Membrane Structure and Function

Transcription

1 Chapter 7 Membrane Structure and Function PowerPoint Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

2 Chapter 7 Membrane Structure and Function PowerPoint Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

3 Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings Controls traffic in and out of cell The plasma membrane exhibits selective permeability allows some substances to cross it more easily than others The ability of the cells to discriminate in its chemical exchanges is fundamental to life.

4 Fig. 7-1 Nerve cell signaling. Protein stores ability of nerve cell to fire again by providing a channel for a stream of K + ions to exit the cell at a precise moment after nerve stimulation

5 Concepts in this Chapter 1. Cellular membranes are fluid mosaics of lipids and proteins 2. Membrane structure results in selective permeability 3. Passive transport is diffusion of a substance across a membrane with no energy investment 4. Active transport uses energy to move solutes against their gradients 5. Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

6 CONCEPT 7.1: CELLULAR MEMBRANES ARE FLUID MOSAICS OF LIPIDS AND PROTEINS

7 Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Lipids and proteins are the staple ingredients of membranes Carbohydrates also present Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules contain hydrophobic and hydrophilic regions Hydrophilic head Hydrophobic tails Phospholipid symbol

8 Fig. 7-2 Hydrophilic head WATER Hydrophobic tail WATER

9 Membrane Models: Scientific Inquiry Membranes have been chemically analyzed and found to be made of proteins and lipids Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer Exist as a stable boundary Hydrophobic tails of phospholupus sheltered from water But did not know where proteins were arranged The fluid mosaic model states that a membrane is a fluid structure with a mosaic of various proteins embedded in it

10 Fig. 7-3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

11 Getting to the fluid mosaic model: Scientific Inquiry In 1935, Hugh Davson and James Danielli proposed a sandwich model Noticed that biologocal membranes adhere better to water than pure phoshopholipid bilayer the phospholipid bilayer lies between two layers of globular proteins Problems Not all membranes are identical Mitochondrial membranes thinner and more protein-filled and have different kinds of phospholipids and other lipids than plasma membrane Different membrane function= different chemical function and structure Proteins have both hydrophobic and hydrophilic regions

12 Getting to the fluid mosaic model: Scientific Inquiry In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, only the hydrophilic regions exposed to water Hydrophobic regions within nonaqueous environment Fluid Mosaic Model Membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids

13 Fig. 7-3 Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

14 Getting to the fluid mosaic model: Scientific Inquiry Freeze-fracture studies of the plasma membrane supported the fluid mosaic model Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer TECHNIQUE Extracellular layer RESULTS Knife Proteins Inside of extracellular layer Plasma membrane Cytoplasmic layer Inside of cytoplasmic layer

15 The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Membrane held together primarily by weak hydrophobic interactions Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane

16 Fig. 7-5a Lateral movement ( 10 7 times per second) Flip-flop ( once per month) (a) Movement of phospholipids

17 The Fluidity of Membranes Membrane proteins move, but a slower rate than lipids larger Some highly directional May use cytoskeletal fibers Some appear immobile Due to attachment to the cytoskeleton Some move by drifting Frye and Edidin, Johns Hopkins Labeled mouse and human plasma membrane proteins with two different markers, mixed, observed

18 Fig. 7-6 RESULTS Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour

19 The Fluidity of Membranes As temperatures cool, 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 that those rich in saturated fatty acids Why? The lipid composition of cell membranes can change as an adjustment to changing temperatures Winter wheat increase unsaturated phosholipids in fall Membranes must be about as fluid as salad oil to work properly

20 Fig. 7-5b Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydrocarbon tails (b) Membrane fluidity

21 The Fluidity of Membranes: Cholesterol The steroid cholesterol acts as a temperature buffer for the membrane Resists changes in membrane fluidity At warm temperatures (such as 37 C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing

22 Fig. 7-5c Cholesterol (c) Cholesterol within the animal cell membrane

23 Membrane Proteins and Their Functions A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer More than 50 kinds of proteins have been found in plasma membrane of red blood cells Proteins determine most of the membrane s specific functions Differ between cells and within various membranes of a cell

24 Fig. 7-7 Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE

25 Membrane Proteins and their Functions There are two major populations of membrane proteins Peripheral proteins On membrane surface Loosely bound Integral membrane proteins Penetrate hydrophobic core of membrane Transmembrane proteins Span the membrane The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices Hydrophilic part exposed to aqueous solutions Some have hydrophilic channel through their center

26 Fig. 7-8 N-terminus EXTRACELLULAR SIDE C-terminus α Helix CYTOPLASMIC SIDE

27 Membrane Proteins and their Functions Six major functions of membrane proteins: Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

