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Chapter 3
*Lecture PowerPoint
Cellular Form and
Function
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Introduction
• All organisms are composed of cells
– Single-cell organisms  humans
• Cells are responsible for all structural and
functional properties of a living organism
– Workings of the human body
– Mechanisms of disease
– Rationale of therapy
2-2
Concepts of Cellular Structure
• Expected Learning Outcomes
– Discuss the development and modern tenets of
the cell theory.
– Describe cell shapes from their descriptive terms.
– State the size range of human cells and discuss
factors that limit their size.
– Discuss the way that developments in microscopy
have changed our view of cell structure.
– Outline the major components of a cell.
2-3
Development of the Cell Theory
• Cytology—scientific study of cells
– Began 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
3-4
Development of the Cell Theory
• Modern cell theory
– All organisms 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.
3-5
Cell Shapes and Sizes
• About 200 types of cells in the human body
• 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
• Note: Some of these cell shapes appear as in tissue
sections, but not their three-dimensional shape
3-6
Cell Shapes and Sizes
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Squamous
Cuboidal
Columnar
Polygonal
Stellate
Spheroid
Discoid
Fusiform (spindle-shaped)
Figure 3.1
Fibrous
3-7
Cell Shapes and Sizes
• Human cell size
– Most from 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
3-8
Cell Shapes and Sizes
• Limitations on cell size
– Cell growth increases volume more than
surface area
– Surface area of a cell is proportional to the
square of its diameter
– Volume of a cell is proportional to the cube of
its diameter
• Nutrient absorption and waste removal utilize surface
area
– If cell becomes too large, may rupture like
overfilled water balloon
3-9
Cell Shapes and Sizes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
20 m
Growth
20 m
Figure 3.2
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)
3-10
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)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Plasma membrane
Nucleus
Nuclear envelope
Figure 3.3
Mitochondria
Golgi vesicle
Golgi complex
Ribosomes
2.0
© K.G. Muri/Visuals Unlimited
m
3-11
Basic Components of a Cell
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a) Light microscope (LM)
From Cell Ultrastructure by William A. Jensen and Roderick B. Park. © 1967 by
Wadsworth Publishing Co., Inc. Reprinted by permission of the publisher
Figure 3.4a
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(b) Transmission electron
microscope (TEM)
2.0
m
From Cell Ultrastructure by William A. Jensen and Roderick B. Park. © 1967 by
Wadsworth Publishing Co., Inc. Reprinted by permission of the publisher
Figure 3.4b
• Magnification of x750 seen through both light
and transmission electron microscopes
• Increased resolution reveals the finer details
3-12
Basic Components of a Cell
3-13
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Cytoplasm
– Organelles
– Cytoskeleton
– Cytosol (intracellular
fluid, ICF)
Apical cell surface
Microfilaments
Microvillus
Terminal web
Desmosome
Secretory vesicle
undergoing
exocytosis
Fat droplet
Secretory vesicle
Intercellular
space
Centrosome
Centrioles
Golgi vesicles
Golgi complex
Lateral cell surface
Free ribosomes
Intermediate filament
Nucleus
Lysosome
Nucleolus
Microtubule
Nuclear
envelope
Rough endoplasmic
reticulum
Smooth endoplasmic
reticulum
• Extracellular fluid (ECF)
– Fluid outside of cell
Mitochondrion
Plasma membranes
Hemidesmosome
Basal cell surface
Figure 3.5
Basement
membrane
3-14
The Cell Surface
• Expected Learning Outcomes
– Describe the structure of the plasma membrane.
– Explain the functions of the lipid, protein, and
carbohydrate components of the plasma membrane.
– Describe a second-messenger system and discuss its
importance in human physiology.
– Describe the composition and functions of the
glycocalyx that coats cell surfaces.
– Describe the structure and functions of microvilli, cilia,
and flagella.
3-15
The Plasma Membrane
• Unit membrane—forms the border of the cell and many of its
organelles
- Appears as a pair of dark parallel lines around cell (viewed with the
electron microscope)
• Plasma membrane—unit 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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Plasma membrane
of upper cell
Intercellular space
Plasma membrane
of lower cell
Nuclear envelope
Nucleus
(a)
100 nm
© Don Fawcett/Photo Researchers, Inc.
