Cell Membrane Structure and Transport Mechanisms | NEET Biology Notes
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| Fluid Mosaic Model showing membrane structure, transport mechanisms, and cell wall components important for NEET Biology. |
Cell Membrane (Fluid Mosaic Model) – Easy NEET Notes (Line by Line)
1. Fluid Mosaic Model (Singer and Nicolson, 1972)
Original Line:
"An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as fluid mosaic model."
Easy Note:
- In 1972, Singer and Nicolson proposed the Fluid Mosaic Model.
- It is the most accepted model of cell membrane structure.
- It explains how lipids and proteins are arranged in the membrane.
NEET Point:
- Fluid Mosaic Model = Singer and Nicolson (1972)
Original Line:
"According to this, the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer."
Easy Note:
- Cell membrane is made mainly of a lipid bilayer.
- Lipids are not fixed; they remain semi-fluid (quasi-fluid).
- Due to this fluid nature, proteins can move sideways (laterally) within the membrane.
Key Term:
- Lateral movement = Side-to-side movement of proteins.
Original Line:
"This ability to move within the membrane is measured as its fluidity."
Easy Note:
- The ease with which molecules move inside the membrane is called fluidity.
- Higher movement = Higher fluidity.
NEET Point:
- Fluidity = Ability of membrane components to move within the membrane.
Importance of Fluid Nature of Membrane
Original Line:
"The fluid nature of the membrane is also important from the point of view of functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc."
Easy Note:
The fluid membrane helps in:
- Cell growth
- Formation of intercellular junctions
- Secretion of substances
- Endocytosis (taking materials inside the cell)
- Cell division
NEET Trick:
GJSED
- Growth
- Junction formation
- Secretion
- Endocytosis
- Division
Transport Across Plasma Membrane
Original Line:
"One of the most important functions of the plasma membrane is the transport of the molecules across it."
Easy Note:
- Plasma membrane controls the movement of substances into and out of the cell.
Original Line:
"The membrane is selectively permeable to some molecules present on either side of it."
Easy Note:
- Plasma membrane is selectively permeable.
- It allows only certain substances to pass through.
- Some substances are allowed, others are restricted.
NEET Point:
- Plasma membrane = Selectively permeable membrane
Passive Transport
Original Line:
"Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport."
Easy Note:
- Movement of substances without ATP energy is called Passive Transport.
Characteristics:
✔ No ATP required
✔ Along concentration gradient
✔ High concentration → Low concentration
Simple Diffusion
Original Line:
"Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient."
Easy Note:
- Neutral molecules move by simple diffusion.
- They move from:
- Higher concentration → Lower concentration
Example:
- Oxygen (O₂)
- Carbon dioxide (CO₂)
NEET Point:
- Diffusion always occurs along concentration gradient.
Osmosis
Original Line:
"Water may also move across this membrane from higher to lower concentration."
Easy Note:
- Water also moves through the membrane by diffusion.
Original Line:
"Movement of water by diffusion is called osmosis."
Easy Note:
- Osmosis = Movement of water molecules through a selectively permeable membrane from higher water concentration to lower water concentration.
Definition for NEET:
Osmosis = Diffusion of water through a selectively permeable membrane.
Facilitated Diffusion
Original Line:
"As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane."
Easy Note:
- Polar molecules cannot directly cross the lipid bilayer.
- They need special membrane proteins called carrier proteins.
- This process is called facilitated diffusion.
Characteristics:
✔ No ATP needed
✔ Carrier proteins required
Examples:
- Glucose
- Amino acids
Active Transport
Original Line:
"A few ions or molecules are transported across the membrane against their concentration gradient."
Easy Note:
- Some substances move:
- Low concentration → High concentration
- This movement is against the concentration gradient.
Original Line:
"Such a transport is an energy dependent process, in which ATP is utilised and is called active transport."
Easy Note:
- Movement against concentration gradient requires ATP energy.
- This process is called Active Transport.
