CIE iGCSE Biology-3.2 Osmosis- Study Notes- New Syllabus

CIE iGCSE Biology-3.2 Osmosis- Study Notes – New syllabus

CIE iGCSE Biology-3.2 Osmosis- Study Notes -CIE iGCSE Biology – per latest Syllabus.

Key Concepts:

Core

Describe the role of water as a solvent in organisms with reference to digestion, excretion and transport
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 osmosis using materials such as dialysis tubing
Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations
State that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall

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 Biology-Concise Summary Notes- All Topics

Role of Water as a Solvent in Organisms

🧠 What Is a Solvent?

A solvent is a substance that can dissolve other substances (called solutes) to form a solution.

In living organisms, water is the universal solvent – most biological reactions happen in water.

💧 Why Is Water So Important in the Body?

Water makes up 60–70% of the human body.
It dissolves nutrients, gases, salts, and wastes.
It allows these substances to be transported, broken down, or removed.
Many enzymes only work when substances are dissolved in water.

🔬 Water as a Solvent in Life Processes

1. Digestion

Function of Water	Explanation
Dissolves digested food	After large molecules (like starch) are broken into smaller ones (e.g. glucose), they dissolve in water.
Enables absorption	Dissolved nutrients can pass through the gut wall into the bloodstream.
Helps enzyme action	Digestive enzymes work in aqueous (watery) solutions.

Example: Glucose and amino acids dissolve in water after digestion → absorbed into blood.

2. Excretion

Function of Water	Explanation
Removes waste products	Waste molecules like urea and salts dissolve in water.
Forms urine	Kidney’s filter waste into water → forms urine.
Prevents toxic buildup	Water carries dissolved wastes out of the body.

Example: Urea is dissolved in water and excreted as urine from the kidneys.

3. Transport

Function of Water	Explanation
Main part of blood plasma	Blood is mostly water, carrying dissolved gases, nutrients, and hormones.
Moves substances around the body	Water helps distribute nutrients and remove waste.
Helps in plants too	Water in xylem dissolves mineral ions from the soil and transports them to leaves.

Examples:
In humans: Oxygen (O₂), carbon dioxide (CO₂), glucose, and hormones are all carried in watery blood plasma.
In plants: Nitrates and magnesium ions dissolve in water → move in xylem.

📌 Summary Table: Water as a Solvent

Process	What Water Dissolves	Why It’s Important
Digestion	Glucose, amino acids, enzymes	Helps absorption and enzyme function
Excretion	Urea, salts	Allows safe removal of waste as urine
Transport	Oxygen, CO₂, nutrients, hormones, minerals	Enables circulation and distribution

🧠 Final Statement:

Water acts as a solvent in living organisms by dissolving nutrients, gases, waste products, and ions. This is essential for digestion (to absorb food molecules), excretion (to remove wastes like urea), and transport (to carry substances in blood or xylem).

Osmosis: Water Movement Across Membranes

🧠 Key Statement:

Water diffuses through partially permeable membranes by osmosis.

🔍 What Is Osmosis?

Osmosis is a special type of diffusion where only water molecules move:

From a region of higher water concentration (or dilute solution)
To a region of lower water concentration (or more concentrated solution)
Through a partially permeable membrane

What Is a Partially Permeable Membrane?

A membrane that allows only small molecules like water to pass through.
It blocks larger solutes like sugar or proteins.
Cell membranes are partially permeable, so osmosis happens in all cells.

🧪 Osmosis in Living Organisms

In Animal Cells:

If water enters too much → the cell may burst (lysis)
If water leaves too much → the cell shrinks (crenation)

In Plant Cells:

Water enters → vacuole swells, cell becomes turgid (firm and supported)
Water leaves → vacuole shrinks, cell becomes flaccid or even plasmolysed

📊 Summary Table:

Term	Explanation
Osmosis	Movement of water by diffusion
Direction	High water concentration → Low water concentration
Membrane type	Must be partially permeable
Energy use	No energy needed (passive process)
Importance	Maintains water balance in cells

Osmosis vs. Regular Diffusion

Osmosis	Diffusion
Only water moves	Any small particles can move
Needs a partially permeable membrane	May or may not need a membrane
From dilute → concentrated solution	From high → low concentration
Very important for cells and tissues	Happens in all fluid or gas situations

Water Movement by Osmosis Through the Cell Membrane

🧠 Key Statement:

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

🔍 What Is Osmosis?

