CIE iGCSE Co-ordinated Sciences-B3.2 Osmosis - Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-B3.2 Osmosis – Study Notes

CIE iGCSE Co-ordinated Sciences-B3.2 Osmosis – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

Key Concepts:

Core

State that water diffuses through partially permeable membranes by osmosis
State that water moves into and out of cells by osmosis through the cell membrane
Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations

Supplement

Describe osmosis as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
Explain the effects on plant cells of immersing them in solutions of different concentrations by using the terms: turgid, turgor pressure, plasmolysis, flaccid
Explain the importance of water potential and osmosis in the uptake and loss of water by organisms

CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Osmosis

📌 Definition

Osmosis = diffusion of water molecules across a partially permeable membrane from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution).

🔑 Key Points

Partially permeable membrane: allows water molecules through, but not larger solute molecules.
Dilute solution: more water, less solute.
Concentrated solution: less water, more solute.
Water always moves down its concentration gradient.

🌱 In Living Cells

Cell membranes act as partially permeable barriers.
If cells are in a dilute solution (freshwater) → water enters → cell swells.
If cells are in a concentrated solution (saltwater) → water leaves → cell shrinks.
In plants, vacuoles gain or lose water by osmosis → affects turgor pressure.

📊 Simple Summary

Term	Meaning
Osmosis	Water diffusion through partially permeable membrane
Direction	From dilute → concentrated solution
Membrane type	Partially permeable (only lets small molecules like water through)
Effect in cells	Water enters or leaves depending on surroundings

⚡ Quick Recap
👉 Osmosis = Water Diffusion
Dilute → Concentrated
Through partially permeable membrane
Cell membranes control osmosis
In dilute → cells gain water
In concentrated → cells lose water

💡 Mnemonic: “Osmosis = Only Small Molecules In Selective Membrane”

Osmosis in Cells

📌 Key Statement

Water moves into and out of cells by osmosis through the cell membrane.

🧾 What is Osmosis?

Osmosis = diffusion of water molecules.
Moves from higher water concentration (dilute) → lower water concentration (concentrated).
Occurs through a partially permeable membrane (lets water through, not large solutes).
Cell membrane = partially permeable, so water enters and exits cells this way.

🌱 In Plant Cells

Cell wall: freely permeable; cell membrane: partially permeable.
In dilute solution: water enters → vacuole swells → cytoplasm pushes against wall → cell turgid (firm). Turgidity supports plant.
In concentrated solution: water leaves → vacuole shrinks → cell flaccid → plant wilts. If extreme → plasmolysis (membrane pulls away from wall).

🐾 In Animal Cells

Only a cell membrane, no wall → more vulnerable.
In dilute solution: water enters → cell swells → may burst (lysis).
In concentrated solution: water leaves → cell shrinks, becomes wrinkled (crenated).

🔬 Experimental Evidence

Dialysis tubing + sugar solution: water enters → liquid level rises.
Potato cylinder experiment:
- In water → gains mass/length (osmosis in).
- In strong sugar solution → loses mass/length (osmosis out).

📊 Summary Table

Condition around cell	Plant cell effect	Animal cell effect
Dilute solution	Water in → Turgid	Water in → Swells, may burst (lysis)
Concentrated solution	Water out → Flaccid / Plasmolysed	Water out → Shrinks (crenated)

⚡ Quick Recap
Osmosis = water diffusion through partially permeable membrane
Cell membrane controls water movement
Plant cells → Turgid in dilute, Flaccid/Plasmolysed in concentrated
Animal cells → Lysis in dilute, Crenated in concentrated
Classic experiments → Dialysis tubing & Potato cylinders

Effects of Solutions on Plant Tissues

📌 Key Idea

Plant tissues respond differently depending on the concentration of the solution they are placed in (osmosis effect).

🧾 Immersing Plant Tissue in Different Solutions

1. In Dilute Solution (Hypotonic):

Water enters cells by osmosis.
Vacuole fills up → cytoplasm pushes against wall.
Cells become turgid → tissues stay firm → stems & leaves supported.

2. In Concentrated Solution (Hypertonic):

Water leaves cells by osmosis.
Vacuole shrinks → cytoplasm pulls away from wall.
Cells become flaccid → tissue loses support → plant wilts.
If extreme → plasmolysis (membrane detaches from wall).

3. In Equal Concentration (Isotonic):

No net movement of water.
Cells remain in normal, slightly soft state (not turgid, not plasmolysed).
Tissues don’t have much support.

📊 Summary Table

Solution around cell	Cell effect	Tissue / Plant effect
Dilute (Hypotonic)	Turgid cells	Plant stiff & supported
Concentrated (Hypertonic)	Flaccid / Plasmolysed cells	Plant limp, wilts
Equal conc. (Isotonic)	Normal cells	Plant soft, not firm

🌿 Real-life Observation

Watered plant: cells turgid → plant upright & firm.
Dehydrated plant: cells lose water → plant wilts.
Re-watered: osmosis restores turgidity → plant recovers.

