Download Introduction to Cellular Structure • All organisms are composed of

Introduction to Cellular Structure
• All organisms are composed of cells
– Single-cell organisms  humans
• Cells are responsible for all structural and
functional properties of a living organism
Development of Cell Theory
• Cytology—scientific study of cells
– Began in 1665 when Robert Hooke coined the word cellulae to
describe empty cell walls of cork
• Theodor Schwann concluded, about two centuries later,
that all animal tissues are made of cells.
• Louis Pasteur established beyond any reasonable doubt
that “cells arise only from other cells”
– Refutes the idea of spontaneous generation—living things arise
from nonliving matter
• Modern cell theory:
– All organisms are composed of cells and cell products.
– The cell is the simplest structural and functional unit of life.
– An organism’s structure and functions are due to the activities
of its cells.
– Cells come only from preexisting cells, not from nonliving
matter.
– Cells of all species have many fundamental similarities in their
chemical composition and metabolic mechanisms.
Cells
• The cell is the structural and functional unit of life
• Human adults are made up of ~100 trillion cells
• Each cell has an outer boundary called the plasma (cell)
membrane which isolates the fluid within the cell from the
fluid that surrounds the cell
• Some cells function individually, while most cells work
together with similar cells forming tissues
• Within each cell is a collection of subcellular components
called organelles which accomplish a specific task for the
cell
– membranous
– nonmembranous (inclusions)
Cell Shapes
• 200 types of cells in the human body:
•
•
•
•
•
•
•
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•
Squamous—thin and flat with nucleus creating bulge
Polygonal—irregularly angular shapes with four or more sides
Stellate—starlike shape
Cuboidal—squarish and about as tall as is wide
Columnar—taller than wide
Spheroid to ovoid—round to oval
Discoid—disc-shaped
Fusiform—thick in middle, tapered toward the ends
Fibrous—threadlike shape
Cell Shapes and Sizes
Squamous
Cuboidal
Columnar
Polygonal
Stellate
Spheroid
Discoid
Fusiform (spindle-shaped)
Fibrous
Cell Sizes
• Human cell size
– approx: 10–15 micrometers (µm) in diameter
• Egg cells (very large) 100 µm diameter
– Barely visible to the naked eye
• Nerve cell at 1 meter long
– Longest human cell
– Too slender to be seen with naked eye
• Limitations on cell size
– Cell growth increases volume more than surface area
– The surface area of a cell is proportional to the square of
its diameter. The volume of a cell is proportional to the
cube of its diameter
• Nutrient absorption and waste removal utilize surface area
– Overgrown cells may rupture
Cells grow mostly in volume:
20 m
Growth
20 m
10 m
10 m
Large cell
Diameter
= 20 mm
Surface area = 20 mm ´ 20 mm ´ 6 = 2,400 mm2
Volume
= 20 mm ´ 20 mm ´ 20 mm = 8,000 mm3
Small cell
Diameter
= 10 mm
Surface area = 10 mm ´ 10 mm ´ 6 = 600 mm2
Volume
= 10 mm ´ 10 mm ´ 10 mm = 1,000 mm3
Effect of cell growth:
Diameter (D) increased by a factor of 2
Surface area increased by a factor of 4 (= D2)
Volume increased by a factor of 8 (= D3)
Basic Components of a Cell
• Light microscope reveals plasma membrane, nucleus, and cytoplasm
– Cytoplasm—fluid between the nucleus and surface membrane
• Resolution (ability to reveal detail) of electron microscopes reveals
ultrastructure
– Organelles, cytoskeleton, and cytosol (ICF)
Plasma membrane
Nucleus
Nuclear envelope
Mitochondria
Golgi vesicle
Golgi complex
Ribosomes
2.0
m
Basic Components of a Cell
• Plasma (cell) membrane
– Surrounds cell, defines boundaries
– Made of proteins and lipids
– Composition and function can vary from one
region of the cell to another
• Cytoplasm
– Organelles
– Cytoskeleton (microtubules)
– Cytosol (intracellular
fluid, ICF)
• Extracellular fluid (ECF)
– Fluid outside of cell
The Plasma Membrane
- Appears as a pair of dark parallel lines around cell (viewed with the
electron microscope)
• Plasma membrane—membrane at cell surface
- Defines cell boundaries
- Governs interactions with other cells
- Controls passage of materials in and
out of cell
- Intracellular face—side that faces
cytoplasm
- Extracellular face—side that faces
outward
Plasma membrane
of upper cell
Intercellular space
Plasma membrane
of lower cell
Nuclear envelope
Nucleus
(a)
100 nm
© Don Fawcett/Photo Researchers, Inc.
