Cell Model - Comprehensive Study Notes

Key Concepts

Cell Structure — Plant vs Animal

Basic Cell Theory:

All living things are made up of cells
Cells are the basic units of life
All cells come from pre-existing cells

Animal Cells:

Generally irregular or round in shape
Smaller in size (10-30 micrometres)
Do NOT have a cell wall, chloroplasts, or large permanent vacuole
Have small temporary vacuoles (if present)
Nucleus is usually centrally located

Plant Cells:

Regular, rectangular/box-like shape due to rigid cell wall
Generally larger than animal cells (10-100 micrometres)
Have THREE additional structures: cell wall, chloroplasts (in green parts), and large permanent vacuole
Nucleus is pushed to the side by the large vacuole

Common Structures in BOTH Plant and Animal Cells:

Cell membrane
Cytoplasm
Nucleus
Mitochondria

Cell Organelles and Functions

Cell Membrane:

Thin, partially permeable outer boundary of the cell
Controls what enters and leaves the cell
Allows some substances to pass through but not others
Found in both plant and animal cells

Cell Wall:

Thick, rigid outer layer made of cellulose
Provides shape and support to the plant cell
Fully permeable (allows all substances to pass through)
Found ONLY in plant cells
Located OUTSIDE the cell membrane

Cytoplasm:

Jelly-like substance that fills the cell
Site where many chemical reactions occur
Contains all the cell organelles
Made mostly of water

Nucleus:

Large, round organelle (usually the largest visible structure)
Contains genetic material (DNA/chromosomes)
Controls all cell activities
Often called the “control centre” of the cell

Mitochondria (singular: mitochondrion):

Small, sausage-shaped organelles
Site of aerobic respiration
Releases energy from food (glucose) for cell activities
Often called the “powerhouse” of the cell
More mitochondria are found in active cells that need more energy

Chloroplasts:

Disc-shaped, green organelles containing chlorophyll
Site of photosynthesis (making food using light energy)
Found ONLY in plant cells, specifically in green parts (leaves, stems)
NOT found in roots as they don’t receive light

Vacuole:

In plant cells: Large, permanent, central vacuole filled with cell sap (water, sugars, salts)
- Provides support to keep the cell firm (turgid)
- Stores substances
In animal cells: Small, temporary vacuoles (if present)
- Store food or waste materials temporarily

Diffusion and Osmosis

Diffusion:

The net movement of particles from a region of higher concentration to a region of lower concentration
Continues until particles are evenly distributed (equilibrium reached)
Does NOT require energy (passive process)
Happens in gases, liquids, and even solids (very slowly)

Factors Affecting Rate of Diffusion:

Concentration gradient — Greater difference in concentration = faster diffusion
Temperature — Higher temperature = faster particle movement = faster diffusion
Distance — Shorter distance = faster diffusion
Size of particles — Smaller particles = faster diffusion
Surface area — Larger surface area = faster diffusion

Osmosis:

A special type of diffusion
The net movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) through a partially permeable membrane
Does NOT require energy (passive process)

Key Understanding:

Osmosis is specifically about WATER movement only
Requires a partially permeable membrane
Water moves from dilute solution → concentrated solution
Water moves from high water concentration → low water concentration

Effects of Osmosis on Cells:

Animal Cells:

Placed in pure water/dilute solution: Water enters by osmosis → cell swells → may burst (lysis)
Placed in concentrated solution: Water leaves by osmosis → cell shrinks (crenation)

Plant Cells:

Placed in pure water/dilute solution: Water enters by osmosis → vacuole expands → cell becomes turgid (firm) → does NOT burst (protected by cell wall)
Placed in concentrated solution: Water leaves by osmosis → vacuole shrinks → cytoplasm pulls away from cell wall → cell becomes plasmolysed (flaccid/limp)

Cell Specialisation

Cell Specialisation (Differentiation):

Cells develop specific structures to perform particular functions
Specialised cells have adaptations that make them efficient at their job

Examples of Specialised Animal Cells:

