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prepared by
Janice Meeking,
Mount Royal College
CHAPTER
3
Cells: The
Living Units:
Part A
Copyright © 2010 Pearson Education, Inc.
Cell Theory
• The cell is the smallest structural and
functional unit of life
• Continuity of life has a cellular basis: all cells
come from pre-existing cells
• Organismal functions depend on individual
and collective cell functions
• Biochemical activities of cells are dictated by
their specific subcellular structures
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Cell Diversity
•
•
Over 200 different types of human cells
Types differ in size, shape, subcellular components, and functions
Erythrocytes
Fibroblasts
Epithelial cells
(a) Cells that connect body parts,
form linings, or transport gases
Skeletal
Muscle
cell
Smooth
muscle cells
(b) Cells that move organs and
body parts
Macrophage
Nerve cell
(e) Cell that gathers information
and control body functions
(f) Cell of reproduction
Sperm
Fat cell
(c) Cell that stores
nutrients
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(d) Cell that
fights disease
Figure 3.1
Basic Features Of All Cells
•
All cells have:
 Plasma membrane
 Semifluid substance called cytosol
 Chromosomes (carry genes)
 Ribosomes (make proteins)
• Human cells have plasma membrane, cytoplasm and
nucleus
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Basic Features Of A Eukaryotic Cell
Chromatin
Nucleolus
Nuclear envelope
Nucleus
Smooth endoplasmic
reticulum
Mitochondrion
Cytosol
Lysosome
Centrioles
Centrosome
matrix
Cytoskeletal
elements
• Microtubule
• Intermediate
filaments
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Plasma
membrane
Rough
endoplasmic
reticulum
Ribosomes
Golgi apparatus
Secretion being
released from cell
by exocytosis
Peroxisome
Figure 3.2
Plasma Membrane
Extracellular fluid
(watery environment)
• Biomolecular double-layered
structures of lipids and
proteins forming the outer
envelope of the cell
• Plays a dynamic role in
cellular activity
• Functions as a selective
barrier between extracellular
fluid (ECF) and intracellular
fluid (ICF), allowing passage
of oxygen, nutrients, and
wastes for the whole volume
of the cell
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Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Outwardfacing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Peripheral
Bimolecular
Inward-facing
proteins
lipid layer
layer of
containing
phospholipids
Nonpolar
proteins
tail of
phospholipid
Cytoplasm
molecule
(watery environment)
Membrane Lipids And Lipid Rafts
LIPIDS
Extracellular fluid
(watery environment)
• 75% phospholipids (lipid bilayer)
•
Phosphate heads: polar and
hydrophilic
•
Fatty acid tails: nonpolar and
hydrophobic (Review Fig. 2.16b)
• 5% glycolipids
•
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
Lipids with polar sugar groups on
outer membrane surface
• 20% cholesterol
•
Polar head of
phospholipid
molecule
Increases membrane stability and
fluidity
LIPID RAFTS
• 20% of the outer membrane surface
• Contain phospholipids, sphingolipids,
and cholesterol
• May function as stable platforms for
cell-signaling molecules
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Outwardfacing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Peripheral
Bimolecular
Inward-facing
proteins
lipid layer
layer of
containing
phospholipids
Nonpolar
proteins
tail of
phospholipid
Cytoplasm
molecule
(watery environment)
Membrane Proteins
• Integral proteins:
Extracellular fluid
(watery environment)
• Firmly inserted into the
membrane (most are
transmembrane)
Functions: Transport proteins
(channels and carriers), enzymes,
or receptors
Polar head of
phospholipid
molecule
Cholesterol
Glycolipid
Glycoprotein
Carbohydrate
of glycocalyx
• Peripheral proteins
• Loosely attached to integral
proteins
• Include filaments on intracellular
surface and glycoproteins on
extracellular surface
Functions: Enzymes, motor
proteins, cell-to-cell links, provide
support on intracellular surface,
and form part of glycocalyx
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Outwardfacing
layer of
phospholipids
Integral
proteins
Filament of
cytoskeleton
Peripheral
Bimolecular
Inward-facing
proteins
lipid layer
layer of
containing
phospholipids
Nonpolar
proteins
tail of
phospholipid
Cytoplasm
molecule
(watery environment)
Functions of Membrane Proteins
1. Transport
(a) Transport
A protein (left) that spans the membrane
may provide a hydrophilic channel across
the membrane that is selective for a
particular solute. Some transport proteins
(right) hydrolyze ATP as an energy source
to actively pump substances across the
membrane.
