Membrane-Bound:
Nucleus
Mitochondria
Peroxisomes
Lysosomes
Endoplasmic Reticulum
Golgi apparatus, etc.
Nonmembranous:
Cytoskeleton
Centrioles
Ribosomes
*Can also be attached to cytoskeleton
Membrane proteins and
lipids
ER
Golgi apparatus.
ER
• Boundary
• Selectively Permeable
• Fluid mosaic
bilayer
• Most abundant
• Amphipathic
• Head
tails
hydrophilic
hydrophobic
Hydrophilic region
of protein
Phospholipid
bilayer
Hydrophobic region of protein
INTERSTITIAL FLUID
Hydrophilic
head
Hydrophobic
tail
CYTOSOL
• Fluid structure
• Phospholipids
• Membrane proteins
laterally
laterally
tails
fluidity
Fluid
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydroCarbon tails
cholesterol
Cholesterol
It keeps the membrane from becoming too solid in colder
temperatures, or too fluid in warmer temperatures.
A membrane is a collage of different proteins embedded in the
fluid matrix of the lipid bilayer
Peripheral proteins
Integral proteins
Are appendages loosely bound
to the surface of the membrane
transmembrane proteins
EXTRACELLULAR
SIDE
Cell-cell recognition
Glycocalyx =
Important biomarker
Membrane structure results in
selective permeability
A cell must exchange
materials with its
surroundings, but is highly
regulated by the membrane.
Hydrophobic molecules
Are lipid soluble and can
pass through the membrane
rapidly
Polar molecules
Cross the membrane slowly
Passive Transport
Active Transport
No ATP required
Energy required (often ATP)
Must go with, or down the concentration Can go against, or up the concentration
gradient (from high to low)
gradient (from low to high)
Simple diffusion (small, uncharged)
Primary Active:
Uses chemical energy (ATP)
Facilitated diffusion (with proteins—
Secondary Active:
channel or carrier)
Uses electrochemical gradient
Osmosis (diffusion of water)
Water can pass through bilayer on own
OR
Through aquaporin (channel) proteins
Net loss of water No net movement Net gain of water
from cell
of water
into the cell
Animal cell will:
Animal cells will:
Animal cells will:
Plasmolyze
Stay the same
Lyse (explode)
(RBCs: “crenate”)
shape & size
• Channel proteins
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
A channel protein (purple) has a channel through which water
molecules or a specific solute can pass.
• Carrier proteins
A carrier protein alternates between two conformations,
moving a solute across the membrane as the shape of
the protein changes.
Can transport the solute in either direction
(net movement down concentration gradient)
Passive transport. Substances diffuse spontaneously
down concentration gradients, crossing membrane with
no expenditure of energy. Rate of diffusion can be
increased by transport proteins in the membrane.
Simple Diffusion
Active transport. Some transport
proteins act as pumps, moving
substances across a membrane against
their concentration gradients. Energy
for this work is usually supplied by ATP.
Facilitated Diffusion
Channel Protein
Carrier Protein
Protein Pump
ATP
• Membrane potential
electrochemical gradient
electrogenic pump
Prominent
Example:
Sodium/
Potassium
Pump
EXTRACELLULAR
FLUID
1. Three cytoplasmic
Na+ ions bind to
sodium-potassium
pump protein.
[Na+] high
[K+] low
Na+
Na+
CYTOPLASM
Na+
[Na+] low
[K+] high
2. Na+ binding stimulates
Na+
Na+
Na+
ADP
Na+ Na
K+ is released and
6.Na+ sites are
receptive again;
ATP binds;
the cycle repeats.
5. Loss of the
phosphate
restores the protein’s
original conformation.
ATP
P
phosphorylation by ATP
(ATP hydrolysis).
+
Na+
3.Phosphorylation causes
K+
the protein to change its
conformation, expelling
Na+ to the outside.
P
K+
P
Pi
4.
K+
K+
K+
K+
P
P
Two extracellular K+
ions bind to the
protein, triggering
release of the
phosphate group.
Proton Pump
–
ATP
EXTRACELLULAR
EXTRACELLULAR
FLUID
FLUID
+
–
+
H+
H+
Proton pump
H+
–
+
H+
H+
+
–
CYTOPLASM
–
+
+
H+
• Cotransport
active
transport
membrane protein
concentration
gradient
• Exocytosis
• Endocytosis
• Pinocytosis
• Phagocytosis
In exocytosis
Transport vesicles
migrate to the plasma
membrane, fuse with
it, and release their
contents
In endocytosis
The cell takes in
macromolecules by
forming new vesicles
from the plasma
membrane
PHAGOCYTOSIS
In phagocytosis, a cell
engulfs a particle by
wrapping pseudopodia
around it.
• Packaged into vacuole
• Vacuole fuses with
lysosome to digest the
material
In pinocytosis, the cell
“gulps” droplets of
extracellular fluid into tiny
vesicles.
• (They want the solutes.)
• Nonspecific
RECEPTOR-MEDIATED ENDOCYTOSIS
• Specific substances can
be acquired in bulk
Coat protein
Receptor
Coated
vesicle
• Specific receptor sites
exposed on outside of
membrane.
Ligand
• Extracellular substances
(ligands) bind to these
receptors, and the “coated
pit” forms a vesicle.
• After this ingested material
is liberated from the
vesicle, the receptors are
recycled.
Coat
protein
Plasma
membrane
Coated
pit
A coated pit
and a coated
vesicle
formed
during
receptormediated
endocytosis
(TEMs).
Tight Junctions
Desmosomes
Gap Junctions
Impermeable junctions
Anchoring junctions bind to
adjacent cells like Velcro
Allow for intercellular
communication
Prevent molecules from passing
through intercellular space
Form internal tension-reducing
network of fibers; plaques on
surface of membrane attach to
protein filaments
Allow ions and small molecules to
pass through channels formed by
connexon protein cylinders
Example:
Example:
Example:
Lining of the digestive tract
Found in tissues subject to stress
like skin; heart muscle
Found in electrically excitable
tissue (heart; smooth muscle) to
synchronize