The Golgi Complex
The Golgi complex or Golgi apparatus consists of 3-20 flattened and stacked saclike structures called
cisternae. A complex network of tubules and vesicles is located at the edges of the cisternae. The Golgi
complex functions to:
1) sort proteins and lipids received from the ER;
2) modify certain proteins and glycoproteins;
3) sort and package these molecules into vesicles for transport to other parts of the cell or secretion
from the cell.
Mitochondria
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Mitochondria are rod-shaped structures ranging from 2 to 8 micrometers in length. They are found
throughout the cytoplasm and may account for up to 20% of the cell's volume. Mitochondria are
surrounded by two membranes. The outer membrane forms the exterior of the organelle while the inner
membrane is arranged in a series of folds called cristae to provide an enormous surface area for
chemical reactions. The space between the inner and outer mitochondrial membranes is called the
intermembrane space while the compartment enclosed by the inner mitochondrial membrane is called
the matrix. Mitochondria replicate giving rise to new mitochondria as they grow and divide. They also
have their own DNA and ribosomes.
Mitochondria function during aerobic respiration to produce ATP through oxidative phosphorylation
The respiratory enzymes and electron carriers for the electron transport system are located within the
inner mitochondria membrane. The enzymes for the citric acid cycle (Krebs cycle) are located in the
matrix.
Chloroplasts
Chloroplasts are disk-shaped structures ranging from 5 to 10 micrometers in length. Like mitochondria,
chloroplasts are surrounded by an inner and an outer membrane. The inner membrane encloses a fluidfilled region called the stroma that contains enzymes for the light-independent reactions of
photosynthesis. Infolding of this inner membrane forms interconnected stacks of disk-like sacs called
thylakoids, often arranged in stacks called grana. The thylakoid membrane, that encloses a fluid-filled
thylakoid interior space, contains chlorophyll and other photosynthetic pigments as well as electron
transport chains. The light-dependent reactions of photosynthesis occur in the thylakoids. The outer
membrane of the chloroplast encloses the intermembrane space between the inner and outer chloroplast
membranes.
The thylakoid membranes contain several pigments capable of absorbing visible light. Chlorophyll is
the primary pigment of photosynthesis. Chlorophyll absorbs light in the blue and red region of the
visible light spectrum and reflects green light. There are two major types of chlorophyll, chlorophyll a
that initiates the light-dependent reactions of photosynthesis, and chlorophyll b, an accessory pigment
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that also participates in photosynthesis. The thylakoid membranes also contain other accessory
pigments. Carotenoids are pigments that absorb blue and green light and reflect yellow, orange, or red.
Phycocyanins absorb green and yellow light and reflect blue or purple. These accessory pigments
absorb light energy and transfer it to chlorophyll.
They are found in plant cells and algae. Like Mitochondria, chloroplasts are surrounded by two
membranes. The outer membrane forms the exterior of the organelle while the inner membrane folds to
form a system of interconnected disclike sacs called thylakoids. The thylakoids are arranged in stacks
called grana. The space enclosed by the inner chloroplast membrane is called the stroma. Chloroplasts
replicate giving rise to new chloroplasts as they grow and divide. They also have their own DNA and
ribosomes.
The thylakoid membranes contain the pigments chlorophyll and carotenoids, as well as enzymes and
the electron transport chains used in photosynthesis (def), a process that converts light energy into the
chemical bond energy of carbohydrates. Energy trapped from sunlight by chlorophyll is used to excite
electrons in order to produce ATP by photophosphorylation. The light-dependent reactions that trap
light energy and produce the ATP and NADPH needed for photosynthesis occur in the thylakoids. The
light-independent reactions of photosynthesis use this ATP and NADPH to produce carbohydrates from
carbon dioxide and water, a series of reactions that occur in the stroma of the chloroplast.
Lysosomes, Peroxisomes, Vacuoles, and Vesicles
1. Lysosomes
Lysosomes, synthesized by the endoplasmic reticulum and the Golgi complex, are membrane-enclosed
spheres typically about 500 nanometers in diameter that contain powerful digestive enzymes. They
function to digest materials that enter by endocytosis. The enzymes are called acid hydrolases because
the function best at a slightly acid pH, maintained by pumping protons into the lysosome. During
endocytosis, the cytoplasmic membrane invaginates and pinches off placing the ingested material in a
vesicle or vacuole called an endosome. Primary lysosomes fuse with the endosome forming a
secondary lysosome where the materials within are digested.
2. Peroxisome
Peroxisomes are membrane-bound organelles containing an assortment of enzymes that catalyze a
variety of metabolic reactions.
3. Proteasome
Proteasomes are cylindrical complexes that use ATP to digest proteins into peptides .
4. Vacuoles and Vesicles
Vacuoles are large membranous sacs; vesicles are smaller. Vacuoles are often used to store materials
used for energy production such as starch, fat, or glycogen. Plant cells often contain large vacuoles
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filled with water. Vacuoles and vesicles also transport materials within the cell and form around
particles that enter by endocytosis .
Ribosome
Ribosomes are the protein-synthesizing machines of the cell.
They translate the information encoded in messenger RNA (mRNA) into a polypeptide.
Ribosomes are
1- roughly spherical.
2- With a diameter of ~20 nm, they can be seen only with the electron microscope. The image on the
right is an electron micrograph showing clusters of ribosomes. These clusters, called polysomes, are
held together by messenger RNA (mRNA).
3- They can make up 25% of the dry weight of cells that specialize in protein synthesis.
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
In eukaryotes,

Ribosomes that synthesize proteins for use within the cytosol (e.g., enzymes of glycolysis) are
suspended in the cytosol.
Ribosomes that synthesize proteins destined for:
o secretion (by exocytosis)
o the plasma membrane (e.g., cell surface receptors)
o lysosomes

are attached to the cytosolic face of the membranes of the endoplasmic reticulum. As the
polypeptide is synthesized, it is extruded into the interior of the endoplasmic reticulum.
Then, before these proteins reach their final destinations, they undergo a series of processing
steps in the Golgi apparatus.

