Do Now 1. What maintains homeostasis for cells?  THE CELL MEMBRANE 2.What is the term that means when the organelle from #1 only lets certain things in and out? SELECTIVELY PERMEABLE

Getting through the barrier of the cell membrane Some substances pass through the membrane without any energy needed from the cell  called passive transport Some substances need energy from the cell to pass through the membrane  called Active Transport

Passive Transport Passive Transport: movement of molecules across a membrane WITHOUT any energy – Three types: 1. Diffusion 2. Facilitated Diffusion 3. Osmosis The Goal: maintain homeostasis using the cell membrane as a barrier.

Diffusion Diffusion = movement of molecules of a substance from an area of HIGH concentration to an area of LOW concentration  Ex: sugar cube in water food coloring home cooking air freshener

Diffusion There is a difference in concentration of the 2 substances (food coloring & water, water & sugar, air freshener & air) The difference in concentration across space/area  concentration gradient When the concentration of a substance is the same all over  Equilibrium! (molecules still moving… at random)

Diffusion Molecules tend to move DOWN the concentration gradient… – more (hi) concentration  to less (low) concentration

Diffusion Across Membranes Some molecules can diffuse across a membrane (with no aid/help) if they are soluble in lipids – NOT all molecules can do this – Also depends on 1. size & type of molecule 2. structure of molecule

Facilitated Diffusion The movement of molecules with the gradient (to an area of lower concentration) using a carrier protein for passage Carrier proteins are molecule specific, so only one type of molecule can move through them Occurs when regular diffusion cannot occur fast enough as needed (large molecules) Can go either into or out of cell depending on gradient

OSMOSIS  Osmosis  Diffusion of water molecules through cell membrane  Direction of osmosis is determined by the SOLUTE concentration

DIRECTION OF OSMOSIS Depends on the concentrations of solutes & water. THE WATER MOVES WHERE THERE IS MORE SOLUTE!

Hypertonic Solution HIGH concentration of SOLUTE OUTSIDE the cell, LOW concentration of SOLUTE INSIDE the cell RESULT: WATER MOVES OUT OF THE CELL.

HYPOTONIC LOW concentration of SOLUTE OUTSIDE the cell HIGH concentration of SOLUTE INSIDE the cell, RESULT: – WATER MOVES INTO THE CELL.

ISOTONIC inside and outside concentrations are equal. equal concentrations of solute & water outside and inside the cell RESULT: water diffuses in and out at constant rate, no net movement

Osmosis in Cells

OSMOSIS

How does osmosis affect cells? Many cells live in isotonic solutions so equilibrium is easily attained But some do not: – Some unicellular freshwater organisms – Plant cells

Unicellular freshwater organisms Organisms live in hypotonic environments (lots of salt inside their cells) so they constantly take in water. If too much water enters  cell dies, called cytolysis They have an organelle called a contractile vacuole = that collects & pumps out the excess water. This requires energy to be expended

Contractile Vacuole of Amoebae

Osmosis in Plants Plant cells also live in hypotonic solutions, so water enters the cell.. They like it Why?... Osmosis continues until vacuole is full & the cell membrane is forced against the cell wall, called  turgor pressure When a plant cell loses turgor pressure, the membrane collapses away from the cell wall, causing the plant to wilt, called  plasmolysis

Plasmolysis