Page 16 - 84 human physiolofy part-1

Chapter 2

and this is termed the fluid mosaic model of the cell membrane.

The phospholipid bilayer is selectively permeable. Only small, uncharged polar molecules can pass
freely across the membrane. Some of these molecules are H O and CO , hydrophobic (nonpolar)
2 2
molecules like O , and lipid soluble molecules such as hydrocarbons. Other molecules need the help of
2
a membrane protein to get across. There are a variety of membrane proteins that serve various
functions:

• Channel proteins: Proteins that provide passageways through the membranes for certain
hydrophilic or water-soluble substances such as polar and charged molecules. No energy is used
during transport, hence this type of movement is called facilitated diffusion.
• Transport proteins: Proteins that spend energy (ATP) to transfer materials across the
membrane. When energy is used to provide passageway for materials, the process is called
active transport.
• Recognition proteins: Proteins that distinguish the identity of neighboring cells. These
proteins have oligosaccharide or short polysaccharide chains extending out from their cell
surface.
• Adhesion proteins: Proteins that attach cells to neighboring cells or provide anchors for
the internal filaments and tubules that give stability to the cell.
• Receptor proteins: Proteins that initiate specific cell responses once hormones or other
trigger molecules bind to them.
• Electron transfer proteins: Proteins that are involved in moving electrons from one
molecule to another during chemical reactions.

Passive Transport Across the Cell Membrane

Passive transport describes the movement of substances down a concentration gradient and does
not require energy use.

• Bulk flow is the collective movement of substances in the same direction in response to a
force, such as pressure. Blood moving through a vessel is an example of bulk flow.
• Simple diffusion, or diffusion, is the net movement of substances from an area of higher
concentration to an area of lower concentration. This movement occurs as a result of the random
and constant motion characteristic of all molecules, (atoms or ions) and is independent from the
motion of other molecules. Since, at any one time, some molecules may be moving against the
gradient and some molecules may be moving down the gradient, although the motion is random,
the word "net" is used to indicate the overall, eventual end result of the movement.
• Facilitated diffusion is the diffusion of solutes through channel proteins in the plasma
membrane. Water can pass freely through the plasma membrane without the aid of specialized
proteins.
• Osmosis is the diffusion of water molecules across a selectively permeable membrane.
When water moves into a body by osmosis, hydrostatic pressure or osmotic pressure may build
up inside the body.
• Dialysis is the diffusion of solutes across a selectively permeable membrane.

16 | Human Physiology