Biology Chapter: Cell Transport & Homeostasis

Cell Transport

• Students should analyze the relationships between cell structures and functions to understand cell transport.
• Investigations should be conducted to determine the role of cell transport (active, passive, and osmosis) in maintaining homeostasis.

Overview of Homeostasis

• Homeostasis is the need of an organism to maintain a stable internal environment.
• A dynamic equilibrium is maintained, meaning conditions stay within a range, not always the same.
• Examples of internal conditions regulated for homeostasis include pH, temperature, and blood sugar.
• Organisms constantly respond to stimuli in the environment to maintain homeostasis.
• Stimulus = a change in the environment
• Response = a change in the organism as a result of the stimulus.

Feedback Mechanisms

• Feedback mechanisms help regulate homeostasis in response to stimuli.
• These mechanisms use the output of a system to signal a change in the input, stabilizing or amplifying the system response.
• Feedback mechanisms can be positive or negative.

Positive Feedback Mechanisms

• In a positive feedback loop, the output intensifies the response.
• Examples include:
  • Childbirth: hormones, contractions, and pressure increase the response.
  • Fruit ripening: ethylene signals surrounding fruit to ripen, accelerating the process.

Negative Feedback Mechanisms

• In a negative feedback loop, the output causes a counter-response to return to a set point.
• Examples include:
  • Body temperature regulation (thermoregulation)
  • Water concentration regulation (osmoregulation)
  • Blood sugar regulation

Maintaining Homeostasis at the Cellular Level: The Cell Membrane

• Homeostasis is maintained from the organ system level down to the cellular level.
• The cell membrane plays a crucial role in controlling the movement of things into and out of the cell.

Cell Membrane Permeability

• The cell membrane is selectively permeable, meaning it's particular about what enters and exits.
• Small, nonpolar, hydrophobic, and neutral substances easily pass through.
• Polar and large molecules do not easily pass.

Types of Cell Transport

• Cell transport is classified as either passive or active.
• Passive transport requires no extra energy.
  • Molecules move from an area of high concentration to an area of low concentration (down the concentration gradient).
    • Examples include simple diffusion, facilitated diffusion, and osmosis.
• Active transport requires extra energy.
  • Molecules move against the concentration gradient (from low to high concentration).
    • Examples include molecular pumps, exocytosis, and endocytosis.

Passive Transport: Simple Diffusion

• Simple diffusion is the movement of small, nonpolar molecules across a membrane until equilibrium is reached.
• Equilibrium is achieved when the concentration is equal on both sides of the membrane.
• Examples include O₂ and CO₂.

Passive Transport: Facilitated Diffusion

• Facilitated diffusion uses transport proteins to help facilitate the movement of molecules that can't pass directly across the cell membrane.
• Molecules move down their concentration gradient.
• Examples include glucose and calcium.

Passive Transport: Osmosis

• Osmosis is the simple diffusion of water across a selectively permeable membrane.
• Water moves from an area of high water concentration to an area of low water concentration until equilibrium is reached.
• High water concentration means low solute concentration
• Low water concentration means high solute concentration

Types of Solutions based on Osmosis

• Hypertonic solution: Water concentration is lower outside the cell, causing net movement of water out of the cell resulting in cell shrinkage/ shriveling.
• Hypotonic solution: Water concentration is higher outside the cell, causing net movement of water into the cell resulting in cell swelling.
• Isotonic solution: Water concentration is equal inside and outside the cell, water movement in and out of cell is balanced.

Active Transport: Molecular Pumps

• A cell uses energy to pump molecules across the membrane, against the concentration gradient through a protein channel.
• This concentrates molecules inside the cell and removes waste quickly.
  • Examples include ions such as potassium (K+), chlorine (Cl-), and sodium (Na+).

Active Transport: Vesicles

• Endocytosis brings large particles into the cell using vesicles.
  • Examples include white blood cells engulfing bacteria
• Exocytosis moves large particles out of the cell by using vesicles.
  • Examples include nerve cells releasing neurotransmitters.

Types of Endocytosis

• Phagocytosis: cell engulfs solids.
• Pinocytosis: cell engulfs liquids.

Summary of Transport

• A table summarizing different types of transport, examples of substances transported, and their roles in maintaining homeostasis.

Practice Questions

• Practice problems related to different types of cell transport.
• Examples from the cell and environmental concentrations(e.g., 5% vs 15% solutions).