Figure 3.6a
The Plasma Membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Figure 3.6b
(b)
• Oily film of lipids with diverse proteins embedded
3-17
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
3-18
Membrane Lipids
• 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
3-19
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 and
extracellular fluid
– Have hydrophobic regions that pass back and forth
through the lipid of the membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Carbohydrate
Transmembrane protein:
Hydrophilic region
Hydrophobic region
Phospholipid
bilayer
Cytoskeletal
protein
Anchoring peripheral
protein
Figure 3.7
3-20
Membrane Proteins
Cont.
– Most are glycoproteins
– Can drift about freely in phospholipid film
– Some anchored to cytoskeleton
• Peripheral proteins
– Adhere to one face of the membrane
– Usually tethered to the cytoskeleton
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Carbohydrate
Transmembrane protein:
Hydrophilic region
Hydrophobic region
Phospholipid
bilayer
Cytoskeletal
protein
Anchoring peripheral
protein
Figure 3.7
3-21
Membrane Proteins
• Functions of membrane proteins include the
following:
– Receptors, second-messenger systems, enzymes, ion
channels, carriers, cell-identity markers, cell-adhesion
molecules
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chemical
messenger
Breakdown
products
CAM of
another cell
Ions
(a) Receptor
A receptor that
binds to chemical
messengers such
as hormones sent
by other cells
(b) Enzyme
An enzyme that
breaks down
a chemical
messenger and
terminates its
effect
(c) Ion Channel
A channel protein
that is constantly
open and allows
ions to pass
into and out of
the cell
(d) Gated ion channel
A gated channel
that opens and
closes to allow
ions through
only at certain
times
Figure 3.8
(e) Cell-identity marker
A glycoprotein
acting as a cellidentity marker
distinguishing the
body’s own cells
from foreign cells
(f) Cell-adhesion
molecule (CAM)
A cell-adhesion
molecule (CAM)
that binds one
cell to another
3-22
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
3-23
Second-Messenger Systems
• Messenger (chemical) binds to a surface
receptor
• Triggers changes within the cell that produce a
second messenger in the cytoplasm
– Involves transmembrane proteins and peripheral
proteins
3-24
Enzymes
• Enzymes in plasma membrane carry out final
stages of starch and protein digestion in small
intestine
• Help produce second messengers (cAMP)
• Break down chemical messengers and hormones
whose job is done
– Stops excessive stimulation
3-25
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
• Channelopathies—family of diseases that
result from defects in channel proteins
3-26
Carriers or Pumps
• Transmembrane proteins bind to glucose,
electrolytes, and other solutes
– Transfer them across membrane
• Pumps consume ATP in the process
3-27
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
3-28
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 the
extracellular material
– Special events: sperm–egg binding; binding of
immune cell to a cancer cell requires CAMs
3-29
Second Messengers
• 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)
3-30
Second Messengers
• cAMP activates a kinase in the cytosol
• Kinases add phosphate groups to other cellular
enzymes
– 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
3-31
Second Messenger System
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1 A messenger such as epinephrine (red triangle)
binds to a receptor in the plasma membrane.
First
messenger
Receptor
G
Adenylate cyclase
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
Figure 3.9
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
3-32
The Glycocalyx
• Unique fuzzy coat external to the plasma
membrane
– Carbohydrate moieties of membrane glycoproteins and
glycolipids
– Unique in everyone but identical twins
• Functions
–
–
–
–
Protection
Immunity to infection
Defense against cancer
Transplant compatibility
– Cell adhesion
– Fertilization
– Embryonic development
3-33
Microvilli
• 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”
– Milking action of actin
• Actin filaments shorten microvilli
– Pushing absorbed contents down into cell
3-34
Microvilli
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Glycocalyx
Microvillus
Actin
microfilaments
(a)
Figure 3.10a
1.0 m
(b)
a: © Ed Reschke; b: Biophoto Associates/Photo Researchers, Inc.
0.1 m
Figure 3.10b
• Actin microfilaments are centered in each microvilli
3-35
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
• 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
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Mucus
Saline
layer
Epithelial
cells
1
2
3
Power stroke
(a)
Figure 3.12a
(b)
4
5
6
7
Recovery stroke
Figure 3.12b
3-36
Cilia
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cilia
(a)
10 µm
a: Custom Medical Stock Photo, Inc.
