AP Biology

Membrane Structure and Function

Plasma Membrane

• Boundary that separates the living cell from nonliving surroundings.

• About 8 nm thick, and is selectively permeable

• Controls traffic into and out of the cell• Ability of the plasma membrane to

move materials is due to its structure.

PhospholipidStructure of thePlasma membrane

Membrane Structure

• Membranes are composed mainly of phospholipids, which are amphipathic molecules (having hydrophilic and hydrophobic areas). Proteins are embedded in the membrane.

• Fluid mosaic model

Fluidity of Membranes• Membrane held together by hydrophobic

interactions• Phospholipids and proteins can move about

within the membrane• Elements of the cytoskeleton may hold

some proteins in place• Membranes remain fluid as temp. decreases

up to the point the phospholipids settle into a close-packed arrangement.

• Cholesterol acts as a buffer to resist changes in temperature.

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Experiment: Membrane protein movement

• Researchers labeled the plasma membrane proteins of a mouse and a human cell, then fused them.

• Results: Mixing the mouse and human proteins produced a hybrid, showing that some proteins move sideways within the membrane.

Membrane proteins and their functions

• Phospholipids form the main fabric of the membrane, but proteins determine its specific function.

• Integral proteins-penetrate the hydrophobic core of the lipid bilayer. Many are transmembrane proteins that completely span the membrane.

• Peripheral proteins- not embedded in the membrane. Appendages loosely bound to membrane surface.

• Structure of atransmembraneprotein

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Six major functions of membrane proteins

• Transport• Enzymatic activity• Signal transduction• Cell-cell recognition• Intercellular joining• Attachment of the cytoskeleton

and ECM

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Membrane surfaces

• Membranes are bifacial, having a side facing inside the cell and a side facing the cytoplasm.

• Two lipid bilayers may vary in composition.

• Membrane synthesis and modification by the ER and Golgi determines the distribution of lipids, proteins, and carbohydrates.

Cell-Cell Recognition• Cell recognition is the ability of a cell to

determine if other cells it encounters are alike or different from itself.

• Essential for sorting embryonic cells into tissues and organs, and rejection of foreign cells by the immune system

• Carbohydrates, glycolipids and glycoproteins, on the external surface of the cell membrane

act as “markers”.• These can vary between species, individuals,

and among the cells of the same individual.

Traffic Across Membranes

• Membrane’s molecular organization results in selective permeability.

• Selective permeability of a membrane depends on solubility characteristics of the phospholipid bilayer and the presence of integral transport proteins.

Permeability of the Lipid bilayer

• Nonpolar (hydrophobic) molecules dissolve in the membrane and cross it easily (hydrocarbons, O2, and CO2)

• Small, polar (hydrophilic) uncharged molecules such as water pass freely

• Large uncharged polar molecules and all ions have difficulty penetrating the hydrophobic layer.

Transport Proteins

• Polar molecules and specific ions can pass through the hydrophobic layer of the cell membrane through transport proteins that span the membrane.

• Some materials pass through protein channels, while others bind to the protein and are physically moved across. Ex) glucose enters liver cells by very selective transport proteins

Passive Transport

• Passive transport is diffusion of a substance across a biological membrane.

• Diffusion- net movement of molecules from high concentration to low concentration (down the concentration gradient)

• Results from kinetic energy of molecules. Does not require energy input.

• Diffusion continues until a dynamic equilibrium is reached (no net directional movement)

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Osmosis

• Osmosis is the passive transport of water across a selectively permeable membrane.

• Relative concentration terms: a) Hypertonic- solution with greater

solute concentration than inside the cell. b) Hypotonic- solution with lower solute

concentration than inside the cell. c) Isotonic- solution with equal solute

concentration compared to that inside a cell.

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Movement of Water• If two solutions are separated by a

selectively permeable membrane that is permeable to water but not to solute, water will diffuse from the hypoosmotic solution to the hyperoosmotic solution.

• Water move to DILUTE!• Direction of osmosis is determined by the

difference in total solute concentration, regardless of the types of solutes in the solutions.

• At equilibrium, water molecules move in both directions at the same rate. (True for isotonic solutions also)

Cell Survival

• Balance of water between the cell and its environment are crucial to organisms.

• Osmoregulation- control of water balance.• Animal cells (no cell walls) have adaptations

for osmoregulation. Ex) Paramecium have contractile vacuoles

• Plants can take in water and become turgid (firm), while in isotonic solutions cells become flaccid (limp). In hypertonic solutions, the plant cell shrivels and plasmolysis occurs.

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Facilitated Diffusion

• Diffusion of polar molecules and ions across a membrane with the aid of transport proteins.

• Proteins have a specialized binding site for the solute they transport.

• Some proteins have gated channels that open or close in response to a stimulus.

Facilitated Diffusion

Active Transport

• Pumping of solutes against the concentration gradient, requiring energy from the cell.

• Sodium-potassium pump allows cells to exchange Na+ and K+ across animal cell membranes.

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Ion Pumps• Because anions and cations are unequally

distributed across plasma membranes, all cells have voltages across their plasma membrane (membrane potential).

• Forces that drive passive transport of ions across membranes include: concentration gradient of the ion (chemical force), and effect of membrane potential (electrical force) on the ion.

• The combination of these forces is called the electrochemical gradient.

Electrogenic Pumps• Transport proteins that generate voltage

across a membrane.• Na+/K+ ATPase is the major electrogenic

pump in animal cells• A proton pump (H+) is the major

electrogenic pump in plants, bacteria, and fungi.

• Mitochondria and chloroplasts use a proton pump to drive ATP synthesis.

• Voltages created by electrogenic pumps are sources of potential energy available to do cellular work.

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Cotransport

• Process where a single ATP-powered pump actively transports one solute and indirectly drives the transport of other solutes against their concentration gradients.

• Example: Plants use the mechanism of sucrose/H+ cotransport to load sucrose produced by photosynthesis into specialized cells in the veins of leaves. Transport proteins can move sucrose into the cell against the concentration gradient only if it travels with the H+ ion.

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Movement of Large molecules

• Water and small molecules cross membranes by passing through the phospholipid bilayer or being carried by a transport protein.

• Large molecules such as proteins and polysaccharides cross membranes by the processes of endocytosis and exocytosis.

Exocytosis

• Process of exporting macromolecules from a cell by fusion of vesicles with the plasma membrane.

• Vesicle usually budded from the ER or Golgi and migrates to plasma membrane

• Used by secretory cells to export products, such as insulin in the pancreas or neurotransmitters from neurons.

Endocytosis

• Process of importing macromolecules into a cell forming vesicles derived from the plasma membrane.

• Vesicle forms from a localized region of plasma membrane that sinks inward, pinches off into the cytoplasm.

• Used by cells to incorporate extracellular substances.

Types of Endocytosis: Phagocytosis

• Endocytosis of solid particles.• Cell engulfs particle with

pseudopodia, and pinches off a food vacuole.

• Vacuole fuses with a lysosome containing hydrolytic (digestive) enzymes that break down the particle.

Phagocytosis of a

bacterial cell by an

amoeba

Pinocytosis

• Endocytosis of fluid droplets• Extracellular fluid is engulfed in

small vesicles• Nonspecific in the substances it

transports. Cell takes in all solutes dissolved in the droplet.

Pinocytosis offluid into celllining a bloodvessel

Receptor-mediated Endocytosis

• More discriminating process than pinocytosis

• Coated pits form vesicles when specific molecules (ligands) bind to receptors on the cell surface.

• After ingested material is released from the vesicle for metabolism, the receptors are recycled to the plasma membrane.

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Receptor-mediated endocytosis

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