Download What is a cell?

What is a cell?
• All living things possess levels of organization.
Organism
Organ System
Organ
Tissue
Cell
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What is a Cell?
• An organism can be unicellular,
unicellular consisting of a single
cell, or multicellular,
multicellular consisting of many cells.
• Cells are the building blocks of all organisms.
– They are the smallest living parts of an organism.
– They are the smallest structures that can be
classified as living.
The Cell Theory
• Cells are very small, so small that they were not
discovered until microscopes were invented in
the mid-seventeenth century.
• In 1665, Robert Hooke examined a piece of cork
using a basic, homemade microscope.
• Hooke observed a
matrix of tiny rooms,
which he called cells.
• Another scientist,
Antoni van
Leeuwenhoek, was the
first to observe and
describe unicellular
organisms.
The Cell Theory
• Advances in microscopy led to the Cell Theory
• The theory states that:
– A cell is the most basic unit of life
– All living things are composed of cells
– All cells arise from pre-existing cells
Why are cells so small?
• The maximum size of a cell is limited by the
amount of surface area it needs to obtain
nutrients and to exchange gases with its
environment.
• In the center of every cell
is the machinery or organelles
(e.g. DNA, mitochondria,
ribosomes,…) that the cell
needs to function.
• All of these components must
communicate with one another.
Surface Area to Volume Ratio
• A cell’s surface provides the
only way to pass substances
into and out of the cell.
• As a cell’s size increases, both
its surface area and volume
increase, as well.
– Surface area is calculated
by squaring the radius.
– Volume is calculated by
cubing the radius.
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Surface Area to Volume Ratio
• Both surface area and volume increase as cell size
increases but volume increases faster than surface
area.
• For a large cell, the ratio between its surface area
and its volume is too small to permit efficient transfer
of materials.
• For a small cell, the ratio between its surface area
and its volume is large enough to permit efficient
transfer of materials.
Prokaryotes vs. Eukaryotes
• All organisms can be divided into two groups,
based on their cell type:
– Prokaryotes
– Eukaryotes
Prokaryotes vs. Eukaryotes
• Prokaryotes are the simplest organisms and
have the simplest cells.
• All Prokaryotes are unicellular and belong to
the domain
– Archaea
or
– Bacteria
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Prokaryotes vs. Eukaryotes
• Eukaryotes include unicellular and multi-cellular
organisms.
• Eukaryotes belong to
the domain Eukarya
– Kingdom Plantae
– Kingdom Fungi
– Kingdom Animalia
– Kingdom Protista
Prokaryotic Cells
• Prokaryotic cells are structurally simpler, and
smaller than eukaryotic cells.
• A prokaryotic cell lacks a nucleus, the membranebound organelle where DNA is stored in eukaryotes.
• A prokaryotic cell is enclosed by a plasma
membrane, but has no distinct interior components.
– No membrane-bound organelles, but do contain
ribosomes.
Eukaryotic Cells
• Eukaryotic cells are larger and structurally more
complex than prokaryotic cells.
• The interior of a eukaryotic cell is divided into
functional compartments.
• Membrane-bound organelles perform specialized
functions within the cell and are connected via an
extensive internal membrane system.
• The organelles are anchored in place by a
cytoskeleton.
Eukaryotic Cells
• Eukaryotic cells are further divided into:
– Plant cells
– Animal cells
Ribosomes
• Ribosomes carry out protein synthesis via the
genetic instructions encoded in the DNA.
• A single cell can contain a few million
ribosomes!
The Nucleus
• One of the most important organelles in a
eukaryotic cell is the nucleus.
• The nucleus is the cell’s control center.
• The nucleus contains most of
the cell’s DNA and controls the
cell’s activities by directing
protein synthesis.
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The Endoplasmic Reticulum
• The endoplasmic reticulum (ER) is an extensive
system of internal membranes surrounding the
nucleus.
• The ER is further differentiated into:
– Rough endoplasmic reticulum
– Smooth endoplasmic reticulum
• The surface of the rough ER is
studded with ribosomes, while
the smooth ER lacks ribosomes.
The Endoplasmic Reticulum
• The ribosomes bound to the rough
ER produce proteins which enter
the ER membrane and are
transported to other organelles or
secreted by the cell.
