THE CELL AND ITS ENVIRONMENT HOMEOSTASIS Homeostasis “ steady state” or balance cells constantly seek homeostasis The Cell Membrane is responsible for maintaining the steady state within cells. Most cells live in some kind of fluid Single celled organisms – ponds, oceans, other bodies!?! Multicellular organism cells are surrounded by body fluid HOMEOSTASIS CONTINUED: Cells have limits to the changes they can survive: Temperature Salt concentrations in water Interstitial fluid – body fluid between cells DIFFUSION Molecules move from a area of high concentration to an area of low concentration Takes place in both solutions and air Gradual spreading out of the molecules The spread out until the reach equilibrium REQUIRES NO ENERGY! DIFFUSION THROUGH MEMBRANES Permeable when membranes allow all substances to pass through Selectively Permeable when membranes allow some substances to pass through, & others not CELL MEMBRANES ARE SELECTIVELY PERMEABLE! Osmosis- diffusion of water through a selectively permeable membrane Remember from high concentrations to low concentrations or… “HIGH to LOW” Q?: What decides what moves into the cell??? A: Compare the solution outside the cell to the solution inside the cell. Hypotonic- a solution having a lower concentration of solutes than the cell Hypertonic- a solution having a higher concentration of solutes than the cell Isotonic- a solution having the same concentration of solutes as the cell RULES FOR DECIDING WHAT WILL HAPPEN TO CELLS… look at the solute concentrations to set up the picture puzzle 2. water moves first and fastest….so 3. look at the water concentrations to decide what will happen to the cell…try some problems …OR “Water Follows Solute” 1. TURGOR PRESSURE term used to describe water pressure within a cell, (usually plant cells) will continue to build up until equilibrium is reached Even if that means EXPLODING! This doesn’t happen often – plant cells are protected by their cell wall PLASMOLYSIS water diffuses out of a cell and turgor pressure is lost plants “wilt” cells shrink CYTOLYSIS when cells burst because water pressure inside the cell is too great This is why sea animals can not live in fresh water (lakes). They are not built to pump out the extra water 2 WAYS SUBSTANCES ARE TRANSPORTED THROUGH MEMBRANES PASSIVE TRANSPORT- the movement of substances through the cell membrane when NO ENERGY taken from the cell Osmosis Diffusion Facilitated Diffusion Has the help of a protein channel These both happen “naturally” SUBSTANCES THAT USE PASSIVE TRANSPORT Water Carbon Dioxide Oxygen Steroids Alcohol Glucose ACTIVE TRANSPORT Transporting substances across membranes USING ENERGY from the cell Examples: Sodium (Na+) Potassium (K+) The cells have high needs of these ions Active Transport moves against natural diffusion Plants have the ability to absorb Na and K when concentrations are higher inside the cell than outside the cell ACTIVE TRANSPORT (CONT.) Endocytosis- taking into a cell Exocytosis- putting out of a cell Phagocytosis- taking in large solid Pinocytosis- taking in or putting out fluid drops excrete or secrete (like sweat!) Cells reach around the particle to be taken in and form a pouch! REVIEW QUESTIONS! Describe the functions of the cell membrane and cell wall. What happens during diffusion? Describe how water moves during osmosis. What is the basic structure of a cell membrane? What is the difference between phagocytosis and pinocytosis? Critical Thinking Comparing and Contrasting What is the main way that active transport differs from diffusion?