28 Fig. 7-9ac Signaling molecule Enzymes Receptor ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction

29 Fig. 7-9df Glycoprotein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

30 The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells need to recognize each other for an organism to function properly Sorting of cells into tissues and organs in animal embryo Rejection of foreign cells by immune system Recognition by binding to surface molecules, often carbohydrates, on the plasma membrane Membrane carbohydrates usually short, branched chains Maybe 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 A, B, AB, O blood groups based on carbohydrates on surface of red blood cells

31 Synthesis and Sidedness of 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

32 Fig Synthesis of membrane proteins and lipids in ER; formation of glycoproteins 2. Further carbohydrate modification to glycolipids 3.Transmembrane protein, glycolipids and secretory protein transported to plasma membrane in vesicles 4. Vesicles fuse with membranes, releases secreted proteins from cell, positions carbohydrates onto outside of membrane. Secretory protein Glycolipid Golgi 2 apparatus Vesicle Secreted protein 3 ER 1 Transmembrane glycoproteins 4 Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Membrane glycolipid

33 Summary Fluid mosaic model Mosaic of Fluid bilayer of Membrane phospholipids move in two ways Laterally Transversely Factors that influence membrane fluidity Type of fatty acid Saturated vs unsaturated Cholesterol Temperature buffer (warm? cool?)

34 Summary Types of membrane proteins Peripheral Not embedded in lipid bilayer Integral Penetrate hydrophobic core Transmembrane proteins Functions Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton and ECM Membrane carbohydrates Glycolipids and glycoproteins Cell-cell recognition

35 CONCEPT 7.2: MEMBRANE STRUCTURE RESULTS IN SELECTIVE PERMEABILITY

36 Concept 7.2: Membrane structure results in selective permeability A cell must exchange materials with its surroundings, a process controlled by the plasma membrane A steady traffic of small molecules and ions moves across the plasma membrane in both directions Plasma membranes are selectively permeable, regulating the cell s molecular traffic Regulates what molecules go in and out of cell Regulates rate of movement of molecules in and out of cell

37 The Permeability of the 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 Hydrophobic core impedes movement Need the help of membrane proteins Proteins built into the membrane may play key roles in regulating transport.

38 Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel For example aquaporins facilitate the passage of water 3 billion water molecules per second compared to a fraction Carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves For example the glucose transporter rejects fructose

39 Summary T/F The plasma membrane is a selective barrier TRUE T/F Polar molecules can cross plasma membrane easily FALSE ; hydrophobic barrier impedes; hydrocarbons can since nonpolar T/F Transport proteins only allow specific substances to cross the membrane TRUE There are two kinds of transport proteins Channel Proteins Carrier Proteins

40 CONCEPT 7.3: PASSIVE TRANSPORT IS DIFFUSION OF A SUBSTANCE ACROSS A MEMBRANE WITH NO ENERGY INVESTMENT

41 Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment Molecules have thermal motion One result of this is diffusion Diffusion is the movement of molecules of any substance so that they 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 Animation: Membrane Selectivity Animation: Diffusion

42 Fig. 7-11a Molecules of dye Membrane (cross section) WATER Net diffusion (a) Diffusion of one solute Net diffusion Equilibrium

43 Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment Substances diffuse down their concentration gradient the difference in concentration of a substance from one area to another Move from from side with higher concentration to side with lower concentration This difference represents potential energy Not affected by concentration differences of other substances No work must be done to move substances down the concentration gradient The diffusion of a substance across a biological membrane is passive transport it requires no energy from the cell to make it happen

44 Fig. 7-11b Net diffusion Net diffusion Equilibrium Net diffusion (b) Diffusion of two solutes Net diffusion Equilibrium

45 Effects of Osmosis on Water Balance Membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

46 Fig Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar H 2 O Selectively permeable membrane Osmosis

47 Water Balance of Cells Without Walls When considering the behavior of a cell in solution two factors must be considered: solute concentration membrane permeability Tonicity is the ability of a solution to cause a cell to gain or lose water Depends in part on the concentration of solutes that cannot cross the membrane relative to that inside the cell If there is a higher concentration of nonpenetrating solutes in the surrounding solution water will leave the cell If there is a lower concentration?

