Movements Through Movements Through the Cell Membranethe Cell Membrane

Chapter 3: CellsChapter 3: Cells

Unit 1: Levels of OrganizationUnit 1: Levels of Organization

Selective PermeabilitySelective Permeability

The cell membrane controls which The cell membrane controls which substances can enter and leave the substances can enter and leave the cell.cell.

– Movement includes:Movement includes:

Passive MechanismsPassive Mechanisms (requires no (requires no energy)energy)

Active MechanismsActive Mechanisms (requires energy) (requires energy)

Passive MechanismsPassive Mechanisms

These mechanisms do not require These mechanisms do not require cellular energy:cellular energy:

– DiffusionDiffusion

– Facilitated DiffusionFacilitated Diffusion

– OsmosisOsmosis

– FiltrationFiltration

DiffusionDiffusion

DiffusionDiffusion (a.k.a. simple diffusion) (a.k.a. simple diffusion) is the process by which molecules is the process by which molecules or ions scatter or spread or ions scatter or spread spontaneously from regions spontaneously from regions where they are in higher where they are in higher concentrations toward regions concentrations toward regions where they are in lower where they are in lower concentrations.concentrations.

How Diffusion WorksHow Diffusion Works

More Diffusion!More Diffusion!

Diffusion of a substance into or out of Diffusion of a substance into or out of the cell can occur only if:the cell can occur only if:

– The cell membrane is The cell membrane is permeablepermeable to that to that substance.substance.

– A A concentration gradientconcentration gradient exists such exists such that the substance is at a higher that the substance is at a higher concentration either outside or inside the concentration either outside or inside the cell.cell.

Here’s a “For Here’s a “For Instance” to Think Instance” to Think About…About… In the body, diffusion is the In the body, diffusion is the

process where oxygen enters process where oxygen enters cells and carbon dioxide leaves cells and carbon dioxide leaves cells, but equilibrium is never cells, but equilibrium is never reached.reached.

– How is this possible?How is this possible?

OxygenOxygen

Intracellular O2 is always low because Intracellular O2 is always low because O2 is constantly being used up by O2 is constantly being used up by metabolic reactions.metabolic reactions.

Extracellular O2 is maintained at a Extracellular O2 is maintained at a high level by the respiratory and high level by the respiratory and cardiovascular systems.cardiovascular systems.

– A concentration gradient always allows A concentration gradient always allows oxygen to diffuse into the body’s cells.oxygen to diffuse into the body’s cells.

Carbon DioxideCarbon Dioxide

Levels of CO2 are always high Levels of CO2 are always high inside the cells.inside the cells.

Respiratory system maintains a Respiratory system maintains a lower extracellular carbon dioxide lower extracellular carbon dioxide level.level.

– A concentration gradient always favors A concentration gradient always favors carbon dioxide diffusing out of cells.carbon dioxide diffusing out of cells.

Facilitated DiffusionFacilitated Diffusion

Substances that are not able to Substances that are not able to pass pass throughthrough the lipid bilayer the lipid bilayer need the help of need the help of membrane membrane proteinsproteins to get across. to get across.

– A process known as A process known as Facilitated Facilitated DiffusionDiffusion..

– Facilitated DiffusionFacilitated Diffusion

Types of Facilitated Types of Facilitated DiffusionDiffusion Ion ChannelsIon Channels

– Smaller polar moleculesSmaller polar molecules

Carrier ProteinsCarrier Proteins

– Molecules that are too large to pass Molecules that are too large to pass through ion channels or the lipid bilayerthrough ion channels or the lipid bilayer

Example: Glucose and amino acidsExample: Glucose and amino acids

– How can the number of carrier molecules in the How can the number of carrier molecules in the cell membrane limit the rate of facilitated cell membrane limit the rate of facilitated diffusion?diffusion?

CommonalitiesCommonalities

Facilitated diffusion is similar to Facilitated diffusion is similar to simple diffusion in that it only simple diffusion in that it only moves molecules from regions of moves molecules from regions of higher concentrations toward higher concentrations toward regions of lower concentrations.regions of lower concentrations.

How is it different???How is it different???

OsmosisOsmosis

OsmosisOsmosis occurs whenever water occurs whenever water molecules diffuse from a region of molecules diffuse from a region of higher higher waterwater concentration to a concentration to a region of lower region of lower waterwater concentration across a selectively concentration across a selectively permeable membrane (i.e. permeable membrane (i.e. phospholipid bilayer)phospholipid bilayer)

Think About Solutions!Think About Solutions!

A higher concentration of solute A higher concentration of solute means a lower concentration of water.means a lower concentration of water.

A lower concentration of solute means A lower concentration of solute means a higher concentration of water.a higher concentration of water.

Why?Why?

Osmosis Animation

Osmotic PressureOsmotic Pressure

The ability of osmosis to generate The ability of osmosis to generate enough pressure to lift a volume of enough pressure to lift a volume of water is called water is called osmotic pressureosmotic pressure..

– The greater the concentration of The greater the concentration of nonpermeablenonpermeable solute particles in solute particles in solution, the solution, the lowerlower the water the water concentration of that solution and the concentration of that solution and the greatergreater the osmotic pressure. the osmotic pressure.

Why does water always tend to diffuse toward solutions of greater osmotic

pressure?

Water and the BodyWater and the Body

Water moves by osmosis Water moves by osmosis throughout the bodythroughout the body– Tries to reach Tries to reach equilibrium equilibrium – ConcentrationConcentration of water and solutes of water and solutes

inside and outside cell is essentially inside and outside cell is essentially the same.the same.

