A&P Lecture 2: Cell Cycle and Cell Division

ANATOMY AND PHYSIOLOGY LECTURE 2

CELL CYCLE AND CELL DIVISION

The life of a cell is called the cell cycle. It is usually divided into five phases or gaps: G0, G1, S, G2, and M. Some cells may not have a G1 phase, and others may not have a G2 stage. However, all cells must grow, replicate their genetic material if they are to divide, and undergo the process of mitosis if they are to replicate. G0 is the stage during which the cell may leave the cell cycle and either remain in a state of inactivity or reenter the cell cycle at another time. G1 is the stage during which the cell is starting to prepare for mitosis through DNA and protein synthesis and an increase in organelle and cytoskeletal elements. The S phase is the synthesis phase, during which DNA replication occurs and the centrioles are beginning to replicate. G2 is the premitotic phase and is similar to G1 as for RNA and protein synthesis. The M phase is the phase during which cell mitosis occurs. Nondividing cells, such as mature nerve cells and cells not preparing for mitosis, are said to be in the G0 phase of the cell cycle .

Cell Division

Cell division, or mitosis, which was first described in 1875, is the process during which a parent cell divides and each daughter cell receives a chromosomal karyotype identical to the parent cell. Cell division gives the body a means of replacing cells that have a limited life span, such as skin and blood cells; increasing tissue mass during periods of growth; and providing for tissue repair and wound healing. Despite the early cytologic description of the four stages of mitosis, it was not until the early 1950s that the importance of the cell cycle was realized. 

Mitosis, which is a dynamic and continuous process, usually lasts from 1 to 11⁄2 hours. It is divided into four stages: prophase, metaphase, anaphase, and telophase (Fig. 4-13). The phase during which the cell is not undergoing division is called interphase. During prophase, the chromosomes become visible because of increased coiling of the DNA, the two centrioles replicate, and a pair moves to each side of the cell. Simultaneously, the microtubules of the mitotic spindle appear between the two pairs of centrioles. Later in prophase, the nuclear envelope and nucleolus disappear. Metaphase involves the organization of the chromosome pairs in the midline of the cell and the formation of a mitotic spindle composed of the microtubules. Anaphase is the period during which separation of the chromosome pairs occurs, with the microtubules pulling one member of each pair of 46 chromosomes toward the opposite cell pole. Cell division, or cytokinesis, is completed after telophase, the stage during which the mitotic spindle vanishes and a new nuclear membrane develops and encloses each complete set of chromosomes.

Cell division is controlled by changes in the intracellular concentrations and activity of three major groups of

intracellular proteins: (1) cyclins, (2) cyclin-dependent kinases, and (3) the anaphase-promoting complex. The central components of the cell cycle control system are the cyclin-dependent kinases, whose activity depends on association with the regulatory units called cyclins. Oscillations in the activity of the various cyclin-dependent kinases lead to initiation of the different phases of the cell cycle. For example, activation of the S-phase cyclin-dependent kinases initiates the S phase of the cell cycle, whereas activation of the M-phase cyclin-dependent kinases triggers mitosis. The anaphase-promoting complex is responsible for the breakdown of the M cyclins and other regulators of mitosis.

Cell division is also controlled by several external factors, including the presence of cytokines, various growth

factors, or even adhesion factors when the cell is associated with other cells in a tissue. In addition, the cell cycle is regulated by several checkpoints that determine whether DNA replication has occurred with a high degree of fidelity. Two of the better understood are the DNA damage and the spindle formation checkpoints. If these biochemical checkpoints are not faithfully met, the cell may default to programmed

cell death or apoptosis.

Anatomy and Physiology Compiled Lecture Notes: The Cell

THE CELL

• Introduction

• The Cell Membrane

• The Nucleus

• The Cytoplasm

• The Ribosomes

• The Endoplasmic Reticulum

• The Golgi Complex

• The Secretory Vesicles

• The Lysosomes

• The Peroxisomes

• The Proteasomes

• The Mitochondria

• The Accessory Parts

• Cillia
• Flagella
• Microvilli
• Centrioles
• Spindle Fibers

A&P Lecture 1.11: Centrioles and Spindle Fibers

This is an in depth  explanation of Centriloes and Spindle fiber as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.11

CENTRIOLES AND SPINDLE FIBERS

Centrioles and Spindle Fibers

The centrosome, a specialized zone of cytoplasm close to the nucleus, is the center of microtubule formation. It contains two centrioles . Each centriole is a small, cylindrical organelle about 0.3–0.5 μm in length and 0.15 μm in diameter, and the two centrioles are normally oriented perpendicular to each other within the centrosome. The wall of the centriole is composed of nine evenly spaced, longitudinally oriented, parallel units, or triplets. Each unit consists of three parallel microtubules joined together.

