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![Cytoplasmic Na+
bonds to
the sodium-potassium pump
CYTOPLASM
Na+
[Na+
] low
[K+
] high
Na+
Na+
EXTRACELLULAR
FLUID
[Na+
] high
[K+
] low
Na+
Na+
Na+
ATP
ADP
P
Na+
binding stimulates
phosphorylation by ATP.
Na+
Na+
Na+
K+
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
P
Extracellular K+
binds
to the protein, triggering
release of the phosphate
group.
P
P
Loss of the phosphate
restores the protein’s
original conformation.
K+
is released and Na+
sites are receptive again;
the cycle repeats.
K+
K+
K+
K+
K+](https://image.slidesharecdn.com/plasmamembrane-150214210618-conversion-gate01/85/Plasma-membrane-40-320.jpg)
Uploaded byAftab Badshah
Plasma membrane
The plasma membrane is a selectively permeable barrier that separates the cell from its external environment. It is composed of a phospholipid bilayer with embedded proteins. Phospholipids are amphipathic molecules with hydrophobic tails and hydrophilic heads that form a fluid bilayer. The fluid mosaic model describes membranes as a fluid structure with various proteins embedded within. Membranes exhibit selective permeability through diffusion, facilitated diffusion, and active transport. Active transport requires ATP and moves molecules against their concentration gradient. The sodium-potassium pump is an example of active transport that helps maintain membrane potential.
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Plasma membrane
- 1. PLASMA MEMBRANE • Theplasma membrane is the boundary that separates the living cell from its nonliving surroundings • The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
- 2. Cellular membranes arefluid mosaics of lipids and proteins • Phospholipids are the most abundant lipid in the plasma membrane • Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions • The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it STRUCTURE
- 3.
- 4. Hydrophilic region of protein Hydrophobicregion of protein Phospholipid bilayer
- 5. The Fluidity ofMembranes • Phospholipids in the plasma membrane can move within the bilayer • Most of the lipids, and some proteins, drift laterally • Rarely does a molecule flip-flop transversely across the membrane
- 6. Lateral movement (~107 times persecond) Flip-flop (~ once per month) Movement of phospholipids
- 7. • As temperaturescool, membranes switch from a fluid state to a solid state • The temperature at which a membrane solidifies depends on the types of lipids • Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids • Membranes must be fluid to work properly; they are usually about as fluid as salad oil
- 8. ViscousFluid Unsaturated hydrocarbon tails withkinks Membrane fluidity Saturated hydro- carbon tails
- 9. • The steroidcholesterol has different effects on membrane fluidity at different temperatures • At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids • At cool temperatures, it maintains fluidity by preventing tight packing • Some proteins in the plasma membrane can drift within the bilayer • Proteins are much larger than lipids and move more slowly
- 10. Cholesterol Cholesterol within theanimal cell membrane
- 11. Membrane Proteins andTheir Functions • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer • Proteins determine most of the membrane’s specific functions • Peripheral proteins are not embedded • Integral proteins penetrate the hydrophobic core and often span the membrane
- 12. Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Microfilaments ofcytoskeleton Cholesterol Integral protein Peripheral proteins CYTOPLASMIC SIDE OF MEMBRANE EXTRACELLULAR SIDE OF MEMBRANE Glycolipid
- 13. • Integral proteinsthat span the membrane are called transmembrane proteins • The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
- 14. • Six majorfunctions of membrane proteins: –Transport –Enzymatic activity –Signal transduction –Cell-cell recognition –Intercellular joining –Attachment to the cytoskeleton and extracellular matrix (ECM)
- 15. Enzymes Signal Receptor ATP Transport Enzymatic activitySignal transduction
- 16. Glyco- protein Cell-cell recognition Intercellularjoining Attachment to the cytoskeleton and extra- cellular matrix (ECM)
- 17. The Role ofMembrane Carbohydrates in Cell- Cell Recognition • Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane • Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) • Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
- 18. Synthesis and Sidednessof Membranes • Membranes have distinct inside and outside faces • The asymmetrical distribution of proteins, lipids and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus
- 19. Plasma membrane: Cytoplasmic face Extracellularface Transmembrane glycoprotein Plasma membrane: Secreted protein Vesicle Golgi apparatus Glycolipid Secretory protein Transmembrane glycoproteins ER
- 20. Membrane structure resultsin selective permeability • A cell must exchange materials with its surroundings, a process controlled by the plasma membrane • Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
- 21. The Permeability ofthe Lipid Bilayer • Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly • Polar molecules, such as sugars, do not cross the membrane easily
- 22. Transport Proteins • Transportproteins allow passage of hydrophilic substances across the membrane • Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel • Channel proteins called aquaporins facilitate the passage of water
- 23. • Other transportproteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane • A transport protein is specific for the substance it moves
- 24. Passive transport isdiffusion of a substance across a membrane with no energy investment • Diffusion is the tendency for molecules to spread out evenly into the available space • Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction • At dynamic equilibrium, as many molecules cross one way as cross in the other direction
- 25. Molecules of dyeMembrane (cross section) WATER Net diffusion Net diffusion Equilibrium Diffusion of one solute
- 26. • Substances diffusedown their concentration gradient, the difference in concentration of a substance from one area to another • No work must be done to move substances down the concentration gradient • The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen
- 27. Net diffusion Netdiffusion Equilibrium Diffusion of two solutes Net diffusion Net diffusion Equilibrium
- 28. Effects of Osmosison Water Balance • Osmosis is the diffusion of water across a selectively permeable membrane • The direction of osmosis is determined only by a difference in total solute concentration • Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration
- 29. Lower concentration of solute (sugar) Higher concentration ofsugar Same concentration of sugar Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can H2O Osmosis
- 30. Water Balance ofCells Without Walls • Tonicity is the ability of a solution to cause a cell to gain or lose water • Isotonic solution: solute concentration is the same as that inside the cell; no net water movement across the plasma membrane • Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water • Hypotonic solution: solute concentration is less than that inside the cell; cell gains water
- 31. • Animals andother organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment • To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance • The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
- 32. Filling vacuole 50 µm 50µm Contracting vacuole
- 33. Water Balance ofCells with Walls • Cell walls help maintain water balance • A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) • If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt • In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
- 34. Animal cell Lysed H2O H2O H2O Normal Hypotonicsolution Isotonic solution Hypertonic solution H2O Shriveled H2OH2OH2OH2OPlant cell Turgid (normal) Flaccid Plasmolyzed
- 35. Facilitated Diffusion: PassiveTransport Aided by Proteins • In facilitated diffusion, transport proteins speed movement of molecules across the plasma membrane • Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane • Carrier proteins undergo a slight change in shape that translocates the solute-binding site across the membrane
- 36.
