Describe the Fluid Mosaic Model of Membrane Structure

Home , Cytosol, Fluid mosaic model, Lipid bilayer, Phospholipid, Vacuole

Module 2C – Membranes and Cell Transport Objective # 13 All cells are surrounded by a plasma membrane. Eukaryotic cells also contain internal membranes and membrane-membrane- Describe the Fluid Mosaic bound organelles. Model of membrane In this module, we will examine the structure. structure and function of cell membranes. We will also look at how materials move within cells and across cell membranes. 1 2

Objective 13 Objective 13

In 1972, Singer and Nicolson Further study has shown that cell membranes proposed the Fluid Mosaic Model of consist of 4 major components: membrane structure. 1) Lipid bilayer According to their model, cell ¾ mainly 2 layers of phospholipids; the non-non- membranes are composed of a lipid poliliiddhlhdlar tails point inward and the polar heads bilayer with globular proteins are on the surface embedded in the bilayer. ¾ contains cholesterol in animal cells ¾ is fluid, allowing proteins to move around within the bilayer 3 4

+ CH2 N (CH3)3 Phospholipids Objective 13 CH2 O

OPO– O Phospholipid Bilayer H

H2C C CH2 OO Extracellular fluid Polar Hydrophilic Heads Hydrophilic Polar C O C O CH2 CH2 Polar hydrophilic heads CH CH ils 2 2

a CH2 CH2

CH2 CH2 CH2 CH2 Nonpolar CH2 CH2 CH2 CH2 hydrophobic tails CH2 CH

CH2 CH c. Icon CH2 CH2 CH2 CH2 Polar hydrophilic heads CH2 CH2

CH2 CH2 CH CH 2 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CH CH Intracellular fluid (cytosol) 2 2 b. Space-filling model Nonpolar Hydrophobic T CH2 CH2

1 Objective 13 Integral proteins 2) Transmembrane proteins or Integral run through both membrane proteins layers of the lipid ¾ globular proteins that run through bilayer. both layers of the lipid bilayer Hydrophobic ¾ have hydrophobic regions that regions of the anchor them in the hydrophobic interior of the lipid bilayer and protein are shaded hydrophilic regions that protrude in blue and from the bilayer hydrophilic regions

Human cell Objective 13 Mouse cell Experiments 3) Interior protein network with cell ¾ proteins associated with the surface fusion show Fuse of the membrane are called cells that integral peripheral membrane proteins proteins can ¾ thihe inter ior sur face o fhf the pl asma Allow time move freely for mixing to occur membrane is structurally supported within the by a network of proteins called fluid lipid spectrins and clathrins

Objective 13 Extracellular matrix protein 4) Cell surface markers Glycoprotein Glycolipid ¾ project from the external surface of the membrane ¾ include glycolipids (lipids with Glycoprotein carbhdbohydrate groups attac hd)hed) an d Integral Actin filaments Cholesterol glycoproteins (proteins with proteins of cytoskeleton carbohydrate groups attached) Intermediate filaments Peripheral of cytoskeleton ¾ function as cell identity markers proteins Fluid Mosaic Model 11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Objective # 14 Functions of Plasma Membrane Proteins

Outside Inside cell cell List and describe the various

functions of membrane Transporter Enzyme Cell surface receptor proteins.

Objective # 15 Objective 15

No cell exists as a closed system. In order to survive, materials must enter Explain the importance of and leave the cell through the plasma cell transport. membrane. Because different processes take place in different parts of the cell, materials must be transported from one part of the cell to another.

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Objective # 16 Objective 16

Explain what passive transport is, and Passive transport: describe the following methods of ¾ During passive transport, substances passive transport across membranes: move according to their own natural a) Sim ple diffusion tendency without an inppgut of energy b) Dialysis from the cell. No ATP is required. c) Osmosis ¾ To understand how passive transport d) Facilitated diffusion works, we need to examine the kinetic theory of matter.

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3 Objective 16 Objective 16a Kinetic theory of matter::matter Diffusion: ¾ All atoms and molecules are in ¾ Diffusion is the net movement of a constant random motion. This energy substance from an area where it has a of motion is called kinetic energyenergy.. higher concentration to an area where it ¾ The higher the temperature, the faster has a lower concentration i.e. down its the atoms and molecules move. concentration gradient. ¾ We detect this motion as heat. ¾ Caused by the constant random motion ¾ All motion theoretically stops at of all atoms and molecules.

absolute zero. 19 20

Objective 16a During diffusion, movement of individual atoms and molecules is always random but net movement of each substance is down its own concentration gradient.

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Objective 16 Objective 16b

In addition to simple diffusiondiffusion,, there Dialysis refers to the diffusion of are 3 special types of diffusion that solutes across a semipermeable involve movement of materials across membrane (i.e. a membrane where a semipermeable membrane::membrane some substances can pass through ¾ dialysis while others cannot). ¾ osmosis The ability of solutes to pass through cell membranes depends mainly on ¾ facilitated diffusion size and electrical charge.

