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1
Cell Structure and Function
2
Microscopes
• Anton Leeuwenhoek invented the microscope in the late 1600’s, which first showed that all living things are composed of cells. Also, he was the first to see microorganisms.
3
Cell Structure• In 1655, the English scientist Robert Hooke coined
the term “cellulae” for the small box-like structures he saw while examining a thin slice of cork under a microscope.
4
Cell Theory
• In 1838 – 1839, German scientists Schleiden and Schwann, proposed the first 2 principles of the cell theory:• All organisms are composed of one or more
cells.• Cells are the smallest living units of all living
organisms.• 15 years later, the German physician Rudolf
Virchow proposed the third principle: • Cells arise only by division of a previously
existing cell.
5
Basic Cell Structure
• All cells have the following basic structure: • A thin, flexible plasma membrane surrounds the
entire cell that regulates the passage of materials between the cell and its surrounding
• The interior is filled with a semi-fluid material called the cytoplasm.
• At some point, all cells contain DNA, the heritable material that directs the cell’s activities
• Also inside some cells are specialized structures called organelles.
6
Cell Theory
• The cell is the lowest level of structure that is capable of performing all the activities of life.
• Regulate its internal environment.• Take in and use energy.• Respond to its local environment.• Develop and maintain its complex
organization.• Divide to form new cells.
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Cell Characteristics
• Two major kinds of cells - prokaryotic cells and eukaryotic cells - can be distinguished by their structural organization.• Eukaryotic cells - Contain membrane-enclosed
organelles, including a DNA-containing nucleus• Prokaryotic cells - Lack such organelles
• The cells of the microorganisms called bacteria and archaea are prokaryotic.
• All other forms of life have the more complex eukaryotic cells
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The Cell
Nucleus(contains DNA)
Eukar yotic cell
Prokar yotic cell
DNA(no nucleus)
Organelles
25
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0
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Multicellular Organisms• Some organisms consist of a single cells, others are
multicellular aggregates of specialized cells. • Multicellular Organisms exhibit three major structural
levels above the cell: • Similar cells are grouped into tissues• Several tissues coordinate to form organs• Several organs form an organ system.
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Generalized Eukaryotic Cell
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Cell Size Limit
• Most cells are relatively small because as size increases, volume increases much more rapidly than surface area.
• longer diffusion time• limit to the volume of cytoplasm that can be
effectively controlled by genes.
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Cell Size Limit
• A cell must exchange materials with its environment. Cell volume determines the amount of materials that must be exchanged, while surface area limits how fast exchange can occur. In other words, as cells get larger the need for materials increases faster than the ability to absorb them.
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Visualizing Cells
14
Prokaryotic Cells
• Simplest organisms• Cytoplasm is surrounded by plasma membrane and
encased in a rigid cell wall composed of peptidoglycan.• No distinct interior compartments• Some use flagellum for locomotion, threadlike structures
protruding from cell surface
15
Eukaryotic Cells
• Characterized by compartmentalization by an endomembrane system, and the presence of membrane-bound organelles.• central vacuole• vesicles• chromosomes• cytoskeleton• cell walls
16
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Infolding of the plasma membrane
DNA Cell wall
Plasma membrane
Prokaryotic cell
Prokaryotic ancestor of eukaryotic cells
Eukaryotic cell
Endoplasmic reticulum (ER)
Nuclear envelope
Nucleus Plasma membrane
17
Endosymbiosis
• Endosymbiotic theory suggests engulfed prokaryotes provided hosts with advantages associated with specialized metabolic activities.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ancestral eukaryotic cell Eukaryotic cell with
mitochondrion
Internal membrane system
Aerobic bacterium
Mitochondrion
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Animal Cell
19
Plant Cell
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Extracellular fluid
Carbohydrate Glycolipid
Transmembraneproteins
Glycoprotein
Peripheralprotein
Cholesterol
Filaments ofcytoskeleton
Cytoplasm
Extracellularmatrix protein
Fluid Mosaic Model
Cell Membrane
21
Nucleus
• Repository for genetic material• Chromatin: DNA and proteins• Nucleolus: Chromatin and ribosomal
subunits - region of intensive ribosomal RNA synthesis
• Nuclear envelope: Surface of nucleus bound by two phospholipid bilayer membranes - Double membrane with pores
• Nucleoplasm: semifluid medium inside the nucleus
22
Nucleus
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Chromosomes
• DNA of eukaryotes is divided into linear chromosomes.• Exist as strands of chromatin, except
during cell division• Histones associated packaging proteins
24
The Nucleus And The Nuclear Envelope
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Nucleolus
• Protein synthesis occurs at tiny organelles called ribosomes.
