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Anatomy of Cells
Introduction to Biology
The Discovery of Cells
• In Holland, Anton van
Leeuwenhoek examined
pond water and a sample
taken from a human
mouth.
• He drew the organisms he
saw—which today we call
bacteria.
• Leeuwenhoek examined
as many types of cells as
he could. He even
observed his own semen!
Overview: The
Importance of Cells
• The early discoveries of cells are summarized in
the cell theory, a fundamental concept of biology.
• The cell theory states:
o All living things are made up of cells.
o Cells are the basic units of structure and function in living things.
o New cells are produced from existing cells.
Origin of Cellular Life
• The Earth formed about 4.6 billion years ago.
o For about 500 million years, the Earth was continually
bombarded by chunks of rock and ice in the solar system.
• The early atmosphere of Earth contained:
o
o
o
o
o
Water vapor H2O
Nitrogen N2
Carbon dioxide CO2
Methane CH4
Ammonia NH3
Origin of Cellular Life
• How did life arise from such a harsh environment?
• Two scientists designed a model of what conditions
were like on Earth at this time.
o This is called the Miller-Urey Apparatus
Miller-Urey Apparatus
• This apparatus simulated three important conditions
on Earth:
– The high amount of lightning
– Heat and gases released by volcanic activity
– Water vapor present in the atmosphere.
Results of Miller-Urey
Apparatus
• Simple compounds including water (H2O), methane
(CH4), ammonia (NH3), and hydrogen (H2) were used to
simulate the atmosphere.
• After 2 weeks, 10-15% of the carbon had been used to
form sugars, amino acids, and parts of nucleic acids.
o These simple organic compounds could have produced the
proteins, lipids, and carbohydrates that make up life today.
The First Cells
• The first life forms on Earth were likely single-celled
prokaryotic organisms.
o Prokaryotic organisms are single-celled organisms that do not
have a nucleus.
• Their DNA or RNA is usually floating freely inside the cell.
o Prokaryotic cells also do not have any membrane bound
organelles.
Pili
Nucleoid
Ribosomes
Plasma
membrane
Bacterial
chromosome
Cell wall
Capsule
0.5 µm
Flagella
A typical
rod-shaped
bacterium
A thin section through the
bacterium Bacillus
coagulans (TEM)
Parts of a Prokaryotic Cell
• Nucleoid – Area where DNA or RNA is located. Not
enclosed in a membrane like a nucleus.
• Ribosomes – Small structures that use DNA or RNA
instructions to produce proteins.
• Pili – Hollow, hair-like structures that can be used to
exchange genes.
• Flagella – Spin to produce movement.
• Cell membrane – Controls what leaves or enters the
cell
Eukaryotic Cells
• Eukaryotes are organisms with
much larger and more
complex cells than
prokaryotes.
• DNA is in a nucleus that is
bounded by a nuclear
membrane.
• Have membrane-bound
organelles
• The largest eukaryotic cells
are 0.1mm to 1.0mm in size.
Why haven’t they evolved
any larger?
LE 6-7
• Volume represents the
size of the cell.
• Surface area represents
the amount of cell
membrane to transport
food, waste, water, and
oxygen.
1
Total surface area
Total volume
Surface-to-volume
ratio
Surface area increases while
Total volume remains constant
5
1
LE 6-7
Surface area increases while
Total volume remains constant
• A cell with a volume of
1mm3 will have a total
surface area of 6mm2.
• This provides plenty of
area for the cell to
absorb what it needs.
5
1
1
Total surface area
(height x width x
number of sides x
number of boxes)
6
Total volume
(height x width x length
X number of boxes)
1
Surface-to-volume
ratio
(surface area  volume)
6
LE 6-7
• A larger cell with a
volume of 125mm3 will
only have a surface area
of 150mm2.
• This cell will not be able to
transport wastes and
nutrients fast enough.
Surface area increases while
Total volume remains constant
5
1
1
Total surface area
(height x width x
number of sides x
number of boxes)
150
Total volume
(height x width x length
X number of boxes)
125
Surface-to-volume
ratio
(surface area  volume)
1.2
LE 6-7
• If the larger cell is instead
broken down into 125
smaller cells, it will once
again have enough
surface area.