Characteristics:
✔ ATP required
✔ Against concentration gradient
✔ Uses carrier proteins
Original Line:
"e.g., Na⁺/K⁺ Pump."
Easy Note:
- Best example of active transport:
- Sodium-Potassium Pump (Na⁺/K⁺ Pump)
NEET Point:
- Na⁺/K⁺ pump uses ATP.
Cell Wall
Original Line:
"A non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants."
Easy Note:
- Cell wall is:
- Non-living
- Rigid
- Present outside plasma membrane
- Found in:
- Plants
- Fungi
NEET Point:
- Animal cells do not have a cell wall.
Functions of Cell Wall
Original Line:
"Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection..."
Easy Note:
Functions:
- Gives shape
- Provides strength
- Protects from injury
- Protects from infection
Original Line:
"It also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules."
Easy Note:
Additional functions: 5. Cell-to-cell communication 6. Prevents entry of unwanted large molecules
Composition of Cell Wall
In Algae
Original Line:
"Algae have cell wall made of cellulose, galactans, mannans and minerals like calcium carbonate."
Easy Note:
Algal cell wall contains:
- Cellulose
- Galactans
- Mannans
- Calcium carbonate
In Higher Plants
Original Line:
"In other plants it consists of cellulose, hemicellulose, pectins and proteins."
Easy Note:
Plant cell wall contains:
- Cellulose
- Hemicellulose
- Pectin
- Proteins
NEET Point:
- Cellulose is the major component.
Primary Wall
Original Line:
"The cell wall of a young plant cell, the primary wall is capable of growth."
Easy Note:
- Young plant cells have a Primary Cell Wall.
- It can expand and grow.
Original Line:
"Which gradually diminishes as the cell matures."
Easy Note:
- As the cell becomes older, growth decreases.
Secondary Wall
Original Line:
"The secondary wall is formed on the inner side of the cell."
Easy Note:
- Mature cells develop a Secondary Cell Wall.
- It is deposited towards the plasma membrane.
- It is thick and rigid.
NEET Point:
- Secondary wall is formed inside the primary wall.
Middle Lamella
Original Line:
"The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together."
Easy Note:
- Middle lamella is present between two adjacent plant cells.
- It acts like cement.
- Made mainly of Calcium Pectate.
NEET Point:
- Middle Lamella = Calcium Pectate
Memory Trick:
"Pectate Pastes Cells Together."
Plasmodesmata
Original Line:
"The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells."
Easy Note:
- Plasmodesmata are tiny channels in plant cell walls.
- They connect the cytoplasm of adjacent cells.
- They allow exchange of materials and communication between cells.
NEET Point:
- Plasmodesmata connect neighbouring plant cells.
Quick NEET Revision Table
| Topic | Key Point |
|---|---|
| Fluid Mosaic Model | Singer & Nicolson (1972) |
| Fluidity | Lateral movement of proteins |
| Passive Transport | No ATP, high → low concentration |
| Diffusion | Movement of molecules along gradient |
| Osmosis | Diffusion of water |
| Facilitated Diffusion | Carrier protein required, no ATP |
| Active Transport | ATP required, low → high concentration |
| Example | Na⁺/K⁺ Pump |
| Cell Wall | Non-living, rigid |
| Major Component | Cellulose |
| Primary Wall | Young, growing wall |
| Secondary Wall | Thick wall in mature cells |
| Middle Lamella | Calcium pectate |
| Plasmodesmata | Cytoplasmic connections between plant cells |
Most Important NEET One-Liners
- Fluid Mosaic Model was proposed by Singer and Nicolson in 1972.
- Plasma membrane is selectively permeable.
- Osmosis = diffusion of water.
- Active transport requires ATP.
- Na⁺/K⁺ pump is an example of active transport.
- Cell wall is non-living and rigid.
- Cellulose is the main component of plant cell wall.
- Middle lamella is mainly composed of calcium pectate.
