Overview of Membrane Structure Transport

Describe the cell membrane within the context of the fluid mosaic modelExplain how spontaneous formation of a membrane could have been important in the origin of lifeDescribe the passage of materials across a membrane with no energy expenditureExplain how osmosis plays a role in maintenance of a cellCopyright 2012 Pearson Education, Inc.Learning outcome

Explain how an imbalance in water between the cell and its environment affects the cellDescribe membrane proteins that facilitate transport of materials across the cell membrane without expenditure of energyDiscuss how energy-requiring transport proteins move substances across the cell membraneDistinguish between exocytosis and endocytosis and list similarities between the two

Copyright 2012 Pearson Education, Inc.Learning outcome

MEMBRANE STRUCTURE AND FUNCTION 2012 Pearson Education, Inc.

5.1 Membranes are fluid mosaics of lipids and proteins with many functionsMembranes are composed of a bilayer of phospholipids with embedded and attached proteinsBiologists described such structure as _____________.The surface appears mosaic because of the proteins embedded in the phospholipids and fluid because the proteins can drift about in the phospholipids

2012 Pearson Education, Inc.

PhospholipidbilayerHydrophobic regionsof proteinHydrophilicregions of proteinThe fluid mosaic model for membranes

5.1 Membranes are fluid mosaics of lipids and proteins with many functionsPhospholipids are made from unsaturated fatty acids that have kinks in their tails.These kinks prevent phospholipids from packing tightly together, keeping them in liquid form. 2012 Pearson Education, Inc.Membrane fluidity

5.1 Membranes are fluid mosaics of lipids and proteins with many functionsIn animal cell membranes, ______________ helpsstabilize membranes at warmer temperatures andkeep the membrane fluid at lower temperatures. 2012 Pearson Education, Inc.Cholesterol within the animal cell membrane

HydrophilicheadWATERHydrophobictailWATERPhospholipid bilayer (cross section)

What is X?What is Y?

WATERWATERWhy are X & Y positioned as shown?

Explain why:0C cells- label stays on mouse side37C cells- label moves over entire cell

What conclusions can you make about the properties of the plasma membrane and /or its components?Incubate at 37oC.

5.1 Membranes are fluid mosaics of lipids and proteins with many functionsMembrane proteins perform many functions.Help maintain cell shape and coordinate changes inside and outside the cell through their attachment to the cytoskeleton and extracellular matrix.Function as receptors for chemical messengers from other cells.Some membrane proteins function as enzymes. 2012 Pearson Education, Inc.

5.1 Membranes are fluid mosaics of lipids and proteins with many functionsSome membrane glycoproteins are involved in cell-cell recognition.Membrane proteins may participate in the intercellular junctions that attach adjacent cells to each other.


Fibers of extracellular matrix (ECM)Enzymatic activityPhospholipidCholesterolCYTOPLASMCYTOPLASMCell-cell recognitionGlycoproteinIntercellular junctionsMicrofilaments of cytoskeletonATPTransportSignal transductionReceptorSignaling moleculeAttachment to the cytoskeleton and extracellular matrix (ECM)Some functions of membrane proteins

2012 Pearson Education, Inc.Animation: Signal Transduction PathwaysRight click on animation / Click play

2012 Pearson Education, Inc.Animation: Overview of Cell SignalingRight click on animation / Click play

5.1 Membranes are fluid mosaics of lipids and proteins with many functionsMembranes may exhibit selective permeability, allowing some substances to cross more easily than others.


2012 Pearson Education, Inc.Animation: Membrane SelectivityRight click on animation / Click play

5.2 EVOLUTION CONNECTION: Membranes form spontaneously, a critical step in the origin of lifePhospholipids, key ingredient of biological membranes, spontaneously self-assemble into simple membranes.Formation of membrane-enclosed collections of molecules was a critical step in evolution of the first cells. 2012 Pearson Education, Inc.

Artificial membrane-bounded sacWhen a mixture of phospholipids and water is shaken, the phospholipids organize into bilayer surroundings water-filled bubbles

WaterWaterThis is a diagram of a section of one of the membrane sacs. Describe its structure.

5.3 Passive transport is diffusion across a membrane with no energy investment__________= tendency of particles to spread out evenly in an available space.Particles move from an area of more concentrated particles to an area of less concentrated.This means particles diffuse down their ________________________Eventually, the particles reach equilibrium where concentration of particles is the same throughout. 2012 Pearson Education, Inc.

