Cytoplasm (Cytosol, Organelles) Structure

Cytoplasm

Cytoplasm is the substance that contains all the cellular contents between the cell membrane and the nucleus. It serves as a matrix substance in which chemical reactions occur. Cytoplasm makes up most of each cell’s volume and is a gel-like material suspending­ the cell’s organelles. It usually appears clear with scattered “specks,” although more powerful magnification reveals that it contains membranous networks, pro-tein frameworks, and a cytoskeleton.

Cytoplasm consists of cytosol and organelles (excluding the nucleus), which are subcellular structures that perform specific functions.

Cytosol

Cytosol is the fluid portion of cytoplasm, containing mostly water as well as glucose, amino acids, fattyacids, ions, lipids, proteins, ATP, and waste products. Cytosol is the site of many chemical reactions that are required for cells to exist. It is the part of the cytoplasm that cannot be removed by centrifugation.

The most important differences between cytosol and extracellular fluid are:

■■ Cytosol contains higher amounts of suspended proteins than does extracellular fluid. Many of these proteins are enzymes that regulate meta-bolic operations; others are involved with the var-ious organelles.

■■ Cytosol also contains higher amounts of potas-sium ions than do extracellular fluid; however, the concentration of sodium ions is much lower in cytosol than in extracellular fluid.

■■ Cytosol usually contains small amounts of lipids, carbohydrates, and amino acids.

Organelles

The cytoplasm receives, processes, and uses nutrients. It contains various types of organelles (nonmembra-nous or membranous). Organelles perform most of the tasks that keep the cell alive and functioning normally. Each organelle accomplishes specific tasks related to cell structure, growth, maintenance, and metabolism. An organelle’s membrane often allows it to unite with the interactive, intracellular endomembrane system. The organelles have specific actions that help the cell to carry out its activities.

Microtubules

Microtubules are hollow tubes found in most cellsthat are constructed from a globular protein called tubulin. Microtubules are about 25 nanometers insize, making them the largest components of the cytoskeleton. They extend out into the cell’s periph-ery from an area near the nucleus called the centro-some. Microtubules are differently distributed andhave different amounts, over time. They form because of the aggregation of tubulin molecules and grow out from their origination at the centrosome. Eventually, microtubules disassemble into individual molecules of tubulin. The functions of microtubules are:

■■ Formation of primary cytoskeleton components:This strengthens cells, makes them more rigid, and anchors the position of major organelles

■■ Disassembly: When microtubules disassemble,they help the cell to change shape, which may assist with cell movement

■■ Movement of vesicles and other organelles inside the cell: This is related to molecular motors, whichare proteins that bind to structures being movedas well as to microtubules, moving along their length. Direction of movement is based on which proteins are involved:

 The proteins dynein and kinesin carry materi-als toward the opposite ends of a microtubule, requiring ATP—these functions are essential to normal cell function

■■ Formation of the spindle apparatus: During celldivision, this process distributes duplicated chro-mosomes to opposite ends of dividing cells

■■ Formation of structural components of organelles: Including centrioles and cilia

Centrioles

Cell division requires a pair of centrioles, which are cylindrical structures composed of short microtubules (FIGURE 3-3). During cell division, the centrioles form the spindle-shaped structure needed for movement of DNA strands. Cardiac muscle cells, skeletal mus-cle cells, mature red blood cells, and typical neurons have no centrioles; therefore, these cells are incapable of dividing.

The centrosome is the cytoplasm surrounding the centrioles. Microtubules of the cytoskeleton usu-ally begin at the centrosome and radiate through the cytoplasm. The centrosome is also known as the cellcenter. It consists of nine microtubule triplets that arearranged like a pinwheel. Each microtubule is con-nected to the next one by nontubulin proteins. The microtubules are arranged to form a hollow tube. The centrioles also form the bases of cilia and flagella.

Microfilaments

The smallest of the cytoskeletal elements, ­microfilaments are composed of the proteins actin and myosin. Similarly,­ larger cytoskeletal elements include the intermediate ­filaments and microtubules. They are typically found in muscle cells. Microfila-ments provide cell movement and contraction via interaction with actin and myosin. This process can also change the shape of the entire cell. Microfilaments as well as intermediate filaments and microtubules are discussed in their relation to the cytoskeleton later in this chapter.

Cilia and Flagella

Cilia, like flagella, extend from certain cell surfaces(FIGURE 3-4). Cilia are short, hair-like structures that move in a coordinated sweeping motion to propel fluids over the surface of tissues (FIGURE 3-5) . They are found in large numbers on cells lining the respi-ratory and reproductive tracts. Cilia are formed when centrioles multiply, lining up beneath the plasma membrane at the cell’s exposed (free) sur-face. The microtubules emerge from each centriole to form the ciliary projections. They accomplish this by causing pressure on the plasma membrane. During this time, the centrioles are referred to as basal bodies. As a cilium moves, it experiences pro-pulsivepower strokes and recovery strokes, which bend and return it to its initial position. It can repeat these two strokes between 10 and 20 times per sec-ond. When one cilium bends, it is soon followed bythe bending of the next cilium, and so forth. This creates a cell surface “current.”

