Cells are the basic unit of life and is one of life's characteristics (See 1.5). Before we can proceed with an investigation of the functions of cells, it helps to have a road map. Understanding cellular structure provides the road map and the focus of this chapter.
There are some basic structural features common to all cells such as a cell membrane, DNA & RNA, and ribosomes. These structures perform physiological functions necessary for cell maintenance, growth and work.
There are characteristics of living organisms that cells have that other non-living forms (viruses, viroids and prions) do not have.
Compare a virus and a bacterium.
There are two fundamentally different types of cells, prokaryotes and eukaryotes. The major distinction between the two basic types of cells is that eukaryotic organisms are physiologically more efficient than prokaryotic organisms because their cells have separate cellular structures (organelles) to provide for a division of labor. Prokaryotes include eubacteria and archaebacteria and eukaryotic forms include protists, fungi, plants, and animals.
Prokaryotes ( = "before nucleus") are more simple, although no living thing can be considered simple. The characteristics of prokaryotes includes:
Most prokaryotes have a cell wall. There are no membrane-bound organelles in prokaryotes.
Eukaryotic cells have a greater complexity indicated by a wider variety of cell structures and organelles.
All of the eukaryotes share some basic structures but also have some structures unique to their group. As such there are are important differences between fungal, plant and animal cells.
View a virtual plant and animal cell.
The following includes descriptions of the major organelles of eukaryotic cells.
The Nucleus (4.5) is often centrally located. Within the nucleus is the genetic material, DNA and RNA. The nuclear membrane or envelope has pores that all for the transport of RNA. The nucleolus is a region of intense RNA synthesis. DNA bonds with protein to form chromatin and the more condensed chromosomes. This is the command center of the cell.
The Endomembrane System (4.6) is a system within the cells that provides space and surface for many of the cell's vital functions. Surfaces are important for the metabolism of a cell.
The Endoplasmic Reticulum or ER (4.7 & 4.8) ranges from the nucleus to the outer membrane. It divides the cell into compartments. There are two types of ER. Rough ER is studded with ribosomes. Ribosomes are the site of protein synthesis. Smooth ER lack ribosomes and is the site of lipid and carbohydrate synthesis (in part). This is the factory of the cell.
The Golgi (4.9) receives the products of the ER, modifies them, and transports them in vacuoles and vesicles. This is the packaging center of the cell - a UPS of the cell ; )
Lysosomes (4.10) are vacuoles that contain powerful digestive enzymes. Other vacuoles have other important functions.
One of the three major themes of Biology is energy transfer. The organelles that transfer energy are similar in structure to the prokaryotes and likely evolved from symbiotic associations with prokaryotes. These organelles are sausage shapes, have their own circular DNA, reproduce on their own, and have two membranes.
Chloroplasts are the site of photosynthesis within eukaryotes (plants and some protistians). Within the chloroplast, the energy from solar photons are transferred into the energy rich bonds of sugars like glucose.
Mitochondria are the site of cellular respiration. Within this organelle the components of glucose are "combusted" by oxygen, their energy transferred to a molecule called ATP.
In the last chapter you learned that proteins have many functions. Among their functions is the construction of fibers and microtubes.
The cytoskeleton is a network of protein tubes that provide structure to the cell.
All organism motion is the result of sliding protein microtubes. Cilia and flagella are organelles composed of sliding microtubes. Flagella propel human sperm and cilia sweep human eggs through the oviduct. Our first journey as cellular organisms were thus at the expense of these organelles : -)