Microtubules appear to influence the distribution of actin and intermediate filaments. Through its control of microtubule formation, the centrosome is closely involved in determining cell shape and movement. The microtubules extending from the centrosomes are very dynamic—constantly growing and shrinking. Before cell division, the two centrioles double in number, the centrosome divides into two, and one centrosome, containing two centrioles, moves to each end of the cell.

Microtubules called
spindle fibers extend out in all directions from the centrosome. These microtubules grow and shrink even more rapidly than those of nondividing cells. If the extended end of a spindle fiber comes in contact with a kinetochore, a specialized region in the centromere of each chromosome, the spindle fiber attaches to the kinetochore and stops growing or shrinking. Eventually, spindle fibers from each centrosome bind to the kinetochores of all the chromosomes. During cell division, the microtubules facilitate the movement of chromosomes toward the two centrosomes

A&P Lecture 1.12: Cilia Flagella and Microvilli

This is an in depth  explanation of Lysosomes as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.12

CILLIA, FLAGELLA AND MICROVILLI

Cilia and Flagella

Cilia  are structures that project from the surface of cells and are capable of movement. They vary in number from one to thousands per cell. Cilia are cylindrical in shape, about 10 μm in length and 0.2 μm in diameter, and the shaft of each cilium is enclosed by the plasma membrane. Two centrally located microtubules and nine peripheral pairs of fused microtubules, the so-called 9 2 arrangement, extend from the base to the tip of each cilium. Movement of the microtubules past each other, a process that requires energy from ATP, is responsible for movement of the cilia. Dynein arms, proteins connecting adjacent pairs of microtubules, push the microtubules past each other. A basal body (a modified centriole) is located in the cytoplasm at the base of the cilium. Cilia are numerous on surface cells that line the respiratory tract and the female reproductive tract. In these regions, cilia move in a coordinated fashion, with a power stroke in one direction and a recovery stroke in the opposite direction. Their motion moves materials over the surface of the cells. For example, cilia in the trachea move mucus embedded with dust particles upward and away from the lungs. This action helps keep the lungs clear of debris.

Flagella  have a structure similar to cilia but are longer (45 μm). Sperm cells are the only human cells to possess flagella and usually only one flagellum exists per cell. Furthermore, whereas cilia move small particles across the cell surface, flagella move the entire cell. For example, each sperm cell is propelled by a single flagellum. In contrast to cilia, which have a power stroke and a recovery stroke, flagella move in a

wavelike fashion.

Microvilli

Microvilli ) are cylindrically shaped

extensions of the plasma membrane about 0.5–1.0 μm in length and 90 nm in diameter. Normally, many microvilli are on each cell, and they function to increase the cell surface area. A student looking at photographs may confuse microvilli with cilia. Microvilli, however, are only one-tenth to one-twentieth the size of cilia. Individual microvilli can usually be seen only with an electron microscope, whereas cilia can be seen with a light microscope. Microvilli do not move, and they are supported with actin filaments, not microtubules. Microvilli are found in the intestine, kidney, and other areas in which absorption is an important

function. In certain locations of the body, microvilli are highly modified to function as sensory receptors. For example, elongated microvilli in hair cells of the inner ear respond to sound.

A&P Lecture 1.10: Mitochondria

This is an in depth  explanation of Mitochondria as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.10

MITOCHONDRIA

All cells in the body, with the exception of mature red blood cells, have from a hundred to a few thousand organelles called mitochondria (singular, mitochondrion ). Mitochondria serve as sites for the production of most of the energy of cells

Mitochondria vary in size and shape, but all have the same basic structure.. Each mitochondrion is surrounded by an inner and outer membrane, separated by a narrow intermembranous space. The outer mitochondrial membrane is smooth, but the inner membrane is characterized by many folds, called cristae, which project like shelves into the central area (or matrix ) of the mitochondrion. The cristae and the matrix compartmentalize the space within the mitochondrion and have different roles in the generation of cellular energy.

