CHAPTER 8MEMBRANE STUCTURE AND
FUNCTION
• Plasma membrane is selectively permeable, (allowing some substances to cross more easily than others)
• PM is flexible – bends and changes shape
Plasma Membrane
Mostly lipids and proteins; some carbohydrates• Lipids are in the form of phospholipids (mostly)• Phospholipids are amphipathic molecules.
– have both hydrophobic regions and hydrophilic regions.
Macromolecules in PM
Phospholipids …
… are amphipathicno
npol
arno
npol
ar
hydrophilic
hydrophilic
hydrophobicamphi- “on both sides”
1895 – PM is made of lipids (water interaction??) 1925 – Phospholipid bilayer (flexibility; adhesion??)1935 – Davson & Danielli – proteins are outside
phospholipid bilayer (hydrophobic proteins, variability???)
Membrane models- history
1950 - EM
Membrane modelsFreeze-fracture studies (SEM) – proteins in the
middle of PM Fluid Mosaic Model – Singer and Nicholson (1972)
Phopholipid makes a bilayer; Integral and peripheral proteins are found ‘within’ or on ‘one side’ of the phospholipid bilayer as the name indicates
Hydrophilic regions of proteins (polar side chains) and phospholipids (polar head)– in contact with water
Hydrophobic tails/nonpolar protein regions are in a nonaqueous environment - tucked away in the middle of the membrane
Weak hydrophobic interactions maintain membrane structure; cholesterol - maintains fluidity;
Glycolipids (antigens) found on outside side of the membrane - provide cell-cell recognition (immune function)
Ion channels are formed by integral proteins to transport polar substances
Some integral/peripheral proteins are enzymes or receptors or junctions - function in cell communication
Fluid Mosaic model
Fig. 8.2b
Fluid Mosaic model
Why ‘Fluid’ and Why ‘Mosaic’?Fluid Mosaic
Proteins and lipids moveLaterally and can also Flip
Cholesterol (restricts it; except at low temp)
Structure is not the same on the inside and outside and from region to region (relate to lipids, integral and peripheral proteins, carbohydrates)
Function is related to structures(AP Theme!); therefore function is a mosaic - elaborate…
• Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other.
• Fluidity depends on unsaturation of fatty acids and presence of cholesterol. More unsaturation, more fluidity (why?).
• At normal and high temp. cholesterol limits fluidity; at low temp. cholesterol actually prevents freezing of membrane (increases fluidity)
Membranes are fluid
• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.
Membranes are mosaics of structure and function
– Peripheral proteins are loosely bonded to the surface (inside or outside) of the PM, often connected to the other membrane proteins.
– Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (a transmembrane protein).
-Glycolipid (antigens) only on Extracellular side
Proteins in MembranesIntegral Proteins form alpha helix coils in the membraneProteins are connected to cytoskeleton (intracellular) and Extra
Cellular Matrix (outside)
Integral Transmembrane Protein: Asymmetry in amino acids
gives the two membrane faces (extracellular and intracellular) their different characteristics
• Membranes have distinctive inside and outside faces.– The two layers may differ
in lipid composition, and proteins in the membrane have a clear direction.
– The outer surface also has carbohydrates.
Mosaic Structure Cont’d
• The proteins in the plasma membrane may provide a variety of major cell functions.
Fig. 8.9
Mosaic Function
Provides a hydrophilicchannel that is selective
for a particular solute
Campbell; Fig 8.9
“active site”
“conformational change”
– important in tissue and organ development.– rejection of foreign cells by the immune system.– Carbohydrates are covalently bonded either to lipids,
forming glycolipids, or, more commonly, to proteins, forming glycoproteins
– Example: The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells
Membrane carbohydrates are important for cell-cell recognition
• Small molecules and ions moves across the plasma membrane in both directions.
Transport Across the Membrane
Sugars, amino acids, oxygen, ions
Wastes, ions, CO2
Easy Access Need Assistance
• Hydrophobic molecules (lipids): hydrocarbons, CO2, and O2
• Small polar molecules - water
• Can get through because of small size or hydrophobic nature
• Ions and polar molecules like glucose, amino acids
• Size + charged nature prevents easy acess
• When do these substances need to get in?
Proteins can assist and regulate the transport of ions and polar molecules
Diffusion
Simple Facilitated
Energy Requirements
Requires no energy other than that of molecular motion
Channel
Proteins
Molecules go through lipid bilayer
Requires TRANSPORT
proteins
PassiveTransport
Carrier
Proteins
Diffusion is the tendency of molecules of any substance to spread out in the available space
driven by the intrinsic kinetic energy of molecules (so temp increase will…….).
a substance will diffuse from where it is more concentrated to where it is less concentrated, i.e. down its concentration gradient.
