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Chapter 3: Cells
• Overview
• Plasma membrane: structure
• Plasma membrane: transport
• Resting membrane potential
• Cell-environment interactions
• Cytoplasm
• Nucleus
• Cell growth & reproduction
• Extracellular materials
• Developmental aspects
Department of Kinesiology and Applied Physiology
WCR
Do exercise scientists need to think about cells?
Exercise in a Pill
Narkar et al.,
•
•
“AMPK and PPARδ Agonists Are Exercise Mimetics” Cell 2008.
AICAR activates intracellular pathways that are also activated
by exercise. Mice taking AICAR look like mice on exercise.
Mice on AICAR plus exercise are supermice.
Department of Kinesiology and Applied Physiology
2
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 (d) Cell that
nutrients
fights disease
Figure 3.1
Generalized Cell
• All cells have some common structures and
functions
• Human cells have three basic parts:
– Plasma membrane—flexible outer boundary
– Cytoplasm—intracellular fluid containing
organelles
– Nucleus—control center
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
• Bimolecular layer of lipids and proteins in a
constantly changing fluid mosaic
• Plays a dynamic role in cellular activity
• Separates intracellular fluid from extracellular
fluid
– Interstitial fluid = ECF that surrounds cells
Extracellular fluid
(watery environment)
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)
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Figure 3.3
Membrane Proteins
• Integral proteins
– Firmly inserted into the membrane (most are
transmembrane)
– Functions:
• Transport proteins (channels and carriers), enzymes, or
receptors
PLAY
Animation: Transport Proteins
Membrane Proteins
• 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
PLAY
Animation: Structural Proteins
PLAY
Animation: Receptor Proteins
(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.
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Figure 3.4a
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
(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
(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
(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
(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
Membrane Junctions
• Three types:
– Tight junction
– Desmosome
– Gap junction
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Interlocking
junctional proteins
Intercellular
space
(a) Tight junctions: Impermeable junctions prevent molecules
from passing through the intercellular space.
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Figure 3.5a
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular space
Plaque
Intermediate
filament (keratin)
Linker glycoproteins
(cadherins)
(b) Desmosomes: Anchoring junctions bind adjacent cells together
and help form an internal tension-reducing network of fibers.
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Figure 3.5b
Plasma membranes
of adjacent cells
Microvilli
Intercellular
space
Basement membrane
Intercellular
space
Channel
between cells
(connexon)
(c) Gap junctions: Communicating junctions allow ions and small molecules to pass from one cell to the next for intercellular communication.
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Figure 3.5c
Membrane Transport:
How things get in and out of cells
Plasma membranes are selectively
permeable: some molecules easily pass
through the membrane; others do not
Types of Membrane Transport
Passive processes
– No cellular energy (ATP) required
– Substance moves down its concentration gradient
Active processes
– Energy (ATP) required
– Occurs only in living cell membranes
Passive Processes
•
What determines whether or not a
substance can passively permeate a
membrane?
1. Lipid solubility of substance
2. Channels of appropriate size
3. Carrier proteins
PLAY
Animation: Membrane Permeability
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Passive Processes
• Simple diffusion
• Carrier-mediated facilitated diffusion
• Channel-mediated facilitated diffusion
• Osmosis
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Passive Processes: Simple Diffusion
• Nonpolar lipid-soluble (hydrophobic)
substances diffuse directly through the
phospholipid bilayer
PLAY
Animation: Diffusion
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Extracellular fluid
Lipidsoluble
solutes
Cytoplasm
(a) Simple diffusion of fat-soluble molecules
directly through the phospholipid bilayer
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Figure 3.7a
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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Lipid-insoluble
solutes (such as
sugars or amino
acids)
(b) Carrier-mediated facilitated diffusion via a protein
carrier specific for one chemical; binding of substrate
causes shape change in transport protein
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Figure 3.7b
Small lipidinsoluble
solutes
(c) Channel-mediated facilitated diffusion
through a channel protein; mostly ions
selected on basis of size and charge
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Figure 3.7c
Passive Processes: Osmosis
• Movement of solvent (water) across a
selectively permeable membrane
• Water diffuses through plasma membranes:
• Through the lipid bilayer
• Through water channels called aquaporins
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Water
molecules
Lipid
billayer
Aquaporin
(d) Osmosis, diffusion of a solvent such as
water through a specific channel protein
(aquaporin) or through the lipid bilayer
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Figure 3.7d
Importance of Osmosis
• When osmosis occurs, water enters or leaves
a cell
• Change in cell volume disrupts cell function
PLAY
Animation: Osmosis
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Tonicity
• Tonicity: How much dissolved material there
is in a solution. Tonicity determines whether a
solution will make cells shrink or swell.
