The Cell: The Basic Unit of Life

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The Cell The Basic Unit of Life

• Life requires a structural compartment separate
  from the external environment in which
  macromolecules can perform unique functions in a
  relatively constant internal environment.
• These living compartments are cells.

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The Cell The Basic Unit of Life

• The cell theory states that
• Cells are the fundamental units of life.
• All organisms are composed of cells.
• All cells come from preexisting cells.

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The Cell The Basic Unit of Life

• Protobionts are aggregates produced from
  molecules made in prebiotic synthesis
  experiments. They can maintain internal chemical
  environments that differ from their surroundings.
• Laboratory experiments suggest a bubble theory
  for the origin of cells.

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The Cell The Basic Unit of Life

• Cell size is limited by the surface
  area-to-volume ratio.
• The surface of a cell is the area that interfaces
  with the cells environment.
• The volume of a cell is a measure of the space
  inside a cell.

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• Surface area-to-volume ratio-
• the surface area divided by the volume
• As cell increases in volume, its surface area
  also increases, but not to the same extent

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• The volume of a cell determines the amount of
  chemical activity it can carry out
• The surface area determines the amount of
  substances the cell can take in and amount of
  waste it can release.

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• Cells are small in volume to maintain a large
  surface area-to-volume ratio
• Large organisms consist of many small cells
  instead of a few huge cells

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The Cell The Basic Unit of Life

• Because most cells are tiny, with diameters in
  the range of 1 to 100 ?m, microscopes are needed
  to visualize them.
• With normal human vision the smallest objects
  that can be resolved (i.e., distinguished from
  one another) are about 200 ?m (0.2 mm) in size.

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Microscopes are needed to visualize cells

• Invention of microscope in the 1600s led to the
  discovery of cells.
• Robert Hooke, described cells in 1665, using a
  light microscope

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• Light microscopes use glass lenses to focus
  visible light and typically have a resolving
  power of 0.2 ?m.
• Resolving power- the distance apart 2 objects
  must be in order for them to be distinguished as
  separate objects
• Resolving power of light microscopes are about
  1000 times that of the human eye

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Electron microscope

• Developed in 1950s
• Instead of light, electron microscopes use
  electron beams which have a shorter wavelength
  than that of light
• the resulting image resolution is far greater
  (about 0.5 nm)
• Enhanced magnification and resolution allow
  researchers to identify subcellular organelles
• Resolution about 400,000 that of the human eye

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Figure 4.2 The Scale of Life
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Figure 4.2 The Scale of Life
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Cells show two organizational patterns

• Prokaryotes have no nucleus or other
  membrane-enclosed compartments. They lack
  distinct organelles.
• Eukaryotes have a membrane-enclosed nucleus and
  other membrane-enclosed compartments or
  organelles as well.

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Prokaryotic Cells

• Prokaryotes inhabit the widest range of
  environmental extremes.
• Hot springs-thermophiles
• Dead Sea-halophiles
• Prokaryotic cells are generally smaller than
  eukaryotic cells.
• Each prokaryote is a single cell, but many types
  can be found in chains or clusters.

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Prokaryotic Cells

• Features shared by all prokaryotic cells
• All have a plasma membrane.
• All have a region called the nucleoid where the
  DNA is concentrated.
• The cytoplasm (the plasma-membrane enclosed
  region) consists of the nucleoid, ribosomes, and
  a liquid portion called the cytosol.

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Figure 4.5 A Prokaryotic Cell
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Prokaryotic Cells

• Specialized features of some prokaryotic cells
• A cell wall just outside the plasma membrane.
• Some bacteria have another membrane outside the
  cell wall, a polysaccharide-rich phospholipid
  membrane.
• Some bacteria have an outermost slimy layer made
  of polysaccharides and referred to as a capsule.

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Prokaryotic Cells

• Photosynthetic membrane
• Some bacteria, including cyanobacteria, can carry
  on photosynthesis. The plasma membrane is
  infolded (to form an internal membrane system)
  and has chlorophyll.
• Flagella
• Some bacteria have flagella, locomotory
  structures shaped like a corkscrew.
• Pili
• Some bacteria have pili, threadlike structures
  that help bacteria adhere to one another during
  mating or to other cells for food and protection.