Mitochondria can migrate through the cytoplasm of a cell and are able to reproduce themselves. Indeed, mitochondria contain their own DNA. All of the mitochondria in a person’s body are derived from those inherited from the mother’s fertilized egg cell. Thus, all of a person’s mitochondrial genes are inherited from the mother. Mitochondrial DNA is more primitive (consisting of a circular, relatively small, doublestranded

molecule) than that found within the cell nucleus. For this and other reasons, many scientists believe that mitochondria evolved from separate organisms, related to bacteria, that invaded the ancestors of animal cells and remained in a state of symbiosis. This symbiosis might not always benefit the host; for example, mitochondria produce superoxide radicals that can provoke an oxidative stress and some scientists believe that accumulations of mutations in mitochondrial DNA may contribute to aging. Mutations in mitochondrial DNA occur at a rate at least ten times faster than in nuclear DNA (probably due to the superoxide radicals),

and there are more than 150 mutations of mitochondrial DNA presently known to contribute to different human diseases. However, genes in nuclear DNA code for 99% of mitochondrial proteins (mitochondrial DNA contains only 37 genes), and so many mitochondrial diseases are produced by mutations in nuclear DNA.

Neurons obtain energy solely from aerobic cell respiration, which occurs in mitochondria. Thus, mitochondrial fission (division) and transport over long distances is particularly important in neurons, where axons can be up to l meter in length. Mitochondria can also fuse together, which may help to repair those damaged by “reactive oxygen species” generated within mitochondria.

A&P Lecture 1.9: Peroxisomes and Proteasomes

This is an in depth  explanation of Peroxisomes and Proteasomes as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.9

PEROXISOMES AND PROTEASOMES

Peroxisomes are membrane-enclosed organelles containing several specific enzymes that promote oxidative reactions. Although peroxisomes are present in most cells, they are particularly large and active in the liver.

All peroxisomes contain one or more enzymes that promote reactions in which hydrogen is removed from particular organic molecules and transferred to molecular oxygen (O 2 ), thereby oxidizing the molecule and forming hydrogen peroxide (H 2 O 2 ) in the process. The oxidation of toxic molecules by peroxisomes in this way is an important function of liver and kidney cells. For example, much of the alcohol ingested in alcoholic drinks is oxidized into acetaldehyde by liver peroxisomes.

The enzyme catalase within the peroxisomes prevents the excessive accumulation of hydrogen peroxide by catalyzing

Proteasomes  consist of large protein complexes, including several enzymes that break down and recycle proteins within the cell. Proteasomes are not surrounded by membranes. They are tunnel-like structures, similar to channel protein complexes; the inner surfaces of the tunnel have enzymatic regions that break down
proteins. Smaller protein subunits close the ends of the tunnel and regulate which proteins are taken into it for digestion.the reaction 2H 2 O 2 → 2 H 2 O + O 2 . Catalase is one of the fastest acting enzymes known (see chapter 4), and it is this reaction that produces the characteristic fizzing when hydrogen peroxide
is poured on a wound.

A&P Lecture 1.8: Lysosomes

This is an in depth  explanation of Lysosomes as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.8

LYSOSOMES

After a phagocytic cell has engulfed the proteins, polysaccharides, and lipids present in a particle of “food” (such as a bacterium), these molecules are still kept isolated from the cytoplasm by the membranes surrounding the food vacuole. The large molecules of proteins, polysaccharides, and lipids must first be digested into their smaller subunits (including amino acids, monosaccharides, and fatty acids) before they can cross the vacuole membrane and enter the cytoplasm.

 The digestive enzymes of a cell are isolated from the cytoplasm and concentrated within membrane-bound organelles called lysosomes, which contain more than 60 different enzymes. A primary lysosome is one that contains only digestive enzymes (about 40 different types) within an environment that is more acidic than the surrounding cytoplasm. A primary lysosome may fuse with a food vacuole (or with another cellular organelle) to form a secondary lysosome in which worn-out organelles and the products of phagocytosis can be digested. Thus, a secondary lysosome contains partially digested remnants of other organelles and ingested organic material. A lysosome that contains undigested wastes is called a residual body. Residual bodies may eliminate their waste by exocytosis, or the wastes may accumulate within the cell as the cell ages.

Partly digested membranes of various organelles and other cellular debris are often observed within secondary lysosomes. This is a result of autophagy, a process that destroys worn-out organelles and proteins in the cytoplasm so that they can be continuously replaced. Lysosomes are thus aptly characterized as the “digestive system” of the cell.

Lysosomes have also been called “suicide bags” because a break in their membranes would release their digestive enzymes and thus destroy the cell. This happens normally in programmed cell death (or apoptosis ), . An example is the loss of tissues that must accompany embryonic development, when earlier structures (such as gill pouches) are remodeled or replaced as the embryo matures

SUMMARY

Lysosomes  are membrane-bound vesicles that pinch off from the Golgi apparatus. They contain a variety of hydrolytic enzymes that function as intracellular digestion systems. Vesicles taken into the cell fuse with the lysosomes to form one vesicle and to expose the endocytized materials to hydrolytic enzymes. Various enzymes within lysosomes digest nucleic acids, proteins, polysaccharides, and lipids. Certain white blood cells have large numbers of lysosomes that contain enzymes to digest phagocytized bacteria. Lysosomes also

digest the organelles of the cell that are no longer functional in a process called autophagia . In other

cells, the lysosomes move to the plasma membrane, and the enzymes are secreted by exocytosis. For example, the normal process of bone remodeling involves the breakdown of bone tissue by specialized bone cells. Enzymes responsible for that degradation are released into the extracellular fluid from lysosomes produced by those cells.