• This is why multicellular
organisms exist!
Surface area increases while
Total volume remains constant
5
1
1
Total surface area
(height x width x
number of sides x
number of boxes)
6
150
750
Total volume
(height x width x length
X number of boxes)
1
125
125
Surface-to-volume
ratio
(surface area  volume)
6
1.2
6
Cell Organization
• The eukaryotic cell can be divided into two major parts: the
nucleus and the cytoplasm.
• The cytoplasm is the fluid portion of the cell outside the
nucleus.
• Prokaryotic cells have cytoplasm as well, even though they
do not have a nucleus.
Eukaryotic Cell Anatomy
• A eukaryotic cell has internal membranes that
partition the cell into organelles.
o Organelles are small structures within cells that have specific jobs.
• Plant and animal cells have most of the same
organelles, although there are a few differences.
ENDOPLASMIC RETICULUM (ER
Nuclear envelope
Flagellum
Rough ER
Smooth ER
NUCLEUS
Nucleolus
Chromatin
Centrosome
Plasma membrane
CYTOSKELETON
Microfilaments
Intermediate filaments
Microtubules
Ribosomes:
Microvilli
Golgi apparatus
Peroxisome
Mitochondrion
Lysosome
In animal cells but not plant cells:
Lysosomes
Centrioles
Flagella (in some plant sperm)
LE 6-9b
Nuclear
envelope
NUCLEUS
Nucleolus
Chromatin
Centrosome
Rough
endoplasmic
reticulum
Smooth
endoplasmic
reticulum
Ribosomes
(small brown dots)
Central vacuole
Golgi
apparatus
Microfilaments
Intermediate
filaments
Microtubules
CYTOSKELETON
Mitochondrion
Peroxisome
Chloroplast
Plasma
membrane
Cell wall
Plasmodesmata
Wall of adjacent cell
In plant cells but not animal cells:
Chloroplasts
Central vacuole and tonoplast
Cell wall
Plasmodesmata
The Nucleus
• The nucleus contains most of the cell’s genes and is
usually the largest organelle.
• The nuclear envelope is a membrane that encloses
the nucleus, separating it from the cytoplasm.
• In the same way that the main office controls a large
factory, the nucleus is the control center of the cell.
• The nucleus contains nearly all the cell’s DNA and,
with it, the coded instructions for making proteins and
other important molecules.
The Nuclear Membrane
• The nuclear envelope is dotted with thousands of
nuclear pores, which allow material to move into and
out of the nucleus.
• The nucleus mainly contains chromatin— the cell’s
DNA instructions joined with proteins.
The Nuclear Membrane
• The nucleus also contains a small
dense region called the
nucleolus.
• The nucleolus produces
ribosomes, which are needed to
build proteins.
Organelles that Build
Proteins
• Because proteins carry out so many of the essential
functions of living things, a big part of the cell is
devoted producing and transporting them.
• Proteins are synthesized on ribosomes, which can be
found in two places:
o Freely floating in the cytoplasm
o Attached to the endoplasmic reticulum
Ribosomes: Protein
Factories
• Ribosomes are particles made of RNA and protein
o Ribosomes produce proteins by following coded instructions
that come from DNA.
o Each ribosome is like a small machine in a factory, turning out
proteins on orders that come from its DNA “boss.”
Endoplasmic Reticulum
• The endoplasmic reticulum (ER) is a huge membrane
that is connected to the nuclear membrane.
• There are two distinct regions of ER:
o Smooth ER, which lacks ribosomes
o Rough ER, with ribosomes studding its surface
Smooth Endoplasmic
Reticulum
• The smooth endoplasmic reticulum:
o
o
o
o
Synthesizes lipids
Metabolizes carbohydrates
Stores calcium
Detoxifies poison
• The smooth endoplasmic reticulum does not contain
any ribosomes, so it is unable to synthesize proteins.
Rough Endoplasmic
Reticulum
• The rough ER
o Holds ribosomes
o Produces any proteins needed by the cell.
The Golgi Apparatus
• The Golgi apparatus is a series of flattened membrane
sacs in the cytoplasm.
