MEMBRANES
 
Carry out crucial functions in cells.
 Boundary of protoplast -- plasma membrane.
 Boundary of compartments -- organelle membranes.

Composition
 Lipid bilayer and associated proteins.
  Phospholipids.
Phospholipids are the principle components of  biological membranes, and are responsible for
 membrane properties.
  A class of lipids with a highly charged, or POLAR, phosphate group attached to one end.
    This adds a specific property to the molecule. It is NEGATIVELY charged.
Because it is charged, this end interacts well with water and is said to be HYDROPHILIC.
The other end consists of two 'tails' each composed of nonpolar or uncharged carbon chains. This end hates or fears water, so it's
  HYDROPHOBIC.

In other words, the phosphate group and tails have very different chemical properties. Phospholipids have a SPLIT PERSONALITY.

This property of phospholipids is quite special, and important.
Membranes consist of two layers of phospholipids ‘cemented’ together. The fatty acid chains, being hydrophobic, interact with each other rather than water on the outside of the cell and in the cytoplasm, and so face the middle of the membrane. The  hydrophilic phosphate groups face the aqueous, or water filled medium to either side of  the membrane.
This arrangement is called a lipid BILAYER. (Fig. 3.7)
 The bilayer has fluid properties, like oil. The phospholipids are mobile -- they move around.

    Proteins are associated with the lipid bilayer.

Other proteins loosely associate with the membrane surfaces, facing the cytoplasm or the outside of the cell. They are relatively loosely bound, and can be easily removed. What are they called?

Membrane functions.

 Transport.
    Some small molecules DIFFUSE across the lipid bilayer.
        Move according to each's concentration gradient.
             PASSIVE TRANSPORT
                Think: what is a concentration gradient?

Other substances are actively PUMPED into cells, against a concentration gradient.
   Called ACTIVE TRANSPORT.
   Cells expend energy to do this.
      It’s WORK!
    Again, specific proteins involved.
     E.g. proton pump in plants.
     Establishes a charge difference across the membrane, with more positive charges outside the cell.
        This is a kind of GRADIENT.
            This in turn creates an electrical charge, or voltage drop, across the membrane.
                In plants, this voltage across the plasma membrane is about 300 mVolts,
                    with the inside of the cell more negative than the outside.
    Cells use such gradients to power other reactions, again like the transport of sucrose into cells.

>>>Keep in mind the energy angle. Cells are highly organized, non-random systems. Lots of order. It takes   
            energy to establish and maintain this state.<<<

Other membrane functions:
 - Recognition.
   Membrane proteins recognize substances or signals in the environment. Those proteins are called RECEPTORS.
    Signals emanating from other organisms, e.g. pathogens, hormones.
      Recognition is linked to a RESPONSE.
        e.g., when a plant turns towards the light
- Cellulose synthesis.
  Specific intregral plasma membrane proteins make cellulose that's then deposited outside the cell in the wall.

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CHEMICAL COMPOSITION OF CELLS

Six elements comprise 99% of the weight of living matter.

 Carbon         C
 Hydrogen     H
 Nitrogen      N
 Oxygen        O
 Phosphorus  P
 Sulfur           S

Easy to remember: SP COHN

The rest of living matter is made up of other elements such as potassium, sodium, magnesium, calcium,  iron, manganese, zinc, molybdenum.

Carbon forms the basic skeletons of ORGANIC molecules.  Organic molecules used to be thought of as special to
living organisms, but we now know there’s nothing special, or ‘vital’ about them. We synthesize them every day
(urea was the first, in the 1800s).
    Now we define organic as chemicals composed principally of carbon.

Carbon forms these skeletons because of the way it BONDS with other atoms.
  (Atoms combine in specific combinations to form  MOLECULES, the smallest unit of a chemical compound,
    with defined characteristics. E.g. H2O, or water)

Each element has a specific number of protons and electrons. It's called the ATOMIC NUMBER.
    For carbon, the atomic number is 6.

Atomic weight is the number of protons and neutrons in the nucleus.
We care only about atomic number, because it's important in determining an element's chemistry!!!

Carbon atom has 6 protons and 6 neutrons in its nucleus, and 6 electrons surrounding the nucleus.
 Electrons are arranged in successive energy levels, or shells.
        Each level can have a maximum number of electrons.

In the case of hydrogen, each atom has one electron, but in a shell whose maximum is 2.

In the case of carbon, its 6 electrons occupy 2 levels.
  The first level has 2 electrons, and an outer level has 4 electrons. The first level is FULL at 2, but the second level,
    with 4 electrons, is only half full -- its  maximum occupancy is 8.

The chemical reactivity of an atom depends on the number of electrons in its outermost shell. Atoms gain or lose electrons depending on this number. In  the case of carbon, with 4 electrons in its outermost shell, it tends to SHARE electrons with  other atoms in order to fill the shell. For example, carbon can share electrons with 4 hydrogen atoms.

 Hydrogen has just one proton and one electron. With one electron, the outermost shell, with a capacity for 2
 electrons, is half full.
    So, two hydrogen atoms can SHARE electrons, so each has two. Creates H2, or molecular hydrogen gas.

Sharing a pair of electrons creates a COVALENT BOND.
    Represented diagramatically as a dashed line.

 Hydrogen has just one proton and one electron. With one electron, the outermost shell, with a capacity for 2
 electrons, is half full. What is hydrogen's atomic number?

So, one electron in carbon’s outer shell can pair with the one electron of a hydrogen atom. When this happens 4 times,
    carbon’s outer shell now has 8 and each hydrogen’s outer shell has 2 electrons.
        These 4 COVALENT BONDS make CH4, or methane.
 Carbon can also form covalent bonds with oxygen, nitrogen and sulfur.

Likewise, oxygen forms covalent bonds with 2 hydrogens to make.....what????

So far, we’ve talked about SINGLE covalent bonds. There are also double and triple bonds.
  E.g. between 2 carbons, or carbon and oxygen -- i.e. carbon dioxide -- and oxygen with itself (O2; Fig 2.6).
    CO2, C2H4 (ethylene - double bond between carbons), C2H2 (acetylene - triple bond between carbons)

Double and triple bonds are very stable. E.g. N2
It takes a lot of energy to break them, e.g. to ‘fix’ N2, or create 2 NH3 molecules from N2 and hydrogen

You can probably see that in bonding with itself, carbon  can form chains of atoms:  C-C-C-C-C-C etc
  The chains also include hydrogen, oxygen, nitrogen, and  often sulfur and phosphorus.

Certain types of carbon chains predominate.
 4 main classes:
  carbohydrates, lipids, proteins, and nucleic  acids.