Download Cells Cell Theory Cell size is limited

Cell Structure
Cells
Chapter 4
• Cells were discovered in 1665 by Robert
Hooke
• Antoni van Leeuwenhoek observed the
first living cells
• Schleiden and Schwann proposed the
Cell Theory
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Cell Theory
Cell size is limited
1. All organisms are composed of cells
2. Cells are the basic unit of life
3. Cells arise only from pre-existing cells
• Most cells are relatively small due reliance
on diffusion of substances in and out of
cells
• Rate of diffusion affected by
• All cells today represent a continuous line
of descent from the first living cells
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– Surface area available
– Temperature
– Concentration gradient
– Distance
– Molecule size
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Surface area-to-volume ratio
• Organism made of many small cells has
an advantage over an organism
composed of fewer, larger cells
• As a cell’s size increases, its volume
increases much more rapidly than its
surface area
• Some cells overcome limitation by being
long and skinny – like neurons
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Microscopes
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2 types
• Not many cells are visible to the naked
eye
• Light microscopes
– Use magnifying lenses with visible light
– Resolve structures that are 200 nm apart
– Most are less than 50 µm in diameter
• Resolution – minimum distance two points
can be apart and still be distinguished as
two separate points
• Electron microscopes
– Use beam of electrons
– Resolve structures that are 0.2 nm apart
– Limit of resolution of human eye is 100 µm
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• Electron
microscopes
Prokaryotes vs. Eukaryotes
– Transmission
electron
microscopes
transmit electrons
through the
material
– Scanning electron
microscopes beam
electrons onto the
specimen surface
All cells contain:
1. DNA
–
Hereditary material
2. Cytoplasm
–
Semifluid matrix of organelles and cytosol
3. Ribosomes
–
Synthesize proteins
4. Plasma membrane
–
Phospholipid bilayer
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Prokaryotic Cells
• Simplest organisms
• Lack a membrane-bound nucleus
– DNA is present in the nucleoid
• Cell wall outside of plasma membrane
• Do contain ribosomes
• Two domains of prokaryotes
– Archaea
– Bacteria
Bacterial cell walls
Flagella
• Most bacterial cells are encased by a strong cell
wall
– composed of peptidoglycan
• Protect the cell, maintain its shape, and prevent
excessive uptake or loss of water
• Susceptibility of bacteria to antibiotics often
depends on the structure of their cell walls
• Archaea lack peptidoglycan
• Present in some prokaryotic cells
– One or multiple or none
• Used for locomotion
• Rotary motion propels the cell
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Eukaryotic Cells
• Membrane-bound nucleus
• More complex than prokaryotic cells
• Hallmark is compartmentalization
– Achieved through use of membrane-bound
organelles
• Cytoskeleton maintains cellular structure
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Nucleus
• DNA
• Nucleolus
– Ribosomal RNA synthesis
– Ribosomes partially assembled
• Nuclear envelope
– 2 phospholipid bilayers
– Nuclear pores – control passage in and out
• In eukaryotes, the DNA is divided into multiple
linear chromosomes
– Chromatin is chromosomes plus protein
Ribosomes
Endomembrane System
•
•
•
•
Protein synthesis machinery
Found in all cell types in all 3 domains
Ribosomal RNA (rRNA)-protein complex
Protein synthesis also requires messenger
RNA (mRNA) and transfer RNA (tRNA)
• Ribosomes may be free in cytoplasm or
associated with internal membranes
• Series of membranes throughout the
cytoplasm
• Divides cell into compartments where
different cellular functions occur
• One of the fundamental distinctions
between eukaryotes and prokaryotes
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Endoplasmic reticulum
• Rough endoplasmic reticulum (RER)
– Attached ribosomes
– Synthesis of proteins destined for secretion,
lysosomes or plasma membrane
• Smooth endoplasmic reticulum (SER)
– Relatively few bound ribosomes
– Carbohydrate and lipid synthesis
– Store Ca2+
– Detoxification
Golgi apparatus
