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Passive Transport and Cell Membrane Diffusion

This chapter explores the concept of passive transport, focusing on diffusion as the simplest form. It discusses how molecules move across a cell membrane based on concentration gradients and kinetic energy. The concept of equilibrium and diffusion across membranes is also explained, along with osmosis and the direction of water movement. The ways in which cells deal with osmosis, such as contractile vacuoles, turgor pressure, and plasmolysis, are discussed. Additionally, facilitated diffusion and diffusion through ion channels are explored, highlighting the role of carrier proteins and ion channels in the movement of molecules and ions across the cell membrane.

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Passive Transport and Cell Membrane Diffusion

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  1. Chapter 5Homeostasis and Transport Section 5.1

  2. Passive Transport • The movement of substances across a cell membrane without any input of energy from the cell

  3. Diffusion • Simplest type of passive transport • Molecules move from an area of higher concentration to an area of lower concentration • Driven by kinetic energy

  4. Equilibrium • When the concentration of the molecules of a substance are the same throughout a space

  5. Diffusion Across Membranes • Cell membranes allow some molecules to pass through, but not others • Depends on size and type of molecule • Also depends on the chemical nature of the membrane

  6. Osmosis • Water molecules diffuse across a cell membrane from an area of higher concentration of H2O to an area of lower concentration

  7. Direction of Osmosis • Hypotonic- the concentration of solute molecules outside the cell is LOWER than the inside of the cell • Water moves INTO the cell

  8. Hypertonic- the concentration of solute molecules outside the cell is HIGHER than the inside of the cell • Water moves OUT OF the cell

  9. Isotonic- the concentrations of solutes outside and inside the cell are equal • No movement of water occurs

  10. How Cells Deal with Osmosis • Contractile vacuole- organelles to remove water • Turgor pressure- pressure of water against the cell wall • Plasmolysis- cells shrink away from cell walls

  11. Facilitated Diffusion • The movement of molecules across a cell membrane through the use of carrier proteins • First, a carrier protein binds to a molecule on one side of the cell membrane

  12. Second, the carrier protein changes shape shielding the molecule from the interior of the membrane • Finally, the molecule is released on the other side of the membrane

  13. Diffusion Through Ion Channels • Ion channels provide small passages for ions to diffuse across the cell membrane • Channels may have “gates” that respond to stretching of the cell membrane, electrical signals, or chemicals in the cytosol

  14. Ions such as Na+, K+, Ca2+, and Cl- are important for a variety of cell functions

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