Download Membrane Structure and Function

Membrane Structure and Function
• The plasma membrane controls traffic into
and out of the cell it surrounds
• It is selectively permeable – allows some
substances to cross it more easily than
others
• Phospholipids
make up most
plasma
membranes
Fluid Mosaic Model
• The membrane is a fluid structure with
various proteins embedded in or
attached to a double layer of
phospholipids
Fluid Mosaic Model
(a) Lipids move laterally in the
membrane, but flip-flopping
across the membrane is rare
(b) Unsaturated hydrocarbon tails
have kinks that keep the molecules
from packing together, enhancing
fluidity
(c) Cholesterol reduces membrane fluidity
at moderate temps. by reducing
phospholipid movement, but at low temps.
it hinders solidification by disrupting the
regular packing of phospholipids
Fluid Mosaic Model
• The main structure to the plasma membrane
is the phospholipid bilayer, but proteins
determine the membrane’s specific function
• Integral proteins – transmembrane proteins
with hydrophobic regions that completely
span the hydrophobic interior of the
membrane
• Peripheral proteins – Protein appendages
loosely bound to the surface of the
membrane and not embedded in the lipid
bylayer
Fluid Mosaic Model
Some Functions of Membrane Proteins
Some Functions of Membrane Proteins
Videos
6 Functions of Membrane Proteins
1. Anchoring proteins (stabilizers):
–
attach to inside or outside structures
2. Recognition proteins (identifiers):
–
label cells normal or abnormal
3. Enzymes:
– catalyze reactions
4. Receptor proteins:
– bind and respond to ligands (ions, hormones)
5. Carrier proteins:
– transport specific solutes through membrane
6. Channels:
– regulate water flow and solutes through membrane
Transport Proteins
• A transmembrane protein that helps a
certain substance to cross the membrane
• The transport protein is specific for the
substances it moves across the membrane
• Some have hydrophilic channels that
substances move through, others hold onto
to the “passengers” and physically move
them across the membrane
• There are two modes of membrane
movement: passive transport and active
transport
The structure of a transmembrane protein
Traffic Across Membranes
• Substances that move across the membrane
do so at different rates
Passive Transport
• Passive transport does not require energy
from the cell
• Diffusion – the tendency for molecules of
any substance to spread out into the
available space
• Substances always diffuse down its
concentration gradient – from an area of
greater concentration to an area of lesser
concentration
• Diffusion is spontaneous because it
decreases free energy
Diffusion
Figure 8.10 The diffusion of solutes across membranes
Osmosis
• Osmosis is the passive transport (diffusion) of
water across a selectively permeable
membrane
• Hypertonic – the solution with a higher
concentration of solutes
• Hypotonic – the
solution with a lower
concentration of
solutes
• Isotonic – solutions of
equal solute
concentration
Water Balance Within Cells
• Osmoregulation is the control of water
balance within living cells
• Animal cells do not have cell walls
– If in a hypotonic solution, the cell takes on
water quickly and can burst (lyse)
– If in a hypertonic solution, the cell loses
water and shrivels usually leading to
death
– Cells usually live in an isotonic
environment
Water Balance Within Cells
Water Balance Within Cells
•Paramecium have a contractile vacuole to get rid
of excess water
Figure 8.13 The contractile vacuole of Paramecium: an evolutionary adaptation for
osmoregulation
Water Balance Within Cells
• Plant, prokaryote, fungi, and some protist
cells do have cell walls
– If in a hypotonic solution, the cell takes on
water, but the cell wall prevents bursting
• Turgor pressure keeps plants upright
– If in a hypertonic solution, the cell loses
water and pulls away from the cell wall
• Plasmolysis usually kills the cell
– In an isotonic environment, water does
not enter the cell
• Causes wilting (cells become flaccid)
Water Balance Within Cells
Water Balance Within Cells
Facilitated Diffusion
• Use of transport proteins to move
substances across the plasma membrane
A channel protein allows
substance to pass through
This transport protein
alternates between two
shapes to move substances
through
Some Functions of Membrane Proteins
Video Transport
Proteins
Active Transport
• The cell must expend energy (ATP) to move
substance against the concentration gradient
(from low concentration to high concentration)
Figure 8.15 The sodium-potassium pump: a specific case of active transport
Comparison of Transport Types
Exocytosis
• Secretion of macromolecules by the
fusion of vesicles with the plasma
membrane
–A transport vesicle that has budded
from the Golgi apparatus moves to
the plasma membrane
–When it reaches the membrane, the
vesicle fuses with it and releases its
contents to the outside of the cell
Exocytosis
Endocytosis
• The cell takes in macromolecules and
particulate matter by forming new
vesicles from the plasma membrane
• Three types
– Phagocytosis – the cell engulfs large
particles by forming a vacuole from
the cell membrane; the vacuole fuses
with a lysosome to be digested
– Pinocytosis – droplets of fluid are
taken into the cell
Endocytosis
Receptor-Mediated Endocytosis
• The movement of specific molecules
into a cell by the inward budding of
vesicles containing proteins with
receptor sites specific to the molecules
being taken in
• This enables a cell to acquire bulk
quantities of specific substances.
Receptor-Mediated Endocytosis
Figure 8.19 The three types of endocytosis in animal cells