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Cell Boundaries: Membrane Functions and Transport Processes

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  1. Chapter 7 7-3 Cell Boundaries

  2. Function of the Cell Membrane • Sometimes called the plasma membrane • Regulates what enters and leaves the cell • Also provides support and protection for the cell

  3. Composition of the Cell Membrane • Nearly all cell membranes are composed of a lipid bilayer, which is a double-layered sheet. • The lipid bilayer gives cell membranes a flexible structure that forms a barrier between the cell and its surroundings. • Proteins (sometimes with carbohydrates attached) are also embedded in the cell membrane with the lipids.

  4. Cell Walls • Cell walls lie outside the cell membrane. • Present in bacteria, algae, fungi and many prokaryotes. • Most cell walls are made from fibers of carbohydrates and proteins. • Plant cell walls are composed mostly of cellulose, the principal component of wood and paper.

  5. Function of the Cell Wall • The main function of the cell wall is to provide support and protection for the cell. • Most cell walls are porous (have holes in them) enough to allow water, oxygen, carbon dioxide, and certain other substances to pass through easily.

  6. Diffusion Through Cell Boundaries • The cell membrane is selectively permeable, which means that some things can pass through it and other things cannot. • The cell exists in a liquid environment that it needs to survive. • The cytoplasm of a cell contains a solution of many different substances in water. • Concentration = the amount of solutes dissolved in a solution

  7. Diffusion • In a solution, particles move constantly. • They collide with one another and spread out randomly. • Diffusion is when the particles move from an area where they are more (higher) concentrated to an area where they are less (lower) concentrated.

  8. Diffusion = No energy • When the concentration of the solute is the same throughout a system, the system has reached equilibrium. • Because diffusion depends on random particle movements, substances diffuse across membranes without requiring the cell to use energy.

  9. Osmosis • Some things (like water) are small enough to fit through the channels in the cell membrane, while others are too big to fit (selectively permeable) • Water will tend to move across the membrane until equilibrium is established. • Osmosis is the diffusion of water through a selectively permeable membrane.

  10. Comparing solutions • When two solutions have the same strength or equal particles, they are called isotonic • When one solution is more concentrated than the other one, it is called hypertonic, which means it is “above strength”. • When one solution is less concentrated than the other one, it is called hypotonic, which means “below strength”.

  11. IMPORTANT RULE! • PARTICLES WILL ALWAYS TEND TO MOVE FROM AN AREA OF HIGHER CONCENTRATION TO AN AREA OF LOWER CONCENTRATION • IT IS ALWAYS TRYING TO REACH EQUILIBRIUM OR EQUAL AMOUNTS IN EACH SOLUTION (ONE INSIDE THE CELL AND ONE OUTSIDE)

  12. Osmotic Pressure • Osmosis can cause problems for cells • Cells have salts, sugars, and proteins inside them • This means that there will almost always be less water in the cell than outside of it • Water will try to move into the cell to balance things out • Can cause cells to swell and burst!

  13. Facilitated Diffusion • Some molecules (like glucose) are large, but they can go through the membrane too • Protein channels in the membrane can help the larger molecules get across • Facilitated diffusion is very specific, so a certain protein channel will only help a certain kind of molecule • Doesn’t need energy to work

  14. Active Transport • Sometimes cells must move materials in the opposite direction (from low to high concentration) • Active transport uses protein pumps and needs energy to move the materials • Materials moved this way include sodium, calcium, and potassium

  15. Other methods of Active Transport • Exocytosis = the vacuole membrane fuses with the cell membrane and gets rid of material • Endocytosis = taking material into the cell by folding the cell membrane

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