Passive transport- 1) Simple Diffusion
O2, and CO2; small polar molecules include ethanol, H2O, and urea.
1) Channel proteins -provide corridors allowing a specific molecule or ion to cross the membrane.
These channel proteins allow fast transportWater channel proteins -aquaprorinsIons move through these channels
Passive transport- 2) Facilitated Diffusion
Channels
Open Closed
Channels that spend almost all of their time in theopen configuration are called
“leak” channels, or pores
Channels that spend almost all of their time in the
closed configuration are called“gated”
Ions, and H2O(SKIP details)
2) Carrier Proteins – Allows movement of
polar compounds
1. Molecules (substrates) actually bind with the carrier (amino acids, sugars, nucleosides, and other small molecules). Know this - ‘glucose’ and ‘aminoacid’ carrier proteins transport these substances into the blood from the small intestine!
2. Protein changes in shape (conformational change)
3. It allows molecules to pass through
Passive transport- 2) Facilitated Diffusion
Transport proteins EnzymesSpecific binding sites
Can become saturated Can be inhibited Catalyze a process
Transport Proteins are like Enzymes
3. Osmosis is the passive transport of water
Tonicity describes how the size of a cell would change if it were placed in a solution
Fig. 8.11
Osmosis is the passive transport of water – it occurs until isotonicity is reached
Osmosis – passive diffusion of water across a semipermeable membrane
Cell survival depends on balancing water uptake and loss
Osmosis is the diffusion of water through a selectively permeable membrane.
Tonicity describes how the size of a cell would change if it were placed in the solution
Isotonic – same solute/water concentrations as inside cells
so cells retain their normalsize and shape
H2O
H2O
Lab - hemodialysis animation
Be sure to know how these calculations were performed (on next test).A) Open system; osmosis
raises water in arm with more solute
B) Plunger pushes and opposes osmosis (back pressure)
C) Plunger keeps pushing, causing water to move against concentration gradient
D) Result: Water rises in arm on opposite side
Wilted(cells may be flaccid or plasmolysed)
Turgid
• Osmoregulation : maintanance of osmotic balance
• Paramecium – protist (contractile vacuole)
Cell survival depends on balancing water uptake and loss
• The cells of plants, prokaryotes, fungi, and some protists have cell walls that contribute to the cell’s water balance.
• Active transport requires the cell to use its own metabolic energy (ATP).
• Active transport is against the Concentration Gradient (solutes move from low concentration to high concentration)
4. Active transport is the pumping of solutes against their gradients
• The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membrane.
All cells maintain a voltage across their plasma membranes using pumps.
The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal distribution of cations (+) and anions (-) on opposite sides of the membrane.
This voltage, the membrane potential, ranges f50 to -200 millivolts.
Some ion pumps generate voltage across membranes
• The membrane potential acts like a battery.• The membrane potential favors the passive
transport of cations (+) into the cell and anions (-) out of the cell.
• Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane: – a chemical force based on an ion’s concentration
gradient – an electrical force based on the effect of the
membrane potential on the ion’s movement.
• Ions diffuse down their electrochemical gradient.• Special transport proteins, electrogenic pumps,
generate the voltage gradients across a membrane• Examples of electrogenic pumps – Na-K pump, proton
(H+) pump
Some ion pumps generate voltage across membranes
• Proton pump actively transports H+ out of the cell.
• Plants, fungi, mitochondria, chloroplast
• These electrogenic pumps store energy that can be used to make ATP.
oncentration gradient.
In cotransport, a membrane protein couples the transport of two solutes
• Small molecules and water Transport proteins• Large macromolecules Vesicles• Exocytosis: a transport vesicle budded from the
Golgi apparatus is moved by the cytoskeleton to the plasma membrane.
Exocytosis and Endocytosis transport large molecules (need ATP)
• Endocytosis - a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane.
• Endocytosis is a reversal of exocytosis.
Fig. 8.19a
• One type of endocytosis is phagocytosis, “cellular eating”.
• In phagocytosis, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large vacuole.
• The contents of the vacuole are digested when the vacuole fuses with a lysosome.
• In pinocytosis, “cellular drinking”, a cell creates a vesicle around a droplet of extracellular fluid.
Fig. 8.19b
• Receptor-mediated endocytosis is very specific in what substances are being transported.
• Extracellular substances bind to special receptors, on the membrane surface, especially near coated pits.
• This triggers the formation of a vesicle
Fig. 8.19c
Transport Overview:• Simple Diffusion• Facilitated diffusion (carrier and channel proteins)• Osmosis – diffusion of wwater• Active Transport• Electrogenic pumps• Cotransport• Exocytosis• Endocytosis and Receptor- mediated endocytosis• Phagocytosis and Pinocytosis
• Why is the plasma membrane structure called the FLUID MOSAIC MODEL?
• Review your Lab
Top Related