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UNIVERSITY OF TECHNOLOGYANATOMY & PHYSOLOGY - DN1 AND MT1

UNIT 1: Cells, Tissues & B !" Flui!s

STRUCTURE OF A TYPICAL ANIMAL CELL

Animal cells are typical of the eukaryotic cell, enclosed by a plasma membrane andcontaining a membrane-bound nucleus and organelles.

ORGANELLES STRUCTURE AND FUNCTIONS

Nucleus

The nucleus is a highly specialized organelle that serves as the information andadministrative center of the cell. This organelle has two major functions. It stores the cell shereditary material, or !"A, and it coordinates the cell s activities, which includeintermediary metabolism, growth, protein synthesis, and reproduction #cell division$.

%enerally there is only one nucleus per cell. The spherical nucleus occupies about &'percent of a cell s volume, making it the cell s most prominent feature. (ost of the nuclearmaterial consists of chromatin, the unstructured form of the cell s !"A that will organize toform chromosomes during mitosis or cell division. Also inside the nucleus is the nucleolus, anorganelle that synthesizes protein-producing macromolecular assemblies called ribosomes.

A double-layered membrane, the nuclear envelope, separates contents of the nucleus fromthe cellular cytoplasm. The envelope is riddled with holes called nuclear pores that allow

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specific types and sizes of molecules to pass back and forth between the nucleus and thecytoplasm. It is also attached to a network of tubules, called the endoplasmic reticulum,where protein synthesis occurs.

Chromatin/Chromosomes - )acked inside the nucleus of every human cell isnearly * feet of !"A, which is divided into +* individual molecules, one for eachchromosome and each about &. inches long. )acking all this material into amicroscopic cell nucleus is an e traordinary feat of packaging. or !"A to function, itcan t be crammed into the nucleus like a ball of string. Instead, it is combined withproteins and organized into a precise, compact structure, a dense string-like fibercalled chromatin.

There are two types of chromatin. /uchromatin is the genetically active portion and isinvolved in transcribing 0"A to produce proteins used in cell function and growth.1eterochromatin contains inactive !"A and is the portion of chromatin that is mostcondensed, since it is not being used.

Nucleolus - The nucleolus is a membrane-less organelle within the nucleus thatmanufactures ribosomes, the cell s protein-producing structures. Through themicroscope, the nucleolus looks like a large dark spot within the nucleus. A nucleusmay contain up to four nucleoli, but within each species the number of nucleoli is

fi ed. After a cell divides, a nucleolus is formed when chromosomes are broughttogether into nucleolar organizing regions. !uring cell division, the nucleolusdisappears. 2ome studies suggest that the nucleolus may be involved with cellularaging and, therefore, may affect the aging of an organism.

Nuclear Envelo e - The nuclear envelope is a double-layered membrane thatencloses the contents of the nucleus during most of the cell s lifecycle. The spacebetween the layers is called the perinuclear space and appears to connect with therough endoplasmic reticulum. The envelope is perforated with tiny holes callednuclear pores. These pores regulate the passage of molecules between the nucleus

and cytoplasm, permitting some to pass through the membrane, but not others. Theinner surface has a protein lining called the nuclear lamina, which binds to chromatinand other nuclear components. !uring mitosis, or cell division, the nuclear envelopedisintegrates, but reforms as the two cells complete their formation and the chromatinbegins to unravel and disperse.

Ri!osomes

All living cells contain ribosomes3 tiny organelles composed of appro imately *' percent0"A and +' percent protein.

0ibosomes are scattered throughout the cytoplasm and are the protein production sites forthe cell.0ibosomes are appro imately 4'nm by 5' nm.They consist of a large ad a small subunit."ot surrounded by a membrane.

/ukaryote ribosomes are produced and assembled in the nucleolus. Three of the fourstrands are produced there, but one is produced outside the nucleolus and transportedinside to complete the ribosome assembly. 0ibosomal proteins enter the nucleolus andcombine with the four strands to create the two subunits that will make up the completedribosome. The ribosome units leave the nucleus through the nuclear pores and unite once inthe cytoplasm.

