid bilayer with proteins that separates the cell from the surrounding environment and functions as a selective barrier for the import and export of materials. 3) Cytoplasm, the rest of the material of the cell within the plasma membrane and excluding the nucleoid region or nucleus. The cytoplasm consists of a fluid portion called the cytosol, and the organelles and other particulates suspended in it. 4) Ribosomes, the structures on which protein synthesis takes place.

Prokaryotic cells have the following features:

1) The genetic material (DNA) is localized to a region called the nucleoid which has no surrounding membrane. 2) The cell contains large numbers of ribosomes that are used for protein synthesis. 3) At the periphery of the cell is the plasma membrane. In prokaryotes, cell respiration occurs in the plasma membrane. 4) Outside the plasma membrane of most prokaryotes is a fairly rigid wall which protects the cell from osmotic lysis. The walls of bacteria consist of peptidoglycans. Sometimes there is also an outer capsule. Note that the cell wall of prokaryotes differs chemically from the eukaryotic cell wall of plant cells and of protists. 5) Some bacteria have flagella which are used for locomotion and/or pili, which are used to connect two cells for genetic material.

Eukaryotic cells Endomembrane sorting materials

Certain materials in the cell, including some proteins, are sorted by the functionally interrelated cellular membranes of the endomembrane system. The membranes consist of phospholipid bilayers with organelle-specific proteins embedded in them. This eukaryotic cell system consists of the nuclear envelope; endoplasmic reticulum (ER) and Golgi apparatus; vesicles and other structures derived from them (e.g., lysosomes, peroxisomes); and the plasma membrane. The various membranes involved, though interrelated, differ in structure and function. The endomembrane system plays a very important role in moving materials around the cell, notably proteins and membranes (the latter is called membrane trafficking). For example, while many proteins are made on ribosomes that are free in the cytoplasm and remain in the cytoplasm, other proteins are made on ribosomes bound to the rough endoplasmic reticulum (RER). The latter proteins are inserted into the lumen of the RER, carbohydrates are added to them to produce glycoproteins, and they are then moved to cis face of the Golgi apparatus in transport vesicles that bud from the ER membrane. Within the Golgi, the protein may be modified further and then be dispatched from the trans face in a new transport vesicle. These vesicles move through the cytoplasm to their final desinations using the cytoskeleton. We can think of the system as analogous to a series of switching yards and train tracks, where materials are sorted with respect to their destinations at the switching yards and sent to those destinations along specific tracks in the cytoskeleton.

Features of Eukaryotic Cells

Eukaryotic cells contain a membrane-bound nucleus and numerous membrane-enclosed organelles (e.g., mitochondria, lysosomes, Golgi apparatus) not found in prokaryotes. The nucleus is bounded by the nuclear envelope, a double membrane with many nuclear pores through which material enters and leaves. Animals, plants, fungi, and protists are all eukaryotes. Eukaryotic cells are more complex than prokaryotic cells and are found in a great many different forms.

Eukaryotic cells Path of Lysosomal Protein

Like secretory proteins and some other proteins, proteins destined for lysosomes are made on ribosomes bound to the RER and move through the endomembrane system. In this case the lysosomal protein-containing vesicle that buds from the trans face of the Golgi apparatus is the lysosome itself.

Eukaryotic cells Path of secretory protein

Proteins destined for secretion are made on ribosomes bound to the RER. The proteins move through the endomembrane system and are dispatched from the trans face of the Golgi apparatus in transport vesicles that move through the cytoplasm and then fuse with the plasma membrane releasing the protein to the outside of the cell. Examples of secretory proteins are collagen, insulin, and digestive enzymes of the stomach and intestine. (In a similar way, proteins destined for a particular cell organelle move to the organelle in transport vesicles that deposit their contents in the organelle by membrane fusion.)