1. The Basis of Life — Cell Structure and Function

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CSS GSA — Paper 7, Part-I, Section II, Sub-topic 1: The Basis of Life — Cell Structures and Functions (Subcellular Organelles such as Nucleus, Mitochondria and Ribosomes)

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Past-paper weightage: asked in 2018, 2020 (MCQ), 2021 (two parts), 2022 (MCQ), 2023 and the 2023 Special paper. The favourite angles are plant vs animal cell, cell wall vs membrane, bacterial vs plant cell, the cell → tissue → organ chain, and "describe cell structure and its functions".

1. What a Cell Is

• Cell: the basic structural and functional unit of life (Latin cellula, "a little room")
• Discovery: Robert Hooke saw cells in cork in 1665; Anton van Leeuwenhoek saw living cells soon after
• Cell theory (Schleiden 1838, Schwann 1839, Virchow 1855)
  • All living things are made of one or more cells
  • The cell is the basic unit of structure and function
  • New cells come only from existing cells
• Unicellular organisms are one cell (bacteria, Amoeba); multicellular organisms have many specialised cells (humans have trillions)

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In simple words: a cell is like a tiny factory. The nucleus is the head office, mitochondria are the power plant, ribosomes are the workers making proteins, and the membrane is the security gate.

2. Prokaryotic vs Eukaryotic Cells

3. Main Parts of a Cell

3.1 Cell membrane (plasma membrane)

• Thin, flexible, living boundary found in all cells
• Made of a phospholipid bilayer with proteins and some carbohydrates; described by the fluid mosaic model (Singer and Nicolson, 1972)
• Selectively permeable: controls entry and exit by diffusion, osmosis and active transport

3.2 Cell wall (plants, fungi, many prokaryotes and some protists)

• Rigid extracellular layer, generally permeable to water and many small solutes
• Gives shape and support and stops the cell bursting when water enters

3.3 Cytoplasm

• Jelly-like fluid (cytosol) between the membrane and the nucleus, holding all organelles
• Site of many reactions, such as glycolysis

4. Subcellular Organelles (Syllabus Focus)

An organelle is a specialized subunit within a biological cell that has a specific, dedicated job—much like an organ does in the human body

4.1 Nucleus (control centre)

• Nuclear envelope: double membrane with nuclear pores that let mRNA and ribosome parts move out
• Nucleoplasm: fluid inside the nucleus
• Chromatin: DNA wound around proteins; it condenses into chromosomes during division (most human nucleated somatic cells have 46; gametes have 23; mature red cells have no nucleus)
• Nucleolus: dense area that makes rRNA and assembles ribosome subunits
• Functions: stores hereditary information, controls all cell activities by deciding which proteins are made, and directs cell division

4.2 Mitochondria ("powerhouse of the cell")

• Double membrane; the inner membrane folds into cristae, which hold the enzymes of the electron transport chain
• Inner fluid is the matrix, where the Krebs cycle runs
• Has its own circular DNA and bacterial-derived ribosomes, which supports the endosymbiotic theory (mitochondria began as free-living bacteria)
• Function: aerobic respiration, producing ATP (glucose + oxygen → CO₂ + water + energy)
• Numerous in active cells such as muscle, liver and sperm cells
• Mitochondrial DNA is inherited from the mother only

4.3 Ribosomes (protein factories)

• Made of rRNA and protein; not membrane-bound
• Two subunits (large and small); found free in the cytoplasm or attached to rough ER
• Function: protein synthesis (translation) — they read mRNA and join amino acids in the right order, brought by tRNA
• Free ribosomes make many cytosolic proteins; rough-ER-bound ribosomes make secreted proteins and many membrane/endomembrane proteins

4.4 Other organelles

4.5 Organelles grouped by membrane

Remember: the cell wall supports and protects the cell, but it is not usually classified as an organelle.

