Class 11 Biology Notes
Complete, exam-ready notes on the cell as the fundamental unit of life: cell theory, prokaryotic and eukaryotic cell structure, the cell envelope, every major organelle and the nucleus — written for CBSE boards and NEET revision.
Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali
The cell is the smallest structural and functional unit of life. All living organisms are made of cells; cells arise from pre-existing cells and carry out all life processes — metabolism, growth, reproduction and response to stimuli.
Cell theory was independently proposed by Matthias Schleiden (1838, plants) and Theodor Schwann (1839, animals). Rudolf Virchow (1855) added the third tenet. Together, the three tenets form the foundation of cell biology.
Remember
Virchow disproved spontaneous generation. Viruses are acellular and not considered living by cell theory — they replicate only inside host cells.
Prokaryotic cells (bacteria, cyanobacteria) are smaller (1–10 µm) and simpler — no membrane-bound nucleus or organelles. Eukaryotic cells (plants, animals, fungi, protists) are larger (10–100 µm) with a true nucleus and membrane-bound organelles. The key difference is the nuclear envelope and internal compartmentalisation.
The cell envelope of bacteria consists of three layers: the innermost plasma membrane (phospholipid bilayer), the cell wall (peptidoglycan — provides shape and rigidity), and the outermost glycocalyx (capsule or slime layer — protection from phagocytosis and desiccation). Gram-positive bacteria have a thick peptidoglycan layer; Gram-negative have a thin layer with an outer membrane containing lipopolysaccharide.
Prokaryote vs Eukaryote ribosomes
70S ribosomes are found in prokaryotes, mitochondria and chloroplasts (evidence of endosymbiotic origin). 80S ribosomes are in the cytoplasm of eukaryotic cells. The 'S' stands for Svedberg unit — a measure of sedimentation rate, not additive.
The cell wall is present in plants, fungi, algae and bacteria — it is absent in animal cells. In plants, it is made of cellulose and gives the cell a definite shape. The plasma membrane (cell membrane) is present in all cells — a selectively permeable phospholipid bilayer that regulates what enters and exits the cell.
The plasma membrane is a fluid phospholipid bilayer with proteins embedded or attached — a mosaic of lipids and proteins. Proteins may be peripheral (on the surface) or integral (spanning the membrane — transmembrane). Cholesterol (in animal membranes) regulates fluidity. Carbohydrate chains on proteins (glycoproteins) and lipids (glycolipids) face outward and are involved in cell recognition.
A network of membrane-bound tubules and sacs (cisternae) continuous with the nuclear envelope. Rough ER (RER) has ribosomes on its surface — synthesises proteins destined for secretion or membrane insertion. Smooth ER (SER) lacks ribosomes — synthesises lipids, detoxifies drugs, and stores calcium ions.
Stacks of flattened, membrane-bound cisternae (cisternae stack ≈ dictyosome). Receives proteins and lipids from ER in vesicles, modifies, packages and labels them (glycosylation, sorting) for secretion or delivery to lysosomes, membrane or other destinations. Also produces glycolipids and forms the cell plate during plant cell division.
Membrane-bound vesicles containing hydrolytic enzymes (acid hydrolases — active at pH ~5). Function: intracellular digestion of worn-out organelles (autophagy), food particles and foreign material. Lysosomal membrane rupture releases enzymes into cytoplasm, digesting the cell — autolysis. Called the 'suicide bags' of the cell.
Large central vacuole in plant cells (bounded by a membrane called the tonoplast) — stores water, nutrients, pigments and waste. Occupies up to 90% of cell volume in mature plant cells; helps maintain turgor. In animals, vacuoles are smaller and food vacuoles (phagosomes) fuse with lysosomes for digestion.
Double-membrane organelle (outer membrane smooth; inner membrane folded into cristae for increased surface area). Matrix (innermost) contains enzymes for the Krebs cycle (TCA cycle). Cristae contain electron transport chain and ATP synthase (oxidative phosphorylation). Has its own circular DNA and 70S ribosomes — evidence for endosymbiotic origin. Function: generates ATP via aerobic respiration.