• Flattened stacks of interconnected
membranes (Golgi bodies)
• Packaging and distribution of proteins and
other molecules
• Cis and trans faces
• Vesicles transport molecules to destination
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Lysosomes
• Membrane-bounded digestive vesicles
• Arise from Golgi apparatus
• Enzymes catalyze breakdown of
macromolecules or organelles
• Destroy cells or foreign matter that the cell
has engulfed by phagocytosis
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Peroxisomes
• Contain enzymes
involved in the
oxidation of fatty
acids
• H2O2 produced as byproduct – rendered
harmless by catalase
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Vacuoles
• Membrane-bounded structures in plants
• Various functions depending on the cell
type
• There are different types of vacuoles:
– Central vacuole in plant cells
– Contractile vacuole of some protists
– Storage vacuoles
Mitochondria
• Found in all types of eukaryotic cells
• Bound by membranes
–
–
–
–
Outer membrane
Intermembrane space
Inner membrane has cristae
Matrix
• On the surface of the inner membrane, and also
embedded within it, are proteins that carry out
oxidative metabolism
• Have their own DNA
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Chloroplasts
• Organelles present in cells of plants and
some other eukaryotes
• Contain chlorophyll for photosynthesis
• Surrounded by 2 membranes
• Thylakoids are membranous sacs within
the inner membrane
– Grana are stacks of thylakoids
• Have their own DNA
Endosymbiosis
• Proposes that some of today’s eukaryotic
organelles evolved by a symbiosis arising
between two cells that were each freeliving
• One cell, a prokaryote, was engulfed by
and became part of another cell, which
was the precursor of modern eukaryotes
• Mitochondria and chloroplasts
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Cytoskeleton
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3 types of fibers
• Microfilaments (actin filaments)
• Network of protein fibers found in all
eukaryotic cells
– Two protein chains loosely twined together
– Movements like contraction, crawling, “pinching”
– Supports the shape of the cell
– Keeps organelles in fixed locations
• Microtubules
– Largest of the cytoskeletal elements
– Dimers of !- and "-tubulin subunits
– Facilitate movement of cell and materials within cell
• Dynamic – constantly forming and
disassembling
• Intermediate filaments
– Intermediate in size
– Very stable – usually not broken down
– E.g., keratin, vimentin, neurofilaments
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Centrosomes
• Region surrounding centrioles in almost
all animal cells
• Microtubule-organizing center
– Can nucleate the assembly of microtubules
• Important for segregation of chromosomes
during cell division
• Animal cells and most protists have a pair
of centrioles
• Plants and fungi lack centrioles
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Cell Movement
• Cell motion is tied to the movement of
actin filaments or microtubules
• Some cells crawl using actin
microfilaments
• Flagella have 9 + 2 arrangement of
microtubules
– Not like prokaryotic flagella
– Cilia are shorter and more numerous
Extracellular matrix (ECM)
• Eukaryotic cell
walls
– Plants, fungi, and
many protists
– Different from
prokaryote
– Plants and protists
– cellulose
– Fungi – chitin
• Animal cells lack cell walls
• Secrete a mixture of glycoproteins into
the space around them
• Collagen may be abundant
• Form a protective layer over the cell
surface
• Integrins link ECM to cell’s cytoskeleton
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Cell-to-cell interactions
Cell connections
• 3 categories based on function
1. Tight junction
• Surface proteins give cells identity
– Cells make contact, “read” each other, and
react
– Glycolipids – most tissue-specific cell
surface markers
– MHC proteins – recognition of “self” and
“nonself” cells by the immune system
– Connect the plasma membranes of adjacent cells in a
sheet – no leakage
2. Anchoring junction
– Mechanically attaches cytoskeletons of neighboring
cells (desmosomes)
3. Communicating junction
– Chemical or electrical signal passes directly from one
cell to an adjacent one (gap junction,
plasmodesmata)
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