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Lecture 2
Cell Biology
Eukaryotic cell structure:
Structure and function of cell organelles
Prepared by Mayssa Ghannoum
Structure and function of cell organelles
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All living organisms are made up of cells, and all living cells
come from preexisting cells.
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The cell: is the basic structural and functional unit of every
living organism.
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An organelle: is a specialized subunit within a cell that has a
specific function, and is usually separately enclosed within its
own lipid bilayer (membrane).
It is called “ little organ”, very small in size
Can only be observed under electron microscope.
Found only in eukaryotic cells.
Protoplasm
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The protoplasm of a cell consists of nucleus, cell
membrane and cytoplasm.
Cytoplasm is a part of the protoplasm of a cell.
The protoplasm is bound by a plasma membrane or cell
membrane on all sides whereas
cytoplasm is the substance which is present surrounding the
nucleus within a cell.
Protoplasm is composed of a mixture of small molecules
such as ions, amino acids, monosaccharides, water and
macromolecules such as nucleic acids, proteins, lipids and
polysaccharides
Protoplasm
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Inside the cell, the space between the organelles is a
semifluid, jellylike substance called cytosol.
The Nucleus: Information central
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Contains most of the genes in the eukaryotic cell.
Usually one per cell.
Control the normal activities of the cell.
The most conspicuous organelle, about 5 µm in
diameter.
Bounded by a nuclear envelope, separating its contents
from the cytoplasm.
The nuclear envelope is a double membrane.
The two membranes, each a lipid bilayer with
associated proteins.
The envelope is perforated by pore structures.
• Within the nucleus, the DNA is organized
into units called chromosomes, structures
that carry the genetic information.
• Each chromosome is made up of a material
called chromatin, a complex of proteins
and DNA.
• The nucleus directs protein synthesis by
synthesizing messenger RNA (mRNA)
according to instructions provided by the
DNA.
• The mRNA is then transported to the
cytoplasm via nuclear pores. Once it’s in
the cytoplasm, ribosomes translate it into a
specific polypeptide.
DNA  RNA  protein.
Ribosomes: Protein Factories
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Are complexes made of ribosomal RNA and protein.
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They Carry out protein synthesis.
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They have two cytoplasmic locales:
1. free ribosomes are suspended in the cytosol
2. bound ribosomes are attached to the rough endoplasmic
reticulum.
The Endoplasmic Reticulum
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The Endoplasmic reticulum membrane separates the
internal compartment of the ER, called ER lumen (cavity)
from the cytosol.
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The ER membrane is continuous with the nuclear
envelope.
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There are two distinct, connected regions of the ER that
differ in structure and function: Smooth ER and Rough
ER.
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Smooth ER: Is so named because its outer surface lacks
ribosomes. Processing and transport of lipids.
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Rough ER: has ribosomes on the outer surface of the
membrane.
Transport of protein.
The Golgi apparatus: Shipping and Receiving Center
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Center of manufacturing, warehousing, sorting and shipping.
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Proteins are modified and stored and then sent to other
destinations.
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Looks like a stack of plates.
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Molecules are transported to and from the Golgi by means
of vesicles.
Lysosomes: Digestive compartments
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Is a membranous sac of hydrolytic enzymes that an
animal cell uses to digest macromolecules, break
wastes down and help recycle large proteins.
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Lysosomal enzymes work best in the acidic
environment found in lysosomes.
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Hydrolytic enzymes and lysosomal membrane are
made by rough ER and then transferred to Golgi
apparatus for further processing.
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Garbage disposal of the cell.
Vacuoles
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Storage container for water, food, enzymes, wastes,
pigments, etc.
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Plant cells contain large central vacuole which can
play many roles:
- Can hold reserves of important organic compounds
- Disposal site for metabolic by-products that would
endanger the cell if they accumulate.
- Some vacuoles contain pigments that color the cell.
- May help protect the plant against predators.
