Cell: The Unit of Life – Class 11 Biology Notes
Introduction
The cell is the basic structural and functional unit of all living organisms. It is the smallest unit capable of performing all the functions of life, such as growth, reproduction, metabolism, and response to stimuli. The study of cells is called cytology, and it helps us understand the functioning of living organisms at a molecular level.
1. Types of Cells
There are two main types of cells:
- Prokaryotic Cells
- Eukaryotic Cells
1.1. Prokaryotic Cells
- Characteristics:
- Lack a defined nucleus (nucleoid region instead).
- Lack membrane-bound organelles.
- Smaller in size (usually 0.1 to 5 micrometers).
- Have a single circular DNA molecule.
- Example: Bacteria and blue-green algae (cyanobacteria).
- Structure:
- Plasma Membrane: Surrounds the cell, providing structure and controlling the movement of substances.
- Nucleoid: Region where the DNA is located.
- Ribosomes: Small structures responsible for protein synthesis.
- Cell Wall: Provides structural support and protection.
- Flagella/Pili: Involved in movement and attachment.
1.2. Eukaryotic Cells
- Characteristics:
- Have a true, membrane-bound nucleus containing the cell’s DNA.
- Contain membrane-bound organelles.
- Larger in size (usually 10 to 100 micrometers).
- Can be unicellular or multicellular.
- Example: Animal cells, plant cells, fungi, and protists.
- Structure:
- Plasma Membrane: A phospholipid bilayer with embedded proteins that regulates the entry and exit of substances.
- Nucleus: Contains the genetic material (DNA) and controls cellular activities.
- Cytoplasm: Jelly-like substance that fills the cell and contains organelles.
- Organelles: Specialized structures within the cytoplasm, each performing specific functions.
2. Structure of a Eukaryotic Cell
2.1. Plasma Membrane
- Structure: Made of a lipid bilayer with embedded proteins, cholesterol, and carbohydrates.
- Function:
- Regulates the movement of ions, nutrients, and waste products into and out of the cell.
- Protects the cell from its environment.
- Facilitates cell communication through receptor proteins.
2.2. Nucleus
- Structure:
- Surrounded by a double membrane known as the nuclear envelope.
- Contains chromatin (DNA and proteins) and nucleolus.
- Function:
- Houses the cell’s genetic material.
- Controls the cell’s activities, including growth, metabolism, and reproduction.
- The nucleolus is responsible for ribosome synthesis.
2.3. Cytoplasm
- Structure: Gel-like substance inside the plasma membrane that contains water, salts, and organic molecules.
- Function:
- Site for various biochemical processes.
- Contains organelles suspended in it.
2.4. Organelles
- Mitochondria:
- The powerhouse of the cell. They generate ATP (energy) through cellular respiration.
- Have a double membrane structure.
- Endoplasmic Reticulum (ER):
- Rough ER: Studded with ribosomes, responsible for protein synthesis and transport.
- Smooth ER: Lacks ribosomes, involved in lipid synthesis, detoxification, and carbohydrate metabolism.
- Golgi Apparatus:
- Composed of stacked, flattened sacs.
- Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
- Lysosomes:
- Contain digestive enzymes.
- Break down waste material, cellular debris, and foreign invaders (e.g., bacteria).
- Ribosomes:
- Tiny, non-membrane-bound organelles.
- Responsible for protein synthesis.
- Found on the rough ER or free in the cytoplasm.
- Cytoskeleton:
- A network of protein filaments (actin filaments, microtubules, intermediate filaments).
- Provides structural support, aids in cell division, and facilitates movement within the cell.
2.5. Plant Cell Special Features
Plant cells, as a type of eukaryotic cell, have a few additional structures:
- Cell Wall: Provides structure and protection, made of cellulose.
- Chloroplasts: Contain chlorophyll and carry out photosynthesis.
- Central Vacuole: A large, membrane-bound sac that stores water, nutrients, and waste products, and maintains turgidity.
3. Differences Between Prokaryotic and Eukaryotic Cells
| Feature | Prokaryotic Cells | Eukaryotic Cells |
|---|---|---|
| Nucleus | Absent (Nucleoid region) | Present (Membrane-bound) |
| Size | Small (0.1-5 micrometers) | Larger (10-100 micrometers) |
| Organelles | Lack membrane-bound organelles | Contain membrane-bound organelles |
| DNA | Circular, free in cytoplasm | Linear, enclosed in the nucleus |
| Reproduction | Asexual (Binary fission) | Sexual and asexual (Mitosis, Meiosis) |
| Examples | Bacteria, Cyanobacteria | Animal cells, Plant cells, Fungi, Protists |
4. Cellular Transport Mechanisms
Cells need to transport substances across their membranes. There are two primary types of transport:
4.1. Passive Transport
- Definition: Movement of molecules across the membrane without using energy.
- Types:
- Diffusion: Movement of molecules from high to low concentration.
- Facilitated Diffusion: Diffusion with the help of transport proteins.
- Osmosis: Movement of water molecules from an area of low solute concentration to high solute concentration.
4.2. Active Transport
- Definition: Movement of molecules against the concentration gradient, requiring energy (ATP).
- Example: Sodium-potassium pump, which moves Na+ out of the cell and K+ into the cell.
MCQs – Cell: The Unit of Life
1. Which of the following is NOT a feature of prokaryotic cells?
a) Nucleoid
b) Membrane-bound organelles
c) Ribosomes
d) Plasma membrane
Answer: b) Membrane-bound organelles
2. The function of mitochondria is to:
a) Synthesize proteins
b) Store genetic information
c) Generate energy (ATP)
d) Control cell division
Answer: c) Generate energy (ATP)
3. Which organelle is responsible for protein synthesis?
a) Nucleus
b) Ribosomes
c) Golgi apparatus
d) Endoplasmic reticulum
Answer: b) Ribosomes
4. The plasma membrane is primarily made of:
a) Proteins
b) Carbohydrates
c) Phospholipids
d) Nucleic acids
Answer: c) Phospholipids
5. The primary function of the Golgi apparatus is:
a) Protein synthesis
b) Packaging and modification of proteins
c) ATP production
d) Lipid synthesis
Answer: b) Packaging and modification of proteins
6. Which of the following is a difference between plant and animal cells?
a) Both have a cell wall
b) Both have chloroplasts
c) Animal cells have a large central vacuole
d) Plant cells have a cell wall and chloroplasts
Answer: d) Plant cells have a cell wall and chloroplasts
7. Osmosis is the movement of:
a) Water from low solute to high solute concentration
b) Solutes from high to low concentration
c) Water from high solute to low solute concentration
d) Solutes from low to high concentration
Answer: a) Water from low solute to high solute concentration
8. The genetic material of eukaryotic cells is located in the:
a) Cytoplasm
b) Nucleus