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Cell Structure and Function

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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.

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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.

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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.

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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.

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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

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,00

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

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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

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Eukaryotic Cells

• Characterized by compartmentalization by an endomembrane system, and the presence of membrane-bound organelles.• central vacuole• vesicles• chromosomes• cytoskeleton• cell walls

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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

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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

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Plant Cell

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Extracellular fluid

Carbohydrate Glycolipid

Transmembraneproteins

Glycoprotein

Peripheralprotein

Cholesterol

Filaments ofcytoskeleton

Cytoplasm

Extracellularmatrix protein

Fluid Mosaic Model

Cell Membrane

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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

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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

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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.

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Ribosomes

• Ribosomes are RNA-protein complexes composed of two subunits that join and attach to messenger RNA.• site of protein synthesis• assembled in nucleoli

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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.

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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.

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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

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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

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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

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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

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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.

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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

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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.

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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

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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.

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(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

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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

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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

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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

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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

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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)

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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

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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

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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