Chapter 5 Movement through the Cell Membrane

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Chapter 5Movement through the Cell Membrane

• How do things get into out of the cell?
• How does the cell maintain homeostasis?

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Just Passing Through

• Just like a country, a cell has borders.
• The cell membrane is the border

http//www.ibiblio.org/virtualcell/amazingbiology/
clipsweb/clipmodelspage.htm
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Cool simulation

• http//www.teachersdomain.org/asset/tdc02_int_memb
  raneweb/

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Homeostasis

• The ability or tendency of an organism or cell to
  maintain internal equilibrium by adjusting its
  physiological processes.
• Cell membranes help organisms maintain
  Homeostasis by controlling what substances may
  enter or leave cells. 

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• Obviously cells need to be able to be different
  than their environment maintain their pH, water
  concentration, certain types of molecules.
• The cell membrane does this job.
• Structure of cell membrane

http//library.thinkquest.org/C004535/cell_membran
es.html
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• The Cell Membrane is SEMIPERMEABLE.
• Only some molecules can pass
• Small molecules like WATER, OXYGEN, AND CARBON
  DIOXIDE can move in and out freely.
• Large molecules like PROTEINS CARBOHYDRATES
  CANNOT.

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What is Equilibrium?

• A state that exists when the concentration of a
  substance is the same throughout a space.
• A condition in which all acting influences are
  canceled by others, resulting in a stable,
  balanced, or unchanging system.
• Reached when molecules are of the same
  concentration, though they keep moving.

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5 basic ways things get into or out of cells

• Simple Diffusion
• Osmosis
• Facilitated Diffusion
• Active Transport
• Vesicles
• passive transport(1-3) active transport (4-5)

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Passive Transport

• Some substances can cross the cell membrane
  without any input of Energy by the cell.  The
  movement of such substances across the membrane
  is known as PASSIVE TRANSPORT.
• 1. Simple Diffusion
• 2. Osmosis
• 3. Facilitated Diffusion

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1. Simple Diffusion

• The movement of molecules from an area of high
  concentration to an area of low concentration

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a. CONCENTRATION GRADIENT
-The difference in concentrations of molecules
across a space.

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b. Kinetic energy of molecules

• Kinetic energy is energy of motion.
• Molecules are in Constant Motion. Molecules move
  in a straight line unless they hit something. If
  no object blocks their path they keep moving-so-
  they move to where there are less molecules.
• Diffusion occurs when molecules move randomly
  away from each other.
• Diffusion is driven entirely by kinetic energy

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c. Simple Diffusion across a membrane

• A few substances can diffuse directly through the
  lipid bilayer part of the membrane.
• The only substances that can do this are
  lipid-soluble molecules such as steroids, -or
  very small molecules, such as H2O, O2 and CO2.
  For these molecules the membrane is no barrier at
  all.
• Since no energy is added -substances can only
  move down their concentration gradient.

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2. Osmosis -the movement of water molecules from
an area of high concentration to an area of low
concentration.
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Osmosis

• Is the diffusion of water across a membrane.
• It is just normal lipid diffusion, but since
  water is so important in cells the diffusion of
  water has its own name - osmosis.
• Cells are like solutions of many different
  solutes.
• Water molecules can diffuse freely across a
  membrane, but always down a concentration
  gradient so water diffuses from a dilute to a
  concentrated solution.

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a. Types of solutions

• There are three possible concentrations of
  solution to consider
• Isotonic solution a solution of equal
  concentration of solutes to a cell
• Hypertonic solution a solution of higher
  concentration of solutes than a cell
• Hypotonic solution a solution of lower
  concentration of solutes than a cell

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Water moves freely across the cell membrane
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Note differences in normal state for
plant/animal cells
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• The more concentrated solution is Hypertonic
  compared to the solution with less solutes. The
  water will always try to rush in to make the more
  concentrated solution less hypertonic.
• The less concentrated solution is referred to as
  Hypotonic, water will attempt to leave this
  compartment decrease concentration of the
  solution with more solute.
• When two compartments are equally concentrated
  they are Isotonic with respect to each other,
  theres no net diffusion of water.

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Salt Sucks

• Salt is a solute, when it is concentrated inside
  or outside the cell, it will draw water in its
  direction.
• This is also why you get thirsty when eating
  something salty and why you cannot drink salt
  water to quench your thirst- it will only make it
  worse.

