Cell Membrane | Anatomy2Medicine
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Cell Membrane

    • Plasma Membrane
      • The lipid bilayer divides the cell into functional compartments.

 

  • The fluid mosaic model is the accepted view of the molecular nature of plasma membranes.(MCQ)

 

      • proteins traverse the lipid bilayer and are incorporated within the lipids.
      • Proteins and lipids can move freely in the plane of the membrane, producing the fluid nature of the membrane.
      • The plasma membrane is composed of phospholipids and proteins.

 

  • Membrane lipids can be classified into three major classes: phospholipids,
  • sphingolipids, and cholesterol.
  • Phospholipids

 

        • most abundant membrane lipids. .(MCQ)
        • They have a bipolar (amphipathic) nature

 

  • contain a charged head group and two hydrophobic (water-insoluble, noncharged) tails.

 

        • The hydrophobic tails face each other, forming a bilayer and exposing the polar head group to the aqueous environment on either side of the membrane.

 

  • Sphingolipids

 

        • have an amphipathic structure similar to phospholipids
        • allows them to insert into membranes.
        • These lipids can be modified by the addition of carbohydrate units at their polar end, creating glycosphingolipids in brain cells.

 

  • Cholesterol

 

          • predominant sterol in human cells
          • it increases the fluidity of the membrane by inserting itself between phospholipids, improving membrane stability.

 

  • Membrane proteins

 

      • span the lipid bilayer are known as integral membrane proteins
      • The majority of integral membrane proteins span the bilayer through the formation of Alpha-helices
      • Protein content of membranes varies from
        • less than 20% for myelin,
        • more than 60% in liver cells

Topic – Gastrointestinal hormones

    • Gastrin
      • secreted by G (gastrin) cells in the antrum of the stomach

 

  • 2 Forms

 

        • a 17-amino acid straight chain peptide
          • is called G17 or “little” gastrin

 

  • the form of gastrin secreted in response to a meal (MCQ)

 

        • 34-amino acid form of gastrin
          • is called G34 or “big” gastrin,

 

  • is secreted during the interdigestive period (between meals) (MCQ)

 

          • is secreted at low basal levels.
          • each form of gastrin has its own biosynthetic pathway, beginning with its own pre- cursor, a progastrin molecule.
      • The minimum fragment necessary for biologic activity of gastrin is the C-terminal tetrapeptide
      • Secretion of gastrin.
        • The physiologic stimuli that initiate gastrin secretion all are related to ingestion of food.
          • products of protein digestion (e.g., small peptides and amino acids),
          • distention of the stomach by food
          • vagal stimulation.
        • Among the products of protein digestion, the amino acids phenylalanine and tryptophan are the most potent stimuli for gastrin secretion. (MCQ)
        • Local vagal reflexes also stimulate gastrin secretion.
        • In these local reflexes, the neurocrine released from vagal nerve endings onto the G cells is gastrin-releasing peptide (GRP), or bombesin. (MCQ)
        • gastrin secretion is inhibited by a (MCQ)

 

  • low pH of the gastric contents
  • somatostatin.

 

    • Actions of gastrin.
      • It stimulates H+ secretion by gastric parietal cells
      • it stimulates growth of the gastric mucosa, a trophic effect
  • Cholecystokinin
    • The functions of cholecystokinin (CCK) are coordi- nated to promote fat digestion and absorption.
    • CCK is a 33-amino acid peptide
      • a member of the “gastrin-CCK family”
      • CCK has some gastrin activity.
      • CCKA receptors are selective for CCK
      • CCKB receptors are equally sensitive to CCK and gastrin.
      • The minimum fragment of CCK necessary for its bio- logic activity is the C-terminal heptapeptide (seven amino acids [CCK-7]).
    • CCK is secreted by the I cells of the duodenal and jejunal mucosa (MCQ)
    • two types of physiologic stimuli
      • monoglycerides and fatty acids (but not tri- glycerides
      • small peptides and amino acids.
    • There are five major actions of CCK, (MCQ)
      • Contraction of the gallbladder with simultaneous relaxation of the sphincter of Oddi
        • ejects bile from the gallbladder into the lumen of the small intestine.
        • Bile is needed for emulsification and solubilization of dietary lipids.
      • Secretion of pancreatic enzymes.
      • Secretion of bicarbonate (HCO3-) from the pan- creas.
      • Growth of the exocrine pancreas and gallbladder.
      • Inhibition of gastric emptying

