Part 3: Homeostasis and Cell Transport (Chapter 5)

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Unit 3 Organization and Development

• Part 3 Homeostasis and Cell Transport (Chapter 5)

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Passive Transport

• Involves the movement of molecules across the
  cell membrane without an input of energy by the
  cell.

• NO ENERGY REQUIRED to move substances across
  membrane -- water, lipids, and other lipid
  soluble substances.

• Types
• Diffusion
• Osmosis
• Facilitated Diffusion
• Diffusion through Ion Channels

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1. Diffusion

• Diffusion is the movement of molecules from an
  area of higher concentration to an area of lower
  concentration (with the concentration gradient)
  until equilibrium is reached
• Concentration gradient the difference in
  concentration across space.
• Equilibrium molecules are eventually evenly
  dispersed OR the concentration of molecules is
  the same throughout the entire space.

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Simple Diffusion diffusion across a membrane

• Cell Membrane semi-permeable
• Some materials can pass through, some cannot
• Molecules will move from high to low
  concentration
• The membrane determine what molecules can pass
  through freely and what molecules need
  assistance

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Diffusion
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2. Osmosis

• Osmosis is the diffusion of water across a
  membrane.

• Direction of Osmosis
• The net direction of osmosis is determined by the
  relative solute concentrations on the two sides
  of the membrane.

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• Direction of Osmosis
• When the solute concentration outside the cell is
  higher than that in the cytosol, the solution
  outside is hypertonic (more solute, less water)
  to the cytosol, and water will diffuse out of the
  cell.
• Plasmolysis cells shrinking due to water loss

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• Direction of Osmosis
• When the solute concentration outside the cell is
  lower than that in the cytosol, the solution
  outside is hypotonic (less solute, more water) to
  the cytosol, and water will diffuse into the
  cell.
• Cytolysis cells bursting due to water gain

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• Direction of Osmosis
• When the solute concentrations outside and inside
  the cell are equal, the solution outside is
  isotonic (same solute, same water), and there
  will be no net movement of water.

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Review of Osmosis terms
Hypotonic
Isotonic
Hypertonic
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Hypertonic, Hypotonic, Isotonic Solutions
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• How Cells Deal With Osmosis
• Cells must compensate for the water that enters
  the cell in hypotonic environments and leaves the
  cell in hypertonic environments.
• Multicellular organisms
• cells respond to hypotonic environments by
  pumping solutes out of the cytosol
• RBCs cannot compensate for changes in solute
  concentration
• Unicellular organisms
• May have Contractile vacuoles organelles that
  regulate water levels

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Real cells
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3. Facilitated Diffusion

• Diffusion of molecules across a membrane when
  they are not soluble in lipids or are too large
  (e.g. glucose) to pass through pores in membrane
• A molecule binds to a carrier protein on one side
  of the cell membrane.
• Carrier Protein
• specific for one type of molecule
• changes its shape and transports the molecule
  down its concentration gradient to the other side
  of the membrane.

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Facilitated Diffusion
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4. Diffusion Through Ion Channels

• Ion channels proteins, or groups of proteins,
  that provide small passageways across the cell
  membrane through which specific ions can diffuse.
• Some are always open
• Some are gated
• Ions
• Charged particles like Na, Ca2, Cl-
• important in cell function
• include sodium, potassium, calcium, and chloride

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Ion Channels
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Active TransportCell Membrane Pumps and Vesicles

• Active transport
• moves molecules across the cell membrane from an
  area of lower concentration to an area of higher
  concentration (against concentration gradient)
• requires cells to expend ENERGY
• Some types of active transport are performed by
  carrier proteins called cell membrane pumps.

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Cell Membrane Pumps

• Sodium-Potassium Pump
• moves three Na ions into the cells external
  environment for every two K ions it moves into
  the cytosol.
• most animal cells must have a higher
  concentration of Na ions outside the cell and a
  higher concentration of K ions inside the cell
• The Na/K- pump allows for the gradient
• ATP supplies the energy that drives the pump.

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More on the Sodium-Potassium Pump

• The exchange of three Na ions for two K ions
  creates an electrical gradient across the cell
  membrane
• Outside becomes positively charged relative to
  the inside, which becomes negative
• Difference in electrical charge is important for
  the conduction of electrical impulses along nerve
  cells

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Sodium-Potassium Pump
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Sodium-Potassium Pump
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Movement in Vesicles

• Endocytosis
• cells ingest external fluid, macromolecules, and
  large particles, including cells by folding
  around them and forming a pouch.
• The pouch then pinches off and becomes a
  membrane-bound organelle called a vesicle.
• Some vesicles fuse with lysosomes, and their
  contents are digested by lysosomal enzymes
• Two main types
• Pinocytosis vesicle contains solutes or fluids
• Phagocytosis vesicle contains large particles or
  whole cells.

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Endocytosis
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Movement in Vesicles

• Exocytosis
• Vesicles made by the cell fuse with the cell
  membrane, releasing their contents into the
  external environment.
• Used to release large molecules, such as
  proteins, waste products, or toxins that would
  damage the cell if they were released within the
  cytosol

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Exocytosis
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Endocytosis and Exocytosis