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Eukaryotic Cells

• Eukaryotes,
• have a membrane-enclosed nucleus in each of their
  cells.
• Include animals, plants, fungi, and protists,

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• Eukaryotic cells
• tend to be larger than prokaryotic cells.
• have a variety of membrane-enclosed compartments
  called organelles.
• have a protein scaffolding called the
  cytoskeleton.

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Compartmentalization is the key to eukaryotic
cell function.

• Each organelle or compartment has a specific
  role defined by chemical processes.
• Membranes surrounding these organelles 1) Keep
  away inappropriate molecules 2) Act as traffic
  regulators for raw materials into and out of
  the organelle

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Tour of the cell

• Organelles that process information
• The endomembrane system
• Organelles that process energy
• Other organelles
• Cytoskeleton

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Organelles that Process Information
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Nucleus

• Nucleus
• contains most of the cells DNA
• site of DNA replication to support cell
  reproduction.
• Nucleolus
• Within the nucleus is a specialized region called
  the nucleolus, where ribosomes are initially
  assembled.

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Nuclear envelope

• Two membrane bilayers form the nuclear envelope
• Nuclear membrane is perforated with nuclear
  pores.
• nuclear pores connect the interior of the nucleus
  with the rest of the cytoplasm.
• A pore complex, consisting of eight large protein
  granules, surrounds each pore.
• RNA and proteins must pass through these pores to
  enter or leave the nucleus.

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Chromatin and chromosomes

• Chromatin
• The chromatin consists of diffuse or very long,
  thin fibers in which DNA is bound to proteins.
• Chromosomes
• Prior to cell division these condense and
  organize into structures recognized as
  chromosomes.

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• Surrounding the chromatin is the nucleoplasm.
• Nucleoplasm is water and dissolved substances
• The nuclear lamina is a meshwork of proteins
  which maintains the shape of the nuclear envelope
  and the nucleus.

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Ribosomes

• Ribosomes are the sites of protein synthesis.
• In eukaryotes, functional ribosomes are found
• free in the cytoplasm,
• in mitochondria,
• bound to the endoplasmic reticulum
• in chloroplasts
• Consist of a type of RNA called ribosomal RNA,
  and more than 50 other proteins.

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The Endomembrane System

• Made up of
• Endoplasmic reticulum
• Golgi bodies
• Lysosomes
• Connections between these structures and the cell
  membrane and nuclear envelope

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Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a network of
  interconnecting membranes distributed throughout
  the cytoplasm.

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• The internal compartment, called the lumen, is a
  separate part of the cell with a distinct protein
  and ion composition.
• The ERs folding generates a surface area much
  greater than that of the plasma membrane.
• At certain sites, the ER membrane is continuous
  with the outer nuclear envelope membrane

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Rough ER

• The rough ER (RER) has ribosomes attached.
• Ribosomes synthesize proteins destined for the ER
  or for incorporation into the ER membrane.
• Some of the proteins that enter the lumen have
  address information which determines their
  final destination.

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Smooth ER

• Smooth ER (SER) is a ribosome-free region of the
  ER.
• SER of liver cells is the site of synthesis and
  hydrolysis of glycogen
• Also SER of liver is site of drug detoxification,
  and cholesterol and steroid synthesis

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• Cells that are specialized for synthesizing
  proteins for extracellular export have extensive
  ER membrane systems.
• Example cells of glands

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Figure 4.11 The Endoplasmic Reticulum
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Golgi Apparatus

• The Golgi apparatus consists of flattened
  membranous sacs and small membrane-enclosed
  vesicles.
• The Golgi apparatus has three roles
• Receive proteins from the ER and further modify
  them.
• Concentrate, package, and sort proteins before
  they are sent to their extracellular
  destinations.
• Some polysaccharides for plant cell walls are
  synthesized in Golgi.

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Golgi apparatus

• In vertebrate organisms, the Golgi is stacked
  like pancakes
• Cis region- closest to the nucleus
• Medial region- middle compartment
• Trans region- closest to plasma membrane
• Each region contains different enzymes and
  performs different functions.
• Individual sacs in the stack are called
  cisternae.

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How do proteins move through the cell?

• Golgi receives proteins from the ER, packages
  them and sends them on their way.
• A piece of ER buds off and travels through the
  cytoplasm to the Golgi.
• This structure is called a vesicle. When it
  reaches the cis region of the Golgi it fuses with
  the membrane of the Golgi and releases the
  protein.
• Vesicles budding off from the Golgi carry
  contents away from the Golgi.