⚡ Quick Recap
Dilute → Water enters → Cells turgid → Plant supported
Concentrated → Water leaves → Cells flaccid/plasmolysed → Plant wilts
Isotonic → No net water movement → Plant soft
Wilting = loss of turgidity in many cells

Osmosis

🌱 Key Points to Remember

Partially permeable membrane: lets water molecules through, not large solutes.
High water potential (WP): dilute solution / pure water.
Low water potential (WP): concentrated solution (solute binds water, less free water).
Water moves until equilibrium is reached.

📌 Examples

Root hair cells: absorb water from soil by osmosis.
Potato experiments: potato gains or loses mass depending on solution.
Red blood cells: swell & burst in pure water, shrink in strong salt solution.

📝 Summary Table

Term	Meaning
Water potential	Tendency of water molecules to move
High WP	Dilute solution / pure water
Low WP	Concentrated solution
Osmosis	Water moves from high WP → low WP via partially permeable membrane

⚡ Quick Recap
Osmosis = water diffusion across partially permeable membrane
From dilute (high WP) → concentrated (low WP)
Classic examples: root hair cells, potato cylinders, red blood cells

Effects of Solutions on Plant Cells (Osmosis)

📌 Introduction

Plant cells respond differently when placed in solutions of different concentrations because of osmosis. The rigid cell wall prevents bursting, unlike animal cells.

🔑 Key Terms

Turgid: Cell has absorbed maximum water, vacuole full, pushing against wall.
Turgor pressure: Pressure of vacuole contents against cell wall, provides support.
Flaccid: Cell loses water, vacuole shrinks, no pressure on wall, cell limp.
Plasmolysis: Extreme water loss, cytoplasm & membrane pull away from wall.

🌊 Effects in Different Solutions

Hypotonic solution (more dilute than cell sap)
- Water enters by osmosis.
- Vacuole swells, presses on cell wall.
- Cell becomes turgid → stem upright, leaves firm.
Isotonic solution (same concentration as cell sap)
- No net water movement.
- Cell remains normal, not fully firm.
Hypertonic solution (more concentrated than cell sap)
- Water leaves cell by osmosis.
- Vacuole shrinks → cell flaccid.
- Extreme case → plasmolysis (cytoplasm pulls from wall).
- Plant wilts (droopy stems & leaves).

📌 Importance of Turgor

Turgor pressure keeps stems upright & leaves spread for light absorption.
Loss of turgor → wilting.
In farming: excess fertilisers/salts ↓ soil water potential → water moves out of roots → wilting & crop loss.

📝 Summary Table

Solution type	Water movement	Cell condition	Notes
Hypotonic (dilute)	In → cell	Turgid	Strong support due to turgor pressure
Isotonic	No net movement	Normal	Not firm, not wilted
Hypertonic (concentrated)	Out → cell	Flaccid / plasmolysed	Plant wilts

⚡ Quick Recap
Water in → turgid → upright plant
Water out → flaccid → wilting
Extreme water loss → plasmolysis
Turgor pressure = main support for plant

Importance of Water Potential & Osmosis in Uptake and Loss of Water by Organisms

📌 Introduction

Water potential (WP): tendency of water to move.

Osmosis: movement of water across a selectively permeable membrane from higher WP → lower WP.

Essential in both plants (uptake, turgidity) and animals (cell survival, homeostasis).

🌱 In Plants

Turgid cells: Vacuole full of water pushes against cell wall → turgor pressure. Keeps stems upright, leaves spread out for photosynthesis.
Flaccid cells: Vacuole loses water → turgor lost → stem droops, wilting.
Root hair cells: absorb water from soil by osmosis (soil WP > cell sap WP).
Fertilizers & salts: lower soil WP → water leaves roots → plants wilt/die.
Irrigation (hot countries): salt build-up ↓ soil WP → crops lose water → land infertile.
Road salt (winters): dissolves in soil → reduces WP → roadside plants lose water → die.

🐾 In Animals

No cell wall: animal cells are highly sensitive to osmotic changes.
Hypotonic fluid (high WP): water enters → cells swell & may burst (haemolysis in RBCs).
Hypertonic fluid (low WP): water leaves → cells shrink (crenation) → reduced O₂ transport.
Isotonic solutions: essential for stability (e.g., medical saline, sports drinks).

📊 Summary Table

Condition	Plants 🌱	Animals 🐾
High external WP (hypotonic)	Cells turgid → firm, upright	Cells swell → may burst
Equal WP (isotonic)	No net movement → cells normal	No change → stable
Low external WP (hypertonic)	Cells lose water → flaccid → plasmolysis possible	Cells shrink (crenated)

🔑 Quick Recap
Turgid = full water, firm, upright
Flaccid = lost water, limp
Plasmolysis = cytoplasm shrinks from wall in strong solution
In plants → WP controls support, growth & survival
In animals → correct osmotic balance prevents bursting/shrinkage

CIE iGCSE Co-ordinated Sciences-B3.2 Osmosis - Study Notes- New Syllabus