Atlas of Plant and Animal Histology
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The cell


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Cell: anatomical and functional unit of living organisms.

Compartment: set of molecules dealing with a function, usually they are confined to a defined space of the cell.

Organelles: membrane-bound compartments.

Main eukaryotic cell compartments: nucleus, cell membrane, cytoplasm.

Cytoplasm: cytosol + organelles.

Cytology (generally known as cell biology) is the topic of this part of the Atlas, and mainly focused on the organization of the cell. But, what is a cell? The following may be a good definition: cells are the anatomical and functional units of living organisms. Cells may be alone or grouped to form multicellular organisms. A cell is the simplest molecular organization considered alive. Three cell lineages are known to be present on Earth: archaea and bacteria, which are unicellular prokaryotes, and eukaryotes, which can be unicellular or form multicellular organisms. Prokaryotes (previous to the presence of a nucleus) do not usually have internal compartments surrounded by membranes, while eukaryotes (with true nucleus) always contain internal membranous organelles. The nucleus is a characteristic compartment of eukaryotes.

animal cells

Main compartments of an animal cell.

plant cell

Main compartments of a plant cell.

Cells, either prokaryote or eukaryote, are highly organized sets of molecules. In fact, cells have many internal compartments with specific functions. Let's say a cellular compartment is a space, delimited or not by membrane, where a necessary or important function for the cell is performed. One of the compartments found in every cell is the cell membrane, also known as plasmalemma or plasma membrane, which encloses all the other cellular compartments, and is a semipermeable barrier that separates the inner cell space from the outer cell space.

Eukaryotic cells have internal compartments delimited by membranes. The nucleus is one of them. It is bounded by a double membrane and contains the genetic material known as DNA. DNA contains the necessary information for the cell to carry out tasks that allow survival and reproduction. The space between the nucleus and the plasma membrane is filled with the cytosol, an aqueous gel containing a variety of molecules involved in structural and metabolic functions, in homeostasis, in signaling, and many others. For example, the ribosomes for protein synthesis, the cytoskeleton for the internal organization of the cell and mobility, many enzymes and co-factors for metabolism. Between the cell membrane and the nucleus there are also many organelles, membrane-bounded compartments, that accomplish functions such as digestion, respiration, photosynthesis, metabolism, intracellular transport, secretion, energy production, storage, etcetera. Mitochondria, chloroplasts, peroxisomes, lysosomes, endoplasmic reticulum, and vacuoles are some of these organelles. The cytoplasm is the cytosol plus all the organelles, excluding the nucleus.

In the following pages we will take a tour through the different parts of the eukaryotic cell, and also by its surroundings. Some aspects of the cell function will not be dealt deeply here, such as gene expression or cellular metabolism. It would need a huge amount of space which would undermine the idea we want to give about the cell. Furthermore, there are many Internet sites dedicated to those subjects. The different "places" of the cell that we are going to "visit" are shown on the right side panel.

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Updated: 2016-11-25. 11:31