2012 Pearson Education, Inc.Animation: DiffusionRight click on animation / Click play

5.3 Passive transport is diffusion across a membrane with no energy investmentDiffusion across a cell membrane does not require energy, so it is called _______________________.The concentration gradient itself represents potential energy for diffusion. 2012 Pearson Education, Inc.True or false: Diffusion requires energyHow do gases move across membrane?

Molecules of dyeMembranePoresNet diffusionNet diffusionEquilibriumPassive transport of one type of molecule

Net diffusionNet diffusionNet diffusionNet diffusionEquilibriumEquilibriumPassive transport of two types of molecule

Which substances cross membrane freely? Why?TestosteroneFructoseO2

5.4 Osmosis is the diffusion of water across a membraneOne of the most important substances that crosses membranes is water.The diffusion of water across a selectively permeable membrane is called osmosis.


2012 Pearson Education, Inc.Animation: OsmosisRight click on animation / Click play

5.4 Osmosis is the diffusion of water across a membraneIf a membrane permeable to water but not a solute separates two solutions with different concentrations of solute,water will cross the membranemoving down its own concentration gradientuntil the solute concentration on both sides is equal.


OsmosisSolute molecule with cluster of water moleculesWater moleculeSelectively permeable membraneSolute moleculeH2OLower concentration of soluteHigher concentration of soluteEqual concentrations of solute

5.5 Water balance between cells and their surroundings is crucial to organisms___________= the ability of a solution to cause a cell to gain or lose water.Depends on concentration of a solute on both sides of membrane.___________solution: concentration of solute is same on both sides of a membrane, cell volume will not change,__________solution: solute concentration is lower outside the cell, water molecules move into the cell & cell will expand and may burst___________solution: solute concentration is higher outside the cell, water molecules move out of the cell & cell will shrink. 2012 Pearson Education, Inc.

5.5 Water balance between cells and their surroundings is crucial to organismsFor an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation, the control of water balance.The cell wall of a plant cell exerts pressure that prevents the cell from taking in too much water and bursting when placed in a hypotonic environment.But in a hypertonic environment, plant and animal cells both shrivel.


Animal cellPlant cellTurgid (normal)FlaccidShriveled (plasmolyzed)Plasma membraneLysedNormalShriveledHypotonic solutionIsotonic solutionHypertonic solutionH2OH2OH2OH2OH2OH2OH2OHow the animal and plant cells react to changes in tonicity

5.6 Transport proteins can facilitate diffusion across membranesHydrophobic substances easily diffuse across a cell membrane.Polar or charged substances do not easily cross cell membranes & move across membranes using specific transport proteins in a process called ____________________, whichdoes not require energyrelies on the concentration gradient. 2012 Pearson Education, Inc.

5.6 Transport proteins can facilitate diffusion across membranesSome proteins function by becoming a hydrophilic tunnel for passage of ions or other molecules. (________________ protein) 2012 Pearson Education, Inc.

5.6 Transport proteins can facilitate diffusion across membranesOther proteins bind their passenger, change shape & release their passenger on the other side. (___________ protein)In both of these situations, the protein is specific for the substrate. i.e. sugars, amino acids, ions, & water. 2012 Pearson Education, Inc.

5.6 Transport proteins can facilitate diffusion across membranesBecause water is polar, its diffusion through a membranes hydrophobic interior is relatively slow.The very rapid diffusion of water into and out of certain cells is because of ____________.Dr. Peter Agre received 2003 Nobel Prize in chemistry for his discovery of aquaporins.

2012 Pearson Education, Inc.Aquaporin in action

Solute moleculeTransport proteinTransport protein providing a channel for diffusion of a specific solute across a membrane

True or false? The lipid layer is a solid surfaceDiffusion only occurs through pores in the membraneAny molecule can diffuse across a membraneDiffusion and osmosis do not occur togetherOsmosis moves any substanceOsmosis works by the opposite principles of diffusionParticles are not moving back and forth during isosmotic conditionsDiffusion is not temperature dependent

5.8 Cells expend energy in the active transport of a soluteIn active transport, a cell must expend energy to move a solute against its concentration gradient.The following figures show the four main stages of active transport. 2012 Pearson Education, Inc.