A&P Lecture 1.6: Golgi Complex

This is an in depth  explanation of the Golgi Complex as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.4

GOLGI COMPLEX

The Golgi complex, also called the Golgi apparatus, consists of a stack of several flattened sacs . This is something like a stack of pancakes, but the Golgi sac “pancakes” are hollow, with cavities called cisternae within each sac. One side of the stack faces the endoplasmic reticulum and serves as a site of entry for vesicles from the endoplasmic reticulum that contain cellular products. The other side of the stack faces the plasma membrane, and the cellular products somehow get transferred to that side. This may be because the products are passed from one sac to the next, probably in vesicles, until reaching the sac facing the plasma membrane. Alternatively, the sac that receives the products from the endoplasmic reticulum may move through the stack until reaching the other side. By whichever mechanism the cell product is moved through the Golgi complex, it becomes chemically modified and then, in the sac facing the plasma membrane, is packaged into vesicles that bud off the sac. Depending on the nature of the cell product, the vesicles that leave the Golgi complex may become lysosomes, or secretory vesicles (in which the product is released from the cell by exocytosis), or may serve other functions.

The reverse of exocytosis is endocytosis, as previously described; the membranous vesicle formed by that process is an endosome. . This reverse pathway is called retrograde transport, because proteins within the extracellular fluid are brought into the cell and then taken to the Golgi apparatus and the endoplasmic reticulum. Some toxins, such as the cholera toxin, and proteins from viruses (including components of HIV) rely on retrograde transport for their ability to infect cells.

A&P Lecture 1.5: Endoplasmic Reticulum

This is an in depth  explanation of the Endoplasmic Reticulum as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.4

ENDOPLASMIC RETICULUM

The endoplasmic reticulum is a continuous a series of membranes distributed throughout the cytoplasm of

the cell located at the he outer membrane of the nuclear envelope. It is consists of broad, flattened, interconnecting sacs and tubules. The interior spaces of those sacs and tubules are called cisternae and are

isolated from the rest of the cytoplasm.

Most cells contain a system of membranes known as the endoplasmic reticulum, or ER. The ER may be either of two types: (1) a granular, or rough, endoplasmic reticulum or (2) an agranular, or smooth, endoplasmic reticulum. A granular endoplasmic reticulum bears ribosomes on its surface, whereas an agranular endoplasmic reticulum does not. The agranular endoplasmic reticulum serves a variety

of purposes in different cells; it provides a site for enzyme reactions in steroid hormone production and inactivation, for example, and a site for the storage of Ca 2 + in striated muscle cells. The granular endoplasmic reticulum is abundant in cells that are active in protein synthesis and secretion, such as

those of many exocrine and endocrine glands.

The rough endoplasmic reticulum is called “rough” because it has ribosomes attached to it. The ribosomes of the rough endoplasmic reticulum are sites where proteins are produced and modified for secretion and for internal use. The amount and configuration of the endoplasmic reticulum within the cytoplasm depend on the cell type and function. Cells with abundant rough endoplasmic reticulum synthesize large amounts of protein that are secreted for use outside the cell.

Smooth endoplasmic reticulum, which is endoplasmic reticulum without attached ribosomes, manufactures lipids, such as phospholipids, cholesterol, steroid hormones, and carbohydrates. Many phospholipids produced in the smooth endoplasmic reticulum help form vesicles within the cell and contribute to the plasma membrane. Cells that synthesize large amounts of lipid contain dense accumulations of smooth endoplasmic reticulum. Enzymes required for lipid synthesis are associated with the membranes of the smooth endoplasmic reticulum. Smooth endoplasmic reticulum also participates in the detoxification processes by which enzymes act on chemicals and drugs to change their structure and reduce their toxicity. The smooth endoplasmic reticulum of skeletal muscle stores calcium ions that function in muscle contraction.