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4 Objective 16c

Osmosis refers to the diffusion of the solvent across a semipermeable membrane. In living systems the solvent is always water, so biologists generally define osmosis as the diffusion of water across a semipermeable membrane:

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Objective 16c Objective 16c

The osmotic pressure of a solution is If 2 solutions have different [solutes]: the pressure needed to keep it in equilibrium with pure H20. ¾ The one with the higher [solutes], and lower [solvent], is hypertonicertonic.. The higher the [solutes] in a solution, the higher its osmotic pressure. ¾ The one with the lower [solutes], and higher [solvent], is hypotonic..hypotonic If 2 solutions have the same [solutes],

they are called isotonic..isotonic 27 28

Objective 16c

What will happen to a cell if it is placed in a hypertonic solution?

Wh at w ill happe n to a ce ll if it is placed in a hypotonic solution?

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Human Red Blood Cells – which cells are in a hypertonic environment?

Cells swell and Shriveled cells Normal cells eventually burst

Hypertonic Isotonic Hypotonic solution solution solution

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Objective 16c Plant cells – which cell is in a hypotonic environment? Cells have developed several ways to Cell body shrinks Normal turgid cell from cell wall Flaccid cell survive in a hypotonic environment: ¾ Pump water out using a contractile vacuole. ¾ Adjust [solutes] so it is isotonic relative Hypertonic Isotonic Hypotonic to the environment. solution solution solution ¾ Develop a thick cell wall that can

Objective 16c Pore proteins have a hydrophilic interior channel that allows ions and polar molecules, Even though water molecules are polar, like water, to pass through the membrane they can pass through the hydrophobic interior of the lipid bilayer because they are so small. However, the flow is fairly limited. Recent studies have shown that movement of water molecules across cell membranes is facilitated by special protein channels called aquaporins..aquaporins hydrophobic lipid bilayer 35 pore protein 36 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

6 Objective 16d Ions and large polar molecules cannot pass Facilitated diffusionrefers to the through the hydrophobic interior of the lipid diffusion of solutes through a bilayer without the help of transport proteins: _ _ + - semipermeable membrane with the help + + _ of special transport proteins. Outside + ¾ NonNon--polarpolar molecules and small polar molecules can diffuse directly through Hydrophobic interior of the lipid bilayer the lipid bilayer of a membrane. ¾ Ions and large polar molecules cannot, they need help from transport proteins. Inside + 37 Transport Protein 38 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Objective 16d Two types of transport proteins can help ions and large polar molecules diffuse through cell membranes: 1) Channel proteins– have a hydrophilic iifiinterior for ions or pol ar mol llecules to pass through. Some channel proteins can be opened or closed in response to a stimulus. These are called gated channelschannels.. 39 40

Objective 16d

2) Carrier proteins– physically bind to the substance being transported on one side of membrane and release it on the other.

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7 Objective 16d Objective # 17

Facilitated diffusion: Explain what active transport is, and ¾ is specific – each channel or carrier describe the following methods of transports certain ions or molecules only active transport across membranes: ¾ isspassve passive – dir ectio n o f net move me nt is a) membrane pppumps always down the concentration gradient b) coupled transport ¾ saturates – once all transport proteins are in use, rate of diffusion cannot be increased further 43 44

Objective 17 Objective 17a

Active transport: We will examine 2 types of active ¾ a cell expends some of its own energy transport: membrane pumps (protein- (protein- (from ATP) to move a substance mediated active transport) and coupled agggpainst its natural tendency e.g. up a transport (cotransport). concentration gradient. With membrane pumps, a carrier ¾ Requires the use of carrier proteins protein uses energy from ATP to (transport proteins that physically bind move a substance across a membrane, to the substance being transported). up its concentration gradient:

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Objective 17b

Coupled transport occurs in 2 stages:

¾ First, a carrier protein uses energy from ATP to move a substance across the membrane, up its concentration gradient. This gradient stores energy.

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8 Objective 17b ¾ Second, a coupled transport protein allows the substance to move back down its concentration gradient. As this happens, the stored energy is released and used to move a second substance up its concentration gradient:

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Objective # 18 Objective 18

Explain what bulk transport is, and Bulk transportallows small particles, describe the following methods of or groups of molecules to enter or bulk transport: leave a cell without actually passing a) Endocytosis includinggp pha gocytosis, through the membrane. pinocytosis, and receptor-receptor-mediatedmediated endocytosis We will examine 2 types of bulk b) exocytosis transport: endocytosis and exocytosis.

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Objective 18a Objective 18a

In endocytosisendocytosis,, part of the plasma A third type of endocytosis is called membrane envelops small particles or receptorreceptor--mediatedmediated endocytosis..endocytosis fluid, then seals on itself to form a In this process, the molecules to be vesicle or vacuole which enters the cell: transppjorted join to s pecific rece ptors on ¾ Phagocytosis – the substance engulfed the membrane. This causes the is a solid particle membrane to indent , and the molecules ¾ Pinocytosis - the substance engulfed is a are engulfed in a coated vesicle which liquid enters the cell.

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9 Objective 18b

The reverse of endocytosis is called exocytosisexocytosis.. During this process, the membrane of a vesicle fuses with the plasma membrane and its contents are released outside the cell:

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Table 5.2

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