CellsCellsCell BoundariesCell Boundaries

What Are We Learning?What Are We Learning? Define the function of the cell Define the function of the cell

membrane and cell wall.membrane and cell wall. Illustrate the arrangement of Illustrate the arrangement of

phospholipids in a lipid bilayer.phospholipids in a lipid bilayer. Describe two characteristics of Describe two characteristics of

a lipid bilayer.a lipid bilayer. Describe the functions of Describe the functions of

proteins in the cell membrane.proteins in the cell membrane. Describe what happens during Describe what happens during

diffusion.diffusion. Explain the processes of Explain the processes of

osmosis, facilitated diffusion, osmosis, facilitated diffusion, and active transport.and active transport.

Cell BoundariesCell Boundaries All cells are surrounded by a All cells are surrounded by a

thin, flexible barrier known as thin, flexible barrier known as the the cell membranecell membrane.. Regulates what enters and Regulates what enters and

leaves the cell.leaves the cell. Provides protection and support.Provides protection and support.

Many cells also produce a Many cells also produce a strong supporting layer around strong supporting layer around the membrane known as a the membrane known as a cell cell wallwall.. The main function of the cell The main function of the cell

wall is to provide support and wall is to provide support and protection for the cell.protection for the cell.

Cell BoundariesCell Boundaries Cell walls are present in Cell walls are present in

plants, algae, fungi, and plants, algae, fungi, and many prokaryotes.many prokaryotes.

Most cell walls are Most cell walls are made from fibers of made from fibers of carbohydrate and carbohydrate and protein.protein.

Plant cell walls are Plant cell walls are composed mostly of composed mostly of cellulose, a tough cellulose, a tough carbohydrate fiber.carbohydrate fiber.

Water and the Cell MembraneWater and the Cell Membrane All cells are surrounded All cells are surrounded

by water.by water. Water is present inside Water is present inside

the cell too.the cell too. All cell organelles All cell organelles

perform their functions perform their functions in water.in water.

The cell membrane is The cell membrane is shaped by the water shaped by the water found inside and outside found inside and outside of the cell.of the cell.

Water and the Cell MembraneWater and the Cell Membrane The basic plan of a cell The basic plan of a cell

membrane begins with a sheet membrane begins with a sheet of lipidsof lipids Lipids are fats and oils.Lipids are fats and oils.

The interaction between water The interaction between water and lipids shapes the cell and lipids shapes the cell membrane.membrane.

What happens when oil is What happens when oil is poured in water?poured in water?

Water and oil will separate Water and oil will separate because water is a polar because water is a polar molecule and oil is nonpolar.molecule and oil is nonpolar.

Cell Membrane ArchitectureCell Membrane Architecture Basic building block of the Basic building block of the

cell membrane is a kind of cell membrane is a kind of lipid molecule called a lipid molecule called a phospholipidphospholipid..

Short “head” is joined to Short “head” is joined to two long “tails.”two long “tails.”

The head contains The head contains phosphorous and nitrogen, phosphorous and nitrogen, which make it polar.which make it polar. Hydrophilic headHydrophilic head

The long tails of fatty acids The long tails of fatty acids are nonpolar.are nonpolar. Hydrophobic tailsHydrophobic tails

Cell Membrane ArchitectureCell Membrane Architecture Water can interact with the Water can interact with the

polar heads and repel the polar heads and repel the nonpolar lipids tails.nonpolar lipids tails.

The polar heads of the The polar heads of the phospholipids point toward phospholipids point toward the water inside and outside the water inside and outside the cell.the cell.

The tails are buried in the The tails are buried in the interior of the membrane, interior of the membrane, directing themselves away directing themselves away from the water.from the water.

This double layer of This double layer of phopholipids forms a flexible phopholipids forms a flexible lipid bilayerlipid bilayer..

Cell Membrane ArchitectureCell Membrane Architecture Lipids bilayers stop polar Lipids bilayers stop polar

molecules. molecules. How?How? Polar molecules cannot interact Polar molecules cannot interact

with the nonpolar tails of the with the nonpolar tails of the phospholipids.phospholipids.

However, most food molecules However, most food molecules and other substances needed by and other substances needed by the cell as polar. the cell as polar.

Why could this be a problem Why could this be a problem for the cell?for the cell?

How does a cell solve that How does a cell solve that problem?problem? Later in the lecture.Later in the lecture.

Cell Membrane ArchitectureCell Membrane Architecture Lipid bilayers and protein Lipid bilayers and protein

molecules are not rigidly fixed in molecules are not rigidly fixed in place.place.

The lipid bilayer is fluid because The lipid bilayer is fluid because proteins move about like icebergs proteins move about like icebergs floating on the surface of the floating on the surface of the ocean.ocean.

Allows phospholipids and protein Allows phospholipids and protein molecules to shift from one molecules to shift from one region of the cell membrane to region of the cell membrane to another.another.

Very important because cell Very important because cell membranes can be structure to fit membranes can be structure to fit the needs of different cell types.the needs of different cell types.