2012 Pearson Education, Inc.Animation: Active TransportRight click on animation / Click play

Transport proteinSoluteADPATPPPPProtein changes shape.Phosphate detaches.Solute bindingPhosphate attachingTransportProtein reversionActive transport of a solute across a membrane

5.9 Exocytosis and endocytosis transport large molecules across membranesA cell uses two mechanisms to move large molecules across membranes.______________is used to export bulky molecules e.g. proteins or polysaccharides._____________is used to import substances useful to livelihood of the cell.In both cases, material to be transported is packaged within a vesicle that fuses with membrane.


2012 Pearson Education, Inc.Animation: Exocytosis and Endocytosis IntroductionRight click on animation / Click play

5.9 Exocytosis and endocytosis transport large molecules across membranes3 kinds of endocytosis.____________- engulfment of a particle by wrapping cell membrane around it, forming a vacuole.____________- same thing except that fluids are taken into small vesicles._________________________- receptors in a receptor-coated pit to interact with a specific protein, initiating the formation of a vesicle. 2012 Pearson Education, Inc.

2012 Pearson Education, Inc.Animation: Exocytosis Right click on animation / Click play

2012 Pearson Education, Inc.Animation: PinocytosisRight click on animation / Click play

2012 Pearson Education, Inc.Animation: PhagocytosisRight click on animation / Click play

2012 Pearson Education, Inc.Animation: Receptor-Mediated EndocytosisRight click on animation / Click play

PhagocytosisPinocytosisReceptor-mediated endocytosisEXTRACELLULAR FLUIDCYTOPLASMPseudopodiumFood or other particleFood vacuoleFood being ingestedPlasma membranePlasma membraneVesicleReceptorSpecific moleculeCoated pitCoated vesicleCoat proteinCoated pitMaterial bound to receptor proteinsThree kinds of endocytosis.

PhagocytosisEXTRACELLULAR FLUIDCYTOPLASMPseudopodiumFood or other particleFood vacuoleFood being ingestedWhat kind of substances are transported?

PinocytosisPlasma membranePlasma membraneVesicleWhat kind of substances are transported?

Receptor-mediated endocytosisPlasma membraneReceptorSpecific moleculeCoated pitCoated vesicleCoat proteinCoated pitMaterial bound to receptor proteinsWhat makes this different from phagocytosis and pinocytosis?

What type of substances are transported?

ExocytosisWhat makes this different from phagocytosis and pinocytosis?

What kind of substances are transported?

True or false? Gases do not diffuseIons easily pass through a membraneAll transport proteins require ATPAll carriers are highly specific to one moleculePinocytosis takes in pure liquids and not solutes

DiffusionRequires no energyPassive transportHigher solute concentration FacilitateddiffusionOsmosisHigher waterconcentrationHigher soluteconcentrationRequires energyActive transportSoluteWaterLower soluteconcentrationLower waterconcentrationLower soluteconcentration

Molecules crosscell membranespassivetransportbybymay bemovingdownmovingagainstrequiresusesdiffusionofpolar moleculesand ionsusesof(a)(c)(d)(b)(e)

Describe the cell membrane within the context of the fluid mosaic modelExplain how spontaneous formation of a membrane could have been important in the origin of lifeDescribe the passage of materials across a membrane with no energy expenditureExplain how osmosis plays a role in maintenance of a cellCopyright 2012 Pearson Education, Inc.You should now be able to

Explain how an imbalance in water between the cell and its environment affects the cellDescribe membrane proteins that facilitate transport of materials across the cell membrane without expenditure of energyDiscuss how energy-requiring transport proteins move substances across the cell membraneDistinguish between exocytosis and endocytosis and list similarities between the two

Copyright 2012 Pearson Education, Inc.You should now be able to

*********Teaching TipsYou might wish to share a very simple analogy that seems to work well for some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy by finding exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)**Campbell, Neil, and Jane Reece, Biology, 8th ed., Figure 7.3 The fluid mosaic model for membranes.

**Teaching TipsYou might wish to share a very simple analogy that seems to work well for some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy by finding exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)**Teaching TipsYou might wish to share a very simple analogy that seems to work well for some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy by finding exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)**Campbell, Neil, and Jane Reece, Biology, 8th ed., Figure 7.2 Phospholipid bilayer (cross section).

