COMPLETE ENGLISH VERSION
1. What is a cell?
A cell is the basic structural and functional unit of life. It is the smallest unit that can perform essential living functions. Just as a building is made of bricks, organisms are made of cells. Cells obtain nutrients, use energy and perform specific functions. In complex organisms, cells work together to form tissues, organs and organ systems.
2. Timeline of important discoveries
----------------------------------------------------------------------- Period Scientist Contribution ----------------------- ----------------------- ----------------------- 1665 Robert Hooke Observed small compartments in a thin slice of cork and named them “cells”.
1670s Antonie van Leeuwenhoek Observed living microscopic organisms.
1831 Robert Brown Described the cell nucleus.
1838 Matthias Schleiden Explained that plants are made of cells.
1839 Theodor Schwann Explained that animals are also made of cells.
Mid-19th century Rudolf Virchow Emphasised that new cells arise from existing cells. -----------------------------------------------------------------------
Three main ideas of cell theory: Living organisms consist of one or more cells; the cell is the basic structural and functional unit; new cells come from pre-existing cells.
3. Unicellular and multicellular organisms
Unicellular organisms consist of one cell that performs all essential life functions. Examples: Amoeba, Chlamydomonas and many bacteria.
Multicellular organisms consist of many specialised cells. Examples: humans, mango trees and paddy plants.
A bacterium is a unicellular organism. A virus has no cellular organisation and is described as acellular.
4. Three basic components of a cell
Cell membrane: Forms the cell’s boundary. It is selectively permeable, meaning that it regulates which substances enter and leave the cell.
Cytoplasm: The material inside the cell membrane, excluding the nucleus. It contains cell organelles. The fluid part is called cytosol.
Nucleus: Acts as the cell’s control centre and contains genetic information in DNA organised as chromatin/chromosomes. It is generally enclosed by a nuclear envelope.
Exam exception: Mature human red blood cells do not have a nucleus. Therefore, “every human cell has a nucleus” is false.
5. Important organelles and their functions
----------------------------------------------------------------------- Organelle Main function Memory clue ----------------------- ----------------------- ----------------------- Mitochondria Major site of ATP Powerhouse production during cellular respiration
Ribosomes Protein synthesis Protein factory; not membrane-bound
Chloroplasts Photosynthesis; contain Food production chlorophyll
Golgi apparatus Modifies, packages and Packaging centre transports materials
Endoplasmic reticulum Production and Transport network (ER) transport of materials within the cell
Lysosomes Break down and recycle Cellular clean-up cellular material
Vacuoles Store water and Storage dissolved materials; help maintain plant cell firmness
Centrioles Help organise cell Division support division in many animal cells -----------------------------------------------------------------------
Rough ER (RER) has ribosomes attached and is involved in protein production. Smooth ER (SER) lacks attached ribosomes and is involved in lipid production. Chloroplasts are not present in every plant cell; for example, many root cells do not have them.
6. Plant cell versus animal cell
----------------------------------------------------------------------- Feature Plant cell Animal cell ----------------------- ----------------------- ----------------------- Cell membrane Present Present
Cell wall Usually present; mainly Absent cellulose
Chloroplasts Present in Absent photosynthetic green cells
Vacuoles Usually one large Small vacuoles may central vacuole occur
Nucleus Present in most living Present in most cells; mature cells mature RBCs are an exception
Mitochondria Present Present
Centrioles Generally absent in Present in many animal higher plant cells cells
Shape Often more regular due More variable to the cell wall -----------------------------------------------------------------------
Remember: A cell wall and a cell membrane are different structures. Plant cells have both; animal cells have a cell membrane but no cell wall.
7. Photosynthesis and cellular respiration
Photosynthesis: Chloroplasts in green plant cells use light energy to make food from carbon dioxide and water, releasing oxygen.
Cellular respiration: Cells release usable energy from food, producing ATP. Important stages of aerobic respiration occur in mitochondria.
Important distinction: Plants carry out both photosynthesis and cellular respiration. Both plant and animal cells contain mitochondria.
8. From cells to an organism
Cell → Tissue → Organ → Organ system → Organism.
Example: Muscle cell → muscle tissue → heart → circulatory system → human being.
Specialised human cells: Red blood cells transport oxygen; white blood cells defend against infection; nerve cells transmit signals; muscle cells contract to produce movement.
9. Microscopes and Tamil Nadu examples
Most cells are too small to see clearly without a microscope. In a school laboratory, a thin onion peel can be examined to identify cell walls and the regular arrangement of plant cells. Green leaf cells can show chloroplasts. Growth in crops such as paddy and banana, cultivated in Tamil Nadu, depends on cell division, cell growth and photosynthesis.
10. Common TNPSC examination traps
- Control centre: Nucleus. Powerhouse: Mitochondria. - Protein synthesis: Ribosomes; they are not membrane-bound. - Photosynthesis: Chloroplasts. Packaging and secretion: Golgi apparatus. - Selective barrier: Cell membrane. Support and protection: Cell wall. - Mature human RBC: No nucleus. Virus: Acellular. - Bacteria: Prokaryotic cells without a membrane-bound nucleus. Plant and animal cells: Eukaryotic.
11. One-minute revision
Cell = basic unit of life. Hooke = 1665. Nucleus = control. Mitochondria = ATP. Ribosomes = protein. Chloroplasts = photosynthesis. Cell membrane = selective permeability. Plant cell = cell wall. Unicellular example = Amoeba. Cells → tissues → organs → organ systems → organism.
12. Textbook references and book-back progress
Primary reference: Tamil Nadu Samacheer Kalvi Class 7 Science, Term 2, Unit 4 – Cell Biology. Relevant sections cover cells as the basic units of life, organelles, plant and animal cells, and levels of biological organisation.
Exercise inventory in a transcription of the 2024 English textbook: I MCQs 5; II Fill in the blanks 5; III True/False 6; IV Matching 5 pairs; V Analogies 4; VI Assertion–Reason 2; VII Very short answers 6; VIII Short answers 6; IX Long answers 3; X Higher-order thinking 1. Total: 43 individually answerable items (provisional 2024 transcription count). Section VIII, question 2 has three comparisons but is counted as one printed numbered item. In-text activities are tracked separately.
