The Bio Brief

Structural Organisation of a Cell

When we observe a cell under a microscope, we usually see three main parts:

 Basic Parts of a Cell

1. Plasma membrane (Cell membrane)
2. Cytoplasm
3. Nucleus

All activities of the cell and interaction with the environment happen because of these parts.

 Flow Chart: Basic Structure of a Cell

Cell

 ↓

Plasma Membrane

Cytoplasm

Nucleus

 Plasma Membrane (Cell Membrane)

 What is Plasma Membrane?

• It is the outermost covering of the cell.
• It separates the cell contents from the external environment.
• It is thin, flexible, and living.

 Functions of Plasma Membrane

• Controls entry and exit of substances
• Protects the cell
• Helps the cell interact with surroundings

 Because it allows some substances to pass and blocks others, it is called a

Selectively Permeable Membrane

Movement of Substances Across Cell Membrane

1. Diffusion

• Movement of substances from high concentration → low concentration
• Does not require energy

Examples:

• Oxygen (O₂) enters the cell
• Carbon dioxide (CO₂) moves out of the cell

 Flow Chart: Diffusion

High Concentration

        ↓

Low Concentration

        ↓

Diffusion (No Energy)

2. Osmosis

• Movement of water molecules through a selectively permeable membrane
• Water moves toward higher solute concentration

 Osmosis is a special type of diffusion

 Definition:

Osmosis is the diffusion of water across a selectively permeable membrane from a region of higher water concentration to lower water concentration.

 Effect of Different Solutions on Cells (Osmosis)

 1. Hypotonic Solution

• Outside solution is dilute
• More water enters the cell
• Cell swells

 2. Isotonic Solution

• Same concentration inside and outside
• No net movement of water
• Cell remains same size

 3. Hypertonic Solution

• Outside solution is concentrated
• Water leaves the cell
• Cell shrinks

Table: Effect of Solutions

Solution Type	Water Movement	Effect on Cell
Hypotonic	Enters cell	Cell swells
Isotonic	Equal in & out	No change
Hypertonic	Leaves cell	Cell shrinks

 Flow Chart: Osmosis Effect

Type of Solution

 ↓

Water Movement

 ↓

Change in Cell Size

Try them out to learn:

 Osmosis Experiment (Egg)

 Egg in Pure Water

• Egg swells
• Water enters by osmosis

 Egg in Salt Solution

• Egg shrinks
• Water moves out

 Raisins / Apricot Experiment

• In plain water → swell
• In salt/sugar solution → shrink

 These experiments prove osmosis.

Importance of Osmosis

• Helps plants absorb water through roots
• Helps maintain cell size
• Important for freshwater organisms

Transport Using Energy

• Some substances need energy to enter or leave the cell
• This is called active transport

 Structure of Plasma Membrane

• Made of lipids and proteins
• Flexible in nature
• Seen clearly only with an electron microscope

Endocytosis

• Cell membrane can engulf food
• Process is called endocytosis
• Seen in Amoeba

 Flow Chart: Endocytosis

Food Outside Cell

        ↓

Cell Membrane Engulfs Food

        ↓

Food Inside Cell

Electron Microscope (Short Note)

• An electron microscope uses a beam of electrons instead of light.
• It has very high magnification and high resolving power.
• It helps us see:
  • Cell membrane structure
  • Cell organelles clearly
• Invented in the 1940s.

 Why important? It helped scientists understand the detailed structure of cells.

 Cell Wall (Only in Plant Cells)

 What is Cell Wall?

• A rigid outer layer present outside the plasma membrane
• Found in plants, fungi, and bacteria
• Absent in animal cells

 Composition of Cell Wall

• Mainly made of cellulose
• Cellulose provides strength and support to plants

Functions of Cell Wall

• Gives shape to the cell
• Protects the cell
• Prevents cell from bursting in hypotonic solution

Plasmolysis

• When a plant cell loses water by osmosis
• The cell content shrinks away from the cell wall
• This process is called plasmolysis

Flow Chart: Plasmolysis

Plant Cell in Concentrated Solution

           ↓

Water Moves Out (Osmosis)

           ↓

Cell Shrinks

           ↓

Plasmolysis

Importance of Cell Wall

• Allows plant cells to survive in hypotonic solutions
• Prevents bursting by balancing pressure

Nucleus

 What is Nucleus?

• A dark, round or oval structure
• Usually present near the centre of the cell
• Called the control centre of the cell

Structure of Nucleus

• Surrounded by double-layered nuclear membrane
• Nuclear membrane has pores
• Pores allow movement of materials between nucleus and cytoplasm

Chromosomes & DNA

• Nucleus contains chromosomes
• Chromosomes are made of:
  • DNA
  • Proteins

DNA & Genes

• DNA stores hereditary information
• Functional parts of DNA are called genes
• Genes control traits passed from parents to offspring

Flow Chart: Genetic Material

Nucleus

  ↓

Chromosomes

  ↓

DNA

  ↓

Genes

Chromatin Material

• In non-dividing cells, DNA is present as chromatin
• Appears as thread-like structures
• During cell division → chromatin forms chromosomes

 Functions of Nucleus

• Controls cell activities
• Helps in cell division
• Directs growth and development

 Prokaryotes vs Eukaryotes

Feature	Prokaryotes	Eukaryotes
Nuclear membrane	Absent	Present
Nucleus	Nucleoid	True nucleus
Cell organelles	Absent	Present
Examples	Bacteria	Plants, animals

Special Case: Prokaryotes

• DNA present in nucleoid
• Chlorophyll (if present) is in membranous vesicles
• No plastids

Cytoplasm

 What is Cytoplasm?

• Jelly-like fluid inside the cell
• Found between:
  • Cell membrane
  • Nucleus

Features of Cytoplasm

• Lightly stained region
• Contains cell organelles
• Site of many chemical reactions

Cell Organelles in Cytoplasm

• Mitochondria
• Ribosomes
• Endoplasmic reticulum
• Golgi apparatus
• Plastids (in plants)

 Flow Chart: Cytoplasm Role

Cytoplasm

   ↓

Contains Organelles

   ↓

Organelles Perform Functions

   ↓

Cell Survives & Works

 Cytoplasm in Prokaryotes vs Eukaryotes

• Prokaryotes: no membrane-bound organelles
• Eukaryotes: membrane-bound organelles present

Viruses – Special Case

• Viruses lack membranes
• Do not show life outside host
• Become active only inside a living cell.