Cell Biology

The plasma membrane is the outer membrane of the cell it delimits the cell itself and a cell interior with specific conditions for the cellular function. Since it is selectively permeable the plasma membrane has an important role for the passage of substances inwards or outwards.

Cell Structure Review - Image Diversity: cell membrane

There are no pluricellular bacteria. All bacteria are unicellular prokaryotic.

In bacteria the genetic material is dispersed in the cytosol and there is no internal membrane that delimits a nucleus.

Ribosomes can be found free in the cytoplasm, adhered to the outer side of the nuclear membrane or associated to the endoplasmic reticulum membrane defining the rough endoplasmic reticulum. Ribosomes are the structures where protein synthesis takes place.

Cell Structure Review - Image Diversity: ribosomes

Cytoplasmic inclusions are cytoplasmic molecular aggregates, such as pigments, organic polymers and crystals. They are not considered cell organelles.

Fat drops and glycogen granules are examples of cytoplasmic inclusions.

Cell Structure Review - Image Diversity: cytoplasmic inclusions

The main constituents of the plasma membrane are phospholipids, proteins and carbohydrates. The phospholipds, amphipathic molecules, are regularly organized in the membrane according to their polarity: two layers of phospholipids form the lipid bilayer with the polar part of the phospholipids pointing to the exterior of the layer and the non polar phospholipid chains in the interior. Proteins can be found embedded in the lipid bilayer and there are also some carbohydrates bound to proteins and to phospholipids in the outer face of the membrane.

Plasma membrane and cell wall is not the same thing. Plasma membrane, also called cell membrane, is the outer membrane common to all living cells and it is made of a phospholipid bilayer, embedded proteins and some appended carbohydrates.

Because cell membranes are fragile, in some types of cells there are even outer structures that support and protect the membrane, like the cellulose wall of plant cells and the chitin wall of some fungi cells. Most bacteria also present an outer cell wall made of peptidoglycans and other organic substances.

Cell Structure Review - Image Diversity: cell wall

The nuclear membrane is also called karyotheca. The nuclear membrane is continuous to the endoplasmic reticulum membrane.

Nucleolus is a region within the nucleus made of ribosomal RNA (rRNA) and proteins. It is not delimited by membrane.

The aqueous fluid that fills the nuclear region is called karyolymph, or nucleoplasm. In the fluid there are proteins, enzymes and other important substances for the nuclear metabolism.

The chromatin, dispersed in the nucleus, is a set of filamentous DNA molecules associated to nuclear proteins called histones. Each DNA filament is a double helix of DNA and thus a chromosome.

Within the cell nucleus the main structures are: the nucleolus, an optically dense region, spherical shaped, where there are concentrated ribosomal RNA (rRNA) associated to proteins (there may be more than one nucleolus in a nucleus); the chromatin, made of DNA molecules dispersed in the nuclear matrix during the cell interphase; the karyotecha, or nuclear membrane, the membrane that delimits the nucleus.

The main cytoplasmic structures of the cell are the centrioles, the cytoskeleton, lysosomes, mitochondria, peroxisomes, the Golgi apparatus, the endoplasmic reticula and ribosomes.

Cell Structure Review - Image Diversity: cell organelles

In bacteria cell wall is made of peptidoglycans; among protists algae have cell wall made of cellulose; in fungi, the cell wall is made of chitin (the same substance that makes the exoskeleton of arthropods); in plants, the cell wall is made of cellulose too.

This is an interesting problem of biological evolution. The most accepted hypothesis asserts that the more simple cell, the prokaryotic cell, appeared early in evolution than the more complex eukaryotic cell. The endosymbiotic hypothesis, for example, affirms that aerobic eukaryotic cells appeared from the mutualist ecological interaction between aerobic prokaryotes and primitive anaerobic eukaryotes.

The walls of the cavities observed by Hooke were the walls of the plant cells that form the tissue. The observation leaded to the the discovery of the cells, a fact only possible after the invention of the microscope. In that work, Hooke established the term “cell”, now widely used in Biology, to designate those cavities seen under the microscope.

Cell Structure Review - Image Diversity: Hooke's cell

The virus is considered the only alive beings that do not have cells. Virus are constituted by genetic material (DNA or RNA) enwrapped by a protein capsule. They do not have membrane and cell organelles neither self-metabolism.

Lipid membranes do not form only the outer cover of cells. Cell organelles, such as the Golgi complex, mitochondria, chloroplasts, lysosomes, the endoplasmic reticula and the nucleus, are delimited by membranes too.

Cell Structure Review - Image Diversity: cell nucleus

The eukaryotic cell can be divided into two main portions: the cell membrane that separates the intracellular space from the outer space phisically delimiting the cell; the cytoplasm, the interior portion filled with cytosol (the aqueous fluid inside the cell); and the nucleus, the membrane-delimited internal region that contains the genetic material.