Ribosomes that synthesize 13 of the proteins destined for the inner membrane of mitochondria
are found within the mitochondrion itself and are quite different in structure from the others.
The ribosomes of bacteria, eukaryotes, and mitochondria differ in many details of their structure.
This table gives some of the data. (S values are the sedimentation coefficient: a measure of the rate at
which the particles are spun down in the ultracentrifuge. S values are not additive. nts = nucleotides.)
Comparison of Ribosome Structure in Bacteria, Eukaryotes, and
Mitochondria
Mitochondrial
Bacterial (70S) Eukaryotic (80S)
(55S)
Large Subunit 50S
60S
39S
23S (2904 nts) 28S (4700 nts) 16S (1560 nts)
rRNAs
5S (120 nts) 5S (120 nts)
(1 of each)
5.8S (160 nts)
Proteins
33
~49
48
Small Subunit 30S
40S
rRNA
16S (1542 nts) 18S (1900 nts)
Proteins
20
~33
5
28S
12S (950 nts)
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The Cytoskeleton
The cytoskeleton is a network of microfilaments, intermediate filaments, and microtubules. The
cytoskeleton functions to:
1) give shape to cells lacking a cell wall;
2) allow for cell movement, the crawling movement of white blood cells and amoebas or the
contraction of muscle cells;
3) movement of organelles within the cell and endocytosis;
4) cell division, the movement of chromosomes during mitosis and meiosis and the constriction of
animal cells during cytokinesis.
a. Microtubules
Microtubules are hollow tubes made of subunits of the protein tubulin. They provide structural support
for the cell and play a role in cell division, cell movement, and movement of organelles within the cell.
Microtubules are components of centrioles, cilia, and flagella.
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b. Microfilaments
Microfilaments are solid, rod like structures composed of actin. They provide structural support, and
play a roll in phagocytosis, cell and organelle movement, and cell division.
c. Intermediate filaments
Intermediate filaments are tough fibers made of polypeptides. They help to strengthen the cytoskeleton
and stabilize cell shape.
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d. Centrioles
Centrioles are located near the nucleus and appear as cylindrical structures consisting of a ring of nine
evenly spaced bundles of three microtubules. Centrioles play a role in the formation of cilia and
flagella. During animal cell division, the mitotic spindle forms between centrioles.
Flagella and Cilia
Flagella are long and few in number whereas cilia are short and numerous. Both consist of 9 fused pairs
of protein microtubules with side arms of the motor molecule dynein that originate from a centriole.
These form a ring around an inner central pair of microtubules that arise from a plate near the cell
surface . The arrangement of microtubules is known as a 2X9+2 arrangement. This complex of
microtubules is surrounded by a sheath continuous with the cytoplasmic membrane. In the presence of
ATP, the dynein side arms of the microtubules in the outer ring elongate and attempt to move along the
neighboring pair, causing the flagellum or the cilium to bend. Flagella and cilia function in locomotion.
Cilia also function to move various material that may surround a cell.
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Vacuole:-
A vacuole is a membrane bound organelle which is present in all plant and fungal cells and some,
animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water
containing inorganic and organic molecules including enzymes in solution, though in certain cases they
may contain solids which have been engulfed. Vacuoles are formed by the fusion of multiple membrane
vesicles and are effectively just larger forms of these. The organelle has no basic shape or size, its
structure varies according to the needs of the cell.
The function and importance of vacuoles varies greatly according to the type of cell in which they are
present, having much greater prominence in the cells of plants, fungi and certain protists than those of
animals and bacteria. In general, the functions of the vacuole include:






Isolating materials that might be harmful or a threat to the cell
Containing waste products
Maintaining internal hydrostatic pressure within the cell
Maintaining an acidic internal pH
Exporting unwanted substances from the cell
Allows plants to support structures such as leaves and flowers due to the pressure of the central
vacuole
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THE ENDOSYMBIOTIC THEORY
It is thought that life arose on earth around four billion years ago. The endosymbiotic theory states that
some of the organelles in today's eukaryotic cells were once prokaryotic microbes.
In this theory, the first eukaryotic cell was probably an amoeba-like cell that got nutrients by
phagocytosis and contained a nucleus that formed when a piece of the cytoplasmic membrane pinched
off around the chromosomes. Some of these amoeba-like organisms ingested prokaryotic cells that then
survived within the organism and developed a symbiotic relationship. Mitochondria formed when
bacteria capable of aerobic respiration were ingested; chloroplasts formed when photosynthetic bacteria
were ingested. They eventually lost their cell wall and much of their DNA because they were not of
benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.
Evidence for this is based on the following:
1. Chloroplasts are the same size as prokaryotic cells, divide by binary fission, and, like bacteria, have
Fts proteins at their division plane. The mitochondria are the same size as prokaryotic cells, divide by
binary fission, and the mitochondria of some protists have Fts homologs at their division plane.
2. Mitochondria and chloroplasts have their own DNA that is circular, not linear.
3. Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S
and 60S.
4. Several more primitive eukaryotic microbes, such as Giardia and Trichomonas have a nuclear
membrane but no mitochondria.
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