• The smooth ER is embedded with
enzymes that aid in the production
of lipids and carbohydrates.
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• Proteins and lipids manufactured on the ER
membranes are transported through ER channels and
packaged into transport vesicles that bud off the ER.
• After biomolecules are produced in the ER, they are
passed along to flattened stacks of membranes known
as Golgi bodies.
• The number of Golgi bodies in a cell ranges from one
or a few in protists, to 20 or more in animal cells, and
several hundred in certain plant cells!
• The vesicles fuse with the Golgi bodies, releasing their
contents.
The Golgi Complex
• Collectively, these Golgi bodies are referred to as the
Golgi complex or Golgi apparatus.
• One side of the Golgi complex receives, sorts,
chemically alters and packages important molecules
manufactured by the ER.
• Within the Golgi complex, many of the proteins and
lipids are tagged with carbohydrates.
• The other side of the Golgi complex serves as a
shipping facility, where vesicles bud off to transport
the prepared materials elsewhere.
Lysosomes
• Organelles known as lysosomes emerge from
the Golgi complex containing a concentrated mix
of enzymes manufactured in the rough ER to
break down macromolecules within the cell.
• Lysosomes are the recycling centers of the cell,
digesting worn-out organelles and cellular
components.
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Mitochondria: ATP Production
• Eukaryotic organisms extract energy from organic
molecules in a process known as cellular respiration,
whereby the chemical energy of food (e.g. glucose) is
converted into the chemical energy of ATP.
• ATP or adenosine triphosphate is the main energy
source for cellular work.
– the molecular currency by which
energy is transferred within
cells for metabolism.
Mitochondria: ATP Production
• The mitochondria are sausage-shaped
organelles, bound by two membranes.
• The outer membrane of the mitochondrion is
smooth, while the inner membrane is bent into
numerous folds called cristae.
• The cristae and the
resulting inner matrix and
surface area make oxidative
metabolism possible.
• The unique characteristics of the mitochondria
provide evidence for endosymbiosis.
• Mitochondria are believed to have once existed
as free-living prokaryotes which were engulfed
and retained by ancient eukaryotic cells
approximately 1.5 billion years ago.
• Chloroplasts, the organelles
responsible for
photosynthesis, are also
believed to have evolved
via endosymbiosis.
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The Cytoskeleton
• Organelles do not float freely within the cell.
• Rather, they are supported and transported by a
dense network of protein fibers called the
cytoskeleton.
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The Cytoskeleton
• The cytoskeleton:
– anchors organelles to fixed locations in the cell.
– provides a framework that supports the shape of
the cell.
– organizes cellular activities by providing a scaffold
that anchors ribosomes and enzymes.
– acts as tracks along which organelles can move.
Protein Fibers of the Cytoskeleton
• The protein fibers of the cytoskeleton are
constantly being formed and disassembled.
• There are three different types of protein fibers:
– Microfilaments
– Intermediate
filaments
– Microtubules
Plant Cells
• Plant cells, as well as those of many protists,
have a sturdy cell wall that surrounds
their plasma membrane.
• The cell wall is made of
cellulose.
• Animal cells do not
possess cell walls.
Plasma membrane
Plant Cells
• Plant cells (and photosynthetic protists) also
possess chloroplasts, the organelle responsible
for photosynthesis.
• Animal and fungal cells
do not contain
chloroplasts, and are
therefore not capable of
performing photosynthesis.
Plant Cells
• Plant cells (and some protist cells) also possess a
large central vacuole.
• The central vacuole may store
water, chemicals), or
waste products.
• The central vacuole usually
occupies ~30% of the cell’s
volume, but can occupy as
much as 80% in specialized cells.
• Central vacuoles help maintain pressure
against the cell wall, which keeps the plant in an
upright position, and pushes the chloroplasts
closer to the membrane, and so closer to the light!
Animal Cells
• In contrast, animal cells do not possess cell
walls, chloroplasts, or central vacuoles.
• Instead, animal cells contain
food vacuoles, which are
involved in the engulfing
of food particles, while
other vacuoles store and
transport lipids and proteins
out of the cell.
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All Cells Contain…
• Regardless of cell type (prokaryotic or
eukaryotic, plant or animal, etc), ALL cells
contain:
– DNA
– Ribosomes
– Plasma membrane