Cell Membrane ArchitectureCell Membrane Architecture

The smooth lipid bilayer is interrupted by proteins The smooth lipid bilayer is interrupted by proteins sticking out from the surface.sticking out from the surface. Some look like boulders.Some look like boulders. Others like tall trees.Others like tall trees.

Proteins that protude from the cell membrane may serve Proteins that protude from the cell membrane may serve as channels, receptors, or markers.as channels, receptors, or markers.

Cell Membrane ArchitectureCell Membrane Architecture Proteins channels are Proteins channels are

formed by doughnut-formed by doughnut-shaped proteins.shaped proteins.

Act as passageways Act as passageways through which only through which only certain molecules can certain molecules can pass.pass.

Each channel will Each channel will admit only certain admit only certain moleculesmolecules

Cell Membrane ArchitectureCell Membrane Architecture Receptor proteins in the cell Receptor proteins in the cell

membrane are shaped like membrane are shaped like boulders.boulders.

Transmit information into the cell Transmit information into the cell by reacting to certain other by reacting to certain other molecules from the external molecules from the external molecules.molecules.

The part of the receptor that sticks The part of the receptor that sticks out from the cell membrane has a out from the cell membrane has a specific shape.specific shape.

Only molecules with the right Only molecules with the right shape can fit into these receptors.shape can fit into these receptors.

When a molecule and a receptor When a molecule and a receptor are locked together, it triggers are locked together, it triggers responses inside the cell.responses inside the cell.

Cell Membrane ArchitectureCell Membrane Architecture Cell surface markers are Cell surface markers are

elongated proteins, often elongated proteins, often with shorth chains of with shorth chains of carbohydrates attached.carbohydrates attached.

Function as the “name Function as the “name tags” of cells, giving tags” of cells, giving each an identity.each an identity.

Every cell of your body Every cell of your body has markers on its has markers on its surface saying that it is a surface saying that it is a part of you and nothing part of you and nothing else.else.

Cell Membrane, A Fluid MozaicCell Membrane, A Fluid Mozaic The composition of all cell membranes is a double-The composition of all cell membranes is a double-

layered sheet called a lipid bilayer.layered sheet called a lipid bilayer. Lipid bilayer gives cell membranes a flexible structure Lipid bilayer gives cell membranes a flexible structure

that forms a strong barrier.that forms a strong barrier. In addition to lipids, most cell membranes contain In addition to lipids, most cell membranes contain

proteins molecules that are embedded in the lipid bilayer.proteins molecules that are embedded in the lipid bilayer. Proteins form channels and pumps that help to move Proteins form channels and pumps that help to move

material across the cell membrane.material across the cell membrane. Carbohydrates molecules are attached to many of these Carbohydrates molecules are attached to many of these

proteins.proteins. Act like chemical identification cards, allowing cards to Act like chemical identification cards, allowing cards to

identify one another.identify one another.

Cell Membrane, A Fluid MozaicCell Membrane, A Fluid Mozaic

A Quick ReviewA Quick Review A solution is a mixture A solution is a mixture

of two or more of two or more substances.substances.

The substances The substances dissolved in the solution dissolved in the solution are called solutes.are called solutes.

The The concentrationconcentration of a of a solution is the mass of solution is the mass of solute in a given volume solute in a given volume of solution, or of solution, or mass/volume.mass/volume.

DiffusionDiffusion Particles in a solution tend to Particles in a solution tend to

move from an area where they move from an area where they are more concentrated to an area are more concentrated to an area where they are less concentrated where they are less concentrated by a process called by a process called diffusiondiffusion..

When the concentration of the When the concentration of the solute is the same throughout a solute is the same throughout a system, the system has reached system, the system has reached equilibrium.equilibrium.

Because diffusion depends Because diffusion depends upon particle movements, upon particle movements, substances diffuse across substances diffuse across membranes without requiring membranes without requiring the cell to use energy.the cell to use energy.

OsmosisOsmosis Most biological membranes Most biological membranes

are selectively permeable, are selectively permeable, meaning that some meaning that some substances can pass across substances can pass across them and others cannot.them and others cannot.

OsmosisOsmosis is the diffusion of is the diffusion of water through a selectively water through a selectively permeable membrane.permeable membrane.

How Osmosis WorksHow Osmosis Works If the concentration of HIf the concentration of H22O is lower inside the O is lower inside the

cell than outside, there will be a net movement of cell than outside, there will be a net movement of HH22O to the inside until equilibrium is reached.O to the inside until equilibrium is reached.

In In isotonicisotonic solutions the concentration of the two solutions the concentration of the two solutions is the same.solutions is the same.

A A hypertonichypertonic solution is the solution with the solution is the solution with the greater concentration of solutes.greater concentration of solutes.

A A hypotonichypotonic is the solution with the lesser is the solution with the lesser concentration of solutes.concentration of solutes.

Osmotic PressureOsmotic Pressure Osmosis exerts a pressure known Osmosis exerts a pressure known

as osmotic pressure on the as osmotic pressure on the hypertonic side of the selective hypertonic side of the selective permeable membrane.permeable membrane.

