CELL STRUCTURE AND FUNCTIONS

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 1. 📘 Introduction

  • The cell is the basic structural and functional unit of all living organisms.

  • It performs all life processes such as nutrition, respiration, excretion, and reproduction.

  • Cells vary in shape and size and may be unicellular or multicellular.

  • Key structural components:

    • Plasma membrane

    • Cell wall (in plants)

    • Nucleus

    • Cytoplasm

    • Cell organelles

    • Cell division



2. 🧫 Plasma Membrane – Structure & Features

  • Also called the cell membrane; surrounds the cell.

  • Composed of a phospholipid bilayer embedded with proteins.

  • It is a semi-permeable membrane, allowing only certain substances to pass through.

  • Present in both plant and animal cells.

  • Maintains the cell's internal environment and interacts with external stimuli.

3. 🧬 Plasma Membrane – Functions

  • Protects the internal cell contents from the external environment.

  • Regulates the movement of substances such as gases, nutrients, and wastes.

  • Plays a key role in cell-to-cell communication and signal transduction.

  • Facilitates osmosis and diffusion, helping maintain homeostasis.

4. 🌿 Cell Wall – Structure

  • Found only in plant cells, fungi, and some prokaryotes.

  • Located outside the plasma membrane.

  • Mainly made of cellulose (in plants), providing rigidity and strength.

  • Porous in nature; allows passage of water, oxygen, and carbon dioxide.

5. 🌱 Cell Wall – Functions

  • Provides mechanical support and helps maintain the shape of the plant cell.

  • Prevents osmotic bursting by controlling water intake.

  • Acts as a protective barrier against physical and chemical damage.

  • Enables intercellular communication through plasmodesmata (tiny channels between cells).

6. 🏛️ Nucleus – Structure

  • Acts as the brain of the cell.

  • Enclosed by a double-layered nuclear envelope with pores for material exchange.

  • Contains:

    • Nucleoplasm – the fluid inside the nucleus.

    • Nucleolus – synthesizes ribosomes.( A ribosome is a complex molecular machine found inside the living cells that produce proteins from amino acids during a process called protein synthesis or translation. )

    • Chromatin – thread-like structures of DNA and proteins that condense into chromosomes during cell division.

7. 🎯 Nucleus – Functions

  • Controls all cellular activities, including growth and metabolism.

  • Stores the genetic information (DNA) required for reproduction and inheritance.

  • Coordinates protein synthesis via messenger RNA.

  • Initiates and regulates cell division.

8. 🧬 Types of Cells Based on Nucleus

  • Prokaryotic Cells:

    • No nuclear membrane.

    • Nucleus not well-defined (e.g., bacteria).

    • DNA lies freely in the cytoplasm (nucleoid).

  • Eukaryotic Cells:

    • Have a well-defined nucleus enclosed in a nuclear membrane.

    • Found in plants, animals, fungi, and protists.

9. 💧 Cytoplasm – Structure

  • Jelly-like fluid that fills the space between the nucleus and the plasma membrane.

  • Made of cytosol (water + organic/inorganic molecules) and suspended organelles.

  • Maintains cell turgor pressure and provides support. (Turgor pressure is the hydrostatic pressure that helps support plant structure and maintain cell rigidity. It is enabled by cytoplasm and vacuole interactions with the cell wall.)

10. 🧪 Cytoplasm – Functions

  • Site of major metabolic reactions like glycolysis and synthesis of amino acids.

  • Suspends organelles and keeps them in place.

  • Acts as a medium for material exchange within the cell.

  • Supports internal movement and cytoplasmic streaming.

11. 🏗️ Cell Organelles – Overview

  • Tiny membrane-bound structures inside eukaryotic cells, each with a specialized function.

  • Important organelles include:

    • Endoplasmic Reticulum (ER)

    • Golgi Apparatus

    • Mitochondria

    • Lysosomes

    • Plastids (in plants)

    • Vacuoles

12. 🛠️ Endoplasmic Reticulum (ER)

  • A network of tubular membranes within the cytoplasm.

  • Rough ER (RER): studded with ribosomes; involved in protein synthesis.

  • Smooth ER (SER): no ribosomes; synthesizes lipids, hormones, and detoxifies harmful substances.

  • Acts as a transport highway for intracellular movement.

13. 📦 Golgi Apparatus

  • Stack of flattened membrane-bound sacs (cisternae).

  • Modifies, stores, and packages proteins and lipids from the ER.

  • Forms lysosomes and vesicles for secretion or internal use.

  • Involved in exocytosis and formation of the cell membrane.

14. ⚡ Mitochondria

  • Known as the “powerhouse of the cell”.

  • Site of aerobic respiration that releases energy (ATP).

  • Has a double membrane and its own DNA – capable of self-replication.

  • More active cells have more mitochondria (e.g., muscle cells).

15. 🧹 Lysosomes

  • Small vesicles containing digestive enzymes.

  • Break down damaged cell parts, bacteria, and wastes.

  • Also called “suicide bags” because they can destroy the entire cell if ruptured.

  • Important in defense and recycling within the cell.

16. 🌞 Plastids (Plant Cells Only)

  • Found only in plant cells and some protists.

  • Types of plastids:

    • Chloroplasts: Green; contain chlorophyll for photosynthesis.

    • Chromoplasts: Coloured; store pigments (yellow, red, orange).

    • Leucoplasts: Colourless; store starch, proteins, and lipids.

17. 💼 Vacuoles

  • Fluid-filled sacs surrounded by a membrane (tonoplast).

  • Large and central in plant cells, smaller in animal cells.

  • Store nutrients, waste products, and water.

  • Help maintain turgor pressure and support plant structure.

18. 🔁 Cell Division – Overview

  • The process by which new cells are formed for growth, repair, and reproduction.

  • Two major types:

    • Mitosis – forms identical cells (asexual reproduction, growth, repair).

    • Meiosis – forms gametes (sperm and egg) with half the chromosome number.

19. 🧩 Mitosis

  • One parent cell divides into two genetically identical daughter cells.

  • Occurs in somatic (body) cells.

  • Involves four phases:

    1. Prophase – Chromosomes condense, spindle fibers form.

    2. Metaphase – Chromosomes line up in the center.

    3. Anaphase – Sister chromatids pulled apart.

    4. Telophase – Two nuclei form.

  • Followed by cytokinesis (division of cytoplasm).

20. 🎲 Meiosis

  • Occurs in reproductive organs to produce gametes.

  • Involves two rounds of division but only one DNA replication.

  • Results in four non-identical cells, each with half the chromosome number.

  • Maintains genetic variation and is essential for sexual reproduction.

21. 📋 Summary

  • Every part of the cell plays a crucial role in maintaining life.

  • The plasma membrane regulates transport.

  • The nucleus controls cellular activities and inheritance.

  • Cytoplasm is the site of metabolic activities.

  • Organelles ensure energy production, storage, synthesis, and waste disposal.

  • Cell division is key to life’s continuity and evolution.

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