Osmosis is the diffusion of water molecules through a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

📌 Passive Process:

Osmosis does not require energy (ATP). It happens naturally using kinetic energy of water molecules.

🔬 The Role of the Cell Membrane

The cell membrane is partially permeable (lets water in/out but blocks large solutes).

Water passes freely depending on concentration inside vs. outside the cell.

When Water Moves Into a Cell:

Organism Type	What Happens
Animal cell	May swell and burst if too much water enters
Plant cell	Becomes turgid (firm and supported by cell wall)

When Water Moves Out of a Cell:

Organism Type	What Happens
Animal cell	Shrinks and becomes crenated
Plant cell	Becomes flaccid, and may undergo plasmolysis

📌 Summary Points:

Osmosis = movement of water only
Occurs through a partially permeable membrane
Always from high to low water concentration
Happens through the cell membrane
Keeps cells hydrated and maintains cell shape

Investigating Osmosis Using Dialysis Tubing

Why Use Dialysis Tubing?

Dialysis tubing is a thin, partially permeable membrane that mimics a cell membrane – it allows small molecules like water to pass but blocks larger solutes like sucrose.

🧪 Materials Needed:

Dialysis tubing (soaked in water)
Sucrose solution (e.g. 10%, 20%)
Distilled water, beakers, string/clips
Measuring cylinders, balance, timer

🧫 Step-by-Step Procedure:

Soak tubing to soften. Tie one end tightly.
Fill with sucrose solution and tie the other end.
Rinse, dry, and record starting mass/volume.
Place in beaker with distilled water. Wait 30–60 min.
Remove, dry gently, measure mass again.
Calculate change in mass to observe osmosis.

📊 What Results to Expect:

External Solution	Inside Tubing	Water Movement	Expected Result
Distilled water	Concentrated sucrose	Water enters tubing	Gains mass
Sucrose solution	Distilled water	Water leaves tubing	Loses mass
Same concentration	Same concentration	No net movement	No change

📌 Osmosis Investigation Recap
Dialysis tubing = model cell membrane
Water moves by osmosis
Change in mass = water gained/lost
Direction depends on solute gradient

Scientific Explanation:

Water moves by osmosis based on the concentration gradient. Since dialysis tubing is partially permeable, only water diffuses, not sugar.

📝 Variables to Consider:

Type of Variable	Examples
Independent	Sucrose concentration
Dependent	Change in mass/volume
Controlled	Time, temperature, volume, tubing size

🔍 Evaluation / Improvements:

Use digital balance for better accuracy
Repeat for reliable results (average)
Test multiple sucrose concentrations
Keep temperature and time constant

Final Statement:

Dialysis tubing is used to model osmosis because it is partially permeable. When filled with a sucrose solution and placed in water, water moves into the tubing by osmosis, causing it to gain mass. The direction of water movement depends on the concentration gradient between the inside and outside solutions.

Investigating the Effects of Osmosis on Plant Tissues

🧠 What Are We Investigating?

We are exploring how plant cells (like potato or beetroot tissue) respond when placed in:

Pure water (high water concentration)
Concentrated sugar/salt solutions (low water concentration)

This experiment models osmosis – the movement of water into or out of plant cells through a partially permeable membrane.

