Membranes organize the chemical activities of cells. Separate Regulate Surround organelles in eukaryotes Are selectively permeable
Membrane phospholipids form a bilayer. Phospholipids have hydrophilic and hydrophobic zones Phospholipids In water In water The hydrophobic interior provides a barrier that regulates passage into and out of the cell
The membrane is a fluid mosaic of phospholipids and proteins. It is a mosaic because of the proteins It is fluid because the individual molecules move about.
Carbohydrates are found on the outside and provide recognition between cells Cholesterol helps stabilize the fluidity at different temperatures. Carbohydrates act as identification tags for cells
Passage Through The Phospholipid Bilayer What molecules can pass through the phsopholipid bilayer? Small non polar Large non polar Small polar What molecules cannot pass freely through the bilayer? Large polar Ions
Passive transport is diffusion across a membrane. What is diffusion?diffusion Diffusion is the tendency for particles to spread out spontaneously from an area of high concentration to an area of low concentration. Diffusion It requires no work. Remember the picture…
What is a concentration gradient? Molecules move from a region of higher to a region of lower concentration. This provides a net direction to their movement It is called going down the concentration gradient At equilibrium, molecules continue to move back and forth, but there is no net directional flow
If a molecule can move freely through the phospholipid bilayer what always controls the direction of its movement? Concentration gradient. Remember that the concentration gradient represents potential energy.
Water, Water Everywhere, But Not a Drop to Drink….
Osmosis What is osmosis? The diffusion of water across a semi- permeable membrane.
The cell can concentrate solutes that are not permeable to the phospholipid bilayer on one side of the membrane. Which way will water move? Water will follow the solutes.
What do the terms hypotonic, hypertonic and isotonic mean? Hypotonic = lower solute concentration and therefore higher water concentration Hypertonic = greater solute concentration and therefore lower water concentration Isotonic = equal solute concentration Using the terms hypotonic and hypertonic, which way will water flow? From hypotonic to hypertonic.
?? Which side is hypotonic and which side is hypertonic? AB
Water balance between cells and their surroundings is crucial to organisms. Cell membranes act as selectively permeable membranes between the cell contents and its surroundings. If a plant or an animal cell is isotonic with its surroundings, no osmosis occurs, and the cells do not change.isotonic with its surroundings Cells in a hypotonic environment Cells in a hypotonic environment An animal cell will gain water and lyse (pop). A plant cell will swell and become turgid (stiff).
Cells in a hypertonic environment Cells in a hypertonic environment An animal cell in a hypertonic solution will lose water and shrivel. This process is called crenation. A plant cell in a hypertonic solution will lose water and lose turgor. This process is called plasmolysis.
Is this cell in a hypotonic, hypertonic, or isotonic environment? Isotonic
Is this cell in a(n) hypotonic, hypertonic, or isotonic environment? Hypotonic
Do Water Molecules Stop Moving in Isotonic Conditions? No. They continue to diffuse, however there is no net movement!
Two solutions equal in solute concentrations so that osmosis does not occur between them are isotonic to each other. However, even in an isotonic solution, water molecules are moving; they move at equal rates in both directions. There is no net movement.
Specific proteins facilitate diffusion across membranes. Facilitated diffusion occurs when a transport protein allows a solute to diffuse. Facilitated diffusion What solutes need facilitated diffusion? Ions The cell does not expend energy, therefore, this process is passive.
Cells expend energy for active transport. Active transport involves the aid of a transport protein Particles move up a concentration gradient (from an area of low concentration to an area of high concentration). Energy is required to help the protein to move the particles. What molecular form does this energy usually have? ATP
Exocytosis and endocytosis transport large molecules. In exocytosis, membrane-bound vesicles (tiny membrane-bounded sacs) containing large molecules fuse with the plasma membrane and release their contents outside the cell.exocytosis
In endocytosis, the plasma membrane surrounds materials outside the cell, closes around the materials, and forms membrane- bound vesicles to contain the materials.endocytosis Two important types of endocytosis are phagocytosis (“cell eating”) and pinocytosis (“cell drinking”).
How Do Large Molecules Move Across a Membrane? Endocytosis or Exocytosis Energy is needed Molecules move using a “pocket” created by the cell “Endo” means “inside”, “exo” means “outside”
What Are The Different Methods In Which Large Molecules May Enter a Cell? Pinocytosis & Phagocytosis Pinocytosis – “cellular drinking” Large molecules in solution are ingested by cell Phagocytosis – “cellular eating” Cell sends out extensions of cytoplasm that surround and engulf large particles