lipid

Examples of lipid in the following topics:

• Connecting Lipids to Glucose Metabolism

  • Lipids can be both made and broken down through parts of the glucose catabolism pathways.
  • Like sugars and amino acids, the catabolic pathways of lipids are also connected to the glucose catabolism pathways.
  • The lipids that are connected to the glucose pathways are cholesterol and triglycerides.

• Lipid-Derived, Amino Acid-Derived, and Peptide Hormones

  • All hormones in the human body can be divided into lipid-derived, amino acid-derived, and peptide hormones.
  • One of the key, distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes whereas the amino acid-derived and peptide hormones cannot.
  • Most lipid hormones are derived from cholesterol, so they are structurally similar to it .
  • The primary class of lipid hormones in humans is the steroid hormones.
  • Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids.

• Digestion and Absorption

  • Emulsification is a process in which large lipid globules are broken down into several small lipid globules.
  • Lipids are hydrophobic substances.
  • The bile salts' hydrophilic side can interface with water, while the hydrophobic side interfaces with lipids, thereby emulsifying large lipid globules into small lipid globules.
  • Vitamins can be either water-soluble or lipid-soluble.
  • It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins.

• Intracellular Hormone Receptors

  • Lipid-soluble hormones diffuse across the plasma membrane of cells, binding to receptors inside the cells where they alter gene expression.
  • Lipid-derived (soluble) hormones such as steroid hormones diffuse across the lipid bilayer membranes of the endocrine cell.
  • At the target cell, the hormones are released from the carrier protein and diffuse across the lipid bilayer of the plasma membrane of the target cells.
  • Other lipid-soluble hormones that are not steroid hormones, such as vitamin D and thyroxine, have receptors located in the nucleus.

• Peroxisomes

  • Peroxisomes neutralize harmful toxins and carry out lipid metabolism and oxidation reactions that break down fatty acids and amino acids.
  • Peroxisomes perform important functions, including lipid metabolism and chemical detoxification.
  • Peroxisomes are membrane-bound organelles that contain an abundance of enzymes for detoxifying harmful substances and lipid metabolism.

• The Endoplasmic Reticulum

  • The endoplasmic reticulum is an organelle that is responsible for the synthesis of lipids and the modification of proteins.
  • The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids.
  • Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions.

• The Golgi Apparatus

  • Before reaching their final destination, the lipids or proteins within the transport vesicles still need to be sorted, packaged, and tagged so that they wind up in the right place.
  • Sorting, tagging, packaging, and distribution of lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranes .
  • As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted.
  • These newly-modified proteins and lipids are then tagged with phosphate groups or other small molecules so that they can be routed to their proper destinations.

• Fluid Mosaic Model

  • The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type.
  • For example, myelin contains 18% protein and 76% lipid.
  • The mitochondrial inner membrane contains 76% protein and 24% lipid.
  • Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane.
  • The principal components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and some of the proteins.

• Plasma Membrane Hormone Receptors

  • Amino acid-derived hormones and polypeptide hormones are not lipid-derived (lipid-soluble or fat-soluble); therefore, they cannot diffuse through the plasma membrane of cells.
  • Lipid-insoluble hormones bind to receptors on the outer surface of the plasma membrane, via plasma membrane hormone receptors.
  • Unlike steroid hormones, lipid-insoluble hormones do not directly affect the target cell because they cannot enter the cell and act directly on DNA.
  • The specific response of a cell to a lipid-insoluble hormone depends on the type of receptors that are present on the cell membrane and the substrate molecules present in the cell cytoplasm.

• Types of Biological Macromolecules

  • Biological macromolecules, the large molecules necessary for life, include carbohydrates, lipids, nucleic acids, and proteins.
  • Each different type of macromolecule, except lipids, is built from a different set of monomers that resemble each other in composition and size.
  • Lipids are not polymers, because they are not built from monomers (units with similar composition).