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2ome ribosomes will remain free-floating in the cytoplasm, creating proteins for the cell suse. 6thers will attach to the endoplasmic reticulum and produce the proteins that will be7e ported7 from the cell.

En"o lasmic ReticulumThe endoplasmic reticulum #/0$ is a network of sacs that manufactures, processes, andtransports chemical compounds for use inside and outside of the cell. The /0 is acontinuous membrane with branching tubules and flattened sacs that e tend throughout thecytoplasm. It is connected to the double-layered nuclear envelope, providing a connectionbetween the nucleus and the cytoplasm.

There are two kinds of /0, rough and smooth. 0ough /0 is covered with ribosomes, giving ita bumpy appearance when viewed through the microscope. This type of /0 is involvedmainly with the production of proteins that will be e ported, or secreted, from the cell. The

ribosomes assemble amino acids into units of proteins, which are transported into the rough/0 for further processing. 6nce inside, the proteins are folded into the correct three-dimensional conformation, as a flattened cardboard bo might be opened up and folded intoits proper shape in order to become a useful bo . 8hemicals, such as carbohydrates orsugars, are added, then the /0 either transports the completed proteins to areas of the cellwhere they are needed, or they are sent to the %olgi apparatus for e port.

2mooth /0 has a smoother appearance than rough /0 when viewed through themicroscope because it does not have ribosomes attached to it. This portion of the /0 isinvolved with the production of lipids #fats$, carbohydrate metabolism, and deto ification ofdrugs and poisons. 2mooth /0 is also involved with metabolizing calcium to mediate somecell activities. In muscle cells, smooth /0 releases calcium to trigger muscle contractions.8ells specializing in lipid and carbohydrate metabolism #brain, muscle$ or deto ification#liver$ usually have more of this type of /0.

Gol#i A aratus

The %olgi apparatus #%A$, also called %olgi body or %olgi comple , is a series of five to eightcup-shaped, membrane-covered sacs that look something like a stack of deflated balloons.The %A is the distribution and shipping department for the cell s chemical products. Itmodifies proteins and lipids #fats$ that have been built in the endoplasmic reticulum andprepares them for e port as outside of the cell. The number of %As in each cell variesaccording to its function, but animal cells generally contain between ten and twenty per cell.

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)roteins and lipids built in the smooth and rough endoplasmic reticulum bud off in tinybubble-like vesicles that move through the cytoplasm until they reach the %A. The vesiclesfuse with the %A membrane and release the molecules into the organelle. 6nce inside, thecompounds are further processed by the %A, which adds molecules or chops tiny pieces offthe ends. 6nce completed, the product is e truded from the %A in a vesicle and directed toits final destination inside or outside the cell. The e ported products are secretions ofproteins or glycoproteins that are part of the cell s function in the organism. 6ther productsare returned to the endoplasmic reticulum or become lysosomes.

L$sosomes9ysosomes break down cellular waste products and debris from outside the cell into simplecompounds, which are transferred out into the cytoplasm as new cell-building materials.

9ysosomes are spherical organelles contained by a single layer membrane. This membraneprotects the rest of the cell from the lysosomes harsh digestive enzymes that wouldotherwise damage it.9ysosomes originate in the %olgi apparatus, but the digestive enzymes are manufactured inthe rough endoplasmic reticulum. 9ysosomes are found in all eukaryotic cells, but are most

numerous in disease-fighting cells, such as white blood cells.2ome human diseases are caused by lysosome enzyme disorders. Tay-sachs disease iscaused by a genetic defect that prevents the formation of an essential enzyme that breaksdown a comple lipid called ganglioside. An accumulation of this lipid damages the nervoussystem, causes mental retardation and death in early childhood. Arthritis inflammation andpain are related to the escape of lysosome enzymes.