🧪 Materials Needed:

Fresh potato cylinders or beetroot slices
Distilled water
Sucrose or salt solutions of different concentrations (e.g. 0%, 5%, 10%, 15%)
Beakers or test tubes
Ruler and/or balance (for measuring length or mass)
Timer
Paper towels

🔬 Step-by-Step Method:

Cut plant tissue into equal-sized pieces (e.g. same length or mass).
Record the initial mass or length of each piece.
Place each piece in a beaker of different solution concentration.
Leave for a fixed time (e.g. 30–60 minutes).
Remove the tissue, gently dry with paper towel.
Record the final mass or length.
Calculate the percentage change.

📊 What Happens to the Tissue?

Solution Type	Water Movement	Effect on Plant Tissue
Distilled water (0%)	Water enters cells	Tissue becomes turgid and may swell
Moderate sucrose (5%)	Water enters slowly	Tissue may slightly increase in mass
Isotonic solution	No net water movement	No visible change
Concentrated sucrose (10–15%)	Water leaves cells	Tissue becomes flaccid and may shrink

🌱 Explanation Using Osmosis:

Plant cells are surrounded by cell membranes and a strong cell wall.
In pure water, water enters the cells → vacuole expands, cell becomes turgid (firm).
In concentrated solution, water leaves → vacuole shrinks, cell becomes flaccid.
If too much water leaves, the cell membrane pulls away from the cell wall → plasmolysis.

📈 Optional Graph (if measuring mass):

Plot % change in mass on the Y-axis and sucrose concentration on the X-axis.
The graph typically shows a curve that crosses zero (no change) at the isotonic point.

📝 Scientific Terms to Use:

Term	Meaning
Turgid	Cell is full of water, firm
Flaccid	Cell has lost water, soft
Plasmolysis	Cell membrane pulls away from cell wall due to water loss
Isotonic	Concentration inside and outside the cell is the same
Hypertonic	Solution outside is more concentrated (water leaves cell)
Hypotonic	Solution outside is less concentrated (water enters cell)

🔍 Variables in the Experiment:

Type of Variable	Examples
Independent	Sucrose/salt concentration
Dependent	Change in mass or length of plant tissue
Controlled	Time, temperature, size of tissue, same plant used

Summary:

When plant tissues are placed in solutions of different concentrations, water moves in or out of their cells by osmosis. In pure water, cells gain water and become turgid. In concentrated solutions, cells lose water and become flaccid or plasmolysed. Measuring the change in mass or length of the tissue helps show the direction and extent of osmosis.

Water Pressure and Support in Plants

Key Idea: Plants stay upright because of the pressure of water inside their cells pushing outwards against the rigid cell wall.

🔬 How It Works

Each plant cell contains a vacuole filled with watery cell sap.
Water enters the cell by osmosis and fills the vacuole.
The swelling vacuole presses the cell membrane against the cell wall.
The stiff cell wall resists, creating internal pressure known as turgor pressure.

What Is Turgor Pressure?

Turgor pressure is the outward push of the cell membrane against the cell wall due to water.
It keeps cells firm and gives structure to leaves and stems.
Especially important in young, green, non-woody parts of the plant.

🌿 What Happens If Water Is Lost?

Cells lose water and become flaccid.
Turgor pressure drops, causing wilting.
In severe cases, the cell membrane pulls away from the wall → plasmolysis.

📌 Summary Table

Water Effect	Result
Water enters by osmosis	Vacuole expands
Vacuole pushes outward	Turgor pressure develops
Turgid cells push on each other	Plant stands upright
Water lost (e.g., dry soil)	Cells become flaccid → Wilting

Osmosis: Movement of Water Based on Water Potential

Key Definition:
Osmosis is 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.

🌊 Understanding Water Potential

Water potential describes how freely water molecules can move.
Pure water has the highest water potential.
Adding solutes (e.g., sugar, salt) lowers water potential.
Water moves from areas of more free water to less free water.

🧬 Why a Partially Permeable Membrane Matters

It allows small water molecules to pass through.
It blocks larger solute particles like starch or sucrose.
This selectivity enables osmosis to happen naturally.