48 Water Balance of Cells Without Walls Isotonic solution: Solute concentration is the same as that inside the cell no net water movement across the plasma membrane Cell volume is stable Hypertonic solution: concentration is greater than that inside the cell cell loses water Shrivels and dies Hypotonic solution: Solute concentration is less than that inside the cell cell gains water Cell lyses and bursts

49 Fig Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O (a) Animal cell Lysed Normal Shriveled

50 Water Balance of Cells Without Walls Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Forces water out of the cell as fast as it gets in Plasma membrane also much less permeable than in other organisms Video: Chlamydomonas Video: Paramecium Vacuole

51 Fig Filling vacuole 50 µm (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell.

52 Water Balance of Cells 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) 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 Bacteria and fungi cell walls too

53 Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

54 Video: Plasmolysis Video: Turgid Elodea Animation: Osmosis

55 Facilitated Diffusion: Passive Transport Aided by Proteins Polar molecules and ions impeded by the plasma membrane can diffuse passively with the help of transport proteins This is called facilitated diffusion Passive transport because solutes move down their concentration gradient Does not alter direction of transport Most transport proteins are very specific There are two kinds of transport proteins Channel Proteins Carrier proteins

56 Channel Proteins Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Have hydrophilic passageways that can allow water or small molecules to pass eaily from one side to the other Channel proteins include Aquaporins facilitated diffusion of water Kidney cells have a high number to reclaim water from urine before its excreted Ion channels Many serve as gated channels open or close in response to a stimulus

57 Fig EXTRACELLULAR FLUID Channel protein (a) A channel protein Solute CYTOPLASM

58 Carrier Proteins Carrier proteins undergo a subtle change in shape translocates the solute-binding site across the membrane Changes in shape may be triggered by the binding and releasing of the transported molecule Example: glucose transporter Carrier protein Solute (b) A carrier protein

59 Facilitated Diffusion: Passive Transport Aided by Proteins Some diseases are caused by malfunctions in specific transport systems for example the kidney disease cystinuria The absence of a carrier protein that transports cysteine and other amino acids across the membranes of kidney cells These amino acids normally reabsorbed by kidney from urine and returned to blood Individuals develop painful stones from amino acids that accumulate and crystallize in the kidneys

60 Summary What is passive transport? Transport across the plasma membrane that does not require energy Diffusion Movement of molecules of a substance so they re spread evenly within a space Concentration gradient The region along which the density of a chemical substance decreases Molecules will diffuse down their concentration gradient Osmosis Diffusion of water across a selectively permeable membrane Osmoregulation The control of water balance

61 Lower concentration of nonpenetrating solutes in environment Same concentration of nonpenetrating solutes in environment Higher concentration of nonpenetrating solutes in environment Hypotonic solution Isotonic solution Hypertonic solution H 2 O H 2 O H 2 O H 2 O (a) Animal cell Lysed Normal Shriveled H 2 O H 2 O H 2 O H 2 O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed

62 Summary What is facilitated diffusion? Passive transport aided by proteins Channel proteins Aquaporins Ion channels Carrier proteins

63 CONCEPT 7.4: ACTIVE TRANSPORT USES ENERGY TO MOVE SOLUTES AGAINST THEIR GRADIENTS

64 Concept 7.4: Active transport uses energy to move solutes against their gradients Facilitated diffusion is still passive because the solute moves down its concentration gradient Some transport proteins, however, can move solutes against their concentration gradients Active transport 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 All are carrier proteins

65 Active transport Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system Animal cells have a higher concentration of K + and lower concentration of Na + compared to surroundings Plasma membrane pumps potassium into cell and sodium out ATP supplies energy for this

66 Fig EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + Na + CYTOPLASM Na + [Na + ] low [K + ] high 1 Cytoplasmic Na + binds to the sodium-potassium pump.

67 Fig Na + Na + Na + ATP P ADP 2 Na + binding stimulates phosphorylation by ATP.

68 Fig Na + Na + Na + 3 Phosphorylation causes the protein to change its shape. Na + is expelled to the outside. P

69 Fig P 4 K + binds on the extracellular side and triggers release of the phosphate group. P

70 Fig Loss of the phosphate restores the protein s original shape.

71 Fig K + is released, and the cycle repeats.

72 Fig EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + Na + Na + Na + Na + Na + Na + Na + CYTOPLASM 1 Na + [Na + ] low [K + ] high 2 P ADP ATP 3 P P P

73 Fig Passive transport Active transport Diffusion Facilitated diffusion ATP

74 How Ion Pumps Maintain Membrane Potential All cells have voltages across their plasma membranes Voltage is electrical potential energy (a separation of opposite charges Cytoplasm negative charge relative to extracellular fluid because of unequal anion and cation distribution Membrane potential is the voltage difference across a membrane Ranges from about -50 to -200 mv Acts as a battery, an energy source that affects traffic of charged particles across membrane Membrane potential favors passive transport of cations into cell and anions out of cell Because inside of cell is negative

75 How Ion Pumps Maintain Membrane Potential The electrochemical gradient a combination of two forces acting on an ion A chemical force (the ion s concentration gradient) An electrical force (the effect of the membrane potential on the ion s movement) These forces drive the diffusion of ions across the membrane Example: Na+ concentration much lower inside cell that outside. When cell is stimulated gated channels open to facilitate Na+ diffusion; ions fall down their electrochemical gradient, driven by 1) the concentration gradient of Na+ and 2) the attractions of these cations to the negative side of the membrane.