5. Plant Cell vs Animal Cell

6. Levels of Organisation

• Cell → Tissue → Organ → Organ system → Organism
• Example: muscle cell → muscle tissue → heart → circulatory system → human

7. Main Functions of Cells in the Human Body

1. Structure: cells build tissues, organs and the whole body
2. Energy: mitochondria release energy from food as ATP
3. Metabolism: all chemical reactions (digestion, respiration, excretion) happen in cells
4. Protein synthesis: ribosomes make enzymes, hormones and antibodies
5. Growth and repair: cells divide by mitosis to grow and heal wounds
6. Reproduction and heredity: DNA is copied and passed on; gametes form by meiosis
7. Transport and communication: the membrane controls exchange; nerve and hormone signals pass between cells
8. Defence: white blood cells destroy germs

8. MCQ Traps

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• Mitochondria = respiration/ATP; chloroplast = photosynthesis (plants only)
• Ribosomes make proteins; they do not store them, and they have no membrane
• The nucleolus makes ribosomes; it is not the same as the nucleus
• Prokaryotes have ribosomes (70S) but no mitochondria or ER
• Cell wall is porous; the membrane provides the main selective permeability
• Mitochondrial DNA comes from the mother

9. Self-Test Questions

1. Write three differences between a plant cell and an animal cell.
2. Which organelle is the "powerhouse of the cell"? Describe its structure and function.
3. Explain the structure of the nucleus, including the nuclear membrane and nucleolus.
4. Briefly explain ribosomes: their structure and function.
5. Differentiate between prokaryotic and eukaryotic cells.

10. CSS Exam Perspective

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• Most repeated areas: plant vs animal cell, cell wall vs membrane, bacterial vs plant cell, organelle functions, cell → tissue → organ
• Likely 2027 angles: nucleus–ribosome teamwork in protein synthesis, mitochondria and the endosymbiotic theory, prokaryote vs eukaryote
• Answer method: match the command word, include a labelled diagram where useful, and pair every structural feature with its function; marks depend on the actual marking scheme.

Syllabus completion — cell processes and answer practice

Coverage: The named nucleus, mitochondria and ribosomes are covered above. The following connects their structures to cell functions and fills the membrane-transport and division gaps.

Transport across the plasma membrane

In a hypotonic solution, animal cells can swell and burst; plant cells become turgid because the wall resists expansion. In a hypertonic solution, cells lose water; plant cells may plasmolyse. Isotonic means no net osmotic water movement.

Growth, division and specialisation

• Mitosis: one nuclear division normally produces two genetically similar daughter nuclei with the same chromosome number; supports growth and repair.
• Meiosis: two divisions after one DNA replication produce haploid cells and genetic variation through crossing over and independent assortment; necessary for sexual reproduction.
• Specialisation: cell structure suits its job—root hairs increase absorption area, neurons carry signals, and mature mammalian red cells lack nuclei and mitochondria and carry oxygen.
• Protein secretion pathway: DNA transcription in nucleus → mRNA → ribosome on rough ER → protein processing → transport vesicle → Golgi → secretory vesicle → exocytosis. Mitochondrial ATP supports energy-demanding steps.

Past-paper application and diagram checklist

Use the saved 2018 Q3(d) for a shared-cell-plan paragraph plus three paired differences; 2021 Q3(a) for the cell–tissue–organ hierarchy with both plant and animal examples; 2021 Q3(b) for wall/membrane comparison; 2023 Q3(c)(i) for prokaryote/eukaryote distinctions; 2023 Special Q5(d) for structure followed by seven separate functions.

For revision, draw a typical plant and animal cell, nucleus and mitochondrion. Label membrane, cytoplasm, nucleus/nucleolus, ribosomes, ER, Golgi and mitochondria; add wall, central vacuole and chloroplast to a photosynthetic plant cell. For a mitochondrion label both membranes, cristae and matrix.

Additional practice—not a past-paper claim: Explain why a secretory cell has abundant rough ER and Golgi. Answer: ribosomes synthesise exported proteins, ER folds/processes them, Golgi modifies/sorts them, and vesicles deliver them to the membrane.

