Interaction of Cell Structures (Biology 521) PDF

521 Interaction of Cell Structures Primary Cell Membrane Function: Homeostasis Conditions in the cell must remain more or less constant under many different external conditions in order for the cell to carry on its life functions. The constant state is called homeostasis. The Cell Memb...

521 Interaction of Cell Structures Primary Cell Membrane Function: Homeostasis Conditions in the cell must remain more or less constant under many different external conditions in order for the cell to carry on its life functions. The constant state is called homeostasis. The Cell Membrane Functions Transport materials into the cell Transport products and wastes out of the cell Prevent entry of unwanted matter into the cell Prevent the escape of materials needed to perform cellular functions  Cell Membrane Structure Lipids Phospholipids – Composed of a hydrophilic head and two hydrophobic tails. Cholesterol – Regulates the rigidity of the membrane over different temperature ranges. Proteins Channel or Gate proteins – Serve to allow materials in and out of the cell. These proteins are often specific to certain molecules. Glycoproteins – Proteins that have a carbohydrate chain on them. Often involved in immune response helping cells to identify one another. Together these make up the phospholipid bilayer.  Fluid Mosaic Model This describes the idea that the membrane has a fluid-like consistency that allows each phospholipid to move independently throughout the membrane. Lipids can move in laterally or flip-flop. All membrane components can move freely as if floating on the surface of the ocean The membrane is able to change shape without damaging the cell  Cholesterol in the Membrane Allows the animal cell membrane to exist in a wide variety of temperatures Warmer temperatures Maintains rigidity of the bilayer – holds it together preventing it from melting Colder temperatures Keeps the membrane fluidic, flexible and functional – prevents cell death from a frozen membrane Note Plant cells have a different lipid with a similar function Membrane Proteins Have a variety of functions, and cells may have several types on their membrane surface Some functions Cell-cell recognition Transport Receptor sites Structural support to cytoskeleton  Maintaining Homeostasis The cell membrane is selectively In multicellular organisms, permeable, allowing some every cell is covered in molecules to pass through, while extracellular fluid made up preventing others of mostly water and dissolved Water is the solvent both inside materials and outside the cell, allowing Wastes eliminated by cells (CO2 materials to be easily dissolved or urea) Substances needed by the cells (O2 or water)  Diffusion and the Cell Membrane Passive Transport – Require no cellular energy The movement of molecules from a region of higher concentration to a region of lower concentration is called diffusion. Occurs along a concentration gradient – high to low Reaches a point of equilibrium – no net change in the concentration of a molecule See figure 2.25 on Rate of diffusion increases with temperature – increasing Brownian page 53 motion (vibrating molecules)  Osmosis: The Diffusion of Water The diffusion of water across a semi-permeable membrane When the membrane does not allow the diffusion of materials, water is still able to cross the membrane from high concentration to low concentration  Cellular Tonicity Isotonic Solution Concentration outside the cell is equal to the concentration inside the cell Hypotonic Solution Higher concentration inside the cell which will pulls (attracts) water into the cell, causing the cell to increase in size. Hypertonic Solution Higher concentration outside the cell which pulls (attracts) water out of the Note: Water moves in or out of the cell cell, causing the cell to decrease in size. down the concentration gradient in an attempt to reach equilibrium Facilitated Diffusion Sometimes materials are too These carrier and channel large to diffuse across the proteins are specific to the membrane without assistance, materials that they are or they may not be soluble in transporting (moving) across the lipids, so they cannot dissolve in membrane by size, shape and the lipid bilayer electrical charge These material need help from a Movement of molecules is still protein going down the concentration Passive transport gradient, but now the carrier No cellular energy is required for protein is helping to move them the carrier protein to function Carrier Protein vs. Channel Protein Channel proteins Have a tunnel that allows ions of a specific charge to move in or out of the cell Ex. Na+ or Cl- Carrier protein Changes shape to move specific molecules in or out of the cell Ex. Glucose  Active Transport The process of moving materials backwards up their concentration gradient FROM LOW CONCENTRATION TO HIGH CONCENTRATION ATP is used to activate the transport protein and pump the material out of the cell. Often a specific shape is required to be set into the protein before the ATP will release energy to open the pump. Similar to pushing an object up a hill Where To Find Active Transport* Kidney cells pump glucose and amino acids out of the urine and back into the blood Intestinal cells pump nutrients from the gut Plant root cells pump nutrients from the soil Fish gill cells pump sodium ions out of the body  The Sodium – Potassium Pump A. Carrier protein + has the shape to allow 3 Na (sodium) ions to attach to it B. ATP molecule splits, releasing its energy and becoming ADP + phosphate (which attaches to the protein) C. Energy causes the carrier protein to change shape, releasing sodium out of the cell D. Protein now has attachment points for two K+ ions E. Phosphate is released, which causes protein to return to its original shape F. Shape change causes 2 K+ ions to be moved into the cell Bulk Membrane Transport Sometimes molecules are too The cell membrane is able to fold large or too polar to cross in on itself and engulf material through the cell membrane into a membrane bubble called a The cell uses a specialized vesicle method of getting these Vesicles may be newly formed materials in or out of the cell. and separate into or out of the cell, or they may simple fuse with the membrane releasing its contents out of the cell  Endocytosis The membrane folds in on itself 2. Phagocytosis trapping matter from the extra Cell “eating” cellular fluid within it. Engulfs extracellular fluid containing bits of matter or There are three types of bacteria endocytosis used by cells Process used by macrophages in depending on what it is engulfing the immune system when they 1. Pinocytosis encounter bacteria Cell “drinking” 3. Receptor-assisted endocytosis Engulfs extracellular fluid Involves intake of specific containing dissolved nutrients molecules that attach to special proteins on the cell membrane This membrane receptors are specific to molecules that bind to them like a key into a lock Endocytosis Receptor-Assisted Endocytosis & Cholesterol Animal cells bring in cholesterol using this method of endocytosis The liver produces cholesterol when levels are low Cholesterol is a lipid and cannot dissolve in the extracellular fluid (which is mostly water) Each molecule is surrounded in a single layer of phospholipids, each with a protein tag This protein tag binds to the receptor side on the cell surface triggering endocytosis to begin Exocytosis Reverse of endocytosis A vesicle from the inside of the cell fuses with the cell membrane The contents of the vesicle are excreted (expelled) into the extracellular fluid Very important to the cells of organs that secrete hormones Ex. The pancreas secretes insulin