Carbohydrates Flashcards
List the major carbohydrates in the diet with examples
Starch - cereals, potatoes, rice
Glycogen - meat
Cellulose and hemicellulose - Plant cell walls - we don’t digest this
Oligosaccharides containing linked glucose - Peas, beans, lentils - are not digested
Lactose, sucrose, maltose - milk, table sugar, beer
Glucose, fructose - fruit, honey
What are the main products of carbohydrate digestion?
Glucose (Glc)
Galactose (Gal)
Fructose (Fru)
What happens to food in the oesophagus in relation to the digestion of carbohydrates?
No significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication.
What does the acidic environment of the stomach do?
Stops the action of the amylase enzyme
Where does the second stage of carbohydrate digestion take place?
In the duodenum of the small intestine
What are the different types of carbohydrates?
Monosaccharides
Disaccharides
Maltose
Lactose
Sucrose
Polysaccharides
Strach
Glycolysis
What are the different types of monosaccharides?
Glucose
Galactose
Fructose
How are disaccharides formed?
From monomers that are linked via glycosidic bonds
What are the three important diasacchrides?
Maltose
Lactose
Sucrose
What is the breakdown product of starch?
Maltose
Where can maltose be found?
Beer (from the starch of barley)
Baby foods (natural sweetener)
What is the main sugar in milk?
Lactose
How is lactose formed?
From a glycosidic and between galactose and glucose
What is sucrose?
Table sugar, made by plants
What are polysaccharides?
Polymers of medium to high molecular weight
What is a homopolysaccharide?
Single monomeric series
What is a hetropolysacchride?
Two or more monomer species
What are the two types of starch?
Amylose
Amylopectin
How does amylopectin from amylose?
Similar structure but it is branched
What is glycogen?
Polymer of glucose linked sub units wit branches every 8 to 12 residues
Where is 90% of glycogen found?
Liver (acts to replenish blood glucose when fasting)
Skeletal muscle (catabolism produces ATP for contraction)
Why should glucose be stored in polymers?
Compactness
Amylopectin and glycogen have non-reducing ends
This allows them to be readily synthesised and degraded to and from monomers respectively - speeding up the formation or degradation
What are glycosaminoglycans?
Un-branched polymers made from repeating units of hexuronic acid and an amino-sugar, which alternate through the chains
Function of glycosaminoglycans?
GAGs play numerous functions in the ECM to regulate mechanical properties of a tissue: cell proliferation, cell adhesion, growth factor signaling, immune cell function, and collagen structure.
What are glycoproteins?
Any of a class of proteins which have carbohydrate groups attached to the polypeptide chain.
Where does the digestion of carbohydrates begin?
In the mouth - savilary enzyme amylase begins the breakdown of food starches into maltose which is a disaccharide.
What happens to chyme from the stomach?
Enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder.
What do pancreatic juices contain?
Amylase which continues the breakdown of starch and glycogen into maltose, a disaccharide.
How are disaccharides broken down?
Are broken down into monosaccharides by enzymes called maltases sucrases, and lactases, which are also present in the brush border of the small intestinal wall.
What is the role of maltase?
Breaks doen maltose into glucose
What breaks down sucrose and lactose?
Sucrase and lactase
What does sucrose break down into?
Sucrase breaks down sucrose into glucose and fructose
What is lactose broken down into?
Glucose and galactose.
What happens to glucose produced from carbohydrate digestion?
The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy.
The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body.
What is GLUT2 and what is it’s role?
A transporter specific for the glucose molecule in the gut
They recognise glucose and allow it to travel down its concentration gradient into the blood stream
How is glucose absorbed?
Indirectly through a ATP powered process
Absorption of monosaccharides
ATP-driven NA+ pump maintains a low cellular Na+ so glucose can continually be moved into the epithelial cells
This system continues to work even if glucose has to be moved into the epithelial cells against its concentration gradient (i.e when the blood is high)
Why can cellulose not be digested?
Cannot be digested by the gut as we don’t have enzymes to do that.
What is the role of cellulose?
They increase faecal gut - what that does is decrease the transit time so it takes less time for food to travel through to your gut which is important as it can potentially better your health.
What can a lack of oligosaccharides do?
Lack of oligosaccharides in diet can lead to poor health for example there are examples of cancer if food is left in the gut for too long.
How are polymers of cellulose be broken down?
Polymers are broken down by gut bacteria which can yield CH4 and H2
What can cause disaccharide deficiencies?
Can be genetic or result from:
- Severe intestinal infection
- Other inflammation of the gut lining
- Drugs injuring the lining
- Surgical removal of the intestinal gut
What characterises disaccharide deficiencies?
Abdominal digestion and cramps
Diagnosis of disaccharide deficiencies
Diagnosis would require enzyme tests of intestinal secretions - usually checking for lactase, maltase or sucrase activity.
What is the most common disaccharide deficiency?
Lactose intolerance
What causes the symptoms of lactose intolerance?
