Chapter 5 – The Fundamental Unit of Life

🔬 Introduction to Cells
●​ Cell: The basic structural and functional unit of life.
●​ All organisms are made up of cells – unicellular (e.g. Amoeba, Bacteria) or multicellular
(e.g. humans, plants).
●​ Every cell originates from a pre-existing cell – Rudolf Virchow (1855).
●​ Cell Theory (Schleiden & Schwann): All living organisms are composed of cells.

🧫 Types of Organisms by Cell Count

●​ Unicellular: One cell does all functions (e.g. Amoeba, Paramecium).
●​ Multicellular: Multiple specialized cells with division of functionality

🧩 Types of Cells
1. Prokaryotic Cells

●​ No true nucleus; DNA lies in nucleoid.

●​ No membrane-bound organelles.
●​ Examples: Bacteria, Blue-green algae.
2. Eukaryotic Cells

●​ Well-defined nucleus with nuclear membrane.

●​ Membrane-bound organelles present.
●​ Examples: Plant, Animal, Fungi cells.

🔹 What is Plasma Membrane?

Plasma Membrane (also called the cell membrane) is the outermost layer of the cell in
animal cells and lies just inside the cell wall in plant cells.

🔑 Key Features:
●​ Made up of: Lipids and proteins (phospholipid bilayer).
●​ Structure: Flexible and dynamic.
●​ Function: Separates the internal contents of the cell from the external environment.

🔹 Why is it Called a Selectively Permeable Membrane?

The plasma membrane is called selectively permeable (or semi-permeable) because:
 ●​ It allows only certain substances (like water, oxygen, carbon dioxide) to enter or exit
the cell.
●​ It restricts the movement of other substances (like large molecules or ions) unless
permitted by the cell.

🔁 This selective control helps maintain the internal balance of the cell.
🔹 What is Diffusion?
Diffusion is the movement of molecules from a region of higher concentration to a region
of lower concentration, until equilibrium is reached.

✅ Characteristics:
●​ No energy is required (passive transport).
●​ Occurs in gases and liquids.
●​ Helps substances like oxygen and carbon dioxide enter or exit the cell across the
plasma membrane.

🧪 Example: When you spray perfume in a corner of a room, it gradually spreads throughout
the room due to diffusion.

🔹 What is Osmosis?
Osmosis is the movement of water molecules through a semi-permeable membrane from a
region of lower solute concentration (more water) to higher solute concentration (less
water).

✅ Characteristics:
●​ Special case of diffusion involving only water.
●​ Requires a semi-permeable membrane.
●​ It is a passive process (no energy needed).
Isotonic, hypotonic solutions, hypertonic solutions
• Isotonic solutions are those which have the same solute and pH concentration as the
surrounding
body fluid or the cytoplasm.
• Hypotonic solutions contain lesser amount of solute concentration compared to the
surrounding
fluid and can force the cell to rupture due to excess input of water into the cell.
• Hypertonic solutions contain a higher concentration of solute compared to the
surrounding fluid
and thus push water out of the cell, shrinking it.

🔹 What is Endocytosis?
Endocytosis is the process by which a cell takes in materials from its external
environment by engulfing them with its plasma membrane.
🔬 Explanation:
●​ During endocytosis, the plasma membrane folds inward to form a
pocket around the target substance (like nutrients, fluids, or other cells).
●​ This pocket then pinches off inside the cell, forming a vesicle that
contains the ingested material.
●​ It allows the cell to absorb large particles (that cannot pass through the
membrane by diffusion or osmosis).

📌 What is the Cell Wall?

The cell wall is a rigid outer covering found in plant cells, fungi, and
bacteria, but not in animal cells.

🧰 Functions of Cell Wall:

●​ Provides structural support and rigidity.
●​ Protects the cell from mechanical damage and pathogens.
●​ Maintains the shape of the cell.
●​ Helps the cell to withstand osmotic pressure (prevents bursting in
hypotonic solutions).

Note -Unlike the plasma membrane, the cell wall is freely permeable—it
allows most substances to pass through it.

What is Cellulose?

Cellulose is a complex carbohydrate made up of glucose molecules. It is the main

component of the plant cell wall.

What is Plasmolysis?

Plasmolysis is the shrinking of the cell membrane away from the cell wall when a
plant cell is placed in a hypertonic solution (a solution with higher solute
concentration).

