The cell is the fundamental building block of living things. Cells can vary greatly from one to the next according to the organism in which a given cell is found and, in more specialized organisms, in relation to the specific physiological function of that cell. But all cells have a few elements in common, including a cell membrane as an external boundary and cytoplasm in the cell’s interior.
Prokaryotic cells – think bacteria – have no nuclei or organelles, and the cytoplasm is hence “everything” visible in the interior. The cytoplasm of eukaryotic cells, which are those in plants, animals and fungi, is “everything” external to the nucleus and any organelles present.
What’s in the Cytoplasm?
First, it is helpful to distinguish between related terms in cell biology. Cytoplasm generally refers to the environment within more complex cells that lies on the interior of the cell but is not part of the organelles of the cell. Eukaryotic cells, in addition to having their genetic material included within a nucleus, feature structures such as mitochondria and Golgi bodies that have their own double plasma membrane, which is similar in construction and content to the cell membrane itself.
The medium in which these organelles sit is considered cytoplasm. Cytosol, on the other hand, is the specific jelly-like substance that makes up cytoplasm, and excludes anything that sits inside it, even smaller components such as enzymes. “Cytoplasm” thus may be regarded as “cytosol plus some impurities," whereas "cytosol" connotes "cytoplasm exclusive of organelles."
The cytoplasm consists mainly of water, salts and proteins. Most of these proteins are enzymes, which catalyze, or help along, chemical reactions. Although the cytoplasm cannot be said to have any one overriding function, it serves as a physical medium for the transportation and processing of molecules within the cell that are vital to the maintenance of life on a moment-to-moment basis.
Prokaryotic cells lack organelles (from the French for “small organs”); the genetic material and the other extra-cytosolic components of those cells' interior “float” freely in the cytoplasm. Plant and animal cells, on the other hand, are virtually always part of multicellular organisms and are correspondingly more complex.
The nucleus is generally not grouped with other organelles owing to its importance, but an organelle is exactly what the nucleus is, double plasma membrane and all. Its size varies, but its diameter might be anywhere between 10 and 30 percent of that of the whole cell. It contains the organism’s chromosomes along with structural and enzymatic proteins needed for the chromosomes to do their job of replicating and ultimately transmitting information to the gamete cells destined to form organisms in the next generation of members of the species.
Organelles in the Cytoplasm
The organelles in a cell are analogous to the various organs and structures in the human body. Humans and other animals do not have a cytosol or cytoplasm, but the fluid that makes up blood plasma and fills much of the space between cells and organs might be regarded as serving the same basic set of functions: A distinct physical scaffolding upon which metabolic and other reactions can occur.
Mitochondria are perhaps the most intriguing organelles. Believed to have once existed as free-standing bacteria in their own right before the advent of eukaryotes, these "power plants" are where the processes of aerobic respiration take place. They are oblong, rather like narrow footballs, and their double membrane includes a great many folds, called cristae, that expand the functional surface of the mitochondria well beyond what a smooth membrane would allow. This is important because of the number and range of the reactions that occur here, among them the well-known tricarboxylic acid cycle (also known as the Krebs or citric-acid cycle). Although mitochondria are found in plants, their role in animals is more often stressed because animals do not participate in photosynthesis.
The endoplasmic reticulum is a shipping network of sorts, with its double plasma membrane continuous with that of the cell as a whole and extending toward the interior ("reticulum" means "little net"). Rough endoplasmic reticulum (RER) has a large number of ribosomes, or miniature protein factories, attached to it, giving it its name, while smooth endoplasmic reticulum has few to no ribosomes studding its length.
Vacuoles are like the storage sheds of a cell, capable of warehousing enzymes, fuel and other substances until they are ready to be used, just as your body can store elements it will later need such as blood cells and glycogen in specific locations.
Golgi apparatus is like a processing center, and it is usually depicted as a stack of pancake-like discs in cell diagrams. If the SER and RER transport the raw products of ribosomal activity (i.e., proteins), the Golgi apparatus refine and modify these products based on where they will eventually wind up in the physical system.
Lysosomes are a manifestation of a cell's need for maintenance and disposal functions. They contain enzymes which can lyse, or chemically digest, the inevitable waste products of metabolic functions and reactions. Just as strong industrial acids are kept in special containers, the cell sequesters away the caustic enzymes deployed by lysosomes in these special vacuoles scattered throughout the cytoplasm.
Finally, chloroplasts are organelles particular to plant cells that include a pigment called chlorophyll, via which sunlight is converted to energy that allows plants to synthesize glucose. Unlike animals, plants obviously cannot get fuel by eating and therefore must manufacture it. Under a microscope, these resemble mitochondria to a considerable extent.
The cytosol, as described, is essentially cytoplasm stripped of organelles. It is a matrix, a gel-like substance that organelles and dissolved substances "float" in. The cytosol contains the cytoskeleton, which is a network of microtubules that help the cell maintain its shape. These microtubules are protein structures made from distinct subunits called tubulins, which are assembled in the centrioles of the cell's two oppositely positioned centrosomes.
In addition to the tubulin-rich microtubules, other elements called microfilaments aid the microtubules in ensuring cells' structural integrity. Despite their name, which perhaps implies a threadlike character, microfilaments are made up of globular proteins called actin, which are also found in the contractile apparatus of muscle cells.
Plants have structures called plasmodesmata running into and through the cytosol of their cells from the outside. These are also small tubes, but they differ from microtubules in that they serve to connect different plant cells to one another. The nonmotile character of plants makes these "living bridges" especially important, as they ensure that processes that might otherwise occur in the course of ordinary animal locomotion can take place.
What’s Dissolved in the Cytoplasm
Less easily visualized on microscopy are the substances in the cytoplasm that help drive cell function, notably enzymes. Just as blood contains a lot more than the red cells and platelets that give it its color and basic consistency, cytosol contains a number of "free-floating" elements and molecules that are metabolically active.
The cytoplasm can be rich in sources of fuel like starch and other carbohydrates, especially in bacterial cells, which lack membrane-bound organelles. A disadvantage of existing outside the system of endoplasmic reticulum and other membranous structures is that materials in the cytoplasm can only move by simple diffusion, meaning that they travel down concentration gradients. Clearly, in situations demanding rapid metabolic changes, items dissolved in the cytoplasm cannot be called upon to react quickly.
The cytosol also contains signalling molecules like the ions calcium, potassium and sodium. These are frequently involved in triggering cell-receptor activity on the surfaces of cells and on the surfaces of the organelles within them, setting in motion cascades of biochemical reactions.s
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