4. STRUCTURE OF CELL
The Endoplasmic Reticulum
The endoplasmic reticulum, or ER, is a network of intracellular membranes continuous with the nuclear
envelope, which surrounds the nucleus. The name endoplasmic reticulum is very descriptive. Endo- means
“within,” plasm refers to the cytoplasm, and a reticulum is a network.
The ER has four major functions:
1. Synthesis. Specialized regions of the ER synthesize proteins, carbohydrates, and lipids.
2. Storage. The ER can store synthesized molecules or materials absorbed from the cytosol without
affecting other cellular operations.
3. Transport. Materials can travel from place to place within the ER.
4. Detoxification. The ER can absorb drugs or toxins and neutralize them with enzymes.
The ER forms hollow tubes, flattened sheets, and chambers called cisternae (singular, cisterna, a reservoir for
water). Two types of ER exist: smooth endoplasmic reticulum and rough endoplasmic reticulum
5. STRUCTURE OF CELL
The amount of endoplasmic reticulum and the proportion of RER to SER vary with the type of cell and its
ongoing activities. For example, pancreatic cells that manufacture digestive enzymes contain an extensive RER,
but the SER is relatively small. The situation is just the reverse in the cells of reproductive organs that
synthesize steroid hormones.
6. The Smooth Endoplasmic Reticulum
The term “smooth” refers to the fact that no ribosomes are associated with the smooth endoplasmic
reticulum (SER). The SER is involved with the synthesis of lipids, fatty acids, and carbohydrates; the
sequestering of calcium ions; and the detoxification of drugs. Important functions of the SER include the
following:
Synthesis of the phospholipids and cholesterol needed for maintenance and growth of the plasma
membrane, ER, nuclear membrane, and Golgi apparatus in all cells
Synthesis of steroid hormones, such as androgens and estrogens (the dominant sex hormones in males and
in females, respectively) in the reproductive organs
Synthesis and storage of glycerides, especially triacyl glycerides, in liver cells and fat cells
Synthesis and storage of glycogen in skeletal muscle and liver cells
In muscle cells, neurons, and many other types of cells, the SER also adjusts the composition of the cytosol
by absorbing and storing ions, such as Ca2+, or large molecules. In addition, the SER in liver and kidney cells
detoxifies or inactivates drugs.
7. The Rough Endoplasmic Reticulum
The rough endoplasmic reticulum (RER) functions as a combination workshop and shipping warehouse. It is
where many newly synthesized proteins are chemically modified and packaged for export to their next
destination, the Golgi apparatus. The fixed ribosomes on the outer surface of the rough endoplasmic
reticulum give the RER a beaded, grainy, or rough appearance. Both free and fixed ribosomes synthesize
proteins using instructions provided by messenger RNA.
The new polypeptide chains produced at fixed ribosomes are released into the cisternae of the RER. Inside
the RER, each protein assumes its secondary and tertiary structures. Some of the proteins are enzymes that
will function inside the endoplasmic reticulum. Other proteins are chemically modified by the attachment of
carbohydrates, creating glycoproteins. Most of the proteins and glycoproteins produced by the RER are
packaged into small membranous sacs that pinch off from the tips of the cisternae. These transport vesicles
then deliver their contents to the Golgi apparatus.
8. Golgi Complex
Most of the proteins synthesized by ribosomes attached to rough ER are ultimately transported to other
regions of the cell. The first step in the transport pathway is through an organelle called the Golgi complex. It
consists of 3 to 20 cisternae, small, flattened membranous sacs with bulging edges. The cisternae are often
curved, giving the Golgi complex a cuplike shape. Most cells have several Golgi complexes, and Golgi
complexes are more extensive in cells that secrete proteins, a clue to the organelle’s role in the cell.
The cisternae at the opposite ends of a Golgi complex differ from each other in size, shape, and enzymatic
activity. The convex entry or cis face is a cisterna that faces the rough ER. The concave exit or trans face is a
cisterna that faces the plasma membrane. Sacs between the entry and exit faces are called medial cisternae.
Transport vesicles (described shortly) from the ER merge to form the entry face. From the entry face, the
cisternae are thought to mature, in turn becoming medial and then exit cisternae.
9.
10.
11. Lysosomes
Cells often need to break down and recycle large organic molecules and even complex structures like
organelles. The breakdown process requires powerful enzymes, and it often generates toxic chemicals that
could damage or kill the cell. Lysosomes (lyso-, a loosening + soma, body) are special vesicles that provide an
isolated environment for potentially dangerous chemical reactions. These vesicles, produced by the Golgi
apparatus, contain digestive enzymes. lysosomes are small, often spherical bodies with contents that look
dense and dark in electron micrographs.
