Cell is the base of life
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How can one define life?
The simplest definition is that any living thing must have
three general properties:
• metabolism
• growth
• reproduction
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Life's Hierarchy
Life on Earth is incredibly extensive and, to make it easier
to study, biologists have broken living systems up into
generalized hierarchical levels:
molecules;
organells;
cells;
tissues;
organs;
organisms;
populations
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Life's Hierarchy
The focus of this course is on the fundamentals of life;
that is, the properties that are held in common among all
living things.
We will concentrate almost exclusively on the molecular
through the cellular level.
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The cell theory
1. Cells are the fundamental units of life, because a cell is
the simplest unit capable of independent existence.
2. All living things are made of cells.
Mattias Schleiden and Theodor Schwann (1839) called cells
“units of life” – cell theory.
This theory still holds true, with the minor caveat that
viruses are only alive while infecting a cell.
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Cell elemental composition
Cells are 90% water.
The remaining molecules, the dry weight is
approximately:
• 50% protein
•
• 15% carbohydrate
• 15% nucleic acid
• 10% lipid
• 10% miscellaneous
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Similarities between the pro- and
eukaryotic types of cells
1. They both have DNA as their genetic material.
2. They are both membrane bound.
3. They both have ribosomes.
4. They have similar metabolism.
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Major differences
1. Eukaryotes have a nucleus and membrane-bound
organelles, while prokaryotes do not.
2. Prokaryotic DNA floats freely around the cell;
eukaryotic is held within its nucleus.
3. Eukaryotic/prokaryotic size is 10/1.
4. The DNA of eukaryotes is much more complex.
5. Prokaryotes have a cell wall composed of
peptidoglycan, a polymer of amino acids and sugar.
Some eukaryotic cells also have cell walls, but none
made of peptidoglycan.
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Eukaryote cells
(protozoa, fungi, higher animals and plants)
Each eukaryote has
•a surrounding membrane - a thin layer of protein and fat
that restricts the flow of substances in and out of the cell and
encloses the
•cytoplasm, a jellylike material containing the nucleus and
other structures (organelles) such as mitochondria.
•the nuclei of some cells contain a dense spherical structure
called the nucleolus, which contains ribonucleic acid (RNA)
for the synthesis of ribosomes.
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Animal cell
structure
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Liver cell
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Liver cell nucleus and nucleolus
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Plant cell
structure
The plant cell is
surrounded by:
a plasma membrane
and contains:
a nucleus,
ribosomes,
ER,
Golgi apparatus,
mitochondria,
peroxisomes,
microfilaments and
microtubules.
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Plant cell
structure
Plant cell also contains a
family of membraneenclosed organelles called
plastids.
The most important type of
plastid is the chloroplast,
which carries out
photosynthesis, converting
sunlight to chemical energy
stored in sugar and other
organic molecules.
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Plant cell
structure
Another prominent
organelle in many plant
cells, especially older
ones, is a large central
vacuole.
The vacuole stores
chemicals, breaks down
macromolecules, and, by
enlarging, plays a major
role in plant growth.
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Plant cell
structure
The vacuole membrane
is called the tonoplast.
Outside a plant cell's
plasma membrane (in
fungi and some protists
as well) is a thick cell
wall.
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Plant cell
structure
It helps to maintain the
cell's shape and protects
the cell from mechanical
damage.
The cytosol of adjacent
cells connects through
trans-wall channels called
plasmodesmata.
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Nucleus
structure
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The nuclear envelope
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Nucleus with Nuclear Pores
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Ribosomes
Ribosomes are the sites of protein synthesis.
They are not membrane-bound and thus occur in both
prokaryotes and eukaryotes.
Eukaryotic ribosomes are slightly larger than prokaryotic
ones.
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Ribosomes
Structurally the ribosome consists of a small and larger
subunit.
Biochemically the ribosome consists of ribosomal RNA
(rRNA) and some 50 structural proteins.
Often ribosomes cluster on the endoplasmic reticulum, in
which case they resemble a series of factories adjoining a
railroad line.
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Structure of the ribosome
Small subunit
Large subunit
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Ribosomes
Both free and bound ribosomes are abundant in this
pancreas cell (TEM).
The pancreas secretes hormones, including the protein
insulin, into the bloodstream, and secretes digestive
enzymes into the intestine.
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Ribosomes
Bound ribosomes, those presently attached to the
endoplasmic reticulum (ER), produce secretory proteins.
Free ribosomes mainly make proteins that will remain
dissolved in the cytosol.
Bound and free ribosomes are identical and can alternate
between these two roles.
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Ribosomes and Polyribosomes in liver cell
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The Endomembrane system
A membranous system of interconnected tubules and
cisternae
Membranes of the endomembrane system vary in structure,
composition, thickness and behavior
The endomembrane system includes:
Nuclear envelope
Endoplasmatic reticulum
Golgi apparatus
Lysosomes
Vacuoles
Plasma membrane (related to endomembrane)
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The Endomembrane system
A membranous system of
interconnected tubules and flattened
sacs called cisternae, the ER is also
a continuous with the nuclear
envelope.
The membrane of the ER encloses a
compartment called the cisternal
space.
Rough ER, which is studded on its
outer surface with ribosomes, can be
distinguished from smooth ER in the
electron micrograph (TEM).
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The Endomembrane system
Endoplasmic reticulum
is a mesh of interconnected membranes that serve a
function involving protein synthesis and transport.
Membranes in the endomembrane system are related either
through:
-direct physical continuity
or
-by the transfer of membrane segments as tiny vesicles.
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Endoplasmatic reticulum (ER)
cisternae
cisternal space
rough ER
ribosomes
smooth ER
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Endoplasmatic reticulum
Rough endoplasmic reticulum (Rough ER) is so-named
because of its rough appearance due to the numerous
ribosomes that occur along the ER.
Rough ER
connects to the nuclear envelope through which the
messenger RNA (mRNA) that is the blueprint for
proteins travels to the ribosomes.
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Rough ER
Newly synthesized proteins fold into its native conformation
Most of the secretory proteins are glycoproteins (proteins
covalently bound to carbohydrates
Secretory proteins are kept separate from the other
proteins to be remained in cytosol
Secretory proteins leave the ER in transport vesicles
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Rough ER
Another very important function of rough ER is the
“production of the membrane”
Proteins and phospholipids are added to the membrane
They are either transported to rough ER or synthesized in
rough ER (phospholipids)
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Smooth ER
Smooth ER
lacks the ribosomes characteristic of Rough ER and
is thought to be involved in transport and metabolic
processes including:
•synthesis of lipids (phospholipids and steroids),
•metabolism of carbohydrates, and
•detoxification of drugs and poisons
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Smooth ER
Among the steroids produced by SER are:
the sex hormones secreted by the adrenal glands;
testes and ovaries are rich in smooth ER
Role of smooth ER in carbohydrate metabolism is well
presented by liver cells:
liver cells store carbohydrate in the form of glycogen;
phosphate molecule is removed from the glucose which
can then live the cell
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Smooth ER
Detoxification of drugs in liver cells mainly occur by adding
hydroxyl groups to drugs.
Hydroxylation makes drugs more soluble and more easier
to flush from liver cells
Unfortunately the exposure to barbiturates or alcohol forces
the growth of smooth ER and subsequent increase
tolerance to one of these agents
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Summary
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Summary
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Reading
Campbell et al. Biology. Ch. 6 A tour of the cell, 110-107.
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