- 37.
- 38. Factors Affecting DiffusionRate • Steepness of concentration gradient – Steeper gradient, faster diffusion • Molecular size – Smaller molecules, faster diffusion • Temperature – Higher temperature, faster diffusion
- 39. Active Transport • Activetransport moves substances against their concentration gradient • Active transport requires energy, usually in the form of ATP • Active transport is performed by specific proteins embedded in the membranes • The sodium-potassium pump is one type of active transport system
- 40. Cytoplasmic Na+ bonds to thesodium-potassium pump CYTOPLASM Na+ [Na+ ] low [K+ ] high Na+ Na+ EXTRACELLULAR FLUID [Na+ ] high [K+ ] low Na+ Na+ Na+ ATP ADP P Na+ binding stimulates phosphorylation by ATP. Na+ Na+ Na+ K+ Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. P Extracellular K+ binds to the protein, triggering release of the phosphate group. P P Loss of the phosphate restores the protein’s original conformation. K+ is released and Na+ sites are receptive again; the cycle repeats. K+ K+ K+ K+ K+
- 41. Diffusion Facilitated diffusion Passivetransport ATP Active transport
- 42. Maintenance of MembranePotential by Ion Pumps • Membrane potential is the voltage difference across a membrane • Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane: – A chemical force (the ion’s concentration gradient) – An electrical force (the effect of the membrane potential on the ion’s movement)
- 43. • An electrogenicpump is a transport protein that generates the voltage across a membrane • The main electrogenic pump of plants, fungi, and bacteria is a proton pump
- 44.
- 45. Cotransport: Coupled Transportby a Membrane Protein • Cotransport occurs when active transport of a solute indirectly drives transport of another solute • Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
- 46.
- 47. • Many membraneproteins are enzymes – This is especially important on the membranes of organelles.
- 48. • Signal transduction(receptor) proteins • bind hormones and other substances on the outside of the cell. – Binding triggers a change inside the cell. • Called signal transduction • Example: The binding of insulin to insulin receptors causes the cell to put glucose transport proteins into the membrane.
- 49.
- 50. Bulk Flow • Vesiclesare used to transport large particles across the PM. – Requires energy • Types: – Exocytosis – Endocytosis • Phagocytosis, pinocytosis, receptor-mediated
- 51.
- 52. Bulk Flow • Exocytosis –Cytoplasmicvesicle merges with the PM and releases its contents –Example: • Golgi body vesicles merge with the PM an release their contents • How nerve cells release neurotransmittors
- 53. Vesicle forming EndocytosisEndocytosis Endocytosis canoccur in three waysEndocytosis can occur in three ways • Phagocytosis ("cell eating")Phagocytosis ("cell eating") • Pinocytosis ("cell drinking")Pinocytosis ("cell drinking") • Receptor-mediated endocytosisReceptor-mediated endocytosis
- 54. Endocytosis • Endocytosis – PMsinks inward, pinches off and forms a vesicle – Vesicle often merges with Golgi for processing and sorting of its contents
- 55. Endocytosis - terms •Phagocytosis – cell eating – Membrane sinks in and captures solid particles for transport into the cell – Examples: • Solid particles often include: bacteria, cell debris, or food • Pinocytosis – cell drinking – Cell brings in a liquid
- 56. Endocytosis - comments •Phagocytosis and pinocytosis are not selective – Membrane sinks inward and captures whatever particles/fluid present. – Vesicle forms and merges with the Golgi body…
- 57. Receptor Mediated Endocytosis •Receptor Mediated Endocytosis is a highly specific form of endocytosis. – Receptor proteins on the outside of the cell bind specific substances and bring them into the cell by endocytosis
- 58. Receptor Mediated Endocytosis 1.Receptor proteins on PM bind specific substances (vitamins, hormones..) 2. Membrane sinks in and forms a pit – Called a coated pit 2. Pit pinches closed to form a vesicle around bound substances • Cytoskeleton aids in pulling in the membrane and vesicle formation
- 59. Fig. 5-9c Coated vesicle Coated pit Specific molecule Receptor-mediated endocytosis Coatprotein Receptor Coated pit Material bound to receptor proteins Plasma membrane
- 61. Fig. 5-9 Phagocytosis EXTRACELLULAR FLUID Pseudopodium CYTOPLASM Food vacuole “Food”or other particle Pinocytosis Plasma membrane Vesicle Coated vesicle Coated pit Specific molecule Receptor-mediated endocytosis Coat protein Receptor Coated pit Material bound to receptor proteins Plasma membrane Food being ingested