Cells

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Cells

• Cell Boundaries

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What Are We Learning?

• Define the function of the cell membrane and cell
  wall.
• Illustrate the arrangement of phospholipids in a
  lipid bilayer.
• Describe two characteristics of a lipid bilayer.
• Describe the functions of proteins in the cell
  membrane.
• Describe what happens during diffusion.
• Explain the processes of osmosis, facilitated
  diffusion, and active transport.

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Cell Boundaries

• All cells are surrounded by a thin, flexible
  barrier known as the cell membrane.
• Regulates what enters and leaves the cell.
• Provides protection and support.
• Many cells also produce a strong supporting layer
  around the membrane known as a cell wall.
• The main function of the cell wall is to provide
  support and protection for the cell.

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Cell Boundaries

• Cell walls are present in plants, algae, fungi,
  and many prokaryotes.
• Most cell walls are made from fibers of
  carbohydrate and protein.
• Plant cell walls are composed mostly of
  cellulose, a tough carbohydrate fiber.

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Water and the Cell Membrane

• All cells are surrounded by water.
• Water is present inside the cell too.
• All cell organelles perform their functions in
  water.
• The cell membrane is shaped by the water found
  inside and outside of the cell.

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Water and the Cell Membrane

• The basic plan of a cell membrane begins with a
  sheet of lipids
• Lipids are fats and oils.
• The interaction between water and lipids shapes
  the cell membrane.
• What happens when oil is poured in water?
• Water and oil will separate because water is a
  polar molecule and oil is nonpolar.

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Cell Membrane Architecture

• Basic building block of the cell membrane is a
  kind of lipid molecule called a phospholipid.
• Short head is joined to two long tails.
• The head contains phosphorous and nitrogen, which
  make it polar.
• Hydrophilic head
• The long tails of fatty acids are nonpolar.
• Hydrophobic tails

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Cell Membrane Architecture

• Water can interact with the polar heads and repel
  the nonpolar lipids tails.
• The polar heads of the phospholipids point toward
  the water inside and outside the cell.
• The tails are buried in the interior of the
  membrane, directing themselves away from the
  water.
• This double layer of phopholipids forms a
  flexible lipid bilayer.

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Cell Membrane Architecture

• Lipids bilayers stop polar molecules. How?
• Polar molecules cannot interact with the nonpolar
  tails of the phospholipids.
• However, most food molecules and other substances
  needed by the cell as polar.
• Why could this be a problem for the cell?
• How does a cell solve that problem?
• Later in the lecture.

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Cell Membrane Architecture

• Lipid bilayers and protein molecules are not
  rigidly fixed in place.
• The lipid bilayer is fluid because proteins move
  about like icebergs floating on the surface of
  the ocean.
• Allows phospholipids and protein molecules to
  shift from one region of the cell membrane to
  another.
• Very important because cell membranes can be
  structure to fit the needs of different cell
  types.

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Cell Membrane Architecture

• The smooth lipid bilayer is interrupted by
  proteins sticking out from the surface.
• Some look like boulders.
• Others like tall trees.
• Proteins that protude from the cell membrane may
  serve as channels, receptors, or markers.

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Cell Membrane Architecture

• Proteins channels are formed by doughnut-shaped
  proteins.
• Act as passageways through which only certain
  molecules can pass.
• Each channel will admit only certain molecules

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Cell Membrane Architecture

• Receptor proteins in the cell membrane are shaped
  like boulders.
• Transmit information into the cell by reacting to
  certain other molecules from the external
  molecules.
• The part of the receptor that sticks out from the
  cell membrane has a specific shape.
• Only molecules with the right shape can fit into
  these receptors.
• When a molecule and a receptor are locked
  together, it triggers responses inside the cell.

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Cell Membrane Architecture

• Cell surface markers are elongated proteins,
  often with shorth chains of carbohydrates
  attached.
• Function as the name tags of cells, giving each
  an identity.
• Every cell of your body has markers on its
  surface saying that it is a part of you and
  nothing else.

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Cell Membrane, A Fluid Mozaic

• The composition of all cell membranes is a
  double-layered sheet called a lipid bilayer.
• Lipid bilayer gives cell membranes a flexible
  structure that forms a strong barrier.
• In addition to lipids, most cell membranes
  contain proteins molecules that are embedded in
  the lipid bilayer.
• Proteins form channels and pumps that help to
  move material across the cell membrane.
• Carbohydrates molecules are attached to many of
  these proteins.
• Act like chemical identification cards, allowing
  cards to identify one another.

