⼍Cell Coverings: Membrane, Wall, and Protoplasm

Why Cell Coverings Matter in Agriculture

Every time a farmer stores potatoes in a cold room or a food scientist processes sugarcane into jaggery, cell coverings play a central role. The cell wall gives sugarcane its rigidity, pectin in the middle lamella is what makes fruit jams set, and the plasma membrane controls which nutrients a root hair absorbs from the soil. Understanding these structures is essential for crop improvement, post-harvest technology, and disease resistance breeding.

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Plasma Membrane (Cell Membrane / Plasmalemma)

The plasma membrane is the outermost living boundary of every cell. It is extremely thin, elastic, and selectively controls what enters and exits the cell.

Structure (Fluid Mosaic Model)

• In plant cells, the membrane lies on the inner side of the cell wall.
• It is a lipoprotein membrane:
  • Protein layer — 20 Å thickness
  • Lipid bilayer — 35 Å
  • Protein layer — 20 Å (monolayer)
• Total thickness: 75–100 Å

The Fluid Mosaic Model describes the membrane as a flexible, fluid lipid bilayer with proteins either embedded within it or attached to its surface.

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Properties and Functions

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Membrane Classification of Organelles

Mnemonic: Double-membrane = “NMC” (Nucleus, Mitochondria, Chloroplast). Both mitochondria and chloroplasts are semi-autonomous because they contain their own DNA.

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Cell Wall

• The cell wall is a non-living, thick, rigid envelope outside the plasma membrane.
• It is permeable and made of cellulose — a long-chain polymer of glucose that gives tensile strength.
• Cell wall is absent in animal cells, which is why animal cells are more flexible in shape.

Agricultural connection: The toughness of jute and cotton fibres comes from the thick cellulose walls of their cells. Breeding for fibre quality in these crops directly involves selecting for cell wall properties.

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Middle Lamella

The middle lamella is the cementing layer that joins the primary cell walls of adjacent cells, holding them together to form tissues.

• Rich in pectin (a polysaccharide used as cementing material and in jelly making).
• Contains Ca²⁺ (trace of Mg) which cross-links pectin molecules to form calcium pectate, providing rigidity.

Agricultural example: When fruits ripen, enzymes break down pectin in the middle lamella, causing the fruit to soften. This is why overripe mangoes become mushy — the cells literally come apart.

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Primary Cell Wall

The primary wall is relatively thin and flexible, allowing cell expansion during growth. Pectin is in the primary cell wall in lower plants but in the middle lamella in higher plants.

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Plant Cell Types Based on Cell Wall

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Lignin

• Chemically it is coniferyl alcohol, solid at room temperature.
• Hardness of woody tissue is due to lignin. ^{IBPS AFO 2012}
• Lignin is the second most abundant organic polymer on Earth (after cellulose). It makes cell walls waterproof and structurally rigid.

Agricultural relevance: Lignin content determines the digestibility of crop residues used as animal fodder. Low-lignin sorghum varieties (brown midrib mutants) are bred specifically for better fodder quality.

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Protoplasm

Protoplasm is all the living content of a cell including the plasma membrane — the cytoplasm, nucleus, and all organelles together.

Exam tip: “Protoplasm = physical basis of life” (Huxley) is one of the most frequently asked one-liners.

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Nucleus

• Word derived from Latin “Kernel” — the central, essential core of the cell.
• Discovered by Robert Brown (1833) while studying orchid cells.
• Surrounded by a double-membrane nuclear envelope (lipoprotein) with nuclear pores.
• Size: 5–25 µm.

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Where Is the Nucleus Absent?

Agricultural connection: Phloem sieve tubes transport sucrose from leaves to developing grains in wheat and rice. Their unique nucleus-free structure allows maximum flow of assimilates.

