Chapter 3
Cells:
The Living
Units
8/19/2015
© Annie Leibovitz/Contact
Press Images
MDufilho
1
3.1 Cells: The Living Units
• Cell theory
– A cell is the structural and functional unit of life
– How well the entire organism functions depends
on individual and combined activities of all of its
cells
– Structure and function are complementary
• Biochemical functions of cells are dictated by shape of
cell and specific subcellular structures
– Continuity of life has cellular basis
• Cells can arise only from other preexisting cells
8/19/2015
MDufilho
2
Figure 3.1 Cell diversity.
Erythrocytes
Fibroblasts
Skeletal
muscle
cell
Smooth
muscle cells
Epithelial cells
Cells that connect body parts, form linings,
or transport gases
Cells that move organs and body parts
Macrophage
Fat cell
Cell that stores
nutrients
Nerve cell
Cell that fights
disease
Cell that gathers information and controls
body functions
Sperm
Cell of reproduction
8/19/2015
3
MDufilho
Figure 3.3 The plasma membrane.
Extracellular fluid
(watery environment
outside cell)
Cholesterol
Polar head of
phospholipid
molecule
Glycocalyx
(carbohydrates)
Glycolipid
Glycoprotein
Nonpolar tail
of phospholipid
molecule
Lipid bilayer
containing proteins
Outward-facing
layer of
phospholipids
Inward-facing layer
of phospholipids
Functions of the
Plasma Membrane:
• Mechanical barrier: Separates two
of the body’s fluid compartments.
• Selective permeability: Determines
manner in which substances enter
or exit the cell.
• Electrochemical gradient:
Generates and helps to maintain
the electrochemical gradient required
for muscle and neuron function.
Filament of
cytoskeleton
Integral
proteins
• Communication: Allows cell-to-cell recognition
(e.g., of egg by sperm) and interaction.
8/19/2015
• Cell signaling: Plasma membrane proteins
interact with specific chemical messengers
and relay messages to the cell interior.
MDufilho
Peripheral
proteins
Cytoplasm
(watery environment
inside cell)
4
Animation: Membrane Structure
8/19/2015
MDufilho
5
Membrane Lipids
• Lipid bilayer is made up of:
– 75% phospholipids, which consist of two parts:
• Phosphate heads: are polar (charged), so are
hydrophilic (water-loving)
• Fatty acid tails: are nonpolar (no charge), so are
hydrophobic (water-hating)
– 5% glycolipids
• Lipids with sugar groups on outer membrane surface
– 20% cholesterol
• Increases membrane stability
8/19/2015
MDufilho
6
Membrane Proteins
• Allow cell communication with environment
• Make up about half the mass of plasma
membrane
• Most have specialized membrane functions
• Some float freely, and some are tethered to
intracellular structures
• Two types:
– Integral proteins; peripheral proteins
8/19/2015
MDufilho
7
Membrane Proteins (cont.)
• Integral proteins
– Firmly inserted into membrane
– Most are transmembrane proteins (span
membrane)
– Have both hydrophobic and hydrophilic regions
• Hydrophobic areas interact with lipid tails
• Hydrophilic areas interact with water
– Function as transport proteins (channels and
carriers), enzymes, or receptors
8/19/2015
MDufilho
8
Membrane Proteins (cont.)
• Peripheral proteins
– Loosely attached to integral proteins
– Include filaments on intracellular surface used for
plasma membrane support
– Function as:
• Enzymes
• Motor proteins for shape change during cell division
and muscle contraction
• Cell-to-cell connections
8/19/2015
MDufilho
9
Figure 3.3 The plasma membrane.
Extracellular fluid
(watery environment
outside cell)
Cholesterol
Polar head of
phospholipid
molecule
Glycocalyx
(carbohydrates)
Glycolipid
Glycoprotein
Nonpolar tail
of phospholipid
molecule
Lipid bilayer
containing proteins
Outward-facing
layer of
phospholipids
Inward-facing layer
of phospholipids
Functions of the
Plasma Membrane:
• Mechanical barrier: Separates two
of the body’s fluid compartments.
• Selective permeability: Determines
manner in which substances enter
or exit the cell.
• Electrochemical gradient:
Generates and helps to maintain
the electrochemical gradient required
for muscle and neuron function.
Filament of
cytoskeleton
Integral
proteins
• Communication: Allows cell-to-cell recognition
(e.g., of egg by sperm) and interaction.
8/19/2015
• Cell signaling: Plasma membrane proteins
interact with specific chemical messengers
and relay messages to the cell interior.
MDufilho
Peripheral
proteins
Cytoplasm
(watery environment
inside cell)
10
Figure 3.4 Membrane proteins perform many tasks.
Enzymatic activity
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.
Enzymes
• A membrane protein may be an enzyme
with its active site exposed to substances
in the adjacent solution.