Cellular Membranes
• The outer boundary of the cell as well as the boundary of many of the
internal organelles is made of a cellular membrane
Extracellular fluid
Peripheral
protein
Glycolipid
Glycoprotein
Carbohydrate
chains
Extracellular
face of
membrane
Phospholipid
bilayer
Channel
Peripheral
protein
Cholesterol
Transmembrane
protein
Intracellular fluid
Proteins of
cytoskeleton
Intracellular
face of
membrane
• Composed
primarily of
phospholipids
that are
arranged in a
bilayer (2
layers) with
proteins,
carbohydrates
and cholesterol
molecules are
integrated
within
Membrane Lipids
• 98% of molecules in plasma membrane are lipids
• Phospholipids
– 75% of membrane lipids are phospholipids
– Amphiphilic molecules arranged in a bilayer
– Hydrophilic phosphate heads face water on each side of
membrane
– Hydrophobic tails—directed toward the center, avoiding
water
– Drift laterally from place to place
– Movement keeps membrane fluid
Phospholipid Models
Cellular Membrane Anatomy
• Cholesterol
– 20% of the membrane lipids
– Holds phospholipids still and can stiffen membrane
• Glycolipids
– 5% of the membrane lipids
– Phospholipids with short carbohydrate chains on
extracellular face
– Contributes to glycocalyx—carbohydrate coating on the
cell surface
Membrane Proteins
• Membrane proteins
– 2% of the molecules in plasma membrane
– 50% of its weight
• Transmembrane proteins
– Pass through membrane
– Have hydrophilic regions in contact with cytoplasm (ICF) and
extracellular fluid (composed of polar amino acids)
– Have hydrophobic regions that pass back and forth through the
lipid of the membrane (composed of nonpolar amino acids)
• Peripheral membrane protein
– associated only with the intracellular surface of the cell
membrane (located in the ICF)
– capable of detaching and moving into the cytosol to
interact with other molecules within the cell
Integral Membrane Protein Structure
Integral Membrane Protein Functions
Membrane Carbohydrates
• The small polysaccharides that are part of the plasma
membrane are always immersed in the ECF
– covalently bound to an integral membrane protein or a
phospholipid head
• 2 varieties
– Glycolipids
• polysaccharides are covalently bound to the polar head
of a phospholipid
– Glycoproteins
• polysaccharides are covalently bound the extracellular
portion of an integral membrane protein
Receptors
• Cell communication via chemical signals
– Receptors—surface proteins on plasma membrane of target
cell
– Bind these chemicals (hormones, neurotransmitters)
– Receptor usually specific for one substrate
3-21
Second-Messenger Systems
• Chemical first messenger (epinephrine) binds to a
surface receptor
– Triggers changes within the cell that produces a second
messenger in the cytoplasm
• Receptor activates G protein
– An intracellular peripheral protein
– Guanosine triphosphate (GTP), an ATP-like compound
• G protein relays signal to adenylate cyclase which
converts ATP to cAMP (second messenger),
activating a signaling cascade.
• cAMP activates a
kinase in the cytosol
1 A messenger such as epinephrine (red triangle)
binds to a receptor in the plasma membrane.
First
messenger
Receptor
G
Adenylate cyclase
• Kinases add phosphate
groups to other cellular
enzymes
G
Pi
2 The receptor releases
a G protein, which
then travels freely in
the cytoplasm and
can go on to step 3
or have various other
effects on the cell.
ATP
Pi
3 The G protein
binds to an enzyme,
adenylate cyclase, in
the plasma membrane.
Adenylate cyclase
cAMP
converts ATP to cyclic (second
AMP (cAMP), the
messenger)
second messenger.
4 cAMP
activates a
cytoplasmic
enzyme called
a kinase.
Inactive
kinase
Activated
kinase
Inactive
enzymes
Pi
5 Kinases add
phosphate groups (Pi)
to other cytoplasmic
enzymes. This activates
some enzymes and
deactivates others, leading
to varied metabolic effects
in the cell.
Activated
enzymes
Various metabolic effects
– Activates some
enzymes, and
inactivates others
triggering a wide
variety of physiological
changes in cells
• Up to 60% of modern
drugs work by altering
activity of G proteins!
