Cell Biology – NDA Biology Notes

Introduction

What is a Cell?

A cell is the smallest unit of life. Every living thing is made of cells. A plant, an animal, a human, a bacterium — all are made of cells. Some living things have only one cell. Humans have trillions of cells.

Robert Hooke discovered the cell in 1665. He used a microscope to look at a thin slice of cork. He saw tiny box-like spaces. He called them “cells.”

Cell Theory has three main ideas:

• All living things are made of cells.
• The cell is the basic unit of life.
• All new cells come from existing cells. [NDA tested via CDS]

The living content of a cell is called protoplasm. It is made of cytoplasm, nucleoplasm, and other organelles. [NDA tested via CDS]

Water is the most abundant substance in the cell. It is the most abundant inorganic constituent of protoplasm. [NDA 2014-I]

Understand the Topic

Two Types of Cells

All cells in the world belong to one of two types.

1. Prokaryotic Cell

“Prokaryote” means “before nucleus.” These cells have no true nucleus. Their DNA floats freely inside the cell. It is not covered by a membrane.

The region where DNA floats is called the nucleoid. [NDA 2020-I] [NDA tested via CDS]

Prokaryotic cells are simple and small. They have no membrane-bound organelles at all. The only structure they have that eukaryotes also have is the ribosome. [NDA tested via CDS]

Examples: Bacteria, Blue-green algae (Cyanobacteria).

2. Eukaryotic Cell

“Eukaryote” means “true nucleus.” These cells have a true nucleus. The nucleus is covered by a membrane. DNA is safely stored inside the nucleus.

Eukaryotic cells are larger and more complex. They have many different organelles, each with a specific job.

Examples: Plant cells, Animal cells, Fungal cells.

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No true nucleus	True nucleus with membrane
DNA location	Nucleoid — floats freely	Inside nucleus
Membrane-bound organelles	Absent	Present
Ribosomes	Present	Present
Examples	Bacteria, Cyanobacteria	Plants, Animals, Fungi

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Plant Cell vs Animal Cell

Both plant and animal cells are eukaryotic. But they have some important differences. This is one of the most frequently tested topics in this chapter.

What is present ONLY in plant cells:

• Cell wall — made of cellulose. It is the hard outer layer outside the cell membrane.
• Chloroplasts — where photosynthesis happens.
• Large central vacuole — stores water.
• Plastids — include chloroplasts and other storage organelles.
• Starch — the form in which plants store energy.

What is present ONLY in animal cells:

• Centrioles — help during cell division.
• Glycogen — the form in which animals store energy.
• Lysosomes are more common in animal cells.

What is present in BOTH plant and animal cells:

• Cell membrane
• Nucleus
• Cytoplasm
• Mitochondria
• Ribosomes
• Endoplasmic reticulum (ER)
• Golgi bodies

Feature	Plant Cell	Animal Cell
Cell wall	Present (cellulose)	Absent
Cell membrane	Present	Present
Chloroplast	Present	Absent
Large vacuole	Present	Small or absent
Centriole	Absent	Present
Plastids	Present	Absent
Lysosomes	Rare	Common and abundant
Starch	Energy storage	Absent
Glycogen	Absent	Energy storage
Shape	Fixed, rectangular	Irregular, flexible

[NDA 2012-III] [NDA tested via CDS]

Cell wall is found in plants, fungi, and bacteria. It is NOT found in animal cells. [NDA tested via CDS]

Starch is present in plant cells. Glycogen is present in animal cells. Both are carbohydrates, but stored in different organisms. [NDA tested via CDS]

Cell Membrane

Every cell has a cell membrane. It is also called the plasma membrane. It is the outer boundary of the cell.

The cell membrane controls what enters and leaves the cell. It is selectively permeable — it allows some things through and blocks others.