Red Blood Cells:
- Function: Transport oxygen around the body
- Adaptations:
  - Biconcave disc shape (increases surface area for oxygen absorption)
  - Contains haemoglobin (protein that binds to oxygen)
  - No nucleus (more space for haemoglobin)
  - Flexible (can squeeze through narrow blood vessels)
Nerve Cells (Neurons):
- Function: Transmit electrical impulses/messages around the body
- Adaptations:
  - Long axon/fibre (carries impulses over long distances)
  - Many branched endings (connect to many other nerve cells)
  - Insulated by fatty sheath (speeds up transmission of impulses)
Muscle Cells:
- Function: Contract and relax to produce movement
- Adaptations:
  - Long and thin (can contract and relax)
  - Many mitochondria (provide energy for contraction)
  - Special proteins that can slide over each other (enable contraction)
Sperm Cells:
- Function: Fertilise the egg cell
- Adaptations:
  - Long tail/flagellum (for swimming)
  - Many mitochondria in middle section (provide energy for swimming)
  - Streamlined head (reduces resistance when swimming)
  - Acrosome in head (contains enzymes to digest egg membrane)

Examples of Specialised Plant Cells:

Root Hair Cells:
- Function: Absorb water and mineral salts from soil
- Adaptations:
  - Long hair-like projection (increases surface area for absorption)
  - Thin cell wall (short diffusion distance)
  - Large permanent vacuole (stores absorbed water and minerals)
  - No chloroplasts (underground, no light)
Palisade Mesophyll Cells:
- Function: Carry out photosynthesis
- Adaptations:
  - Many chloroplasts (maximum light absorption for photosynthesis)
  - Arranged near top of leaf (receive maximum light)
  - Regular, cylindrical shape with large surface area
  - Thin cell walls (allow carbon dioxide to diffuse in easily)
Xylem Vessels:
- Function: Transport water and mineral salts from roots to leaves
- Adaptations:
  - Hollow tube with no end walls (allows continuous water flow)
  - No cytoplasm or nucleus (dead cells = no blockage to water flow)
  - Thick walls strengthened with lignin (provides support, prevents collapse)
  - Walls have pits (allow water to move sideways between vessels)
Phloem Cells:
- Function: Transport sugars (food) from leaves to other parts of plant
- Adaptations:
  - Sieve tubes with sieve plates (allow dissolved sugars to flow through)
  - Companion cells (provide energy for transport)
  - Living cells with cytoplasm but no nucleus

Important Definitions

Cell: The basic structural and functional unit of all living organisms.

Cell Membrane: A thin, partially permeable membrane that surrounds the cell and controls the movement of substances in and out of the cell.

Cell Wall: A rigid outer layer made of cellulose found only in plant cells that provides support and shape.

Cytoplasm: The jelly-like substance inside the cell where chemical reactions take place.

Nucleus: The control centre of the cell that contains genetic material (DNA) and controls all cell activities.

Mitochondria: Organelles where aerobic respiration occurs to release energy from glucose.

Chloroplasts: Green organelles found only in plant cells where photosynthesis occurs.

Vacuole: A fluid-filled space; plant cells have a large permanent vacuole containing cell sap, while animal cells may have small temporary vacuoles.

Diffusion: The net movement of particles from a region of higher concentration to a region of lower concentration until evenly distributed.

Osmosis: The net movement of water molecules from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane.

Partially Permeable Membrane: A membrane that allows some molecules (like water) to pass through but not others (like large solute molecules).

Concentration Gradient: The difference in concentration between two regions.

Turgid: A plant cell that is firm and swollen due to water entering by osmosis.

Plasmolysed: A plant cell that has become limp/flaccid because water has left by osmosis and the cytoplasm has pulled away from the cell wall.

Cell Specialisation: The process by which cells develop specific structures and features to perform particular functions efficiently.

Cellulose: The strong material that makes up plant cell walls.

Cell Sap: The solution of water, sugars, and salts found in the permanent vacuole of plant cells.

Diagrams and Structures

Diagram 1: Animal Cell

How to draw:

Draw an irregular oval/circular shape
Draw the cell membrane as the outer boundary (single thin line)
Inside, draw the nucleus as a large circle (usually central or slightly off-centre)
Inside the nucleus, draw a smaller dark circle (nucleolus)
Fill the space between nucleus and membrane with dots to represent cytoplasm
Draw 4-6 small oval shapes scattered in the cytoplasm (mitochondria)
Optional: Draw 1-2 small circles (temporary vacuoles)

Labels required:

Cell membrane (pointing to outer boundary)
Cytoplasm (pointing to jelly-like area)
Nucleus (pointing to large central structure)
Mitochondria (pointing to small oval shapes)

Diagram 2: Plant Cell

How to draw:

