Diffusion, Osmosis and Active Transport
Why This Matters
This lesson explores the fundamental mechanisms by which substances move across cell membranes: diffusion, osmosis, and active transport. Understanding these processes is crucial for comprehending how cells maintain homeostasis, absorb nutrients, and excrete waste products.
Key Words to Know
1. Diffusion: Passive Movement Down a Gradient
Diffusion is a passive process, meaning it does not require metabolic energy from the cell. It is driven by the random motion of particles. Substances move from an area where they are in high concentration to an area where they are in low concentration, effectively moving 'down' their concentration gradient. This process continues until the particles are evenly distributed, reaching a state of dynamic equilibrium where net movement ceases.
Factors affecting the rate of diffusion include:
- Concentration gradient: A steeper gradient leads to a faster rate.
- Surface area: A larger surface area allows more particles to diffuse simultaneously.
- Distance (thickness of membrane): A shorter diffusion distance results in a faster rate.
- Temperature: Higher temperatures increase kinetic energy of particles, leading to faster diffusion.
- Size of diffusing molecule: Smaller molecules diffuse faster.
Examples in biology include oxygen diffusing from alveoli into the blood, and carbon dioxide diffusing from blood into alveoli.
2. Facilitated Diffusion: Aided Passive Transport
Facilitated diffusion is a type of passive transport that also does not require metabolic energy. However, unlike simple diffusion, it involves the assistance of carrier proteins or channel proteins embedded within the cell membrane. These proteins provide specific pathways for molecules that are too large, too polar, or charged to pass directly through the lipid bilayer.
Channel proteins form hydrophilic pores through the membrane, allowing specific ions or small polar molecules to pass. They can sometimes be gated, opening or closing in response to specific stimuli. Carrier proteins bind to specific molecules on one side of the membrane, undergo a conformational change, and release the molecule on the other side.
Like simple diffusion, facilitated diffusion occurs down a concentration gradient. It exhibits specificity (each protein transports only certain molecules) and saturation (there are a limited number of transport proteins, so the rate can reach a maximum). Examples include glucose uptake into red blood cells and ion movement through specific channels.
3. Osmosis: The Movement of Water
Osmosis is a special case of diffusion involving the net movement of water molecules across a partially permeable membrane. Water moves from a region of higher water potential (more free water molecules, lower solute concentration) to a region of lower water potential (fewer free water molecules, higher solute concentration).
Water potential (Ψ) is measured in kilopascals (kPa) and is influenced by solute potential (Ψs) and pressure potential (Ψp). Pure water has a water potential of 0 kPa. Adding solutes lowers the water potential, making it negative.
Effects of osmosis on cells:
- Animal cells: In a hypotonic solution (higher water potential outside), water enters, causing the cell to swell and potentially lyse (burst). In a hypertonic solution (lower water potential outside), water leaves, causing the cell to crenate (shrink). In an isotonic solution, there is no net movement.
- Plant cells: In a hypotonic solution, water enters, pushing the cell membrane against the cell wall, making the cell turgid (firm). In a hypertonic solution, water leaves, causing the protoplast to pull away from the cell wall (plasmolysis). The cell wall prevents bursting.
4. Active Transport: Energy-Dependent Movement
Active transport is the process by which cells move substances across their membranes against their concentration grad...
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Exam Tips
- 1.Clearly distinguish between passive (diffusion, facilitated diffusion, osmosis) and active transport, focusing on the requirement for energy and movement relative to the concentration gradient.
- 2.For osmosis questions, always refer to 'water potential' and 'partially permeable membrane'. Avoid using terms like 'concentration of water' or 'strong/weak solution' without also linking to water potential.
- 3.When describing factors affecting diffusion/transport rates, explain *why* each factor has its effect (e.g., higher temperature increases kinetic energy, leading to more frequent collisions and faster diffusion).
- 4.Be prepared to interpret diagrams of cell membranes showing different transport mechanisms and to predict the movement of substances in various scenarios (e.g., what happens to a plant cell in a hypertonic solution?).
- 5.Remember specific biological examples for each transport mechanism, such as oxygen exchange in lungs (diffusion), glucose uptake (facilitated diffusion/active transport), and mineral ion uptake by roots (active transport).