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Nuclear Contents

• Nucleoplasm (nuclear sap/karyolymph) — gel-like matrix supporting chromatin and nucleolus.
• Chromatin — tangled thread-like mass of DNA + histone protein; condenses into chromosomes during cell division.
• Genes — stretches of DNA that carry information for protein synthesis. Genes are the hereditary units; DNA is the hereditary material.

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Nucleolus

• A spheroidal, non-membrane-bound, dense organelle within the nucleus.
• Discovered by Fontana (1781). Rich in RNA but also contains DNA.
• Main function: synthesise ribosomal RNA (rRNA).
• Attached to a specific chromosomal site called the nucleolar organiser region (NOR).

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RNA Polymerases in Eukaryotes

Exam tip: Remember “I = ribosomal, II = messenger, III = transfer” — the numbering matches the size of product (rRNA is largest, tRNA is smallest).

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Chromosome

• First seen by Strasburger in 1875 as fine threads.
• Named “Chromosome” (chroma = colour + soma = body) by Waldeyer in 1888 using basic dye.
• Chromosomes are carriers of hereditary units (genes) — established by Morgan using Drosophila. ^{UPPSC 2021}
• Homozygous = identical alleles for a trait; Heterozygous = different alleles. ^{RRB SO 2021}

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• Chromosome number is species-specific: human 2n = 46, rice 2n = 24.
• In prokaryotes: single circular chromosome called genophore (without histones).
• In eukaryotes: rod-shaped, linear chromosomes wrapped around histone proteins.

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Genetic Material: DNA and RNA

Building Blocks

Nitrogenous Bases

Mnemonic: Pure As Gold = Purines are Adenine and Guanine. The rest (C, T, U) are pyrimidines.

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Key Discoveries

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Structure of DNA (Watson-Crick Model)

• Two antiparallel polynucleotide chains wound helically (one runs 5’→3’, the other 3’→5’).
• Sugar-phosphate backbone on the outside; bases on the inside.
• Chargaff’s Rules: A = T and G = C; total purines (A+G) = total pyrimidines (T+C).
• (A+T)/(G+C) = Base pair ratio — unique to each species.

Exam tip: Higher G-C content = higher thermal stability (melting temperature). This matters when designing PCR primers in agricultural biotechnology.

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Structure of RNA

• Usually single-stranded (not helical like DNA).
• In most plant viruses, genetic material is RNA.
• Non-genetic RNAs: mRNA, tRNA, rRNA — synthesised on DNA template.
• Genetic RNA of viruses is self-replicating via RNA-dependent RNA synthesis using the enzyme RdRp.

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Gene Concepts

Benzer’s three units:

Modern refinement: One gene → One polypeptide (not all genes code for enzymes, and some proteins have multiple polypeptide chains).

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Genetic Code

• Deciphered by Holley, Khorana & Nirenberg (Nobel Prize 1968).
• Triplet code: 4 bases taken 3 at a time = 4³ = 64 codons (enough for 20 amino acids).

Mnemonic for stop codons: “U Are Annoying, U Are Gone, U Go Away” — UAA, UAG, UGA.

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Mitochondria

• Power house of the cell — primary site of ATP production through aerobic respiration.
• ATP = Energy currency of the cell.
• First identified by Altman (1886) as Bioplast.
• Named “mitochondria” by C. Benda (1898).

• Contains own DNA (0.02%), RNA (3–4%), and 70S ribosomes → semi-autonomous (supports Endosymbiotic Theory).

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Plastids

Classified by Schimper (1885) based on pigment content:

Types of Leucoplasts

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Chlorophyll and Plant Pigments

• Carotene + Xanthophyll = carotenoid pigments (accessory pigments; provide photoprotection).
• Chromoplasts contain only carotenoid pigments.

Pigment Classification

Agricultural note: Anthocyanin content in crops like purple cabbage, black rice, and jamun is a marker for antioxidant-rich varieties — an active area of breeding research.

