Metabolism: Catabolic and Anabolic Pathways
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Questions and Answers

What is the primary enzyme responsible for carbohydrate digestion in the small intestine?

  • Salivary amylase
  • Maltase
  • Pancreatic amylase (correct)
  • Sucrase
  • At what pH level is amylase active?

  • 8.0 - 9.0
  • 5.8 – 7.1 (correct)
  • 4.0
  • 1 - 2
  • Which disaccharide is broken down by lactase?

  • Lactose (correct)
  • Maltose
  • Sucrose
  • Galactose
  • How is fructose transported into epithelial cells of the villi?

    <p>Facilitated diffusion</p> Signup and view all the answers

    What happens to excess glucose in the human body?

    <p>It is stored as glycogen</p> Signup and view all the answers

    During fasting, which organ primarily degrades glycogen to release glucose into the bloodstream?

    <p>Liver</p> Signup and view all the answers

    What is the preferred energy source for the brain?

    <p>Glucose</p> Signup and view all the answers

    Which type of cells primarily require glucose as an energy source due to having very few mitochondria?

    <p>Red blood cells</p> Signup and view all the answers

    What is the primary fate of glucose when the body requires energy?

    <p>Oxidized to CO2 and H2O</p> Signup and view all the answers

    Which process converts excess glucose into glycogen?

    <p>Glycogenesis</p> Signup and view all the answers

    What happens to liver glycogen when blood glucose levels are low?

    <p>Reconverted to glucose and released into the blood</p> Signup and view all the answers

    Which glucose transporter is primarily involved in glucose uptake from the blood?

    <p>GLUT-4</p> Signup and view all the answers

    In which mechanism is glucose transported against a concentration gradient?

    <p>Na+-monosaccharide co-transporter system</p> Signup and view all the answers

    What results from the partial degradation of glucose during muscle contraction?

    <p>Formation of lactic acid</p> Signup and view all the answers

    Which type of glucose transport does not require energy?

    <p>Na+-independent facilitated diffusion</p> Signup and view all the answers

    What percentage of glucose can be used for synthesizing ribose and deoxyribose?

    <p>Small amounts</p> Signup and view all the answers

    What is the main purpose of catabolic reactions in metabolism?

    <p>To break down complex molecules and release energy</p> Signup and view all the answers

    What type of reaction is involved in the synthesis of glycogen from glucose?

    <p>Anabolic reaction</p> Signup and view all the answers

    Which of the following is NOT a source of glucose for the human body?

    <p>Lipids</p> Signup and view all the answers

    Which of these carbohydrates is a disaccharide?

    <p>Lactose</p> Signup and view all the answers

    What is the primary function of anabolic pathways?

    <p>To synthesize complex molecules from simpler ones</p> Signup and view all the answers

    How does fructose convert to glucose in the body?

    <p>By enzymatic conversion in the liver</p> Signup and view all the answers

    Which statement about digestion of carbohydrates is accurate?

    <p>Digestion requires enzymes from saliva, pancreatic and intestinal juices</p> Signup and view all the answers

    Which carbohydrate is primarily derived from a plant source?

    <p>Starch</p> Signup and view all the answers

    What type of transport is mediated by the sodium-dependent-glucose transporter (SGLT)?

    <p>Secondary active transport</p> Signup and view all the answers

    Which glucose transporter is known for its high affinity for glucose and is primarily found in neurons?

    <p>GLUT-3</p> Signup and view all the answers

    Which glucose transporter is abundant in adipose tissue and skeletal muscle and is regulated by insulin?

    <p>GLUT-4</p> Signup and view all the answers

    Where is GLUT-2 primarily expressed?

    <p>Liver and kidney</p> Signup and view all the answers

    Which glucose transporter is responsible for glucose transport in the blood-brain barrier?

    <p>GLUT-1</p> Signup and view all the answers

    What is the primary function of SGLT in the body?

    <p>Absorption and reabsorption of glucose</p> Signup and view all the answers

    Which glucose transporter does not respond to insulin?

    <p>GLUT-2</p> Signup and view all the answers

    Which glucose transporter facilitates bidirectional transport of glucose, particularly influenced by hormones?

    <p>GLUT-2</p> Signup and view all the answers

    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

    • Body needs for energy: glucose is completely oxidized to CO2, H2O, and energy via glycolysis and the citric acid cycle.

    • Excess glucose may be converted to glycogen and stored in the liver and muscle tissues (glycogenesis).

    • To maintain blood glucose levels, liver glycogen can be reconverted to glucose and released into the blood (glycogenolysis).

    • Excess glucose after glycogen conversion can be converted to fatty acids and stored in adipose tissue as triglycerides (lipogenesis).

    • Small amounts of glucose may be utilized for the synthesis of ribose and deoxyribose for nucleic acid synthesis.

    • 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

    • 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)
    • 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.

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