- Undigested lactose is broken down by gut bacteria causing gas builds up and irritant acids
- Lactose is osmotically active, thus drawing water from the gut into the lumen causing diarrhoea
How can symptoms of lactose intolerance be controlled?
- Avoiding milk products
- Using milk products treated with fungal lactase
- Supplementing diet with lactase
Where does glucose diffuse through to?
Glucose diffuses through the intestinal epithelium cells into the portal blood and on to the liver.
What is glucose phosphorylated into?
Glucose is immediately phosphorylated into glucose 6-phosphate by the hepatocytes or any other cell glucose enters
Why can glucose-6-phosphate not able to diffuse out of the cell?
GLUT transporters won’t recognise it - traps the glucose in the cell
What are the Km (for glucose) and Vmax for glucokinase?
Km - high
Vmax - low
What are the Km (for glucose) and Vmax for hexokinase?
Km - Low
Vmax - Low
What does high glucokinase Vmax mean?
High glucokinase Vmax, means that it can phosphorylate all the glucose quickly, thus most absorbed glucose is trapped in the liver
What does Hexokinase low Km mean?
Hexokinase low Km means that even at low glucose, tissues can grab glucose effectively
What does Hexokinase low Vmax mean?
Hexokinase low Vmax means tissues are “easily satisfied” so don’t keep “grabbing” glucose.
What is the role of the liver in maintaining steady blood glucose levels?
The liver plays a central role in maintaining steady blood glucose levels by converting excess glucose into glycogen through a process known as glycogenesis.
What is glycogenesis?
When there is bodily demand for glucose, the glycogen can be converted back into glucose through a process known as glycogenolysis.
What % of the monosaccharide load absorbed by the small intestine is comprised of glucose?
80% - which is then delivered to the hepatocytes by the portal vein.
What else is absorbed into the small intestine and what happens to it
Fructose and galactose - converted into glucose in the liver.
What happens when glucose is isolated?
Once glucose is isolated in the hepatocyte, a sequence of reactions occurs to synthesise glycogen.
Where do glycogenesis and glycogenolysis take place?
In the liver
Stages of glycogenolysis
Firstly, a molecule of glucose is phosphorylated to remove it from the glycogen chain by glycogen phosphorylase (assisted by a debranching enzyme) creating glucose-1-phosphate
Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase
Glucose-6-P is converted to glucose by glucose-6-phosphatase
Glucose-6-phosphate can also enter the glycolysis pathway at this point, which culminates in the formation of pyruvate.
How are glycogenesis and glycogenolysis regulated?
Through allosteric regulation and hormonal regulation
What is the key regulatory enzyme in glycogenesis?
Glycogen synthase.
What is the key regulatory enzyme in glycogenesis?
Glycogen synthase.
What is the key regulatory enzyme in glycogenolysis?
Glycogen phosphorylase
What act via secondary messengers to control the rate and direction of glycogen metabolism?
Glucagon, adrenaline and insulin.
What is the role of glycogen?
Provides energy to cell
Uses store of glygoen in liver to monitor blood glucose levels
What is glycolysis?
Metabolism of glucose into two pyruvate molecules, net generation of 2 ATP and 2 NADH.
What type of reaction is glycolysis?
Glycolysis is an anaerobic reaction, and in low oxygen conditions is the cell’s sole source of ATP.
How many parts is glycolysis considered to have?
Glycolysis can be considered as a two-part process.
What are the two parts of glycolysis?
Energy investment phase
Energy payout phase
What happens in the energy investment phase?
Requires two ATP molecules to produce high energy intermediates
What happens in the energy investment phase?
Requires two ATP molecules to produce high energy intermediates
What happens in the energy payout phase?
The intermediate is metabolised, producing four ATP molecules and two NADH molecules
What is the net effect of glycolysis?
Net effect is that 2 ATP and 2 NADH are produced
What reactions of glycolysis and unidirectional and are therefore key regulatory steps?
1
3
10
What happens for one glucose passes through the preparatory phase?
Two molecules of G3P formed to enter the payoff phase?
ATP generated for each glucose?
2 ATP used in preparatory phase
4 ATP used in the payoff phase
NET GAIN 2 ATP
How can substrates enter the glycolysis pathway?
- Dietary glucose
- Glycogenolysis
- Other monosaccharides
How do substrates enter the glycolysis pathway via dietary glucose?
Glucose is directly absorbed into the bloodstream from the gastrointestinal tract and enters the pathway.
How do substrates enter the glycolysis pathway via glycogenesis?
Glucose is released from hepatic stores of glycogen and enters the pathway.
How do substrates enter the glycolysis pathway via monosaccharides?
galactose and fructose enter the glycolysis pathway at various levels via common intermediates.
What needs to happen in order for circulating glucose to be used by cells?
Needs to pass from the extracellular space (blood stream) into the intracellular space.
What transports glucose into cells?
GLUT-1
GLUT-2
GLUT-4
What is the fate of pyruvate?
Pyruvate is a versatile molecule which feeds into numerous pathways. After glycolysis, it can be converted to acetyl-CoA, which has numerous metabolic destinations, including the TCA cycle.
