Cell Organelles

All cells, whether they are prokaryotic or eukaryotic, have some common features. These common features are:

DNA, the genetic material contained in one or more chromosomes and located in a nonmembrane bound nucleoid region in prokaryotes and a membrane-bound nucleus in eukaryotes

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

Cytoplasm, the rest of the material of the cell within the plasma membrane, excluding the nucleoid region or nucleus, that consists of a fluid portion called the cytosol and the organelles and other particulates suspended in it

Ribosomes, the organelles on which protein synthesis takes place

Prokaryotic cells are fundamentally different in their internal organization from eukaryotic cells. Notably, prokaryotic cells lack a nucleus and membranous organelles. 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 some prokaryotes the plasma membrane folds in to form structures called mesosomes, the function of which is not clearly understood.

4. Outside the plasma membrane of most prokaryotes is a fairly rigid wall which gives the organism its shape. 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 may be used to pull two cells in close contact, and perhaps to facilitate the transfer of genetic material.

Eukaryotic cells contain a membrane-bound nucleus and numerous membrane-enclosed organelles (e.g., mitochondria, lysosomes, Golgi apparatus) not found in prokaryotes. 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.

The nucleus contains most of the genetic material (DNA) of the cell. Additional DNA is in the mitochondria and (if present) chloroplasts. The nuclear DNA is complexed with proteins to form chromatin, which is organized as a number of linear chromosomes. Genetic control of the cell is carried out by the production of RNA in the nucleus (the process of transcription) and the subsequent transfer of this RNA to a ribosome in the cytoplasm, where protein synthesis (the process of translation) is directed. The resulting proteins carry out cell functions. Also located in the nucleus is the nucleolus or nucleoli, organelles in which ribosomes are assembled. The nucleus is bounded by a nuclear envelope, a double membrane perforated with pores and connected to the rough endoplasmic reticulum membrane system.

The cytoskeleton consists of microtubules, intermediate fibers, and microfilaments, which together maintain cell shape, anchor organelles, and cause cell movement. The microtubules and microfilaments are frequently assembled and disassembled according to cellular needs for movement and maintaining cell shape. Intermediate filaments are more permanent than microtubules and microfilaments.

The cell diagrams shown here represent intestinal epithelial cells with fingerlike projections, the microvilli. The location and appearance of cytoskeletal fibers in different cell types will vary.

A ribosome is the site of protein synthesis in the cell. Each ribosome consists of a large subunit and a small subunit, each of which contains rRNA (ribosomal RNA) and ribosomal proteins. In protein synthesis the mRNA (messenger RNA) moves through the ribosome while amino acids attached to tRNAs (transfer RNAs) are brought to the ribosome. The amino acids are joined to produce the protein. You may access more information on From Gene to Protein: Translation. Ribosomes exist free in the cytoplasm and bound to the endoplasmic reticulum (ER). Free ribosomes synthesize the proteins that function in the cytosol, while bound ribosomes make proteins that are distributed by the membrane systems, including those which are secreted from the cell.

The plasma membrane (also called the cell membrane) is a phospholipid bilayer with embedded proteins that encloses every living cell. This membrane blocks uncontrolled movements of water-soluble materials into or out of the cell. The various proteins embedded in the phospholipid bilayer penetrate into and through the bilayer three-dimensionally. It is the proteins of the membrane that are responsible for the specific functions of the plasma membrane. These functions include controlling the flow of nutrients and ions into and out of the cells, mediating the response of a cell to external stimuli (a process called signal transduction), and interacting with bordering cells. All membranous eukaryotic cell organelles have the common feature of a phospholipid bilayer, although the proteins differ in each case.

Mitochondria (singular = mitochondrion) are the sites of cellular respiration, a process that generates ATP from substrates in reactions using oxygen. All eukaryotic cells contain mitochondria, often many hundreds per cell. Each mitochondrion is about 1-10 um long. Mitochondria contain the enzymes and other components needed for the enzyme complexes that catalyze respiration. The primary function of mitochondria is to synthesize ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and Pi (inorganic phosphate).

Mitochondria are large organelles containing DNA and surrounded by a double membrane. The inner membrane is highly convoluted, with deep folds called cristae. The membranes divide the mitochondrion into two compartments, the central matrix, and the intermembrane space. DNA, in the form of a circular or linear molecule, is found in the matrix. The mitochondrial DNA encodes many of the components for mitochondrial function, while nuclear DNA encodes the remaining components. Components of the protein synthesizing machinery specific for mitochondria-ribosomes, tRNAs and specific proteins and enzymes-are also found in the matrix.

All eukaryotic cells have within them a functionally interrelated membrane system, the endomembrane system which consists of the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, vesicles and other organelles derived from them (for example, lysosomes, peroxisomes), and the plasma membrane. Many materials, including some proteins, are sorted by the functionally cellular membranes of the endomembrane system. 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.

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.)

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.

The figure below illustrates at a glance the structures that are common to both animal and plant cells, as well as the structures that are unique to each. Structures that are common to both plant and animal cells are labeled between the cells; structures that are unique to plants are labeled on the left of the cells and those unique to animals are labeled on the right.

Chloroplasts are plant cell organelles that contain chlorophyll and the enzymes required for photosynthesis, the light-dependent synthesis of carbohydrates from carbon dioxide (CO2) and water (H2O). Oxygen (O2) is a product of the photosynthesis process, and is released into the atmosphere. Chloroplasts are large organelles bounded by a double membrane and containing DNA. Unlike the mitochondrial double membrane, the inner membrane is not folded. Distinctly separate from the double membrane is an internal membrane system consisting of flattened sacs and called thylakoids. The space between the thylakoid and the outer membranes is called the stroma. The stroma contains the chloroplast DNA as well as components of the protein synthesizing machinery specific for the chloroplast, namely the ribosomes, tRNAs, and specific proteins and enzymes. Most of the components of photosynthesis are located in the thylakoids. The thylakoid membranes are organized into stacks called grana. The interior of the thylakoid is the lumen.

© 2007 Wilhelm S. Cruz, All Rights Reserved