Flagella are longer than cilia and often exist asonly a single flagellum. The only example of a flagel-lum is the “tail” of a sperm cell (FIGURE 3-6 ). The key difference between cilia and flagella is that cilia pro-pel other substances, whereas flagella propel the cells to which they are attached. There are also nonmotilecilia (primary cilia), which are actually present just asone single cilium on the surface of most cells in the body. Primary cilia act as antennae, which examine the external environment for recognizable molecules. They coordinate various intracellular pathways regu-lating embryonic development and maintain healthy tissues in later life.

Microvilli

Microvilli are tiny, finger-like extensions of theplasma membrane. They project from exposed cell surfaces, increasing the plasma membrane surface area to a large degree. Microvilli are usually found on absorptive cell surfaces, such as in the kidney tubules and intestines. A core of actin filaments, in bundles, extends into the terminal web of the cytoskeleton. In the microvilli, actin appears to have a mechanically stiffening function.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a network of intracellular membranes connected to the nuclear envelope, which surrounds a cell’s nucleus. It has interconnected tubules and parallel membranes that enclose fluid-filled cisterns (cavities). The ER coils and twists through the cytosol and is con-tinuous with the outer nuclear membrane. Nearly 50% of the cell’s membranes are made up of the ER. The two types of ER are the smooth endoplasmicreticulum (SER) and the rough endoplasmic reticulum (RER). The SER does not have ribo-somes on its outer surface, whereas fixed ribo-somes appear on the RER’s outer surface, giving it“studded” appearance (­FIGURE 3-7). Proteins on these ribosomes are threaded into the ER cisterns. The SER can synthesize phospholipids and cho-lesterol, which are needed for the cell membrane’s growth and maintenance. It is continuous with the RER, consisting of a network of looped tubules.

Its enzymes catalyze many different reactions. These reactions are used for many functions, including metabolizing lipids, synthesizing steroid-based hormones, detoxification of drugs and chemicals, breaking down stored glycogen to form free glucose, and for fat absorption, synthesis, and transport.

Cardiac­ and skeletal muscle cells have an elaborate SER (the sarcoplasmic reticulum) that helps to store and release calcium during muscle contraction. Overall, most body cells contain very little SER. The RER can synthesize ­proteins, and newly made pro-teins are enclosed in vesicles when they move to the Golgi apparatus for additional processing. In most secretory cells, liver cells, and antibody-producing plasma cells, the RER is very well developed. The RER is the cell’s membrane factory, manufacturing integral proteins and phospholipids that form parts of cellular membranes. On the external face of the ER membrane, enzymes required for lipid synthe-sis have active sites. Both free and fixed ribosomes synthesize proteins via instructions from messen-ger RNA. The amount of ER, along with the propor-tion of RER to SER, is varied between different cells and their activities. One example is the pancreatic cells that make digestive enzymes. They contain an extensive RER, but have a relatively small SER. The opposite situation exists in the reproductive organ cells that synthesize the steroid hormones.

Golgi Apparatus

The Golgi apparatus, also called the Golgi complex, consists of a stack of several flattened sacs. These “pancake­-like” structures are hollow, with cavities called cisternae inside them. The flattening of these sacs is caused by a protein complex that pulls them, when they contain newly synthesized proteins, off the Golgi. Vesicles from the RER fuse with the con-vex receiving side of the Golgi, which is known as the cis face. Glycoproteins are modified inside, with sugargroups being added or deleted and sometimes with phosphate groups being added. Three or more types of vesicles bud from the concave trans face of the Golgi apparatus. Those that contain proteins to be exported pinch off assecretory vesicles (granules). They migrate to the plasma membrane, discharging their contents from the cell via exocytosis. The enzyme-producing pancreatic cells are examples of specialized secretory cells that have a prominent Golgi apparatus. Other vesicles that contain lipids and transmembrane pro-teins are pinched off by the Golgi apparatus and sent to the plasma membrane or other membranous organelles. Digestive enzymes are packaged by the Golgi apparatus into membranous lysosomes that remain in the cell (FIGURE 3-8). The Golgi apparatus has three main functions: (1) modifying­ and packag-ing secretions (such as hormones or enzymes) that are released via exocytosis, (2) packaging special enzymes inside ­vesicles for use in the cytosol, and (3) renewing or modifying the cell membrane.