Draw a regular rectangular shape with straight edges
Draw the cell wall as the thick outer boundary (double line or thick single line)
Just inside, draw the cell membrane (thinner line, parallel to cell wall)
Draw a large central vacuole (large circle or oval taking up most of the cell space)
The vacuole pushes the nucleus to one side — draw nucleus as a circle at the edge
Inside nucleus, draw nucleolus (small dark circle)
Draw cytoplasm as a thin layer between cell membrane and vacuole
Draw 6-10 small oval green shapes in the cytoplasm (chloroplasts)
Draw 4-6 small sausage shapes in cytoplasm (mitochondria)

Labels required:

Cell wall (pointing to thick outer boundary)
Cell membrane (pointing to thin line inside cell wall)
Cytoplasm (pointing to jelly-like layer)
Nucleus (pointing to circular structure at side)
Large permanent vacuole (pointing to large central space)
Chloroplasts (pointing to green oval structures)
Mitochondria (pointing to small sausage shapes)

Diagram 3: Comparison Table Structure

Create a table showing:

Feature	Animal Cell	Plant Cell
Shape	Irregular/round	Regular/rectangular
Size	Smaller (10-30 μm)	Larger (10-100 μm)
Cell wall	Absent	Present (cellulose)
Cell membrane	Present	Present
Nucleus	Present (central)	Present (at side)
Cytoplasm	Present	Present
Mitochondria	Present	Present
Chloroplasts	Absent	Present (in green parts)
Vacuole	Small, temporary	Large, permanent, central

Diagram 4: Osmosis in Plant Cells

Three scenarios to draw:

Scenario A - Turgid Cell (in dilute solution/water):

Draw rectangular plant cell
Vacuole is large, pressing against cell wall
Cytoplasm pressed firmly against cell membrane
Label: “Water enters by osmosis”
Arrow pointing inward showing water movement

Scenario B - Normal Cell (in isotonic solution):

Draw rectangular plant cell
Vacuole occupies normal space
Cytoplasm fills cell normally
Label: “No net water movement”

Scenario C - Plasmolysed Cell (in concentrated solution):

Draw rectangular plant cell
Vacuole is shrunken and small
Cytoplasm pulled away from cell wall (gaps visible)
Label: “Water leaves by osmosis”
Arrow pointing outward showing water movement

Diagram 5: Specialised Cells

Root Hair Cell:

Draw irregular rectangular shape with one long thin projection (root hair)
Large vacuole in main body
Nucleus at one side
No chloroplasts
Label all parts

Red Blood Cell:

Draw biconcave disc shape (looks like a disc with indents on both sides)
No nucleus (important!)
Label “Contains haemoglobin”

Nerve Cell:

Draw central cell body with nucleus
Long extension from one end (axon)
Multiple branched endings at both ends
Label: cell body, nucleus, axon, nerve endings

Worked Examples

Example 1: Identifying Cell Type from Description

Question: A student observes a cell under the microscope with the following features:

Regular rectangular shape
Has a cell wall
Contains many green disc-shaped structures
Has a large central structure pushing other contents to the sides

Is this a plant or animal cell? Give reasons for your answer.

Solution:

Step 1: Identify the key features mentioned

Regular rectangular shape ✓
Cell wall ✓
Green disc-shaped structures = chloroplasts ✓
Large central structure = large permanent vacuole ✓

Step 2: Compare with known cell types

Cell wall is found ONLY in plant cells
Chloroplasts are found ONLY in plant cells
Large permanent vacuole is found ONLY in plant cells
Regular shape is characteristic of plant cells

Step 3: Write complete answer

Answer: This is a plant cell.

Reasons:

It has a cell wall, which is only present in plant cells
It contains chloroplasts (green disc-shaped structures), which are only found in plant cells
It has a large permanent vacuole in the centre, which is characteristic of plant cells
It has a regular rectangular shape due to the rigid cell wall

(This answer would earn full marks as it correctly identifies the cell type and provides multiple valid reasons with proper terminology.)

Example 2: Explaining Osmosis Effects

Question: A piece of potato is placed in a concentrated salt solution for 30 minutes. Explain what happens to the potato cells and why. Use the term osmosis in your answer.

Solution:

Step 1: Identify the water concentration

Inside potato cells: high water concentration (cells contain dilute cell sap)
Outside in salt solution: low water concentration (concentrated solution)

Step 2: Determine direction of water movement

Water moves from high concentration → low concentration
Water moves from inside cells → outside into salt solution

Step 3: Identify the process

This movement is called osmosis (water movement through partially permeable membrane)

Step 4: Explain the effect on cells

Water leaves the vacuole
Vacuole shrinks
Cytoplasm pulls away from cell wall
Cells become plasmolysed (flaccid/limp)

Step 5: Write complete answer

Answer: The potato cells will become plasmolysed (flaccid/limp).