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Chloroplast Structure

• RuBISCO is the most abundant protein on Earth.
• Chloroplasts contain own DNA (0.5%), RNA (3–4%), and 70S ribosomes → semi-autonomous (Endosymbiotic Theory).
• Grana are interconnected by stroma lamellae (intergranal lamellae).

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Endoplasmic Reticulum (ER)

• Dense network of double-membrane structures running through the cytoplasm.
• Ultrastructure first reported by Porter (1948). Origin from nuclear membranes.
• Dynamic — can be broken down and reconstructed based on cellular needs.

Functions of ER

• Provides mechanical support (endoskeleton) to the cell.
• Increases surface area for metabolic reactions.
• Intracellular transport of molecules.
• Formation of cell plate and nuclear membrane during division.
• SER produces lipids for membrane biogenesis.
• SER in liver cells detoxifies drugs and poisons.

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Ribosomes

• Composition: rRNA (40–60%) + Protein (40–60%) — no lipid (membrane-less organelle).
• First observed by Claude (1943); term coined by R.B. Robert (1958).
• Detailed ultrastructure by Palade (1956, Nobel Prize).

• Mg²⁺ ions promote subunit association; low Mg²⁺ causes dissociation.

Types of RNA

Protein Synthesis Steps

1. Transcription — DNA → mRNA (in nucleus)
2. Translation — mRNA → Protein (at ribosomes)

Explore More

• Protein Synthesis: Transcription and Translation

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Golgi Body (Dictyosome)

• Discovered by Camillo Golgi (1898) using silver staining in nerve cells.
• Stacks of flattened sacs called cisternae; called dictyosomes in plants.
• Has polarity: cis face (receiving, near ER) → trans face (shipping, near plasma membrane).
• Origin from ER (transition vesicles).
• Acrosomes on sperm cells are derived from Golgi complex.

Functions

• Store, modify, package and condense proteins (the cell’s “post office”).
• Form the cell plate during plant cell division.
• Add sugars to proteins (glycosylation → glycoproteins).
• Produce lysosomes.

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Lysosome

• Lysis = digestion; Soma = body → “digestive bodies” of the cell.
• Single membrane-bound vesicles containing hydrolytic enzymes.
• Formed from Golgi complex (directly) and ER (indirectly).
• Discovered by De Duve (1955, Nobel Prize 1974). Mainly found in animals; also in some plants like Neurospora.

Functions

• Intracellular digestion of large molecules.
• Defense against bacteria and viruses (fuse with phagosomes).
• During starvation: digest own organelles (autophagy) → called "suicidal bag".

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Other Organelles and Structures

Spherosomes

• Single membrane-bound; mainly in plants.
• Function: fat metabolism — abundant in oilseeds (groundnut, mustard, sunflower).

Microsomes

• Artificial structures formed when cells are broken in the lab (ER fragments + ribosomes).
• Used as in vitro models to study ER functions and protein synthesis.

Vacuole

• Most prominent in mature plant cells; may occupy up to 90% of cell volume.
• Single membrane-bound (membrane = tonoplast); contains cell sap.
• Functions: osmoregulation, storage, maintaining turgor pressure (cell rigidity).

Agricultural relevance: Turgor pressure keeps crop plants upright. When water supply drops, cells lose turgor and the plant wilts — visible as leaf rolling in rice or drooping in sunflower.

Plasmodesmata

• Microscopic channels found only in plants; named by Strasburger (1903).
• Origin from ER.
• Allow direct cell-to-cell communication and transport of molecules.
• All connected protoplasts form the symplast.

Centrosome

• Present near nucleus in all animal cells and some plant groups (Chlamydomonas, gymnosperms).
• Contains two centrioles (nine triplets of microtubules, 9+0 arrangement).
• Functions as the microtubule organising centre (MTOC); produces astral rays during mitosis.

Ergastic Substances

• Non-living cell inclusions: starch, sugar, organic acids, fats, oils, pigments, crystals, tannins, resins.
• These are metabolic products, not part of living protoplasm.

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Summary Table

Summary Cheat Sheet