Passive TransportPassive Transport

Section 4-1Section 4-1

Alabama State Course Objectives Alabama State Course Objectives to Be Coveredto Be Covered

AL COS 2AL COS 2Describe cell processes necessary for Describe cell processes necessary for achieving homeostasis, including active and achieving homeostasis, including active and passive transport, osmosis, diffusion, passive transport, osmosis, diffusion, exocytosis, and endocytosis.exocytosis, and endocytosis.

AL COS 2.2AL COS 2.2Comparing the reaction of plant and animal Comparing the reaction of plant and animal cells in isotonic, hypotonic, and hypertonic cells in isotonic, hypotonic, and hypertonic solutions.solutions.

Diffusion is Caused by the Diffusion is Caused by the Random Movement of ParticlesRandom Movement of Particles

RECALL… When an organism adjusts RECALL… When an organism adjusts internally because the external environment internally because the external environment is changing, this is HOMEOSTASIS.is changing, this is HOMEOSTASIS.

One way cells maintain homeostasis is by One way cells maintain homeostasis is by controlling the movement of substances controlling the movement of substances across the cell membrane.across the cell membrane.

Random Motion and Random Motion and ConcentrationConcentration

Passive TransportPassive Transport – movement across the – movement across the cell membrane that DOES NOT require cell membrane that DOES NOT require energy.energy.

GradientGradient – a difference in the concentration – a difference in the concentration of a substance across a spaceof a substance across a space

EquilibriumEquilibrium – a condition in which the – a condition in which the concentration of a substance is equal concentration of a substance is equal throughout a spacethroughout a space

Movement of SubstancesMovement of Substances If there is a concentration gradient in the If there is a concentration gradient in the

solution, the substance will move from an solution, the substance will move from an area of high concentration to an area of area of high concentration to an area of lower concentration.lower concentration.

DiffusionDiffusion – movement of a substance from – movement of a substance from an area of high concentration to low an area of high concentration to low concentration caused by the random motion concentration caused by the random motion of particlesof particles

Movement of Substances (con’t)Movement of Substances (con’t) Many substances (molecules and ions) dissolved Many substances (molecules and ions) dissolved

in cytoplasm and in fluid outside the cell in cytoplasm and in fluid outside the cell enter/leave cells by diffusionenter/leave cells by diffusion

1. Inside cell – concentration of most of these 1. Inside cell – concentration of most of these substances are different from concentration substances are different from concentration

outside cell. This forms a outside cell. This forms a concentration concentration gradient across the cell gradient across the cell membrane. membrane.

2. To diffuse “down” the concentration 2. To diffuse “down” the concentration gradient, gradient, the molecule/ion goes from an the molecule/ion goes from an area area of high of high concentration to an area of low concentration to an area of low concentration concentration across the cell membrane.across the cell membrane.

Movement of Substances Movement of Substances (con’t)(con’t) Cell Membrane is selectively permeable.Cell Membrane is selectively permeable.

1. Nonpolar interior of lipid bilayer repels 1. Nonpolar interior of lipid bilayer repels ions ions and most polar molecules. This and most polar molecules. This prevents prevents diffusion across the cell diffusion across the cell

membrane of these membrane of these substances.substances.2. Very small molecules or nonpolar 2. Very small molecules or nonpolar molecules can diffuse across molecules can diffuse across cell cell membrane down their concentration membrane down their concentration gradient.gradient.

Diffusion of small or nonpolar molecules is Diffusion of small or nonpolar molecules is the simplest type of passive transport.the simplest type of passive transport.

Water Diffuses into and out of Water Diffuses into and out of Cells by OsmosisCells by Osmosis

OsmosisOsmosis – diffusion of water through a – diffusion of water through a selectively permeable membraneselectively permeable membrane

Osmosis involves the movement of water Osmosis involves the movement of water down its concentration gradient.down its concentration gradient.

The direction of water movement across cell The direction of water movement across cell membrane depends on relative membrane depends on relative concentrations of free water molecules in concentrations of free water molecules in cytoplasm and in fluid outside of cell.cytoplasm and in fluid outside of cell.

