Reality → Life → Cell → Components
All eukaryotic cells consist of a cell membrane, the cytoplasm, and one or more nuclei. The decisive distinction between animal and plant cells is that the latter have, in addition to the cell membrane, a supporting and protecting outer cell wall, which contains cellulose as a prime component.
Amphiphilic lipid bilayers self-assemble into thin membranes of only 2 molecules thickness (see Lipids, Notes 5 and 6), building a barrier that blocks water soluble substances but normally allows lipid substances to pass.
The term 'cytoplasm' is sometimes also used to describe the cell substance without organelles and may be confused with the term 'cytosol'. Here, cytoplasm means the cell content without nucleus and cell membrane, and cytosol means the plasm's watery component with all dissolved substances. The distinction is somewhat arbitrary because major colloidal components may change, through hydration, their solubility and hence the plasm's gel properties.
The cytoskeleton is a complex and dynamic network of protein filaments in the cytoplasm. Important components are microtubules, actin, and myosin. These structures provide mechanical support for the cell and are involved in cell division, intracellular transport, cell movements (as for instance exhibited by amoebae), and contractions (as exhibited by muscle cells). The functions can be traced back to rapid dynamic changes at the molecular level (e.g., polymerization/depolymerization, sliding of parallel molecular chains, and other interactions).
The decomposition of ATP (adenosine triphosphate, see Carbohydrates, Note 5) to diphosphate releases 32kJ/mol of energy (64 kJ/mol if decomposed to monophosphate). Regeneration of triphosphate in the mitochondrion is enabled by the membrane enzyme ATP synthase, an incredibly complex and sophisticated rotating molecular machine (see animations).
The mitochondrial DNA resembles closely the DNA of a prokaryotic cell (85% of its genes are identical with bacterial genes). This supports a hypothesis that at an early stage of life's evolution a proto-eukaryotic cell engulfed a prokaryotic cell that then developed into the mitochondria of eukaryotic life.
In plants, mitochondria play a role in the complex light-dependent reactions of photosynthesis taking place in the thylakoids of chloroplasts.
Vesicles are small bubbles encapsulated by lipid bilayers. They protect and transport macromolecules for various functions of the cell. Vesicles are grown from the membranes of larger organelles. They can fuse with membranes of other organelles or with the cell membrane and thereby release their content.
The nuclear membrane (also called nuclear envelope) consists of an inner and outer lipid bilayer. Large protrusions of the outer layer form walls of the ER which enclose cavities contiguous with the space between inner and outer layer of the nuclear membrane.
The nuclear lamina consists of a dense (30-100 nm thick) network of filaments with associated membrane proteins. Besides supporting the nuclear envelope and anchoring the nuclear pore complexes, the lamina is believed to also play a regulatory role in DNA replication and cell division. Lamina, the attached nuclear envelope, and the pore complexes disassemble before cell division and reassemble thereafter.
The nucleolus is the largest and most important sub-organelle of the nucleoplasm. The nucleolus consists mainly of RNA and protein, has no membrane, and is not visible during cell division. The nucleolus plays an important role in the synthesis of ribosomal RNA, the main component of ribosomes, and is often called the ribosome factory (see Protein biosynthesis for the role of ribosomes).