**Figure 5.14 Bilayer structure formed by self-assembly of phospholipids in an aqueous environment

***Lateral movement***Teaching TipsYou might wish to share a very simple analogy that seems to work well for some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy by finding exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)**Teaching TipsYou might wish to share a very simple analogy that seems to work well for some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy by finding exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)**Figure 5.1 Some functions of membrane proteins**Teaching TipsYou might wish to share a very simple analogy that seems to work well for some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy by finding exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)**Teaching TipsYou might wish to share a very simple analogy that seems to work well for some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy by finding exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)**Teaching TipsYou might wish to share a very simple analogy that seems to work well for some students. A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into the sandwich represent proteins variously embedded partially into or completely through the membrane. Transport proteins would be like the jellybeans that poke completely through the sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy by finding exceptions. (For example, this analogy does not include a model of the carbohydrates on the cell surface.)**Student Misconceptions and ConcernsFor students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives.Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sourcesthe smell of dinner on the stove, the scent of perfume or cologne from a bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away.2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Alternatively, release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration.**Teaching TipsThe hydrophobic and hydrophilic ends of a phospholipid molecule create a lipid bilayer. The hydrophobic edges of the layer will also seal to other such edges, eventually wrapping a sheet into a sphere that can enclose water. Furthermore, because of these hydrophobic properties, lipid bilayers are naturally self-healing. All of these properties emerge from the structure of phospholipids.

**Figure 5.2 Artificial membrane-bounded sacs**Figure 5.2Q Structure of membrane sacs**Student Misconceptions and ConcernsFor students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives.Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sourcesthe smell of dinner on the stove, the scent of perfume or cologne from a bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away.2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Alternatively, release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration.**Student Misconceptions and ConcernsFor students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives.Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sourcesthe smell of dinner on the stove, the scent of perfume or cologne from a bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away.2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Alternatively, release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration.**Student Misconceptions and ConcernsFor students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives.Teaching Tips 1. Students often benefit from reminders of diffusion in their lives. Smells can usually be traced back to their sourcesthe smell of dinner on the stove, the scent of perfume or cologne from a bottle, the smoke drifting away from a campfire. These scents are strongest nearest the source and weaker as we move away.2. Consider demonstrating simple diffusion. A large jar of water and a few drops of dark-colored dye work well over the course of a lecture period. Alternatively, release a strong scent of cologne or peppermint or peel part of an orange in the classroom and have students raise their hands as they first detect the smell. Students nearest the source will raise their hands before students farther away. The fan from an active overhead projector or overhead vent may bias the experiment a bit, so be aware of any directed movements of air in your classroom that might disrupt this demonstration.**Figure 5.3A Passive transport of one type of molecule**Figure 5.3B Passive transport of two types of molecules****Student Misconceptions and ConcernsFor students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives.Teaching TipsYour students may have noticed that the skin of their fingers wrinkles after taking a long shower or bath, or after washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Through osmosis, water moves into the epidermal skin cells. Our skin is hypertonic to these solutions, producing the swelling that appears as large wrinkles. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water because the soap removes the natural layer of oil from our skin.

**Student Misconceptions and ConcernsFor students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives.Teaching TipsYour students may have noticed that the skin of their fingers wrinkles after taking a long shower or bath, or after washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Through osmosis, water moves into the epidermal skin cells. Our skin is hypertonic to these solutions, producing the swelling that appears as large wrinkles. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water because the soap removes the natural layer of oil from our skin.

**Student Misconceptions and ConcernsFor students with limited science backgrounds, concepts such as diffusion and osmosis can take considerable time to fully understand and apply. Instructors often struggle to remember a time in their lives when they did not know about such fundamental scientific principles. Consider spending extra time to illustrate and demonstrate these key processes to the class. Consider short interactive class exercises in which students create analogies or think of examples of these principles in their lives.Teaching TipsYour students may have noticed that the skin of their fingers wrinkles after taking a long shower or bath, or after washing dishes. The skin wrinkles because it is swollen with water but still tacked down at some points. Through osmosis, water moves into the epidermal skin cells. Our skin is hypertonic to these solutions, producing the swelling that appears as large wrinkles. Oils inhibit the movement of water into our skin. Thus, soapy water results in wrinkling faster than plain water because the soap removes the natural layer of oil from our skin.**Figure 5.4 Osmosis, the diffusion of water across a membrane**Student Misconceptions and ConcernsStudents easily confuse the term hypertonic and hypotonic. One challenge is to get them to understand that these are relative terms, such as heavier, darker, or fewer. No single object is heavier, no single cup of coffee is darker, and no single bag of M & Ms has fewer candies. Such terms only apply when comparing two or more items. A solution with a higher concentration than another solution is hypertonic to that solution. However, the same solution might also be hypotonic to a third solution.Teaching Tips1. The word root hypo means below. Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations of solutes below that of the other solution(s).2. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A salmon might swim from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____ environment. (Answers: hypertonic, hypotonic)3. The effects of hypertonic and hypotonic solutions can be demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations.