Water Diffuses into and out of Water Diffuses into and out of Cells by Osmosis (con’t)Cells by Osmosis (con’t)

In the solutions on either side of the cell In the solutions on either side of the cell membrane, many ions and polar molecules are membrane, many ions and polar molecules are dissolved in water. When these substances dissolved in water. When these substances dissolve in water, water molecules are attracted to dissolve in water, water molecules are attracted to them and so are no longer free to move around. If them and so are no longer free to move around. If solutions on either side of cell membrane have solutions on either side of cell membrane have different concentrations of dissolved particles, they different concentrations of dissolved particles, they will also have different concentrations of “free” will also have different concentrations of “free” water molecules. Osmosis will occur as free water water molecules. Osmosis will occur as free water molecules move into the solution with the lower molecules move into the solution with the lower concentration of free water molecules.concentration of free water molecules.

http://highered.mcgraw-hill.com/sites/0072495855/http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosisstudent_view0/chapter2/animation__how_osmosis_works.html_works.html

Three Possibilities for Direction of Three Possibilities for Direction of Water MovementWater Movement

Water Moves OutWater Moves OutWhen water diffuses out of the cell, CELL When water diffuses out of the cell, CELL

SHRINKSSHRINKSHypertonicHypertonic – solution that causes a cell to – solution that causes a cell to

shrink because of osmosis.shrink because of osmosis.If fluid outside cell has a higher concentration of If fluid outside cell has a higher concentration of

dissolved particles than cytoplasm, outside fluid dissolved particles than cytoplasm, outside fluid has lower concentration of free water molecules has lower concentration of free water molecules than cytoplasm.than cytoplasm.

Three Possibilities for Direction of Three Possibilities for Direction of Water Movement (con’t)Water Movement (con’t)

Water Moves InWater Moves InWhen water diffuses into cell, CELL SWELLSWhen water diffuses into cell, CELL SWELLSHypotonicHypotonic – a solution that causes a cell to – a solution that causes a cell to

swell because of osmosisswell because of osmosisIf fluid outside the cell has a lower concentration If fluid outside the cell has a lower concentration

of dissolved particles than cytoplasm has, then of dissolved particles than cytoplasm has, then the outside fluid also has a higher concentration the outside fluid also has a higher concentration of free water molecules than cytoplasmof free water molecules than cytoplasm

Three Possibilities for Direction of Three Possibilities for Direction of Water Movement (con’t)Water Movement (con’t)

No Net Water MovementNo Net Water MovementCytoplasm and fluid outside cell have same Cytoplasm and fluid outside cell have same

concentration of free water molecules - concentration of free water molecules - isotonicisotonic, water diffuses into and out of the cell , water diffuses into and out of the cell at equal rates.at equal rates.

Results in no net movement of water across cell Results in no net movement of water across cell membranemembrane

Cell stays the same sizeCell stays the same size

http://www.tvdsb.on.ca/westmin/science/sbi3a1/Chttp://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htmells/Osmosis.htm

Animal Cells and Plant CellsAnimal Cells and Plant Cells If left unchecked, a hypotonic solution can cause a If left unchecked, a hypotonic solution can cause a

cell to burst. Different cells have evolved different cell to burst. Different cells have evolved different methods to deal with this.methods to deal with this.

Plants and Fungi – rigid cell walls to prevent cell Plants and Fungi – rigid cell walls to prevent cell from expanding too muchfrom expanding too much

Unicellular Eukaryotes – contractile vacuoles – Unicellular Eukaryotes – contractile vacuoles – collect excess water and force it outcollect excess water and force it out

Animal Cells – avoid swelling by osmosis by Animal Cells – avoid swelling by osmosis by removing dissolved particles from cytoplasm. This removing dissolved particles from cytoplasm. This increases concentration of free water molecules increases concentration of free water molecules inside cell.inside cell.

Proteins Help Some Substances Proteins Help Some Substances Cross the Cell MembraneCross the Cell Membrane

Ions and polar molecules cross the cell Ions and polar molecules cross the cell membrane with the help of transport membrane with the help of transport proteinsproteins

Transport Proteins form channels or polar Transport Proteins form channels or polar passagewayspassageways

Each channel allows only specific Each channel allows only specific substances to pass through cell membranesubstances to pass through cell membrane

This selectivity enables a cell to control what This selectivity enables a cell to control what enters and leaves.enters and leaves.

Diffusion Through Ion ChannelsDiffusion Through Ion Channels

Ions such as NaIons such as Na++ (sodium), K (sodium), K++ (potassium), (potassium), CaCa2+2+ (calcium), and Cl (calcium), and Cl-- (chloride) are (chloride) are involved in many important cell functions.involved in many important cell functions.

