The Cell

The Cell

Many living things are unicellular – they consist of a single cell. Other living things are multicellular, consisting of many cells – many trillions of cells in the case of human adult. The cell is in several senses the basic unit of a living organism. There are more than 200 distinct kinds of cells in the human body. All living things are composed of cells, and all cells arise from preexisting cells.

Plasma membrane
Every living cell is surrounded by a plasma membrane that regulates the entry and exit of molecules and ions and that contains receptors for various external signals.

Eukaryotic cell structure
Compared with a prokaryotic cell, a Eukaryotic cell is full of membranous structures. Unlike a prokaryotic cell, a Eukaryotic cell contains a nucleus and numerous membrane – bounded organelles. But , like a prokaryotic cell, it contains many ribosomes for protein assembly.

Organelles
Organelles are subcellular structures with specialized functions. Among the kinds of organelles are the ribosomes, the mitochondria,.. many organelles are membrane – bounded, but ribosomes are not.

Nucleus
The nucleus is the information center of a eukaryotic cell. A double-membraned nuclear envelope bounds the nucleus with its enclosed, linear chromosomes. The nucleoplasm is the fluid material within the nucleus, suspending the chromosomes and nucleoli.

Nuclear envelope
The nucleus of a cell (a Eukaryotic cell, that is … recall that prokaryotic cells lack nuclei) is surrounded by a double membrane, the nuclear envelope is perforated by nuclear pores. The nuclear envelope breaks down at the onset of prometaphase in mitosis or meiosis, and it forms again during telophase.
Nuclear lamina
In Eukaryotic a nuclear lamina lines the inside of the inner membrane of the nuclear envelope. Intermediate filaments, woven together, constitute the nuclear lamina. These intermediate filaments are made of proteins called nuclear lamins. Breakdown of the nuclear lamina precedes the disappearance of the nuclear envelope at the start of prometaphase.

Nuclear pore
The nuclear envelope is perforated by nuclear pore … channels that allow the passage of certain molecules between nucleus and cytoplasm. Each pore is part of a nuclear pore complex consisting of over 100 different proteins.

Chromatin
Chromatin is the DNA-protein complex found in eukaryotic chromosomes. It is replicated during the S phase of the cell cycle.

Chromosome
Chromosome contains all or almost all the hereditary material of a cell. A single Chromosome contains a large number of genes, each with its own locus (place) on the Chromosome. A Chromosome consists of a large molecule of DNA and a number of protein molecules. Chromosome change dramatically in structure during the cell cycle.

Nucleolus
A Nucleolus is a small, generally spherical body found within the nucleus of eukaryotic cells. Transcription of ribosomal RNA (rRNA) takes place in the nucleoli, as does the assembly of ribosomal subunits. The nucleoli contain ribosomal RNA (rRNA) and many copies of the genes that encode rRNA. The number of nucleoli in a nucleus is characteristic of a species, one is the most common number.
Nucleoplasm
Nucleoplasm is the fluid material within the nucleus. Chromosomes, nucleoli, and other particulate components of the nucleus, as well as the nuclear envelope, are distinct from the Nucleoplasm.

Cytoplasm
The cytoplasm is the contents of the cell, excluding the nucleus. The cytoplasm includes many membrane-bounded organelles.

Cytosol

The liquid (non-particulate) part of the cytoplasm is called the cytosol. The organelles and cytoskeleton lie in the cytosol.

Ribosome
A ribosome is a tiny organelle composed of rRNAs and ribosomal protiens. It is not surrounded by a membrane. It is a “factory” for the synthesis of proteins, using information provided by mRNAs. A ribosome consists of tow ribosomal subunits that combine only in the presence of mRNAs and an appropriate charged transfer RNA. Both eukaryotic and prokaryotic cells contain ribosomes.

Mitochonderion
Mitochondria are large organelles in eukaryotic cells. They are the sites of cellular respiration. They contain the enzymes of the citric acid cycle, the respiratory chain, and oxidative phosphorylation. They are bounded by a double membrane. The inner mitochondrial membrane folds, producing cristae. Mitochondria produce most of the ATP in eukaryotic cells.

Crista
Cristae are infoldings of the interior mitochondrial membrane. The cristae are the sites of oxidative phosphorylation in cells. Each crista contains many complexes that cause proton pumping that is the basis for chemiosomatic production of ATP.

Mitochonderil Matrix
The mitochondrial matrix is the fluid filled interior of the mitochondrion. Most of the enzymes of the citric acid cycle are contained in the mitochondrial matrix.

Endomembrane System
The endomembrane system is an extensive set for closely interrelated membranes inside the cell. The components of the endomembrane system exchnge materials by means of vesicles. The chief components of the endomembrane system are the endoplasmic reticulum and the Golgi apparatus.

Endoplasmic Reticulum
Eukaryotic cells contain an endoplasmic reticulum; a network of membranous structure, in the cytoplasm. The ER is of tow types: rough endoplasmic reticulum and smooth endoplasmic reticulum. Skeletal muscle fibers contain a modified ER called the sarcoplasmic reticulum, that controls the concentration of calcium ions around the contractile machinery.

Rough Endoplasmic Reticulum
Rough endoplasmic reticulum studded with ribosomes. Rough ER participates in the synthesis and later processing of proteins destined to be secreted or to be associated with membranes.

Smooth Endoplasmic Reticulum
Smooth endoplasmic reticulum lacks ribosomes. It is where lipids are synthesized.

Golgi Apparatus
The Golgi apparstus is a packaging and processing station for molecular traffic among parts of the cell. It is a system of folded membranes, arranged like stacks of pocket bread (pita), in eukaryotic cells. In a Golgi apparatus, there are important differences between the cis and trans regions; materials are processed through the apparatus from the cis region to the trans region.