Biology: Water Potential and Cell Movement

Study Notes

Water potential is the tendency of water to move from one area to another.
Water moves from an area of higher water potential to an area of lower water potential.
Water potential is influenced by solute concentration and pressure.
Areas with higher solute concentration have lower water potential.
Increased pressure, such as turgor pressure in plant cells, raises water potential.

### Effects of Water Potential and Molarity on Cell Movement

Hypotonic Solution:
- Solution has lower solute concentration (higher water potential) than the cell's cytoplasm.
- Water moves into the cell, causing it to swell.
- Plant cells gain turgor pressure.
- Animal cells may lyse (burst) due to excessive swelling.
Hypertonic Solution:
- Solution has higher solute concentration (lower water potential) than the cell's cytoplasm.
- Water moves out of the cell, causing it to shrink.
- Plant cells undergo plasmolysis (plasma membrane detaches from the cell wall).
- Animal cells undergo crenation (shriveling).
Isotonic Solution:
- Solute concentration (and water potential) is equal inside and outside the cell.
- No net movement of water.

Movement of water molecules across a selectively permeable membrane from an area of high water concentration to an area of lower water concentration.
Movement continues until water concentration is equal on both sides of the membrane.
Direction and rate of osmosis are determined by the difference in solute concentration across the membrane.
Water moves towards the region of higher solute concentration to equalize concentrations.

Describes the ability of a surrounding solution to cause a cell to gain or lose water through osmosis.
Hypotonic: Lower solute concentration outside the cell, water moves into the cell, causing it to swell. Plant cells thrive in hypotonic environments. Animal cells risk bursting (lysis).
Hypertonic: Higher solute concentration outside the cell, water moves out of the cell, causing it to shrink. Plant cells undergo plasmolysis. Animal cells shrink and lose structural integrity.
Isotonic: Equal solute concentration, no net movement of water. Animal cells function optimally in isotonic environments.

Active regulation of osmotic pressure within an organism's cells and tissues to maintain water balance.
Prevents excessive uptake or loss of water.
Organisms with cell walls (plants, fungi, bacteria, protists) have rigid cell walls that provide structural support and prevent bursting in hypotonic environments.
Turgor pressure is crucial for cell growth and function.

Amphipathic molecules with hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.
Hydrophilic head: glycerol molecule linked to a phosphate group.
Hydrophobic tails: two fatty acid chains attached to glycerol.
Phospholipids spontaneously arrange into a bilayer in an aqueous environment.
Hydrophilic heads face outward, interacting with the watery environment.
Hydrophobic tails are sequestered in the middle of the bilayer, shielded from the aqueous environment.

Proposed by S.J. Singer and G.L. Nicolson in 1972.
Describes the cell membrane as a dynamic and fluid structure.
Phospholipids and other components can drift laterally within their layer.
Fluidity is crucial for membrane function: flexibility, self-repair, movement of proteins and other molecules within the membrane.

### Proteins

Integral Proteins:
- Embedded within the phospholipid bilayer.
- Often span the entire membrane (transmembrane proteins).
- Have hydrophobic regions that interact with the phospholipid tails and hydrophilic regions that interact with the aqueous environment.
Peripheral Proteins:
- Located on the surfaces of the membrane.
- Often attached to integral proteins or the polar heads of phospholipids.

Transporters: Facilitate the movement of molecules and ions across the membrane. Can be passive (no energy required) or active (requires energy).

### Endosymbiotic Theory

Proposed origin of mitochondria and chloroplasts - they were once free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

Structural similarities:
- Size and shape of mitochondria and chloroplasts similar to prokaryotic cells.
- Double membrane of these organelles resembles prokaryotic cell membranes.
Functional similarities:
- Mitochondria and chloroplasts replicate independently through a process similar to binary fission.
- They possess their own DNA and ribosomes, allowing them to synthesize their own proteins.
Genetic evidence:
- Analyses of mitochondrial and chloroplast DNA sequences show similarities to specific groups of free-living bacteria.

Surface Area to Volume Ratio:
- A cell's size is limited by the relationship between its surface area and volume.
- Larger cells have smaller surface area-to-volume ratio, hindering efficient transport of nutrients and waste products across the plasma membrane.
Plasma Membrane:
- Defines the cell's boundary.
- Fluid mosaic model structure allows it to be selectively permeable.