🧪 Process:
1.​ When the plant cell is placed in a hypertonic solution, water moves out of the
cell via osmosis.
2.​ The cytoplasm shrinks and pulls away from the cell wall.
IMPORTANT QUESTIONS

1. Why do plant cells like Rheea leaf or Tradescantia shrink when placed in
a hypertonic solution?

✅ Answer:
When plant cells like Rheea leaf are placed in a hypertonic solution (solution with higher solute
concentration than the cell sap), the following happens:

●​ Water moves out of the cell by osmosis (from high water potential inside the cell to low water
potential outside).
●​ The cytoplasm shrinks, and the plasma membrane pulls away from the cell wall.
●​ This process is known as plasmolysis.

2. What happens to Red Blood Cells (RBCs) when placed in different solutions?

3. What happens when dry raisins are placed in water and in sugar
solution?

✅ Answer:
a. When dry raisins are placed in plain water (hypotonic solution):

●​ Water enters the raisins by osmosis.

●​ Raisins swell up because water moves from outside (low solute concentration) into the raisin (higher solute concentration).

b. When swollen raisins are placed in a concentrated sugar or salt solution (hypertonic):

●​ Water comes out of the raisins by osmosis.

●​ Raisins shrink as water moves from inside (more water) to outside (more solute).
1. Nucleus: The Control Center of the Cell
The nucleus is a double membrane-bound organelle found in most eukaryotic cells.

✅ Functions of the Nucleus:

●​ Controls all cellular activities (metabolism, growth, etc.).
●​ Regulates cell division and reproduction.
●​ Carries genetic material in the form of DNA.
●​ Transmits hereditary information from parent to offspring.

🧬 Components of the Nucleus:

●​ Nuclear Membrane: Double-layered; has pores for material exchange.
●​ Nucleoplasm: Jelly-like fluid inside the nucleus.
●​ Nucleolus: Helps in ribosome synthesis.
●​ Chromatin: Thread-like DNA + proteins (forms chromosomes during division).

●​

🧬 2. Chromosomes, DNA, and Genes

📌 Chromosomes:
●​ Long, thread-like structures made of DNA and proteins.
 ●​ Visible during cell division.
●​ Humans have 46 chromosomes (23 pairs) in each cell.

📌 DNA (Deoxyribonucleic Acid):

●​ Genetic material present in all living cells.
●​ Stores and transfers hereditary information.
●​ Has a double-helix structure.

📌 Gene:
●​ A segment of DNA that controls a specific trait.
●​ Acts as a code for making proteins.
●​ Each gene has information for making a particular protein or enzyme.

📌 Inheritance:
●​ The process by which genetic information is passed from one
generation to the next.
●​ Controlled by genes on chromosomes.

. Role of Nucleus in Cellular Reproduction

●​ Reproduction involves cell division (mitosis or meiosis).
●​ The nucleus duplicates its DNA before cell division.
●​ During mitosis, chromosomes are equally distributed to daughter cells.
●​ Ensures each daughter cell has the same genetic information.
●​ In meiosis, half the chromosomes are passed to gametes for sexual reproduction.

Define chromosome.

Answer:​
Chromosomes are thread-like structures in the nucleus made of DNA and proteins that carry
genetic information.

Q3: What is a gene?

Answer:​
A gene is a specific segment of DNA that contains instructions for making a particular protein or
trait.

Q4: What is a nucleotide?

Answer:​
A nucleotide is the basic structural unit of DNA, consisting of a sugar, a phosphate group, and a
nitrogenous base.

Q5: How does the nucleus help in inheritance?

Answer:​
The nucleus contains DNA with genes that are passed from parents to offspring during
reproduction, ensuring inheritance of traits.

Q6: How does the nucleus play a central role in reproduction?

Answer:​
Before cell division, the nucleus replicates its DNA. During mitosis or meiosis, chromosomes are
distributed to daughter cells, ensuring the transmission of genetic information.

Q7: What is the difference between chromatin and chromosomes?

Answer:

●​ Chromatin: Uncoiled, thread-like DNA present when the cell is not dividing.
●​ Chromosomes: Condensed, visible form of chromatin seen during cell division.

Cytoplasm: The fluid content inside the plasma membrane is referred to as

cytoplasm. It's a vicious jelly-like substance that covers the entire cell save the
nucleus. It also contains a variety of specialised cell organelles, each of which
serves a specific purpose for the cell.
●​ Endoplasmic Reticulum
The endoplasmic reticulum is a membrane-bound cell organelle that plays an integral role
in the
interpretation of the genetic information present in the nucleus.
Rough ER
Rough ER are the ones that have ribosomes on them. The ribosome is made up of nucleic
acids and
proteins. They are the site of protein synthesis. The Rough ER is also involved in the
modification and
folding of proteins.
Smooth ER
Smooth ER do not have ribosomes and thus are not involved in protein synthesis. They
are, however,
involved in lipid metabolism and detoxifying of poisonous molecules.