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Figure 4.12 The Golgi Apparatus
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The Endomembrane System

• Lysosomes are vesicles containing digestive
  enzymes
• Some originate in the Golgi.
• Site of breakdown of macromolecules into monomers
• Food and foreign objects are taken up into the
  cell by phagocytosis.
• A pocket forms around particles which becomes a
  small vesicle which moves into the cytoplasm as a
  phagosome.

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• Phagosome fuses with lysosome and digestion
  occurs.
• Waste products are expelled when lysosome fuses
  with cell membrane.

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Figure 4.13 Lysosomes Isolate Digestive Enzymes
from the Cytoplasm
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Autophagy

• Lysosomes are also the sites where digestion of
  spent cellular components occurs, a process
  called autophagy.
• Recycles organelles by breaking components into
  monomers.
• Monomers then can be reused by cell.

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Organelles that Process Energy

• Mitochondrion
• Plastids
• Chloroplasts
• Chromoplasts
• Leucoplasts

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Mitochondrion

• Primary function of mitochondria
• convert the potential chemical energy of fuel
  molecules into a form that the cell can use
  (ATP).
• Partially degraded fuel molecules such as glucose
  enter the mitochondrion and the stored chemical
  energy is converted to ATP.
• The production of ATP is called cellular
  respiration.

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Structure of mitochondrion

• Two membranes
• Outer membrane
• Smooth and protective
• Lipid bilayer
• Inner membrane
• highly folded
• Provides more surface area

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• Folds of the inner membrane give rise to the
  cristae, which contain large protein molecules
  used in cellular respiration.
• The region enclosed by the inner membrane is
  called the mitochondrial matrix.
• Matrix also contains ribosomes and DNA that are
  used to make some of the proteins needed for
  cellular respiration.

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Figure 4.14 A Mitochondrion Converts Energy from
Fuel Molecules into ATP (Part 2)
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Plastids

• Plastids are organelles found only in plants and
  some protists.
• Chloroplasts, the sites where photosynthesis
  occurs, are one type of plastid.
• Other plastids function in the storage of
  pigments or polysaccharides
• Chromoplasts- red, orange or yellow pigments in
  plants
• Leucoplasts- storage for starches and fats

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Structure of a chloroplast

• Chloroplasts are surrounded by two layers, and
  have an internal membrane system.
• The internal membranes are arranged as thylakoids
  and grana.
• Grana are stacks of thylakoids.
• The fluid in which the grana are suspended is
  called the stroma.

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Figure 4.15 The Chloroplast The Organelle That
Feeds the World
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Organelles that Process Energy

• Endosymbiosis may explain the origin of
  mitochondria and chloroplasts.
• According to the endosymbiosis theory, both
  organelles were formerly prokaryotic organisms
  that somehow became incorporated into a larger
  cell.

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Endosymbiosis

• Proposal for origin of chloroplasts
• Chloroplasts were once independent photosynthetic
  prokaryotic organisms.
• This prokaryote was engulfed by a larger one and
  not digested.
• Successive generations of the cell contained the
  photosynthetic cell.

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The Endosymbiosis Theory
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Evidence for endosymbiosis

• Many biochemical similarities between
  chloroplasts and modern photosynthetic bacteria.
• DNA sequencing shows similarities between
  chloroplasts and photosynthetic bacteria
• Outer membrane could have come from the engulfing
  cells membrane and the inner membrane from the
  photosynthetic cells membrane

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• Today, both mitochondria and chloroplasts have
  DNA and ribosomes, and are self-duplicating
  organelles.

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Other Organelles

• Peroxisomes, also called microbodies, are small
  organelles that are specialized to
  compartmentalize toxic peroxides and break them
  down.
• Glyoxysomes are structurally similar organelles
  found only in plants.

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Vacuoles

• Function of plant vacuoles
• Storage of toxic by-products and waste materials
• Toxins may deter animals from eating plants
  because of their bad taste
• Turgor pressure
• a swelling that helps the plant cell maintain
  support and rigidity.
• Pigment storage
• Digestion

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Function of protistan vacuoles

• Food vacuoles are formed in single-celled
  protists.
• Cells engulf food particles by phagocytosis and
  create a vacuole
• Contractile vacuole
• Many freshwater protists have a contractile
  vacuole that helps eliminate excess water and
  restore proper salt balance

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The Cytoskeleton

• The cytoskeleton
• maintains cell shape and support.
• provides the mechanisms for cell movement.
• acts as tracks for motor proteins that help
  move materials within cells.

