The cytosol and the cytoskeleton

Inside a eukaryotic cell, the organelles float in a watery, protein-packed fluid. The cytosol is the region inside the plasma membrane but outside all the organelles. Don’t confuse it with the cytoplasm, which means everything inside the plasma membrane: the cytosol plus the organelles.

The cell’s main chemical workshop

Metabolism is the sum of the chemical reactions a cell uses to make materials and harness energy, and the cytosol coordinates many of them. Reactions usually run in a series of steps, a metabolic pathway, and each step is sped up by its own enzyme, a protein that makes a specific reaction go faster.

Pathways run in two directions:

• Catabolism breaks molecules down, releasing usable energy and supplying building blocks.
• Anabolism builds larger molecules from smaller ones, such as polysaccharides from sugars or polypeptides from amino acids.

Building polypeptides is one of the cytosol’s big jobs. Ribosomes, which do the building, sit free in the cytosol, on the outer surface of the nucleus and the endoplasmic reticulum (a membrane network next to the nucleus), and inside mitochondria and chloroplasts.

The cytoskeleton: three kinds of fibers

The cytoskeleton is a network of protein filaments, mostly in the cytosol, that shapes the cell, organizes its contents, and moves things around. Each filament is built from many small protein subunits snapped together, a clear case of protein-protein interactions.

Microtubules

A microtubule is built from two proteins, α-tubulin and β-tubulin, and has a plus end and a minus end. It grows only at the plus end but can shrink from either end. A single microtubule often flips between growing and shrinking, a behavior called dynamic instability that helps sort chromosomes during cell division.

Microtubules grow out from a microtubule-organizing center (MTOC). In nondividing animal cells, that’s the centrosome near the nucleus, which holds a pair of centrioles set at right angles and anchors the microtubules’ minus ends. Most plant cells (and many protists) have no centrosome or centrioles, and their microtubules start at scattered sites, including spots along the nuclear membrane.

Intermediate filaments

Intermediate filaments, found in many but not all animals, are tough fibers that bear tension and keep cells from being pulled out of shape. Unlike the other two filaments, they stay a fairly fixed length. Keratins form them in skin, intestine, and kidney cells (dead, keratin-packed cells make up hair and nails), and lamins form a mesh of them lining the inside of the nucleus.

Actin filaments

Actin filaments, also called microfilaments, are the thinnest fibers. Like microtubules, they have plus and minus ends and grow at the plus end. They cluster just under the plasma membrane, supporting it and giving the cell shape and strength.

Actin also lets cells crawl. A pseudopodium (plural, pseudopodia) is a temporary, finger-like bulge of the plasma membrane used for moving or for engulfing particles. Actin rapidly assembles into a gel-like network that pushes the membrane outward, and the rest of the cell follows. This amoeboid movement is used by amoebas, white blood cells, and some cancer cells.

Motor proteins

A motor protein uses energy from ATP to move along a filament. (ATP is the cell’s main energy-carrying molecule; splitting it into ADP and phosphate releases energy.) Each motor has three parts:

• a head, which binds the filament and splits ATP
• a hinge, which bends when ATP binds and is split
• a tail, which attaches to cargo or other proteins (hinge plus tail form the lever arm)

What actually moves depends on what’s held in place:

Cilia and flagella

Flagella and cilia are whip-like extensions that move cells through fluid. Flagella are usually longer and come singly or in pairs, like a sperm cell’s single flagellum. Cilia are shorter and often cover much of the surface; a paramecium beats hundreds of them in coordinated waves. Both send bends along their length that push against the fluid.

Both share the same core, the axoneme, made of microtubules, dynein motors, and linking proteins. Its microtubules form a 9 + 2 array: nine outer doublets (each a complete microtubule fused to a partial one) around two single central microtubules. Each cilium or flagellum grows from a basal body just inside the plasma membrane. Like a centrosome, the basal body is where microtubules start growing; its microtubules form triplets, and it closely resembles a centriole.

Check yourselfAn engineer wants a fabric that resists being pulled apart. Which cytoskeletal filament is the best model, and why?

Intermediate filaments. They are tension-bearing fibers whose main job is to keep cells from being stretched out of shape.