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11. Past-Paper Long Questions — Exam-Ready Answers

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These are exam-ready study answers for the questions recorded in this page. Year labels and wording follow the saved collection; they have not all been authenticated against original papers. Coverage splits are suggested planning guides, not official FPSC marking schemes. Use the marks printed on your paper. For combined questions without an explicit sub-split, divide space according to the tasks asked.

• CSS 2023 Special Q5(d): Describe cell structure. Elaborate seven main functions of a cell in human beings. (5 marks)

  Suggested coverage: structure 2; seven functions 3.

  Model answer

  The cell is the basic structural and functional unit of life. A typical human cell has a plasma membrane, cytoplasm containing organelles, and a nucleus.

  Structure: The selectively permeable phospholipid membrane regulates exchange and communication. Cytoplasm contains mitochondria for ATP production, ribosomes for protein synthesis, ER for synthesis and transport, Golgi apparatus for modification and packaging, and lysosomes for intracellular digestion. The nucleus contains DNA; its nucleolus assembles ribosomal subunits.

  Seven functions:

  1. Support: Cells form the tissues and organs of the body.
  2. Energy production: Cells convert energy from nutrients into ATP.
  3. Metabolism: Cellular enzymes build and break down molecules.
  4. Protein synthesis: Ribosomes make structural proteins, enzymes and other proteins.
  5. Growth and repair: Cell growth and division replace damaged cells.
  6. Heredity and reproduction: DNA stores information and is passed to daughter cells.
  7. Defence: Specialised immune cells recognise and eliminate pathogens.

  These functions are distributed among specialised cells; mature human red cells, for example, lack a nucleus and mitochondria.

  Diagram: Draw a typical animal cell and label membrane, cytoplasm, nucleus, mitochondria, ribosomes, ER and Golgi.

• CSS 2018 Q3(d): Describe cell structure and give at least three differences between animal and plant cells. (5 marks)

  Suggested coverage: basic structure 2; comparison 3.

  Model answer

  A cell is the basic unit of life. Plant and animal cells are eukaryotic: their DNA is enclosed in a nucleus, while their cytoplasm contains mitochondria, ribosomes, ER and Golgi apparatus. The cell membrane regulates entry and exit; organelles perform specialised metabolic tasks.

  Feature	Plant cell	Animal cell
  Wall	Cellulose wall outside the membrane provides support	No cell wall
  Plastids	Chloroplasts in photosynthetic cells	No chloroplasts
  Vacuole	Usually a large central vacuole maintaining turgor	Usually smaller vesicles/vacuoles
  Shape	Often relatively fixed because of the wall	Often flexible or irregular
  Centrioles	Absent from most higher-plant cells	Usually present

  Both cell types respire; plant cells do not rely on photosynthesis instead of respiration. Chloroplasts are generally absent from root cells.

  Diagram: Draw two simple labelled cells; highlight wall, chloroplast and central vacuole in the plant cell.

• CSS 2021 Q3(a): Explain with examples the relationship between cells, tissues and organs. (5 marks)

  Suggested coverage: definitions 2; linked examples and hierarchy 3.

  Model answer

  Multicellular organisms show increasing levels of organisation: cell → tissue → organ → organ system → organism.

  A cell is the basic living unit. A tissue is an organised group of cells, with any associated extracellular material, performing related functions. An organ consists of different tissues working together for a particular function.

  Animal example: Cardiac muscle cells form cardiac muscle tissue. In the heart, this tissue works with connective tissue, vascular tissue and nervous tissue to pump blood. The heart and blood vessels together form the circulatory system.

  Plant example: Epidermal cells form epidermal tissue. In a leaf, epidermis, mesophyll and vascular tissues cooperate in photosynthesis and gas exchange. Xylem supplies water, mesophyll fixes carbon, and the epidermis limits water loss.

  Thus, cells specialise, tissues coordinate related work, and organs integrate several tissue functions.

