Podcast
Questions and Answers
What is the primary enzyme responsible for carbohydrate digestion in the small intestine?
At what pH level is amylase active?
Which disaccharide is broken down by lactase?
How is fructose transported into epithelial cells of the villi?
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What happens to excess glucose in the human body?
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During fasting, which organ primarily degrades glycogen to release glucose into the bloodstream?
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What is the preferred energy source for the brain?
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Which type of cells primarily require glucose as an energy source due to having very few mitochondria?
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What is the primary fate of glucose when the body requires energy?
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Which process converts excess glucose into glycogen?
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What happens to liver glycogen when blood glucose levels are low?
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Which glucose transporter is primarily involved in glucose uptake from the blood?
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In which mechanism is glucose transported against a concentration gradient?
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What results from the partial degradation of glucose during muscle contraction?
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Which type of glucose transport does not require energy?
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What percentage of glucose can be used for synthesizing ribose and deoxyribose?
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What is the main purpose of catabolic reactions in metabolism?
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What type of reaction is involved in the synthesis of glycogen from glucose?
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Which of the following is NOT a source of glucose for the human body?
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Which of these carbohydrates is a disaccharide?
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What is the primary function of anabolic pathways?
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How does fructose convert to glucose in the body?
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Which statement about digestion of carbohydrates is accurate?
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Which carbohydrate is primarily derived from a plant source?
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What type of transport is mediated by the sodium-dependent-glucose transporter (SGLT)?
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Which glucose transporter is known for its high affinity for glucose and is primarily found in neurons?
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Which glucose transporter is abundant in adipose tissue and skeletal muscle and is regulated by insulin?
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Where is GLUT-2 primarily expressed?
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Which glucose transporter is responsible for glucose transport in the blood-brain barrier?
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What is the primary function of SGLT in the body?
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Which glucose transporter does not respond to insulin?
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Which glucose transporter facilitates bidirectional transport of glucose, particularly influenced by hormones?
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Study Notes
Metabolism
- The sum of chemical changes happening in a cell, tissue or the body
- Can be classified as either catabolic (degradative) or anabolic (synthetic)
Catabolic Reactions
- Break down complex molecules into simple ones
- Examples include breaking down proteins, polysaccharides, and lipids into CO2, NH3 (ammonia), and water
Anabolic Pathways
- Form complex molecules from simple precursors
- Example is the synthesis of glycogen from glucose
- Require energy (endergonic), which is generally provided by the breakdown of ATP to adenosine diphosphate (ADP) and inorganic phosphate (Pi)
Catabolic Pathways
- Capture chemical energy in the form of adenosine triphosphate (ATP)
- This energy is captured from the degradation of energy-rich fuel molecules
- Convert molecules in the diet (or nutrient molecules stored in cells) into building blocks needed for the synthesis of complex molecules
Anabolic Pathways
- Combine small molecules to form complex ones
- Example is combining amino acids to form proteins
- Often involve chemical reductions where reducing power is provided by NADPH
Sources of Glucose
- Carbohydrates in the diet are the primary source, especially after meals
- Excess glucose is stored as glycogen in the liver and skeletal muscles
Sources of Glucose in the Carbohydrate Diet
- Free monosaccharides, such as glucose and fructose. Fructose is converted into glucose in the liver.
- Disaccharides, such as sucrose (glucose and fructose), lactose (glucose and galactose), and maltose (glucose and glucose). These are digested into monosaccharides in the intestine. Fructose and galactose are converted into glucose in the liver.
- Polysaccharides, such as starch (plant source e.g. rice, potato, flour) and glycogen (animal source). These are digested into glucose in the gastrointestinal tract.
Digestion and Absorption
- Digestion of carbohydrates is achieved by enzymes found in saliva, pancreatic juice, and intestinal juice.