 

    • Secretin
      • Secretin, a 27-amino acid peptide
      • structurally homologous to glucagon (MCQ)
      • a member of the secretin-glucagon family
      • In contrast to gastrin and CCK, which have active fragments, all 27 amino acids of secretin are required for its biologic activity.

 

  • For activity, the entire secretin molecule must fold into its tertiary structure, an alpha helix.

 

      • Secretin is secreted by the S cells (secretin cells) of the duodenum(MCQ)
      • Produced in response to H+ and fatty acids in the lumen of the small intestine.

 

  • Thus, secretion of secretin is initiated when the acidic gastric contents (pH < 4.5) arrive in the small intestine. (MCQ)

 

      • The function of secretin
        • to promote the secretion of pancreatic and biliary HCO3-, which then neutra- lizes H+ in the lumen of the small intestine.
          • Neutralization of H+ is essential for fat digestion
          • pancreatic lipases have pH optimums between 6 and 8,
          • they are inactivated or denatured when the pH is less than 3. (MCQ)
        • Secretin also inhibits the effects of gastrin on the parietal cells (H+ secretion and growth).
    • Glucose-Dependent Insulinotropic Peptide (GIP),
      • a 42-amino-acid peptide
      • member of the secretin-glucagon family
      • GIP produce most of the actions of secretin. (MCQ)
      • GIP is secreted by K cells of the duodenal and jeju- nal mucosa.
      • It is the only gastrointestinal hormone that is secreted in response to all three types of nutrients: glucose, amino acids, and fatty acids.
      • The major physiologic action of GIP is stimulation of insulin secretion by the pancreatic b cells. (MCQ)

 

  • The other action of GIP is inhibition of gastric H+ secretion.

 

  • Candidate Hormones
    • Motilin
      • a 22-amino acid peptide
      • secreted from the upper duodenum during fasting states. (MCQ)
      • Motilin initiates the interdigestive myoelectric complexes that occur at 90-minute intervals.
    • Pancreatic polypeptide
      • 36-amino acid peptide
      • secreted by the pancreas in response to ingestion of carbohydrates, proteins, or lipids. inhibits pancreatic secretion of HCO3- and enzymes(MCQ)
    • Enteroglucagon
      • released from intestinal cells
      • released in response to a decrease in blood glucose concentration.
      • It then directs the liver to increase glycogenolysis and gluconeogenesis. (MCQ)

 

    • Paracrines
      • are synthesized in endocrine cells of the gastrointestinal tract.
      • The paracrines do not enter the systemic circulation but act locally, reaching their target cells by diffusing over short distances.
      • Somatostatin
        • secreted by D cells (both endocrine and paracrine) of the gastrointestinal mucosa (MCQ)
        • secreted in response to decreased luminal pH. (MCQ)
        • inhibits secretion of the other gastrointestinal hormones
        • inhibits gastric H+ secretion. (MCQ)

 

  • In addition to these paracrine functions in the gastrointestinal tract, somatostatin is secreted by the hypothalamus and by the delta (d) cells of the endocrine pancreas. (MCQ)

 

    • Histamine
      • secreted by endocrine-type cells of the gastrointestinal mucosa, particularly in the H+-secreting region of the stomach.
      • Histamine, along with gastrin and ACh, stimulates H+ secretion by the gastric parietal cells. (MCQ)
  • NEUROCRINES
    • Neurocrines are synthesized in cell bodies of gastrointestinal neurons