🔋 1. Mitochondria — Powerhouse of the Cell

Mitochondria are double membrane-bound organelles that act as the sites of aerobic respiration. They release stored energy from food molecules and convert it into ATP (Adenosine Triphosphate), the universal energy currency of the cell.

🧠 Why Mitochondria Are Called Powerhouses

  • ATP produced here powers all cellular activities like muscle contraction, active transport, synthesis, and cell division.

🔑 Memory Line:

Energy of food → ATP in mitochondria → life activities.

🧱 Structure of Mitochondria

Shape

  • Usually sausage-shaped or cylindrical.

Double Membrane System

1️⃣ Outer Membrane

  • Smooth and freely permeable to small molecules.

2️⃣ Inner Membrane

  • Folded inward to form cristae.
  • Cristae increase surface area for enzymes of respiration.

🔑 Key Point:

More cristae = more ATP production.

🧪 Internal Compartments

  • Inter-membrane space: Between outer and inner membranes.
  • Matrix: Inner dense fluid-filled space.

Matrix Contains:

  • Enzymes of Krebs’ cycle
  • Circular DNA
  • RNA molecules
  • 70S ribosomes

🔑 Key Point:

Mitochondria have their own DNA and ribosomes.

🔁 Reproduction of Mitochondria

  • Multiply by binary fission.

🔑 Concept Line:

New mitochondria arise from pre-existing mitochondria.

🧠 Semi-Autonomous Nature

Because mitochondria:

  • Have their own DNA
  • Have 70S ribosomes
  • Can synthesize some proteins independently

But still depend on the nucleus for most proteins.

🔑 Exam Line:

Mitochondria are semi-autonomous, not fully independent.

🌿 2. Plastids — The Plant Cell Speciality

Plastids are double membrane-bound organelles found in plant cells and euglenoids. They are responsible for photosynthesis, pigmentation, and storage of food.

🧬 Types of Plastids

1️⃣ Chloroplasts 🌱

  • Green plastids containing chlorophyll.
  • Site of photosynthesis.

🔑 Memory Line:

Chloroplast = Kitchen of the cell.

2️⃣ Chromoplasts 🎨

  • Contain carotenoid pigments like carotene and xanthophyll.
  • Provide yellow, orange, red colours to fruits and flowers.

🔑 Function:

Attract pollinators and seed dispersers.

3️⃣ Leucoplasts 📦 (Storage Plastids)

Colourless plastids mainly involved in storage.

Type Stored Material Example
Amyloplast Starch Potato
Elaioplast Oils & fats Seeds
Aleuroplast Proteins Endosperm

🔑 One-Line:

Leucoplasts = storage specialists.

🧱 Structure of Chloroplast

Shape

  • Lens-shaped / oval / discoid.

Double Membrane

  • Outer membrane
  • Inner membrane (less permeable)

Internal Parts

🧪 Stroma

  • Fluid inside inner membrane.
  • Contains enzymes, circular DNA, 70S ribosomes.

🧩 Thylakoids

  • Flattened membranous sacs.
  • Contain chlorophyll pigments.

🪙 Grana

  • Stacks of thylakoids like coins.

🔗 Stroma Lamellae

  • Connect grana to each other.

🔹 Lumen

  • Space inside thylakoid.

🔑 Key Diagram Line:

Photosynthesis light reaction → thylakoid | Dark reaction → stroma.

🧠 Semi-Autonomous Nature of Chloroplast

  • Possess circular DNA
  • Contain 70S ribosomes
  • Can synthesize some proteins

🔑 Exam Tip:

Both mitochondria and chloroplasts support endosymbiotic theory.

🧠 One-Page Memory Capsule

  • Mitochondria → ATP, respiration
  • Plastids → photosynthesis, storage, colour
  • Chloroplast → grana + stroma
  • Leucoplast → storage
  • Ribosomes → protein synthesis
  • Mitochondria & chloroplast → semi-autonomous

💡 Questions & Answers

Q1. Why are mitochondria called powerhouses of the cell?

A1. Mitochondria are called powerhouses because they produce ATP, which provides energy for all cellular activities.

🔑 Key Points:

  • Site of aerobic respiration
  • ATP synthesis
  • Energy currency of cell

Q2. What is the role of cristae in mitochondria?

A2. Cristae increase the surface area of the inner membrane, allowing more enzymes to function and produce more ATP.

🔑 Key Points:

  • Inner membrane folds
  • Increase surface area

Q3. Why are mitochondria and chloroplasts called semi-autonomous?

A3. They are semi-autonomous because they have their own DNA and ribosomes and can synthesize some proteins independently.

🔑 Key Points:

  • Circular DNA
  • 70S ribosomes
  • Partial independence

Q4. What are plastids? Name their types.

A4. Plastids are plant cell organelles involved in photosynthesis, storage, and pigmentation. They are of three types: chloroplasts, chromoplasts, and leucoplasts.

🔑 Key Points:

  • Found in plants
  • Three main types

Q5. Differentiate between 80S and 70S ribosomes.

A5. 80S ribosomes are found in eukaryotic cytoplasm, while 70S ribosomes are found in prokaryotes and inside mitochondria and chloroplasts.

🔑 Key Points:

  • 80S → eukaryotic cytoplasm
  • 70S → prokaryotes & organelles

Q6. What is the function of chlorophyll?**

A6. Chlorophyll absorbs light energy and converts it into chemical energy during photosynthesis.

🔑 Key Points:

  • Light absorption
  • Photosynthesis