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b. Examples of problems that living cells face
all the time.

• Simple animal cells (protozoans, ie. paramecium)
  in fresh water habitats are surrounded by a
  hypotonic solution and constantly need to expel
  water using contractile vacuoles to prevent
  swelling lysis.
• Cells in marine environments are surrounded by a
  hypertonic solution, and must actively pump ions
  into their cells to reduce their water potential
  and so reduce water loss by osmosis.

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Marine Freshwater fish-homeostasis
responseFreshwater fish- pump water out to keep
from exploding. Marine fish must take in
ions(salts) to keep all the water from leaving
their body.
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Plant example

• Plant roots may be in a hypotonic environment.
  Since more water/ less solute is outside roots
  cells, water rushes into the plant cells (by
  osmosis). Cells swell against the cell wall.
• TURGOR PRESSURE is the pressure that water
  molecules exert against the cell wall. This force
  gives the plant rigidity, and provides structural
  support.
• You have probably experienced this with lettuce
  and celery that have been in the fridge too long.
  They are wilted and no longer crisp because they
  have lost their Turgor Pressure.

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sun.menloschool.org
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3.Facilitated Diffusion
-The transport of substances across a membrane by
a trans-membrane protein molecule. -No energy
is involved -substances can only move down their
concentration gradient.
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2 kinds of pump proteins

• Carrier Proteins have a binding site for a
  specific solute and constantly flip between two
  states so that the site is alternately open to
  opposite sides of the membrane. The substance
  will bind on the side where it at a high
  concentration and be released where it is at a
  low concentration.
• ION Channel Proteins form a water-filled pore or
  channel in the membrane. This allows charged
  substances (usually ions) to diffuse across
  membranes. (Most channels can be gated (opened or
  closed), allowing the cell to control the entry
  and exit of ions.)

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Active Transport

• -when cells must move materials up their
  concentration gradient, from an area of lower to
  an area of higher concentration
• 2 ways
• Cell Membrane pumps -.
• -Sodium-potassium pump
• Vesicles - Macromolecule exchange
• - Endocytosis
• - Exocytosis

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1. Active Transport (pumping)

• Pumping of substances across a membrane by a
  trans-membrane protein pump molecule.
• The protein binds a molecule of the substance to
  be transported on one side of the membrane,
  changes shape, and releases it on the other side.
  
• REQUIRES ENERGY An active process,
• .
• Use ATP -into ADP P- use the energy released to
  change shape and pump the molecule.

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• The NaK Pump. This transport protein is present
  in the cell membranes of all animal cells and is
  the most abundant and important of all membrane
  pumps.
• 3 Sodiums (Na) out for every 2 Potassium's (K) in

                                                  
                                                  
                       
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Exchange of 3 NA for 2 K -creates an electrical
gradient across the cell membrane
The outside of the membrane becomes positively
charged () the inside negatively (-)
charged.-Important for understanding electrical
impulses like in nerve cells.
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2. Vesicles

• The processes described so far only apply to
  small molecules.
• Large molecules (such as proteins,
  polysaccharides and nucleotides) and even whole
  cells are moved in and out of cells by using
  membrane vesicles.
• Endocytosis Exocytosis- both require energy- so
  are active transports

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Endocytosis-the process by which cells ingest
external fluid, macromolecules large
particles
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Endocytosis

• Materials enclosed by a fold of the cell
  membrane, which then pinches shut to form a
  closed vesicle.
• When the materials and the vesicles are small
  (such as a protein molecule) the process is known
  as pinocytosis (cell drinking)
• If the materials are large (such as a white blood
  cell ingesting a bacterial cell) the process is
  known as phagocytosis (cell eating)

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• Here a White Blood Cell eats some bacteria. How
  does the cell get the bacteria get inside the
  cell?

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Pseudopodia of a phagocytic cell
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Exocytosis

• Essentially endocytosis in reverse.
• Vesicles in the cytoplasm fuse with the cell
  membrane empty their contents outside the cell.

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Summary of Membrane Transport
method uses energy uses proteins specific controllable
Simple Diffusion N N N N
Osmosis N N Y N
Facilitated Diffusion N Y Y Y
Active Transport Y Y Y Y
Vesicles Y N Y Y

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