76 How Ion Pumps Maintain Membrane Potential Some membrane proteins involved in active transport contribute to the membrane potential This kind of protein that generates voltage across a membrane is called an electrogenic pump The sodium-potassium pump is the major electrogenic pump of animal cells Pumps 3 Na+ out of the cell for every 2 K+ it pumps into the cell Net transfer of 1+ out of cell, a process that stores energy as voltage

77 Fig EXTRACELLULAR FLUID [Na + ] high [K + ] low Na + Na + Na + Na + Na + Na + Na + Na + CYTOPLASM 1 Na + [Na + ] low [K + ] high 2 P ADP ATP 3 P P P

78 How Ion Pumps Maintain Membrane Potential The main electrogenic pump of plants, fungi, and bacteria is a proton pump Actively transports H+ out of cell By generating voltages across membranes, electrogenic pumps store energy that can be tapped for cellular work

79 Fig EXTRACELLULAR FLUID ATP + H + H + Proton pump H + + CYTOPLASM + H + H + H + + H +

80 Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of another solute A substance that has been pumped across a membrane can do work as it moves back across the membrane by diffusion Like water that has been pumped uphill can do work as it moves back down Or a cotransporter separate from the pump can couple the downhill diffusion of this substance to the uphill transport of another substance against its concentration gradient Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

81 Fig ATP + + H + H + H + Proton pump H + + H + Sucrose-H + cotransporter + H + H + Diffusion of H + H + Sucrose + + Sucrose

82 Summary Cotransport Active transport moves molecules against/down concentration gradient It is performed by.. Proteins Carrier proteins Example? Membrane potential Voltage difference across membranes Electrochemical gradient Chemical Electrical

83 CONCEPT 7.5: BULK TRANSPORT ACROSS THE PLASMA MEMBRANE OCCURS BY EXOCYTOSIS AND ENDOCYTOSIS

84 Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy

85 Exocytosis The cell secretes certain biological molecules by exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Neurons use exocytosis to release neurotransmitters to signal other neurons or muscle cells Plant cells use exocytosis to deliver proteins and carbohydrates from golgi vesicles to outside of cell to make cell walls Animation: Exocytosis

86 Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis: Phagocytosis ( cellular eating ) Pinocytosis ( cellular drinking ) Receptor-mediated endocytosis Animation: Exocytosis and Endocytosis Introduction

87 Phagocytosis In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle Animation: Phagocytosis

88 Fig. 7-20a EXTRACELLULAR FLUID Pseudopodium CYTOPLASM PHAGOCYTOSIS 1 µm Pseudopodium of amoeba Food or other particle Food vacuole Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM)

89 Pinocytosis In pinocytosis, molecules are taken up when extracellular fluid is gulped into tiny vesicles Animation: Pinocytosis

90 Fig. 7-20b PINOCYTOSIS Plasma membrane 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle

91 Receptor-mediated Endocytosis In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule Familial hypercholesterolemia Very high cholesterol levels in blood LDL receptor proteins defective or missing LDL particles cannot enter cells Cholesterol accumulates in blood, causes artherosclerosis, narrows blood vessels, impedes blood flow Animation: Receptor-Mediated Endocytosis

92 Fig. 7-20c RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated vesicle Ligand Coated pit Coat protein A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) Plasma membrane 0.25 µm

93 Endocytosis and exocytosis can also be a mechanism for plasma membrane rejuvenation or remodeling Endocytosis and exocytosis occur continually in most eukaryotic cells, but the plasma membrane in a nongrowing cell remains constant

94 Summary Exocytosis transport vesicles migrate to the membrane, fuse with it, and release their contents Endocytosis cell takes in macromolecules

95 Fig. 7-20a EXTRACELLULAR FLUID Pseudopodium CYTOPLASM PHAGOCYTOSIS 1 µm Pseudopodium of amoeba Food or other particle Food vacuole Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM)

96 Fig. 7-20b PINOCYTOSIS Plasma membrane 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle

97 Fig. 7-20c RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Coat protein Coated vesicle Ligand Coated pit Coat protein A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) Plasma membrane 0.25 µm

98 You should now be able to: 1. Define the following terms: amphipathic molecules, aquaporins, diffusion 2. Explain how membrane fluidity is influenced by temperature and membrane composition 3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions

99 4. Explain how transport proteins facilitate diffusion 5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps 6. Explain how large molecules are transported across a cell membrane