Students who have fully met the prescribed learning outcomes (PLO’s) are able to:
B9. Analyze the structure and function of the cell membrane.
apply knowledge of organic molecules – including phospholipids, proteins, glycoproteins, glycolipids, carbohydrates, and cholesterol – to explain the structure and function of the fluid-mosaic membrane model.
Analyze the structure and function of the cell membrane. (continued)
identify the hydrophobic and hydrophilic regions of the phospholipid bilayer.
explain why the cell membrane is described as “selectively permeable”.
describe passive transport processes including diffusion, osmosis, and facilitated transport.
explain factors that affect the rate of diffusion across a cell membrane (e.g., temperature, size of molecule, charge of molecule, concentration gradient, pressure gradient).
predict the effects of hypertonic, isotonic, and hypertonic environments on osmosis in animal cells.
And a little bit more….
describe active transport processes including active transport, endocytosis (Phagocytosis and pinocytosis), and exocytosis.
compare specific transport processes – including diffusion, osmosis, facilitated transport, active transport, endocytosis and exocytosis – in terms of:
use of channel or carrier protein
use of energy
types/sizes of molecules transported
devise an experiment using the scientific method (e.g. to investigate the tonicity of cells). Spud Lab!
B10. Explain why cells divide when they reach a particular surface area-to-volume ratio.
differentiate between cells that have a high or low surface area-to-volume ratio.
demonstrate an understanding of the significance of surface-area-to-volume ratio in cell size.
http://highered.mcgraw-hill.com/sites/0072421975/student_view0/chapter4/ (Mader’s Student Edition Website Support for Chapter 4; Animations, quizzes, flashcards, Thinking Scientifically etc.)
http://www.mhhe.com/biosci/genbio/espv2/data/cells/003/index.html (Essential Study Partner: CM & Cell Transport)
http://www.ibiblio.org/virtualcell/textbook/chapter3/cms1.htm (Cell membrane structure text & animation)
http://sps.k12.ar.us/massengale/biology%20class%20notes2.htm (A terrific site with animations, self-quiz, notes etc. Click on the various notes for Effect of Solutions on Cells, Web Tutorials on Passive & Active Transport, Scientific Method, and Designing an Experiment )
http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transport.htm (Animation of Cell Transport)
http://curriculum.calstatela.edu/courses/builders/lessons/less/les4/cellsize.html (Limits to Cell Size)
And a few more
(Terrific power points on Scientific Method and Identifying Controls & Variables )
The ‘fluid-mosaic model’ is a model that describes the structures of the CM, i.e. a fluid lipid layer in a mosaic made up of many different types of molecules. Cell membranes include a flexible bilayer of phospholipids interspersed with large protein molecules that aid in membrane transport and cholesterol for rigidity.
1.) Phospholipid bilayer
Make up the basic structural unit of the cell membrane.
Contains a charged polar head ( hydrophilic /‘H 2 O- loving’) and non-polar , fat-soluble tails ( hydrophobic /‘H 2 O-fearing’).
Allows lipid-soluble molecules such as alcohol, O 2 , CO 2 and lipid soluble molecules, i.e. lipids, steroids, (and vitamins A & E) to pass through the phospholipid bilayer.
Provides flexibility and fluidity thus allowing vesicles to form.
Prevents non-lipid soluble molecules , e.g. water, ions, amino acids and monosaccharides to pass through.
Note: Lack of permeability to ions sets up membrane potential (See C11: Nerve transmission).
Form a “mosaic” pattern/scattered throughout the phospholipid bilayer.
Largely determine the cell membrane’s functions.
Embedded in cell membrane and on the surfaces; 3 o structures act as carriers, pumps, channels, receptors and cell recognition proteins.
Some act as carriers bringing amino acids , ions , and glucose in/out, e.g. Na+/K+ pump in nerve transmission; protein carriers bringing in glucose for RBC’s; or concentrating molecules/ions inside or outside of cell, e.g. nerve cells actively pumping Na+ & K+ or thyroid cells concentrating iodine.