Figure 3.11a
3-37
Cilia
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Axoneme—core of cilia that is the
structural basis for ciliary
movement
• Has 9 + 2 structure of
microtubules
Axoneme:
Peripheral microtubules
Central microtubules
Dynein arms
Shaft of
cilium
Basal
body
– Nine pairs form basal body inside
the cell membrane
• Anchors cilium
– Dynein arms ―crawl‖ up adjacent
microtubule, bending the cilia
• Uses energy from ATP
Plasma membrane
(b)
Dynein
arm
Central
microtubule
Axoneme
Figure 3.11b,d
Peripheral
microtubules
(d)
Cilia
• Saline layer
– Chloride pumps pump Cl- into
ECF
– Na+ and H2O follows
– Cilia beat freely in saline layer
Microvilli
Dynein
arm
Central
microtubule
Peripheral
microtubules
(c)
0.15 m
c: © Biophoto Associates/Photo Researchers, Inc.
Figure 3.11c
3-38
Cystic Fibrosis
• Saline layer at cell surface due to
chloride pumps move Cl- out of
cell. Na+ ions and H2O follow
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Mucus
• Cystic fibrosis—hereditary
disease in which cells make
chloride pumps, but fail to install
them in the plasma membrane
Saline
layer
Epithelial
cells
– Chloride pumps fail to create
adequate saline layer on cell surface
(a)
Figure 3.12a
• Thick mucus plugs pancreatic
ducts and respiratory tract
– Inadequate digestion of nutrients and
absorption of oxygen
– Chronic respiratory infections
– Life expectancy of 30
3-39
Flagella
• Tail of a sperm—only functional flagellum
• Whiplike structure with axoneme identical to cilium
– Much longer than cilium
– Stiffened by coarse fibers that support the tail
• Movement is more undulating, snakelike
– No power stroke or recovery stroke as in cilia
3-40
Membrane Transport
• Expected Learning Outcomes
– Explain what is meant by a selectively permeable
membrane.
– Describe the various mechanisms for transporting
material through the plasma membrane.
– Define osmolarity and tonicity and explain their
importance.
3-41
Membrane Transport
• Plasma membrane—a barrier and a gateway between the
cytoplasm and ECF
– Selectively permeable—allows some things through, and prevents
other things from entering and leaving the cell
• Passive transport mechanisms require no ATP
– Random molecular motion of particles provides the necessary energy
– Filtration, diffusion, osmosis
• Active transport mechanisms consumes ATP
– Active transport and vesicular transport
• Carrier-mediated mechanisms use a membrane protein to
transport substances from one side of the membrane to the
other
3-42
Filtration
• Filtration—process in which
particles are driven through
a selectively permeable
Solute
membrane by hydrostatic Water
pressure (force exerted
Capillary wall
on a membrane by water)
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Blood pressure in capillary
forces water and small
solutes such as salts through
narrow clefts between
capillary cells.
Red blood
cell
• Examples
– Filtration of nutrients through
gaps in blood capillary walls
into tissue fluids
– Filtration of wastes from the
blood in the kidneys while
holding back blood cells and
proteins
Clefts hold back
larger particles
such as red blood
cells.
Figure 3.13
3-43
Simple Diffusion
• Simple diffusion—the net
movement of particles from
area of high concentration
to area of low concentration
– Due to their constant,
spontaneous motion
• Also known as movement
down the concentration
gradient—concentration of
a substance differs from
one point to another
Down
gradient
Up
gradient
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Figure 3.14
3-44
Simple Diffusion
• Factors affecting diffusion rate through a
membrane
– Temperature:  temp.,  motion of particles
– Molecular weight: larger molecules move slower
– Steepness of concentrated gradient: difference,  rate
– Membrane surface area:  area,  rate
– Membrane permeability:  permeability,  rate
3-45
Simple Diffusion
• Diffusion through lipid bilayer
– Nonpolar, hydrophobic, lipid-soluble substances
diffuse through lipid layer
• Diffusion through channel proteins
– Water and charged, hydrophilic solutes diffuse
through channel proteins in membrane
• Cells control permeability by regulating
number of channel proteins or by opening and
closing gates
3-46
Osmosis
• Osmosis—flow of water
from one side of a selectively
permeable membrane to the
other
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Side A
Side B
– From side with higher water
Solute
