Transcript The cell
The cell Chapter 3 Cell membrane composition • Cell membrane is the boundary between a cell and the outside environment and controls the passage of materials into and out of the cell • Made of a double layer of phospholipids along with a variety of other materials. • A phospholipid is made of a charged phosphate group, a glycerol, and two fatty acid chains • Phosphate and glycerol make up the head • The fatty acids make up the tail • The head contains a charge and is therefore a “polar” portion • This allows the head to form hydrogen bonds with water • The tails are “nonpolar” and can’t form hydrogen bonds • Some of the other molecules attached to the membrane are: Cholesterol, Proteins, and Carbohydrates • The cholesterol helps to strengthen the membrane • Proteins help to allow material to pass through the membrane, while others are components of the cytoskeleton • Carbohydrates serve as ID tags for the cell and help with cellular communication. Fluid Mosaic model • Model that was developed by scientist to help describe the cell membrane. • The fluid aspects helps to describe that the membrane is flexible and not rigid • The mosaic aspects helps to describe how the membrane is made of many different things Selective permeability • The cell membrane also shows selective permeability which allows certain things to pass through the membrane while keeping others out • This property allows the cell to maintain homeostasis • Material can pass the membrane in a few ways. Some of these require energy while others do not. • This depends on the size, polarity, and concentration of the material in the cell. Chemical signals transmitted across the membrane • In order to receive signals the cell must have receptors. • These receptors bind with a certain molecule at a certain time to insure the correct action. • The molecule a receptor binds to is called a LIGAND. • Once they bind the receptor will change shape and this is important because it effects how the receptor will react with other molecules. 2 major receptors • Intracellular and Membrane • Intracellular- will be inside the cell. Many hormones fit into this category. These can only react in certain cells and will cause a certain reaction to occur • Membrane- Will be located on the outside of the cell. Will bind and then send a signal to the inside of the cell. The entire receptor changes shape when this occurs. Starts a chain of events inside the cell 3.4 Diffusion and Osmosis • Diffusion and Osmosis are both examples of passive transport. • Passive transport is when there is no ATP needed in order for a process to take place. • Diffusion is the movement of molecules in a fluid or gas from an area of higher concentration to an area of lower concentration. • As this happens the molecules will move down the concentration gradient . • In cells, diffusion plays an important role in molecules passing across the cell membrane. • The end result of diffusion is for the particles to end up in a state of equilibrium. Osmosis • Is very similar to diffusion except it is dealing exclusively with water. • In this case the higher the number of dissolved particles in a solution will effect the concentration of water. • 3 states of osmosis: ISOTONIC, HYPERTONIC, HYPOTONIC • Isotonic- same concentration in the cell and in the solution • Hypertonic- solution has a higher concentration of dissolved particles than cell. Water will flow out of the cell and cause the cell to shrink. • Hypotonic- solution has a lower concentration than in the cell. Water will move into the cell and cause the cell to swell. Transport proteins • The use of proteins in order to bring material into the cell is called Facilitated Diffusion. • This is still a form of passive transport even though proteins are being used. • Most protein channels only allow certain molecules to pass through such as ions. Active transport, Endocytosis, and Exocytosis • Some times we must move material across the membrane even if it goes against the concentration gradient. • When this occurs we must use pumps to bring the material across the membrane. • In this process energy must be used and this is called ACTIVE TRANSPORT • All transport proteins will span the membrane. • Most will change shape when they bond to a target molecule. • Can bind to one type of molecule or many • Can move in one direction or in multiple directions. • Must use ATP as energy • Ex: Sodium Potassium pump, Proton pump Endocytosis • Process of bringing material into the cell by engulfing them in the membrane. • Steps of endocytosis: • Cell membrane makes a pocket around the substance • Pocket breaks off inside the cell and forms a vesicle • Vesicle attaches with Lysosome in order to break the material down for use. • Phagocytosis is the process of the cell engulfing large particles. • The term literally means “cell eating” • This process is used by the white blood cells in the immune system to fight off infection and disease Exocytosis • Opposite of Endocytosis • Steps of endocytosis: • Vesicle will form around the material in the cell • Fuse with the cell membrane • Material is removed from the cell Used by nerve cells in order to have muscles move. Impulses travel as chemical signals that are then transported by vesicles from one nerve cell to another