Year 9 Science: Osmosis, Diffusion, and Active Transport

What substances move into and out of living cells?

Nutrients (such as glucose and amino acids), gases (like oxygen for respiration and carbon dioxide for removal), and waste products (like urea in animals or oxygen in plants during photosynthesis).

How is the cell surface membrane described?

The cell surface membrane is partially permeable, meaning it allows certain small molecules (e.g., water and gases) to pass through while blocking larger molecules. This helps maintain the cell's internal environment.

What is diffusion?

The net movement of particles from an area of high concentration to an area of low concentration until they are evenly distributed. It is a passive process that does not require energy from the cell.

Is diffusion a passive or active process?

Diffusion is passive, meaning it occurs naturally and does not require ATP or energy input from the cell.

What effect does temperature have on the rate of diffusion?

Higher temperatures increase the kinetic energy of particles, making them move faster and thus speeding up diffusion.

How does the concentration gradient affect the rate of diffusion?

A steeper concentration gradient (a bigger difference in concentration between two areas) results in faster diffusion because particles have a stronger 'driving force' to move.

Give an example of diffusion in the lungs.

Oxygen diffuses from the air in the alveoli (high concentration) into the blood in capillaries (low concentration), while carbon dioxide diffuses in the opposite direction.

Give an example of diffusion in plants.

Carbon dioxide diffuses into the leaves (where it is lower in concentration due to photosynthesis), and oxygen diffuses out into the surrounding air.

What is osmosis?

Osmosis is the movement of water molecules from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration) through a partially permeable membrane.

What is a solution?

A solution is made up of a solute (e.g., salt or sugar) dissolved in a solvent (e.g., water).

What is a common solvent in biology?

Water, often referred to as the 'universal solvent' because it dissolves a wide range of substances necessary for life.

What happens to free water molecules when solute is added to water?

Adding solute reduces the number of free water molecules available, lowering the water potential.

What is the relationship between solute concentration and free water molecules?

Higher solute concentration results in fewer free water molecules, reducing water potential.

What is a hypertonic solution?

A solution that has a higher solute concentration than another solution, causing water to move out of a cell placed in it (e.g., salty water).

What is a hypotonic solution?

A solution with a lower solute concentration than another solution, causing water to move into a cell placed in it (e.g., distilled water).

What is an isotonic solution?

A solution with an equal solute concentration compared to another solution, meaning there is no net movement of water.

What happens to animal cells in a hypotonic solution?

Water enters the cell by osmosis, causing it to swell and possibly burst (lysis) because animal cells lack a cell wall.

Hypertonic solution effect on animal cells

Water leaves the cell by osmosis, causing the cell to shrink and shrivel (crenation).

Hypotonic solution effect on plant cells

Water enters the cell, causing it to become turgid (firm). The cell wall prevents it from bursting, maintaining structure and support.

Isotonic solution effect on plant cells

The cell becomes flaccid because there is no net movement of water, and the plant may wilt as turgor pressure decreases.

Hypertonic solution effect on plant cells

Water leaves the cell, causing it to plasmolyse. The cell membrane pulls away from the cell wall, and the plant may die if this continues.

Passive transport

Does not require energy and moves substances down a concentration gradient.

Active transport

Requires ATP to move substances against a concentration gradient.

Molecule movement during active transport

Molecules move from an area of low concentration to an area of high concentration using energy and specific carrier proteins in the cell membrane.

Requirements for active transport

Energy in the form of ATP and specialized transport proteins in the membrane.

Source of ATP for active transport

ATP is produced during cellular respiration in mitochondria.

Example of active transport in plants

Root hair cells use active transport to absorb mineral ions like nitrates from the soil, even when the concentration of ions is higher in the cell than in the soil.

Example of active transport in animals

Glucose is actively transported from the small intestine into the bloodstream, especially when glucose concentrations in the intestine are lower.

Surface area to volume (SA:vol) ratio

The ratio of the total surface area of an object compared to its volume, which affects the efficiency of substance exchange.

Effect of size on SA:vol ratio

Smaller objects have a high SA:vol ratio, making diffusion more efficient. Larger objects have a low SA:vol ratio, making diffusion less efficient.

Importance of high SA:vol ratio for cells

To allow efficient exchange of materials like oxygen, carbon dioxide, and nutrients with their environment.

Need for specialized exchange surfaces in multicellular organisms

Because their low SA:vol ratio limits the efficiency of diffusion, so specialized structures (e.g., lungs or gills) increase the surface area for exchange.

Example of an exchange surface in humans

The alveoli in the lungs, which are thin and have a large surface area for gas exchange.

Example of an exchange surface in fish

Gills, which have thin filaments and a large surface area for oxygen uptake and carbon dioxide release.

Example of an exchange surface in plants

Roots, which absorb water and nutrients, and leaves, which allow gas exchange for photosynthesis.

Required practical for osmosis

Investigating the effect of different concentrations of salt or sugar solutions on the mass of plant tissues (e.g., potato cylinders).

Calculating percentage change in mass in osmosis practical

(Final mass - Initial mass) ÷ Initial mass × 100.