Cell Transport Mechanisms in Frogs

• Cell transport involves mechanisms for moving substances across the plasma membrane.
• Cell transport types include passive transport, active transport, and bulk transport.

Plasma Membrane

• It separates the living cell from its surroundings.
• It exhibits selective permeability, allowing some substances to cross more easily than others.
• Phospholipids are the most abundant lipid.
• Phospholipids are amphipathic, containing hydrophobic and hydrophilic regions.
• The fluid mosaic model describes the membrane as a fluid structure with a "mosaic" of embedded proteins.
• Proteins aren't randomly placed; their distribution is specific.
• Phospholipids can move within the bilayer, drifting laterally.
• Lipids rarely flip-flop transversely across the membrane.

Proteins

• Proteins determine most of the membrane's specific functions.
• Peripheral proteins are bound to the membrane's surface.
• Integral proteins penetrate the hydrophobic core.
• Integral proteins spanning the membrane are transmembrane proteins.
• Hydrophobic regions of integral proteins consist of nonpolar amino acid stretches, often coiled into alpha helices.
• Six major functions are transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and ECM.

Membrane Carbohydrates

• They allow cells to recognize each other through binding.
• They are covalently bonded to lipids (glycolipids) or proteins (glycoproteins).
• Carbohydrates vary among species, individuals, and even cell types.
• Membranes have distinct inside and outside faces.
• The asymmetrical distribution of proteins, lipids, and associated carbohydrates is determined during membrane construction in the ER and Golgi apparatus.

Selective Permeability

• Cells exchange materials with their surroundings.
• Plasma membranes are selectively permeable, controlling molecular traffic.
• Hydrophobic (nonpolar) molecules dissolve and pass through the lipid bilayer rapidly.
• Hydrophilic molecules, including ions and polar molecules, don't cross the membrane easily.

Passive Transport

• It involves the diffusion of a substance across a membrane without energy investment.
• Diffusion is the molecule's tendency to spread out into available space.
• Diffusion of a population of molecules may be directional.
• At dynamic equilibrium, the rate of molecules crossing the membrane is the same in both directions.
• Substances diffuse down their concentration gradient, moving from areas of high to low density.
• It does not require work by the cell.

Osmosis

• It involves the diffusion of water across a selectively permeable membrane.
• Water diffuses from regions of lower solute concentration to higher solute concentration until it's equal on both sides.

Tonicity

• It is the ability of surrounding solutions to cause cells to gain or lose water.
• Isotonic solution: Solute concentration is the same inside/ outside the cell; water movement is equal.
• Hypertonic solution: Solute concentration is greater outside than inside the cell; the cell loses water.
• Hypotonic solution: Solute concentration is less outside than inside the cell; the cell gains water.
• Hypertonic or hypotonic environments cause osmotic problems for organisms.
• Osmoregulation controls solute concentrations and water balance is a necessary adaptation for life.
• Some protists like Paramecium possess contractile vacuoles, which help pump excess water out of the cell.

Simple Diffusion

• Molecules flow directly through the membrane.
• Seen in skin structure for topical applications, cutaneous respiration in Amphibians, and alveolar gas exchange.

Facilitated Diffusion

• It speeds passive movement of molecules across the plasma membrane with transport proteins
• Channel proteins provide corridors for specific molecules or ions to cross.
  • Aquaporins facilitate water diffusion.
  • Ion channels facilitate ion diffusion.
  • Gated channels open or close in response to a stimulus.
  • Ligand-gated channels open by neurotransmitters
  • Mechanically-gated channels open by pressure
  • Some channels are always open
  • Voltage-gated channels open by voltage change
• Carrier proteins undergo shape changes to translocate solute-binding sites across the membrane
• Glucose transporter type 1(GLUT 1) transports glucose.

Filtration

• Forces small molecules across plasma membranes using hydrostatic (water) pressure.
• An example of this is seen as blood pressure forces water and dissolves wastes out of blood vessels and into the kidney tubules in the first step of urine formation.

Active Transport

• Moves substances against the concentration gradients. It requires energy, usually in the form of ATP, and is performed by specific proteins embedded in the membranes.
• Uniporters transport a single of molecule type across the membrane
• Symporters transport two molecules/ions in the same direction
• Antiporters transport two molecules/ions in opposite directions
• Allows cells to maintain concentration gradients that differ from their surroundings.
• The sodium-potassium pump is an example.

Cotransport

• Occurs when active transport of a solute indirectly drives the transport of other substances.

Secondary Active transport : Sodium Glucose Linked Transport (SGLT)

• Example of cotransport, involving sodium-potassium pumps (Na+-K+), sodium-glucose co-transporter protein, and channel Protein

Exocytosis and Endocytosis

• Small molecules and water enter or leave the cell via transport proteins or through the lipid bilayer. Large molecules such as proteins and polysaccharides, cross the membrane in bulk via vesicles and requires energy
• In Exocytosis, transport vesicles migrate to and fuse with the membrane, releasing their contents outside of the cell.
  • Many secretory cells export their product from the cell via exocytosis
• In Endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
• Utilizes Motor proteins, kinesin and dynein
• Phagocytosis is cell eating solid matter
• Pinocytosis is cell drinking liquid matter
• Receptor-mediated endocytosis is receptor coated endocytosis of LDL that may undergo several stages.

Membrane potential

• Is the voltage difference across a membrane. Voltage is created by differences in the distribution of positive and negative ions across the membrane.
• electrochemical gradient, drives the diffusion of ions across a membrane
  • A chemical force (the ion's concentration gradient)
  • An electrical force (the effect of the membrane potentialon the ion's movement)
• Electrogenic pump is a transport protein , generates voltage across the membrane
  • The sodium-potassium pump is the major electrogenic pump of animal cells
  • The main electrogenic pump of plants, fungi, and bacteria is a proton pump

Explore cell transport mechanisms in frogs including endocytosis, exocytosis, and osmosis. Understand how frog cells maintain balance and transport molecules. This covers active and passive transport in frog cells.