Cell Specialization

Source: Wikipedia and some random week 2 notes

Prokaryotes:

Prokaryotes are organisms without a cell nucleus. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. In 1977, Carl Woese proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the significant genetic differences between the two. This arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the three-domain system replacing the traditional two-empire system.

Ev0lution of Prokaryotes:

It is generally accepted that the first living cells were some form of prokaryote. Fossilized prokaryotes approximately 3.5 billion years old have been discovered, and prokaryotes are perhaps the most successful and abundant organism even today. In contrast the eukaryote only appeared between approximately 1.7 and 2.2 billion years ago.

Eukaryotes:

A eykaryote is an organism with complex cells that have nuclei containing DNA. Eukaryotes include animals, plants and fungi, as well as various unicellular organisms such as protists.

The Special Features of Eukaryotic cells

Unlike prokaryotic cells (such as bacteria), eukaryotic cells are larger and show internal compartments, the nucleus and the cytoplasm. The existence of these compartments allows a eukaryote to separate various metabolic functions inside the cell, for example the regulation of DNA synthesis within the nucleus, and the production of proteins within the cytoplasm. This function, where specialised activities occur within defined regions of a single cell, is characteristic of special multicullular organisms.

The Symbiotic Theory

A critical step for the specialization of eukaryotic cells is beleived to be the symbiotic acquisiton of mitochondria from aerobic bacteria and in the case of plants, or chloroplasts from photosynthetic bacteria. The theory supposes that the separation of both respiration and energy metabolism into a mitochondrion may have provided the ancestral eukaryote with an abundant source of energy.

Cell Specialization

Eukaryotic cells have developed internal membranes that provide an enormous surface area for metabolic reactions that perform hundreds or thousands of different functions inside the cell:
- Cytoskeleton: allows for arrangement of membranes, alterations of cell shape and motility
- Nucleus: packages genetic material, separating DNA and RNA sequences from cytoplasm and provides for independant control of the chromosomes
- Cell plasma membrane: specialized to provide for adhesion, receptors for external stimuli, and control over the entry and exit of substances

Multicellular Organisms

Increasing cell specialization, uncluding the ability to control cell multiplication, has led to the complexity and diversity of many different types of cells that are reorganized as tissues, organs and organ systems. Unicellular organisms can form colonies, e.g. Volvox (green algae), where around 50,000 individual cells form colonies consisting of hollow balls in which the cells are embedded in a gelatinous matrix. The colony is structurally and functionally polarised - it can swim towards light and its reproductive cells reside at one region of the colony. Volvox displays specialized cells that cooperate in function, an essential feature of multicellular organisms.

Additional key features of multicellularity:
- Cohesions between cells and with extracellular molecules
- Complete cell division, or incomplete cytokinesis through cytoplasmic bridges
- Formation of multicellular sheets, or epithelium
- Inner and outer layers of epithelium
- Cell-cell communication, cell responds to signals

Gene Expression

Cenral to the developnment of multicellular organisms and the specialized cell types which they produce, is the process of differentiation. This is where single cells or groups of cells undergo structure and metabolic changes which distinguish them functionally from other cells in the developing organism. The instructions are contained in the DNA, and when activated, this gene expression induces important phases of biochemical reactions throughout development.