Ch 5 Membrane Ch 5 Membrane Structure and FunctionStructure and Function

Control the movement of materials Control the movement of materials into and out of the cell.into and out of the cell.

Membrane StructureMembrane Structure

PhospholipidsPhospholipids: arranged in : arranged in a bilayer due to polarity of a bilayer due to polarity of molecule.molecule.

GlycolipidsGlycolipids: Similar to : Similar to phospholipids mono or phospholipids mono or polysaccharides instead of polysaccharides instead of phosphates. Cell to cell phosphates. Cell to cell communicationcommunication

CholesterolCholesterol: lipid found in : lipid found in animal plasma animal plasma membranes, reduces membranes, reduces permeability and provides permeability and provides rigidity.rigidity.

Membrane Structure ContdMembrane Structure Contd

Membrane ProteinsMembrane Proteins: Integral proteins : Integral proteins largely determine the membrane’s largely determine the membrane’s function(s). function(s). Channel proteins, Carrier proteins, Receptor Channel proteins, Carrier proteins, Receptor

proteins, Enzymatic proteins, Glycoproteins. proteins, Enzymatic proteins, Glycoproteins. Learn these!Learn these!

Membrane structureMembrane structure Fluid Mosaic ModelFluid Mosaic Model: The composition of : The composition of

phospholipids will contribute to the degree phospholipids will contribute to the degree of permeability, and the proteins are free to of permeability, and the proteins are free to move laterally w/in the plasma membrane. move laterally w/in the plasma membrane.

QODQOD

1. How would you expect the saturation 1. How would you expect the saturation levels of membrane fatty acids to differ in levels of membrane fatty acids to differ in plants adapted to cold environments and plants adapted to cold environments and plants adapted to hot environments?plants adapted to hot environments?

AnswersAnswers

1.1. Plants adapted to cold environments would be Plants adapted to cold environments would be expected to have more unsaturated fatty acids expected to have more unsaturated fatty acids in their membranes, since those remain fluid at in their membranes, since those remain fluid at lower temperatures. Plants adapted to hot lower temperatures. Plants adapted to hot environments would be expected to have more environments would be expected to have more saturated fatty acids, which would allow the saturated fatty acids, which would allow the fatty acids to “stack” more closely, making the fatty acids to “stack” more closely, making the membranes less fluid and therefore helping membranes less fluid and therefore helping them to stay intact at higher temperatures. them to stay intact at higher temperatures.

Permeability of the Permeability of the Plasma MembranePlasma Membrane

Selective PermeabilitySelective Permeability

Some substances can freely move across Some substances can freely move across the membrane whereas others cannot.the membrane whereas others cannot.

Hydrophobic (nonpolar) molecules, such Hydrophobic (nonpolar) molecules, such as hydrocarbons, carbon dioxide, and as hydrocarbons, carbon dioxide, and oxygen, can cross with ease.oxygen, can cross with ease.

Small Polar molecules (HSmall Polar molecules (H22O) can pass O) can pass

between the phospholipids between the phospholipids

Selective Permeability contdSelective Permeability contd

Macromolecules cannot pass through Macromolecules cannot pass through membranemembrane Transported by vesicle formationTransported by vesicle formation

Ions and charged molecules cannot Ions and charged molecules cannot Carrier and channel proteins transport these.Carrier and channel proteins transport these.

Transport across the plasma membrane Transport across the plasma membrane includes:includes:

Passive transportPassive transport Facilitated transportFacilitated transport Active transportActive transport Membrane assisted Membrane assisted

Passive TransportPassive Transport

DiffusionDiffusion: The spontaneous tendency of a : The spontaneous tendency of a substance to move down its concentration substance to move down its concentration gradient from a more concentrated to a gradient from a more concentrated to a less concentrated area. less concentrated area. The diffusion of a substance across a The diffusion of a substance across a

biological membrane = passive transportbiological membrane = passive transport

Note that each substance diffuses down its own concentration gradient, unaffected by the concentration differences of other substances

Passive Transport contdPassive Transport contd

OsmosisOsmosis: The diffusion of water across : The diffusion of water across the plasma membrane. the plasma membrane.

Solutions: a liquid that is a homogeneous Solutions: a liquid that is a homogeneous mixture of two or more substances. Has mixture of two or more substances. Has 2 parts2 parts

1.1. SolventSolvent: The dissolving agent of a : The dissolving agent of a solution (liquid portion) . Water is the solution (liquid portion) . Water is the most versatile solvent known. most versatile solvent known.

2.2. SoluteSolute: A substance that is dissolved in a : A substance that is dissolved in a solution. solution.

Passive Transport contdPassive Transport contdTonicityTonicity: The ability of a solution to cause a cell : The ability of a solution to cause a cell

within it to gain or lose water. 3 typeswithin it to gain or lose water. 3 types HypertonicHypertonic: A solution with a higher : A solution with a higher

concentration of solute than inside the cell.concentration of solute than inside the cell. Water diffuses out. Water diffuses out.