Found only in plant cells and algae. Double-membrane, with their own DNA and 70S ribosomes. Three types: chloroplasts (green — photosynthesis, contain chlorophyll and carotenoids), chromoplasts (coloured — pigments in flowers and fruits), leucoplasts (colourless — storage; amyloplasts store starch, elaioplasts store lipids, proteinoplasts store protein).
Non-membrane-bound organelles made of rRNA and proteins. Site of protein synthesis. Eukaryotic cytoplasmic ribosomes are 80S (60S + 40S); prokaryotic ribosomes are 70S (50S + 30S). Ribosomes are also found free in the cytoplasm and attached to the RER.
The nucleus is the control centre of the cell — it houses the genetic material and directs all cellular activities. It is bounded by a double membrane (nuclear envelope) perforated by nuclear pores (for RNA and protein transport between nucleus and cytoplasm).
Prokaryotic nucleus
Prokaryotes have no nuclear envelope — their circular DNA lies in a region called the nucleoid. They also lack a nucleolus. This is the single biggest difference between prokaryotic and eukaryotic cells.
Example: Explain why the mitochondria and chloroplasts are considered semi-autonomous organelles.
Solution: Both mitochondria and chloroplasts have their own circular DNA and 70S ribosomes (the same size as prokaryotic ribosomes). They can synthesise some of their own proteins independently of the nuclear genome. This, along with their double-membrane structure, supports the endosymbiotic theory — that both organelles evolved from free-living bacteria engulfed by an ancestral eukaryotic cell. Because they depend on the nucleus for most of their proteins but retain partial genetic independence, they are called semi-autonomous.
Example: A cell is treated with a drug that disrupts ribosome function. Predict which organelles and processes will be directly affected.
Solution: Ribosomes are the site of protein synthesis, so the most direct effect is cessation of protein synthesis. This hits the rough ER (which has attached 80S ribosomes) and free cytoplasmic ribosomes first — secretory and membrane proteins stop being made, and cytoplasmic proteins deplete. Over time, the Golgi body (dependent on ER-derived vesicles) is also affected. Mitochondria and chloroplasts have their own 70S ribosomes — they may be partially shielded initially if the drug specifically targets 80S ribosomes, but sustained disruption affects overall ATP production and cell viability.
Revision
Memorise these before attempting numericals — most exam questions hinge on one of them.
Cell theory tenet 3
Prokaryotic ribosome
Eukaryotic ribosome
Prokaryote vs Eukaryote size
Fluid mosaic model
Axoneme arrangement
Centriole arrangement
Exam tips
Where this topic appears in CBSE, JEE Main and NEET papers.
FAQ
Prokaryotic cells (bacteria) lack a membrane-bound nucleus and organelles — their DNA is in a nucleoid region. Eukaryotic cells have a true nucleus enclosed by a nuclear envelope and membrane-bound organelles (ER, Golgi, mitochondria). Prokaryotes are smaller (1–10 µm) with 70S ribosomes; eukaryotes are larger (10–100 µm) with 80S ribosomes.
Mitochondria are the site of aerobic respiration — the Krebs cycle in the matrix and oxidative phosphorylation (electron transport chain + ATP synthase) on the cristae together generate most of the cell's ATP. A single cell may contain hundreds of mitochondria to meet its energy demands.
The endosymbiotic theory states that mitochondria and chloroplasts evolved from free-living prokaryotes that were engulfed by an ancestral eukaryotic cell. Evidence includes their own circular DNA, 70S ribosomes, double membranes and semi-autonomous division — features shared with bacteria.
The nucleolus is a dense region inside the nucleus where ribosomal RNA (rRNA) is synthesised and ribosomal subunits (60S + 40S in eukaryotes) are assembled. It is not bounded by a membrane and its size reflects the cell's protein-synthetic activity.
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