• Ribosomes are composed of a large subunit and a small subunit.
• Ribosomes can be found alone in the cytoplasm, in groups called polyribosomes, or attached to the endoplasmic reticulum.
26
Ribosomes
• Ribosomes are RNA-protein complexes composed of two subunits that join and attach to messenger RNA.• site of protein synthesis• assembled in nucleoli
27
Endomembrane System• Compartmentalizes cell, channeling passage
of molecules through cell’s interior.• Endoplasmic reticulum
• Rough ER - studded with ribosomes• Smooth ER - few ribosomes
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Rough ER• Rough ER is especially abundant in cells that secrete proteins.
• As a polypeptide is synthesized on a ribosome attached to rough ER, it is threaded into the cisternal space through a pore formed by a protein complex in the ER membrane.
• As it enters the cisternal space, the new protein folds into its native conformation.• Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is
attached.• Secretory proteins are packaged in transport vesicles that carry them to their next stage.
• Rough ER is also a membrane factory.• Membrane-bound proteins are synthesized directly into the membrane.• Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.• As the ER membrane expands, membrane can be transferred as transport vesicles to other
components of the endomembrane system.
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Smooth ER• The smooth ER is rich in enzymes and plays a role in a variety of metabolic processes.• Enzymes of smooth ER synthesize lipids, including oils, phospholipids, and steroids.• These include the sex hormones of vertebrates and adrenal steroids.• In the smooth ER of the liver, enzymes help detoxify poisons and drugs such as
alcohol and barbiturates.• Smooth ER stores calcium ions.
• Muscle cells have a specialized smooth ER that pumps calcium ions from the cytosol and stores them in its cisternal space.
• When a nerve impulse stimulates a muscle cell, calcium ions rush from the ER into the cytosol, triggering contraction.
30
The Golgi apparatus• The Golgi apparatus is the shipping and receiving center for cell
products.• Many transport vesicles from the ER travel to the Golgi apparatus for
modification of their contents.• The Golgi is a center of manufacturing, warehousing, sorting, and
shipping.• The Golgi apparatus consists of flattened membranous sacs—
cisternae—looking like a stack of pita bread.• The Golgi sorts and packages materials into transport vesicles.
31
Functions Of The Golgi Apparatus
TEM of Golgi apparatus
cis face(“receiving” side ofGolgi apparatus)
Vesicles movefrom ER to Golgi Vesicles also
transport certainproteins back to ER
Vesicles coalesce toform new cis Golgi cisternae
Cisternalmaturation:Golgi cisternaemove in a cis-to-transdirection
Vesicles form andleave Golgi, carryingspecific proteins toother locations or tothe plasma mem-brane for secretion
Vesicles transport specificproteins backward to newerGolgi cisternae
Cisternae
trans face(“shipping” side ofGolgi apparatus)
0.1 0 µm16
5
2
3
4
Golgiapparatus
32
Membrane Bound Organelles• Lysosomes – vesicle
containing digestive enzymes that break down food/foreign particles
• Vacuoles – food storage and water regulation
• Peroxisomes - contain enzymes that catalyze the removal of electrons and associated hydrogen atoms
(a) Phagocytosis: lysosome digesting food
1 µm
Lysosome containsactive hydrolyticenzymes
Food vacuole fuses with lysosome
Hydrolyticenzymes digestfood particles
Digestion
Food vacuole
Plasma membraneLysosome
Digestiveenzymes
Lysosome
Nucleus
33
Energy Organelles
• Mitochondria • bounded by exterior and interior
membranes• interior partitioned by cristae
• Chloroplasts• have enclosed internal compartments of
stacked grana, containing thylakoids• found in photosynthetic organisms
• Both organelles house energy in the form of ATP
34
Mitochondria• Mitochondria are found in plant and animal cells.• Sites of cellular respiration, ATP synthesis• Bound by a double membrane surrounding fluid-filled matrix.• The inner membranes of mitochondria are cristae• The matrix contains enzymes that break down carbohydrates and
the cristae house protein complexes that produce ATP
35
Chloroplasts• A chloroplast is bounded by two membranes enclosing
a fluid-filled stroma that contains enzymes.• Membranes inside the stroma are organized into
thylakoids that house chlorophyll.• Chlorophyll absorbs solar energy and carbohydrates
are made in the stroma.
36
Cytoskeleton
• The eukaryotic cytoskeleton is a network of filaments and tubules that extends from the nucleus to the plasma membrane that support cell shape and anchor organelles.