These ions diffuse across the cell These ions diffuse across the cell membrane through ion channels.membrane through ion channels.

Ion Channel – doughnut-shaped transport Ion Channel – doughnut-shaped transport protein with a polar pore through which ions protein with a polar pore through which ions can pass.can pass.

Diffusion Through Ion Channels Diffusion Through Ion Channels (con’t)(con’t)

Pores of some ion channels always open. Pores of some ion channels always open. Others, the pores can be closed by ion Others, the pores can be closed by ion channels gateschannels gates

Ion channel gates may open or close in Ion channel gates may open or close in response to several stimuli (ex. Stretching of response to several stimuli (ex. Stretching of the cell membrane, change in electrical the cell membrane, change in electrical charge, or the binding of specific molecules charge, or the binding of specific molecules to the ion channel).to the ion channel).

Stimuli able to control ability of particular Stimuli able to control ability of particular ions to cross cell membraneions to cross cell membrane

Diffusion Through Ion Channels Diffusion Through Ion Channels (con’t)(con’t)

Passive transport because no use of energy Passive transport because no use of energy because ions move down their because ions move down their concentration gradientsconcentration gradients

The effect of electrical charge on ion The effect of electrical charge on ion transporttransportRate of movement of a substance across cell Rate of movement of a substance across cell

membrane is generally determined by the membrane is generally determined by the concentration gradient of the substance.concentration gradient of the substance.

Movement of a charged particle across cell Movement of a charged particle across cell membrane influenced by particle’s positive or membrane influenced by particle’s positive or negative charge.negative charge.

Diffusion Through Ion Channels Diffusion Through Ion Channels (con’t)(con’t)

Inside Cell – negativeInside Cell – negativeOutside Cell – positiveOutside Cell – positiveOpposite charges attract, so positive ions Opposite charges attract, so positive ions

typically diffuse into cell and negative ions typically diffuse into cell and negative ions typically diffuse out of cell.typically diffuse out of cell.

Direction of movement caused by an ion’s Direction of movement caused by an ion’s concentration gradient may oppose the direction concentration gradient may oppose the direction of movement caused by ion’s electrical charge.of movement caused by ion’s electrical charge.

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ody6/index.htmlody6/index.html

Facilitated DiffusionFacilitated Diffusion

Carrier ProteinsCarrier Proteins – bind to specific – bind to specific substances on one side of the cell substances on one side of the cell membrane and carry the substance to the membrane and carry the substance to the other side. It then releases the substance.other side. It then releases the substance.

Facilitated DiffusionFacilitated Diffusion – type of passive – type of passive transport because it moves substances transport because it moves substances down their concentration gradient without down their concentration gradient without using the cell’s energy.using the cell’s energy.

Facilitated Diffusion (con’t)Facilitated Diffusion (con’t) Figure 4-4 p. 80Figure 4-4 p. 80 Carrier protein binds substance and Carrier protein binds substance and

changes shapechanges shape Change in shape of carrier protein exposes Change in shape of carrier protein exposes

substance to other side of cell membrane. substance to other side of cell membrane. Molecule is transported across cell Molecule is transported across cell membrane.membrane.

Carrier protein shields molecule from interior Carrier protein shields molecule from interior of cell membrane. Molecule released from of cell membrane. Molecule released from carrier protein. Carrier protein back to carrier protein. Carrier protein back to original shape.original shape.

Facilitated DiffusionFacilitated Diffusion

ReviewReview Passive transport is the movement of Passive transport is the movement of

substances across the cell membrane substances across the cell membrane without the use of energy by the cell.without the use of energy by the cell.

Diffusion is the movement of a substance Diffusion is the movement of a substance from an area of high concentration to an from an area of high concentration to an area of lower concentration.area of lower concentration.

Osmosis is the diffusion of free water Osmosis is the diffusion of free water molecules across a selectively permeable molecules across a selectively permeable membrane. membrane.

Ion channels are proteins that have a pore Ion channels are proteins that have a pore through which ions can cross the cell through which ions can cross the cell membrane.membrane.

ReviewReview

In facilitated diffusion, a carrier protein In facilitated diffusion, a carrier protein transports a substance across the cell transports a substance across the cell membrane down the concentration gradient membrane down the concentration gradient of the substance.of the substance.

HOMEWORKHOMEWORK

Section 4-1 Review QuestionsSection 4-1 Review QuestionsPage 80Page 80

Questions 1-5Questions 1-5