Cell Membranes How substances move into and out

Cell Membranes How substances move into and out of the cell

Objectives: � 1. Identify the main functions of the cell membrane � 2. Describe what happens during diffusion � 3. Explain the processes of osmosis, facilitated diffusion, and active transport

Cell Membrane � Regulates what enters and leaves the cell and also provides protection and support � In: food and water, Out: waste � Lipid bilayer = strong, flexible barrier ◦ Lipids – hydrophobic tails face each other ◦ Proteins – some are channels and pumps ◦ Carbohydrates – attached to surface of some proteins for recognition

Diffusion � Moves substances across the membrane without use of energy � The cytoplasm inside of cells is a solution (liquid mixture of substances) � Substances dissolved in liquid = solutes

Diffusion � Concentration = amount of solute per volume of solution (example: g/L) � Molecules are moving, and diffuse from high concentration to low concentration = diffusion � Equilibrium = same concentration of solute throughout solution

Osmosis � Membranes are “selectively permeable” = some substances can pass through, others cannot � Osmosis = the diffusion of water through a selectively permeable membrane (through proteins called “aquaporins”) � Water moves in both directions, but the net movement is in the direction of the most solute particles

Osmosis

Osmosis � Osmotic pressure is handled in a number of ways… � Cells are surrounded by isotonic solution (blood) � Cells are surrounded by a cell wall (plants) � Cells use a contractile vacuole to pump excess water out of the cell (single-celled organisms)

Movement of Substances Across the Membrane � Protein channels allow substances to cross that normally could not ◦ Ions, sugars, salts, etc. ◦ Net movement from higher to lower concentration ◦ No energy required Facilitated Diffusion Movement against the concentration gradient � Requires energy input � � “Pumps” (for small molecules or ions) Endocytosis = taking in large amount of material by folding inward � Exocytosis = removal of large amounts of material by a vacuole fusing with the membrane � Active Transport