Explanation:

Water moves out of the potato cells by osmosis
This is because water moves from a region of higher water concentration (inside the cells) to a region of lower water concentration (in the concentrated salt solution) through the partially permeable cell membrane
As water leaves, the vacuole shrinks and the cytoplasm pulls away from the cell wall
This causes the potato to become soft and limp

(This answer includes the key terms and explains the process step-by-step, earning full marks.)

Example 3: Linking Structure to Function

Question: Explain how two features of a red blood cell are adapted to its function of transporting oxygen.

Solution:

Step 1: Identify the function

Function: Transport oxygen around the body

Step 2: Identify adaptations

Feature 1: Biconcave disc shape
Feature 2: No nucleus
Feature 3: Contains haemoglobin (Choose any two)

Step 3: Link each feature to how it helps the function

Answer:

Adaptation 1: Biconcave disc shape

This increases the surface area of the cell
A larger surface area allows more oxygen to be absorbed quickly and efficiently

Adaptation 2: No nucleus

The absence of a nucleus means there is more space inside the cell
This allows the cell to carry more haemoglobin, which can bind to and transport more oxygen

Alternative Adaptation: Contains haemoglobin

Haemoglobin is a protein that binds to oxygen
This allows the red blood cell to pick up oxygen in the lungs and release it to body cells that need it

(Any two adaptations with clear explanations linking structure to function would earn full marks. The key is to explain HOW each feature helps the cell perform its function.)

Common Mistakes to Avoid

Mistake 1: Confusing Plant and Animal Cell Features

❌ Saying animal cells have chloroplasts or cell walls
❌ Saying plant cells don’t have mitochondria
✓ Remember: Cell wall, chloroplasts, and large vacuole are ONLY in plant cells
✓ Remember: BOTH cell types have cell membrane, cytoplasm, nucleus, and mitochondria

Mistake 2: Incorrect Osmosis Definition

❌ Saying “movement of particles” (this is diffusion)
❌ Forgetting “partially permeable membrane”
❌ Saying water moves from concentrated to dilute solution
✓ Correct: “Net movement of water molecules from higher water concentration to lower water concentration through a partially permeable membrane”

Mistake 3: Wrong Direction of Water Movement

❌ Thinking concentrated solution has high water concentration
✓ Remember: Concentrated solution = LOW water concentration (lots of solute, less water)
✓ Remember: Dilute solution = HIGH water concentration (little solute, more water)

Mistake 4: Confusing Turgid and Plasmolysed

❌ Mixing up which happens in water vs. salt solution
✓ Turgid: Happens when plant cell is in WATER/dilute solution (cell becomes firm)
✓ Plasmolysed: Happens when plant cell is in CONCENTRATED solution (cell becomes limp)

Mistake 5: Incomplete Structure-Function Answers

❌ Just stating the feature without explaining HOW it helps
❌ Example: “Red blood cells have no nucleus” (incomplete)
✓ Correct: “Red blood cells have no nucleus, which provides more space to carry haemoglobin, allowing them to transport more oxygen”

Mistake 6: Drawing Errors in Diagrams

❌ Drawing chloroplasts in animal cells
❌ Drawing plant cells with irregular shapes
❌ Not showing the cell wall as separate from cell membrane
✓ Always check your diagram matches the cell type required

Mistake 7: Confusing Cell Wall and Cell Membrane

❌ Saying cell wall controls what enters/leaves (that’s the cell membrane’s job)
❌ Saying cell membrane provides support (that’s the cell wall’s job)
✓ Cell wall: Support and shape (fully permeable)
✓ Cell membrane: Controls entry/exit of substances (partially permeable)

Mistake 8: Wrong Terminology for Mitochondria

❌ Saying “mitochondria produces energy”
✓ Correct: “Mitochondria releases energy from glucose during respiration”
✓ Energy cannot be produced or destroyed, only converted/released

Mistake 9: Incomplete Osmosis Explanations

❌ Not mentioning the partially permeable membrane
❌ Not specifying water concentration differences
❌ Not explaining what happens to the cell after osmosis
✓ Include: water movement direction, concentration gradient, membrane type, effect on cell

Mistake 10: Forgetting About Root Location

❌ Drawing chloroplasts in root hair cells
✓ Remember: Roots are underground, no light, so NO chloroplasts needed

