Membrane Chapter 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 the movement in and out of the 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 Cholesterol 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 higher solute concentration Or lower water 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 Common solutes in the cell or cell’s cytoplasm? Solvent Substance doing the dissolving What is the most common solvent in the cell?

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

– + H+ ATP H+ – + H+ H+ – + H+ H+ – + H+ H+ – + – + Diffusion of H+ Fig. 7-19 – + H+ ATP H+ – + Proton pump H+ H+ – + H+ H+ – + H+ Diffusion of H+ Sucrose-H+ cotransporter Figure 7.19 Cotransport: active transport driven by a concentration gradient H+ – Sucrose + – + Sucrose