Movement Into & Out of Cells

Definition: Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Mechanism: Molecules move down a concentration gradient due to their random movement.
Importance: Diffusion enables living organisms to obtain necessary substances, dispose of waste products, and conduct gas exchange for respiration.

The movement of substances is influenced by the presence of the cell membrane, which is a partially permeable membrane.
Partially Permeable Membrane: Allows certain molecules to pass through easily while restricting others based on size and other factors.

Organisms rely on diffusion to:
- Obtain essential nutrients.
- Remove waste products.
- Facilitate gas exchange necessary for respiration.

Substances: Organisms obtain oxygen (essential for respiration) and carbon dioxide (necessary for photosynthesis) via diffusion.
Energy Source: Diffusion is fueled by the kinetic energy from random movement of molecules, a phenomenon known as Brownian motion.

The larger a cell or structure, the smaller its surface area relative to its volume, slowing down diffusion.
Adaptations:
- Root Hair Cells: Increased surface area to absorb water and minerals.
- Ileum Cells: Adapted to enhance absorption of digested products.

Shorter distances yield faster transport rates.
Example: Blood capillaries and alveoli have walls only one cell thick to facilitate rapid diffusion.

Higher temperatures increase molecular movement and collision frequency, enhancing diffusion rates.

A greater difference in concentration across the membrane leads to faster diffusion, as more molecules collide with the membrane.
Examiner Tips: Be prepared to describe or conduct experiments demonstrating these factors in exams.

Importance: Water is vital for living organisms as a solvent, allowing various substances to dissolve.
Functions:
- Facilitates the transport of dissolved substances, e.g., nutrients in plants (xylem and phloem) and digestive products in blood.
- Helps remove toxic wastes like urea and excess salts from the body.
- Acts as a critical component of cytoplasm for metabolic reactions.

Definition: Osmosis is the diffusion of water molecules from a dilute solution (low solute concentration) to a concentrated solution (high solute concentration) through a partially permeable membrane.
Water moves from areas of high water concentration to lower water concentration across the cell membrane.
Important consideration: Osmosis includes understanding water potential gradients for extended study.

Common experiments include immersing cylinders of root vegetables (e.g., potatoes) in different concentrations of sucrose solution.
- Procedure:
1. Weigh cylinders before immersion.
2. Place in varying sucrose solutions.
3. After 20-30 minutes, remove, dry, and reweigh.
4. Analyze changes in mass to determine water movement.
Mass Gain: Water moves into plant tissue from a lower concentration sucrose solution.
Mass Loss: Water moves out, indicating the surrounding solution is more concentrated than the plant tissue.
No Change in Mass: Equal concentrations in plant tissue and surrounding solution.
Dialysis Tubing Experiment: Demonstrates osmosis using a non-living semi-permeable membrane.

Water entering a plant cell enlarges the vacuole, increasing turgor pressure, which maintains cell rigidity and provides structural support.
If water is scarce, cells become flaccid, leading to wilting.

Water Potential:
- High water potential = dilute solution, low solute concentration (vica versa for low water potential).

Animal cells exhibit more severe osmosis effects due to the absence of cell walls.
- Crenation: Occurs in a concentrated sugar solution (cell loses water).
- Lysis: Occurs in distilled water (cell gains water excessively and may burst).

Definition: Active transport is the movement of particles through a cell membrane from a region of lower concentration to a higher concentration, utilizing energy from respiration.

Vital for molecule and ion movement against concentration gradients, including:
- Glucose uptake by intestinal epithelial cells and kidney tubules.
- Uptake of ions from soil water by root hair cells in plants.

Utilizes carrier proteins embedded in the membrane to translocate specific molecules against their concentration gradient:
1. Substance binds to carrier protein.
2. Carrier protein transports substance across the membrane using energy to change shape.
3. Substance is released into the cell.
Examiner Tips: No need to know phospholipid bilayer details for active transport.