The cell membrane is made of:

• Phospholipid molecules (a double layer — called a bilayer)
• Protein molecules embedded in the layer
• Cholesterol molecules [NDA 2024-I]

The cell membrane is flexible. It can stretch and bend. This allows the cell to engulf materials from outside. [NDA tested via CDS]

The cell membrane is NOT made of cellulose. Cellulose is the material of the cell wall. [NDA tested via CDS]

Cell Organelles — What They Do

Organelles are small structures inside the cell. Each organelle has one specific job. The table below gives you everything you need for the exam.

Organelle	Main Function	Key Exam Facts
Nucleus	Controls all cell activities. Stores DNA.	Contains chromosomes = DNA + histone proteins [NDA tested via CDS]
Mitochondria	Produces energy (ATP) for the cell.	“Powerhouse of the cell.” ATP made at inner membrane [NDA 2021-I]. Has own DNA and ribosomes [NDA 2018-II]. Can make own proteins [NDA 2019-III]
Chloroplast	Carries out photosynthesis.	Only in plant cells. Contains chlorophyll. Has own DNA and ribosomes [NDA 2018-II] [NDA tested via CDS]
Ribosome	Makes proteins.	Found in ALL cells — prokaryotic and eukaryotic. [NDA tested via CDS]
Rough ER	Transports proteins made by ribosomes.	Has ribosomes on its surface — that is why it looks “rough” under microscope. [NDA tested via CDS]
Smooth ER	Makes lipids. Detoxifies harmful substances.	No ribosomes on surface. Does NOT make proteins. [NDA 2018-III] [NDA 2022-I]
Golgi Body	Packages proteins and sends them to the right place.	“Post office of the cell.” Forms lysosomes. [NDA tested via CDS]
Lysosome	Digests and destroys foreign material and old organelles.	“Suicide bag.” Rich in hydrolytic enzymes. [NDA 2019-II] Most lysosomes found in WBCs. [NDA tested via CDS] Formed from Golgi bodies. [NDA tested via CDS]
Vacuole	Stores water, food, and waste.	Large in plant cells. In unicellular organisms, expels excess water. [NDA 2020-I]
Centriole	Helps in cell division.	Found only in animal cells.
Cytoskeleton	Maintains cell shape.	Framework of protein fibres inside cytoplasm. [NDA tested via CDS]

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Three Organelles That Contain DNA

This is one of the most repeated facts in this chapter. Only three organelles contain DNA:

1. Nucleus
2. Mitochondria
3. Chloroplast

Golgi bodies, lysosomes, endoplasmic reticulum — these do NOT contain DNA. [NDA 2018-II] [NDA tested via CDS]

Mitochondria and chloroplast also have their own ribosomes. They can make their own proteins. This is why they are sometimes called semi-autonomous organelles. [NDA tested via CDS]

Osmosis — How Water Moves In and Out of Cells

Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration. This movement happens through a semi-permeable membrane. [NDA tested via CDS]

The cell membrane is semi-permeable.

Three situations:

• Hypotonic solution — water concentration outside is higher than inside. Water enters the cell. Cell swells. If too much water enters, the cell may burst (lysis). Example: RBCs in plain water burst. [NDA tested via CDS]
• Hypertonic solution — water concentration outside is lower than inside. Water leaves the cell. Cell shrinks.
• Isotonic solution — water concentration is equal inside and outside. No net movement. Cell size does not change. Example: RBCs in normal saline stay the same size. [NDA tested via CDS]

It is very important for the body to maintain a constant water potential (concentration) inside cells. If this is not maintained, cells shrink or burst. [NDA 2013-I]

When excess fertiliser is applied to a plant without water, water is drawn out of plant cells by osmosis. The cell membrane pulls away from the cell wall. This is called plasmolysis. The plant dies. [NDA tested via CDS]

DNA and RNA — Structure and Function

What is DNA?

