CELL MEMBRANE, MEMBRANE TRANSPORT, BODY FLUIDS, & IONIC BASIS
OF EXCITATION
CELL MEMBRANE
Cell membranes are bilayers from a mixture of phospholipids and proteins.
A. Lipid bilayer
1. Phospholipids have a glycerol backbone, which is the hydrophilic (water-soluble) head, and two
fatty acid tails, which are hydrophobic (water-insoluble). The hydrophobic tails face each other and
form a bilayer.
2. Lipid-soluble substances such as gases--oxygen and carbon dioxide---and steroid hormones cross
cell membranes because they can dissolve in the hydrophobic lipid bilayer.
3. Water-soluble substances such as sodium, chloride, glucose, and water cannot dissolve in the lipid
of the membrane, but may cross through water-filled channels, or pores, or may be transported by
carriers (Costanzo, 2006).
(Koeppen & Stanton, 2008)
(Guyton, 2011)
B. Proteins
The proteins are responsible for the dynamic aspects of membrane function. There are two main types of
membrane proteins.
Integral membrane proteins: They are embedded in the lipid bilayer and cannot be removed without
disrupting the bilayer. They include channels, pumps, carriers, and receptors.
, the cell membrane). These peripheral proteins function almost entirely as enzymes or as controllers of
transport of substances (Costanzo, 2006).
Membrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of
glycoproteins or glycolipids. Most of the integral proteins are glycoproteins, and about one tenth of the
membrane lipid molecules are glycolipids. The “glyco” portions of these molecules almost invariably protrude
to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called
proteoglycans—which are mainly carbohydrate substances bound to small protein cores—are loosely attached
to the outer surface of the cell as well. The entire outside surface of the cell often has a loose carbohydrate
coat called the glycocalyx (Guyton, 2011).
C. Intercellular connections
1. Tight junctions (zonula occludens)
• Are the attachments between cells (often epithelial cells)
• May be an intercellular pathway for solutes, depending on the size, charge, and characteristics
of the tight junction
• May be “tight” (impermeable), as in the renal distal tubule, or “leaky” (permeable), as in the
renal proximal tubule and gallbladder.
2. Gap junctions
• the attachments between cells that permit intercellular communication
• Seen in myocardial cells that permit current flow and electrical coupling (Costanzo, 2006)
What is the importance of the cell membrane? It contributes to homeostasis because other than acting as a
barrier, the proteins serve many roles to maintain the functions and integrity of the cell most especially through
its transport processes. The effects of cell injury and subsequent inflammation are seen on the cell membrane
and membranes of the mitochondria and lysosomes. Drugs are able to take effect through the cell membrane.
MEMBRANE TRANSPORT
Diffusion
• The only form of transport that is not carrier-mediated
• Occurs down an electrochemical gradient described as “downhill.”
• Does not require energy and thus is passive
Factors that affect the rate of diffusion of a substance between two compartments separated by a
membrane:
1. Concentration gradient. The greater the concentration gradient, the greater the rate of diffusion.
2. Surface area. The greater the surface area, the greater the rate of diffusion. (For example, exercise
opens additional pulmonary capillaries increasing the surface area for exchange. Emphysema
decreases the surface area for exchange.)
3. Solubility in the membrane or permeability Factors that increase permeability:
• The more lipid-soluble is the solute, the more permeable The smaller the radius of the solute, the more
permeable.
• The thinner the membrane, the more permeable since the diffusion distance is decreased
4. Thickness of the membrane. The thicker the membrane, the slower the rate of diffusion, (e.g., lung
fibrosis).
5. Molecular weight. This factor is not important clinically.
DIFFERENT TYPES OF TRANSPORT
TYPE ELECTROCHEMICAL CARRIER- METABOLIC Na+ Gradient
GRADIENT MEDIATED ENERGY
Simple diffusion Downhill No No No
OF EXCITATION
CELL MEMBRANE
Cell membranes are bilayers from a mixture of phospholipids and proteins.
A. Lipid bilayer
1. Phospholipids have a glycerol backbone, which is the hydrophilic (water-soluble) head, and two
fatty acid tails, which are hydrophobic (water-insoluble). The hydrophobic tails face each other and
form a bilayer.
2. Lipid-soluble substances such as gases--oxygen and carbon dioxide---and steroid hormones cross
cell membranes because they can dissolve in the hydrophobic lipid bilayer.
3. Water-soluble substances such as sodium, chloride, glucose, and water cannot dissolve in the lipid
of the membrane, but may cross through water-filled channels, or pores, or may be transported by
carriers (Costanzo, 2006).
(Koeppen & Stanton, 2008)
(Guyton, 2011)
B. Proteins
The proteins are responsible for the dynamic aspects of membrane function. There are two main types of
membrane proteins.
Integral membrane proteins: They are embedded in the lipid bilayer and cannot be removed without
disrupting the bilayer. They include channels, pumps, carriers, and receptors.
, the cell membrane). These peripheral proteins function almost entirely as enzymes or as controllers of
transport of substances (Costanzo, 2006).
Membrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of
glycoproteins or glycolipids. Most of the integral proteins are glycoproteins, and about one tenth of the
membrane lipid molecules are glycolipids. The “glyco” portions of these molecules almost invariably protrude
to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called
proteoglycans—which are mainly carbohydrate substances bound to small protein cores—are loosely attached
to the outer surface of the cell as well. The entire outside surface of the cell often has a loose carbohydrate
coat called the glycocalyx (Guyton, 2011).
C. Intercellular connections
1. Tight junctions (zonula occludens)
• Are the attachments between cells (often epithelial cells)
• May be an intercellular pathway for solutes, depending on the size, charge, and characteristics
of the tight junction
• May be “tight” (impermeable), as in the renal distal tubule, or “leaky” (permeable), as in the
renal proximal tubule and gallbladder.
2. Gap junctions
• the attachments between cells that permit intercellular communication
• Seen in myocardial cells that permit current flow and electrical coupling (Costanzo, 2006)
What is the importance of the cell membrane? It contributes to homeostasis because other than acting as a
barrier, the proteins serve many roles to maintain the functions and integrity of the cell most especially through
its transport processes. The effects of cell injury and subsequent inflammation are seen on the cell membrane
and membranes of the mitochondria and lysosomes. Drugs are able to take effect through the cell membrane.
MEMBRANE TRANSPORT
Diffusion
• The only form of transport that is not carrier-mediated
• Occurs down an electrochemical gradient described as “downhill.”
• Does not require energy and thus is passive
Factors that affect the rate of diffusion of a substance between two compartments separated by a
membrane:
1. Concentration gradient. The greater the concentration gradient, the greater the rate of diffusion.
2. Surface area. The greater the surface area, the greater the rate of diffusion. (For example, exercise
opens additional pulmonary capillaries increasing the surface area for exchange. Emphysema
decreases the surface area for exchange.)
3. Solubility in the membrane or permeability Factors that increase permeability:
• The more lipid-soluble is the solute, the more permeable The smaller the radius of the solute, the more
permeable.
• The thinner the membrane, the more permeable since the diffusion distance is decreased
4. Thickness of the membrane. The thicker the membrane, the slower the rate of diffusion, (e.g., lung
fibrosis).
5. Molecular weight. This factor is not important clinically.
DIFFERENT TYPES OF TRANSPORT
TYPE ELECTROCHEMICAL CARRIER- METABOLIC Na+ Gradient
GRADIENT MEDIATED ENERGY
Simple diffusion Downhill No No No