concentration to side with
Water
lower water concentration
– Reversible attraction of water
to solute particles forms
(a) Start
hydration spheres
– Makes those water molecules
less available to diffuse back to
the side from which they came
3-47
Osmosis
• Aquaporins—channel proteins
in plasma membrane
specialized for passage of water
– Cells can increase the rate of
osmosis by installing more
aquaporins
– Decrease rate by removing them
• Significant amounts of water
diffuse even through the
hydrophobic, phospholipid
regions of the plasma
membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Side A
Side B
Solute
Water
(a) Start
Figure 3.15a
3-48
Osmosis
• Osmotic pressure—
amount of hydrostatic
pressure required to stop
osmosis
• Reverse osmosis—
pressure applied to one
side, overrides pressure,
drives against
concentration gradient
– Heart drives water out of
capillaries by reverse
osmosis—capillary
filtration
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Osmotic
pressure
Hydrostatic
pressure
(b) 30 minutes later
Figure 3.15b
3-49
Osmolarity and Tonicity
• One osmole = 1 mole of dissolved particles
– 1 M NaCl (1 mole Na+ ions + 1 mole Cl- ions) thus 1 M
NaCl = 2 osm/L
• Osmolarity—number of osmoles of solute per liter of
solution
3-50
Osmolarity and Tonicity
• Osmolality—number of osmoles of solute per kilogram of
water
• Physiological solutions are expressed in milliosmoles per
liter (mOsm/L)
– Blood plasma = 300 mOsm/L
– Osmolality similar to osmolarity at concentration
of body fluids: less than 1% difference
• Tonicity—ability of a solution to affect fluid volume and
pressure in a cell
– Depends on concentration and permeability of solute
3-51
Osmolarity and Tonicity
• Hypotonic solution
– Has a lower concentration of nonpermeating solutes than
intracellular fluid (ICF)
• High water concentration
– Cells absorb water, swell, and may burst (lyse)
• Hypertonic solution
– Has a higher concentration of nonpermeating solutes
• Low water concentration
– Cells lose water + shrivel (crenate)
• Isotonic solution
– Concentrations in cell and ICF are the same
– Cause no changes in cell volume or cell shape
– Normal saline
3-52
Effects of Tonicity on RBCs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a) Hypotonic
(b) Isotonic
(c) Hypertonic
© Dr. David M. Phillips/Visuals Unlimited
Figure 3.16a
Figure 3.16b
Figure 3.16c
Hypotonic, isotonic, and hypertonic solutions affect the fluid
volume of a red blood cell. Notice the crenated and swollen cells.
3-53
Carrier-Mediated Transport
• Transport proteins in the plasma membrane that
carry solutes from one side of the membrane to the
other
• Specificity
– Transport proteins specific for a certain ligand
– Solute binds to a specific receptor site on carrier protein
– Differs from membrane enzymes because carriers do not
chemically change their ligand
• Simply picks them up on one side of the membrane, and releases
them, unchanged, on the other
• Saturation
– As the solute concentration rises, the rate of transport
rises, but only to a point—transport maximum (Tm)
• Two types of carrier-mediated transport
– Facilitated diffusion and active transport
3-54
Carrier-Mediated Transport
Rate of solute transport
(molecules/sec passing
through plasma membrane)
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Transport maximum (Tm)
Figure 3.17
Concentration of solute
• Transport maximum—transport rate when all
carriers are occupied
3-55
Carrier-Mediated Transport
• Uniport
– Carries only one solute at a time
• Symport
– Carries two or more solutes simultaneously in same direction
(cotransport)
• Antiport
– Carries two or more solutes in opposite directions
(countertransport)
– Sodium-potassium pump brings in K+ and removes Na+ from cell
• Carriers employ two methods of transport
– Facilitated diffusion
– Active transport
3-56
Carrier-Mediated Transport
• Facilitated diffusion—carrier-mediated transport of
solute through a membrane down its concentration
gradient
• Does not consume ATP
• Solute attaches to binding site on carrier, carrier
changes confirmation, then releases solute on other
side of membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ECF
Figure 3.18
ICF
1 A solute particle enters
the channel of a membrane
protein (carrier).
2 The solute binds to a receptor
site on the carrier and the
carrier changes conformation.
3 The carrier releases the
solute on the other side of
the membrane.