A&P Lecture 1.4: Ribosome

This is an in depth  explanation of the Ribosome as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.4

RIBOSOME

Ribosomes  are the sites of protein synthesis. Each ribosome is composed of a large subunit and a smaller one. The ribosomal subunits, which consist of ribosomal RNA (rRNA) and proteins, are produced separately in the nucleolus of the nucleus. The ribosomal subunits then move through the nuclear pores into the cytoplasm, where the ribosomal subunits assemble with mRNA to form the functional ribosome during protein synthesis. Ribosomes can be found free in the cytoplasm or associated with an intracellular membrane complex called the endoplasmic reticulum. Free ribosomes primarily synthesize proteins used inside the cell, whereas ribosomes attached to the endoplasmic reticulum u

sually produce proteins that are secreted from the cell.

Ribosomes are often called the “protein factories” of the cell because it is here that proteins are produced according to the genetic information contained in messenger RNA. The ribosomes are quite tiny, about

25 nanometers in size, and can be found both free in the cytoplasm and located on the surface of an organelle called the endoplasmic reticulum.

Each ribosome consists of two subunits, which are designated 30S and 50S after their sedimentation rate in

a centrifuge (this is measured in Svedberg units, from which the “S” is derived). Each of the subunits is composed of both ribosomal RNA and proteins. Contrary to earlier expectations of most scientists, it now appears that the ribosomal RNA molecules serve as enzymes (called ribozymes ) for many of the reactions

in the ribosomes that are required for protein synthesis.

A&P Lecture 1.2: Cytoplasm

This is an in depth  explanation of the Cytoplasm as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE NOTE 1.2

CYTOPLASM, CYTOSOL, CYTOSKELETON, 

AND CYTOPLASMIC INCLUSION

CYTOPLASM

Cytoplasm, the cellular material outside the nucleus but inside the plasma membrane, is about half cytosol and half organelles.

CYTOSOL

Cytosol consists of a fluid portion, a cytoskeleton, and cytoplasmic inclusions. The fluid portion of cytosol is a solution with dissolved ions and molecules and a colloid with suspended molecules, especially proteins. Many of these proteins are enzymes that catalyze the breakdown of molecules for energy or the synthesis of sugars, fatty acids, nucleotides, amino acids, and other molecules.

CYTOSKELETON

The cytoskeleton supports the cell and holds the nucleus andother organelles in place. It is also responsible for cell movements,such as changes in cell shape and the movement of cell organelles. The cytoskeleton consists of three groups of proteins: microtubules, actin filaments, and intermediate filaments ( figure 3.27 ).

Microtubules are hollow tubules composed primarily of protein units called tubulin. The microtubules are about 25 nanometers (nm) in diameter, with walls about 5 nm thick. 

Microtubules vary in length but are normally several micrometers (μm) long. Microtubules play a variety of roles within cells. They help provide  support and structure to the cytoplasm of the cell, much like an internal scaffolding. They are involved in the process of cell division and in the transport of intracellular materials, and they form essential components of certain cell organelles, such as centrioles, spindle fibers, cilia, and flagella.

Actin filaments, or microfilaments, are small fibrils about 8 nm in diameter that form bundles, sheets, or networks in the cytoplasm of cells. These filaments have a spiderweb-like appearance within the cell. Actin filaments provide structure to the cytoplasm and mechanical support for microvilli. Actin filaments support the plasma membrane and define the shape of the cell. Changes in cell shape involve the breakdown and reconstruction of actin filaments. These changes in shape allow some cells to move about. Muscle cells contain a large number of highly organized actin filaments, which are responsible for the muscle’s contractile capabilities.

Intermediate filaments are protein fibers about 10 nm in diameter. They provide mechanical strength to cells. For example, intermediate filaments support the extensions of nerve cells, which have a very small diameter but can be a meter in length. 

CYTOPLASMIC INCLUSION

The cytosol also contains cytoplasmic inclusions, which are aggregates of chemicals either produced by the cell or taken in by the cell.For example, lipid droplets or glycogen granules store energy-rich molecules; hemoglobin in red blood cells transports oxygen; melanin is a pigment that colors the skin, hair, and eyes; and lipochromes are pigments that increase in amount with age. Dust, minerals, and dyes can also accumulate in the cytoplasm.

A&P Lecture 1.3: Nucleus

This lecture note on Anatomy and Physiology is linked to A&P Lecture 1: The Cell to further explain the physiology of the Nucleus.

ANATOMY AND PHYSIOLOGY LECTURE 1.3

NUCLEUS

The nucleus is a large, membrane-bound structure usually located near the center of the cell. It may be spherical, elongated, or lobed, depending on the cell type. All cells of the body have a nucleus at some point in their life cycle, although some cells, such as red blood cells, lose their nuclei as they develop. Other cells, such as skeletal muscle cells and certain bone cells, called osteoclasts, contain more than one nucleus.

