UNIT 3 BIOLOGY UNIT 3 TOPICS • Cell membranes and organelles • Protein synthesis • Gene regulation • Enzymes • Photosynthesis • Respiration • Cellular Signals • Infection, pathogens, and defence mechanisms • Immunity UNIT 4 TOPICS • Mutations • Changes in populations over time • Changes in biodiversity over time • Methods to determine relatedness between species • Hominin evolution • DNA manipulation and genetic technologies RESOURCES AND TASKS TO COMPLETE • PowerPoints available on Compass • Learning tasks on Compass • Edrolo videos • Douchy’s Biology Podcast • Past exam questions (one set per topic) THE PLASMA MEMBRANE KEY KNOWLEDGE BLAH BLAH • The fluid mosaic model of the structure of the plasma membrane and the movement of hydrophilic and hydrophobic substances across it based on their size and polarity KEY KNOWLEDGE • Purpose and function of the plasma membrane • Structure of the plasma membrane • Phospholipids • Membrane proteins • Methods of crossing the plasma membrane • Factors affecting transport across the membrane • Surface-area to volume ratio PURPOSE OF THE PLASMA MEMBRANE • Protect the cell • Keeps the internal contents together • Keeps out foreign molecules • Communication with other cells • Contains markers to identify the cell • Control the osmotic environment of the cell • Let certain substances in and out THE FLUID MOSAIC MODEL • The fluid mosaic model describes the structure of the plasma membrane • The plasma membrane is composed of: • Phospholipids • Membrane proteins • Sterols (cholesterol in animal cells, also other sterols in plants and fungi) • Fluid because the phospholipids can move around, and the membrane can break and reform • Mosaic because proteins embedded within the phospholipids form mosaic-like patterns THE FLUID MOSAIC MODEL PHOSPHOLIPIDS • Composed of a hydrophilic, polar head and a hydrophobic, nonpolar tail • Hydrophilic head contains phosphate • Hydrophobic tail is made of fatty acid chains MEMBRANE PROTEINS RECEPTOR PROTEINS • Receive signal molecules (ligands) from other cells • These proteins are usually just called receptors • For example, some receptors receive signals from other cells in the form of hormones, which triggers the cell to do something • Each receptor is specific for the signal molecule it receives RECOGNITION PROTEINS • Called MHC proteins • Act as markers to identify the cell as belonging to the individual • This prevent the body’s immune system from attacking the cell • More on that in AOS 2… ADHESION PROTEINS • In multicellular organisms, adhesion proteins link cells together CHANNEL PROTEINS • Allow for transport across the cell membrane • Form narrow passageways within the cell membrane through which molecules can move • Cross the plasma membrane CARRIER PROTEINS • Also allow for transport across the cell membrane • Bind to a specific molecule on one side of the plasma membrane • After binding, the protein changes its shape and releases the molecule on the other side METHODS OF CROSSING THE PLASMA MEMBRANE SIMPLE DIFFUSION • Movement of a substance across the membrane from an area of high concentration to low concentration • Moves down its concentration gradient • No energy required • No transport proteins required – the molecules freely move through the phospholipid bilayer • Small, non-polar, non-charged molecules do this pretty well OSMOSIS • Simple diffusion of water • It’s actually kind of hard for water to move through the plasma membrane, as it is repelled by the hydrophobic fatty-acid tails • But as it is a small molecule, if there is a strong enough concentration gradient it can move through KNOW THESE! HERE’S A BETTER EXAMPLE FACILITATED DIFFUSION • Some substances can’t cross the membrane even though there is a concentration gradient • They may be too large, polar, charged, etc • Facilitated diffusion is the movement of a substance down its concentration gradient with the assistance of carrier proteins or channel proteins • Example: Glucose requires carrier proteins • Example: Most ions, like magnesium and sodium, require channel proteins. Sometimes they are referred to as ion channels in this case ACTIVE TRANSPORT • The movement of a substance against its concentration gradient • Requires carrier proteins (always carrier proteins, not channel proteins, for active transport) • Requires energy in the form of ATP • Example: Movement of sodium and potassium ions across the membrane of nerve cells, to build up an action potential BULK TRANSPORT • The movement of molecules that are too large to pass through the membrane or via a membrane protein • Cells make vesicles to transport these substances across the membrane – a vesicle is a small membrane-bound package • The membrane for the vesicle comes from the cell membrane • Bulk transport requires energy TYPES OF BULK TRANSPORT • Endocytosis is bulk transport into the cell • Phagocytosis is bulk transport of solids into the cell • Pinocytosis is bulk transport of liquid into the cell • Exocytosis is bulk transport of solid or liquid out of the cell ENDOCYTOSIS FACTORS AFFECTING MEMBRANE TRANSPORT • Size • Polarity • Hydrophobic/hydrophilic • Charge • Concentration gradient MOLECULAR SIZE Phospholipid tails repel charged particles High concentration Low concentration SUMMARY • The plasma membrane protects and contains cell contents, denotes cell identity, and communicates with other cells • It consists of a phospholipid bilayer, membrane proteins, and sterols • Facilitated diffusion and simple diffusion do not require energy • Active transport and bulk transport require energy • Osmosis is simple diffusion of water • Size, charge, polarity and size of concentration gradient affect the ease at which a substance will cross the plasma membrane