Figure 3.4a
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Functions of Membrane Proteins
2. Receptors for Signal Transduction
Signal
Receptor
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(b) Receptors for signal transduction
A membrane protein exposed to the
outside of the cell may have a binding
site with a specific shape that fits the
shape of a chemical messenger, such
as a hormone. The external signal may
cause a change in shape in the protein
that initiates a chain of chemical
reactions in the cell.
Figure 3.4b
Functions of Membrane Proteins
3. Attachment to Cytoskeleton and
Extracellular Matrix (ECM)
(c) Attachment to the cytoskeleton
and extracellular matrix (ECM)
Elements of the cytoskeleton (cell’s
internal supports) and the extracellular
matrix (fibers and other substances
outside the cell) may be anchored to
membrane proteins, which help maintain
cell shape and fix the location of certain
membrane proteins. Others play a role in
cell movement or bind adjacent cells
together.
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Figure 3.4c
Functions of Membrane Proteins
4. Enzymatic Activity
(d) Enzymatic activity
Enzymes
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A protein built into the membrane may
be an enzyme with its active site
exposed to substances in the adjacent
solution. In some cases, several
enzymes in a membrane act as a team
that catalyzes sequential steps of a
metabolic pathway as indicated (left to
right) here.
Figure 3.4d
Functions of Membrane Proteins
5. Intercellular Joining
(e) Intercellular joining
Membrane proteins of adjacent cells
may be hooked together in various
kinds of intercellular junctions. Some
membrane proteins (CAMs) of this
group provide temporary binding sites
that guide cell migration and other
cell-to-cell interactions.
CAMs
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Figure 3.4e
Functions of Membrane Proteins
6. Cell-cell Recognition
(f) Cell-cell recognition
Some glycoproteins (proteins bonded
to short chains of sugars) serve as
identification tags that are specifically
recognized by other cells.
Glycoprotein
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Figure 3.4f
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
Glycoprotein
Major
(d) Cell-cell recognition (e) Intercellular joining (f) Attachment to
the cytoskeleton
and extracellular
Functions Of Membrane Proteins
matrix (ECM)
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Membrane Junctions
1. Tight Junctions: Impermeable junctions that prevent fluids and most
molecules from moving through the intercellular space (space between cells)
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Interlocking
junctional proteins
Intercellular
space
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Figure 3.5a
Membrane Junctions
2. Desmosomes: Anchoring junctions (like “rivets” or “spot welds”)
bind adjacent cells together and help form an internal tension-reducing
network of fibers.