Channel Proteins
• Transmembrane proteins with pores that allow
water and dissolved ions to pass through membrane
– Some are constantly open
– Some are gated channels that open and close in response
to stimuli
• Ligand (chemically)-regulated gates
• Voltage-regulated gates
• Mechanically regulated gates (stretch and pressure)
• Play an important role in the timing of nerve signals
and muscle contraction
Carriers or Pumps
• Transmembrane proteins bind to glucose, electrolytes, and other
solutes
– Transfer them across membrane
• Pumps consume ATP in the process
Cell-Identity Markers
• Glycoproteins contribute to the glycocalyx
– Carbohydrate surface coating
– Acts like a cell’s “identification tag”
• Enables our bodies to identify which cells belong to it and which
are foreign invaders.
Cell-Adhesion Molecules (CAMs)
• Adhere cells to each other and to extracellular material
• Cells do not grow or survive normally unless they are
mechanically linked to extracellular material
– Special events: sperm–egg binding; binding of immune cell to a cancer
cell requires CAMs
Microvilli
Glycocalyx
Microvillus
Actin
microfilaments
(a
1. 0 m
Extensions of
membrane (1–2 mm)
Serves to increase
cell’s surface area
Best developed in cells
specialized in
absorption
Gives 15 to 40 times
more absorptive
surface area
On some cells they are
very dense and appear
as a fringe—“brush
border”
Cilia
• Hairlike processes 7–10 mm long
–
–
–
–
Single, nonmotile primary cilium found on nearly every cell
“Antenna” for monitoring nearby conditions
Sensory in inner ear, retina, nasal cavity, and kidney
Composed of microtubules
• Motile cilia—respiratory tract, uterine tubes, ventricles of the brain,
efferent ductules of testes
– Beat in waves
– Sweep substances across surface in same direction
– Power strokes followed by recovery strokes
Mucus
Saline
layer
Epithelial
cells
1
2
3
Power stroke
(a)
(b)
4
5
6
7
Recovery stroke
Cilia
Cilia
(a)
10 µm
Cilia
• Axoneme—core of cilia that is the structural basis for ciliary movement
• Composed of : microtubules (anchor
the cilia) and Dynein (a protein which
uses ATP to bend the microtubule).
Shaft of
cilium
• Saline layer
– Chloride pumps pump Cl- into ECF
– Na+ and H2O follows
– Cilia beat freely in saline layer
Flagella
• Tail of a sperm—only functional flagellum
• Whiplike structure with axoneme identical to the cilia
– Much longer than cilium
– Stiffened by coarse fibers that support the tail
• Movement is more undulating, snakelike
Cystic Fibrosis
• Saline layer at cell surface due to
chloride pumps move Cl- out of
cell. Na+ ions and H2O follow
Mucus
• Cystic fibrosis—hereditary
disease in which cells make
chloride pumps, but fail to install
them in the plasma membrane
– Chloride pumps fail to create
adequate saline layer on cell surface
• Thick mucus plugs pancreatic
ducts and respiratory tract
– Inadequate digestion of nutrients
and absorption of oxygen
– Chronic respiratory infections
– Life expectancy of 30
Saline
layer
Epithelial
cells
(a)
The Cytoskeleton
• Cytoskeleton—collection of filaments and cylinders
– Cell scaffolding
• Composed of:
– Microfilaments: 6 nm thick, actin, forms terminal web
– Intermediate fibers: 8–10 nm, support, strength, and
structure
– Microtubules: 25 nm, tubulin, movement
3-32
Cytoskeleton
Microvilli
Microfilaments
Secretory
vesicle in
transport
Desmosome
Terminal web
Lysosome
Kinesin
Microtubule
Intermediate
filaments
Centrosome
Microtubule
undergoing
disassembly
Intermediate
filaments
Microtubule
in the process
of assembly
Nucleus
Mitochondrion
(a)
(b)
Basement
membrane
Hemidesmosome
15 m
Organelles
• Internal structures,
carry out specialized
metabolic tasks
• Membranous
organelles
– Nucleus, mitochondria,
lysosomes,
peroxisomes,
endoplasmic reticulum,
and Golgi complex
• Nonmembranous
organelles
– Ribosomes,
centrosomes,
centrioles, basal bodies
The Nucleus
• Largest organelle of a cell
– only intracellular organelle visible with a compound light
microscope (plasma membrane is also visible)
• Consists of 3 parts:
– nuclear envelope
– Chromatin (threadlike matter composed of DNA and
protein)
– nucleolus (ribosome production)
• Some cells are anuclear (no nuclei) and some are
multinucleated
Nuclear Structure
Nuclear
pores
Nucleolus
Nucleoplasm
Nuclear
envelope
(a) Interior of nucleus
2 mm
(b) Surface of nucleus
1.5 mm
ER
• Interconnected maze of membranous tubes and sacs
• Smooth ER
– Storage site of intracellular calcium (Ca2+)
– Location of enzymes which:
• synthesize lipids
– steroids
– phospholipids
• hydrolyze drugs and toxins
• Rough ER
– ribosomes on the outer surface of the ER synthesize
proteins that are then moved to the Golgi complex for
modification
Endoplasmic Reticulum (ER)
Ribosomes
• Site of protein synthesis in a cell
• Move around the cell between 2 locations
– floating in the ICF, ribosomes are “free”
• synthesize proteins that remain within the cell
– temporarily attached to the membrane of the endoplasmic
reticulum, ribosomes are “membrane-bound”
• synthesize:
– integral membrane proteins
– secreted proteins which are exported out of the
cell into the ECF to go elsewhere in the body
• once the ribosome finishes making the protein, it
detaches from the ER becoming “free” again
Golgi Complex
• Layers of flattened membranous sacs
• Modifies proteins synthesized at the rough ER
– addition of carbohydrates to make glycoproteins
– the removal of some amino acids
• makes these proteins biologically active
• packages the protein into membrane-bound Golgi
vesicles
Golgi Complex
Golgi vesicles
Golgi
complex
600 nm
Centrioles
• a short cylindrical
assembly of
microtubules.
Microtubules
Protein
link
Cross
section
0.1 m
(a) Cross section (TEM)
Microtubules
(b) Pair of centrioles
– Play role in cell
division
Longitudinal
view
Lysosomes
• Spherical membranous bags containing hydrolytic
(digestive) enzymes
– hydrolyze bacteria and viruses that infect a cell
– hydrolyze old, worn out organelles
• allows cells to “recycle” the macromolecules that make up
organelles
Lysosomes
Peroxisomes
• Peroxisomes—resemble lysosomes but contain
different enzymes and are not produced by the Golgi
complex
• General function is to use molecular oxygen to oxidize
organic molecules
– These reactions produce hydrogen peroxide (H2O2)
– Catalase breaks down excess peroxide to H2O and O2
– Neutralize free radicals, detoxify alcohol, other drugs, and a
variety of blood-borne toxins
– Break down fatty acids into acetyl groups for mitochondrial
use in ATP synthesis
• In all cells, but abundant in liver and kidney
Mitochondria
• Mitochondria—organelles specialized for
synthesizing ATP
• Variety of shapes: spheroid, rod-shaped,
kidney-shaped, or threadlike
• Surrounded by a double membrane
– Inner membrane has folds called cristae
(increase surface area)
– Spaces between cristae called matrix
• Matrix contains ribosomes, enzymes
used for ATP synthesis, small circular
DNA molecule
– Mitochondrial DNA (mtDNA)
• “Powerhouses” of the cell
– Energy is extracted from organic
molecules and transferred to ATP
Matrix
Outer membrane
Inner membrane
Mitochondrial
ribosome
Intermembrane
space
Crista
Evolution of the Mitochondrion
• It is a virtual certainty that mitochondria
evolved from bacteria that invaded another
primitive cell, survived in the cytoplasm, and
became permanent residents
– Its two unit membranes suggest that the original
bacterium provided the inner membrane, and the
host cell’s phagosome provided the outer membrane
– Mitochondrial ribosomes are more like bacterial
ribosomes
– Has its own mtDNA
• Small circular molecule resembling bacterial DNA
• Replicates independently of nuclear DNA
– When a sperm fertilizes the egg, any mitochondria
introduced by the sperm are usually destroyed, and only
those provided by the egg are passed on to the developing
embryo
• Mitochondrial DNA is almost exclusively inherited through the
mother
– Mutates more readily than nuclear DNA
• No mechanism for DNA repair
• Produces rare hereditary diseases
• Mitochondrial myopathy, mitochondrial encephalomyopathy, and
others