Cells of the human body Essay

This essay has a total of 4369 words and 24 pages.

cells of the human body



Cells are the basic living units of all plants and animals. The cell is the structural
and functional unit of all living organisms. There are a wide variety of cell types, such
as nerve, muscle, bone, fat, and blood cells. Each cell type has many characteristics,
which are important to the normal function of the body as a whole. One of the important
reasons for maintaining hemostasis is to keep the trillions of cells that form the body
functioning normally. An averaged size cell is one-fifth the size of the smallest dot you
can make on a sheet of paper with a sharp pencil.

Although cells may have quite different structures and functions, all cells share some
common characteristics. The plasma, or cell membrane, forms the outer boundary of the
cell through which the cells interacts with its external environment. The nucleus is
usually located centrally and functions to direct cell activities, most of which take
place in the cytoplasm, located between the plasma membrane and the nucleus.


PLASMA (CELL) MEMBRANE

The plasma membrane is the outer part of a cell. The plasma membrane is made up of 45% -
50% lipids, 45% - 50% proteins, and 4% - 8% carbohydrates. The main lipids are
phospholipids and cholesterol. Phospholipids easily come together to form a lipid
bilayer, a double layer of lipid molecules, because they have a polar head and a nonpolar
tail. The charged water-loving heads are exposed to water inside and outside the cell,
whereas the uncharged water-fearing tails face one another in the interior of the plasma
membrane. The other major lipid in the plasma membrane is cholesterol, which is mixed
among the phospholipids and makes up about a third of the total lipids in the plasma
membrane. Cholesterol is too hydrophobic to extend to the hydrophilic surface of the
membrane but lies within the hydrophobic region of the phospholipids. The amount of
cholesterol in a given membrane is a major factor in determining the fluid nature of the
membrane, which is important to its function.

The fluid-mosaic model suggests that the plasma membrane is highly flexible and can change
its shape and composition through time. The lipid bilayer functions as a liquid in which
other molecules such as proteins "float". The fluid nature of the lipid bilayer is very
important. It provides an important means of distributing molecules within the plasma
membrane. In addition, slight damage to the membrane can be repaired because the
phospholipids tend to reassemble around damaged sites and seal them closed. The fluid
nature of the lipid bilayer enables membranes to fuse with one another.

Although the basic structure of the plasma membrane is determined mainly by its lipids,
the functions of the plasma membrane are determined mainly by its proteins. Integral, or
intrinsic proteins, penetrate the lipid bilayer from one surface to the other.
Peripheral, or extrinsic proteins, are attached to either the inner or outer surfaces of
the lipid bilayer. Integral proteins consist of regions made up of amino acids with
hydrophobic R groups and other regions of amino acids with hydrophilic R groups. The
hydrophobic regions are located within the hydrophobic part of the membrane, and the
hydrophilic regions are located at the inner or outer surface of the membrane or line
channels through the membrane. Peripheral proteins are usually bound to integral
proteins. Some membrane proteins form channels through the membrane or act as carrier
molecules. Other membrane proteins are receptors, markers, enzymes, or structural
supports in the membrane. The ability of membrane proteins to function depends on their
three-dimensional shape.

Channel proteins are one or more integral proteins arranged so that they form a tiny
channel through the plasma membrane. The hydrophobic regions of the proteins face outward
toward the hydrophobic part of the cell membrane, and the hydrophilic regions of the
proteins line channel. Small molecules or ions of the right shape, size, and charge can
pass through the channel. The charges in the hydrophilic part of the channel protein
determine which typed of ions can pass through the channel.

The function of a channel protein is determined by its shape. The channel can be open or
closed, depending on the shape of the channel proteins. Some channel proteins change
shape to open the channel when a ligand binds to a specific receptor site on the protein.
This is called a ligand-gated channel. Other channel proteins change shape to open the
channel when there is a change in charge across the cell membrane. This is called a
voltage-gated channel.

Receptor molecules are proteins in the cell membrane with an exposed binding site on the
outer cell surface, which can attach to specific ligand molecules. The receptors and the
ligands they bind are part of an intercellular communication system that controls
coordination of cell activities. The binding acts as a signal that triggers a response,
such as contraction in the muscle cell. The same chemical messenger would have no effect
on another cell that lacks the receptor molecule. Some receptor molecules function by
means of a G protein complex located on the inner surface of the cell membrane. G
proteins may function in one of several ways. For example, when a ligand such as a
hormone attaches to the receptor molecule, the G protein complex binds guanosine
triphosphate (GTP) and is activated. The activated G protein, in turn, activated
adenylate cyclase, which catalyzes the conversion of adenosine triphosphate (ATP) to
cyclic adenosine monophosphate (cAMP). cAMP functions as a second messenger inside the
cell, stimulating a variety of cell functions.

Marker molecules are cell surface molecules that allow cells to identify and attach to
each other. They are mostly glycoproteins or glycolipids.



THE NUCLEUS

The nucleus, which contains most of the genetic information of the cell, 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 is surrounded
by a nuclear envelope composed of surface of the nuclear envelope, the inner and outer
membranes fuse to form porelike structures, the nuclear pores. Molecules move between the
nucleus and the cytoplasm through these nuclear pores.

Deoxyribonucleic acid (DNA) and associated proteins are mixed throughout the nucleus as
thin strands about 4-5 nanometers (nm) in diameter. The proteins include histones and
other proteins that play a role in the regulation of DNA function. The DNA and protein
strands can be stained with dyes and are called chromatin. Chromatin is distributed
throughout the nucleus but is more condensed and more readily stained in some areas than
in others. The more highly condensed chromatin apparently is less functional than the
more evenly distributed chromatin, which stains lighter. During cell division the
chromatin condenses to form the more solid bodies called chromosomes.

