Mechanisms for Transporting Small Molecules and Ions Across Biomembranes

PROPERTYPASSIVE DIFFUSION

FACILITATED DIFFUSION

PRIMARY ACTIVE TRANSPORT

SECONDARY ACTIVE TRANSPORT

Requires transport protein

- + + +

Works against gradient

- - + +

Coupled to ATP hydrolysis

- - + (directly) - (indirectly)

Powered by movement of cotransported ion

- - - +

Examples of transported molecules

O2, CO2, many drugs, steroid hormones

Glucose and amino acids (most cells); Cl- and

exchange (red blood cells)

Na+/K+, Ca2+ Glucose and amino acids (intestine and kidney tubule); Ca2+ (cardiac muscle)

Major Trimeric G Proteins

Gα TYPE FUNCTION* COUPLED RECEPTORS

Gs Stimulates adenylate cyclase (↑ cAMP)

Dopamine (D1), epinephrine (β1, β2), glucagon histamine (H2), vasopressin (V2)

Gi Inhibits adenylate cyclase (↓ cAMP) Dopamine (D2), epinephrine (α2)

Gq Stimulates phospholipase C (↑ IP3, DAG)

Angiotensin II, epinephrine (α1), oxytocin, vasopressin (V1)

Gt (transducin)

Stimulates cGMP phosphodiesterase (↑ cGMP)

Rhodopsin (light sensitive)

Hexose Transport Proteins

TRANSPORTER PRIMARY TISSUE LOCATION SPECIFICITY AND PHYSIOLOGIC FUNCTIONS

GLUT1 Most cell types (e.g., brain, erythrocytes, endothelial cells, fetal tissues) but not kidney and small intestinal epithelial cells

Transports glucose (high affinity) and galactose but not fructose; mediates basal glucose uptake

GLUT2 Hepatocytes, pancreatic β cells, epithelial cells of small intestine and kidney tubules (basolateral surface)

Transports glucose (low affinity), galactose, and fructose; mediates high-capacity glucose uptake by liver at high blood glucose levels; serves as glucose sensor for β cells (insulin independent); exports glucose into blood after its uptake from lumen of intestine and kidney tubules

GLUT3 Neurons, placenta, testes Transports glucose (high affinity) and galactose but not fructose; mediates basal glucose uptake

GLUT4 Skeletal and cardiac muscle, adipocytes

Mediates uptake of glucose (high affinity) in response to insulin stimulation, which induces translocation of GLUT4 transporters from the Golgi apparatus to the cell surface

GLUT5 Small intestine, sperm, kidney, brain, muscle, adipocytes

Transports fructose (high affinity) but not glucose or galactose

GLUT7 Membrane of endoplasmic reticulum (ER) in hepatocytes

Transports free glucose produced in ER by glucose-6-phosphatase to cytosol for release into blood by GLUT2

SGLUT1 (Na+/K+ symporter)

Epithelial cells of small intestine and kidney tubules (apical surface)

Cotransports glucose or galactose (but not fructose) and Na+ in same direction; mediates uptake of sugar from lumen against its concentration gradient powered by coupled transport of Na+ down its gradient

OVERVIEW OF ENERGY METABOLISM

GLYCOLYSIS AND PYRUVATE DEHYDROGENASE

TRANSPORTER PRIMARY TISSUE LOCATION SPECIFICITY AND PHYSIOLOGIC FUNCTIONS

GLUT1 Most cell types (e.g., brain, erythrocytes, endothelial cells, fetal tissues) but not kidney and small intestinal epithelial cells

Transports glucose (high affinity) and galactose but not fructose; mediates basal glucose uptake

GLUT2 Hepatocytes, pancreatic β cells, epithelial cells of small intestine and kidney tubules (basolateral surface)

Transports glucose (low affinity), galactose, and fructose; mediates high-capacity glucose uptake by liver at high blood glucose levels; serves as glucose sensor for β cells (insulin independent); exports glucose into blood after its uptake from lumen of intestine and kidney tubules

GLUT3 Neurons, placenta, testes Transports glucose (high affinity) and galactose but not fructose; mediates basal glucose uptake

GLUT4 Skeletal and cardiac muscle, adipocytes

Mediates uptake of glucose (high affinity) in response to insulin stimulation, which induces translocation of GLUT4 transporters from the Golgi apparatus to the cell surface

GLUT5 Small intestine, sperm, kidney, Transports fructose (high affinity) but not glucose or

brain, muscle, adipocytes galactose

GLUT7 Membrane of endoplasmic reticulum (ER) in hepatocytes

Transports free glucose produced in ER by glucose-6-phosphatase to cytosol for release into blood by GLUT2

SGLUT1 (Na+/K+ symporter)

Epithelial cells of small intestine and kidney tubules (apical surface)

Cotransports glucose or galactose (but not fructose) and Na+ in same direction; mediates uptake of sugar from lumen against its concentration gradient powered by coupled transport of Na+ down its gradient

GALACTOSA METABOLISM

FRUCTOSA METABOLISM