**Student Misconceptions and ConcernsStudents easily confuse the term hypertonic and hypotonic. One challenge is to get them to understand that these are relative terms, such as heavier, darker, or fewer. No single object is heavier, no single cup of coffee is darker, and no single bag of M & Ms has fewer candies. Such terms only apply when comparing two or more items. A solution with a higher concentration than another solution is hypertonic to that solution. However, the same solution might also be hypotonic to a third solution.Teaching Tips1. The word root hypo means below. Thus, a hypodermic needle injects substances below the dermis. Students might best remember that hypotonic solutions have concentrations of solutes below that of the other solution(s).2. After introducing the idea of hypertonic and hypotonic solutions, you may wish to challenge your students with the following: A salmon might swim from the ocean up a freshwater stream to reproduce. The salmon is moving from a _____ environment to a _____ environment. (Answers: hypertonic, hypotonic)3. The effects of hypertonic and hypotonic solutions can be demonstrated if students soak carrot sticks, long slices of potato, or celery in hypertonic and hypotonic solutions. These also make nice class demonstrations.**Figure 5.5 How animal and plant cells react to changes in tonicity**Teaching TipsThe text notes that The greater the number of transport proteins for a particular solute in a membrane, the faster the solutes rate of diffusion across the membrane. This is similar to a situation that might be more familiar to your students. The more ticket-takers present at the entrance to a stadium, the faster the rate of movement of people into the stadium.

***Teaching TipsThe text notes that The greater the number of transport proteins for a particular solute in a membrane, the faster the solutes rate of diffusion across the membrane. This is similar to a situation that might be more familiar to your students. The more ticket-takers present at the entrance to a stadium, the faster the rate of movement of people into the stadium.



***Figure 5.6 Transport protein providing a channel for the diffusion of a specific solute across a membrane****Teaching Tips1. Active transport uses energy to move a solute against its concentration gradient. Challenge your students to think of the many possible analogies to this situation, for example, bailing out a leaky boat by moving water back to a place (outside the boat) where water is more concentrated. An alternative analogy might be the herding of animals, which requires work to keep the organisms concentrated and counteract their natural tendency to spread out.2. Students familiar with city subway toll stations might think of some gate mechanisms that work similarly to the proteins regulating active transport. A person steps up to a barrier and inserts payment (analogous to ATP input), and the gate changes shape, permitting passage to the other side. Even a simple turnstile system that requires payment is generally similar.

**Teaching Tips1. Active transport uses energy to move a solute against its concentration gradient. Challenge your students to think of the many possible analogies to this situation, for example, bailing out a leaky boat by moving water back to a place (outside the boat) where water is more concentrated. An alternative analogy might be the herding of animals, which requires work to keep the organisms concentrated and counteract their natural tendency to spread out.2. Students familiar with city subway toll stations might think of some gate mechanisms that work similarly to the proteins regulating active transport. A person steps up to a barrier and inserts payment (analogous to ATP input), and the gate changes shape, permitting passage to the other side. Even a simple turnstile system that requires payment is generally similar.

**Figure 5.8_s4 Active transport of a solute across a membrane (step 4)**Teaching TipsStudents carefully considering exocytosis may notice that membrane from secretory vesicles is added to the plasma membrane. Consider challenging your students to identify mechanisms that balance out this enlargement of the cell surface. (Endocytosis subtracts area from the cell surface. It is a major factor balancing out the additional membrane supplied by exocytosis.)

**Teaching TipsStudents carefully considering exocytosis may notice that membrane from secretory vesicles is added to the plasma membrane. Consider challenging your students to identify mechanisms that balance out this enlargement of the cell surface. (Endocytosis subtracts area from the cell surface. It is a major factor balancing out the additional membrane supplied by exocytosis.)






**Figure 5.9 Three kinds of endocytosis**Figure 5.9_1 Three kinds of endocytosis (part 1)**Figure 5.9_2 Three kinds of endocytosis (part 2)**Figure 5.9_3 Three kinds of endocytosis (part 3)*************

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Overview of Membrane Structure Transport