Describe the covalent bond in a disaccharide
Hydroxyl group of one monosaccharide forms a covalent bond with an anomeric carbon of another monosaccharide
What is meant by an anomeric carbon?
Carbon 1 of one monosaccharide is the mirror image of the other carbon on another monosaccharide
Name three important disaccharides
Maltose
Lactose
Sucrose
What is the direct breakdown product of starch?
Maltose
Why is maltose considered a reducing sugar?
Anomeric Carbon 1 is available for oxidation
Is sucrose reducing or non-reducing?
Non-reducing?
What are the two forms of starch?
Amylopectin
Amylose
What proportion of starch is amylopectin?
75-80%
Describe the structure of amylose
D-glucose units with alpha 1-4 linkage
Describe the structure of amylopectin
Branches occur every 24-30 monomers
Branches linked with alpha 1-6 glycosidic bonds
Describe the overall shape of amylose and amylopectin
Form alpha helices
What is the difference in structure between amylopectin and glycogen?
In glycogen, branches occur much more frequently (every 8 to 12 units)
Where can you find the glycogen in the body?
90% liver
10% skeletal muscles
Amylopectin and glycogen are said to have many non-reducing ends, why might this be beneficial?
Synthesis and degradation to and from individual monomers becomes easier and therefore fast
What is the possible effect of a carbohydrate on the protein?
Increase the proteins solubility
Influence protein folding and conformation
Protect it from degradation
Act as communication between cells
What are glycosaminoglycans ?
Unbranched polymers made from repeating units of hexuronic acid and an amino sugar - found in mucus and synovial fluid around the joints
What are proteoglycans?
A compound consisting of protein bonded to glycosaminoacid groups (mucopolysaccharide groups)
There is more protein than carbohydrate usually
Where can you find glycosamionglycans?
Found on the surface of cells or in between cells in the extracellular matrix
What type of tissue are proteglycans involved in?
Connective tissue
What type of proteins might be glycoproteins?
Cytoplasmic and nuclear proteins
What does salivary amylase hydrolyse?
The 1,4 glycosidic bonds in starch
Where and what is the duodenum?
It is the shortest part of the small intestine where most chemical digestion takes place. It precedes the jejunum.
Where are carbohydrates digested?
Absorbed into the blood stream and transported into the liver for digestion
What enzyme works in the duodenum to break down carbohydrates?
Pancreatic amylase - hydrolyses 1,4 bonds as in mouth
What happens in the jejunum?
Final digestion by mucosal cell surface enzymes
What enzymes are contained within the jejunum and what are their functions?
Isomaltase - hydrolyses the 1,6 bonds
Glucoamylase - releases glucose from non-reducing ends
Sucrase - Hydrolyses sucrose
Lactase - Hydrolyses lactose
What is the function of high sodium levels in glucose transport within the intestinal lumen?
Glucose symporter - High sodium levels drive glucose across the membrane
How does the glucose leave the epithelial cell into the blood?
Through a glucose uniporter - facillitates downhill eflux (diffusion)
Which other monosaccharide uses other gradients to facilitate its transport?
Galactose
What is the function of cellulose within the gut?
Increases faecal bulk and decreases transit time
What are cellulose polymers broken down by gut bacteria to produce?
CH4 and H2
Why does lactase deficiency cause diarrhoea?
Lactose is osmotically active - drawing water from the gut in to the intestinal lumen
How would you diagnose disaccharidase deficiencies?
Enzyme tests of intestinal secretions - checking for lactase maltase and sucrase activity low levels of enzymes might reflect low levels of disaccharide?
What happens to glucose after it is absorbed by the intestinal epithelium cells?
Enters hepatic portal vein and travels to the liver
What is the effect of hepatocytes on absorbed glucose?
They immediately phospohorylate them to into glucose-6 phosphate
Why can’t glucose 6 phosphate leave the cell?
Glucokinase (liver) and hexokinase (other cells)
When is most glucose trapped in the liver?
After a meal, glucokinase phosphorylates all the glucose quickly.
How many molecules of G-3-P enter the payoff phase?
3
What is the first reaction of glycolysis?
Phosphorylation of glucose
How much ATP is used during the first stage of glycolysis? (phosphorylation of glucose) And what enzyme is used?
1 ATP
Hexokinase
Why does the second step of glycolysis proceed in both directions?
Reaction has a low free energy
Where does NAD+ come from in the cell?
Niacin - an essential vitamin
What is the difference between substrate level phosphorylation and respiration linked phosphorylation ?
Substrate level - requires soluble enzymes and chemical intermediates
Respiration linked means membrane bound enzymes and gradients of protons
What compound is used to replenish NAD+?
The different fates of pyruvate
How is pyruvate converted into ethanol?
Pyruvate + pyruvate carboxylase = Acetaldehyde
Acetaldehyde + Alcohol dehydrogenase + NADH = Ethanol + NAD+
What is pyruvate converted into in aerobic conditions?
Acetyl Co-A, in mitochindria, NADH is formed
