Cells and Cell Structure

Cell types

      Cells can be placed into two basic categories:  Prokaryotic cells have the simplest basic structure.  These cells contain no nucleus, and very few of the structures associated with more complex forms.  The archebacteria, eubacteria and cyanobacteria all are prokaryotic organisms.
Prokaryotes are among the smallest of living things, so most must be viewed with high powered microscopes.  The bacterial cell has a cell wall, composed of a complex carbohydrate compound called peptidoglycan, and a cell membrane called the plasmalemma.  Some bacteria have both inner and outer cell membranes, with the cell wall between the two.  This can be visualized through the use of a differential staining technique called Gram staining.  In the Gram staining process, a primary stain (crystal violet) is applied to the cell, which is bonded to the cell wall with iodine, then destained with alcohol.  A secondary stain (safranin) is then applied.  If the cell has both inner and outer membranes, the primary stain is removed when the outer membrane is dissolved by the destain.  These cells retain the red secondary stain and are said to be Gram negative. Gram positive cells have no outer membrane, thus they stain blue since the blue primary stain forms an insoluble complex with the iodine mordant and cell wall, which cannot be removed by destain.  This information can be used to help classify bacterial cells.

     Inside the cell, there is cytoplasm, composed of water and large molecules of dissolved compounds.  The graininess of the cytoplasm is caused by the presence of large protein molecules, lipids, carbohydrates, special structures called ribosomes which serve in the synthesis of proteins for the cell, and DNA (deoxyribonucleic acid) the molecule which carries the genetic information necessary for the production of protein compounds essential to the organism's survival.  Bacterial DNA appears normally in the form of a large ring called a chromosome, but can also exist as very small rings called plasmids, which carry genetic information about the production of various reproductive structures, special enzymes, or resistance to antibiotic compounds.

     The three basic morphological forms of bacteria most commonly recognized are the coccus, which is round and non-motile, the bacillus, which is rod-shaped and can move, and the spirillum, which is spiral- shaped and able to move.  Bacterial cells can also occur as vibrios, which are comma-shaped.

      Eukaryotic cells are generally larger than prokaryotic cells and contain many more specialized cell structures, called organelles, than do prokaryotic cells.  These cell types are very similar in terms of their structure, but differ in some of the organelles each contain.  All animal cells are heterotrophic, meaning that they must obtain nutrients from an outside source, and thus are incapable of making their own food.  Many plant cells; however, are autotrophic, which means they can produce their own food, and have specialized organelles, called chloroplasts, which perform in this function.  The organelles which both animal and plants contain are described below:

     The plasmalemma or cell membrane separates the cytoplasm from the outside environment, and serves to allow passage of molecules into and out of the cell.  It is composed of a double layer of phospholipids, punctuated by various proteins.  A phospholipid is composed of a single phosphate molecule bonded to two fatty acid molecules.  The phosphate group is a polar molecule which mixes easily with water, and is thus referred to as hydrophilic (water loving), while the two fatty acids are nonpolar, thus do not attract water and are said to be hydrophobic (water fearing).  The major consequence of this structure is that the hydrophilic phosphate group faces outward toward the watery environment or inward toward the cytoplasm, while the hydrophobic lipids face inward toward one another.  The rather fluid structure created by this arrangement of autonomous phospholipid molecules is hypothesized to be held together by sterol molecules such as cholesterol in mammal cells and ergosterol in fungal cells, which lie between the hydrophobic lipids.  Small molecules such as water and some ions, can easily pass between the phospholipids into and out of the cell.  The plasmalemma is thus said to be a semi- permiable membrane, allowing the passage of small molecules, but denying that of large molecules.

     The surface of the plasmalemma sometimes is folded into a series of finger like projections called microvilli which increase its absorptive area.  The membrane can also fold inward to form pinocytic vesicles which can be used to take some of the fluid outside of the cell into the cell.  Some cells can fold the plasma membrane inward to ingest large molecules in a process called phagocytosis.  The membrane can then be pinched off to form a vacuole, which serves to store large particles for eventual digestion.

     The nucleus contains DNA and RNA (ribonucleic acid) and is the primary center of cellular control.  It is composed of a nuclear membrane which is punctuated by openings called nuclear pores, an inner cytoplasm-like colloid called the nucleoplasm and several darkly staining bodies called nucleoli which function in the production of ribosomes.  The nuclear membrane is composed of the same basic material as the endoplasmic reticulum, and is often associated with it.

     Ribosomes are small organelles composed of RNA and protein, and divided into two subunits.  Prokaryotes and eukaryotes both contain ribosomes, but those of prokaryotic organisms are considerably less dense.  Ribosomes function in the production of proteins for the cell.  The endoplasmic reticulum (ER) is composed of a series of long, tube like structures which serve in the packaging and transport of protein compounds.  There are two types of endoplasmic reticulum, rough ER, which is covered with ribosomes and serves in the packaging and transport of proteins for export from the cell, and smooth ER which is not associated with ribosomes and serves in the synthesis of new cell lipids.  Mitochondria are structures responsible for the production of energy-carrying molecules in the cell.  Mitochondria are composed of a membrane which forms a series of folds inward called cristae, and an inner colloidal compound called the matrix.  The golgi apparatus is a series of folded sacs which functions in the production of cellular membranes, and the packaging of protein compounds for export out of the cell.  Lysosomes are membrane-bound containers filled with digestive enzymes. These can be used to break down large food compounds taken into the cell actively during phagocytosis, the ingestion of large particles.  Vacuoles are membrane-bound containers of water or food particles.  Microtubules and microfilaments are proteinaceous strands which lie within the cell and serve to give the cell structure and to move organelles from one part of the cell to another.  The network of microtubules and microfilaments which support the cell are collectively referred to as the cytoskeleton.  Microfilaments known as spindle fibers drag chromosomes apart from one another during cell division, and groups of microtubules form structures called cilia and flagella which organisms use for movement or to produce currents in fluid environments.  Centrioles, structures found in animal cells but not in plant cells house the spindle fibers used during the process of cell reproduction.

     Plant cells contain additional organelles.  The cell wall is a rigid structure composed of the complex carbohydrate cellulose.  The cell wall gives the plant cell support and structure.  Plastids, such as chloroplasts are structures composed of a membranous outer layer, flat inner membranes called thylakoids,which lie in stacks called grana, and a colloidal inner fluid called the stroma.  Between the grana are the light sensitive pigments called chlorophylls, which are active in the process of photosynthesis.  Chlorophylls reflect green light, and thus are responsible for the green color of photosynthetic plants.

Cooperative Learning Exercises



1.  We can correctly think of the cell as a kind of factory, where materials are taken in, processed,
     and recycled into new forms.  Large biomolecules are broken down into smaller component parts,
     which can then be used to construct new molecules.  Based on the information you have
     concerning the structure of the eukaryotic cell, describe the process that is used to produce a
     protein for export from the cell.  Be sure to include each organelle utilized in this process and
     explain its function.

2.  Make a table to compare and contrast the structures of eukaryotic and prokaryotic cells.

  Test Yourself - Take this quiz on cell structure.

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