PASSIVE TRANSPORT One way cells maintain homeostasis is by controlling the movement of substances across their cell membrane. Cells want to reach “equilibrium”. Equilibrium – is a condition in which the concentration of a substance is equal throughout a space

Equilibrium:

Passive Transport – the movement of substances across a cell membrane that does not require energy from the cell **passive transport does not require energy because substance are moving along or down the concentration gradient concentration gradient – the difference in the concentration of a substance across a space

1. Diffusion – the movement of a substance from an area of high concentration to an area of lower concentration (substance)

2. Osmosis – the diffusion of water through a selectively permeable membrane

Three possibilities for the direction of water movement:  hypertonic – the fluid outside the cell has a higher concentration of dissolved particles (or less water molecules) than the inside of the cell therefore, water molecules leave the cell (diffuse out) and the cell shrinks  hypotonic – the fluid outside the cell has a lower concentration of dissolved particles (or more water molecules) than the inside of the cell therefore, water diffuses into the cell and the cell swells  isotonic – the inside of the cell and the fluid outside have the same concentration of free water molecules

Types of Solutions Lower concentration of particles (or more water molecules) outside the cell. Higher concentration of particles (or less water molecules) outside the cell. Equal concentration of particles (and water molecules) inside and outside the cell. 90% Water 80% Water

Increased Particles Increased Particles Equal Particles Effects of Hypotonic, Isotonic, and Hypertonic Solutions Decreased Particles Equal Particles Decreased Particles

3. Transport Proteins allow molecules that can’t pass the membrane by simple diffusion to pass. ex: ion channels – they are a transport protein which allow ions to pass

Some transport proteins are referred to as carrier proteins because they bind to specific substances on one side of the cell membrane then carry the substance across releasing it on the other side. This is called facilitated diffusion.

Passive Transport Summary:  Diffusion (diffusion of any substance)  Osmosis (diffusion of water)  Transport Proteins or Carrier Proteins The movement of a substance down a concentration gradient, from high to low concentration, therefore no energy is required from the cell! Three Examples:

Only molecules that are either very small, can dissolve in fluid, or are nonpolar can diffuse across the cell membrane. Other molecules enter the cell through transport proteins. Because only some substances are allowed to pass through the cell membrane, the cell membrane is said to be selectively permeable or semipermeable.