2. - All cells are surrounded by a cell membrane -A.k.a. “plasma membrane” -Cell membrane = thin, flexible barrier -Regulates what enters and exits cell -“selectively permeable” -Provides protection and support -Made of two layers of lipids: lipid bilayer

4.  Lipid bilayer  Protein molecules with attached carbohydrates ◦ Proteins form channels and pumps ◦ Move material across membrane ◦ Carbohydrates = “chemical identification cards”  Allows cell to identify one another

5. - Some cells have a cell wall -Strong supporting layer -Porous -Lies outside cell membrane -Found in plants, algae, bacteria

6.  Made from fibers of protein and carbohydrates  Plant cell walls are made of cellulose ◦ Cellulose is a carbohydrate fiber  Makes up wood and paper

7.  Liquid environments  Allow cells to survive  What is the Most Important Function of the Cell Membrane? ◦ To regulate movement of dissolved molecules from the liquid on one side of the membrane… ◦ …to the liquid on the other side of the membrane

8.  Solution ◦ Mixture of two or more substances  Cytoplasm ◦ Gel-like area outside of nucleus ◦ Within cell membrane ◦ Contains a solution of many different dissolved substances in H 2 O  The dissolved substances in the solution are “solutes”

9.  The mass of a solute in a given volume of solution ◦ Mass/volume ◦ Example:  If you dissolved 12 grams of salt in 3 liters of water, what would the concentration be?  Which is more concentrated – 3 grams/liter or 4 grams/2 liters?

10.  In a solution, particles  Move constantly  Collide with each other  Spread out randomly  This leads to…  Diffusion ◦ Particles move from an area of high concentration to an area of low concentration ◦ Does not require energy

11.  Equilibrium ◦ When concentration of solute is the same throughout a system

12.  How do diffusion and equilibrium work within cell membranes?  Not all substances can cross the cell membrane ◦ If a substance can cross the cell membrane, particles will move toward the area where it is less concentrated  until equilibrium is reached

13.  Even when equilibrium is reached… ◦ particles of a solution will continue to move across the membrane in both directions  However, because almost equal numbers of particles move in each direction, there is no further change in concentration  Remember, diffusion does NOT require energy

15.  If a substance can diffuse across the cell membrane…  The membrane is PERMEABLE to the substance  If a substance cannot diffuse across the cell membrane…  The membrane is IMPERMEABLE to the substance  Most cell membranes are selectively permeable ◦ Some substances can cross, others cannot

16.  A.k.a. “semipermeable”  Many substances can cross biological membranes  However, if the substance is too large or highly charged, it will not cross

17.  Water can diffuse through the cell membrane  Osmosis ◦ The diffusion of water through a selectively permeable membrane ◦ Water will tend to cross the membrane until equilibrium is reached

19.  Isotonic – concentration of two solutions is the same  When concentrations of two solutions are NOT the same: ◦ Hypertonic – the solution with the greater concentration of solutes “above strength” ◦ Hypotonic – the solution with the lower concentration of solutes “below strength”

20.  Osmosis ◦ Net movement of water across a selectively permeable membrane ◦ Driven by difference in solute concentrations on the two sides of a cell membrane ◦ KEY DIFFERENCE FROM DIFFUSION: Water flows from solution with lower solute concentration to solution with higher solute concentration

22.  Cells placed in ◦ Isotonic solutions  Cells neither gain nor lose water ◦ Hypertonic solutions  Animal cells shrink, plant vacuoles collapse ◦ Hypotonic solutions  Animal cells burst, plant vacuoles swell and push contents of cell against cell wall

23.  Fortunately, cells in large organisms are not in danger of bursting ◦ Most animal cells do not come into contact with fresh water  This would make too much water enter the cell because solute concentration is higher inside the cell  Remember, water goes where solute concentration is higher ◦ Most animal cells are bathed in isotonic solutions  Example: blood  Has concentrations of dissolved materials that are similar to that of the cell

24.  Plants and bacteria DO come into contact with fresh water ◦ Cell walls protect against osmotic pressure  Prevent cells from expanding  However, increased osmotic pressure makes cells vulnerable to injuries to their cell walls

25.  Lipid bilayer normally keeps large and highly charged molecules out  Glucose passes through cell membrane even though it is large and highly charged  Why is this?

26.  “Facilitate” means “to help”  Cell membranes have protein channels ◦ Protein channels act as carriers ◦ Help glucose molecules cross cell membrane ◦ Also known as “carrier facilitated diffusion” ◦ Protein channels can help other molecules diffuse across cell membrane

27.  Because it is a form of diffusion… ◦ Molecules will only cross the cell membrane if there is a higher concentration of solute on one side ◦ It does NOT require cell’s energy

28.  Known as Active Transport ◦ Requires cell’s energy ◦ Uses transport proteins  “Pumps” found on membrane ◦ Transports molecules from area of low solute concentration to high solute concentration ◦ Types of active transport:  Molecular Transport  Endocytosis  Exocytosis

29.  Small molecules and ions are carried across membranes  Carried by proteins ◦ Proteins act as energy-requiring pumps  Examples of molecules transported: ◦ Calcium, Potassium, and Sodium ions

30.  Process of taking material into cell through pockets of the cell membrane ◦ Examples of molecules taken in through pockets:  Large food molecules, whole cells

31.  Phagocytosis  “cell eating”  Extensions of cytoplasm surround a particle and package it within a food vacuole  Pinocytosis  How many cells take up liquid from environment  Tiny pockets form along cell membrane  Fill with liquid  Pinch off to form vacuoles Vacuole

32.  Cells also release material from inside the cell  During exocytosis, membrane of vacuole containing material fuses with cell membrane  Forces contents out