• A team of several enzymes in a membrane
may catalyze sequential steps of a
metabolic pathway as indicated (left to
right) here.
ATP
Receptors for signal transduction
Signal
Intercellular joining
• A membrane protein exposed to the
outside of the cell may have a binding site
that fits the shape of a specific chemical
messenger, such as a hormone.
• When bound, the chemical messenger
may cause a change in shape in the
protein that initiates a chain of chemical
reactions in the cell.
• Membrane proteins of adjacent cells may
be hooked together in various kinds of
intercellular junctions.
• Some membrane proteins (cell adhesion
molecules or CAMs) of this group provide
temporary binding sites that guide cell
migration and other cell-to-cell
interactions.
Receptor
CAMs
Cell-cell recognition
Attachment to the cytoskeleton
and extracellular matrix
• Some glycoproteins (proteins bonded to
short chains of sugars which help to make
up the glycocalyx) serve as identification
tags that are specifically recognized by
other cells.
• Elements of the cytoskeleton (cell’s internal
supports) and the extracellular matrix
(fibers and other substances outside the
cell) may anchor to membrane proteins,
which helps maintain cell shape and fix the
location of certain membrane proteins.
• Others play a role in cell movement or bind
adjacent cells together.
Glycoprotein
8/19/2015
11
MDufilho
Membrane Transport Definitions
• Concentration = grams of
solutes/100 ml water
= % or osmoles
• Concentration gradient – difference
in concentration
• Equilibrium – no difference in
concentration
8/19/2015
MDufilho
12
How do substances move across the plasma
membrane?
• Plasma membranes are selectively permeable
– Some molecules pass through easily; some do
not
• Two ways substances cross membrane
– Passive processes: no energy required
– Active processes: energy (ATP) required
8/19/2015
MDufilho
13
3.3 Passive Membrane Transport
• Passive transport requires no energy
• Two types of passive transport
– Diffusion
• Simple diffusion
• Carrier- and channel-mediated facilitated diffusion
• Osmosis
– Filtration
• Type of transport that usually occurs across capillary
walls
8/19/2015
MDufilho
14
Diffusion
• Collisions between molecules in areas of high
concentration cause them to be scattered into
areas with less concentration
– Difference is called concentration gradient
– Diffusion is movement of molecules down their
concentration gradients (from high to low)
• Energy is not required
• Speed of diffusion is influenced by size of
molecule and temperature
8/19/2015
MDufilho
15
Figure 3.6 Diffusion.
Dye pellet
8/19/2015
Diffusion occurring
Dye evenly distributed
16
MDufilho
Figure 3.7a Diffusion through the plasma membrane.
Extracellular fluid
Lipidsoluble
solutes
Cytoplasm
Simple diffusion
of fat-soluble
molecules directly
through the
phospholipid bilayer
8/19/2015
MDufilho
17
Diffusion (cont.)
• Facilitated diffusion
– Certain hydrophobic molecules (e.g., glucose,
amino acids, and ions) are transported passively
down their concentration gradient by:
• Carrier-mediated facilitated diffusion
– Substances bind to protein carriers
• Channel-mediated facilitated diffusion
– Substances move through water-filled channels
8/19/2015
MDufilho
18
Figure 3.7b Diffusion through the plasma membrane.
Lipid-insoluble solutes
(such as sugars or
amino acids)
Shape
change
releases
solutes
Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes transport
protein to change shape
MDufilho
Figure 3.7c Diffusion through the plasma membrane.
Small lipidinsoluble
solutes
Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
MDufilho
Diffusion (cont.)
• Osmosis
– Movement of solvent, such as water, across a
selectively permeable membrane
– Water diffuses through plasma membranes
• Through lipid bilayer (even though water is polar, it is
so small that some molecules can sneak past
nonpolar phospholipid tails)
• Through specific water channels called aquaporins
(AQPs)
– Flow occurs when water (or other solvent)
concentration is different on the two sides of a
membrane
8/19/2015
MDufilho
21
Figure 3.7d Diffusion through the plasma membrane.
Water
molecules
Lipid
bilayer
Aquaporin
Osmosis, diffusion of
a solvent such as water
through a specific
channel protein
(aquaporin) or through
the lipid bilayer
MDufilho
Diffusion (cont.)
• Osmolarity: measure of total concentration of
solute particles
• Water concentration varies with number of
solute particles because solute particles
displace water molecules
– When solute concentration goes up, water
concentration goes down, and vice versa
• Water moves by osmosis from areas of low
solute (high water) concentration to high areas
of solute (low water) concentration
8/19/2015
MDufilho
23
Figure 3.8a Influence of membrane permeability on diffusion and osmosis.
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:
Right
compartment:
Solution with
lower osmolarity
Solution with
greater osmolarity
Both solutions have the
same osmolarity: volume
unchanged
H2O
Solute
Freely
permeable
membrane
8/19/2015
MDufilho
Solute
molecules
(sugar)
24
Figure 3.8b Influence of membrane permeability on diffusion and osmosis.
