Membrane Function
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
Membrane Function
• Working cells must control the flow of materials to and
from the environment.
• A cell membrane or wrapper made of phospholipids is
relatively impermeable
• Proteins embedded in the membrane provide for selective
permeability and transport of materials into and out of the
cell, among other things
Membrane Selectivity
Cell or Plasma Membrane
Phospholipid
bilayer
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)
Figure 3.3
Functions of Membrane Proteins
Figure 5.11
The Working Cell: Membrane Function
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
Passive Transport: Diffusion Across Membranes
• Molecules contain heat energy.
– They vibrate and wander randomly.
• Diffusion is movement of molecules from regions
of high concentration to low concentration
– Molecules tend to spread into the available space.
• Diffusion Across A Membrane by passive transport
– Simple diffusion of small or hydrophobic
molecules
– Facilitated diffusion of larger and polar
molecules
Diffusion
Examples of Passive Transport Across a Phospholipid Bilayer
The Working Cell: Membrane Function
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
Osmosis and Water Balance in Cells
• Osmosis is the facilitated transport of water
across a selectively permeable membrane.
• Water moves across a membrane from high
concentration (high purity) to low concentration (low
purity)
Terms Used to Compare the Purity of Water Solutions
• A hypertonic solution
– Has a higher concentration of dissolved
substances (solute) like salts, proteins,
etc.
5%
salt
1%
salt
• A hypotonic solution
– Has a lower concentration of dissolved
substances (solute)
1%
salt
5%
salt
• An isotonic solution
– Has an equal concentration of
dissolved substances (solute).
1%
salt
1%
salt
Cells Must Regulate Salt and Water Balance
(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).
Turgid Elodea
Plasmolysis
(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).
The Working Cell: Membrane Function
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
Active Transport: The Pumping of Molecules Across Membranes
• Active transport is the movement of molecules from low
concentration to high concentration, against a concentration gradient
• Active transport requires energy to pump molecules across
a membrane, into more a more crowded space
Concentration gradient
Active Transport
The Sodium-Potassium Pump Creates a Gradient Using ATP
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.
Active Transport Can Create Electrical AND Chemical Gradients
Both chemical and electrical gradients represent stored energy, and can
be used to perform work (e.g. pumping something through the
membrane by co-transport, or making ATP)
The Working Cell: Membrane Function
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
Bulk Transport: Exocytosis and Endocytosis
• Exocytosis
– Dumping molecules out of the cell (export)
Exocytosis and Endocytosis Introduction
Bulk Transport: Exocytosis and Endocytosis
•
Endocytosis
–
Bringing molecules into the cell (import)
–
Includes pinocytosis, phagocytosis, and receptor-mediated endocytosis
Pinocytosis: cell “drinking”
Phagocytosis: cell “eating”
Receptor-mediated endocytosis
Receptor-Mediated Endocytosis
The Working Cell: Membrane Function
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
The Role of Membranes in Cell Signaling
•
Cellular
communication
1.
Begins with the
reception of an
extracellular signal.
2.
The signal is
transduced or
passed across the
membrane
3.
A response is
elicited from the
transduced signal
The Working Cell: Membrane Function
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling