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nucleus
Most eukaryotic DNA is nuclear; mitochondria and chloroplasts also contain DNA.
Class 9 · English Version · Biology
Cells, organisation and biological molecules
Relate cell structure and biological molecules to their functions.
Shared cross-level core and stretch unit. Start with the core questions, then use the stretch example for deeper work. The same unit is available in related class and board pathways; it is not a full grade-specific or examination-specification course. A teacher should select the appropriate depth and further topics.
Learn the idea
Cells have specialised structures. The membrane controls exchange; ribosomes build proteins. Eukaryotic cells have a nucleus, while bacteria do not have a membrane-bound nucleus. Proteins, carbohydrates and lipids have different structures and roles; no single cell diagram represents every cell type.
Worked example
A root hair cell has an extended surface for absorption. A palisade cell contains many chloroplasts for photosynthesis. Magnification = image size/actual size when both sizes use the same units.
- Name the cell type.
- Connect a structure to a mechanism.
- Convert units before magnification calculations.
Stretch example
An enzyme’s active site interacts selectively with substrates. Temperature and pH can alter activity; high temperature can disrupt the protein structure. A low reaction rate alone does not reveal which factor is limiting.
Your written practice
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ribosomes
Ribosomes translate messenger RNA.
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1000
20/0.02 = 1000.
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They increase surface area in contact with the soil solution.
Connect shape to exchange.
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Bacteria lack a membrane-bound nucleus, and some specialised eukaryotic cells also lack a nucleus.
Give an accurate counterexample.
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protein
Most enzymes are proteins; some RNA molecules also catalyse reactions.
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An inhibitor competes with substrate for the active site, reducing the rate under the stated conditions.
Distinguish competition from permanent destruction of the enzyme.
Structure-function cards
Prepare four cell-structure cards with drawings and explanations.
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- Name the cell and organelle accurately.
- Explain a mechanism.
- State one limitation of the simplified drawing.
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