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Major types of cytoskeletal components

• microfilaments,
• intermediate filaments
• microtubules

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Microfilaments

• made of the two chains of protein actin
• Microfilaments are needed for cell contraction,
  as in muscle cells, and add structure to the
  plasma membrane and shape to cells.
• They are involved in cytoplasmic streaming, and
  the formation of pseudopodia.

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Intermediate filaments

• Found only in multicellular organisms, forming
  ropelike assemblages in cells.
• They have two major structural functions
• to stabilize the cell structure
• resist tension.
• In some cells, intermediate filaments maintain
  the positions of the nucleus and other organelles
  in the cell.

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Microtubules

• Hollow cylinders made from 2 tubulin protein
  subunits.
• Alpha-tubulin and beta-tubulin
• Microtubules provide a rigid intracellular
  skeleton for some cells, and they function as
  tracks that motor proteins can move along in the
  cell.
• They lengthen and shorten by adding or
  subtracting tubulin dimers.

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Figure 4.21 The Cytoskeleton (Part 1)
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Cilia and Flagella

• Cilia and flagella, common locomotary appendages
  of cells, are made of microtubules.
• Flagella are typically longer than cilia, and
  cells that have them usually have only one or
  two.
• Cilia are shorter and usually present in great
  numbers.

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• The microtubules in cilia and flagella are
  arranged in a 9 2 array.
• 9 fused doublets form outer cylinder
• 2 unfused microtubules run up center of cylinder
• At the base of each flagellum or cilium is a
  basal body. The nine pairs extend into the basal
  body.

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• Centrioles are found in an organizing center near
  the cell nucleus. Centrioles are similar to basal
  bodies, but are located in the center of the cell
  and help in the movement of chromosomes during
  cell division.

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Figure 4.23 Cilia are Made up of Microtubules
(Part 2)
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Review of cytoskeleton

• Microfilaments
• Actin
• Movement in cell division, cytoplasmic streaming
  and pseudopod extension
• Intermediate filaments
• Keratin
• Rope-like structures
• Hold organelles in place and add strength to
  cellular attachments
• Microtubules
• Tubulin dimers
• Structure and function of cilia and flagella

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Extracellular Structures

• Materials external to plasma membrane
• Provide protection, support, attachment for cells

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Extracellular Structures

• Plant cell wall
• composed of cellulose fibers embedded in a matrix
  of other complex polysaccharides and proteins.
• provides a rigid structure for the plasma
  membrane under turgor pressure, giving important
  support.
• Plant cells connected by membrane lined channels
  called plasmodesmata.

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Extracellular matrix in multicellular animals

• Extracellular matrix composed of fibrous
  proteins, such as collagen, and glycoproteins

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Examples of extracellular matrices

• Bone cells are embedded in an extracellular
  matrix consisting of collagen and calcium
  phosphate
• Proteoglycan
• Huge molecule of proteins and polysaccharides
• Component of extracellular matrix
• Epithelial cells, which line the human body
  cavities, have a basement membrane of
  extracellular material called the basal lamina.
• Connects and separates different cells and
  provide strength

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Practice Quiz-Cells

• _______ are a model of how cells may have
  originated.
• In biology, we call the basic unit of life the
  _______.
• Membranous compartments with distinctive shapes
  and functions are termed _______.
• _______ is the process whereby light energy is
  converted into chemical bonds.
• The _______ is the organelle with many folds
  called cristae.

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• The _______ is an organelle that serves as a
  sort of postal depot where some of the proteins
  synthesized on ribosomes and rough ER are
  processed.
• RNA carries information for protein synthesis
  from the DNA in the nucleus to the ribosomes in
  the cytoplasm. To get from the nucleoplasm to
  the cytoplasm, RNA must pass through _______.
• All organisms are composed of cells all cells
  come from preexisting cells. These statements are
  called _______.

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• The DNA in a prokaryotic cell can be found in
  the _______ region.
• The _______ of some bacteria help them avoid
  being detected by the human immune system.
• The side of the Golgi facing the ER is the
  _______ face.
• The substances that enter the Golgi come from the
  _______.

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• Toxic peroxides that are formed unavoidably as
  side products of important cellular reactions are
  found and neutralized in _______.
• The _______ is the cytoskeletal component with
  the smallest diameter.
• Keratin is classified as an _______ type of
  filament.