1.3 & 1.4 Membrane Structure and Membrane Transport

Phospholipids PHOSPHOLIPIDS: are the foundations of the cell membrane. The are amphipathic (are both hydrophilic and hydrophobic) Hydrophilic: polar, water loving head Hydrophobic: nonpolar, water fearing tails, made of 2 fatty acid chains

Lipid Bilayer The phospholipids form a lipid bilayer (2 layers of phospholipids, with the adjacent hydrophobic tails) This prevents the tail from having contact with “water” environments

Fluid Mosaic Model of The Cell Membrane The cell membrane is fluid (ie not rigid) Thus, it can change shape Cholesterol molecules in the bilayer allow for this fluidity (At moderate temperatures, cholesterol makes the bilayer less fluid. At colder temperatures, it makes it slightly more liquid. This helps maintain functionality and strength of the membrane over a wide range of temperatures)

The fluid mosaic model proposes that proteins float in the fluid lipid bilayer (like boats on a pond) Some are anchored and cannot move These tend to be Integral Proteins, which span the bilayer Some can float freely and can move within the membrane These tend to be Peripheral or Surface Proteins, which are found only on one side of the bilayer

Proteins found in the Plasma Membrane

Transport Protein These are protein pores which allow ions and molecules to pass into or out of the cell. They can be: Protein Channels (which do not require energy to transport material) Protein Pumps (require energy to transport material)

Enzymes Biological Catalysts (speed up chemicals reactions) Membranes provide a convenient surface for enzymes to be embedded. Enzymes for related reactions are organized next to each other in order to organize the reactions for greater efficiency.

Cell Surface Receptors Many proteins have a “lock and key” surface to which only specific substances can fit. When the appropriate substance binds to the proteins, it may initiate a reaction in the cell or cause the substance to be brought into the cell (receptor mediated transport) This is how some large food molecules, chemical messengers, or even viruses enter cells

Cell Surface Identity Markers Each cell carries its own ID markers (glycoproteins – proteins with carbohydrate molecules attached to them) These molecules identify the cell as belonging to that individual organism

Cell Adhesion Proteins Allow adjacent cells to stick together via interlocking proteins on their membranes

Membrane Permeability Permeable: all molecules can pass though (ie: pores in the membrane) Non-permeable: nothing can pass through (ie: no pores in the membrane) Semi-permeable: allows only small molecules to pass through (because there are small pores in the membrane) The cell membrane is semi-permeable

Cell Transport How do molecules move through the cell membrane? PASSIVE TRANSPORT Movement of molecules from high to low concentration Does not require energy ACTIVE TRANSPORT Movement of molecules from low to high concentration Requires energy (ATP energy)

Passive Transport Diffusion Random movement of any molecule from high to low concentration (with the concentration gradient)

Animations http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/Osmosis_Animation.htm http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

Passive Transport 2. Osmosis Diffusion of water across a membrane Water moves from high concentrations of water to low concentrations of water OR Water move from low solute concentration to high solute concentration

Osmosis

Terms Associated with Osmosis ISOTONIC (iso = equal / tonic = solute) 2 solutions (inside and outside the cell) with equal concentrations of solute and water. Net flow of water between the 2 is equal. We say both are in equilibrium. H2O diffusion still occurs in both directions at equilibrium

Cells want to reach equilibrium (have an equal concentration of solutes inside and outside the cell) If the concentration of solutes is too high inside the cell or too low, the cell will either take in or release water

Terms Associated with Osmosis HYPERTONIC (hyper = more / tonic = solute) Any solution having more solute (less H2O) than inside cell. Water will flow out of cell so cell shrinks. Ex: If a cell was put into a salt water solution, water would move out of the cell to “dilute” the extracellular fluid

Terms Associated with Osmosis HYPOTONIC (hypo = less / tonic = solute) Any solution having less solute (more H2O) than the cell contents. Water moves into the cell (which has more solute, less H2O) so cell expands. (Think of a hippopotamus with bloated cheeks)

Animations http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

Passive Transport 3. Facilitated Diffusion Passage of molecules from high to low concentration (diffusion) through a protein channel The protein channels are unique in size and shape and only allow particular molecules to move through them. Used when the molecules are too large to diffuse directly through the phospholipid bilayer or do not have the proper charge http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

ACTIVE TRANSPORT Movement of materials AGAINST the concentration gradient (from low to high concentration) Requires ATP enery 1. Endocytosis Movement of molecules INTO the cell

Types of Endocytosis Phagocytosis Movement of large particles into the cell by the formation of vacuoles Cell “eating” This is how some unicellular organisms (ie amoeba) eat Involves the formation of pseudopods (false feet) Usually receptor mediated endocytosis (the molecule entering the cell activates endocytosis by binding to receptors)

http://academic.brooklyn.cuny.edu/biology/bio4fv/page/rectpr.htm http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000098&ptid=17

Types of Endocytosis b) Pinocytosis Cell “drinking” The movement of liquid material or small particles into the cell by vacuole formation http://academic.brooklyn.cuny.edu/biology/bio4fv/page/endocytb.htm

Active Transport 2. Exocytosis Movement of materials out of the cell Vacuole passes plasma membrane to dump contents out of the cell

Active Transport 3. Protein Pump Transport protein that moves materials in or out of the cell (against the concentration gradient) but requires energy (ATP) to be activated Ex: Sodium-Potassium Pump Moves 3 Na+ out of the cell and 2 K+ into the cell

http://highered. mcgraw-hill http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Homework Read about the Davson-Danielli Model (of the membrane) pages 26-28