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
Selectively
permeable
membrane
8/19/2015
MDufilho
Solute
molecules
(sugar)
25
Diffusion (cont.)
• Movement of water causes pressures:
– Hydrostatic pressure: pressure of water inside
cell pushing on membrane
– Osmotic pressure: tendency of water to move
into cell by osmosis
• The more solutes inside a cell, the higher the osmotic
pressure
8/19/2015
MDufilho
26
Diffusion (cont.)
• Tonicity
– Ability of a solution to change the shape or tone
of cells by altering the cells’ internal water
volume
• Isotonic solution has same osmolarity as inside the
cell, so volume remains unchanged
• Hypertonic solution has higher osmolarity than
inside cell, so water flows out of cell, resulting in cell
shrinking
– Shrinking is referred to as crenation
• Hypotonic solution has lower osmolarity than inside
cell, so water flows into cell, resulting in cell swelling
– Can lead to cell bursting, referred to as lysing
8/19/2015
MDufilho
27
Figure 3.9 The effect of solutions of varying tonicities on living red blood cells.
Isotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as
inside cells; water moves in
and out).
MDufilho 8/19/2015
Hypertonic solutions
Cells lose water by osmosis and
shrink in a hypertonic solution
(contains a higher concentration
of nonpenetrating solutes than
are present inside the cells).
Hypotonic solutions
Cells take on water by osmosis
until they become bloated and
burst (lyse) in a hypotonic
solution (contains a lower
concentration of nonpenetrating
solutes than are present
inside cells).
28
Passive Membrane Transport: Filtration
• The passage of water and solutes through a
membrane by hydrostatic pressure
• Pressure gradient pushes solute-containing
fluid from a higher-pressure area to a lowerpressure area
• Does not occur into or out of cell, but through
filtration membrane made of rows of cells.
MDufilho
8/19/2015
29
3.4 Active Membrane Transport
• Two major active membrane transport
processes
– Active transport
– Vesicular transport
• Both require ATP to move solutes across a
plasma membrane for any of these reasons:
– Solute is too large for channels, or
– Solute is not lipid soluble, or
– Solute is not able to move down concentration
gradient
8/19/2015
MDufilho
30
Active Transport
• Requires carrier proteins (solute pumps)
• Moves solutes against their concentration
gradient (from low to high)
– This requires energy (ATP)
• Two types of active transport:
– Primary active transport
• Required energy comes directly from ATP hydrolysis
– Secondary active transport
• Required energy is obtained indirectly from ionic
gradients created by primary active transport
8/19/2015
MDufilho
31
Active Transport (cont.)
• Primary active transport
– Energy from hydrolysis of ATP causes change in
shape of transport protein
– Shape change causes solutes (ions) bound to
protein to be pumped across membrane
– Example of pumps: calcium, hydrogen (proton),
Na+-K+ pumps
8/19/2015
MDufilho
32
Slide 7
Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using
energy supplied directly by ATP. The action of the Na+-K+ pump is an important example of primary active transport.
Extracellular fluid
Na+
Na+ –K+
pump
ATP
ATP-binding site
Na+ bound
K+
Cytoplasm
1 Three cytoplasmic Na+ bind to
pump protein.
ATP
P
K+ released
ADP
6 Pump protein binds ATP; releases
K+ to the inside, and Na+ sites are ready
to bind Na+ again. The cycle repeats.
2 Na+ binding promotes hydrolysis
of ATP. The energy released during this
reaction phosphorylates the pump.
Na+
released
K+ bound
P
Pi
5 K+ binding triggers release of
the phosphate. The dephosphorylated
pump resumes its original
conformation.
K+
3 Phosphorylation causes the
pump to change shape, expelling
Na+ to the outside.
P
4 Two extracellular K+ bind to pump.
8/19/2015
MDufilho
33
Active Transport (cont.)
• Secondary active transport
– Depends on ion gradient that was created by
primary active transport system
– Energy stored in gradients is used indirectly to
drive transport of other solutes
8/19/2015
MDufilho
34
Slide 3
Figure 3.10 Secondary active transport is driven by the concentration gradient created by primary active transport.
Extracellular fluid
Na+
Na+
Na+-glucose
Na+
Na+
Glucose
Na+
Na+
Na+
Na+
K+
Na+-K+
pump
symport
transporter
loads glucose
from extracellular
fluid
Na+
Na+
Na+-glucose
symport transporter
releases glucose
into the cytoplasm
Na+
ATP
Cytoplasm
1 Primary active transport
Na+-K+
The ATP-driven
pump
stores energy by creating a steep
concentration gradient for Na+
entry into the cell.
