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 & OsmosisDiffusion & 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 (O2, CO2), 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 concentrationIsotonic = 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