Biology 12

30 MHR • Unit 1 Metabolic Processes

Called a phospholipid, this molecule interacts with

water in a way that spontaneously results in the

structure shown in Figure 1.23(b). This

phospholipid bilayer is the foundation for the

semi-permeable membrane that surrounds cells.

Some molecules can pass freely through the

membrane, while others require assistance to enter.

The phospholipid bilayer is virtually impermeable

to macromolecules, relatively impermeable to

charged ions, and quite permeable to small, lipid-

soluble molecules. Molecules that move through

the membrane do so at differing rates, depending

on their ability to enter the hydrophobic interior

of the membrane bilayer.

Many small, non-polar solute molecules, such

as oxygen and carbon dioxide, pass through the

bilayer of the cell membrane with least resistance.

They enter by means of diffusion, a form of passive

transport. As you learned in previous studies, in

this method of cellular transport, molecules move

from regions of high concentration to those of low

concentration. Water, a small polar molecule, can

travel through the cell membrane freely in the

process of osmosis. This process involves the

movement of the solvent water from an area of

higher concentration of water to an area of lower

concentration of water.

Some molecules are too large to diffuse

unassisted across the cell membrane. These

molecules enter the cell by means of specialized

proteins called carrier proteins — they move and

change shape to create an opening into the cell.

Large uncharged hydrophilic molecules such as

glucose make use of these proteins in order to enter

cells (see Figure 1.24). No cellular energy is

required for this facilitated diffusion process, so it

is a form of passive transport. Appendix 5 shows

several other examples of passive transport through

the cell membrane. In the next chapter, you will

see how cells use energy to move larger molecules

across the cell membrane.

Figure 1.24Molecules such as glucose move into the cell

when carrier proteins in the cell membrane change shape.

There may be a new way to move peptides and nucleic

acids in synthetic DNA through the hydrophobic inner

region of the cell membrane. Researchers in Pennsylvania

have created hydrocarbon “umbrellas” that close around

molecules when they are within the hydrophobic area of the

membrane, but open in the hydrophilic areas, helping to

move the molecules into the cell where they can do their

work. This development holds promise for medical

procedures such as gene therapy.

BIO FACT

carrier

protein

glucose

SECTION REVIEW

1. Describe a redox reaction. Draw a simple diagram
   of such a reaction between a sodium atom and a
   chlorine atom. Provide an example of a redox
   reaction that takes place in all cells.


2. Explain how molecular models have helped us
   to better understand atoms, bonds, and molecules.
   Prepare a list of possible applications of molecular
   modelling in scientific research.
   3. Identify the functional group involved in the
   condensation reaction that forms the long chains
   of glucose molecules that make up starch.
   4. What is the monosaccharide product of the
   hydrolysis of starch?
   5. Explain what happens in a neutralization reaction.
   6. What are buffers, and why are they important
   for cellular processes?

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