Lecture 10 Movement

Water movement is governed by three basic processes:
  1. Bulk Flow
  2. Diffusion
  3. Osmosis

Definition: Bulk flow (also called mass flow) refers to the movement of water or other liquid from areas of high concentration to areas of lower concentration because of differences in potential energy.
Potential energy: The stored energy of an object, which is dependent on its position.
Key Concept:
- Higher water potential corresponds to higher potential energy, while lower water potential indicates lower potential energy.
- Example of potential energy: A waterfall, which is driven by gravity.
Water potential: The potential energy of water per unit area compared to pure water.
- The water potential of pure water is defined as 0.

Definition: Diffusion occurs via random kinetic movement of particles.
Equilibrium: Net diffusion stops when there is a uniform distribution of particles.
Energy Requirement: The energy required for diffusion is 0. However, molecules continue to move after equilibrium is achieved, without affecting the net concentration.

In cells:
  - Main method of movement for molecules.
  - Cytoplasmic streaming (cyclosis): An effective mechanism to facilitate rapid transport of materials within cells.
  - Efficient diffusion requires:
    - A steep concentration gradient (substantial concentration difference).
    - Short distances between sources and destinations.
Diffusion capabilities:
- Hydrophobic, small, and uncharged molecules can diffuse across membranes.
- Large, uncharged polar molecules and ions cannot diffuse across membranes.

Definition: Osmosis is the movement of water across a selectively permeable membrane.
Selectively permeable membranes: These membranes permit the passage of certain substances while blocking others.
Direction of water movement: Water typically moves from a dilute solution to a more concentrated solution.

Diagram: Shows net movement of water from Solution B to Solution A until both achieve equal concentrations of solute.

Optimal Conditions: Organisms function best when their water potential is similar to that of their surrounding medium.
Adaptations in Organisms: In organisms such as Euglena and Paramecium, a contractile vacuole collects water and pumps it out rhythmically to maintain water balance.

Definition: Turgor pressure is the pressure within a plant cell that develops from osmosis or imbibition.
- Imbibition describes the uptake of water by tissues, causing swelling, for example during seed germination.
- Effect on plant cells: Contributes to their stiffness.
Plasmolysis: The process where the protoplast (the living part of a plant cell) shrinks due to water loss in a hypertonic solution.
Types of Solutions:
  - Hypertonic: A solution with more solute and less water.
  - Hypotonic: A solution with less solute and more water.
  - Isotonic: Solutions with equal concentrations of solute.

Lipid Bilayer: Composed of phospholipids.
- Hydrophobic tails face inward; hydrophilic heads face outward.
- The presence of sterols (e.g., stigmasterol) in plant membranes, emphasizing that plants do not contain cholesterol.
Proteins:
- Peripheral proteins: Do not penetrate the membrane.
- Integral proteins: Transmembrane proteins that have both hydrophobic and hydrophilic zones.
Carbohydrates: Attach to proteins (forming glycoproteins) or lipids (forming glycolipids), contributing to the fluid mosaic model of the membrane.

Single transmembrane domain: Proteins that pass through the membrane once (single pass).
Multiple transmembrane domains: Proteins that pass through the membrane multiple times (multipass).

Simple diffusion:
- Applicable for small, non-polar molecules like O₂, CO₂, and small, uncharged molecules like water.
- Occurs down a concentration gradient.
Facilitated diffusion:
   - Via carrier or channel proteins that bind solutes and undergo conformational changes to facilitate passage.
   - Allows movement of larger, membrane-insoluble compounds (e.g., sugars, amino acids, ions).
   - Characteristics: Passive transport, occurs down concentration/electrochemical gradient, requires no energy, and is fully reversible.
Active transport:
   - Definition: Movement of substances across a membrane against a concentration/electrochemical gradient, which consumes energy.
   - Energy source: Hydrolysis of ATP to ADP and inorganic phosphate (Pi).
   - Function: Pumps specific molecules in or out of the cell using specific integral membrane proteins.

Purpose: Facilitates the transport of large molecules (proteins, polysaccharides), large particles, microorganisms, and cellular debris across membranes.
Exocytosis: The process of expelling materials from cells via vesicles.
Endocytosis: The process of bringing materials into cells via vesicles.

Phagocytosis:
   - Ingestion of large particles (e.g., bacteria) through the formation of large vesicles from the plasma membrane.
   - Origin of terms:
     - ‘phagein’ (Gk.) - to devour
     - ‘kytos’ (Gk.) - cell
     - ‘osis’ (Gk.) - process
Pinocytosis:
- Similar mechanism to phagocytosis; ingestion of liquids via small vesicles.
Receptor-mediated endocytosis:
- Occurs in specialized areas of the plasma membrane that are coated with peripheral proteins (clathrin). Receptors in the coating are key for recognizing transported substances, forming coated vesicles.

Signal Transduction: The process through which cells utilize chemical messengers for communication.
Definition: Signal transduction refers to the conversion of an extracellular signal into a cellular response (e.g., calcium signaling).
Plasmodesmata: Specialized structures in plant cells that enable cell-to-cell communication.
The concept of the symplast: Refers to the interconnected protoplasts of plant cells alongside their plasmodesmata.
Contrast with the apoplast: The continuum of cell walls and intercellular spaces.
Desmotubule: A tubular strand of the endoplasmic reticulum that passes through plasmodesmata, tightly constricted.

Experiment: Injecting a hair from the stamen of Setcreasea purpurea with fluorescent dye demonstrated the movement of dye through plasmodesmata, as the plasma membrane is impermeable to the dye.
- Time observation: Movement tracked at 2 minutes and 5 minutes intervals.

Water moves following its water potential, through bulk flow or diffusion.
Osmosis is specifically the movement of water across selectively permeable membranes.
Membranes are composed of a lipid bilayer along with proteins.
Small molecules traverse membranes via simple diffusion, facilitated diffusion, or active transport.
Large molecules and particles utilize vesicle-mediated transport through endocytosis and exocytosis.
Signal transduction employs chemical messengers for cellular communication.
Plasmodesmata are crucial for intercellular communication among plant cells.