Membrane Structure and Function
Chapter 7A. P. Biology
Plasma Membrane• Also called the plasmalemma.• Plasma Membrane =
Phospholipid Bilayer + Transmembrane Proteins + Supporting Fibers + Glycoproteins and Glycolipids
Scientists studying the plasma membrane– Reasoned that it must be a phospholipid
bilayer
Figure 7.2
HydrophilicheadHydrophobictail
WATER
WATER
Phospholipid Bilayer• Glycerol + 2 Fatty Acids +
Phosphorylated Alcohol = Phospholipid• Hydrophilic or Polar Region =
Phosphate• Hydrophobic or Nonpolar region = Fatty
Acids
The Davson-Danielli sandwich model of membrane structure
–Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers.
–Was supported by electron microscope pictures of membranes
In 1972, Singer and Nicolson– Proposed that membrane proteins are dispersed
and individually inserted into the phospholipid bilayer
Figure 7.3
Phospholipidbilayer
Hydrophobic region of protein
Hydrophobic region of protein
Freeze-fracture studies of the plasma membrane– Supported the fluid mosaic model of membrane structure
Figure 7.4
A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.
Extracellular layer Cytoplasmic layer
APPLICATION A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior.
TECHNIQUE
Extracellularlayer
Proteins
Cytoplasmic layer
Knife
Plasmamembrane
These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer.
RESULTS
Lipid Bilayer• Nonpolar interior prevents passage of
water-soluble, polar compounds.• Only very small, uncharged molecules
like O2 and H2O can enter through the lipid bilayer.
• Also, allows nonpolar compounds to freely enter.
The Fluidity of Membranes• Phospholipids in the plasma membrane
– Can move within the bilayer
Figure 7.5 A
Lateral movement(~107 times per second)
Flip-flop(~ once per month)
(a) Movement of phospholipids
The type of hydrocarbon tails in phospholipids– Affects the fluidity of the plasma membrane
Figure 7.5 B
Fluid Viscous
Unsaturated hydrocarbontails with kinks
Saturated hydro-Carbon tails
(b) Membrane fluidity
The steroid cholesterol– Has different effects on membrane fluidity at
different temperatures
Figure 7.5 (c) Cholesterol within the animal cell membrane
Cholesterol
Figure 7.7
Glycoprotein
Carbohydrate
Microfilamentsof cytoskeleton Cholesterol Peripheral
proteinIntegral
proteinCYTOPLASMIC SIDE
OF MEMBRANE
EXTRACELLULAR
SIDE OFMEMBRANE
Glycolipid
Membrane Proteins and Their Functions• A membrane
– Is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
Fibers of
extracellularmatrix (ECM)
Lipid Bilayer is Fluid• Fluid = Moving, Dynamic.• Each lipid can rotate, move laterally• Fluidity depends on temperature and
type of fatty acid used.• Unsaturated fatty acids are more fluid.• Fluid Mosaic Model
Transmembrane Proteins (Integral Proteins)
• Part of the protein that extends through the bilayer is nonpolar (several nonpolar amino acids in this region).
• Usually is an alpha helix or beta barrel.• Used to anchor protein in the membrane.• Beta-barrels = form a pore and are called
a porin protein.
Integral proteins– Penetrate the hydrophobic core of the lipid bilayer– Are often transmembrane proteins, completely
spanning the membrane
EXTRACELLULARSIDE
Figure 7.8
N-terminus
C-terminus
HelixCYTOPLASMICSIDE
Extracellular Side
• An overview of six major functions of membrane proteins
Figure 7.9
Transport
Enzymatic activity
Signal transduction
(a)
(b)
(c)
ATP
Enzymes
Signal
Receptor
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeletonand extracellular matrix(ECM).
(d)
(e)
(f)
Glyco-protein
Figure 7.9
Transmembrane Proteins• Channels = passive transport of molecules across
membrane.• Carriers = transport of molecules against the gradient.• Receptors = transmit information into the cell.• Cell Adhesion Proteins = connect cells to each
other.• Cytoskeleton Attachment Proteins = to attach actin.
Membrane Receptors Conduct Signals
Integral Proteins Laterally Diffuse in the Membrane
Movement Across the Membrane
• Diffusion = random motion of molecules that causes a net movement from areas of high concentration to areas of low concentration.
• Osmosis = diffusion of water across a selectively permeable membrane.
Passive Diffusion
Factors that Affect the Direction of Diffusion
• The concentration gradient; High Low.
• Temperature; High heat Low Heat.
• Pressure; High Pressure Low Pressure
Factors that Affect the Rate of Diffusion
• The steepness of the gradient.
• The molecular weight of the solute.
Concentrations• Osmotic concentrations = concentrations of all
solutes in a solution.• If unequal concentrations…
Hyperosmotic = solution with the higher solute concentration.Hypoosmotic = solution with the lower solute concentration.Isosmotic = solutions with the same osmotic or solute concentration.
Plasmolyzes
Crenate
Osmotic Pressure• If a cell’s cytoplasm is hyperosmotic to
the extracellular fluid, then water diffuses into the cell and it swells. Pressure of the cytoplasm pushing out against the membrane- hydrostatic pressure.
• Osmotic pressure is the pressure needed to stop the osmotic movement of water across a membrane.
How do living things maintain osmotic balance?