Cells are filled with salts, sugars, Cells are filled with salts, sugars, proteins, and other molecules, so proteins, and other molecules, so they are almost always hypertonic they are almost always hypertonic to fresh water.to fresh water.

As a consequence, osmotic As a consequence, osmotic pressure should produce a net pressure should produce a net movement of water into a typical movement of water into a typical cell that is surrounded by fresh cell that is surrounded by fresh water.water.

Facilitated DiffusionFacilitated Diffusion A few molecules seem to pass A few molecules seem to pass

through the cell membrane much through the cell membrane much more quickly than they should.more quickly than they should.

Cell membranes have protein Cell membranes have protein channels that make it easy for channels that make it easy for certain molecules to cross the certain molecules to cross the membrane in a process known as membrane in a process known as facilitated diffusionfacilitated diffusion..

Hundreds of different protein Hundreds of different protein channels have been found that channels have been found that allow particular substances to allow particular substances to cross different membranes.cross different membranes.

Facilitated diffusion does not Facilitated diffusion does not require the use of cell’s energy.require the use of cell’s energy.

Active TransportActive Transport During During active transportactive transport cells cells

move materials against a move materials against a concentration difference.concentration difference. Requires energy.Requires energy. Carried out by transport proteins Carried out by transport proteins

found in the cell membrane.found in the cell membrane. Moves molecules in only one Moves molecules in only one

direction.direction. Many cells use such proteins to Many cells use such proteins to

move Camove Ca2+2+, K, K++, and Na, and Na++.. Enables cells to concentrate Enables cells to concentrate

substances in a particular location, substances in a particular location, even against the forces of even against the forces of diffusion.diffusion.

Sodium-Potassium PumpSodium-Potassium Pump The NaThe Na++-K-K++ moves sodium moves sodium

out of the cells and out of the cells and potassium into cells.potassium into cells.

This flow of ions is vital for This flow of ions is vital for restoring a balance needed restoring a balance needed to keep a nerve cell to keep a nerve cell functioning.functioning.

It also helps transport sugars It also helps transport sugars and amino acids into your and amino acids into your cells.cells.

Every second, each channel Every second, each channel can transport more than 300 can transport more than 300 sodium ions out of the cell.sodium ions out of the cell.

Cystic FibrosisCystic Fibrosis Cystic fibrosis causes the body to produce very thick Cystic fibrosis causes the body to produce very thick

mucus, which build up in the lungs, pancreas and liver.mucus, which build up in the lungs, pancreas and liver. Extreme difficulty breathing.Extreme difficulty breathing. Cannot produce the enzymes needed to completely digest Cannot produce the enzymes needed to completely digest

food.food. Increase chances of infections.Increase chances of infections.

People with cystic fibrosis have very salty sweat, an People with cystic fibrosis have very salty sweat, an indication of abnormally high levels of sodium and indication of abnormally high levels of sodium and chloride ions.chloride ions. Causes thick mucus to accumulate in patients lungs due to Causes thick mucus to accumulate in patients lungs due to

the high levels of salt in their cells draw water out of the the high levels of salt in their cells draw water out of the mucus.mucus.

What Causes Cystic Fibrosis?What Causes Cystic Fibrosis? Test of cystic fibrosis patients Test of cystic fibrosis patients

revealed that their cells are revealed that their cells are unable to export chloride unable to export chloride ions.ions.

Searching for the gene that Searching for the gene that encode Clencode Cl-- channels, channels, researches isolated the gene researches isolated the gene that causes cystic fibrosis.that causes cystic fibrosis. The genes encodes for a The genes encodes for a

misshapen protein channel misshapen protein channel that cannot function that cannot function properly.properly.

EndocytosisEndocytosis EndocytosisEndocytosis is the process of is the process of

taking material into the cell by taking material into the cell by means of infoldings, or pockets, of means of infoldings, or pockets, of the cell membrane.the cell membrane.

The resulting pocket breaks loose The resulting pocket breaks loose from the outer portion of the cell from the outer portion of the cell membrane and form a vacuole membrane and form a vacuole within the cytoplasm.within the cytoplasm. In In phagocytosisphagocytosis extensions of extensions of

cytoplasm surround a particle and cytoplasm surround a particle and package within a food vacuole.package within a food vacuole.

In In pinocytosispinocytosis, many cells take up , many cells take up liquid from the surrounding liquid from the surrounding environment.environment.

ExocytosisExocytosis Many cells also release large Many cells also release large

amounts of material from the amounts of material from the cell, a process known as cell, a process known as exocitosis.exocitosis.

During During exocytosisexocytosis, the , the membrane of the vacuole membrane of the vacuole surrounding the material surrounding the material fuses with the cell fuses with the cell membrane, forcing the membrane, forcing the contents out of the cell.contents out of the cell. Removal of water by means Removal of water by means

of contractile vacuole.of contractile vacuole.

Endocytosis and ExocytosisEndocytosis and Exocytosis

Endocytosis and ExocytosisEndocytosis and Exocytosis