Lysosomes

Lysosomes are tiny spherical sacs that begin asendosomes with inactive enzymes. They dispose cell wastes, using enzymes to break down nutrients or foreign particles (such as bacteria). They also destroy older parts of the cell. This breakdown pro-cess requires the use of powerful enzymes. It often generates toxic chemicals capable of damaging or killing the cell. Lysosomes are specialized vesicles that provide an isolated environment for potentially dangerous chemical reactions (FIGURE 3 -9). They are produced close to the Golgi apparatus and contain digestive enzymes. In phagocytes, lysosomes are large and plentiful, able to digest nearly every type of biological molecule. They are most effective in acidic ­conditions and are known as acid hydrolases. The lysosomal membrane contains hydrogen pro-ton pumps. These ATPases collect hydrogen ions from surrounding cytosol that maintain the acidic pH of the organelle. The lysosomal membrane also traps dangerous acid hydrolases, as it allows finaldigestive product to leave for use by the cell or excretion. Because of lysosomes, sites are provided where digestion can occur safely inside a cell. The many ­functions of lysosomes also include digestion of bacteria, viruses, toxins, and other particles taken in by endocytosis; performing glycogen breakdown and release and other metabolic functions; breaking down bone to release calcium into the blood; degrad-ing organelles that are nonfunctional or “worn out”; and breaking down non-useful tissues, for example, the uterine lining during menstruation. Although mostly stable, the lysosomal membrane becomesfragile when the cell is deprived of oxygen, has too much vitamin A, or is injured. Rupture of lysosomes causes the cell to digest itself (autolysis), which assists in desirable destruction of cells.

Mitochondria

Mitochondria are thread-like or bean-shaped com-plex membranous organelles. All cells in the body, with the exception of mature red blood cells, have between 100 and a few thousand mitochondria (singularly­ called a mitochondrion). In a living cell, the mitochondria­ move and change shape on an almost continuous basis. Mitochondria have double membranes that play a central role in the production of energy (via ATP). Mitochondria are the “powerhouses” of cells (FIGURE­ 3-10).

Mitochondria areusually­ clustered where most cellular activity occurs. The liver, kidneys, and muscles have a large number of mitochondria in their cells because they use ATP at a high rate. A mitochondrion is ­surrounded by two membranes that are similar in structure to the plasma membrane. The outer mitochondrial membrane is smooth. The inner mitochondrial membrane has a series of folds called cristae that protrude into the cen-tral fluid-filled cavity (the matrix), which is enclosed by the inner membrane and cristae.

■■ The number of mitochondria in a particular cell varies, based on the cell’s energy demands.They can migrate through the cytoplasm of a cell and are able to reproduce themselves. Mito-chondria contain their own DNA, but in a more primitive form than that found within the cell nucleus. They also contain their own RNA and ribosomes.

■■Glucose and other food fuel products are broken­ down by enzymes to water and carbon diox-ide. Some of these dissolve in the mitochondrial matrix, whereas others form part of the crista membrane. During oxidization of metabolites, some released energy is captured and then used to form ATP by attaching phosphate groups to ade-nosinediphosphate molecules (a process known as aerobic cellular respiration).

■■Approximately, 37 mitochondrial genes con-trol synthesis of 1% of the proteins needed for mitochondrial function. The remaining proteins needed for cellular respiration are encoded by the DNA of the cell nucleus. As the cell requires more ATP, the mitochondria either halve them-selves (fission) or synthesize more cristae. This increases their number, and they grow to their former size. Mitochondria are similar to the purple bacteria phylum. Mitochondrial DNA is also bacteria-like.

Peroxisomes

Peroxisomes are spherical sacs with enzymes(primarily,­ oxidases and catalases) that speed up many biochemical reactions. They are abundant in the liver and kidney cells, and their diverse actions include synthesis­ of bile acids, detoxification of hydro-gen peroxide or alcohol, and breaking down lipids and ­biochemicals. Oxidases use molecular oxygen to detoxify alcohol, formaldehyde, and other harmful substances­. Most important, oxidases neutralize freeradicals, which are highly reactive chemicals. Free­radicals have unpaired electrons that can ruin the structure of biological molecules. Oxidases convert free radicals to hydrogen ­peroxide, which catalyzes quickly into water. Although hydrogen ­peroxide and free radicals are normal cellular metabolic byproducts, they can greatly harm cells if they accumulate in exces-sive numbers. Peroxisomes also aid in energy metab-olism via the breakdown and synthesis of fatty acids. They appear similar to small lysosomes that usually form by budding off of the ER via special processes.

Ribosomes

Ribosomes are small, dark-staining granules that aremade up of ribosomal RNA and proteins. They are found on the outer membrane of the rough ER whereprotein synthesis occurs; they may also be scattered through the cytoplasm. Their functions involve the formation of proteins, and they are therefore called the “protein factories” of the cell. They have globular subunits (two per ribosome) that fit together to form structures that resemble acorns. Protein synthesis­ is shared by two different types of ribosomes. Freeribosomes float freely in cytoplasm, making solubleproteins that function, whereas other proteins are transported to the mitochondria and certain organ-elles. Membrane-bound ribosomes form the RER and synthesize proteins that will be incorporated into cell membranes or lysosomes. These proteins may also be exported out of the cell. Subtypes of ribosomes can change functions. They can attach to ER membranes as well as detach from them, based on the type of pro-tein they are making.

Vesicles

Vesicles are also known asvacuoles.These sacs areformed when a part of a cell membrane folds inward, establishing a bubble-like structure within the cyto-plasm. Vesicles contain various liquid or solid materials that formerly existed outside the cell membrane. TABLE 3-1 summarizes the structures and func-tions of organelles.

1. What are the major differences between cytosol and extracellular fluid?

2. Identify the differences between RER and SER.

3. What is the role of the mitochondria?

4. Compare ribosomes and lysosomes