Cells constitute the structural and functional basis of all living forms. A cell is a membrane-bound aqueous entity where the biochemical and metabolic reactions, fundamental for life, take place. Throughout evolution, two types of cells differing by their internal organization came to exist: Prokaryotes and Eukaryotes.
Prokaryotes: such as the ones forming the unicellular bacteria, exhibit a simple structure with a cytoplasm, the inner space within the cell membrane, devoid of a nucleus. Their genetic material, the DNA (deoxyribonucleic acid) is suspended in the cytoplasm as a nucleoprotein complex. Eukaryotic cells: such as yeast, plant or animal cells, are more elaborate and contain in their cytoplasm a nucleus limited by its own membrane. The space inside the nucleus is called nucleoplasm and harbors the majority of the eukaryotic DNA (1).
The differences between the cytoplasm and the nucleoplasm: will be reviewed in the following sections.
Cytoplasm : is composed of the cytosol, a concentrated fluid component surrounded by the plasma membrane of the cell. The cytosol contains soluble molecules as well as a highly organized fibrous protein structure, the cytoskeleton.
In eukaryotic cells, a number of distinct organelles are suspended in the cytoplasm. Each organelle is delimited by its own membrane and exhibits a different structure and function. Their number varies according to the cell type. In animal cells, five types of organelles have been described: mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes. The mitochondria contain their own DNA and are replaced by chloroplasts in plant cells.
The nucleoplasm: similarly to the cytoplasm, is characterized by a fluidic matrix, the nucleosol. It is separated from the rest of the cytoplasm by the nuclear envelope and contains as well soluble molecules and a structure of proteins, but no organelles.
Nuclear processes are localized in distinct regions of the nucleoplasm. These regions such as the nucleolus, the site of ribosomal activity necessary for protein synthesis, appear as condensed structures and are not surrounded by a membrane. However, the most important feature of the nucleoplasm is the presence of the DNA arranged in highly ordered chromosomes (2).
Role in cellular metabolism
The protein filaments forming the cytoskeleton in the cytoplasm give the cell its shape and play the major role in cellular movements, intracellular transport between the different organelles and the nucleus, and the extracellular transport between the cell and its surrounding environment. The nucleoplasmic matrix with its protein structure, even though not as complex as the cytoskeleton, plays also an important role in maintaining the shape of the nucleus, and in coordinating the different biochemical processes occurring in the nucleoplasm.
In the cytoplasm, each organelle has a distinct function (3):
. Mitochondria constitute the power machine of the cell and are responsible for the
synthesis of ATP (adenosine triphosphate), the energy unit used by the cell for active
metabolic processes such as protein synthesis or muscle contraction. Mitochondria are
thought to have originated from bacteria through endosymbiosis and contain their own
. Chloroplasts, similar to mitochondria, produce ATP by photosynthesis using sunlight. They
originated from cyanobacteria and harbor as well their own DNA.
. The single membrane vesicles of the Golgi apparatus secrete the cell products, for instance
proteins, to the exterior of the cell and help form the plasma membrane.
. The endoplasmic reticulum constitutes the major site of protein and lipid synthesis.
. Peroxisomes are involved in oxidation reactions and lipid biosynthesis.
. Lysosomes are small vesicles acting as intracellular digestive organs, degrading
materials ingested by the cell or any cellular components following cell death.
While the cytoplasm organelles are each well defined and play different roles in the cellular metabolism, the nucleoplasm functional regions are not so clearly defined and lack the presence of a limiting membrane (4):
. The nucleolus is the condensed region where ribosomal RNA (ribonucleic acid) is
synthesized and processed.
. Linear chromosomes formed by the DNA, the inherited genetic material, harbor 99,5%
of the genetic code of the animal cell, the rest being in the mitochondria.
. Other nucleoplasmic regions such as the coiled bodies or the PML Bodies remain less well
characterized, and are believed to be involved in ribonucleoprotein synthesis.