🔁 Osmosis in Real Situations

Situation	Water Movement
Root hair cells absorbing water	Soil (dilute) → Root cell sap (concentrated)
Rehydrating dried fruits	Water → Into fruit cells
Kidney tubules concentrating urine	Tubule → Capillaries (if surrounding fluid is more concentrated)

🧠 Scientific Vocabulary

Term	Meaning
Dilute solution	High water potential (more free water, few solutes)
Concentrated solution	Low water potential (less free water, more solutes)
Net movement	Overall direction water flows
Partially permeable	Allows water but blocks solutes

Effects of Different Solutions on Plant Cells

🧪 What Happens in Different Solutions?

Plant cells behave differently in dilute or concentrated solutions due to osmosis – the movement of water in or out of the vacuole through a partially permeable membrane.

🧊 In a Dilute Solution (e.g., pure water)

Water enters the cell by osmosis.
Vacuole fills with water → pushes cell membrane against the cell wall.
Cell becomes turgid (firm and swollen).
Turgor pressure builds up inside the cell.
The rigid cell wall prevents bursting.

Turgid: A plant cell full of water, firm due to internal pressure.
Turgor Pressure: Pressure from water pushing the cell membrane outward against the cell wall.

🧪 In a Concentrated Solution (e.g., salty/sugary water)

Water leaves the cell by osmosis.
Vacuole shrinks → cell membrane pulls away from the cell wall.
Cell becomes flaccid (soft and weak).
Plasmolysis may occur in extreme cases.

Flaccid: A plant cell that has lost water and become soft.
Plasmolysis: When the cell membrane detaches from the cell wall due to extreme water loss. Usually irreversible.

🌱 Summary Table

Solution Type	Water Movement	Effect on Cell	Key Term(s)
Dilute (hypotonic)	Water enters	Firm, swollen cell	Turgid, Turgor Pressure
Concentrated (hypertonic)	Water leaves	Soft or shrunken cell	Flaccid, Plasmolysis

When a plant cell is placed in a dilute solution, it becomes turgid due to water entering by osmosis and building up turgor pressure. In a concentrated solution, water leaves the cell, causing it to become flaccid, and in extreme cases, plasmolysis occurs as the membrane pulls away from the wall.

Importance of Water Potential and Osmosis in Water Uptake and Loss

🧠 What Is Water Potential?

Water potential measures how likely water is to move from one area to another.

Water moves from high water potential (more water, dilute solution)
To low water potential (less water, concentrated solution)
Across a partially permeable membrane by osmosis

🔍 Why Osmosis Matters in Living Organisms

Osmosis is vital for maintaining water balance and cell health. It:

Controls water entry and exit in cells
Maintains shape and structure of cells
Supports nutrient movement and prevents damage

🌱 In Plants

Function	How Osmosis Helps
Water uptake by root hair cells	Water moves from soil (high potential) → root (low potential)
Turgor pressure	Water enters vacuole → cells become turgid → support leaves/stems
Wilting prevention	Steady water intake keeps cells from becoming flaccid or plasmolysed

Without proper osmosis, plants lose turgor and struggle to transport minerals.

🧍 In Animals

Function	How Osmosis Affects It
Maintaining cell hydration	Water moves in or out to balance body fluid levels
Preventing cell damage	Too much water → cells burst (lysis) Too little water → cells shrink (crenation)
Kidney function	Osmosis controls water reabsorption → keeps body water balanced

Osmosis in animal cells must be tightly regulated, as they have no cell wall to prevent bursting or shrinkage.

📌 Summary Points

Water potential drives osmosis → controls water flow in and out of cells.
In plants: Osmosis supports water uptake, turgidity, and survival.
In animals: Osmosis balances fluid levels and protects cells from bursting or shrinking.

Water potential and osmosis are essential for the movement of water into and out of cells. In plants, they enable water absorption, turgor pressure, and nutrient transport. In animals, osmosis helps maintain hydration, prevent cell damage, and regulate functions such as water reabsorption in the kidneys.

CIE iGCSE Biology-3.2 Osmosis- Study Notes

CIE iGCSE Biology-3.2 Osmosis- Study Notes- New Syllabus