• Functions of the Golgi apparatus:
o Modifies, sorts, and packages materials into transport vesicles
for storage or transport out of the cell.
o A typical path for a protein produced by the cell:
o Rough ER → Golgi → Cell membrane → Released by cell
LE 6-16-1
Nucleus
Rough ER
Smooth ER
Nuclear envelope
LE 6-16-2
Nucleus
Rough ER
Smooth ER
Nuclear envelope
cis Golgi
Transport vesicle
trans Golgi
LE 6-16-3
Nucleus
Rough ER
Smooth ER
Nuclear envelope
cis Golgi
Transport vesicle
Plasma
membrane
trans Golgi
Organelles that Store,
Clean Up, and Support
• These are organelles that help the cell maintain its
shape, clean up wastes, and store material needed
later.
o Vacuoles
o Lysosomes
o Cytoskeleton
Vacuoles
• Vesicles and vacuoles are membrane-bound sacs
that store many materials.
• Plant cells often have one large central vacuole. This
fills with water, making the cell rigid.
o When they are empty and dry, plants wilt!
Lysosomes
• Lysosomes serve as the cell’s cleanup crew.
• A lysosome is full of enzymes that can digest proteins,
lipids, polysaccharides, and nucleic acids.
o Can also breakdown old organelles so they can be re-used.
Animation: Lysosome Formation
Cytoskeleton
• The cytoskeleton is a network of protein filaments that
give the cell shape.
o Can also help transport materials across the cell.
• Centrioles are part of the cytoskeleton that help move
chromosomes during cell division.
Organelles that Capture
and Release Energy
• All life requires energy.
• Organisms either can get their energy from sunlight
via photosynthesis, or by eating other organisms via
cell respiration.
• Photosynthesis occurs in chloroplasts.
• Cell respiration occurs in mitochondria.
Mitochondria
• Mitochondria are the power plants of the cell.
• They convert the chemical energy stored in food into
smaller molecules for the cell to use.
• Mitochondria have two membranes, outer and inner.
• The inner membrane is folded up to increase the
amount of surface area to do chemical reactions.
Chloroplasts
• Chloroplasts contain the green pigment
chlorophyll, as well as enzymes and other
molecules that function in photosynthesis
• Chloroplasts are found in leaves and other
green organs of plants and in algae
Plasma Membrane
• The plasma membrane is a selective barrier.
o Allows passage of oxygen, nutrients into the cell,
and waste out of the cell.
• The general structure of a biological
membrane is a double layer of phospholipids
o This allows the cell to control what goes in and
out.
Cell Wall
• The cell wall is made of cellulose and serves as
support and protection for the cell.
• Animals do not have cell walls, but plants, fungi, and
algae do.
• The cell wall is outside of the cell membrane.
Plants: Plasmodesmata
• The cell wall is so thick that oxygen, nutrients, water,
and waste cannot travel easily through.
• Plasmodesmata are channels that perforate plant cell
walls
• Through plasmodesmata, water and other small
molecules can enter the cell.
Animals: Tight Junctions,
Desmosomes, and Gap Junctions
• Although animal cells do not have cell walls, they also
have special structures within their cell membranes.
• At tight junctions, membranes of neighboring cells are
pressed together, preventing leakage of extracellular
fluid.
o Example: Lining of small intestines
• Desmosomes (anchoring junctions) fasten cells
together into strong sheets
o Example: Layers of outer skin cells
• Gap junctions (communicating junctions) provide
cytoplasmic channels between adjacent cells
o Example: Cardiac muscle cells
LE 6-31
Tight junctions prevent
fluid from moving
across a layer of cells
Tight junction
0.5 µm
Tight junction
Intermediate
filaments
Desmosome
1 µm
Space
between
cells
Gap
junctions
Plasma membranes
of adjacent cells
Gap junction
Extracellular
matrix
0.1 µm
The Cell: A Living Unit Greater Than the
Sum of Its Parts
• Cells rely on the integration of structures and
organelles in order to function
• For example, a macrophage’s ability to destroy
bacteria involves the whole cell, coordinating
components such as the cytoskeleton, lysosomes, and
plasma membrane