HypotonicHypotonic: A solution with a lower concentration : A solution with a lower concentration of solute than inside the cell.of solute than inside the cell. Water diffuses into.Water diffuses into.

IsotonicIsotonic: A solution with equal concentrations of : A solution with equal concentrations of solute on both sides of the membrane.solute on both sides of the membrane. No net movement of water into or out of. No net movement of water into or out of.

TurgidTurgid: A walled cell becomes turgid if it has a : A walled cell becomes turgid if it has a greater solute concentration than its surroundings, greater solute concentration than its surroundings, resulting in entry of water. (turgor pressure) resulting in entry of water. (turgor pressure) (Tonicity?)(Tonicity?)

PlasmolysisPlasmolysis: : A phenomenon in walled cells in A phenomenon in walled cells in which the cytoplasm shrivels and the plasma which the cytoplasm shrivels and the plasma membrane pulls away from the cell wall when the membrane pulls away from the cell wall when the cell loses water to a hypertonic environment. cell loses water to a hypertonic environment.

Crenate

Transport by ProteinsTransport by Proteins

Molecules that cannot diffuse across the Molecules that cannot diffuse across the plasma membrane, can be transported by plasma membrane, can be transported by integral membrane proteins.integral membrane proteins.

Channel & Carrier ProteinsChannel & Carrier Proteins: Specific to the : Specific to the molecule they transportmolecule they transport Required for facilitated and active transport.Required for facilitated and active transport.

Facilitated TransportFacilitated Transport

An integral protein (channel or carrier) An integral protein (channel or carrier) assists the movement of a molecule down assists the movement of a molecule down

its concentration gradient. its concentration gradient.

Facilitated DiffusionFacilitated Diffusion

Increases the rate Increases the rate at which the at which the solute crosses the solute crosses the plasma plasma membranemembrane Faster than Faster than

diffusiondiffusion

Active TransportActive Transport

The movement of molecules against their The movement of molecules against their concentration gradientconcentration gradient

Active TransportActive Transport

Movement from low concentration to high Movement from low concentration to high concentration.concentration.

Requirements:Requirements: Transport protein (specific) aka pumpsTransport protein (specific) aka pumps Energy (ATP) Energy (ATP)

The sodium–potassium pump: a The sodium–potassium pump: a specific case of active transport.  specific case of active transport. 

-pumps ions against steep -pumps ions against steep concentration gradientsconcentration gradients

-Sodium ion concentration -Sodium ion concentration (represented as [Na+]) is high (represented as [Na+]) is high outside the cell and low inside, while outside the cell and low inside, while potassium ion concentration ([K+]) potassium ion concentration ([K+]) is low outside the cell and high is low outside the cell and high inside. inside.

-The pump oscillates between two -The pump oscillates between two conformational states in a pumping conformational states in a pumping cycle that moves three sodium ions cycle that moves three sodium ions out of the cell for every two out of the cell for every two potassium ions pumped into the cell.potassium ions pumped into the cell.

- ATP powers the changes in - ATP powers the changes in conformation by phosphorylating the conformation by phosphorylating the transport protein (that is, by transport protein (that is, by transferring a phosphate group to transferring a phosphate group to the protein). the protein).

Fig 5.11Fig 5.11

Proton pumps, the main electrogenic pumps of plants, fungi, and bacteria, are membrane proteins that store energy by generating voltage (charge separation) across membranes.

Membrane Assisted Membrane Assisted TransportTransport

The formation of vesicles by the plasma The formation of vesicles by the plasma membrane to enable the transport of membrane to enable the transport of

macromolecules (too big for transport proteins) macromolecules (too big for transport proteins) - Bulk transport- Bulk transport

Membrane Assisted TransportMembrane Assisted Transport ExocytosisExocytosis: “out of : “out of

cell” Vesicles cell” Vesicles (formed by Golgi) (formed by Golgi) fuse with plasma fuse with plasma membrane to membrane to secrete specific secrete specific molecules to molecules to external external environment. environment.

Membrane Assisted TransportMembrane Assisted Transport

EndocytosisEndocytosis: “into cell” invagination and : “into cell” invagination and pinching off of plasma membrane to form pinching off of plasma membrane to form a vesicle to take in certain substances into a vesicle to take in certain substances into cell.cell.

Types:Types: PhagocytosisPhagocytosis: “cell eating” solids such as food : “cell eating” solids such as food

particles or other cells.particles or other cells. PinocytosisPinocytosis: “cell drinking” liquids. : “cell drinking” liquids.