The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 3

and it is the site for protein synthesis, metabolism, and various other reactions.
Although diffusion governs the movement of soluble material through the cytosol,
larger molecules, complexes, and organelles move around by active transport.
Nucleus
The nucleus is the control center of the cell. A bilayer membrane with pores encloses
the nuclear material and forms a formidable barrier to any unwanted molecular pene-
tration. Material transport is permitted through the nuclear pores, about 60-100 nm in
dimension, and is regulated by more than 50 different proteins called nucleoporins.
These allow easy unregulated passage of molecules of 5000 Da while progressively
restricting the larger molecules and regulating their transport. The nucleus contains
the genetic information of the cells in the form of deoxyribonucleic acid (DNA),
which in turn is packaged as chromosomes and associated with proteins called his-
tones. Within the nucleus is the nucleolus, a spherical body composed of DNA, pro-
teins, and ribonucleic acid (RNA); the nucleolus is the site of RNA synthesis.
Endoplasmic Reticulum
Endoplasmic Reticulum (ER) is the meshlike network of flattened tubular sacs found
throughout the cytosol and also extending to the nuclear membrane. The structure at
places may be bound with ribosomes (rough ER); in other regions it may be smooth
in texture (smooth ER). Smooth ER is the site of lipid synthesis, whereas the rough
ER is the site of protein synthesis. Smooth ER also transports proteins and peptides
by forming transport vesicles. Endoplasmic reticulum provides the machinery for
metabolic functions of the cells.
Lysosomes
These membrane-enclosed organelles, termed the “suicide bags of the cell,” perform
the function of breaking down macromolecules and ingested material. The intracom-
partmental pH of around 5 aids in hydrolytic enzymatic activity. The intracellular
digestive activity of lysosomes recycles some of the macromolecules by breaking
them down into constituting monomers, and lysosomes also help rid the cell of toxic
molecules and damaged cellular structures.
Mitochondria
Mitochondria are the powerhouses of the cell. They are double-membrane-bound
structures having their own DNA and ribosomes, and hence are self-replicative. They
are one of the most abundant organelles in the cytosol. Mitochondria are the resi-
dence site for many oxidative enzymes, which extract energy from sugars and fats to
generate adenosine triphosphate (ATP).