DNA ultimately determines the structure of proteins. Many structural components of the
cell and all the enzymes, which regulate most chemical reactions in the cell, are
proteins. By determining protein structure, DNA therefore ultimately controls the
structural and functional characteristics of the cell. DNA does not leave the nucleus,
but works by means of an intermediate, ribonucleic acid (RNA), which can leave the
nucleus. DNA determines the structure of messenger RNA (mRNA), ribosomal RNA (rRNA), and
transfer RNA (tRNA). mRNA moves out of the nucleus through the nuclear pores into the
cytoplasm, where it determines the structure of proteins.

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 individual person survives.

A nucleolus is a somewhat rounded, dense region within the nucleus that lacks a
surrounding membrane. There is usually one nucleolus per nucleus, but several smaller,
accessory nucleoli may also be seen in some nuclei, especially during the latter phases of
cell division. The nucleolus contains portions of 10 chromosomes, called nucleolar
organizer regions. These regions contain DNA from which rRNA is produced. Within the
nucleolus, the subunits of ribosomes are manufactured.



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 and organelles in place. It is
also responsible for cell movements, such as changes in cell shape or movement of cell
organelles. The cytoskeleton consists of three groups of proteins: microtubules, actin
filaments, and intermediate filaments.

Microtubules are hollow tubules composed primarily of protein units called tubulin. The
microtubules are about 25 nm in diameter, with walls that are about 5 nm thick.
Microtubules vary in length but are normally several micrometers (um) 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 form essential parts 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. 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. Actin filaments are involved in cell
movement. Cell movement in cells that can move about is accomplished by changes in cell
shape controlled by the actin cytoskeleton. Muscle cells contain a large number of highly
organized actin filaments 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 INCLUSIONS
The cytosol also contains cytoplasmic inclusions, which are collections of chemicals
either produced by the cell or taken in by the cell. Dust, minerals, and dyes can also
accumulate in the cytoplasm.


ORGANELLES
Organelles are small structures within cells that are specialized for particular
functions, such as manufacturing proteins or producing ATP. Most organelles have
membranes that are similar to the plasma membrane. The membranes separate the organelles
from the rest of the cytoplasm, creating a subcellular compartment with its own enzymes
that is able to carry out its own unique chemical reactions. The nucleus is an example of
an organelle.

The number and type of cytoplasmic organelles within each cell are related to the specific
structure and function of the cell. Cells secreting large amounts of protein contain
well-developed organelles that synthesize and secrete protein. Cells actively
transporting substances such as sodium ions across their plasma membrane contain highly
developed organelles that produce ATP. The following sections describe the structure and
main functions of the major cytoplasmic organelles found in cells.


RIBOSOMES
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 assembled separately in the nucleolus of the nucleus. The ribosomal
subunits then move through the nuclear pores into the cytoplasm, where they come together
to form the functional ribosome during protein synthesis. Ribosomes can be found free in
the cytoplasm or associated with a membrane called the endoplasmic reticulum. Free
ribosomes primarily synthesize proteins used inside the cell, whereas endoplasmic
reticulum ribosomes can produce proteins that are secreted from the cell.


ENDOPLASMIC RETICULUM
The outer membrane of the nuclear envelope is continuous with a series of membranes
distributed throughout the cytoplasm of the cell, referred to as the endoplasmic
reticulum. The endoplasmic reticulum 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.

Rough endoplasmic reticulum is endoplasmic reticulum with attached ribosomes. The
ribosomes of the rough endoplasmic reticulum produce proteins for secretion for internal
use. The amount and make up 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,
produces lipids, such as phospholipids, cholesterol, steroid hormones, and carbohydrates
such as glycogen. 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.


GOLGI APPARATUS
The Golgi apparatus is composed of flattened membranous sacs, containing cisternae, that
are stacked on each other like dinner plates. The Golgi apparatus modifies, packages, and
distributes proteins and lipids manufactured by the rough and smooth endoplasmic reticula.
Proteins produced at the ribosomes of the rough endoplasmic reticulum are surrounded by a
vesicle, or little sac, that forms from the membrane of the endoplasmic reticulum. The
vesicle moves to the Golgi apparatus, fuses with the membrane of the Golgi apparatus, and
releases the protein into the cisterna of the Golgi apparatus. The Golgi apparatus
concentrates and, in some cases, chemically modifies the proteins by synthesizing and
attaching carbohydrate molecules to the proteins to form glycoproteins or attaching lipids
to proteins to form lipoproteins. The proteins are then packaged into vesicles that pinch
off from the margins of the Golgi apparatus and are distributed to various locations.
Some vesicles carry proteins to the plasma membrane where the proteins are secreted from
the cell by exocytosis; other vesicles contain proteins that become part of the plasma
membrane; and still other vesicles contain enzymes that are used within the cell.

The Golgi apparatuses are most numerous and most highly developed in cells that secrete
large amounts of protein or glycoproteins, such as cells in the salivary glands and the
pancreas.


SECRETORY VESICLES
The membrane-bound secretory vesicles that pinch off from the Golgi apparatus move to the
surface of the cell, their membranes fuse with the plasma membrane, and the contents of
the vesicle are released to the exterior by exocytosis. The membranes of the vesicles are
then incorporated into the plasma membrane.

Secretory vesicles accumulate in many cells, but their contents frequently are not
released to the exterior until a signal is received by the cell. For example, secretory
vesicles that contain the hormone insulin do not release it until the concentration of
glucose in the blood increases and acts as a signal for the secretion of insulin from the
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