The Properties of Water and Cell Structure
Water’s Vital Role and Cell Structure
The Remarkable Properties of Water
Density of Ice
Ice, the solid form of water, is less dense than liquid water, allowing it to float. This unique property enables aquatic life to thrive beneath ice sheets in cold environments.
Water’s Functions
Water’s properties contribute to its diverse roles:
- Metabolic Function: Water dissolves substances, facilitating metabolic reactions and acting as a reagent in processes like hydrolysis during digestion. It also provides hydrogen for plant photosynthesis.
- Transport Function: Water’s liquid state and excellent solvent properties make it an ideal vehicle for transporting substances within and between organisms.
- Structural Function: Water’s cohesive and adhesive forces shape cells lacking rigid membranes, regulating cytoplasmic changes and deformations.
- Lubricant and Buffer Function: Water’s low viscosity allows it to act as a lubricant, reducing friction between surfaces.
- Thermoregulatory Function: Water’s high specific heat and heat of vaporization regulate body temperature, mitigating abrupt external temperature changes and maintaining constant internal temperature in endothermic homeotherms.
Osmosis
Osmosis is the process by which water moves across a semipermeable membrane, equalizing the concentration of two solutions with different initial concentrations. Water flows from the more dilute (hypoosmotic or hypotonic) solution to the more concentrated (hyperosmotic or hypertonic) solution until equilibrium (isotonic or isoosmotic) is reached. Osmotic pressure is the pressure required to stop this water flow.
Cell membranes act as semipermeable membranes. A cell in a hypertonic medium loses water, potentially leading to dehydration and death in animal cells or plasmolysis in plant cells. In a hypotonic medium, water enters the cell, causing swelling (turgor). Animal cells may burst (hemolysis) without a cell wall, while plant cells and bacteria are protected by their cell walls. In an isotonic medium, water exchange is balanced.
Ionization of Water and pH Scale
Pure water weakly dissociates into H+ and OH– ions. The ion product of water at 25°C is [H+] * [OH–] = 10-14. In pure water, [H+] = [OH–] = 10-7. Acids increase [H+], while bases decrease it. The pH scale measures acidity: pH = -log[H+]. A pH of 7 is neutral, <7 is acidic, and >7 is basic. The logarithmic scale means a one-unit change represents a tenfold change in [H+].
Buffer Systems
Buffer solutions maintain a stable pH in the internal environment, preventing abrupt changes during metabolic processes. These solutions, typically a weak acid and its salt, resist pH shifts by reacting with excess H+ or OH– ions.
Cell Theory and Evolution
Historical Development
- Anton van Leeuwenhoek: Built the first simple microscope, enabling early cell observations.
- Robert Hooke: Coined the term “cell” after observing cork cells.
- Robert Brown: Discovered the nucleus in plant cells.
- Matthias Schleiden and Theodor Schwann: Formulated the cell theory, stating all living organisms are composed of cells.
- Rudolf Virchow: Added that every cell originates from another cell (“omnis cellula ex cellula”).
- Santiago Ramón y Cajal: Proposed the neuron theory, solidifying the cell theory’s universality.
Principles of Cell Theory
- All living things are made of cells (anatomical unit).
- The cell is the basic unit of life (physiological unit).
- All cells arise from pre-existing cells.
- The cell is the genetic unit of life, containing hereditary information.
Cellular Origin and Evolution
- Formation of biomolecules: Occurred in Earth’s early history.
- Formation of protocells (progenotes or protobionts): These precursors had membranes and replication capabilities.
- Endosymbiont theory (Lynn Margulis): Explains the origin of eukaryotic cells through symbiotic relationships between prokaryotes.
Cell Structure and Organization
Common Features
- Cell membranes: Separate the cell from its environment.
- Cytoplasm: Contains cytosol and organelles.
- Genetic material (DNA): Directs cell activities.
Prokaryotic Cells
- Smaller and simpler than eukaryotes.
- Lack a nucleus and membrane-bound organelles.
- Genetic material is a circular DNA molecule in the nucleoid region.
- Divide by binary fission.
- Include bacteria and cyanobacteria.
Eukaryotic Cells
- Larger and more complex than prokaryotes.
- Have a nucleus and membrane-bound organelles.
- Genetic material is linear DNA organized into chromosomes.
- Divide by mitosis or meiosis.
- Include protists, fungi, plants, and animals.
Animal Cells
- Lack a cell wall.
- Have smaller vacuoles.
- Contain centrioles.
- Store glycogen.
Plant Cells
- Have a cell wall made of cellulose.
- Have larger vacuoles.
- Contain plastids (e.g., chloroplasts).
- Store starch.
Cell Organelles and Their Functions
1. Plasma Membrane
Structure
- Fluid mosaic model: Lipids and proteins are arranged in a fluid bilayer.
- Asymmetric: Different molecules are distributed unevenly.
Functions
- Selective permeability: Controls substance exchange.
- Cell signaling: Contains receptors for external signals.
- Endocytosis and exocytosis: Transport of large molecules.
2. Endoplasmic Reticulum (ER)
Rough ER
- Has ribosomes attached.
- Synthesizes and modifies proteins.
Smooth ER
- Lacks ribosomes.
- Synthesizes lipids and detoxifies substances.
3. Golgi Apparatus
Structure
- Dictyosomes: Stacks of flattened sacs.
- Cis and trans faces: Involved in processing and transport.
Functions
- Modifies, sorts, and packages molecules.
- Forms lysosomes.
4. Lysosomes
Functions
- Contain digestive enzymes.
- Break down macromolecules and cellular debris.
5. Vacuoles
Plant Vacuoles
- Maintain cell turgor.
- Store various substances.
Animal Vacuoles
- Smaller and less numerous than in plants.
- Involved in digestion.
6. Ribosomes
Structure
- Two subunits (large and small).
- Composed of rRNA and proteins.
Function
- Synthesize proteins.
7. Cytoplasmic Inclusions
- Deposits of various substances.
- Examples: Starch grains, glycogen granules, pigment granules.
8. Cilia and Flagella
Structure
- 9+2 arrangement of microtubules.
- Dynein arms: Generate movement.
Function
- Cell motility or movement of fluids.
9. Centrosome
Structure
- Contains two centrioles.
- Microtubule organizing center.
Function
- Forms the spindle during cell division.