IP Biology Notes: Movement of Substances (Upper Sec 02)

Study guide26 Nov 2025, 00:00 ZUpdated 17 Jul 2026

Free IP Biology notes on diffusion, osmosis, and active transport for Sec 3 to Sec 4, with water potential language, root hair uptake, and osmosis applications.

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For Integrated Programme students: Your current school materials, teacher instructions, and assessment scope take precedence because IP topic sequence and depth vary by school. This is an Eclat IP guide, not the O-Level / SEC G3 exam-track guide.

How this chapter applies

Eclat core: diffusion, osmosis, active transport, concentration gradients, water movement, and experimental interpretation form the main route.
School-sensitive extension: water-potential notation, quantitative water-potential work, membrane-protein detail, and unfamiliar graph modelling should be used only where the current school requires them.
2027 national comparison: K325 Topic B2 covers diffusion in nutrient uptake and gaseous exchange, osmosis in plant and animal tissues, and energy-consuming active transport against a concentration gradient.
Check your school: definitions, diagram conventions, and the expected depth of water-potential reasoning can differ. Match the current task wording.
Exam-track route: use the separate O-Level and SEC G3 Biology notes for K325 topic ownership.

The core idea is simple: Substances move by diffusion, osmosis, or active transport.

Use it as a working check: Diffusion and osmosis move down a gradient without energy. Active transport uses energy to move substances against a gradient.

Then go one layer deeper: Example: water enters a root hair cell by osmosis, but nitrate ions may need active transport if the soil has a lower nitrate concentration than the cell.

What you must know

Diffusion: net movement of particles down concentration gradient; no energy needed; faster with higher temperature/steeper gradient.
Osmosis: net movement of water across partially permeable membrane from higher to lower water potential (dilute → concentrated).
Active transport: movement against gradient using energy from respiration and carrier proteins (e.g., mineral ions into root hair, glucose in ileum).
Osmosis effects: plant cells become turgid in dilute solutions, flaccid/plasmolysed in concentrated; animal cells lyse in very dilute, crenate in concentrated.

Choose the Transport Method

Use the question cues before you write the process name.

Cue in the question

Process to name

Reviewed by

Ezekiel Tan·Academic Advisor (Biology)

Process	Step 1: name the moving substance	Step 2: state the gradient	Step 3: add the required condition	Step 4: finish with the result
Diffusion	Oxygen, carbon dioxide, or another particle	From higher concentration to lower concentration	No energy required	Net movement continues until the gradient becomes smaller.
Osmosis	Water molecules only	From higher water potential to lower water potential	Across a partially permeable membrane	Cell gains water, loses water, becomes turgid, flaccid, plasmolysed, lysed, or crenated.
Active transport	Mineral ions, glucose, amino acids, or another solute	From lower concentration to higher concentration	Energy from respiration and carrier proteins	Substance accumulates inside the cell or is absorbed despite the gradient.

School-work symptom	Missing reasoning step	Practice fix
Osmosis answers use water concentration loosely	Water-potential comparison	State which side has higher water potential before naming the direction.
Active transport is named whenever absorption occurs	Gradient proof	Show that the solute moves from lower to higher concentration.
Plant and animal cell outcomes are mixed up	Cell-wall consequence	Decide water direction first, then apply the correct plant or animal term.
Practical explanations stop at mass change	Cell-level mechanism	Link the potato strip result to water movement through partially permeable membranes.

Question clue	Gradient to prove	Process to name	Sentence to write
Nitrate ions enter a root hair cell even though the cell already has a higher nitrate ion concentration than the soil	From lower concentration in soil to higher concentration in the cell	Active transport	Nitrate ions move against their concentration gradient using energy from respiration and carrier proteins.
Glucose enters an ileum epithelial cell when glucose concentration in the lumen is lower than in the cell	From lower concentration in the lumen to higher concentration in the cell	Active transport	Glucose is absorbed against its concentration gradient, so energy from respiration is required.
Oxygen enters a respiring cell from the blood or air spaces	From higher concentration outside the cell to lower concentration inside	Diffusion	Oxygen diffuses down its concentration gradient, so no energy is required.
Water enters a root hair cell from dilute soil solution	From higher water potential outside to lower water potential inside	Osmosis	Water moves by osmosis through a partially permeable membrane.

Wording in the question	Water-potential comparison	Direction of water movement	Common trap
Cell is placed in a dilute solution	Outside has higher water potential than the cell sap	Water enters the cell by osmosis	Saying dilute means weak, so nothing happens.
Cell is placed in concentrated sucrose or salt solution	Outside has lower water potential than the cell sap	Water leaves the cell by osmosis	Saying "more concentrated" means higher water potential.
Plant cell gains water	Cell contents become more turgid as vacuole expands against the cell wall	Water moved into the cell	Calling it lysis even though the cell wall resists bursting.
Animal cell loses water	Cell shrinks because there is no cell wall to hold shape	Water moved out of the cell	Using plasmolysed for animal cells instead of crenated.

Situation	Water movement to prove first	Correct outcome word	Why this word fits
Onion epidermal cell in concentrated salt solution	Water leaves the cell by osmosis.	Plasmolysed	The cell membrane pulls away from the cell wall after water loss.
Red blood cell in concentrated salt solution	Water leaves the cell by osmosis.	Crenated	The animal cell shrinks because it has no cell wall.
Leaf cell in distilled water	Water enters the cell by osmosis.	Turgid	The vacuole expands and pushes the cytoplasm against the cell wall.
Red blood cell in distilled water	Water enters the cell by osmosis.	Lysed	The membrane may burst because there is no cell wall to resist swelling.

IP Biology Notes: Movement of Substances (Upper Sec 02)

How this chapter applies

What you must know

Choose the Transport Method

Sources

Answer Frame Checkpoint

Movement-of-substances tuition checkpoint

Active transport justification checkpoint

Detailed notes

Worked walkthroughs

Water-potential direction checkpoint

Cell outcome diagram checkpoint

Potato Strip Osmosis Example

Pitfalls and fixes

Practice drills

Quick applications

Exam cues

Next steps

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