If someone asked you, "What's the primary job of almost all living cells?" and demanded an answer within five seconds, what would you say? "Carry on genes to the next generation" is a reasonable answer, but this is really more an attribute of cells than a function they perform. "Divide into two equal cells" is a defensible reply, too, but this is something cells by definition do at the very ends of their own lives, not during them.
The primary job of cells is really to make things, mostly proteins. Using instructions from the same DNA (deoxyribonucleic acid) that carries the genetic code for the entire organism, structures called ribosomes manufacture individual proteins. Some proteins become incorporated into cells, tissues and organs. Others are destined to become enzymes.
In eukaryotes (plants, fungi and animals), many of these ribosomes are attached to a "highway-like" membrane-heavy feature called the endoplasmic reticulum. This comes in two types, "smooth" and "rough." The cells of the liver, ovaries and testes have a high density of smooth endoplasmic reticulum (smooth ER, or simply SER), whereas organs that secrete a great deal of protein, such as the pancreas, have cells rich in rough endoplasmic reticulum (rough ER, or simply RER).
The Cell, Explained
Before exploring what any particular component of a cell does, it is worth reviewing what cells as a whole are and how they differ between types of organisms.
Cells are called the building blocks of life because they are the tiniest individual things that include the major properties associated with living things in general. Even the simplest cells have four physical features: a cell membrane to protect and hold together the cell; cytoplasm to make up the bulk of its mass and offer a matrix in which reactions can occur, ribosomes to make proteins; and genetic material in the form of DNA.
While organisms in the domain Prokaryota often have cells that include essentially just these components, and also consist only of a single cell, organisms in the other domain, Eukaryota, have more complex and diverse cells. Eukaryotic cells, as they're known, have various organelles such as mitochondria, chloroplasts, Golgi bodies and the endoplasmic reticulum; they also isolate their DNA inside a nucleus, which also has a membrane and may itself be considered an organelle.
Eukaryotic Organelles in Detail
Prokaryotes have been around for about 3.5 billion years, which means they arose "only" about a billion years after the Earth itself was fully formed. Eukaryotes are believed to have followed within the next billion years, and evidence suggests that they got their start thanks to a mostly chance encounter between a large, anaerobic bacteria and a much smaller aerobic bacteria.
- In this endosymbiont theory, the large bacteria "ate" the smaller one, with both surviving. The result was a large aerobic bacteria with bacteria-turned-organelles called mitochondria now responsible for supplying most of these cells' energy needs.
The nucleus contains DNA separated into a number of chromosomes, with the total number varying between species (humans have 46). During the process of mitosis, the nuclear membrane dissolved, chromosomes that have already been duplicated in pairs are pulled apart, and the nucleus and cell divide into daughter structures one after the other.
Golgi bodies are structures resembling small membrane-enclosed stacks of pancakes. They participate in the processing of proteins and other newly synthesized molecules, and can shuttle such substances between the endoplasmic reticulum and other organelles, like tiny taxicabs.
Basic Features of the Endoplasmic Reticulum
About half of the total membrane surface of a typical animal cell (including the outer cell membrane) consists of the organelle known as endoplasmic reticulum. It consists of many layers of the same double plasma membrane, or phospholipid bilayer, that forms the boundaries of all organelles and of the cell as a whole.
While, as noted, endoplasmic reticulum is divided into smooth ER and rough ER, this distinction actually refers to different compartments-within-compartments of the same organelle. Thus the standard rough ER definition and smooth ER definition are slightly misleading. They suggest that each one is completely separate from the other, micro-anatomically speaking, when in fact they are part of the same larger membranous network.
Both types of endoplasmic reticulum function to process and move products of anabolism, in one case proteins and in the other case lipid (and some steroid hormones). At times, portions of the endoplasmic reticulum can be followed from the nuclear membrane on the inside of the cell to the cell membrane on the distant cell border.
Smooth ER Function and Appearance
Under a microscope you view a cell with an extensive smooth endoplasmic reticulum present. What would you see and how would you describe it?
Smooth ER gets its name, as do so many things in anatomy and microanatomy, not from how it would really feel or taste but from its appearance. Because smooth ER does not have a high density of ribosomes (which appear dark on microscopy) embedded in its membranes, it looks like what it is: a tiny network of interconnected tubes. ER of all types is at its heart a sort of hollow subway system through the "gooey" cytoplasm, allowing things to move more quickly throughout the cell.
Functions: Smooth ER has a number of important functions. It synthesizes carbohydrates, lipids and steroid hormones (including testosterone in the testis). It aids in the detoxification of ingested chemicals, from prescription medications to household poisons. It serves as a storage depot of calcium ions in muscle cells, where a specialized type of smooth ER called the sarcoplasmic reticulum stores up the calcium ions that are needed to initiate muscle-cell contractions.
Rough ER Function and Appearance
Rough ER gets its name from its characteristic appearance, which resembles a convoluted ribbon "studded" with dark dots, in some places very closely spaced and at others spaced farther apart. The "dots" are ribosomes, or the "protein factories" of all living things. Ribosomes themselves are made of proteins plus a special kind of nucleic acid.
The flattened "bags" that make up rough ER are attached to the nuclear membrane, so the density of this type of ER in the cell is highest closer to the center, where the nucleus tends to be. As in all organelles, the membrane surrounding the many folds of the rough ER is a double plasma membrane; the ribosomes are attached to the outer portion of this membrane, that is, the side facing the cell cytoplasm.
Functions: Along with the ribosomes themselves, the rough ER participates in getting amino acids and polypeptides to the site of translation, or protein synthesis, on the ribosome. After a protein is fully synthesized and released by the ribosome into rough ER, a number of things may happen. The protein may be "tagged" with a chemical "label" on the inner membrane of the ER before it even enters the lumen, or space, inside. It may instead be processed in the lumen itself.
Parts of the rough ER consists of what are called protein folding units, which do as exactly as their name suggests. When proteins are first made, they exist as a strand, a chain of amino acids. But the ultimate shape of a protein includes a great deal of bending and folding and often bonds between amino acids in different parts of the now-twisted chain.
- Scitable By Nature Education: What Is a Cell?
- B.C. Campus Open Textbooks Anatomy and Physiology: The Cytoplasm and Cellular Organelles
- British Society for Cell Biology: Endoplasmic Reticulum (Rough and Smooth)
- LibreTexts Biology: The Endoplasmic Reticulum
- Scitable By Nature Education: Endoplasmic Reticulum, Golgi Apparatus, and Lysosomes
- NCBI Bookshelf: The Cell: A Molecular Approach (2nd Edition): The Endoplasmic Reticulum