– Osmotic pressure of the intracellular Osmotic pressure of the intracellular and extracellular fluids is the same.and extracellular fluids is the same.

Types of SolutionsTypes of Solutions

Isotonic SolutionsIsotonic Solutions

Hypertonic SolutionsHypertonic Solutions

Hypotonic SolutionsHypotonic Solutions

Isotonic SolutionsIsotonic Solutions

Any solution that has the same osmotic Any solution that has the same osmotic pressure as body fluids is called pressure as body fluids is called IsotonicIsotonic..

– No net movement of water!No net movement of water!

Hypertonic SolutionsHypertonic Solutions

Solutions with Solutions with higher osmotic higher osmotic pressure than pressure than body fluids are body fluids are called called HypertonicHypertonic..

– Net movement of Net movement of water out of the water out of the cell.cell.

Hypotonic SolutionsHypotonic Solutions

Solutions with a Solutions with a mower osmotic mower osmotic pressure than pressure than body fluids are body fluids are called called hypotonichypotonic..

– Net movement of Net movement of water into the cell.water into the cell.

Example: Red Blood Example: Red Blood CellsCells If RBCs are placed in distilled water If RBCs are placed in distilled water

((hypotonichypotonic to cells), water diffuses to cells), water diffuses into the cells and they into the cells and they hemolyzehemolyze..

If RBCs are exposed to 0.9% NaCl If RBCs are exposed to 0.9% NaCl (normal saline), they do not change in (normal saline), they do not change in shape because the solution is shape because the solution is isotonicisotonic to human cells.to human cells.

If RBCs are placed in a If RBCs are placed in a hypertonichypertonic solution they solution they crenatecrenate. .

ImplicationsImplications

The concentration of solute in The concentration of solute in solutions that are infused into solutions that are infused into body tissues or blood must be body tissues or blood must be controlled.controlled.

Otherwise, osmosis can cause Otherwise, osmosis can cause cells to swell or shrink, impairing cells to swell or shrink, impairing their function.their function.

FiltrationFiltration

The process of filtration forces The process of filtration forces molecules through membranes.molecules through membranes.

– Commonly used to separate solids Commonly used to separate solids from water.from water.

Example: Making coffee (the “drip” Example: Making coffee (the “drip” method)method)

In your body, hydrostatic pressure In your body, hydrostatic pressure causes filtrationcauses filtration

In Your BodyIn Your Body

Filtration is always occurring in Filtration is always occurring in your body.your body.– Pressure in your arteries forces Pressure in your arteries forces

fluids out of vesselsfluids out of vessels– Fluid can build up in tissuesFluid can build up in tissues

Condition known as Condition known as edemaedema Balanced by maintaining a higher Balanced by maintaining a higher

osmotic pressureosmotic pressure inside blood vessels inside blood vessels

In Your BodyIn Your Body

QuestionQuestion

What prevents a condition called What prevents a condition called EdemaEdema from occurring in the from occurring in the body?body?

– Edema is a condition associated with Edema is a condition associated with excess tissue fluids building up excess tissue fluids building up within tissue spaces.within tissue spaces.

Active MechanismsActive Mechanisms

These mechanisms do require cellular These mechanisms do require cellular energy:energy:

– Active transportActive transport

– EndocytosisEndocytosis

– ExocytosisExocytosis

Active TransportActive Transport

Active TransportActive Transport is a process that is a process that moves particles through moves particles through membranes from a region of membranes from a region of lowerlower concentration to a region of concentration to a region of higherhigher concentration.concentration.

– Requires energy which comes from Requires energy which comes from cellular metabolism and, specifically, cellular metabolism and, specifically, from a molecule called from a molecule called Adenosine Adenosine TriphosphateTriphosphate (ATP) (ATP)

Carrier MoleculesCarrier Molecules

Carrier Molecules for A.T. are proteins Carrier Molecules for A.T. are proteins with binding sites that combine with with binding sites that combine with the particles being transported.the particles being transported.

– Also called “Pumps” because they Also called “Pumps” because they transport substances transport substances against the against the concentration gradientconcentration gradient..

– Examples of particles that are actively Examples of particles that are actively transported across the membrane are transported across the membrane are sodium, potassium, calcium, and hydrogen sodium, potassium, calcium, and hydrogen ions.ions.

Sodium Potassium Pump

When Things are too When Things are too Large for ProteinsLarge for Proteins Two processes that use cellular Two processes that use cellular

energy to move substances into energy to move substances into or out of a cell without actually or out of a cell without actually crossing the cell membrane are:crossing the cell membrane are:

– EndocytosisEndocytosis

– ExocytosisExocytosis

EndocytosisEndocytosis

Molecules or other particles that are Molecules or other particles that are too large to enter a cell by diffusion, too large to enter a cell by diffusion, facilitated diffusion, or active transport facilitated diffusion, or active transport are conveyed into cell within a are conveyed into cell within a vesiclevesicle formed from a section of the cell formed from a section of the cell membrane.membrane.

– 3 types of endocytosis:3 types of endocytosis:

PinocytosisPinocytosis

PhagocytosisPhagocytosis

Receptor-mediated EndocytosisReceptor-mediated Endocytosis

Receptor-Mediated Receptor-Mediated EndocytosisEndocytosis

Moves very specific kinds of Moves very specific kinds of particles into the cell by binding particles into the cell by binding them with receptors.them with receptors.

– The combination of the substance with The combination of the substance with the receptor site protein stimulates the receptor site protein stimulates the cell membrane to indent.the cell membrane to indent.