TRANSPORT THROUGH CELL MEMBRANES Diffusion and Osmosis

Learning Outcomes: B9 - Analyse the structure and function of the cell membrane: describe passive transport processes including diffusion, osmosis, and facilitated transport explain factors that affect the rate of diffusion across a cell membrane predict the effects of hypertonic, isotonic, and hypotonic environments on osmosis in animal cells

Recall that the cell membrane is selectively permeable, allowing only certain things to enter and exit the cell Several mechanisms are involved in the movement of materials in and out of the cell Some of these are passive (do not require energy), while others are active (require energy)

Diffusion & Osmosis Passive - don’t require energy - “goes with the flow” Movement of substances with the concentration gradient (from high to low concentration)

Diffusion Movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached

Substances that can pass freely through the cell membrane will do so by diffusion, if a concentration gradient is present (higher concentration on one side of the membrane) Examples - gases (O 2, CO 2 ), lipid-soluble (non- polar) molecules

Rate of diffusion Factors that affect the rate of diffusion: Temperature Size of molecules Concentration gradient (how would each of these affect the rate?)

Osmosis Is the diffusion of water across a differentially permeable membrane Water moves in and out of cells by osmosis through the cell membrane

Membrane requirements Membrane must be permeable to water but impermeable to the solute Must have different solute concentrations on either side of the membrane

Water moves toward higher solute concentration (ex. toward the salt) Solute stays where it is (“salt sucks!”)

Osmosis in cells

Osmotic pressure is pressure due to movement of water across a membrane This is important in the functioning of many living systems (ex. circulatory system)

Tonicity Refers to the relative solute concentrations of solutions in two places Determines the direction of water movement (ex. In or out of cells)

Hypertonic = higher concentration Hypotonic = lower concentration Isotonic = equal concentrations These terms are used to compare two solutions

Cell in a hypertonic solution Solute concentration is greater outside the cell Net movement of water OUT of the cell (cell shrinks)

Cell in a hypotonic solution Solute concentration is less outside than inside the cell Net movement of water INTO the cell (cell expands and may burst)

Cell in an isotonic solution Solute concentrations are equal inside and outside of the cell No net movement of water (equal movement in both directions)

Animal and Plant Cells

Blood Cells