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. Amphipathic: hydrophobic & hydrophilic regions Singer-Nicolson: fluid mosaic model

COMPONENTS OF CELL MEMBRANE Phospholipids: membrane fluidity Cholesterol: membrane stabilization “Mosaic” Structure: Integral proteins: transmembrane proteins Peripheral proteins: surface of membrane Membrane carbohydrates : cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins

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 Transport: what can enter/leave cell. Serve as enzymes Signal transduction (ie. Hormones) Intercellular joining Cell-cell recognition (T-cells) ECM attachment

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 Hypertonic solutions – more dissolved solute. (less water) Hypotonic solutions – less dissolved solute. (more water) Isotonic solutions – the same dissolved solute. QUESTION: What happens to the cell in each situation?

Osmoregulation Osmoregulation: control of water balance Hypertonic: higher concentration of solutes Hypotonic: lower concentration of solutes Isotonic: equal concentrations of solutes Cells with Walls: Turgid (very firm) Flaccid (limp) Plasmolysis: plasma membrane pulls away from cell wall

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 – (also known as passive diffusion) no energy is needed to move particles. Facilitated diffusion – embedded proteins act as tunnels allowing particles to “fall” through. Requires the use of transport proteins

Ion channels: specialized transport proteins 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. 135) 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 SHOWS HOW YOU CAN COUPLE TRANSPORTS TO SAVE ENERGY

Sodium-potassium pump

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 Receptor-mediated endocytosis: ligands (molecules that bind to a specific receptor site) induce endocytosis

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

WATER POTENTIAL On the AP Exam, you will have to understand a couple of formulas that deal with water potential: Ψ = Ψ P + Ψ S Ψ = Free energy associated with water potential Ψ P = Pressure potential (force from water pressure) Ψ S = Potential dependent on the solute concentration (how many particles of material are in solution

WATER POTENTIAL Water always moves from an area of high water potential to low water potential (osmosis) In an open beaker (atmospheric pressure only) of PURE water, the water potential is zero (Ψ = 0) No difference is solute concentration No external pressure (gravity, turgor pressure, etc) Increasing the Ψ P (pressure potential), increases the water potential (Ψ > 0) The water wants to “move” to an area of lower pressure (potential)

WATER POTENTIAL Increasing the Ψ S (solute potential), lowers the water potential If you put more particles in solution, you make the solution hypertonic The water wants to enter the system to equalize the water potential (Ψ < 0) The total water potential results from a combination of water pressure and solute concentration

SOLUTE POTENTIAL Solute potential (Ψ S ) has its own formula Ψ S = -iCRT i = ionization constant (different based on the material used for the solute) C = the molar concentration (molarity = moles/L) R = pressure constant ( liters*bars/mole*K) T = temperature in Kelvin (K = ̊C + 273) Bar = 1 atm (at sea level)

EXAMPLE PROBLEM A sample of 0.15M sucrose at atmospheric pressure (Ψ P = 0) and 25 ̊C has what water potential? Since sucrose dose not break apart into ions (i = 1).

SOLUTION Ψ = ? Ψ P = 0 Ψ S = -iCRT i = 1 C = 0.15M R = liters*bars/mole*K T = 25 ̊C = 298K Ψ = Ψ P +Ψ S Ψ = o +Ψ S Ψ S = -iCRT Ψ S = - (1)(0.15M)(0.0831)(298K) Ψ S = -3.7 bars Ψ = o +Ψ S Ψ = o -3.7bars Ψ = -3.7bars