3-57
Carrier-Mediated Transport
• Active transport—carrier-mediated transport of
solute through a membrane up (against) its
concentration gradient
• ATP energy consumed to change carrier
• Examples of uses:
– Sodium–potassium pump keeps K+ concentration
higher inside the cell
– Bring amino acids into cell
– Pump Ca2+ out of cell
3-58
Carrier-Mediated Transport
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Each pump cycle consumes one
ATP and exchanges three Na+
for two K+
Glucose
Na+
Apical surface
SGLT
• Keeps the K+ concentration
higher and the Na+
concentration lower within the
cell than in ECF
• Necessary because Na+ and K+
constantly leak through
membrane
– Half of daily calories utilized for
Na+−K+ pump
Cytoplasm
Na+– K+
pump
ATP
ADP + Pi
Basal surface
Na+
3-59
Carrier-Mediated Transport
• Secondary active transport
– Steep concentration gradient maintained between one
side of the membrane and the other (water behind a
dam)
– Sodium–glucose transport protein (SGLT)
simultaneously binds Na+ and glucose and carries
both into the cell
– Does not consume ATP
• Regulation of cell volume
– ―Fixed anions‖ attract cations causing osmosis
– Cell swelling stimulates the Na+−K+ pump to
 ion concentration,  osmolarity and cell swelling
3-60
Carrier-Mediated Transport
• Maintenance of a membrane potential in all
cells
– Pump keeps inside more negative, outside more
positive
– Necessary for nerve and muscle function
• Heat production
– Thyroid hormone increases number of Na+−K+
pumps
– Consume ATP and produce heat as a by-product
3-61
Vesicular Transport
• Vesicular transport—processes that move large particles,
fluid droplets, or numerous molecules at once through the
membrane in vesicles—bubblelike enclosures of membrane
• Endocytosis—vesicular processes that bring material into
the cell
– Phagocytosis—―cell eating,‖ engulfing large particles
• Pseudopods; phagosomes; macrophages
– Pinocytosis—―cell drinking,‖ taking in droplets of ECF
containing molecules useful in the cell
• Pinocytic vesicle
– Receptor-mediated endocytosis—particles bind to specific
receptors on plasma membrane
• Clathrin-coated vesicle
• Exocytosis—discharging material from the cell
• Utilizes motor proteins energized by ATP
3-62
Vesicular Transport
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Particle
7 The indigestible
residue is voided by
exocytosis.
1 A phagocytic cell encounters a
particle of foreign matter.
Pseudopod
Residue
2 The cell surrounds
the particle with its
pseudopods.
Nucleus
Phagosome
6 The phagolysosome
fuses with the
plasma membrane.
Lysosome
Vesicle fusing
with membrane
3 The particle is phagocytized
and contained in a
phagosome.
Phagolysosome
5 Enzymes from the
lysosome digest the
foreign matter.
4 The phagosome fuses
with a lysosome and
becomes a phagolysosome.
Figure 3.21
3-63
Phagocytosis keeps tissues free of debris and infectious microorganisms
Vesicular Transport
• Pinocytosis or ―cell-drinking‖
– Taking in droplets of ECF
– Occurs in all human cells
• Membrane caves in, then pinches off into the
cytoplasm as pinocytotic vesicle
3-64
Vesicular Transport
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Extracellular
molecules
Receptor
Coated
pit
Clathrincoated
vesicle
Clathrin
1 Extracellular molecules bind to
receptors on plasma membrane;
receptors cluster together.
2 Plasma membrane sinks inward,
forms clathrin-coated pit.
(all): Company of Biologists, Ltd.
3 Pit separates from plasma
membrane, forms clathrin-coated
vesicle containing concentrated
molecules from ECF.
Figure 3.22
Receptor-mediated endocytosis
3-65
Vesicular Transport
• Receptor-mediated endocytosis
– More selective endocytosis
– Enables cells to take in specific molecules that bind to
extracellular receptors
• Clathrin-coated vesicle in cytoplasm
– Uptake of LDL from bloodstream
3-66
Vesicular Transport
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Capillary endothelial cell
Intercellular cleft
Capillary lumen
Pinocytotic vesicles
Muscle cell
Figure 3.23
Tissue fluid
© Don Fawcett/Photo Researchers, Inc.
0.25 m
• Transport of material across the cell by capturing it on one
side and releasing it on the other
• Receptor-mediated endocytosis moves it into the cell and
exocytosis moves it out the other side
– Insulin
3-67
Vesicular Transport
• Exocytosis
– Secreting material
– Replacement of plasma membrane removed by
endocytosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Dimple
Fusion pore
Secretion
Plasma
membrane
Linking
protein
Secretory
vesicle
(a)
(b)
1 A secretory vesicle approaches
2 The plasma membrane and
vesicle unite to form a fusion
the plasma membrane and docks
pore through which the vesicle
on it by means of linking proteins.
contents are released.
