Lab Exam

• a membrane-spanning protein with 7 alfa helices  (belongingto the 7-TM receptor family).
• is a light-sensitive chromoprotein.
• It consistsof opsin (the proteinaceouspart)   and 11–cis-retinal bound to the e-amino group of a lysine residue.

1. storage form of enegy in the body 
2. serving as cell membrane components 
3. structural component of many orgaism (e.g. cell walls of bacteria, exoskeleton of insects + fibrous cellulose of plants) 

• componds with the same chemical formula but different structures
• e.g. fructose, glucose, mannose, galactose same formula C₆H₁₂O₆

• Carbohydrate isomers that differ in configuration around only one specific C-Atom 
• e.g. glucose + galactose on C4
• e.g. glucose + mannose on C2

1. Glycogen
2. Starch 
3. Sucrose
4. Lactose
5. free pentoses & hexoses 

Mouth and Intestine 

1. Glucose
2. Galactose
3. Fructose

salivary alpha amylase 

→ hydrolyzes 1,4- glycosidic bonds of starch + glycogen

getting oligosaccharides

- activator is Cl^-

• takes part of digestion of carbohydrates 
• acts in the mouth 
• hydrolyzes 1,4- glycosidic bonds of starch + glycogen
• activated by Cl^-

• Pancreas secretes bicarbonate to neutralize
• pancreatic alpha amylase continues the process of starch digestion getting short disaccharides/ maltotriose/ maltose/ isomaltose 

• acts in the small intestine
• coninues the process of carbohydrate digestion into smal disaccharides/ maltotrioses/ maltose/ isomaltose
• activator: Cl^- / Ca²⁺

• Final digestive process occur primalily at mucosal lining of upper jejunum
• includes action of several disaccharidases: 

For example:

• isomaltase - isomaltose = glucose 
• maltase - maltose/ maltotriose = glucose 
• sucrase - sucrose = glucose + fructose 
• lactase (beta- galactosidase) - lactose = galactose + glucose

getting Monosaccharides Glucose/ galactose / Fructose 

the upper jejunum absorbs the bulk of monosaccharides & they are transported to portal circulation to liver 

digestive enzyme deficiencies - lead to diarrhea, cramps 

• genetic defect of specific disaccharidases result in intolerance 
• alterations in disaccharide degradation can also be caused by intestinal diseases, malnutrition, drugs

• caused by small variations in DNA sequence on chromosome 2 that controls expresssion of gene for lactase 
• treatment: reduced milk consumption, eating green vegetables, use lactase treated products or take lactase in pill form prior to eating 

different sugars have different mechanisms for absorption! 

• they are taken into enterocytes by secondary active transport 
• this requires concurrent uptake of sodium 
• transport protein is sodium dependent glucose transporter 1 (SGLT-1) 

• Fructose utilizes an energy - and Na⁺ -independent monosaccharide transporter (GLUT-5) 

Glucose can not diffuse directly into cells, but can enter on two transport mechanisms:

1. Na⁺-indepenent, facilitated diffusion transport system 
2. Na⁺- monosaccharide cotransporter system 

• 14 glucose transporters in cell membranes GLUT-1 to GLUT14 
• passive system mediated by GLUT1,4
• monomeric and contain 12 transmembran helices with carbohydrate chain on outer surface 
• exist in 2 conformational states 
• extracellular glucose binds to the transporter, which alters it confirmation and transport gucose across the cell membrane 
• UNIPORTER 

abundant in RBC & blood brain barrier but low in adult muscle 

Function: primarily in glucose uptake from the blood 

abundant in Liver, kidney, pancreatic beta cells, intestine 

Function: can either transport glucose into cell when glucose level is high or transport glucose from cells to the blood when blood glucose level is low

Primary in Neurons (Brain)

Function: glucose uptake from blood 

SENSITIVE TO INSULIN 

in muscle and adipose tissue, heart 

Frunction: glucose uptake from blood 

muscle & spermatozoa 

Function: primary transporter of FRUCTOSE in small intestine and testes 

• Sodium and ATP dependent cotransport system
• energy requiring process - transports glucose againts a concentration gradiemt 
• movement of glucose is coupled to concentration of Na⁺
• transports glucose against concentration gradient, while Na⁺ goes with its conetration gradient 
• occurs in epithelial cells of intestine, renal tubules and choroid plexus 

• peptide hormone produced by the beta cells of pancreas (most glucose- sensing cells in the body 
• most important hormone coordinating the ues of fuels by tissues 
• promotes glucose storage, most prominent in liver, muscle & adipose tissue 

decreases the production of glucose through the inhibition of glyconeolysis and gluconeogenesis

• effectively traps the sugar as cyclic glucose-6-phosphate because there are no specific transmembrane carriers for phosphorylated monosaccharide
• too polar to diffuse through lipid core of membrane 

insulin is released from the pancreas - signals the tissues to uptake glucose-glucose enters the cells via glucose transporters, there is a chance for them to leave the cell.

If we are in need of energy- don't want glucose to leave the cell.- glucose is phosphorylated by ATP to become glucose-6-phosphate, which now bears a charge. This disqualifies it from leaving through glucose transporters.

Glucose catalyzed by isozymes of hexokinase to Glucose-6-phosphate 

by using ATP to ADP

Hexokinase 1-3 

Hexokinase 4 (glucokinase)

• inhibited by endproduct G-6-P
• low Km - high affinity for glucose 
• low Vmax 

• Glucokinase 
• in liver parenchymal cell & pancreatic beta cells
• High Km
• High Vmax
• Not inhibited by G-6-P