Cell Membrane and its Permability

SELECTIVE PERMEABILITY OF CELL MEMBRANES

Permeability refers to whether or not a particular molecule can cross the cell membrane. The cell membrane is selective in that it is able to regulate the movement of a number of substances. However, some substances can move freely across the membrane, without the cell's control

Permeability of the cell membrane to a particular molecule depends upon a number of factors:

a) the size of the molecule
b) whether the molecule is hydrophilic (water-loving) or hydrophobic
( water-hating, lipid soluble)
c) whether the molecule is electrically charged or not

These factors also influence the method of transport that the cell uses to move the molecules across the membrane:

1. Passive Transport (PT) - a) Diffusion or b) Osmosis
2. Facilitated Diffusion(FD) - Protein carriers
3. Active Transport(AT) - a) Endocytosis- Pinocytosis or Phagocytosis
b) Exocytosis
c) Protein carriers that use energy

PERMEABILITY
TYPE OF MOLECULE METHOD OF TRANSPORT
1. HIGHEST Small, hydrophilic or Diffusion/Osmosis
hydrophobic uncharged molecules Freely pass between
eg. O2, H20, CO2 phospholipids of bilayer


2. HIGH Hydrophobic, large, uncharged Diffusion
eg. benzene, hydrophobic a.a. dissolve in lipid bilayer


3. MODERATE Hydrophilic, large, uncharged Facilitated diffusion or
TO LOW eg. glucose, sugars, Active transport protein
hydrophilic amino acids carriers



4. LOW Charged ions Facilitated diffusion
eg. H+, Na +, K +, Ca2+, Cl- or Active transport
charged amino acids


5. VERY LOW Very large Macromolecules Active transport
(cannot cross) eg. proteins, starch (endocytosis)


PERMEABILITY OF THE CELL MEMBRANE CONTINUED:

1. the size of the molecule

• the cell membrane is very permeable to small molecules such as
water, carbon dioxide and oxygen
• these molecules freely DIFFUSE across the membrane because they are small
enough to pass between the phospholipid molecules of the lipid bilayer
• the cell CANNOT control the diffusion of these molecules, and their movement
is determined by the concentration gradient
• such as glucose and amino acids, cannot enter by simple diffusion and must
pass through PROTEIN CHANNELS or pores which are large enough to accommodate
them
• such as proteins, or starch, cannot pass through the membrane at all, and
must used ENDOCYTOSIS (membrane shape changes) to enter into a food vacuole.


2. whether the molecule is hydrophilic (water loving) or hydrophobic (water hating or lipid soluble)

HYDROPHOBIC molecules
• such as ether, can pass through the membrane rapidly by
diffusion , regardless of size, because they are able to dissolve in the
lipid bilayer (lipid soluble)

HYDROPHILIC molecules
• such as glucose and amino acids, or ions, must avoid the lipid bilayer, since
they are not lipid soluble.
• therefore, they must enter through a hydrophilic protein channel


3. very small molecules

• molecules which carry a charge are called IONS eg. sodium ion Na+, potassium
ion K+
• the charge makes the molecule hydrophilic and unable to pass through the
lipid bilayer, even though they might be small
• therefore, these ions must enter the cell through a protein channel

MACROMOLECULE PRACTICE QUESTIONS

For all of those who needed a little extra practise. Here are a few BASIC question to help prepare you for the test. Obviously the test will be more indepth, but if you can handle these few question you are on the right path for studying.


Examine the chemical processes below, and then determine if they are undergoing dehydration synthesis or hydrolysis.

The production of a dissacharide from glucose and fructose. ________________________________. (1)

The breakdown of a fat into a glycerol and 3 fatty acids. _______________________________. (1)

The breaking down of maltase into two molecules of glucose.
__________________________. (1)


Answer the following questions concerning the pictures below.

( There should be a fatty acid witha double bond in it here)
(Sorry I can't post pictures...yet!...I'm still learning how!)

The above molecule is a: protein, carbohydrate, lipid, or nucleic acid. (1)

What is the double bonded oxygen, hydrogen end group called? (1)

Is the above molecule saturated or unsaturated?____________________________.(1)

Draw the chemical structural formula of an amino acid in the space provided below.(1)






Circle and label the 3 groups on the above diagram (3)

What components make up a nucleotide? (1)


GOOD LUCK TONIGHT AND AGAIN 7:45 am I will hold another tiny answer period for questions incase you missed out 1 1/12 hr review after school yesterday.

CARBOHYDRATES fill in the blanks- UNIVERSITY BIOLOGY

CARBOHYDRATES fill in the blanks

POLYSACCHARIDE: (poly which means many)
o Are carbohydrates that are made-up of long chains of sugars.
o Polysaccharides are made-up of three or more monosaccharides.
o Starches perform an important function of energy storage in plants. Starches can be found in such food sources as bread, pasta, and potatoes. Plants convert excess sugars into Starches for long-term storage.

o Glycogen performs the same function as starches but it in animals. Glycogen consists of hundreds of glucose molecules strung together in a highly branched chain. Animals store Glucose in the form of polysaccharide glycogen in the liver and muscles to be used as quick energy.


Q. Note the difference between the structures of starches to that of the glycogen structure. What’s the difference? WHY?

A. The larger amount of branching in glycogen means that glycogen molecules pack more glucose units into a cell that do starch molecules.


o Cellulose which is produced by plants contains an even greater amount of glucose molecules than that of glycogen macromolecule. Plants use cellulose to build their cell walls. Cellulose gives the strength and rigidity to plant cells and is considered a structural molecule because it protects and provides support for the entire plant. Cellulose is a made up of glucose units therefore makes it have a high storage capacity of energy.


Only a few bacterial species produce the digestive chemicals needed to break cellulose down into glucose units and release its energy. These bacteria can be found in the stomach and intestines of certain animals and creatures. However, humans do not host these bacteria; therefore the food energy in cellulose is not directly available to us.
please see text page 12 for diagrams