- Plasmodesmata connect neighbouring plant cells.
- Primary wall is present in young cells; secondary wall develops in mature cells.
CBSE Class 11 Biology: Cell Membrane and Cell Wall Question Bank with Answers
A. Multiple Choice Questions (MCQs)
1. The Fluid Mosaic Model was proposed by:
a) Robert Hooke
b) Schleiden and Schwann
c) Singer and Nicolson
d) Watson and Crick
Answer: c) Singer and Nicolson
2. The plasma membrane is:
a) Freely permeable
b) Impermeable
c) Selectively permeable
d) Non-permeable
Answer: c) Selectively permeable
3. Movement of water through a selectively permeable membrane is called:
a) Diffusion
b) Active transport
c) Osmosis
d) Endocytosis
Answer: c) Osmosis
4. Which process requires ATP?
a) Diffusion
b) Osmosis
c) Facilitated diffusion
d) Active transport
Answer: d) Active transport
5. Middle lamella is mainly composed of:
a) Cellulose
b) Calcium pectate
c) Hemicellulose
d) Proteins
Answer: b) Calcium pectate
6. The major component of plant cell wall is:
a) Lipid
b) Protein
c) Cellulose
d) Chitin
Answer: c) Cellulose
7. Which structure connects neighbouring plant cells?
a) Ribosome
b) Centrosome
c) Plasmodesmata
d) Lysosome
Answer: c) Plasmodesmata
8. Example of active transport:
a) Osmosis
b) Diffusion
c) Na⁺/K⁺ Pump
d) Facilitated diffusion
Answer: c) Na⁺/K⁺ Pump
B. Very Short Answer Questions (1 Mark)
1. Who proposed the Fluid Mosaic Model?
Answer: Singer and Nicolson (1972).
2. Define osmosis.
Answer: Osmosis is the diffusion of water through a selectively permeable membrane.
3. What is fluidity of membrane?
Answer: The ability of membrane components to move within the membrane.
4. Name the main component of middle lamella.
Answer: Calcium pectate.
5. What is the nature of cell wall?
Answer: Non-living and rigid.
6. What is plasmodesmata?
Answer: Cytoplasmic connections between neighbouring plant cells.
C. Short Answer Questions (2–3 Marks)
1. What is passive transport?
Answer: Passive transport is the movement of substances across the membrane without expenditure of energy (ATP). It occurs along the concentration gradient from higher concentration to lower concentration.
2. Write any three functions of cell wall.
Answer:
- Gives shape to the cell.
- Protects the cell from mechanical injury.
- Prevents infection and provides support.
3. Differentiate between diffusion and osmosis.
| Diffusion | Osmosis |
|---|---|
| Movement of molecules | Movement of water molecules only |
| May occur with or without membrane | Requires selectively permeable membrane |
| From higher to lower concentration | From higher water concentration to lower |
4. What is active transport?
Answer: Active transport is the movement of substances across a membrane against the concentration gradient with the help of ATP energy.
Example: Na⁺/K⁺ Pump.
D. Long Answer Questions (5 Marks)
1. Explain the Fluid Mosaic Model of plasma membrane.
Answer:
- The Fluid Mosaic Model was proposed by Singer and Nicolson in 1972.
- According to this model, the plasma membrane consists of a phospholipid bilayer.
- Proteins are embedded within the lipid bilayer.
- Lipids are fluid in nature, allowing proteins to move laterally.
- This movement gives flexibility and fluidity to the membrane.
- The membrane is selectively permeable and regulates transport of substances.
2. Explain the structure and functions of the cell wall.
Answer: The cell wall is a non-living rigid structure present outside the plasma membrane in plant cells.
Structure:
- Made mainly of cellulose.
- Contains hemicellulose, pectin and proteins.
- Consists of primary wall, secondary wall and middle lamella.
Functions:
- Provides shape and support.
- Protects against mechanical damage.
- Prevents infection.