DNA stands for Deoxyribonucleic Acid. It is the genetic material of all living organisms. It carries the instructions for making proteins and for all cell activities. [NDA tested via CDS]

Structure of DNA:

• DNA is a double helix — like a twisted ladder.
• It has two strands running in opposite directions (anti-parallel). [NDA tested via CDS]
• The two strands are held together by hydrogen bonds. [NDA tested via CDS]
• Each strand is made of nucleotides.
• One DNA nucleotide = nitrogenous base + deoxyribose sugar + phosphate group. [NDA tested via CDS]

Base pairing in DNA:

• Adenine (A) pairs with Thymine (T)
• Guanine (G) pairs with Cytosine (C)
  [NDA 2025-I]

Where is DNA found?

• Nucleus
• Mitochondria
• Chloroplasts [NDA 2018-II]

Important: Not all segments of DNA make proteins. Some parts of DNA are non-coding. [NDA tested via CDS]

What is RNA?

RNA stands for Ribonucleic Acid. RNA carries the message from DNA to the ribosome to make proteins.

Feature	DNA	RNA
Strands	Double stranded	Single stranded
Sugar	Deoxyribose	Ribose
Base T or U	Has Thymine (T)	Has Uracil (U) instead of T
Location	Nucleus, mitochondria, chloroplast	Nucleus, cytoplasm, ribosomes, mitochondria, chloroplast [NDA tested via CDS]
Function	Stores genetic information	Helps make proteins

The three important processes:

• Transcription — copying DNA into RNA. [NDA tested via CDS]
• Translation — using RNA to build a protein.
• Replication — copying DNA to make more DNA.
• Mutation — a change in the DNA sequence.

Chromosomes and Histones

Chromosomes are made of DNA and proteins — specifically histone proteins. [NDA tested via CDS]

Histones are found in the nucleus. They are associated with DNA. They help pack the long DNA strand into a compact structure. [NDA tested via CDS]

Before cell division, the loose chromatin (DNA + histones) condenses into rod-shaped chromosomes. [NDA 2024-I]

Cell Division

Mitosis — produces two identical cells. Used for growth and repair of body tissues. Growth and repair involve mitotic cell division only. [NDA 2014-II]

Meiosis — produces four cells with half the number of chromosomes. Used to make sex cells (sperm and egg). [NDA tested via CDS]

Prokaryotes and eukaryotes do NOT use the same process for cell division. [NDA 2024-I]

Syngamy — when sperm and egg fuse together during fertilisation. This forms a diploid zygote (full set of chromosomes). [NDA tested via CDS]

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Sex Determination

The sex of a baby is determined by the chromosome from the father. [NDA 2013-I]

• Mother always gives an X chromosome.
• Father gives either X (daughter) or Y (son).
• XX = female, XY = male.

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Genetics Basics

Alleles — different versions of the same gene are called alleles. Example: T (tall) and t (short) are alleles of the height gene. [NDA 2021-II]

Mendel’s cross — TT × tt:

• Parents: TT (pure tall) × tt (short)
• F1 generation: all Tt (hybrid tall — look tall, carry short gene)
• F2 generation: 1 TT + 2 Tt + 1 tt
• Phenotype ratio: 3 tall : 1 short
• Pure tall (TT) : short (tt) = 1 : 1 [NDA tested via CDS]

Variation in genetic material of offspring comes from:

• Sexual reproduction
• Mutations
• Epigenetic changes
  It does NOT come from asexual reproduction alone. [NDA tested via CDS]

DNA fingerprinting is done using the Southern Blotting technique. Not ELISA, not Northern Blotting, not RIA. [NDA tested via CDS]

Microbodies and Other Cell Facts

Microbodies are small structures found in both plant and animal cells. They contain enzymes. [NDA tested via CDS]

Germplasm includes genetic resources, seeds or tissues for breeding, and egg and sperm repositories. [NDA tested via CDS]

Protoplasm = cytoplasm + nucleoplasm + all organelles together. [NDA tested via CDS]