AP Biology 2.1: Cell Structure and Function

Every cell, from a bacterium to a neuron in your brain, has ribosomes. A ribosome is made of ribosomal RNA (rRNA) and protein. It has no membrane around it, so it is a subcellular structure, not a membrane-bound organelle.

A ribosome's job is to synthesize proteins. It follows the sequence of a messenger RNA (mRNA) and joins amino acids in the order that sequence sets.

Ribosomes are found in cells in all forms of life: Bacteria, Archaea and eukaryotes alike. That reflects the common ancestry of all known life. A structure that every lineage still carries is most simply explained as an inheritance from one ancestral cell.

In a eukaryotic cell, some ribosomes are free in the cytoplasm and others are bound to the membrane of the endoplasmic reticulum. Both kinds build proteins the same way.

Many of a eukaryotic cell's membranes work together as one system, the endomembrane system: the endoplasmic reticulum (ER), the Golgi complex, lysosomes, vacuoles and transport vesicles, the nuclear envelope and the plasma membrane. Together they modify, package and transport polysaccharides, lipids and proteins.

The ER is a network of membranes that gives the cell mechanical support, helping it keep its shape, and plays a role in moving materials within the cell. Rough ER has ribosomes bound to its membrane, so it helps carry out protein synthesis, and its membranes divide the cell into compartments. Smooth ER has no bound ribosomes; its jobs include synthesizing lipids and detoxifying the cell.

The Golgi complex is a series of flattened membrane sacs. It correctly folds and chemically modifies newly made products, then packages proteins for trafficking.

A protein the cell will export takes a set path. Its mRNA leaves the nucleus through the nuclear envelope. A ribosome on the rough ER builds the protein, a transport vesicle carries it to the Golgi complex to be modified and packaged, and another vesicle carries it to the plasma membrane, fuses with it and releases the protein outside.

Mitochondria carry out the reactions of aerobic cellular respiration, which make most of a eukaryotic cell's ATP. A mitochondrion has a double membrane, and the two membranes provide separate compartments for different metabolic reactions.

The two membranes are not alike. The outer membrane is smooth. The inner membrane is highly convoluted: it folds in on itself again and again. Those folds pack far more inner membrane into the same small organelle, which lets ATP be synthesized more efficiently. Cells that use ATP quickly, such as muscle cells, usually have many mitochondria.

Chloroplasts are specialized organelles found in plants and photosynthetic algae. They also have a double membrane, and they are the location of photosynthesis. Most plant cells have mitochondria as well: chloroplasts capture energy from light, and mitochondria release energy from sugars for the cell's work.

Lysosomes are membrane-enclosed sacs that contain hydrolytic enzymes, enzymes that break molecules apart by hydrolysis. A lysosome digests material: particles the cell has taken in, worn-out organelles, and molecules the cell no longer needs. Lysosomes also play a role in programmed cell death, or apoptosis, the orderly way a cell destroys itself when it is damaged or no longer needed.

Vacuoles are membrane-bound sacs that play many different roles, and plant and animal cells use them differently. In a plant cell, one specialized large vacuole stores water and nutrients. As it fills, it presses outward against the cell wall. That pressure, turgor pressure, keeps the cell firm and helps hold the plant upright. In an animal cell, vacuoles are smaller and more plentiful than in a plant cell, and they store cellular materials.