- Helps in cell-to-cell interaction.
- Acts as a barrier against undesirable substances.
E. Assertion and Reason Questions
1.
Assertion (A): Plasma membrane is selectively permeable.
Reason (R): It allows only specific substances to pass through it.
a) Both A and R are true and R is the correct explanation of A.
b) Both A and R are true but R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.
Answer: a)
2.
Assertion (A): Active transport requires ATP.
Reason (R): Substances move against the concentration gradient.
Answer: a)
3.
Assertion (A): Middle lamella acts as cement between cells.
Reason (R): It is mainly composed of calcium pectate.
Answer: a)
4.
Assertion (A): Osmosis requires ATP.
Reason (R): Water moves from higher to lower concentration.
Answer: c) Assertion is false but Reason is true.
F. Fill in the Blanks
-
The Fluid Mosaic Model was proposed by __________ and __________.
Answer: Singer, Nicolson
-
Movement of water by diffusion is called __________.
Answer: Osmosis
-
Active transport requires energy in the form of __________.
Answer: ATP
-
The major component of plant cell wall is __________.
Answer: Cellulose
-
Middle lamella is mainly made of __________.
Answer: Calcium pectate
-
Cytoplasmic connections between plant cells are called __________.
Answer: Plasmodesmata
G. Statement-Based Questions
Read the statements carefully:
Statement I:
The plasma membrane is selectively permeable.
Statement II:
All molecules can freely pass through the plasma membrane.
Choose the correct option:
a) Both statements are true
b) Both statements are false
c) Statement I is true but Statement II is false
d) Statement I is false but Statement II is true
Answer: c)
Statement I:
Primary wall is found in young plant cells.
Statement II:
Secondary wall is deposited on the inner side of the primary wall.
Answer: Both statements are true.
H. Match the Columns
Match Column A with Column B
| Column A | Column B |
|---|---|
| A. Osmosis | 1. ATP required |
| B. Active Transport | 2. Calcium pectate |
| C. Middle Lamella | 3. Water movement |
| D. Plasmodesmata | 4. Cell connections |
Answers:
A → 3
B → 1
C → 2
D → 4
Match Column A with Column B
| Column A | Column B |
|---|---|
| A. Singer and Nicolson | 1. Cell wall |
| B. Cellulose | 2. Fluid Mosaic Model |
| C. Na⁺/K⁺ Pump | 3. Active transport |
| D. Osmosis | 4. Water diffusion |
Answers:
A → 2
B → 1
C → 3
D → 4
I. Case Study Questions
Case Study 1
A scientist observed that proteins embedded in a membrane could move from one place to another within the membrane. He concluded that the membrane is fluid in nature.
Questions:
1. Which model explains this observation?
Answer: Fluid Mosaic Model.
2. Who proposed this model?
Answer: Singer and Nicolson.
3. What property allows proteins to move laterally?
Answer: Fluidity of lipids.
4. Name one function dependent on membrane fluidity.
Answer: Endocytosis (or cell division/secretion).
Case Study 2
A plant cell was observed under a microscope. A layer made of calcium pectate was found between two neighbouring cells.
Questions:
1. Name the structure.
Answer: Middle lamella.
2. What is its function?
Answer: It binds neighbouring cells together.
3. Which substance is present in it?
Answer: Calcium pectate.
4. Name the channels connecting adjacent cells.
Answer: Plasmodesmata.
Important CBSE Exam Questions
Frequently Asked 1-Mark Questions
- Define osmosis.
- What is active transport?
- Name the scientist who proposed Fluid Mosaic Model.
- What is middle lamella?
- Define plasmodesmata.
Frequently Asked 3-Mark Questions
- Explain passive transport.
- Differentiate between diffusion and osmosis.
- State three functions of cell wall.
Frequently Asked 5-Mark Questions
- Explain Fluid Mosaic Model.
- Describe structure and functions of cell wall.
- Explain different modes of transport across plasma membrane.
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