The plasma membrane caves in
at that point to meet the vesicle.
Courtesy of Dr. Birgit Satir, Albert Einstein College of Medicine
Figure 3.24a,b
3-68
The Cell Interior
• Expected Learning Outcomes
– List the main organelles of a cell, describe
their structure, and explain their functions.
– Describe the cytoskeleton and its functions.
– Give some examples of cell inclusions and
explain how inclusions differ from organelles.
3-69
The Cell Interior
• Structures in the cytoplasm
– Organelles, cytoskeleton, and inclusions
– All embedded in a clear gelatinous cytosol
3-70
The Cytoskeleton
• Cytoskeleton—collection of filaments and cylinders
– Determines shape of cell, lends structural support,
organizes its contents, directs movement of
substances through the cell, and contributes to the
movements of the cell as a whole
• Composed of:
– Microfilaments: 6 nm thick, actin, forms terminal web
– Intermediate fibers: 8–10 nm, support, strength, and
structure
– Microtubules: 25 nm, tubulin, movement
3-71
Cytoskeleton
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Microvilli
Microfilaments
Terminal web
Secretory
vesicle in
transport
Desmosome
Lysosome
Kinesin
Microtubule
Intermediate
filaments
Intermediate
filaments
Microtubule
in the process
of assembly
Centrosome
Microtubule
undergoing
disassembly
Nucleus
Mitochondrion
(a)
Basement
membrane
Hemidesmosome
Figure 3.25a
3-72
EM and Fluorescent Antibodies
Demonstrate Cytoskeleton
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(b)
15 m
© K.G. Murti/Visuals Unlimited
Figure 3.25b
3-73
Microtubules
• A microtubule is a cylinder of 13 parallel strands called
protofilaments
– Each protofilament is a long chain of globular proteins called tubulin
• Microtubules radiate from centrosome and hold organelles in place,
form bundles that maintain cell shape and rigidity, and act somewhat
like railroad tracks
– Motor proteins ―walk‖ along these tracks carrying organelles and
other macromolecules to specific locations in the cell
• Not permanent structures, they come and go moment by moment
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(a)
(b)
(c)
Microtubule
Protofilaments
Dynein arms
Tubulin
Figure 3.26
3-74
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
3-75
The Nucleus
• Nucleus—largest organelle (5 mm in diameter)
– Most cells have one nucleus
– A few cells are anuclear or multinucleate
• Nuclear envelope—two unit membranes surround
nucleus
– Perforated by nuclear pores formed by rings of protein
• Regulate molecular traffic through envelope
• Hold two unit membranes together
3-76
The Nucleus
Cont.
– Supported by nuclear lamina
•
•
•
•
Web of protein filaments
Supports nuclear envelope and pores
Provides points of attachment and organization for chromatin
Plays role in regulation of the cell life cycle
• Nucleoplasm—material in nucleus
– Chromatin (threadlike matter) composed of DNA and
protein
– Nucleoli—one or more dark masses where ribosomes
are produced
3-77
The Nucleus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nuclear
pores
Nucleolus
Nucleoplasm
Nuclear
envelope
(a) Interior of nucleus
2 mm
(b) Surface of nucleus
1.5 mm
a: © Richard Chao; b: © E.G. Pollock
Figure 3.27a
Figure 3.27b
3-78
Endoplasmic Reticulum
• Endoplasmic reticulum—system of
interconnected channels called cisternae enclosed
by unit membrane
• Rough endoplasmic reticulum—composed of
parallel, flattened sacs covered with ribosomes
– Continuous with outer membrane of nuclear envelope
– Adjacent cisternae are often connected by perpendicular
bridges
– Produces the phospholipids and proteins of the plasma
membrane
– Synthesizes proteins that are packaged in other
organelles or secreted from cell
3-79
Endoplasmic Reticulum
• Smooth endoplasmic reticulum
– Lack ribosomes
– Cisternae more tubular and branching
– Cisternae are thought to be continuous with those of
rough ER
– Synthesizes steroids and other lipids
– Detoxifies alcohol and other drugs
– Manufactures all membranes of the cell
• Rough and smooth ER are functionally different
parts of the same network
3-80
Smooth and Rough ER
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Rough endoplasmic