• Protein fibers• Actin filaments - cell movement• Microtubules - centrioles• Intermediate filaments
37
Centrioles
• Centrioles are short cylinders with a 9 + 0 pattern of microtubule triplets.
• Centrioles may be involved in microtubule formation and disassembly during cell division and in the organization of cilia and flagella.
38
Cilia and Flagella• Contain specialized arrangements of microtubules• Are locomotor appendages of some cells• Cilia and flagella share a common ultrastructure
(a)
(c)
(b)
Outer microtubuledoublet
Dynein arms
CentralmicrotubuleOuter doublets cross-linkingproteins inside
Radialspoke
Plasmamembrane
Microtubules
Plasmamembrane
Basal body
0.5 µm
0.1 µm
0.1 µm
Cross section of basal body
Triplet
39
Cilia and Flagella• Cilia (small and numerous) and flagella (large and single) have a
9 + 2 pattern of microtubules and are involved in cell movement. • Cilia and flagella move when the microtubule doublets slide past
one another.• Each cilium and flagellum has a basal body at its base.
40
(a) Motion of flagella. A flagellum usually undulates, its snakelike motion driving a cell in the same direction as the axis of the flagellum. Propulsion of a human sperm cell is an example of flagellatelocomotion (LM).
1 µm
Direction of swimming
Cilia and Flagella
(b) Motion of cilia. Cilia have a back- and-forth motion that moves the cell in a direction perpendicular to the axis of the cilium. A dense nap of cilia, beating at a rate of about 40 to 60 strokes a second, covers this Colpidium, a freshwater protozoan (SEM).
15 µm
41
Cell Junctions• Long-lasting or permanent connections between
adjacent cells, 3 types of cell junctions:
Tight junctions prevent fluid from moving across a layer of cells
Tight junction
0.5 µm
1 µm
Spacebetweencells
Plasma membranesof adjacent cells
Extracellularmatrix
Gap junction
Tight junctions
0.1 µm
Intermediatefilaments
Desmosome
Gapjunctions
At tight junctions, the membranes ofneighboring cells are very tightly pressedagainst each other, bound together byspecific proteins (purple). Forming continu-ous seals around the cells, tight junctionsprevent leakage of extracellular fluid acrossA layer of epithelial cells.
Desmosomes (also called anchoringjunctions) function like rivets, fastening cellsTogether into strong sheets. IntermediateFilaments made of sturdy keratin proteinsAnchor desmosomes in the cytoplasm.
Gap junctions (also called communicatingjunctions) provide cytoplasmic channels fromone cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars,amino acids, and other small molecules maypass. Gap junctions are necessary for commu-nication between cells in many types of tissues,including heart muscle and animal embryos.
TIGHT JUNCTIONS
DESMOSOMES
GAP JUNCTIONS
42
Tight Junctions
• Connect cells into sheets. Because these junctions form a tight seal between cells, in order to cross the sheet, substances must pass through the cells, they cannot pass between the cells.
Tightjunction
43
Anchoring Junctions
• Attach the cytoskeleton of a cell to the matrix surrounding the cell, or to the cytoskeleton of an adjacent cell.
Cell1
Inter-cellularspace
Extracellular matrix
Intracellular attachment
proteins
Plasmamembranes
Transmembranelinking proteins
Cell2
Cytoskeletalfilament
44
Communicating Junctions
• Link the cytoplasms of 2 cells together, permitting the controlled passage of small molecules or ions between them.
Two adjacent connexonsform a gap junction
Adjacent plasmamembranes
Connexon
Intercellular space
45
Prokaryotes EukaryotesOrganisms Monera (bacteria) All other
organisms
Size Very small
(1 – 5 μm)
Much larger
(10 – 100 μm)
Complexity Relatively simple Complex
Cell wall Usually present (contains peptidoglycan)
Sometimes present (lacks peptidoglycan)
46
Prokaryotes Eukaryotes
Plasma membrane
Always present Always present
Internal membranes
May contain infoldings of the plasma membrane but usually lack internal membranes
Complex system of internal membranes divides cell into specialized compartments
47
Prokaryotes EukaryotesMembrane-bound organelles
Absent Present
Ribosomes Smaller and free in the cytoplasm
Larger and may be bound to ER
Cytoskeleton Absent Present
Flagella Solid flagellin; rotate
Microtubules; bend
48
Prokaryotes Eukaryotes
Structure of genetic material
Single, naked, circular DNA molecule
Many linear chromosomes, each made of 1 DNA molecule joined with protein
Location of genetic material
In an area of the cytoplasm called the nucleoid
Inside a membrane-bound nucleus
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