Cellular Transport: Active and Passive Mechanisms

Cellular Transport Mechanisms

• Transport mechanisms facilitate the entry of substances into the cell.

• Transport mechanisms allow for:

  • Maintaining and regulating body homeostasis
  • Digestion and nutrient absorption
  • Metabolism and energy transfer
  • Transporting hormones, enzymes, proteins, and ions

• Catalysts, enzymes, and hormones can influence cell transport.

• Exchange that occurs along the concentration gradient doesn't need added energy.

• Exchange against the concentration gradient needs extra metabolic energy.

Active vs. Passive Transport

• Active transport requires energy to move a substance across a membrane and against its concentration gradient, from low to high concentration.
• Active transport includes processes like endocytosis and exocytosis.
• Passive transport moves substances across the membrane without energy, from high to low concentration down the concentration gradient.
• Passive transport includes processes like diffusion, facilitated diffusion, and osmosis.

Active Transport

• Active transport uses cellular energy (ATP) to move molecules against a gradient, polar repulsion, or resistance.

• Active transport moves molecules across a cell membrane from a region of lower concentration to a region of higher concentration.

• Active transport is transport of material "against the concentration gradient."

• Active transport requires energy input, usually ATP.

• Active transport moves a solute against its concentration gradient to the concentrated side of the membrane.

• Active transport examples include:

  • The sodium-potassium pump exchanges sodium and potassium ions across cell walls.
  • Amino acids move along the human intestinal tract.
  • Calcium ions move from cardiac muscle cells.
  • Glucose moves in or out of a cell.
  • A macrophage ingests a bacterial cell.

• Active transport occurs when ions or molecules get transported across a membrane of higher concentration and uses enzymes while expending energy.

  • Glucose in the small intestine is absorbed by microvilli, passed into the bloodstream, and circulated to all organs.

Endocytosis

• Endocytosis involves part of the cell membrane engulfing a desired macromolecule, such as food particles, and taking them inside to form new food vesicles.
• Endocytosis brings particles into the cell that are too large to passively cross the cell membrane.
• Endocytosis can be:
  • Phagocytosis
  • Pinocytosis
  • Receptor-mediated endocytosis
• Endocytosis is a type of active transport.
  • A cell absorbs a membrane by invagination to form a vacuole.
  • Pregnancy occurs where an egg cell engulfs a sperm cell during fertilization and as the baby grows in the womb from absorbing food from the mother via the amniotic sac.
  • The process of solidifying engulfed substances is called phagocytosis.
  • The process of absorbing liquid is called pinocytosis

Phagocytosis

• Phagocytosis is the process by which a cell engulfs material to destroy it, feed on it, or get information from it, known as "Cellular eating."
• Through phagocytosis, food is taken by the cell and becomes a food vacuole in the cytoplasm.

Pinocytosis

• Pinocytosis is the ingestion of liquid into a cell by the budding of small vesicles from the cell membrane.
• Pinocytosis is also known as "Cellular drinking."

Receptor-Mediated Endocytosis

• Receptor-Mediated Endocytosis is a process by which cells absorb metabolites, hormones, and other proteins.
• Receptor-Mediated Endocytosis' specificity is mediated by receptor proteins located on depressed areas of the cell membrane called coated pits.

Exocytosis

• Exocytosis transports molecules, such as proteins, out of the cell by expelling them in an energy-using process.
• Waste materials and other secretory products are packaged by the golgi bodies in sacs called vesicles.
• These membrane-bound secretory vesicles are carried to the cell membrane.
• The contents are expelled out of the cell during exocytosis.
• Crying, the secretion of salty solution from tear glands, is an example of exocytosis.
• Exocytosis occurs when contents of a cell vacuole fuse with the plasma membrane for expulsion outside the cell.
  • Menstruation, where the egg cell combines with uterine lining tissues, hormones, and blood, is also an example of exocytosis.

Passive Transport

• With passive transport, substances move from an area of higher concentration to an area of lower concentration without needing energy.
• Large molecules are transported across the cell membranes by carrier proteins.

Kinds of Passive Transportation

• Diffusion
• Facilitated diffusion
• Osmosis

Diffusion

• The rate of diffusion for different substances is impacted by membrane permeability.
• Diffusion moves material from an area of high concentration to one of lower concentration.
• Moving solutes "down the concentration gradient" describes the process of diffusion.
• Diffusion happens when particles move from an area of high concentration to an area of low concentration.
  • Purple dye dissolving in water is an example of diffusion.
  • Diabetes involves excess glucose diffusing to cells of the blood stream, destroying beta cells and hampering insulin hormone production.

Facilitated Diffusion

• Facilitated diffusion is the spontaneous passage of molecules or ions across a biological membrane through specific transmembrane integral proteins.

Osmosis

• Osmosis is the diffusion of fluids through a cell membrane.
• Fluids, such as water, pass through a semi-permeable membrane in osmosis.
• The effect of osmosis on cells depends on whether the solution is hypertonic, isotonic, or hypotonic.
  • Hypertonic has a higher solution osmosis to the cell and will cause the cell to be flaccid and shrink. It then becomes plasmolyzed, ultimately dying by dehydration.
  • Isotonic solution osmosis won't see any change in equal gradient value.
  • Hypotonic solution osmosis will add excess water and cause the cell to swell.

Hypertonic

• Hypertonic means "Over," where the solute is over the amount of solvent.
• Hypertonic causes a net outflow of H2O, which deflates and shrinks the cell.

Hypotonic

• Hypotonic means "Under," where the amount of solute is under the amount of solvent.
• Hypotonic cells experience a net inflow of H2O that make the cell bloated.

Isotonic

• "Iso" means "Equal," and the amount of solute is equal to the amount of solvent.
• Isotonicity has no net flow and maintains the cell's normal shape.