• Isotonic: A solution with the same solute
concentration as the inside of a normal cell
• Hypertonic: A solution with a greater solute
concentration than than a normal cell
• Hypotonic: A solution with a lesser solute
concentration than a normal cell
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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
Membrane Transport: Active Processes
• Two types of active processes:
• Active transport
• Vesicular transport
• Both use ATP to move solutes across a living
plasma membrane
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Active Transport
• Requires carrier proteins (solute pumps)
• Moves solutes against a concentration
gradient
• Types of active transport:
• Primary active transport
• Secondary active transport
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Primary Active Transport
• Energy from breakdown of ATP causes shape change in
transport protein to “pump” molecules across the
membrane
• Example: Sodium-potassium pump (Na+-K+ ATPase)
• Located in all plasma membranes
• Involved in primary and secondary active transport of
nutrients and ions
• Maintains “electrochemical gradients” essential for
functions of muscle and nerve tissues
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Extracellular fluid
Na+
Na+-K+ pump
Na+ bound
K+
ATP-binding site
Cytoplasm
1 Cytoplasmic Na+ binds to pump protein.
P
ATP
K+ released
ADP
6 K+ is released from the pump protein
and Na+ sites are ready to bind Na+ again.
The cycle repeats.
2 Binding of Na+ promotes
phosphorylation of the protein by ATP.
Na+ released
K+ bound
P
Pi
K+
5 K+ binding triggers release of the
phosphate. Pump protein returns to its
original conformation.
3 Phosphorylation causes the protein to
change shape, expelling Na+ to the outside.
P
4 Extracellular K+ binds to pump protein.
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Figure 3.10
Secondary Active Transport
• Energy stored in ionic gradients is used indirectly to drive
transport of other solutes
• Always involves cotransport – transport of more than one
substance at a time
• Two substances transported in same direction (Na+,
glucose)
• Two substances transported in opposite directions
(Na+, H+)
Mod WCR
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Extracellular fluid
Glucose
Na+-K+
pump
Na+-glucose
symport
transporter
loading
glucose from
ECF
Na+-glucose
symport transporter
releasing glucose
into the cytoplasm
Cytoplasm
1 The ATP-driven Na+-K+ pump
2 As Na+ diffuses back across the
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
membrane through a membrane
cotransporter protein, it drives glucose
against its concentration gradient
into the cell. (ECF = extracellular fluid)
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Figure 3.11
Vesicular Transport
• Transport of large particles, macromolecules, and fluids
across plasma membranes
• Requires cellular energy (e.g., ATP)
• Functions:
• Exocytosis—transport out of cell
• Endocytosis—transport into cell (receptor mediated;
phago-; pino-)
• Transcytosis—transport into, across, and then out of cell
• Vesicular transport within a cell (see the video)
Mod WCR
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Phagosome
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(a) Endocytosis:
Phagocytosis
The cell engulfs a large
particle by forming projecting pseudopods (“false
feet”) around it and enclosing it within a membrane
sac called a phagosome.
The phagosome is
combined with a lysosome.
Undigested contents remain
in the vesicle (now called a
residual body) or are ejected
by exocytosis. Vesicle may
or may not be proteincoated but has receptors
capable of binding to
microorganisms or solid
particles.
Figure 3.13a
(b) Endocytosis:
Pinocytosis
The cell “gulps” drops of
extracellular fluid containing
solutes into tiny vesicles. No
receptors are used, so the
process is nonspecific. Most
vesicles are protein-coated.
Vesicle
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Figure 3.13b
Vesicle
Receptor recycled
to plasma membrane
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(c) Receptor-mediated
endocytosis
Extracellular substances
bind to specific receptor
proteins in regions of coated
pits, enabling the cell to
ingest and concentrate
specific substances
(ligands) in protein-coated
vesicles. Ligands may
simply be released inside
the cell, or combined with a
lysosome to digest contents.
Receptors are recycled to
the plasma membrane in
vesicles.
Figure 3.13c
Exocytosis
Plasma membrane
Extracellular
SNARE (t-SNARE)
fluid
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
Fusion pore formed
1 Membrane-
bound vesicle
migrates to
plasma membrane.
2 Proteins at
vesicle surface (vFused SNAREs) bind with
v- and t-SNAREs (plasma
t-SNAREs membrane
proteins).
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3 Vesicle and
plasma membrane
fuse and pore
opens up.
4 Vesicle contents
released to cell
exterior.
Figure 3.14a
Summary of Active Processes
Process
Energy Source
Example
Primary active transport
ATP
Pumping of ions across
membranes
Secondary active
transport
Ion gradient
Movement of polar or charged
solutes across membranes
Exocytosis
ATP
Secretion of hormones and
neurotransmitters
Phagocytosis
ATP
White blood cell phagocytosis
Pinocytosis
ATP
Absorption by intestinal cells
Receptor-mediated
endocytosis
ATP
Hormone and cholesterol uptake
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