They can contain as many as 60 kinds of powerful digestive and hydrolytic enzymes that can break down a
wide variety of molecules once lysosomes fuse with vesicles formed during endocytosis. Because lysosomal
enzymes work best at an acidic pH, the lysosomal membrane includes active transport pumps that import
hydrogen ions (H+). Thus, the lysosomal interior has a pH of 5, which is 100 times more acidic than the pH of
the cytosol (pH 7). The lysosomal membrane also includes transporters that move the final products of
digestion, such as glucose, fatty acids, and amino acids, into the cytosol.
12. Lysosomes
Lysosomes have several functions.
One is to remove damaged organelles. Primary lysosomes contain inactive enzymes. When these lysosomes
fuse with the membranes of damaged organelles (such as mitochondria or fragments of the ER), the
enzymes are activated, and secondary lysosomes are formed.
The process by which entire worn-out organelles are digested is called autophagy (auto-self; phagy-eating).
In autophagy, the organelle to be digested is enclosed by a membrane derived from the ER to create a vesicle
called an autophagosome; the vesicle then fuses with a lysosome.
Lysosomes also destroy bacteria (as well as liquids and organic debris) that enter the cell from the
extracellular fluid. The cell encloses these substances in a small portion of the plasma membrane, which is
then pinched off to form a transport vesicle, or endosome, in the cytoplasm. Then a primary lysosome
fuses with the vesicle, forming a secondary lysosome. Activated enzymes inside break down the contents
and release usable substances, such as sugars or amino acids. In this way, the cell both protects itself against
harmful substances and obtains valuable nutrients.
13. Lysosomes
Lysosomes also do essential cleanup and recycling
inside the cell. For example, when muscle cells are
inactive, lysosomes gradually break down their
contractile proteins. (This mechanism accounts for
the reduction in muscle mass that accompanies
aging.) The process is usually precisely controlled,
but in a damaged or dead cell, the regulatory
mechanism fails. lysosomes then disintegrate,
releasing enzymes that become activated within
the cytosol. These enzymes rapidly destroy the
cell’s proteins and organelles in a process called
autolysis.
14.
15. Peroxisomes
Peroxisomes are similar to lysosomes, but smaller in structure. Peroxisomes, also called microbodies,
contain several oxidases, enzymes that can oxidize various organic substances. For instance, amino acids
and fatty acids are oxidized in peroxisomes as part of normal metabolism. In addition, enzymes in
peroxisomes oxidize toxic substances, such as alcohol. Thus, peroxisomes are very abundant in the
liver, where detoxification of alcohol and other damaging substances occurs. A by-product of the
oxidation reactions is hydrogen peroxide (H2O2), a potentially toxic compound, and associated free
radicals such as superoxide. However, peroxisomes also contain the enzyme catalase, which
decomposes H2O2. Because production and degradation of H2O2 occur within the same organelle,
peroxisomes protect other parts of the cell from the toxic effects of H2O2. Peroxisomes also contain
enzymes that destroy superoxide. Without peroxisomes, byproducts of metabolism could accumulate
inside a cell and result in cellular death. Peroxisomes can self-replicate. New peroxisomes may form
from preexisting ones by enlarging and dividing. They may also form by a process in which components
accumulate at a given site in the cell and then assemble into a peroxisome.
16. Mitochondria
Mitochondria are referred to as the “powerhouses” of the cell as they generate most of the ATP through
aerobic respiration. A cell may have as few as a hundred or as many as several thousand mitochondria,
depending on its activity. Active cells, such as those found in the muscles, liver, and kidneys, which use ATP at
a high rate, have a large number of mitochondria. For example, regular exercise can lead to an increase in the
number of mitochondria in muscle cells, and this allows muscle cells to function more efficiently.
Mitochondria are usually located within the cell where oxygen enters the cell or where the ATP is used, for
example, among the contractile proteins in muscle cells.
A mitochondrion consists of an outer mitochondrial membrane and an inner mitochondrial membrane with a
small fluid-filled space between them. Both membranes are similar in structure to the plasma membrane. The
inner mitochondrial membrane contains a series of folds called mitochondrial cristae. The central fluid-filled
cavity of a mitochondrion, enclosed by the inner mitochondrial membrane, is the mitochondrial matrix. The
elaborate folds of the cristae provide an enormous surface area for the chemical reactions that are part of the
aerobic phase of cellular respiration, the reactions that produce most of a cell’s ATP.
17. Mitochondria
The enzymes that catalyze these reactions are located on the cristae and in the matrix of the
mitochondria.
