The Plasma Membrane and Homeostasis FLUID MOSAIC MODEL

Homeostasis – Maintaining a Balance Cells must keep the proper concentration of nutrients and water and eliminate wastes. The plasma membrane is selectively permeable – it will allow some things to pass through, while blocking other things.

Structure of the Plasma Membrane Lipid bilayer – two sheets of lipids (phospholipids). Found around the cell, the nucleus, vacuoles, mitochondria, and chloroplasts. Embedded with proteins and strengthened with cholesterol molecules.

What’s a Phospholipid? It’s a pair of fatty acid chains and a phosphate group attached to a glycerol backbone. Polar (water-soluble) heads face out and the nonpolar fatty acids hang inside.

Membrane Proteins 1. Determine what particles can pass through the membrane. 2. Serve as enzymes (may speed reactions). 3. Act as markers that are recognized by chemicals and molecules from the inside and the outside of the cell (the immune system).

Cellular Transport Diffusion – movement of particles from an area of high concentration to an area of low concentration. Caused by Brownian motion (movement of particles because of the movement of their atoms). Continues until an equilibrium is reached (no gradient). Dynamic equilibrium – particles move freely and are evenly distributed.

Osmosis Diffusion of water across a selectively permeable membrane. Occurs until water is balanced on both sides of the membrane.

Cell Concentrations 1. Hypertonic solutions (external environment) – more dissolved solute. (less water) 2. Hypotonic solutions (external environment)– less dissolved solute. (more water) 3. Isotonic solutions (external environment) – the same dissolved solute. QUESTION: What happens to the cell in each situation?

ANSWER 1. Hypertonic solutions (external environment) – water leaves the cell. If severe enough, the cell shrivels and dies (plasmolysis) 2. Hypotonic solutions (external environment)– water enters the cell. If severe enough, the cell swells and bursts (cytolysis) 3. Isotonic solutions (external environment) – water passes back and forth equally. There is no adverse effect

Overcoming Osmosis Contractile vacuoles – expel excess water from bacterial cells that live in water. Turgor pressure – water pressure in a plant cell. Loss of turgor pressure causes wilting (plasmolysis).

Cellular Transport Passive transport – no energy is needed to move particles. Facilitated diffusion – embedded proteins act as tunnels allowing particles to “fall” through. Occurs when molecules are too big to fit through the plasma membrane

Ion channels Many ions are not soluble in lipids To enter the cell, they need to go through a protein “tunnel” to get into the cell Examples: Na +, K +, Ca +2, Cl - These protein “tunnels” have “gates” that open or close to allow ions into the cell or to leave the cell Again, this depends on the concentration gradient Stimuli in the cell determine when the gates open or close

Cellular Transport Active transport – energy is needed to move particles. Carrier proteins – embedded proteins change shape to open and close passages across the membrane. This system allows the cell to move substances from a lower concentration to a higher concentration

Example: Sodium-potassium pump The sodium-potassium pump is one of the active transport mechanisms used in the conduction of a nerve impulse. How it works: (open book to pg. 104) – FOLLOW ALONG 1. Three Na + ions (inside the cell) bind to a protein in the cell membrane 2. You must use energy to move the Na + ions out of the cell so an ATP molecule is used (energy molecule) to change the shape of the carrier protein 3. With a phosphate is bound to the carrier protein it has “space” for two K + to bind to the protein

Sodium-potassium pump 4. When the two K + bind to the carrier protein, the protein again changes shape by releasing the phosphate and allows the K + to enter the cell NOTE: Another driving force for the pump is an attempt to maintain a balanced electric charge You lose 3+ so it’s easier to add + into the cell IN SHORT: You need ATP to change the shape of proteins so that molecules can enter or leave the cell.

ENDOCYTOSIS VS EXOCYTOSIS There are two other ways to move substances into and out of the cell: Endocytosis: the cell ingests external substances (macromolecules, external fluid, other cells) The cell membrane engulfs the substance and forms a vesicle The substance inside the vesicle is kept separate from the rest of the cell by the phospholipid bilayer of the vesicle These substances can be transported to the lysosome for digestion or other membrane-bound organelles for other functions

ENDOCYTOSIS – CONT. Types of endocytosis Pinocytosis: this creates a vesicle that is transporting fluids Phagocytosis: creates a vesicle that transports large particles or other cells Example: Your immune system creates a type of phagocyte (cell that digests foreign bacteria) called a macrophage that helps to fight off bacterial infections

EXOCYTOSIS Exocytosis: when a substance is released from the cell by binding a vesicle to the plasma membrane This process is basically the reverse of endocytosis This process is used for Elimination of large molecules from the cell (they are large enough that they would damage the cell membrane if allowed to leave through the plasma membrane) Elimination of toxins that need to be kept separate from cell interior Many endocrine cells use this method to release hormones