The nucleus consists of nucleoplasm surrounded by a nuclear envelope composed of two membranes separated by a space. At many points on the surface of the nuclear envelope, the inner and outer membranes fuse to form porelike structures called nuclear pores. Molecules move between the nucleus and the cytoplasm through these nuclear pores.

Deoxyribonucleic acid (DNA) is mostly found within the nucleus , although small amounts of DNA are also found within mitochondria. Nuclear DNA and associated proteins are organized into discrete structures called chromosomes. The proteins include histones, which are important for the structural organization of DNA, and other proteins that regulate DNA function. During most of the life cycle of a cell, the chromosomes are dispersed throughout the nucleus as delicate filaments referred to as chromatin. During cell division, the dispersed chromatin becomes densely coiled, forming compact chromosomes. At the beginning of cell division, each chromosome consists of two chromatids , which are attached at a single point called the centromere. The kinetochore, a protein structure within the centromere, provides a point of attachment for microtubules during cell division.

DNA determines the structural and functional characteristics of the cell by specifying the structure of proteins. Proteins form many structural components of the cell and all the enzymes, which regulate most chemical reactions in the cell. DNA establishes the structure of proteins by specifying the sequence of their amino acids. DNA is a large molecule that does not leave the nucleus but functions by means of an intermediate, ribonucleic acid (RNA) , which can leave the nucleus through nuclear pores. DNA determines the structure of messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA) (all described in more detail on p. 86). A sequence of nucleotides in a DNA molecule that specifies the structure of a protein or RNA molecule is called a gene.

Because mRNA synthesis occurs within the nucleus, cells without nuclei accomplish protein synthesis only as long as the mRNA produced before the nucleus degenerates remains functional. The nuclei of developing red blood cells are expelled from the cells before the red blood cells enter the blood, where they survive without a nucleus for about 120 days. In comparison, many cells with nuclei, such as nerve and skeletal muscle cells,

survive as long as the person survives. 

A nucleolus is a somewhat rounded, dense region within the nucleus that lacks a surrounding membrane . Usually, one nucleolus exists per nucleus, but several nucleoli may also be seen in the nuclei of rapidly dividing cells. The nucleolus incorporates portions of 10 chromosomes (5 pairs), called nucleolar organizer regions. These regions containDNA from which rRNA is produced. Within the nucleolus, the subunits of ribosomes are manufactured

A&P Lecture 1.1: Cell Membrane

This lecture note on Anatomy and Physiology is linked to A&P Lecture 1: The Cell to further explain the physiology of the Cell Membrane. 

ANATOMY AND PHYSIOLOGY LECTURE NOTE 1.1

CELL MEMBRANE

The outermost component of the cell is the plasma membrane. It is a boundary that separates the substances inside the cell, which are intracellular , from substances outside the cell, which are extracellular.It determines what moves into and out of cells. As a result, the intracellular contents of cells is different from the extracellular environment. The membrane encloses and supports the cell contents. It attaches cells to the extracellular environment or to other cells. The cells’ ability to recognize and communicate with each other takes place through the plasma membrane.

The regulation of ion movement by cells results in a charge difference across the plasma membrane called the membrane potential. The outside of the plasma membrane is positively charged, compared with the inside, because there are more positively charged ions immediately on the outside of the plasma membrane and more negatively charged ions and proteins inside. This concept is paramount in understanding the Sodium Potassium Pump that controls the electrophysiology of the heart, transmission of nerve impulses and contraction of muscles.

The plasma membrane consists of 45%–50% lipids, 45%– 50% proteins, and 4%–8% carbohydrates ( figure 3.2 ). The carbohydrates combine with lipids to form glycolipids and with proteins to form glycoproteins. The glycocalyx (glı¯-ko¯-ka¯ liks) is the collection of glycolipids, glycoproteins, and carbohydrates on the outer surface of the plasma membrane. The glycocalyx also contains molecules absorbed from the extracellular environment, so there is often no precise boundary where the plasma membrane ends and the extracellular environment begins.

Indepth Medical Physiology

The membrane that surrounds cells is a notable structure. It is made up of lipids and proteins and is semipermeable meaning- it allows some substances to pass through it and excluding others. However, its permeability can also be varied because it contains numerous regulated ion channels and other transport proteins that can change the amounts of substances moving across it. It is generally referred to as the plasma membrane. The nucleus and other organelles in the cell are bound by similar membranous structures.

