Cell Structure and Organelles

Why this topic matters for NDA

The NDA General Ability Test (GAT) Paper allots a fixed share of marks to General Science, and within that, Biology questions are among the most predictable on the whole paper. Cell structure appears almost every year — often as a one-line question like "Which organelle is the powerhouse of the cell?" or "Which of the following is absent in a plant cell?"

These are pure recall marks. You do not need calculation, reasoning, or analysis — just a tidy mental picture of the cell and a short list of facts. Compare this with Mathematics, where one mistake can cost you the whole sum. Here, if you know the fact, the mark is guaranteed.

Because cutoffs for NDA are competitive, every such guaranteed mark matters. Many candidates skip Biology thinking it is "random", and then lose 3–4 sure marks to guesswork. Spend an hour on this single chapter and you protect those marks for good. The chapter also forms the foundation for later topics such as tissues, reproduction, and human physiology, so understanding cells pays off across the whole syllabus.

What exactly is a cell?

A cell is the basic structural and functional unit of every living organism. "Structural" means it is the building block that the body is made of; "functional" means it is the smallest unit that can actually carry out all the activities of life on its own — nutrition, respiration, growth, response to stimuli, and reproduction. Just as bricks build a house, cells build a body.

The word "cell" comes from the Latin cellula, meaning "a little room". The British scientist Robert Hooke coined it in 1665 while looking at a thin slice of cork under his self-made microscope; the box-like empty compartments reminded him of the small rooms (cells) in a monastery. Importantly, what Hooke saw were dead cell walls of cork — not living cells.

Organisms made of a single cell are unicellular (e.g. Amoeba, Paramoecium, bacteria), where that one cell performs every life function by itself. Organisms made of many cells are multicellular (e.g. humans, mango tree), where groups of cells divide the work — this is called division of labour. Similar cells that work together form a tissue, tissues form organs, and organs form organ systems. This step-by-step organisation begins, in every case, with the humble cell.

The cell theory

The cell theory was proposed by two German scientists — Matthias Schleiden (a botanist, 1838) and Theodor Schwann (a zoologist, 1839). It was later expanded by Rudolf Virchow (1855).

So no cell appears from nothing — every new cell is born when an existing cell divides into two. This single idea ties together growth (more cells), healing of wounds (replacing damaged cells), and reproduction (a new organism starts from a single fertilised cell). One exception NDA may test: the cell theory in its original form did not apply to viruses, because viruses are not made of cells and cannot reproduce on their own outside a host. That is why viruses are often called "on the borderline between living and non-living".

Shape and size of cells

Cells come in many shapes — round, spindle-shaped, branched, or irregular — depending on their job. A nerve cell is long and branched to carry signals, while a red blood cell is disc-shaped to flow easily.

Size is unrelated to organism size — an elephant's cells are not bigger than a mouse's; the elephant simply has far more cells. Note also that the unit micrometre (µm), equal to one-millionth of a metre, is used to measure cells, because most are too small to see with the naked eye. The egg of a bird is a single cell large enough to see, which is why it is a popular exam example of "largest cell".

Prokaryotic vs eukaryotic cells

This is the most-tested distinction in the chapter. The difference is all about the nucleus.

Prokaryotic cells

Have no true (membrane-bound) nucleus and no membrane-bound organelles. Their genetic material lies freely in the cytoplasm in a region called the nucleoid. Examples: bacteria and blue-green algae (cyanobacteria).

Eukaryotic cells

Have a true nucleus enclosed by a nuclear membrane and contain membrane-bound organelles like mitochondria and the Golgi body. Examples: all plants, animals, fungi, and protists.

Both cell types share three things: a plasma membrane, cytoplasm, and ribosomes (the site of protein synthesis). Ribosomes are the only organelle found in prokaryotes. Prokaryotic cells are also generally smaller (1–10 µm) and simpler, while eukaryotic cells are larger (10–100 µm) and more complex.

A quick way to remember: pro means "before" and karyon means "nucleus" — so prokaryote means "before the nucleus existed", i.e. no proper nucleus. Eu means "true", so eukaryote means "true nucleus". If a cell in a question has mitochondria, a Golgi body, or a nuclear membrane, it must be a eukaryote.

Plant cell vs animal cell

Both plant and animal cells are eukaryotic and share a nucleus, cytoplasm, mitochondria, ribosomes, ER and a Golgi body. But a few features set them apart — another favourite NDA question, usually framed as "Which of these is found in a plant cell but not an animal cell?"

Cell membrane and cell wall

The plasma membrane (cell membrane) is the outermost boundary of an animal cell and lies just inside the wall in a plant cell. It is made mainly of lipids and proteins and is selectively permeable — it decides what enters and leaves the cell.

Movement across it happens by:

• Diffusion — movement of substances from high to low concentration.
• Osmosis — movement of water across the membrane from high to low water concentration.

The cell wall (plants, fungi and bacteria) is a tough, non-living layer that protects the cell and lets it withstand pressure changes without bursting. In plants it is made of cellulose, while in fungi it is made of chitin. Because the wall is rigid, plant cells keep a fixed shape, whereas wall-less animal cells stay flexible.

Nucleus and cytoplasm

The nucleus is the control centre of the cell — it directs all activities and carries the hereditary material (DNA) packed into structures called chromosomes. It was discovered by Robert Brown.

Inside the nucleus floats a dense body, the nucleolus, which makes ribosomes. The thread-like network of DNA inside the nucleus is called chromatin; when the cell is about to divide, this chromatin condenses into rod-like chromosomes. Humans have 46 chromosomes (23 pairs) in each body cell — a fact NDA has asked directly. The whole nucleus is wrapped in a double-layered nuclear membrane with tiny pores that allow controlled exchange with the cytoplasm.

The cytoplasm is the jelly-like fluid filling the cell between the membrane and the nucleus. All the organelles float in it, and many chemical reactions of the cell happen here.

The major organelles and their jobs

Organelles are tiny "organs" inside the cell, each doing a special job, just as organs do inside your body. They float in the cytoplasm and are found mainly in eukaryotic cells. Learn these one-liners — they are pure marks and appear in almost every science paper.

Worked example

Notice how just two clues — presence of a true nucleus and presence of a cell wall — let you classify almost any cell. Ask: "Does it have a nucleus?" If no, it is a prokaryote (bacterium). If yes, ask "Does it have a cell wall and chloroplast?" If yes, plant; if no, animal. That two-question trick covers the entire chapter and almost every classification question NDA can throw at you.

Previous-year style question

Quick revision

Revise this box the night before your exam and these marks are safely in your pocket. Good luck from The Cavalier!