Exam Tips

For Structure/Diagram Questions:

Use a ruler for plant cells — they have straight edges
Label lines must be straight and should not cross each other
Labels must touch the part they’re identifying
Draw in pencil first, then trace over in pen if allowed
Always include: cell membrane, cytoplasm, nucleus, mitochondria for both cell types
Add cell wall, chloroplasts, large vacuole for plant cells ONLY

For Definition Questions:

Key phrases to include for Osmosis:

“Net movement of water molecules” (not just “water” or “particles”)
“From region of higher water concentration” (specify “water”)
“To region of lower water concentration”
“Through a partially permeable membrane”
All four components needed for full marks

Key phrases for Diffusion:

“Net movement of particles/molecules”
“From higher concentration to lower concentration”
“Until evenly distributed/equilibrium reached”

For Compare/Contrast Questions:

When comparing plant and animal cells:

Make a clear table or list
State what animal cells have AND what plant cells have
Use “whereas” or “but” to show differences
Example: “Animal cells have small temporary vacuoles whereas plant cells have large permanent vacuoles”

For Function Questions:

Always link structure to function:

Feature → How it helps → What the cell does
Use connecting words: “allows,” “enables,” “helps to,” “so that”
Example: “Many mitochondria provide energy so that muscle cells can contract”

For Osmosis Effect Questions:

Mark-earning phrases:

“Water moves by osmosis from…”
“Through the partially permeable cell membrane”
“From higher water concentration to lower water concentration”
Describe what happens to the vacuole
State whether cell becomes turgid/plasmolysed
Describe visible effect (firm/limp/burst)

Keywords to Use for Specialised Cells:

Red Blood Cells:

Surface area, haemoglobin, no nucleus, flexible, transport oxygen

Nerve Cells:

Long axon, transmit impulses, branched endings, fatty sheath, connect

Root Hair Cells:

Long projection, increases surface area, absorb water and minerals, thin wall

Palisade Cells:

Many chloroplasts, photosynthesis, maximum light absorption, near top of leaf

General Exam Strategy:

Read questions carefully — “plant cell” vs “animal cell” makes a big difference
Use correct scientific terms — not “energy-making things” but “mitochondria”
Check the marks — 3 marks usually means 3 distinct points needed
Give reasons/explanations — don’t just state facts
Use “because,” “so that,” “this allows” to show understanding
For 4-mark questions, usually need to:
- State the process (e.g., osmosis)
- Explain the direction of movement
- Give the reason (concentration difference)
- Describe the effect on the cell

Command Words:

State/Name: Just give the answer, no explanation needed
Describe: Say what happens (no need to explain why)
Explain: Say what happens AND why it happens
Compare: Give similarities AND differences
Suggest: Use your knowledge to work out an answer (not directly taught)

✓ Cell Structure: Know all organelles in plant cells (cell wall, cell membrane, cytoplasm, nucleus, mitochondria, chloroplasts, large permanent vacuole) vs animal cells (cell membrane, cytoplasm, nucleus, mitochondria, small temporary vacuoles)

✓ Three structures ONLY in plant cells: Cell wall, chloroplasts (in green parts), large permanent vacuole

✓ Function of each organelle: Cell membrane (controls entry/exit), cytoplasm (site of reactions), nucleus (control centre with DNA), mitochondria (releases energy), chloroplasts (photosynthesis), vacuole (storage and support in plants)

✓ Diffusion definition: Net movement of particles from higher to lower concentration until evenly distributed

✓ Osmosis definition: Net movement of water molecules from higher to lower water concentration through partially permeable membrane

✓ Water concentration: Dilute solution = HIGH water concentration; Concentrated solution = LOW water concentration

✓ Effects on plant cells: In water → turgid (firm); In concentrated solution → plasmolysed (limp, cytoplasm pulls away from wall)

✓ Effects on animal cells: In water → swells, may burst; In concentrated solution → shrinks

✓ Cell specialisation: Cells develop specific features for specific functions

✓ Red blood cells: Biconcave shape (increases surface area), no nucleus (more space for haemoglobin), contains haemoglobin (binds oxygen)

✓ Root hair cells: Long projection (increases surface area for absorption), no chloroplasts (underground, no light), thin wall (short diffusion distance)

✓ Palisade mesophyll cells: Many chloroplasts (maximum photosynthesis), located near top of leaf (maximum light exposure)

Remember: Understanding WHY cells have certain features is just as important as knowing WHAT features they have. Always link structure to function in your exam answers!