🌿 Eukaryotic Cells

Eukaryotic cells are structurally complex and highly organised cells. They include all Protists, Plants, Animals, and Fungi. Their complexity allows division of labour, making them efficient and specialised.

🔑 Key Characteristics of Eukaryotic Cells

  • Presence of a true nucleus surrounded by a nuclear membrane.
  • Cytoplasm is compartmentalised by membrane-bound organelles like mitochondria, ER, Golgi apparatus, lysosomes, etc.
  • Genetic material (DNA) is organised into chromosomes.

🔑 Memory Line:

Eukaryotic cell = true nucleus + membrane-bound organelles.

1️⃣ The Cell Membrane (Plasma Membrane)

The cell membrane is a thin, living, flexible boundary that separates the cell contents from the external environment.It is the selectively permeable boundary of every cell (both prokaryotic and eukaryotic). Its structure became clear only after the use of electron microscopes.

🧬 Chemical Composition of Cell Membrane

The membrane is made up of lipids, proteins, and carbohydrates.

(a) Lipids

  • Arranged in a bilayer.
  • Hydrophilic (polar) heads face outward.
  • Hydrophobic (non-polar) tails face inward.
  • Main lipids: phosphoglycerides.
  • Cholesterol (especially in animal cells) maintains membrane stability and fluidity.

🔑 Key Point:

Lipid bilayer is the basic framework of the plasma membrane.

(b) Proteins

Based on their position:

  • Integral (Intrinsic) proteins: Embedded partially or completely in the lipid bilayer.
  • Peripheral (Extrinsic) proteins: Loosely attached on the membrane surface.

🔑 Key Point:

Proteins are responsible for transport, receptors, and enzymatic functions.

(c) Carbohydrates

  • Present as glycoproteins and glycolipids.
  • Mostly found on the outer surface of the membrane.

🔑 Key Point:

Carbohydrates help in cell recognition and cell–cell interaction.

🧠 Fluid Mosaic Model (1972)

Proposed by Singer and Nicolson, this is the most accepted model of plasma membrane structure.

Concept

  • Lipids form a quasi-fluid bilayer.
  • Proteins float like icebergs in a sea of lipids, giving a mosaic appearance.
  • Lateral movement of lipids and proteins gives the membrane fluidity.

Importance of Fluidity

Fluidity is essential for:

  • Cell growth
  • Formation of intercellular junctions
  • Secretion
  • Endocytosis and exocytosis
  • Cell division

🔑 One-Line Memory Trick:

Fluid lipids + floating proteins = Fluid Mosaic Model.

🚦 Functions of Cell Membrane

1. Selective Permeability

The membrane allows some substances to pass while restricting others.

2. Transport Mechanisms

(a) Passive Transport (No Energy Required)
  • Simple diffusion: Small, non-polar molecules (O₂, CO₂).
  • Osmosis: Movement of water across a semipermeable membrane.
(b) Active Transport (Energy Required)
  • Movement against concentration gradient.
  • Requires ATP and carrier proteins.
  • Example: Na⁺/K⁺ pump.

🔑 Key Point:

Passive = no ATP, Active = ATP required.

2️⃣ Cell Wall (Plants & Fungi)

The cell wall is a non-living, rigid outer covering found outside the plasma membrane in plant and fungal cells.

🧱 Functions of Cell Wall

  • Gives shape and rigidity to the cell.
  • Protects from mechanical damage and pathogens.
  • Prevents osmotic bursting.
  • Helps in cell-to-cell interaction.

🔑 Key Point:

Cell wall provides strength; plasma membrane provides flexibility.

🧪 Composition of Cell Wall

  • Algae: Cellulose, galactans, mannans, minerals (CaCO₃).
  • Plants: Cellulose, hemicellulose, pectins, proteins.

🔑 High-Yield Fact:

Cellulose is the most abundant organic polymer on Earth.

🧩 Layers of Plant Cell Wall

  1. Primary Wall:
  • Thin, elastic, present in young growing cells.
  1. Secondary Wall:
  • Thick, rigid, formed inside primary wall in mature cells.
  1. Middle Lamella:
  • Cementing layer between adjacent cells.
  • Made mainly of calcium pectate.

🔑 Memory Line:

Middle lamella glues plant cells together.

🔗 Plasmodesmata

  • Microscopic cytoplasmic connections between adjacent plant cells.
  • Traverse cell wall and middle lamella.

Function:

  • Allow direct cell-to-cell transport.
  • Enable communication and coordination.

🔑 Key Point:

Plasmodesmata create a living cytoplasmic network (symplast).

💡 Questions & Answers

Q1. What is the fluid mosaic model of plasma membrane?

A1. The fluid mosaic model explains that the plasma membrane is made of a fluid lipid bilayer in which proteins are embedded and can move laterally. This structure allows flexibility and efficient functioning of the membrane.

🔑 Key Points:

  • Proposed by Singer and Nicolson (1972)
  • Lipids are fluid
  • Proteins form mosaic pattern
  • Essential for membrane functions

Q2. How do neutral solutes and polar molecules move across the plasma membrane?

A2. Neutral molecules like oxygen and carbon dioxide move by simple diffusion without energy. Polar molecules cannot cross directly; they move either by facilitated diffusion or active transport using carrier proteins.

🔑 Key Points:

  • Neutral → simple diffusion
  • Polar → carriers required
  • Active transport needs ATP

Q3. Differentiate between plant cells and animal cells based on cell wall.

A3. Plant cells have a rigid cell wall made of cellulose, while animal cells do not have a cell wall and are bounded only by a plasma membrane.

🔑 Key Points:

  • Plant cells → cell wall present
  • Animal cells → cell wall absent
  • Cell wall gives rigidity

Q4. What is the middle lamella and what is it made of?

A4. The middle lamella is a thin layer between two plant cells that holds them together. It is mainly made of calcium pectate.

🔑 Key Points:

  • Acts as cementing layer
  • Rich in calcium pectate

Q5. What are plasmodesmata? State their function.

A5. Plasmodesmata are cytoplasmic channels connecting neighbouring plant cells. They allow direct transport and communication between cells.

🔑 Key Points:

  • Cytoplasmic connections
  • Enable cell-to-cell communication
  • Important for coordination in plants