Although the chemical structures of membranes and their properties vary considerably from one location to another, they have certain common features. They are generally about 7.5 nm thick. The major lipids are phospholipids such as phosphatidylcholine, phosphotidylserine, and phosphatidylethanolamine. The shape of the phospholipid molecule reflects its solubility properties: the “head” end of the molecule contains the phosphate portion and is relatively soluble in water  and the “tail” ends are relatively insoluble. The possession of both hydrophilic and hydrophobic properties makes the lipid an amphipathic molecule. In the membrane, the hydrophilic ends of the molecules are exposed to the aqueous environment that bathes the exterior of the cells and the aqueous cytoplasm; the hydrophobic ends meet in the water-poor interior of the membrane. In prokaryotes, the membranes are relatively simple, but in eukaryotes, cell membranes contain various glycosphingolipids, sphingomyelin, and cholesterol in addition to phospholipids and phosphatidylcholine.

Many different proteins are embedded in the membrane. They exist as separate globular units and many pass through or are embedded in one leaflet of the membrane- integral proteins, whereas others- peripheral proteins, are associated with the inside or outside of the membrane. The amount of protein varies significantly with the function of the membrane but makes up on average 50% of the mass of the membrane; that is, there is about one protein molecule per 50 of the much smaller phospholipid molecules.

The proteins in the membrane carry out many functions. Some are cell adhesion molecules that anchor cells to their neighbors or to basal laminas. Some proteins function as pumps, actively transporting ions across the membrane. Other proteins function as carriers, transporting substances down electrochemical gradients by facilitated diffusion. Still others are ion channels, which, when activated, permit the passage of ions into or out of the cell. The role of the pumps, carriers, and ion channels in transport across the cell membrane is discussed below. Proteins in another group function as receptors that bind ligands or messenger molecules, initiating physiologic changes inside the cell. Proteins also function as enzymes, catalyzing reactions at the surfaces of the membrane. 

The uncharged, hydrophobic portions of the proteins are usually located in the interior of the membrane, whereas the charged, hydrophilic portions are located on the surfaces. Peripheral proteins are attached to the surfaces of the membrane in various ways. One common way is attachment to glycosylated forms of phosphatidylinositol. Proteins held by these glycosylphosphatidylinositol (GPI) anchors include enzymes such as alkaline phosphatase, various antigens, a number of CAMs, and three proteins that combat cell lysis by complement. Over 45 GPI-linked cell surface proteins have now been described in humans. Other proteins are lipidated, that is, they have specific lipids attached to them. Proteins may be myristoylated, palmitoylated, or prenylated (ie, attached to geranylgeranyl or farnesyl groups).

The protein structure—and particularly the enzyme content—of biologic membranes varies not only from cell to cell, but also within the same cell. For example, some of the enzymes embedded in cell membranes are different from those in mitochondrial membranes. In epithelial cells, the enzymes in the cell membrane on the mucosal surface differ from those in the cell membrane on the basal and lateral margins of the cells; that is, the cells are polarized. Such polarization makes directional transport across epithelia possible. Th e membranes are dynamic structures, and their constituents are being constantly renewed at different rates. Some proteins are anchored to the cytoskeleton, but others move laterally in the membrane.

Underlying most cells is a thin, “fuzzy” layer plus some fibrils that collectively make up the basement membrane or, more properly, the basal lamina. Th e basal lamina and, more generally, the extracellular matrix are made up of many proteins that hold cells together, regulate their development, and determine their growth. These include collagens, laminins, fibronectin, tenascin, and various proteoglycans.

A&P Lecture 1: The Cell

"You need to know the normal before you can determine the abnormal." 

Knowledge in Anatomy and Physiology is paramount before tackling advance Nursing Courses. It is the reason why Anatomy and Physiology class is taken first before professional courses like Pathophysiology, Medical Surgical Nursing, Maternal and Child Nursing and Psychiatric Nursing.Anatomy and Physiology is a good foundation when you already try to study illnesses. This A&P Lecture Series is designed to help students develop a solid understanding of the concepts of anatomy and physiology and to use this knowledge to solve problems.

ANATOMY AND PHYSIOLOGY LECTURE 1

THE CELL

The Cell is the basic unit of life. Cells are the smallest parts of an organism, such as a human, that have the characteristics of life. Although cells may have quite different structures and functions, they share several characteristics which includes:

• Cell metabolism and energy use
• Synthesis of molecules
• Communication
• Reproduction and inheritance

Plasma Membrane

Structure: Lipid bilayer composed of phospholipids and cholesterol with proteins that extend across or are embedded in either surface of the lipid bilayer

Function: Outer boundary of cells that controls entry and exit of substances; receptor molecules function in intercellular communication; marker molecules enable cells to recognize one another.

