MembraneChapter 7

Cell membrane

Cell Membrane Plasma membrane Selective permeability Surrounds all living cells 2 molecules thick.

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Plasma Membrane Interacts with environment Exchanges information Manages movement in & out of

cell

Fluid mosaic model Revised in 1972 by Singer &

Nicolson Globular proteins imbedded in the

membrane Not surrounding the lipid layer Proteins “float” in the membrane

like boats in a pond.

Plasma membrane Lipid membrane (phospholipids) Protein passageways Allow water-soluble substances to

pass

Membrane Structure 1. Phospholipid layer 2. Cholesterol 3. Transmembrane proteins 4. Interior protein network 5. Cell surface markers

Membrane structure

Triglycerides Water insoluble Non-polar

Phospholipid Glycerol (3 carbon alcohol) 2 fatty acids Phosphate attached to 3rd carbon

Phospholipid bilayer Orients polar head toward water

molecules Non-polar tail away from water. Non-polar barrier to water soluble

molecules

Phospholipid bilayer Hydrophobic interactions hold

membrane together Phospholipids & unanchored

proteins move freely within the membrane

Cholesterol Located between lipids Increases or decreases fluidity Depends on temperature.

Cholesterol

Structures of membrane proteins 1. Peripheral proteins Attached to the surface membrane 2. Integral proteins Extend through the membrane Transmembrane proteins Partway

Structures of membrane proteins Non-polar section of protein Contact with the non-polar interior

(hydrophobic) Polar section of the protein Extends out of membrane

Transmembrane proteins 1. Single-pass anchors 2. Multiple-pass channels and

carriers 3. Pores

Aquaporins

Functions of membrane proteins 1. Transport Allow specific substances to enter or

leave 2. Enzymes Carry out reactions on interior

surface 3. Cell surface receptors Sensitive to chemical messages

Functions of membrane proteins

4. Cell surface identity markers Cell recognition between cells Glycoproteins 5. Cell adhesion proteins Proteins stick to each other Temporary or more permanently 6. Attachments to the cytoskeleton

Membrane Proteins

Membrane structure

Membrane formation Proteins & lipids made in ER Processed in Gogi apparatus Vesicles Fuse with membrane Correct orientation

Movement across the membrane 1. Passive transport 2. Bulk transport 3. Active transport

Passive transport Diffusion High concentration to areas of

lower concentration Concentrations are the same in all

regions

Passive transport Two conditions determine the

direction of the flow of ions 1. Relative concentration of the ion

on each side of the membrane 2. The voltage across the

membrane

Diffusion

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Osmosis Net movement of water across a

membrane Moves towards an area of lower

water concentration Or higher solute concentration Passive transport of water

Osmosis Osmoregulation: Control of water balance

Osmosis Hyperosmostic Solution with higher concentration of solute Has less water Hypoosmostic Solution with the lower concentration of solute Has more water Isosmostic Solutions have equal concentration of solutes

Solutions Solute Substance being dissolved Solvent Substance doing the dissolving

Solutions Hypertonic: Solution with higher concentration of

solute Hypotonic: Solution with lower concentration of

solute Isotonic Solutions with equal solute

concentrations

Facilitated diffusion Carriers transport Ions, aa or sugars across the

membrane Specific for type of solute Moves from high concentration to

low The carrier facilitates the process

Examples RBC diffusion of the bicarbonate

ion RBC diffusion of glucose

Facilitated diffusion 1. It is specific 2. It is passive 3. It saturate Carriers are saturated Transport rate cannot increase Despite the concentration gradient

Bulk transport Endocytosis Membrane envelops food particles 1. Phagocytosis 2. Pinocytosis 3. Receptor-mediated endocytosis

Bulk transport Phagocytosis: The cell engulfs particles Pinocytosis: The cell engulfs liquid Receptor-mediated: Transported molecules bind specific

receptors in the plasma membrane

Phagocytosis

Pinocytosis

Receptor-mediated

Exocytosis The reverse of endocytosis Discharge of materials from the

cell Secretion of many hormones,

neurotransmitters, digestive enzymes and other substances

Active transport Movement of substances against or

up the concentration gradient Low concentration to high Requires energy. Requires the use of ATP. Cell takes up substances into

cytoplasm that are already hypertonic to the extracellular fluid.

Sodium-potassium pump Protein transports Na and K ions

across the membrane. Every cycle transports 3 Na ions

out Exchange for 2 K ions into the cell Against concentration gradients.

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Coupled transport Molecule moves up its

concentration gradient Uses the drive from another

substance moving down its concentration gradient.

Na ions and glucose

Fig. 7-19

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Sucrose