Cell Membranes

• Elodea

an aquatic plant of a genus that includes the ornamental waterweeds

• Hypertonic

describes a solution in which the solute concentration is greater than on the outer side of a semipermeable membrane

• Hypotonic

describes the solution in which the solute concentration is less than on the other side of a semipermeable membrane

• Isotonic

the solute concentration and the water concentration both inside and outside the cell are equal - no net gain or net loss of water across the permeable membrane because there is no difference in concentration of the water

• Aqueous solution

An _________ ____________ is a solution in which the solvent is water. It is usually shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be represented as NaCl(aq)

• Explain the process of osmosis in living cells, both plant and animal, exposed to different extracellular solute concentrations and how the outcomes are similar and different

Animal Cell: example = usually isotonic to surrounding blood plasma, hypotonic (solute concentration is lower than the cell= may cause the cell to burst as water moves by osmosis into red blood cell) hypertonic = surroundings have a higher concentration of solutes than inside a cell's cytoplasm , cells may shrivel and die for lack of water Plant Cell: plants roots are hypertonic(roots have a higher concentration of solutes, therefore drawing water in from the soil-) water rushes in , central vacuoles expand until the cell walls constrain the growth - turgor pressure is the resulting force of water against the cell wall low turgor pressure can cause plant cells to wilt, but can be reversed when the plant cell is exposed to water (in a hypotonic solution)

• Dialysis

In biochemistry, ____________ is the process of separating molecules in solution by the difference in their rates of diffusion through a semipermeable membrane, such as dialysis tubing

• Provide examples of molecules that can diffusion through the cell membrane and molecules that cannot diffusion through the membrane

In order for a cell to function effectively, it needs to be able to control which substances can enter and exit through its membrane. The cell membrane's main trait is its selective permeability, which means that it allows some substances to cross it easily, but not others. Small molecules that are nonpolar (have no charge) can cross the membrane easily through diffusion, but ions (charged molecules) and larger molecules typically cannot. Nonpolar and small polar molecules can pass through the cell membrane, so they diffuse across it in response to concentration gradients. Carbon dioxide and oxygen are two molecules that undergo this simple diffusion through the membrane. The simple diffusion of water is known as osmosis The cell membrane is made of a bilayer of phospholipids, with an inner and outer layer of charged,hydrophilic "heads" and a middle layer of fatty acid chains, which are hydrophobic, or uncharged. Charged ions cannot permeate the cell membrane for the same reason that oil and water don't mix: uncharged molecules repel charged molecules. Even the smallest of ions -- hydrogen ions -- are unable to permeate through the fatty acids that make up the membrane. If ions "want" to enter the cell due to a high concentration of that type of ion on one side of the cell, they can do so by entering through the protein channels that are embedded between the lipids

• Explain how cells use transport proteins and vesicular transport to move molecules across the membrane

Occurs in Active transport - net movement is against concentration gradient; requires transport proteins and energy input, often from ATP - vesicle carries large particles into or out of a cell; requires energy input - membrane engulfs substance and draws it into cell - vesicle fuses with cell membrane, releasing substances outside of the cell

• Explain how temperature influences the movement of a substance

things move faster as they are heated up

• Solute

a chemical that dissolves in a solvent, forming a solution

Explain the purpose of wearing gloves for this experiment

to avoid the indicator test chemicals from coming into contact with your skin

• Concentration

a measure of the amount of dissolved substance (the solute) contained per unit of volume

• Osmotic equilibrium

________ __________ is the term used to indicate that the concentration of a solute in water is the same on both sides of a semi-permeable membrane

• Solvent

a chemical in which other substances dissolve, forming a solution

• Describe the function of a controlled experiment

an experiment or trial that uses controls, usually separating the subjects into one or more control groups and experimental groups in this case the positive controls were testing the iodine with starch, benedicts with simple sugars, and silver nitrate with salt- so we know what positive results will look like when we compare them to the substnces being tested

• Describe the use of indicator chemicals to test for the movement of molecules and to determine the direction of diffusion

benedict's test - simple sugars iodine - starch silver nitrate test - salt

• Describe the appearance of elodea cells in culture water, 20% saline solution, and DI water solution

culture water = chloroplasts distributed evenly throughout cell 20 percent saline solution = chloroplasts packed tightly together near the middle of the cell DI water solution = chloroplasts spread out against the boundary of the cell membrane and wall

• State the direction the water is moving for each of the environments the elodea is in (i.e. culture water, 20% saline solution, and DI water solution)

culture water = isotonic , water not moving in or out 20 percent saline solution = hypertonic, solute concentration is greater on the outside- shrinks elodea cell DI water solution = hypotonic, solute concentration is less on the outside, swells elodea cell

• Explain how the concentration of a substance influences the movement of a substance

diffusing molecules always move outward from an area of high concentration to low concentration - the difference in concentration levels between 2 locations is known as the concentration gradient

• Explain the concepts of diffusion and osmosis and why they are important to cells physiology

diffusion = tendency of all molecules to scatter evenly throughout the environment, move from higher to lower concentration, requires no energy - there must be a difference in concentration (concentration gradient) osmosis = movement of solvent (water) across a selectively permeable membrane

• Explain what dialysis tubing bag represents in the diffusion experiment

it mimics a cell's permeability - large molecules will not pass, small molecules will pass

• Dialysis tubing

mimics a cell's property of selective permeability (based on size alone) - large molecules will not pass, small molecules will pass

• State the tests used for the diffusion experiment and the positive and negative outcomes

salt = + starch = - glucose = +

• State which solutions moved through the membrane in the dialysis tubing experiment and which solutions did not

salt and glucose moved through b/c small (hypotonic solution) starch did not move through b/c it was too large

• Describe the outcomes of the diffusion experiment (i.e. which solutions can diffuse through the membrane and which solutions cannot diffuse through the membrane)

salt and glucose were able to diffuse through the membrane, while starch was not able to

• Explain how molecular size influences the movement of a substance

smaller size = faster movement

• Silver nitrate test for the presence of chloride ions

tests for NaCl (salt)

• Iodine test

tests for polysaccharides (starch)

• Benedict's test

tests for simple sugars (glucose)

• Concentration gradient

the difference in concentration on either side of a semi-permeable membrane (like a cell membrane)

• Molecular weight

the lighter molecular weight will diffuse much faster, the heavier molecular weight will diffuse slower

• Selectively permeable membrane/Cell membrane

the membrane is "choosy" - some substances pass freely through the bi-layer, but others - such as the sugar from a digested energy bar - require proteins

• Molecular motion

the more molecular motion, (more heat) results in molecules moving faster