***Read more about the Plasma Membrane here

Cytoplasm

Fluid Part

Structure: Water with dissolved ions and molecules; colloid with suspended proteins

Function:Contains enzymes that catalyze decomposition and synthesis reactions; ATP is produced in glycolysis reactions

Cytoskeleton/ Microtubules

Structure: Hollow cylinders composed of the protein tubulin; 25 nm in diameter

Function: Support the cytoplasm and form centrioles, spindle fibers, cilia, and flagella; responsible for movement of structures in the cell

Actin Filaments

Structure: Small fibrils of the protein actin; 8 nm in diameter

Function: Provide structural support to cells, support microvilli, responsible for cell movements

Intermediate Filaments

Structure:Protein fibers; 10 nm in diameter

Function: Provide structural support to cells

Cytoplasmic Inclusions

Structure:Aggregates of molecules manufactured or ingested by the cell; may be membrane-bound

Function: Function depends on the molecules: energy storage (lipids, glycogen), oxygen transport (hemoglobin), skin color (melanin), and others

*** Read More about the Cytosol and its components here

Organelles, Nucleus

Nuclear envelope

Structure:Double membrane enclosing the nucleus; the outer membrane is continuous with the endoplasmic reticulum; nuclear pores extend through the nuclear envelope

Function: Separates nucleus from cytoplasm and regulates movement

of materials into and out of the nucleus

Chromatin

Structure:Dispersed, thin strands of DNA, histones, and other proteins; condenses to form chromosomes during cell division

Function: DNA regulates protein (e.g., enzyme) synthesis and therefore the chemical reactions of the cell; DNA is the genetic, or hereditary, material

Nucleolus

Structure: One or more dense bodies consisting of ribosomal RNA and proteins

Function: Assembly site of large and small ribosomal subunits

***Read more about the Nucleus and it's components here

Cytoplasmic Organelles

Ribosome

Structure:Ribosomal RNA and proteins form large and small subunits; attached to endoplasmic reticulum or free ribosomes are distributed throughout the cytoplasm

Function: Site of protein synthesis
 ***Read more about Ribosome here

Rough endoplasmic reticulum

Structure: Membranous tubules and flattened sacs with attached ribosomes

Function: Protein synthesis and transport to Golgi apparatus

*** Read more about the Endoplasmic Reticulum here

Smooth endoplasmic reticulum

Structure: Membranous tubules and flattened sacs with no attached ribosomes

Function: Manufactures lipids and carbohydrates; detoxifies harmful chemicals; stores calcium
*** Read more about the Endoplasmic Reticulum here

Golgi apparatus

Structure: Flattened membrane sacs stacked on each other

Function: Modifies, packages, and distributes proteins and lipids for secretion or internal use

***Read more about the Golgi apparatus here

Secretory vesicle

Structure: Membrane-bound sac pinched off Golgi apparatus

Function: Carries proteins and lipids to cell surface for secretion

***Read more about the Secretory vesicle here

Lysosome

Structure: Membrane-bound vesicle pinched off Golgi apparatus

Function: Contains digestive enzymes
***Read more about the Lysosomes  here

Peroxisome

Structure: Membrane-bound vesicle

Function: One site of lipid and amino acid degradation; breaks down hydrogen peroxide

***Read more about Peroxisome here

Proteasomes

Structure: Tubelike protein complexes in the cytoplasm

Function: Break down proteins in the cytoplasm
***Read more about Proteasomes here

Mitochondria

Structure: Spherical, rod-shaped, or threadlike structures; enclosed by double membrane; inner membrane forms projections called cristae

Function: Major site of ATP synthesis when oxygen is available
***Read more about the Mitochondria here

Centrioles

Structure: Pair of cylindrical organelles in the centrosome, consisting of triplets of parallel microtubules 

Function: Centers for microtubule formation; determine cell polarity during cell division; form the basal bodies of cilia and flagella

***Read more about the Centriole here

Spindle fibers

Structure:Microtubules extending from the centrosome to chromosomes and other parts of the cell (i.e., aster fibers)

Function: Assist in the separation of chromosomes during cell division

***Read more about the Spindle fiber here

Cilia

Structure: Extensions of the plasma membrane containing doublets of parallel microtubules; 10 μm in length

Function: Move materials over the surface of cells
***Read more about the Cilia here

Flagellum

Structure: Extension of the plasma membrane containing doublets of parallel microtubules; 55 μm in length

Function: In humans, responsible for movement of spermatozoa

***Read more about the Flagellum here

Microvilli

Structure: Extension of the plasma membrane containing microfilaments

Function: Increase surface area of the plasma membrane for absorption and secretion; modified to form sensory receptors

***Read more about the Microvillli here