Chapter 7 7-3 Cell Boundaries

Function of the Cell Membrane Sometimes called the plasma membrane Regulates what enters and leaves the cell Also provides support and protection for the cell

Composition of the Cell Membrane Nearly all cell membranes are composed of a lipid bilayer, which is a double-layered sheet. The lipid bilayer gives cell membranes a flexible structure that forms a barrier between the cell and its surroundings. Proteins (sometimes with carbohydrates attached) are also embedded in the cell membrane with the lipids.

Cell Walls Cell walls lie outside the cell membrane. Present in bacteria, algae, fungi and many prokaryotes. Most cell walls are made from fibers of carbohydrates and proteins. Plant cell walls are composed mostly of cellulose, the principal component of wood and paper.

Function of the Cell Wall The main function of the cell wall is to provide support and protection for the cell. Most cell walls are porous (have holes in them) enough to allow water, oxygen, carbon dioxide, and certain other substances to pass through easily.

Diffusion Through Cell Boundaries The cell membrane is selectively permeable, which means that some things can pass through it and other things cannot. The cell exists in a liquid environment that it needs to survive. The cytoplasm of a cell contains a solution of many different substances in water. Concentration = the amount of solutes dissolved in a solution

Diffusion In a solution, particles move constantly. They collide with one another and spread out randomly. Diffusion is when the particles move from an area where they are more (higher) concentrated to an area where they are less (lower) concentrated.

Diffusion = No energy When the concentration of the solute is the same throughout a system, the system has reached equilibrium. Because diffusion depends on random particle movements, substances diffuse across membranes without requiring the cell to use energy.

Osmosis Some things (like water) are small enough to fit through the channels in the cell membrane, while others are too big to fit (selectively permeable) Water will tend to move across the membrane until equilibrium is established. Osmosis is the diffusion of water through a selectively permeable membrane.

Comparing solutions When two solutions have the same strength or equal particles, they are called isotonic When one solution is more concentrated than the other one, it is called hypertonic, which means it is “above strength”. When one solution is less concentrated than the other one, it is called hypotonic, which means “below strength”.

IMPORTANT RULE! PARTICLES WILL ALWAYS TEND TO MOVE FROM AN AREA OF HIGHER CONCENTRATION TO AN AREA OF LOWER CONCENTRATION IT IS ALWAYS TRYING TO REACH EQUILIBRIUM OR EQUAL AMOUNTS IN EACH SOLUTION (ONE INSIDE THE CELL AND ONE OUTSIDE)

Osmotic Pressure Osmosis can cause problems for cells Cells have salts, sugars, and proteins inside them This means that there will almost always be less water in the cell than outside of it Water will try to move into the cell to balance things out Can cause cells to swell and burst!

Facilitated Diffusion Some molecules (like glucose) are large, but they can go through the membrane too Protein channels in the membrane can help the larger molecules get across Facilitated diffusion is very specific, so a certain protein channel will only help a certain kind of molecule Doesn’t need energy to work

Active Transport Sometimes cells must move materials in the opposite direction (from low to high concentration) Active transport uses protein pumps and needs energy to move the materials Materials moved this way include sodium, calcium, and potassium

Other methods of Active Transport Exocytosis = the vacuole membrane fuses with the cell membrane and gets rid of material Endocytosis = taking material into the cell by folding the cell membrane