Mitochondria also play an important and early role in apoptosis, the orderly, genetically programmed
death of a cell. In response to stimuli such as large numbers of destructive free radicals, DNA damage,
growth factor deprivation, or lack of oxygen and nutrients, certain chemicals are released from
mitochondria following the formation of a pore in the outer mitochondrial membrane. One of the
chemicals released into the cytosol of the cell is cytochrome c, which while inside the mitochondria is
involved in aerobic cellular respiration. In the cytosol, however, cytochrome c and other
substances initiate a cascade of activation of protein-digesting enzymes that bring about apoptosis. Like
peroxisomes, mitochondria self-replicate, a process that occurs during times of increased cellular energy
demand or before cell division. Synthesis of some of the proteins needed for mitochondrial functions
occurs on the ribosomes that are present in the mitochondrial matrix.
18. Mitochondria
Mitochondria even have their own DNA, in the form of multiple copies of a circular DNA molecule that
contains 37 genes. These mitochondrial genes control the synthesis of 2 ribosomal RNAs, 22 transfer
RNAs, and 13 proteins that build mitochondrial components.
Although the nucleus of each somatic cell contains genes from both your mother and your father,
mitochondrial genes are inherited only from your mother. This is due to the fact that all mitochondria in
a cell are descendants of those that were present in the oocyte (egg) during the fertilization process.
The head of a sperm (the part that penetrates and fertilizes an oocyte) normally lacks most organelles,
such as mitochondria, ribosomes, endoplasmic reticulum, and the Golgi complex, and any sperm
mitochondria that do enter the oocyte are soon destroyed. Since all mitochondrial genes are inherited
from the maternal parent, mitochondrial DNA can be used to trace maternal lineage (in other words, to
determine whether two or more individuals are related through their mother’s side of the family).
19.
20.
21. STRUCTURE OF CELL
Nucleus
The nucleus is a spherical or oval-shaped structure that usually is the most prominent feature of a cell. Most
cells have a single nucleus, although some, such as mature red blood cells, have none. In contrast, skeletal
muscle cells and a few other types of cells have multiple nuclei. A double membrane called the nuclear
envelope separates the nucleus from the cytoplasm. Both layers of the nuclear envelope are lipid bilayers
similar to the plasma membrane. The outer membrane of the nuclear envelope is continuous with rough ER
and resembles it in structure. Many openings called nuclear pores extend through the nuclear envelope.
Each nuclear pore consists of a circular arrangement of proteins surrounding a large central opening that is
about 10 times wider than the pore of a channel protein in the plasma membrane. Nuclear pores control the
movement of substances between the nucleus and the cytoplasm. Small molecules and ions move through
the pores passively by diffusion. Most large molecules, such as RNAs and proteins, cannot pass through the
nuclear pores by diffusion.
22. Nucleus
Instead, their passage involves an active transport process in which the molecules are recognized and
selectively transported through the nuclear pore into or out of the nucleus. For example, proteins needed for
nuclear functions move from the cytosol into the nucleus; newly formed RNA molecules move from the
nucleus into the cytosol in this manner.
The fluid contents of the nucleus are called the nucleoplasm. The nucleoplasm contains the nuclear matrix, a
network of fine filaments that provides structural support and may be involved in the regulation of genetic
activity. The nucleoplasm also contains ions, enzymes, RNA and DNA nucleotides, small amounts of RNA, and
DNA.
Inside the nucleus are one or more spherical bodies called nucleoli that function in producing ribosomes. Each
nucleolus is simply a cluster of protein, DNA, and RNA; it is not enclosed by a membrane. Nucleoli are the sites
of synthesis of rRNA and assembly of rRNA and proteins into ribosomal subunits. Nucleoli are quite prominent
in cells that synthesize large amounts of protein, such as muscle and liver cells. Nucleoli disperse and
disappear during cell division and reorganize once new cells are formed.
23. Nucleus
Within the nucleus are most of the cell’s hereditary units, called genes, which control cellular structure and
direct cellular activities. Genes are arranged along chromosomes. Human somatic (body) cells have 46
chromosomes, 23 inherited from each parent. Each chromosome is a long molecule of DNA that is coiled
together with several proteins. This complex of DNA, proteins, and some RNA is called chromatin. The total
genetic information carried in a cell, or an organism is its genome.
In cells that are not dividing, the chromatin appears as a diffuse, granular mass. Electron micrographs reveal
that chromatin has a beads-on-a-string structure. Each bead is a nucleosome that consists of double-stranded
DNA wrapped twice around a core of eight proteins called histones, which help to organize the coiling and
folding of DNA. The string between the beads is called linker DNA, which holds adjacent nucleosomes
together. In cells that are not dividing, another histone promotes coiling of nucleosomes into a larger-
diameter chromatin fiber, which then folds into large loops. Just before cell division takes place, however, the
DNA replicates (duplicates) and the loops condense even more, forming a pair of chromatids. Shortly, during
cell division a pair of chromatids constitutes a chromosome.