- Has a major role in the growth of plant cells.
Mitochondria: Chemical energy conversion
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Sites of cellular respiration, the metabolic process that
generates ATP by extracting energy from sugars, fats, and
other fuels with the help of oxygen.
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Enclosed by two membranes, each a phospholipid bilayer
with a unique collection of embedded proteins.
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The outer membrane is smooth
the inner membrane is convoluted, with infoldings called
cristae.
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Power house of the cell.
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Has its own strand of DNA
Chloroplasts: Capture of light energy
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Found only in plant cells and algae.
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Sites of photosynthesis.
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They convert solar energy to chemical energy by absorbing
sunlight and using it to drive the synthesis of organic
compounds such as sugars from CO2 and water.
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Contain the green pigment:
Chlorophyll.
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Bound by double membrane.
Peroxisomes: Oxidation
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Specialized metabolic compartments
bounded by a single membrane.
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Roughly spherical and have a granular
core that is a dense collection of enzyme
molecules.
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Contain enzymes that transfer hydrogen
from various substrates to oxygen (O2).
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Produce hydrogen peroxide (H2O2) as a
by-product.
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Peroxisomes have many functions:
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Some peroxisomes use oxygen to break down fatty acids
down into smaller molecules that can be then transported
to mitochondria as fuel for cellular respiration.
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Peroxisomes in the liver detoxify alcohol and other
harmful compounds by transferring hydrogen from the
poisons to oxygen.
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H2O2 formed by peroxisomes is itself toxic, but the
organelle also contains an enzyme that converts H2O2 to
water.
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Peroxisomes may increase in number by splitting in two
when they reach a certain size.
The cytoskeleton
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Network of fibers extending throughout the cytoplasm.
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Plays a major role in organizing the structures and activities of
the cell.
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It gives mechanical support to the cell and maintain its shape.
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It’s especially important for animal cells, which lack walls.
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The strength and resilience of the cytoskeleton is based on its
architecture.
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It provides anchorage for many organelles and cytosolic
enzymes.
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The cytoskeleton is more dynamic than an animal
skeleton.
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Functions in cell motility (changes in cell location).
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Involved in the regulation of biochemical activities in the
cell in response to mechanical stimulation.
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Composed of three types of molecular structures:
microtubules, microfilaments, and intermediate filaments.
Cytoskeleton
Microtubules
Thick
Responsible for cell
motility, and separation
of chromosome during
cell division.
(Tubulin protein)
Microfilaments
Thin
Support cell motility
and transport
materials within the
cell.
(Actin protein)
Intermediate
filaments
Middle
Reinforcing the cell
shape and fixing
position of organelles.
(Fibrous protein)
Centrosomes and Centrioles
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Found only in animal cells.
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Centrosome is a region located near the nucleus and
considered “microtubule-organizing center”.
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Within the centrosome are a pair of centrioles, each
composed of 9 sets of triplet microtubules arranged in a ring.
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Before an animal cell divides,
the centrioles replicate.
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Aids in cell division.
Cilia and flagella
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They both act as locomotors appendages.
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When cilia or flagella extend from cells that are held in
place as part of a tissue layer, they can move fluid over the
surface of the tissue.
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Motile cilia usually occur in large number on the cell
surface.
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Flagella are usually limited to just one or few per cell.
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Cilia are about 0.25 µm in diameter and 2-20 µm long.
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Flagella are the same diameter but longer, 10-200 µm.
A comparison of the beating of flagella and cilia
Motion of flagella:
a flagellum usually undulates,
it’s snakelike motion driving a
cell in the same direction as the
axis of the flagellum.
E.g.: propulsion of a human
sperm cell.
Motion of cilia:
cilia have a back-and-forth
motion. The rapid power
stroke moves the cell in a
direction perpendicular to the
axis of the cilium. Then,
during slower recovery
stroke, the cilium bends and
sweeps sideways, closer to
the surface.