Example: Found in cardiac or smooth muscle cells for spread of ions
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular space
Plaque
Intermediate
filament (keratin)
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Linker glycoproteins
(cadherins)
Figure 3.5b
Membrane Junctions
3. Gap Junctions: Transmembrane proteins that form communicating
junctions to allow ions and small molecules to pass from one cell to the
next for intercellular communication
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular
space
Channel
between cells
(connexon)
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Figure 3.5c
Membrane Transport
• Plasma membranes are selectively permeable, allowing the passage
of some, but not all molecules
• Transport of molecules across cell membranes occurs by passive or
active processes
 Passive processes
 No cellular energy (ATP) required
 Substance moves down its concentration gradient
 Active processes
 Energy (ATP) required
 Occurs only in living cell membranes
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Passive Processes Of Membrane Transport
•
Whether or not a substance can passively permeate a
membrane is determined by:
1. Lipid solubility of the substance
2. Size of membrane channels
3. Carrier proteins in the membranes
• Passive processes can be categorized as:
1. Simple diffusion
2. Carrier-mediated facilitated diffusion
3. Channel-mediated facilitated diffusion
4. Osmosis
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Passive Processes: Simple Diffusion
• Nonpolar lipid-soluble (hydrophobic) substances diffuse directly
through the phospholipid bilayer
Extracellular fluid
Lipidsoluble
solutes
Cytoplasm
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Passive Processes: Facilitated Diffusion
• Certain lipophobic molecules (e.g., glucose, amino acids,
and ions) use carrier proteins or channel proteins, both
of which:
• Exhibit specificity (selectivity)
• Are saturable; rate is determined by number of carriers
or channels
• Can be regulated in terms of activity and quantity
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Facilitated Diffusion Using Carrier Proteins
• Transmembrane integral proteins transport specific polar molecules
(e.g., sugars and amino acids)
• Binding of substrate causes shape change in carrier
Lipid-insoluble
solutes (such as
sugars or amino
acids)
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Facilitated Diffusion Using Channel Proteins
• Aqueous channels formed by transmembrane proteins selectively transport ions
or water
• Two types: (a) Leakage channels (always open); (b) Gated channels: controlled
by chemical or electrical signals
Small lipidinsoluble
solutes
Channel-mediated facilitated diffusion through a channel protein; mostly ions
selected on basis of size and charge
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Passive Processes: Osmosis
• Movement of solvent
(water) across a
selectively permeable
membrane
Water
molecules
• Water diffuses through
plasma membranes:
Lipid
billayer
 Through the lipid
bilayer
 Through water
channels called
aquaporins (AQPs)
Aquaporin
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Osmosis, diffusion of a solvent such as
water through a specific channel protein
(aquaporin) or through the lipid bilayer
Passive Processes: Osmosis
• Water concentration is determined by solute
concentration because solute particles displace water
molecules
• Osmolarity: The measure of total concentration of solute
particles
• When solutions of different osmolarity are separated by a
semi-permeable membrane, osmosis occurs until
equilibrium is reached
Importance of Osmosis:
• When osmosis occurs, water enters or leaves a cell
• Change in cell volume disrupts cell function
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Passive Processes: Diffusion
(a)
Membrane permeable to both solutes and water
Solute and water molecules move down their concentration gradients
in opposite directions. Fluid volume remains the same in both compartments.
Left
compartment:
Solution with
lower osmolarity
Right
compartment:
Solution with
greater osmolarity
Both solutions have the
same osmolarity: volume
unchanged
H2O
Solute
Membrane
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Solute
molecules
(sugar)
Figure 3.8a
Passive Processes: Osmosis
(b)
Membrane permeable to water, impermeable to solutes
Solute molecules are prevented from moving but water moves by osmosis.
Volume increases in the compartment with the higher osmolarity.
Left
compartment
Right
compartment
Both solutions have identical
osmolarity, but volume of the
solution on the right is greater
because only water is
free to move
H2O
Membrane
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Solute
molecules
(sugar)
Figure 3.8b
Tonicity
• Tonicity: The ability of a solution to cause a
cell to shrink or swell
• Isotonic: A solution with the same solute
concentration as that of the cytosol
• Hypertonic: A solution having greater solute
concentration than that of the cytosol
• Hypotonic: A solution having lesser solute
concentration than that of the cytosol
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Effect Of Tonicity On Red Blood Cells
(a)
Isotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
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(b)
Hypertonic solutions
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of solutes than are present inside
the cells).
(c)
Hypotonic solutions
Cells take on water by osmosis until
they become bloated and burst (lyse)
in a hypotonic solution (contains a
lower concentration of solutes than
are present in cells).
Figure 3.9
Summary of Passive Processes
Process
Simple
diffusion
Facilitated
diffusion
Osmosis
Energy
Source
Kinetic
energy
Kinetic
energy
Kinetic
energy
• Also see Table 3.1
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Example
Movement of O2 through
phospholipid bilayer
Movement of glucose into
cells
Movement of H2O through
phospholipid bilayer or
AQPs