- Slides: 16
II. The Cell Membrane
A. Cell Membrane Function 1. Transport Proteins are selectively permeableregulate what materials enter and exit the cell 2. Enzymes are often attached to membranes with the active sites exposed 3. Receptor Proteins (signal transduction)- receive information from a chemical messenger 4. Cell-cell Recognition- glycoproteins sever as ID tags that are recognized by other cells 5. Intercellular Joining- membrane proteins join to cells together 6. Attachment Proteins- attach to the cytoskeleton that maintains the cell’s shape
B. Membrane Structure Phospholipid Oligosaccharide Chains Enzyme Proteins Cholesterol
Cell Membrane Structure 1. The cell membrane consists of a phospholipid bilayer in which globular proteins are embedded. 2. The phospholipids are amphipathic - they have both hydrophilic and hydrophobic ends. Membrane is stabilized by hydrogen bond which causes membranes to be self sealing
3. Phospolipids are Fluid -The lipids and protein are free to move within the plane of the membrane. 4. Two types of Membrane proteins: a) Integral Proteins - extend through one or both lipid layers. b) Peripheral Proteins- on membrane surface
5. Cholesterol molecules are found in the hydrophobic lipid layer of the membrane. Cholesterol helps to maintain membrane structure: a) Cholesterol acts like a speed bump to slow down phospholipids when they are moving to fast due to increased heat b) Cholesterol helps spread out phospholipids when they are cold and prevents the hydrophobic tails from sticking together
6. Membrane Carbohydrates Allow Cell Recognition a) Cells recognize each other by the pattern of oligosaccharides attached to the cell membrane 1) Glycolipids – carbohydrates attached to lipids 2) Glycoproteins - carbohydrates attached to proteins b) Oligosaccharides- contain 15 or fewer sugars
C. Passive Transport Mechanisms- move molecules across the membrane using no ATP 1. Diffusion- movement of molecules from areas of high concentration to areas of low concentration. Diffusion is due to Brownian Movement -random motion of small particles. Factors that affect diffusion are: a) Concentration - the higher the concentration of solute, the greater the rate of diffusion b) Temperature - increased temperature increases the kinetic energy of the particles causing them to move faster and collide more thus increasing diffusion c) Pressure - the more particles in a fixed volume, the greater chance for molecular collision, therefore increasing diffusion
1. 2. Facilitated Diffusion- the use of protein channels to move specific molecules across the membrane with their concentration gradient. Transport may be: a) Uniport- one molecule or ion one way. b) Symport- two different solutes one way c) Antiport- two different types of molecules are transported opposite ways.
1. 3. Osmosis-diffusion of water from an area of high water concentration to an area of low water concentration. 2. a) The concentration of dissolved solutes and water is inversely proportional. Therefore, diffusion and osmosis are opposites. OSMOSIS High Solute concentration High H 2 O concentration Low solute concentration b) Cells may be in One of Three Conditions: DIFFUSION
1) Isotonic Environment- equal amount of solute and water on both sides of a membrane Cell 5% solute 95% H 2 O Beaker with 5% solute and 95% water Water will move equally in and out of the cell
2) Hypertonic Environment- more solute outside the cell and more water in the cell Cell 5%Shriveled solute 95% H 2 O Beaker with 10% solute and 90% water Water will move out of the cell
3) Hypotonic Environment- less solute and more water outside the cell. Cells with Cell no 5% cell wall solute Beaker with 0% solute and 100% water Burst! 95% H 2 O Water will move into the cell
D. Water balance in cells: 1. Cells lacking cell walls must be isotonic with their environment or have a mechanism to remove water hypotonic situations. Ex. Contractile vacuoles 2. Plants, prokaryotes, fungus, and some protists have cell walls a) In hypotonic solution they are turgid which is ideal and gives them support b) In isotonic solutions they are flaccid or wilty c) In hypertonic solutions plant cells plasmolyze. This is lethal. The cell membrane is pulled from the cell wall.
E. Mediated Transport Systems transport systems that require energy. 1. Active Transport- using transport proteins to bind with specific substances to move them across the cell membrane AGAINST the concentration gradient. Example: Sodium- potassium pump 2. Ion Transport- diffusion of charged solutes depend on a chemical and electrical gradient called the electrochemical gradient a) b) Electrochemical gradient determines the net direction of ionic diffusion Membrane Potential- the voltage across a membrane is determined by ion distribution. The sodiumpotassium pump and proton pump generate voltage across a membrane.
3. Transport of Large Molecules a) Endocytosis- movement of large particle into the cell 1) Phagocytosis - engulfing solids 2) Pinocytosis- engulfing liquids for the desired solutes 3) Receptor Mediated Endocytosis- receptor sites on membrane proteins bind with specific substrates in an area called a coated pit 1) when an appropriate number of substantes have been captured, the coated pit is engulfed 2) The cell can aquire bulk quatities of specific solutes this way b) Exocytosis - moves large materials out of the cell