How does the role of energy differ between diffusion and active transport?

Diffusion is a passive process that relies solely on the kinetic energy of particles to move down a concentration gradient. Active transport requires chemical energy released during respiration to power protein pumps that move particles against their concentration gradient.

Compare the impact of distilled water on animal cells versus plant cells.

Animal cells gain water and burst because they only have a thin cell membrane that cannot withstand the internal pressure. Plant cells become turgid because their rigid cellulose cell wall provides structural support, preventing the cell from bursting even as internal pressure increases.

What is the distinction between 'water potential' and 'water concentration'?

Water potential is a more accurate term that describes the tendency of water to move due to solute concentration and pressure. A dilute solution has a high water potential (more free water molecules), while a concentrated solution has a low water potential.

Why is it incorrect to say that osmosis is the movement of 'solute' from a dilute to a concentrated solution?

Osmosis refers specifically to the movement of water (the solvent), not the solute. In osmosis, water moves across a partially permeable membrane to balance the concentrations, while the solute molecules are typically too large to pass through.

What is a common misconception regarding the 'random movement' of particles in diffusion?

While individual particle movement is random and directionless, the net result of millions of these movements is a predictable flow from high to low concentration. Students often mistakenly think the particles 'know' where to go, but the spreading out is purely statistical.

What mistake do students often make when describing the cell wall's role in osmosis?

Many students think the cell wall is partially permeable like the cell membrane, but it is actually usually fully permeable to water and solutes. The cell wall's primary role is providing physical strength to prevent bursting, not acting as a selective barrier for osmosis.

Define a 'partially permeable membrane' in the context of cell transport.

A partially permeable membrane is a biological barrier that allows only certain substances, typically small molecules like water or oxygen, to pass through while blocking larger molecules like starch or proteins. This selectivity is vital for maintaining the cell's internal composition.

What is the formula for calculating Surface Area to Volume Ratio for a cube?

The ratio is calculated by dividing the total surface area ($6 \times \text{side}^2$) by the volume ($\text{side}^3$). For a cube with side length $L$, the ratio simplifies to $6/L$, showing that as $L$ increases, the ratio decreases.

Define 'plasmolysis' in plant cells.

Plasmolysis occurs when a plant cell loses water via osmosis in a concentrated solution, causing the vacuole to shrink. This results in the cell membrane pulling away from the cell wall, leading to a loss of turgor and causing the plant to become flaccid.

Why does an increase in temperature accelerate the rate of diffusion?

Higher temperatures provide particles with more kinetic energy, causing them to move faster and collide more frequently. This increased speed of random movement allows the particles to spread out and cross membranes more rapidly than at lower temperatures.

Diffusion, Osmosis & Active Transport | Pearson Edexcel IGCSE Biology

2. Structure & Function in Living Organisms

Diffusion, Osmosis & Active Transport

Summary

The movement of substances across biological membranes is governed by passive processes like diffusion and osmosis, which rely on kinetic energy, and active processes like active transport, which require metabolic energy to move molecules against concentration gradients. Understanding these mechanisms is essential for grasping how cells maintain homeostasis, acquire nutrients, and eliminate waste products efficiently through specialized structural adaptations.

1. Fundamental Mechanisms of Passive Transport

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration until they are evenly distributed. This process is powered by the innate kinetic energy of particles, meaning it is a passive process that requires no cellular energy input.
Osmosis is a specific type of diffusion involving the movement of water molecules from a region of higher water potential (a dilute solution) to lower water potential (a concentrated solution). For osmosis to occur, the movement must take place across a partially permeable membrane that restricts the passage of larger solute molecules while allowing small water molecules through.
The concept of water potential is critical; pure water has the highest potential, and adding solutes like salts or sugars lowers this potential. Scientists prefer this term over 'concentration' because it more accurately describes the physical pressure and chemical potential driving water movement.

Diagram illustrating osmosis where water molecules move from a dilute solution (left) to a concentrated solution (right) across a partially permeable membrane.

2. Active Transport and Energetic Requirements

Active Transport is the movement of substances against their concentration gradient, traveling from a region of lower concentration to a region of higher concentration. Unlike diffusion, this process requires metabolic energy released from cellular respiration to function.
This mechanism utilizes specialized protein pumps embedded within the cell membrane. These proteins act as gates that actively move specific ions or molecules across the bilayer using energy (ATP) to change shape or drive the transport process.
Vital examples include the absorption of mineral ions into plant root hair cells, where soil concentrations are often lower than internal levels, and the uptake of glucose in the human small intestine into the bloodstream.

3. Factors Regulating the Rate of Movement

4. Key Distinctions: Passive vs. Active Transport

5. Impact of Osmosis on Animal and Plant Cells

6. Exam Strategy & Tips