Mouth
- Salivary glands secrete saliva
- Saliva contains: a- amylase (ptyalin), water and glycoprotein
- PH of a- amylase = 5.8 – 7.1, less than 4.0 is inactive
Stomach
- No digestion of carbohydrates occurs in the stomach
- Amylase is inactive due to the acidic PH of the stomach (1-2)
Small intestine
- This is the major site of carbohydrate digestion
- Pancreatic amylase is the primary enzyme here
- The optimum PH for amylase = 7.1
- Intestinal mucosal (brush border) cells have membrane-bound enzymes that complete the process of breaking down disaccharides:
- Maltose is broken down by maltase into glucose + glucose
- Sucrose is broken down by sucrase into glucose + fructose
- Lactose is broken down by lactase into glucose + galactose
Absorption of Carbohydrates
- 1. Transport into epithelial cells: glucose and galactose are actively transported, while fructose is transported by facilitated diffusion.
- 2. Transport from epithelial cells into the blood stream: facilitated diffusion is used.
Fate of Glucose after Absorption
- In the liver, glucose undergoes various chemical changes depending on the body's needs.
Glucose in the Body
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Body needs for energy: glucose is completely oxidized to CO2, H2O, and energy via glycolysis and the citric acid cycle.
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Excess glucose may be converted to glycogen and stored in the liver and muscle tissues (glycogenesis).
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To maintain blood glucose levels, liver glycogen can be reconverted to glucose and released into the blood (glycogenolysis).
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Excess glucose after glycogen conversion can be converted to fatty acids and stored in adipose tissue as triglycerides (lipogenesis).
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Small amounts of glucose may be utilized for the synthesis of ribose and deoxyribose for nucleic acid synthesis.
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During muscle contraction, only partial degradation of glucose may occur, resulting in the formation of lactic acid which is disposed of by the liver.
Glucose Transport into Cells
- Glucose cannot diffuse directly into cells, but is transported using one of two mechanisms:
- Na+-independent, facilitated diffusion
- Na+-monosaccharide cotransporter
Types of Glucose Transporters
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1. Na+-independent facilitated diffusion:
- Transport happens with the concentration gradient so doesn't require energy (ATP).
- Carried out by a group of at least 14 glucose transporters (GLUT-1 to 14)
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2. Na+-monosaccharide cotransporter system:
- Glucose is transported against a concentration gradient from low external concentrations to higher internal concentrations.
- This is an energy-requiring process that uses a carrier called a sodium-dependent-glucose transporter or SGLT.
Glut Protein Functions
- GLUT proteins are found in various tissues and have specialised functions.
- GLUT-1, GLUT-3, and GLUT-4 are primarily involved in glucose uptake from the blood.
- GLUT-2, found in the liver and kidney, transports glucose into these cells when blood glucose levels are high and out of these cells when levels are low (e.g., during fasting).
Glut Proteins are Tissue-Specific
- GLUT-1 is abundant in erythrocytes and the blood brain barrier, but low in adult muscle.
- GLUT-2 is present in the liver and kidney (also found in pancreatic beta cells).
- GLUT-3 is the primary glucose transporter in neurons.
- GLUT-4 is abundant in adipose tissue and skeletal muscle.
Glucose Transporter Table
| Transporter | Tissues | Type of transport | Notes | Sensitive to insulin? |
|---|---|---|---|---|
| SGLT | Renal tubules, intestinal epithelia (apical membrane) | Secondary active transport | Responsible for the absorption (intestine) and reabsorption (renal tubule cells) of glucose. | No |
| GLUT1 | Pancreatic beta cells, hepatocytes | Facilitated diffusion | Pancreatic beta cells: important for gauging blood glucose levels in humans. | No |
| GLUT2 | Pancreatic beta-cells, hepatocytes, intestinal epithelium, kidney | Facilitated diffusion | Hepatocytes: bi-direction transport of glucose when influenced by hormones, such as thyroid hormone. | No |
| Hepatocytes: important for the bi- directional transport of glucose with regards to hepatic glucose metabolism. | ||||
| GLUT3 | CNS | Facilitated diffusion | Very high affinity for glucose. | No |
| GLUT4 | Skeletal muscle, cardiac muscle, adipose tissue | Facilitated diffusion | Expression regulated by insulin. |
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Description
Explore the fundamental processes of metabolism, including catabolic and anabolic reactions. Understand how complex molecules are broken down and synthesized, and learn about the energy transformations involved in these pathways. This quiz will test your knowledge of key metabolic concepts and examples.