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Cell Membrane, A Fluid Mozaic
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A Quick Review

• A solution is a mixture of two or more
  substances.
• The substances dissolved in the solution are
  called solutes.
• The concentration of a solution is the mass of
  solute in a given volume of solution, or
  mass/volume.

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Diffusion

• Particles in a solution tend to move from an area
  where they are more concentrated to an area where
  they are less concentrated by a process called
  diffusion.
• When the concentration of the solute is the same
  throughout a system, the system has reached
  equilibrium.
• Because diffusion depends upon particle
  movements, substances diffuse across membranes
  without requiring the cell to use energy.

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Osmosis

• Most biological membranes are selectively
  permeable, meaning that some substances can pass
  across them and others cannot.
• Osmosis is the diffusion of water through a
  selectively permeable membrane.

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How Osmosis Works

• If the concentration of H2O is lower inside the
  cell than outside, there will be a net movement
  of H2O to the inside until equilibrium is
  reached.
• In isotonic solutions the concentration of the
  two solutions is the same.
• A hypertonic solution is the solution with the
  greater concentration of solutes.
• A hypotonic is the solution with the lesser
  concentration of solutes.

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Osmotic Pressure

• Osmosis exerts a pressure known as osmotic
  pressure on the hypertonic side of the selective
  permeable membrane.
• Cells are filled with salts, sugars, proteins,
  and other molecules, so they are almost always
  hypertonic to fresh water.
• As a consequence, osmotic pressure should produce
  a net movement of water into a typical cell that
  is surrounded by fresh water.

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Facilitated Diffusion

• A few molecules seem to pass through the cell
  membrane much more quickly than they should.
• Cell membranes have protein channels that make it
  easy for certain molecules to cross the membrane
  in a process known as facilitated diffusion.
• Hundreds of different protein channels have been
  found that allow particular substances to cross
  different membranes.
• Facilitated diffusion does not require the use of
  cells energy.

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Active Transport

• During active transport cells move materials
  against a concentration difference.
• Requires energy.
• Carried out by transport proteins found in the
  cell membrane.
• Moves molecules in only one direction.
• Many cells use such proteins to move Ca2, K,
  and Na.
• Enables cells to concentrate substances in a
  particular location, even against the forces of
  diffusion.

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Sodium-Potassium Pump

• The Na-K moves sodium out of the cells and
  potassium into cells.
• This flow of ions is vital for restoring a
  balance needed to keep a nerve cell functioning.
• It also helps transport sugars and amino acids
  into your cells.
• Every second, each channel can transport more
  than 300 sodium ions out of the cell.

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Cystic Fibrosis

• Cystic fibrosis causes the body to produce very
  thick mucus, which build up in the lungs,
  pancreas and liver.
• Extreme difficulty breathing.
• Cannot produce the enzymes needed to completely
  digest food.
• Increase chances of infections.
• People with cystic fibrosis have very salty
  sweat, an indication of abnormally high levels of
  sodium and chloride ions.
• Causes thick mucus to accumulate in patients
  lungs due to the high levels of salt in their
  cells draw water out of the mucus.

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What Causes Cystic Fibrosis?

• Test of cystic fibrosis patients revealed that
  their cells are unable to export chloride ions.
• Searching for the gene that encode Cl- channels,
  researches isolated the gene that causes cystic
  fibrosis.
• The genes encodes for a misshapen protein channel
  that cannot function properly.

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Endocytosis

• Endocytosis is the process of taking material
  into the cell by means of infoldings, or pockets,
  of the cell membrane.
• The resulting pocket breaks loose from the outer
  portion of the cell membrane and form a vacuole
  within the cytoplasm.
• In phagocytosis extensions of cytoplasm surround
  a particle and package within a food vacuole.
• In pinocytosis, many cells take up liquid from
  the surrounding environment.

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Exocytosis

• Many cells also release large amounts of material
  from the cell, a process known as exocitosis.
• During exocytosis, the membrane of the vacuole
  surrounding the material fuses with the cell
  membrane, forcing the contents out of the cell.
• Removal of water by means of contractile vacuole.

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Endocytosis and Exocytosis
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Endocytosis and Exocytosis