• CSS 2021 Q3(b): Compare the structure and function of a cell wall and a cell membrane. (5 marks)

  Suggested coverage: definitions 1; paired structural and functional differences 4.

  Model answer

  The cell membrane is the living cell’s selectively permeable boundary. A cell wall is an additional extracellular layer outside the membrane in plants, fungi and many prokaryotes.

  Feature	Cell wall	Cell membrane
  Occurrence	Plants, fungi and most bacteria; absent in animals	All cells
  Composition	Cellulose in plants; chitin in fungi; peptidoglycan in most bacteria	Lipid bilayer with proteins
  Position	Outside the plasma membrane	Surrounds cytoplasm; inside the wall if present
  Mechanical character	Relatively rigid and supportive	Thin, flexible and dynamic
  Permeability	Porous to water and many small solutes	Selectively controls movement using bilayer and transport proteins
  Main roles	Shape, protection and resistance to osmotic bursting	Transport, signalling, recognition and internal regulation

  The wall provides mechanical support, whereas the membrane controls the cell’s chemical exchange. A plant cell requires both structures.

• CSS 2023 Q3(c)(i): How is a bacterial cell different from a plant cell? (Short part; saved allocation 2.5 marks)

  Suggested coverage: one direct distinction followed by four paired differences.

  Model answer

  A bacterial cell is prokaryotic, whereas a plant cell is eukaryotic.

  Feature	Bacterial cell	Plant cell
  DNA	Nucleoid without a nuclear envelope; usually a circular chromosome	Linear chromosomes enclosed in a nucleus
  Organelles	No membrane-bound organelles	Mitochondria, ER, Golgi and, in photosynthetic cells, chloroplasts
  Wall	Usually peptidoglycan; wall-less exceptions exist	Mainly cellulose
  Ribosomes	70S	80S in cytosol; organelle ribosomes differ
  Division	Usually binary fission	Mitosis; meiosis during sexual reproduction

  Both possess a cell membrane, cytoplasm, DNA and ribosomes.

12. Past-Paper MCQ Quiz (FPSC)

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MCQs retained from the saved collection. Answers are study explanations, not verified official keys; historical source limits are recorded in the Biology hub.

1. (CSS 2020) Which organelle is called the powerhouse of the cell?

• (A) Mitochondria (B) Ribosome (C) Plastid (D) None of these
• ✅ Answer: (A) Mitochondria — they carry out aerobic respiration and make most of the cell's ATP. Ribosomes make proteins; plastids (chloroplasts) do photosynthesis.

2. (CSS 2019) What organelles do plant cells have?

• (A) Cell wall (B) Vacuoles (C) Both A and B (D) None of these
• ✅ Answer: (C) Both A and B — plant cells have a cellulose cell wall and a large central vacuole (strictly, the wall is a structure rather than an organelle; (C) is the best fit to these transcribed options).

3. (CSS 2022) What does RNA stand for?

• (A) Ribonucleic Acid (B) Russian National Assembly (C) Red Nitric Acid (D) None of these
• ✅ Answer: (A) Ribonucleic Acid — RNA carries DNA's instructions to the ribosomes (mRNA), brings amino acids (tRNA) and forms part of ribosomes (rRNA).

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13. Textbook Coverage Check — Mitochondria, Heredity, Nucleus and Ribosomes

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Added from textbook pages 51–53. These notes expand the existing organelle sections and provide exam-ready answers to the specific questions shown in the photographs. They are placed here to avoid duplicating the main notes.

13.1 CSS-style question: Which organelle is called the powerhouse of the cell? Explain its structure and functions.

Answer / outline: Definition → double-membrane structure → cristae and matrix → ATP production → other roles.

Mitochondria — powerhouse of the cell

• Definition: Mitochondria are organelles that convert energy released from food into ATP, the usable energy currency of the cell. They are major sites of aerobic cellular respiration in eukaryotic cells.
• Shape: Often bean-shaped or rod-shaped, although shape varies and mitochondria can change dynamically.