Golgi Apparatus (Golgi Complex / Golgi Body)

🔬 Structure:
●​ Stack of membrane-bound flattened sacs called cisternae.
●​ Found near the nucleus and associated with the ER.

✅ Functions:
●​ Modifies, stores, and packages proteins and lipids received from the ER.
●​ Forms vesicles for transporting materials within and outside the cell.
 ●​ Helps in the formation of lysosomes.
●​ Produces certain hormones and enzymes.
●​ Helps in the secretion of materials like mucus and cell wall materials in plants.

🧠 Key Point:
Acts like the post office of the cell – it receives, processes, packs, and dispatches.

Lysosomes – Suicidal Bags of the Cell

🔬 Structure:
●​ Small, spherical, single-membrane-bound organelles.
●​ Contain powerful digestive enzymes (hydrolytic enzymes).
●​ Formed by the Golgi Apparatus.

✅ Functions:
●​ Break down waste material, damaged organelles, or foreign substances like
bacteria.
●​ Play a key role in cell defense and cell cleaning.
●​ Help in intracellular digestion.
●​ During cell damage, lysosomes may burst, causing the cell to self-destruct.

🧠 Key Point:
They are called "suicidal bags" because their enzymes can digest the cell itself if
released.

Mitochondria – The Powerhouse of the Cell

🔬 Structure:
●​ Double membrane-bound organelle.
●​ Inner membrane is highly folded into structures called cristae (increase surface
area).
●​ Contain mtDNA (mitochondrial DNA) and ribosomes.

✅ Functions:
 ●​ Site of aerobic respiration – produces ATP (adenosine triphosphate), the
energy currency of the cell.
●​ Helps in energy production for cellular activities.
●​ Can replicate independently due to their own DNA.

🧠 Key Point:
Mitochondria are semi-autonomous (can make some of their own proteins) and are more
active in muscle cells, which require more energy.

Vacuoles – Storage Sacs

🔬 Structure:
●​ Membrane-bound sac filled with fluid.
●​ Surrounded by a membrane called the tonoplast.

✅ Functions:
●​ Storage of water, food, nutrients, and waste materials.
●​ In plant cells, large central vacuole provides turgor pressure to maintain cell
rigidity.
 ●​ In animals, smaller vacuoles are involved in transport, storage, or waste isolation.
●​ In some unicellular organisms (e.g. Amoeba), vacuoles help in expelling excess
water and digestion.

🧠 Key Point:
Vacuoles are large and central in plant cells but small or absent in animal cells.

Why Are Lysosomes Called Suicidal Bags?

✅ Reason:
Lysosomes contain powerful digestive enzymes (hydrolytic enzymes) that can break
down:

●​ Worn-out organelles
●​ Harmful substances
●​ Invading bacteria or viruses
●​ Even the entire cell when it's damaged

🔥 Suicidal Action:
●​ If the cell is damaged or under stress, lysosomes burst open and release
enzymes.
●​ These enzymes digest the cell itself, causing cell death.
●​ Therefore, lysosomes are called "suicidal bags" of the cell.
What is Stroma in Plastids?
✅ Stroma:
●​ Stroma is the dense, colorless fluid inside chloroplasts (a type of plastid).

🧬 Functions of Stroma:
●​ Site of the dark reaction of photosynthesis
●​ Contains enzymes, DNA, ribosomes, and starch grains.
●​ Helps in sugar synthesis using the products of light reaction (ATP and NADPH).

What is ATP in Mitochondria?

✅ ATP:
●​ Full form: Adenosine Triphosphate
●​ Known as the "energy currency of the cell".

🏭 Where is it made?
●​ ATP is produced in mitochondria during aerobic respiration through the Krebs
Cycle.

🔄 Role:
●​ Provides energy for various cellular processes like muscle contraction, transport,
and biosynthesis.

Where Are Proteins Synthesized in the Cell?

✅ Site of Protein Synthesis:
●​ Ribosomes are the site of protein synthesis.

🏭 Ribosomes can be:

 ●​ Free in cytoplasm → Make proteins for use inside the cell.
●​ Attached to Rough Endoplasmic Reticulum (RER) → Make proteins for
export or for use in membranes and organelles.

What is Cell Division?

Cell division is the process by which a parent cell divides to form new daughter cells.​
It is essential for:

●​ Growth of organisms
●​ Repair of damaged tissues
●​ Reproduction (asexual and sexual)