Chapter 3: Exchanging Materials with the Environment

3.3 Diffusion & Osmosis Diffusion The movement of molecules from an area of higher concentration to an area of lower concentration

3.3 Diffusion & Osmosis Why does diffusion happen? Diffusion Animation Concentration gradient – a difference in concentration of molecules Molecules are described as moving down a concentration gradient (from high to low)

3.3 Diffusion & Osmosis A concentration gradient forms across a cell membrane because the membrane is selectively permeable, so certain molecules are not allowed to pass freely through the membrane. This can store a great amount of potential energy (if a great concentration gradient builds up and then movement of the molecules is allowed)

3.3 Diffusion & Osmosis Osmosis A special type of diffusion, in which water moves across a selectively permeable membrane Osmosis Animation

How does this differ from an animal cell? 3.3 Diffusion & Osmosis Osmosis Osmotic pressure = important for plant cells They have a vacuole & when water flows into the vacuole, it exerts pressure on the cell wall, giving the cell firmness  TURGOR PRESSURE How does this differ from an animal cell?

3.3 Diffusion & Osmosis Osmosis An animal cell can burst  CYTOLYSIS  if placed in pure water, causing water to flow into the cell. So, why does water move into or out of the cell?

Solutions Both the internal & external environment of cells can be considered solutions. Solutions are made of solute and a solvent. Solvent – liquid into which something (the solute) is dissolved (usually water) Solute - substance that is dissolved or put into the solvent; “STUFF” Example – Lemonade; Water = Solvent Sugar = Solute

Tonicity (Amounts of solute) ~Relative Term  Must be comparing 2 environments Hypotonic Solution – the solution that has a lower concentration of solute than another. Hypertonic Solution – the solution that has a higher concentration of solute than another. Isotonic Solution - both solutions have same concentrations of solute.

Tonicity & Osmosis

3.4 Active & Passive Transport 2 Types of transport across a cell membrane: 1. Passive transport – processes that take no energy; molecules move freely through the membrane based on concentration gradients (diffusion & osmosis) 2. Active transport – processes that require energy in order to carry molecules across the membrane

Passive Transport In addition to diffusion & osmosis: Facilitated diffusion – the movement of molecules down the concentration gradient (like diffusion) but they use protein channels to move No energy is needed

Active Transport Used if cells have to transport a substance across the cell membrane against the normal concentration gradient. Membrane proteins actively pump molecules in the direction that the cell wants them to go. This process requires energy.

Active Transport Two special types for large molecules: Exocytosis – the removal of waste materials from cell in which the cell membrane folds around the waste to transport it out Endocytosis – the formation of a pocket of the cell membrane to bring materials (food) into a cell

Cell Membrane Transport

Endocytosis & Exocytosis