Structure

1. Outer membrane: Smooth outer boundary. It contains proteins, including porins, that allow many small molecules and ions to cross.
2. Intermembrane space: The narrow space between the outer and inner membranes. During respiration, protons accumulate here, creating a gradient used to produce ATP.
3. Inner membrane: Selectively permeable membrane containing the electron-transport chain and ATP synthase.
4. Cristae: Folds of the inner membrane. They increase the surface area available for the electron-transport chain and ATP production.
5. Matrix: The fluid-filled innermost compartment. It contains enzymes for the link reaction and Krebs (citric acid) cycle, as well as mitochondrial DNA and ribosomes.

Functions

• ATP production: Oxidative phosphorylation at the inner membrane produces most ATP during aerobic respiration.
• Krebs cycle: Enzymes in the matrix release high-energy electrons from products derived from food.
• Energy-demanding cells: Mitochondria are abundant in many active cells, including muscle and liver cells. Numbers vary with cell type and energy demand.
• Lipid metabolism: Mitochondria participate in the synthesis and breakdown of certain lipids, but they are not the cell's sole or general site of lipid synthesis; the smooth ER performs much of that work.
• Additional roles: They help regulate programmed cell death and calcium balance.

Overall respiration equation (simplified):

Glucose + oxygen → carbon dioxide + water + energy (captured partly as ATP)

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Remember: Matrix = Krebs cycle; inner membrane/cristae = electron-transport chain and ATP synthase; intermembrane space = proton accumulation. Cristae increase the inner membrane's surface area.

13.2 CSS-style question: Where is hereditary material located in the cell, and in what form?

Answer

• In human and other eukaryotic cells, most hereditary material is found in the nucleus, in the form of DNA associated with proteins.
• DNA and its associated proteins form chromatin. When a cell divides, chromatin condenses into visible chromosomes.
• The nucleolus is not where the cell's main hereditary material is stored. It is a region inside the nucleus where ribosomal RNA (rRNA) is made and ribosome subunits begin to assemble.
• Mitochondria also contain a small amount of their own DNA (mitochondrial DNA). In humans, it is usually inherited from the mother.
• Mature mammalian red blood cells lack a nucleus and therefore lack nuclear DNA; this is an important exception to the phrase “nearly every cell contains DNA.”

DNA (deoxyribonucleic acid)

• DNA stores genetic instructions used in development, cell functioning and reproduction.
• It is built from four kinds of nucleotide bases: adenine (A), thymine (T), cytosine (C) and guanine (G).
• In double-stranded DNA, A pairs with T and C pairs with G.
• The order of bases carries genetic information, much as the order of letters forms words.
• A typical human body cell has 46 chromosomes arranged in 23 pairs: one chromosome of each pair is usually inherited from the mother and the other from the father. Gametes (egg and sperm) normally have 23 chromosomes each.
• Human mitochondrial DNA is separate from the 46 nuclear chromosomes.

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Common exam correction: The textbook passage says hereditary material is inside the nucleolus. The accurate answer is that most hereditary material (DNA) is in the nucleus, organised as chromatin and chromosomes. The nucleolus mainly makes rRNA and assembles ribosomal subunits.

13.3 Explain the structure of the nucleus, including the nuclear membrane and nucleolus.

Definition: The nucleus is a membrane-bound organelle that stores most of a eukaryotic cell's DNA and helps regulate cell activities through gene expression.

Main parts

1. Nuclear envelope: A double membrane surrounding the nucleus.
2. Nuclear pores: Protein complexes in the envelope that regulate movement of RNA, proteins and other materials between the nucleus and cytoplasm.
3. Perinuclear space: The narrow space between the two membranes of the nuclear envelope.
4. Outer nuclear membrane: Continuous with the endoplasmic reticulum and may have ribosomes attached.
5. Nucleoplasm: The fluid-like material inside the nucleus, containing chromatin and other nuclear components.
6. Chromatin: DNA associated with proteins, especially histones. It condenses into chromosomes during cell division.
7. Nucleolus: A dense, non-membrane-bound region where rRNA is produced and ribosomal subunits are assembled.