reticulum
Ribosomes
Cisternae
(c)
Smooth
endoplasmic
reticulum
Figure 3.28c
3-81
Ribosomes
• Ribosomes—small granules of protein and
RNA
– Found in nucleoli, in cytosol, and on outer surfaces of
rough ER, and nuclear envelope
• They ―read‖ coded genetic messages
(messenger RNA) and assemble amino acids
into proteins specified by the code
3-82
Golgi Complex
• Golgi complex—a small system of cisternae that
synthesize carbohydrates and put the finishing
touches on protein and glycoprotein synthesis
– Receives newly synthesized proteins from rough ER
– Sorts them, cuts and splices some of them, adds
carbohydrate moieties to some, and packages the
protein into membrane-bound Golgi vesicles
• Some become lysosomes
• Some migrate to plasma membrane and fuse to it
• Some become secretory vesicles for later release
3-83
Golgi Complex
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Golgi vesicles
Golgi
complex
Figure 3.29
600 nm
Visuals Unlimited
3-84
Lysosomes
• Lysosomes—package of enzymes bound by a
single unit membrane
– Extremely variable in shape
• Functions
– Intracellular hydrolytic digestion of proteins, nucleic
acids, complex carbohydrates, phospholipids, and other
substances
– Autophagy—digest and dispose of worn out
mitochondria and other organelles
– Autolysis—―cell suicide‖: some cells are meant to do a
certain job and then destroy themselves
3-85
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
3-86
Lysosome and Peroxisomes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Mitochondria
Peroxisomes
Lysosomes
Smooth ER
Golgi
complex
(a) Lysosomes
1 mm
(b) Peroxisomes
0.3 mm
© Don Fawcett/Photo Researchers, Inc.
Figure 3.30a
Figure 3.30b
3-87
Mitochondria
• Mitochondria—organelles specialized
for synthesizing ATP
• Variety of shapes: spheroid, rod-shaped,
kidney-shaped, or threadlike
• Surrounded by a double unit membrane
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
– Inner membrane has folds called cristae
– Spaces between cristae called matrix
• Matrix contains ribosomes, enzymes
used for ATP synthesis, small circular
DNA molecule
– Mitochondrial DNA (mtDNA)
Matrix
Outer membrane
Inner membrane
Mitochondrial
ribosome
Intermembrane
space
Crista
• ―Powerhouses‖ of the cell
– Energy is extracted from organic
molecules and transferred to ATP
Figure 3.31
3-88
Mitochondrion
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Matrix
Outer membrane
Inner membrane
Mitochondrial
ribosome
Figure 3.31
Intermembrane
space
Crista
1 m
3-89
(Left): Dr. Donald Fawcett & Dr. Porter/Visuals Unlimited
Evolution of 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
3-90
Evolution of Mitochondrion
– 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
3-91
Centrioles
• Centriole—a short cylindrical assembly of
microtubules arranged in nine groups of
three microtubules each
• Two centrioles lie perpendicular to each
other within a small, clear area of
cytoplasm called the centrosome
– Play role in cell division
3-92
Centrioles
• Cilia and flagella formation
– Each basal body of a cilium or flagellum is a single
centriole oriented perpendicular to plasma membrane
– Basal bodies originate in centriolar organizing center
• Migrate to plasma membrane
– Two microtubules of each triplet elongate to form the
nine pairs of peripheral microtubules of the axoneme
– Cilium reaches full length in less than an hour
3-93
Centrioles
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 3.32a,b
Microtubules
Protein
link
Longitudinal
view
Cross
section
0.1 m
(a) Cross section (TEM)
Microtubules
(b) Pair of centrioles
3-94
a: From: Manley McGill, D.P. Highfield, T.M. Monahan, and B.R. Brinkley Effects of Nucleic Acid Specific Dyes on Centrioles
of Mammalian Cells, published in the Journal of Ultrastructure Research 57, 43-53 (1976), pg. 48, fig. 6, with permission from Elsevier
Inclusions
• Two kinds of inclusions
– Stored cellular products
• Glycogen granules, pigments, and fat droplets
– Foreign bodies
• Viruses, intracellular bacteria, dust particles, and other debris
phagocytized by a cell
• Never enclosed in a unit membrane
• Not essential for cell survival
3-95
Inclusions
3-96