Diffusion, Osmosis, and Active Transport

The Cell Membrane Recall that the cell membrane is the structure found in both plant and animal cells that controls the movement of materials both into and out of the cell

Functions of the Cell Membrane 1. Separates the contents of the cell from the outside environment 2. Serves as a barrier for which substances can enter and exit a cell 3. Recognizes chemical signals (messages) which will trigger the cell to react in a particular way

Selective Permeability The cell membrane is considered selectively permeable, which means that some molecules can pass through, and some molecules cannot It maintains balance both inside and outside the cell

Selective Permeability

Structure of the Membrane Fluid Mosaic Model- a model of the cell membrane The membrane is a double lipid layer (bilayer) with large proteins embedded in it

Structure of the Cell Membrane Made up of a double layer of “phospholipids” A phospholipid is made of two parts: Phosphate head- hydrophilic, or “water-loving” Lipid Tails- hydrophobic, or “water-fearing” (think oil, a substance that does not dissolve in water)

Structure of the Cell Membrane The “water-loving” phosphate heads face the outside environment and the inside of the cell The “water-fearing” lipid tails face each other on the inside of the membrane

Structure of the Cell Membrane Proteins are embedded within the membrane because most molecules cannot easily pass through the “water- fearing” lipid tails A protein channel enables molecules to pass through without interacting with the lipids

Passage of Materials through Cell Membranes In addition to receiving information, cells need a way to move molecules and other substances through their membranes Some molecules, such as water, pass freely. This is called passive transport. Others that do not pass freely must be carried through channels. This is called active transport

Diffusion Movement of molecules from an area of HIGH concentration to an area of LOW concentration Form of passive transport because NO ENERGY is required! Diffusion Animation

Facilitated Diffusion Movement of molecules down their concentration gradient with the help of transport proteins embedded within the cell membrane No energy is required! Animation

Osmosis The diffusion of water through a membrane Water molecules move from an area of high concentration to low concentration NO ENERGY is required

Hypotonic Solution When the inside of a cell has a much higher salt (solute) concentration than the outside of a cell Water will diffuse into the cell to “dilute” the inside contents Result = Cell Swells and sometimes bursts (lyses) Video Demo

Hypertonic Solution When the outside of a cell has a much higher salt (solute) concentration than the inside of a cell Water will diffuse out of the cell to “dilute” it’s surroundings Result = Cell Shrivels (shrinks) Video Demo (Start at 3:10)

Isotonic Solution When the solute concentration outside the cell is equal to the solute concentration inside the cell No net movement of water occurs! Video Demo (Start at 4:25)

Summary

Name that solution! IsotonicHypertonicHypotonic

Active Transport Movement of molecules through a cell membrane from an area of LOW concentration to an area of HIGH concentration Always requires ENERGY (ATP)! Often uses a protein “pump” embedded within the membrane to move molecules

Types of Active Transport Endocytosis- cells take in large particles by engulfing them- cell membrane pinches in to form a vesicle Exocytosis- cells get rid of particles by enclosing them in a vesicle (a membrane) and fusing that membrane with the cell membrane

Passive vs. Active Transport Passive Transport Active Transport Does not require energy Diffusion, Osmosis Water, Oxygen, Carbon dioxde Like riding a bike downhill! Requires Energy Protein pumps, endocytosis, exocytosis Large and charged particles! Like riding a bike uphill!