Functions of the nucleus

• Stores and protects most of the cell's hereditary information.
• Regulates cell activities by controlling which genes are expressed and which proteins are produced.
• Coordinates DNA replication before cell division.
• Supports ribosome production through the nucleolus.

Distinguish these terms

13.4 Briefly explain ribosomes, their structure and function.

Definition: Ribosomes are non-membrane-bound structures made of ribosomal RNA (rRNA) and proteins. They are the sites of protein synthesis.

Structure

• Each ribosome has a small subunit and a large subunit.
• Ribosomes are assembled from rRNA and proteins; in eukaryotic cells, assembly of their subunits begins in the nucleolus.
• They occur freely in the cytoplasm or attached to the rough endoplasmic reticulum and the outer nuclear membrane.
• Ribosomes in bacteria are generally 70S; ribosomes in the cytoplasm of eukaryotic cells are generally 80S. Chloroplast ribosomes are generally 70S; mitochondrial ribosomes vary between organisms (55S in mammals). Structural evidence for mammalian 55S ribosomes.

Function: protein synthesis (translation)

1. The ribosome reads the instructions carried by messenger RNA (mRNA).
2. Transfer RNA (tRNA) brings amino acids that match the mRNA code.
3. The ribosome joins amino acids with peptide bonds to build a polypeptide chain, which folds and may be modified to become a functional protein.

Free vs attached ribosomes

• Free ribosomes mainly make proteins used in the cytosol or delivered to certain internal compartments.
• Ribosomes attached to rough ER mainly make proteins that enter the secretory pathway, become membrane proteins, or are sent to certain organelles.

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Memory chain: DNA stores instructions → mRNA carries a copy of the instructions → ribosome reads mRNA → tRNA brings amino acids → a protein is assembled.

13.5 Quick revision checklist

• I can label the outer membrane, inner membrane, cristae, matrix and intermembrane space of a mitochondrion.
• I can explain why cristae increase the efficiency of ATP production.
• I know that the nucleus contains most cellular DNA; the nucleolus mainly produces rRNA and assembles ribosomal subunits.
• I can distinguish DNA, chromatin, chromosomes and the nucleolus.
• I know the four DNA bases: A, T, C and G.
• I can describe the two ribosomal subunits and explain translation.
• I can distinguish free ribosomes from rough-ER-bound ribosomes.
• I can explain the usual 46 chromosomes in human body cells and the 23 chromosomes in human gametes.

Labelled figures — cells and protein synthesis

Typical animal and plant cells — OpenStax Biology 2e, Figure 4.8

Figure source: OpenStax, Eukaryotic Cells, Figure 4.8. Identify nucleus, nucleolus, mitochondria, ribosomes, ER and Golgi in both cells. The plant diagram adds a wall, chloroplasts and a large central vacuole. Chloroplasts occur in photosynthetic plant cells; root cells generally lack them.

Figure: how a secreted protein is made — a process schematic, not an anatomical drawing.

flowchart TD
 A["DNA in nucleus"] -->|"transcription"| B["mRNA"]
 B -->|"through nuclear pore"| C["Ribosome on rough ER"]
 C -->|"translation"| D["Protein enters ER"]
 D --> E["Transport vesicle"]
 E --> F["Golgi: modify and sort"]
 F --> G["Secretory vesicle"]
 G -->|"exocytosis"| H["Protein outside cell"]

Supporting revision: typical human somatic cells are diploid (46 chromosomes); gametes are haploid (23). Mitosis ordinarily preserves chromosome number; meiosis halves it. DNA is replicated before division, not between meiosis I and II. Mature human red cells have neither nucleus nor mitochondria. These distinctions prevent the misleading statement that every human cell has 46 chromosomes.

Diagram check: draw a nucleus with a double envelope and pores; a mitochondrion with outer membrane, folded inner membrane/cristae and matrix; a ribosome with two subunits and no surrounding membrane.