Some have channels/gates/pores to allow passage of H 2 O, dissolved ions, and small molecules.
Some are receptors that a specific molecule can bind to, e.g. hormones such as insulin, thyroxin, aldosterone, estrogen, testosterone etc. or neurotransmitters such as acetylcholine (ACh), norepinephrine (NE). http://www.mhhe.com/biosci/genbio/espv2/data/cells/003/index.html
3.) Glycoproteins and Glycolipids
Attached to outside of cell membrane
* NOT found on intra-cellular membranes (e.g. mitochondria, ER, Golgi, etc.)
Made up of carbohydrate-chains attached to a phospholipid head or a protein.
Function : - Serves as recognition sites allowing organisms to recognize foreign cells/molecules.
Abundant lipids found wedged between the phospholipids.
Function : - Stiffen CM and provides more flexibility.
Note: Cytoskeleton (Recall Unit B1) is an internal framework of protein fibres that gives the cytoplasm strength and flexibility and provides movement of organelles. Some protein filaments attaches to the integral proteins of the CM. Attachment of the Cytoskeleton
Summary of Cell Membrane Structures and Functions Amino acids Allows/selects certain molecules in/out of cell through channels/gates/pores . E.g. H 2 O, O 2 , CO 2 , ions. Carries molecules selectively in/out of cell by carriers. E.g. Na + , K + , C 6 H 12 O 6 , áá’s, HCO 3 - , Ca 2+ . Proteins Unit molecules Functions Molecules
Protein Function Cont
Catalyzes reactions on cell surface by enzymatic proteins. E.g. Enzymes for ATP metabolism or breakdown of ATP for Na+ transport.
Provides receptor sites by receptor proteins. E.g. Hormones having the same shape as receptor proteins to bind to it.
Backbone of 4 fused C-H rings Stiffen CM and provide flexibility Cholesterol Chains of carbohydrates attached to lipid or protein (a) Used in cell identification . (b) Glycoproteins can form a carbohydrate coat that envelops the cell membrane (glycocalyx). Glycolipids and Glycoproteins 2 fatty acids (saturated/ unsaturated) & 1 glycerol, N and phosphate group (a) Allows for diffusion of lipid soluble molecules. E.g. O 2 , CO 2 , and alcohol. (b) Allows for flexibility & fluidity of cell membrane. E.g. Vesicle formation. (c) Excludes H 2 O and ions. (d) Acts as a boundary keeping organelles within the cell. Phospholipids
B9 – Selectively Permeable Cell Membrane
Cell membrane is described as selectively permeable because it allows only certain molecules or ions to pass through it, i.e. it limits what can move across.
*Functions of Cell Membrane
Regulates the exit/entrance of molecules/ions via pinocytosis/phagocytosis/endocytosis/exocytosis.
Regulates the exit/entrance of molecules/ions via diffusion/facilitated transport/active transport/osmosis.
Used in cell identification via glycoproteins and glycolipids.
Catalyzes reactions on the cell surface via enzymatic proteins.
Provides receptor sites via receptor proteins for certain molecules.
Acts as a cell boundary keeping the organelles within the cell.
B9 – Entrance/Exit of Molecules
Introduction: Terms to know
Solvent - the dissolving agent, usually H 2 O.
Solute - molecules, ions dissolved in the solvent.
Concentration - # of molecules in a given unit of volume.
Gradient - the physical difference between two regions which cause molecules to move from one region to the other and tends to equalize the difference.
Concentration gradient - the difference in solute concentration between two regions resulting in molecular movement.
Osmotic pressure - pressure generated by H 2 O moving by osmosis into or out of a cell.
Note---ie. Pay Attention!!
Osmotic pressure always moves water toward the hypertonic side (the side containing the least amount of water) of a membrane.
Some more terms….
Hydrostatic pressure – pressure exerted by the weight of H 2 O/fluid pushing against a surface, e.g. capillary.
Tonicity - total solute concentration of the solution outside the cell; causes the cell to gain or lose H 2 O.
Turgor (‘to swell’)- the rigid state of a cell (especially plants) caused by osmotic pressure of the cytoplasm against the cell wall or membrane.