• Some oceanic eukaryotes adjust internal [solutes]- they are isosmotic.
• Animals – circulate an isosmotic fluid around their cells. Must constantly monitor the fluid’s [solute] Ex. Humans secrete albumin into the plasma to match the body cells.
• Protozoa- are hyperosmotic, so use extrusion to remove excess water; may have special organelles-contractile vacuoles.
• Plants- are hyperosmotic, but do not circulate an isosmotic solution; are usually under osmotic pressure- turgor pressure-presses the plasma membrane against the cell wall.
• Water balance in cells with walls
Plant cell. Plant cells are turgid (firm) and generally healthiest ina hypotonic environ-ment, where theuptake of water iseventually balancedby the elastic wallpushing back on thecell.
(b)
H2OH2OH2OH2O
Turgid (normal) Flaccid Plasmolyzed
Figure 7.13
Bulk Movement through Membranes
• Endocytosis- the cytoskeleton extends the membrane outward toward food particles. Bulk transport into cell.
• Extended membrane encircles the particle, fuses with itself, and contracts.
• Forms a vesicle around particle.
Three Kinds of Endocytosis• Phagocytosis - “cell eating”- large, amounts of
organic material;white blood cells and protists.• Pinocytosis- “cell drinking”- liquid material
brought into cell; mammalian ova and follicle cells.
• Receptor-mediated Endocytosis- use receptors in the membrane for specific transport into cell.
Receptor-Mediated Endocytosis• Have indentations on the plasma membrane.• Indentations = are clathrin-coated pits.• Pits have receptor proteins on the extracellular side
= trigger• When receptor binds to target molecule, clathrin
proteins on the cytoplasmic side begins endocytosis.
• Forms a clathrin -coated vesicle. • Very specific.
Receptor-mediated Endocytosis
Exocytosis• The release of material from vesicles
at the cell surface.• Examples: protists using a
contractile vacuole to release water, gland cells secreting hormones, neurons releasing neurotransmitters.
Secretion Vesicles
Exocytosis and Neurons
Problems with Bulk Transport
• Endocytosis and Exocytosis are energy-intensive.
• Not highly selective.
Channel \Proteins– Provide corridors that allow a specific
molecule or ion to cross the membrane
Figure 7.15
EXTRACELLULARFLUID
Channel proteinSolute
CYTOPLASM
A channel protein (purple) has a channel through which water molecules or a specific solute can pass.
(a)
Selectively Permeable Transport• Channels- proteins in the cell
membrane that transport specific ions into and out of the cell.
• Water-filled pores span the membrane. • Ions do not interact with the channel
protein. • Diffusion is passive, [high] --> [low].
Channels are somewhat selective
Selectively Permeable Transport
• Carriers- proteins that transport specific ions, sugars, and amino acids into and out of the cell.
• The proteins facilitate movement by binding to the solute-facilitated diffusion.
• The proteins bind to the solute on one side of the membrane and release them on the other.
Uniport Facilitated Diffusion
Facilitated Diffusion• It is specific, only certain
molecules transported by a given carrier.
• It is passive, net movement is [high]-->[low].
• It may become saturated if all protein carriers are occupied.
Active Transport• A method of transporting specific ions,
sugars, amino acids, nucleotides against conc. gradient.
• Involves protein carriers in the membrane and energy (ATP).
• How cells accumulate molecules internally.
Active Transport
The Sodium-Potassium PumpIs one type of active transport system
An Example of Active Transport• Sodium-Potassium Pump- Active
transport of Na+ and K+ ions.• Normally, inside the cell:
the [Na+] is low the [K+] is high
• The cell maintains this by actively pumping Na+ out and K+ in.
Sodium-Potassium Pump• The protein uses ATP as an energy source for
this movement against the gradient [low]--> [high].
• See Fig. 6.21, p. 135 for how.• Uses ~1/3 of all ATP in resting cell.• This pump can transport 300 Na+ ions/second.• All animals use it.
Cotransport: active transport driven by a concentration gradient
Figure 7.19
Proton pump
Sucrose-H+
cotransporter
Diffusionof H+
Sucrose
ATP H+
H+
H+
H+
H+
H+
H+
+
+
+
+
+
+–
–
–
–
–
–
Cotransport and Countertransport• Many amino acids and sugars are
transported into the cell through coupled channels.
• Their active transport is coupled with the movement of Na+ inside the cell.
• [Na+] high --> [Na+] low into cell.• [Amino acid] low--> [Amino acid] high.
An Electrogenic Pump– Is a transport protein that generates the voltage
across a membrane
Figure 7.18
EXTRACELLULARFLUID
+
H+
H+
H+
H+
H+
H+Proton pump
ATP
CYTOPLASM
+
+
+
+–
–
–
–
–
+
The Proton Pump• A transmembrane protein that moves H+
against their concentration gradient, from [low] --> [high] outside of cells or organelles.
• Example: mitochondria move H+ across the inner membrane during electron transport.
• Energy to power this pump comes from NADH and FADH molecules.
Proton Pump• The proton pump moves H+ out of the matrix,
through the inner membrane.• ATP synthase channels H+ back into the matrix,
DOWN the gradient. PROVIDES ENERGY! • ATP synthesis is coupled to H+ movement.• Almost all of the energy for cells is made this
way.
Electron Transport Chain