Biology: Tonicity and Osmosis Quiz

Tonicity

• Biologists use tonicity to quickly compare the relative concentration of solutes in different solutions.
• Hypertonic solutions have a higher solute concentration (osmolarity) compared to the reference solution.
• Hypotonic solutions have a lower solute concentration (osmolarity) compared to the reference solution.
• Isotonic solutions have the same solute concentration (osmolarity) as the reference solution.

Osmosis and Cell Membranes

• Water molecules easily move through cell membranes due to their small size and low polarity.
• Aquaporins are specialized protein channels that enhance the movement of water across cell membranes.

Diffusion

• Cells rely on diffusion to transport water and nutrients, but it's slower over longer distances.
• Diffusion is essential for supplying cells with oxygen and removing carbon dioxide during aerobic respiration.
• Even multicellular organisms can rely on diffusion for nutrient transport if their bodies are thin enough to maintain a favorable surface area to volume ratio.

Eukaryotic Cell Transport

• Phagocytosis is a form of active transport where cells engulf large food particles for internal digestion.
• Eukaryotic cells utilize the cytoskeleton and transport vesicles to rapidly distribute nutrients throughout the cell, allowing them to grow larger than prokaryotic cells.

Active Transport in Organisms

• Active transport involves the movement of solutes against their concentration gradients, requiring energy.
• Organisms use active transport to regulate water movement by adjusting solute concentrations in cells or tissues.
• Multicellular organisms have specialized vascular tissues to rapidly transport large volumes of water and dissolved substances using pressure gradients.

Vascular Tissues and Bulk Flow

• Vascular tissues (xylem and phloem) are responsible for conducting fluids in multicellular organisms like plants and fungi.
• Xylem transports water and minerals from roots to shoots.
• Phloem transports sugars from leaves or storage tissues to other areas of the plant.
• Sap movement in vascular plants is a combination of active and passive processes: active solute transport creates osmotic gradients that water passively follows.

Xylem and Transpiration

• Xylem is composed of dead, hollow cells that act like straws, facilitating water flow.
• Two types of xylem cells:
  • Tracheids (thin, tapered cells found in both gymnosperms and angiosperms)
  • Vessel elements (larger cells with greater water flow capacity, mainly found in angiosperms)
  • The cell walls of both tracheids and vessel elements are reinforced with cellulose and lignin to prevent collapse under pressure.
• Transpiration, the process by which water evaporates from plant leaves, drives the upward pull of water through the xylem, creating a chain of water molecules.
• Transpiration is a wasteful process because most of the water taken up by plants is lost to evaporation.

Stomata and Water Conservation

• Stomata, openings on plant leaves, regulate gas exchange and water loss.
• Guard cells surrounding stomata regulate opening and closing based on water content.
• Plants actively transport solutes into or out of guard cells to control the flow of water and optimize water conservation.
• To reduce transpiration, stomata are often located on the lower, shaded surfaces of leaves and remain mostly closed during hot periods.