There are two main categories of transport across selectively permeable membranes:
1.) Passive transport
Movement of molecules from [high] to [low] (along a concentration gradient).
Does not require energy; some use specific CM proteins; does not use vesicles; involves movement of small molecules, water (osmosis), ions, C 6 H 12 O 6 , amino acids, fatty acids.
Second type of transport
2.) Active transport
Movement of molecules from [low] to [high] and energy (ATP) is required (against a concentration gradient).
Requires energy; uses carrier or receptor proteins; can use vesicle formation; involves movement of small & large molecules, ions, cells, microorganisms.
Materials/molecules can move by 3 processes:
(a) Diffusion .
(b) Facilitated transport/diffusion .
(c) Osmosis .
examples of lab equipment which demonstrate diffusion and osmosis, i.e. the osmometer and U-tube apparatus .
Net movement of molecules/ions from an area of [ high ] to [ low ].
Requires no energy and molecules must be lipid soluble .
Large molecules, proteins, polysaccharides, nucleic acids, and charged ions cannot diffuse across.
Small, uncharged molecules (CO 2 , O2) can enter easily, e.g. O 2 and CO 2 diffuse across membranes of alveoli (See C10); H 2 O as well via channel proteins/ “ aquaporins ” (osmosis).
C 6 H 12 O 6 , amino acids, fatty acids and ions diffuse via CM proteins, e.g. neurotransmitters (NT’s) diffusing across the synaptic cleft during nerve impulses/synapse OR
Ca 2+ diffusing across the presynaptic membrane during nerve impulses/synapse (See C11); monomers diffusing into epithelial cells of villi (See C1).
Isotonic If the concentration of solute (salt) is equal on both sides, water will move back in forth but it won't have any result on the overall amount of water on either side. " ISO " means the same.
Hypotonic " HYPO " means less. There are less solute (salt) molecules outside the cell, which causes water to move into the cell by osmosis (high to low). The cell will gain water & grow larger. In plant cells, the central vacuole will fill and the plant becomes stiff and rigid, i.e. turgor. The cell wall keeps the plant from bursting. In human animal cells, the cell is in danger of lysis or bursting.
" HYPER " means more. There are more solute (salt) molecules outside the cell, which causes the water to leave the cell by osmosis (high to low).
In plant cells, the central vacuole loses water; the cells shrink, i.e. plasmolysis causing wilting.
In animal cells, the cells also shrink. In both cases, the cell may die.
It is dangerous to drink sea water. People marooned at sea will speed up dehydration (and death) by drinking sea water. This is also why "salting fields" was a common tactic during war, it would kill the crops, thus causing food shortages.
iso – equal tonicity – solute
The solution has the same/equal [solute] as the [solute] inside the cell.
This results in the cell not gaining or losing H 2 O.
Isotonic 95% H 2 0 5% salt 95% H 2 O 5% salt Isotonic Solution 95% H 2 O 5% salt H 2 O moves in/out of red blood cells (RBC) at equal rates and the shape remains as biconcave.
hypo – less tonicity – solute
The solution has a lower [solute] than the [solute] inside the cell.
This results in the movement of water (osmosis) INTO the cell causing it to swell and burst causing lysis .
H 2 O moves into the RBC by osmosis, causing it to swell and lysis to occur.
95% H 2 0 5% salt 98% H 2 O 2% salt Hypotonic Solution
Example of a RBC placed in a hypotonic solution . RBC shape in the beginning of the experiment? After 2 minutes?
hyper – more tonicity – solute
The solution has a highe r [solute] than the [solute] inside the cell.
This results in the movement of water (osmosis) OUT of the cell causing it to shrivel or in the case of RBC’s, crenation occurs; in plant cells, plasmolysis occurs.
Hypertonic H 2 O moves OUT of the RBC’s by osmosis causing it to shrivel and undergo crenation .
In plant cells , H 2 O moves OUT of the cytoplasm by osmosis causing the cell membrane to pull away from the cell wall. This is termed plasmolysis .