8/19/2015
MDufilho
2 Secondary active transport
As Na+ diffuses back across the membrane
through a membrane cotransporter protein, it
drives glucose against its concentration gradient
into the cell.
35
Vesicular Transport
• Involves transport of large particles, macromolecules, and
fluids across membrane in membranous sacs called
vesicles
• Requires cellular energy (usually ATP)
• Processes:
– Endocytosis: transport into cell
• phagocytosis, pinocytosis, receptor-mediated
endocytosis
– Exocytosis: transport out of cell
– Transcytosis: transport into, across, and then out of cell
– Vesicular trafficking: transport from one area or
organelle in cell to another
8/19/2015
MDufilho
36
Figure 3.11 Events of endocytosis mediated by protein-coated pits.
1 Coated pit
ingests substance.
Extracellular fluid
Plasma
membrane
Protein coat
(typically clathrin)
Cytoplasm
2 Protein-coated
vesicle detaches.
3 Coat proteins are recycled
to plasma membrane.
Transport
vesicle
Uncoated
endocytic
vesicle
Endosome
4 Uncoated vesicle fuses with
a sorting vesicle called an
endosome.
Lysosome
5 Transport vesicle
containing membrane
components moves to
the plasma membrane
for recycling.
6 Fused vesicle may (a) fuse
(a)
8/19/2015
MDufilho
with lysosome for digestion of
its contents, or (b) deliver its
contents to the plasma
membrane on the opposite
side of the cell (transcytosis).
(b)
37
Vesicular Transport (cont.)
• Phagocytosis: type of endocytosis that is
referred to as “cell eating”
– Membrane projections called pseudopods form
and flow around solid particles that are being
engulfed, forming a vesicle which is pulled into
cell
– Formed vesicle is called a phagosome
– Phagocytosis is used by macrophages and
certain other white blood cells
• Phagocytic cells move by amoeboid motion where
cytoplasm flows into temporary extensions that allow
cell to creep
8/19/2015
38
MDufilho
Figure 3.12a Comparison of three types of endocytosis.
Receptors
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 protein-coated
but has receptors capable of binding to
microorganisms or solid particles.
Phagosome
8/19/2015
MDufilho
39
Vesicular Transport (cont.)
• Pinocytosis: type of endocytosis that is
referred to as “cell drinking” or fluid-phase
endocytosis
– Plasma membrane infolds, bringing extracellular
fluid and dissolved solutes inside cell
• Fuses with endosome
– Used by some cells to “sample” environment
– Main way in which nutrient absorption occurs in
the small intestine
– Membrane components are recycled back to
membrane
8/19/2015
MDufilho
40
Figure 3.12b Comparison of three types of endocytosis.
Pinocytosis
The cell “gulps” a drop of extracellular fluid
containing solutes into tiny vesicles. No receptors
are used, so the process is nonspecific. Most
vesicles are protein-coated.
Vesicle
8/19/2015
MDufilho
41
Vesicular Transport (cont.)
• Receptor-mediated endocytosis involves
endocytosis and transcytosis of specific molecules
– Many cells have receptors embedded in clathrin-coated
pits, which will be internalized along with the specific
molecule bound
• Examples: enzymes, low-density lipoproteins (LDL), iron,
insulin, and, unfortunately, viruses, diphtheria, and cholera
toxins may also be taken into a cell this way
– Caveolae have smaller pits and different protein coat
from clathrin, but still capture specific molecules (folic
acid, tetanus toxin) and use transcytosis
8/19/2015
MDufilho
42
Figure 3.12c Comparison of three types of endocytosis.
Vesicle
8/19/2015
MDufilho
Receptor-mediated endocytosis
Extracellular substances bind to specific receptor
proteins, 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.
43
Exocytosis
• Process where material is ejected from cell
– Usually activated by cell-surface signals or
changes in membrane voltage
• Substance being ejected is enclosed in
secretory vesicle
• Protein on vesicle called v-SNARE finds and
hooks up to target t-SNARE proteins on
membrane
– Docking process triggers exocytosis
• Some substances exocytosed: hormones,
neurotransmitters, mucus, cellular wastes
8/19/2015
MDufilho
44
Figure 3.13a Exocytosis.
Extracellular
fluid
Secretory
vesicle
Plasma membrane
SNARE (t-SNARE)
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
Fused
v- and
t-SNAREs
The process of
exocytosis
1 The membranebound vesicle migrates
to the plasma
membrane.
2 There, proteins at
the vesicle surface
(v-SNAREs) bind with
t-SNAREs (plasma
membrane proteins).
Fusion pore formed
3 The vesicle and
plasma membrane fuse
and a pore opens up.
4 Vesicle contents
are released to the cell
exterior.
8/19/2015
MDufilho
45
Mediated transport
•
•
•
•
Specificity
Competition
Saturation
Transport maximum
MDufilho
8/19/2015
46