Cell: Structure and Functions - Class 11 NCERT Biology Guide

Overview of Cell Structure and Its Importance

A cell is the basic structural and functional unit of all living organisms. Understanding the cell: structure and functions is essential for Class 11 students as it forms the foundation for advanced biology topics.

• Cells can be broadly classified into prokaryotic and eukaryotic types.
• Eukaryotic cells, which include plant and animal cells, have membrane-bound organelles.
• The cell’s structure enables it to perform vital functions such as metabolism, growth, and reproduction.

Studying the cell helps us understand how life operates at a microscopic level and connects to larger biological systems.

Cell Membrane: The Selective Barrier

The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that separates the cell’s interior from its external environment.

• It mainly consists of a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails inward.
• Cholesterol molecules within the bilayer provide stability and fluidity.
• Proteins embedded in the membrane are of two types:
• Integral proteins (partially or fully embedded)
• Peripheral proteins (attached to the surface)

The fluid mosaic model describes this dynamic structure, allowing lateral movement of lipids and proteins essential for cell functions like growth, secretion, and division.

Transport Mechanisms

• Passive transport: Diffusion and osmosis without energy
• Facilitated diffusion: Carrier proteins help polar molecules cross
• Active transport: Energy-dependent processes like the Na+/K+ pump

Understanding the cell membrane is crucial for grasping how substances enter and exit cells.

Nucleus: Control Centre of the Cell

The nucleus is a prominent organelle that controls all cellular activities by regulating gene expression.

• Discovered by Robert Brown in 1831.
• Surrounded by the nuclear envelope, which has two membranes separated by a perinuclear space (10–50 nm).
• The envelope contains nuclear pores that allow selective exchange of RNA, proteins, and molecules between nucleus and cytoplasm.
• Inside the nucleus, the nucleolus is the site of ribosomal RNA synthesis.
• Chromatin (DNA and proteins) condenses into visible chromosomes during cell division.

The nucleus stores genetic information and directs protein synthesis, making it essential for cell function and inheritance.

Cytoplasm and Cell Organelles: The Functional Units

The cytoplasm is the jelly-like fluid that fills the cell and holds all organelles in place.

Key organelles and their functions include:

• Mitochondria: Powerhouses producing energy (ATP) via cellular respiration.
• Ribosomes: Sites of protein synthesis, found free or attached to the endoplasmic reticulum.
• Endoplasmic Reticulum (ER): Rough ER has ribosomes for protein synthesis; Smooth ER synthesizes lipids.
• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
• Lysosomes: Contain enzymes for digestion of cellular waste.
• Chloroplasts (in plants): Conduct photosynthesis to produce food.

Each organelle performs specific functions that contribute to the cell’s survival and efficiency.

Plant vs Animal Cells: Key Differences

Understanding the differences between plant and animal cells is important for Class 11 students studying cell: structure and functions.

These differences reflect the distinct roles plant and animal cells play in their respective organisms.

Worked Example: Calculating Osmosis Rate in Cells

Osmosis is the movement of water across a selectively permeable membrane from a region of low solute concentration to high solute concentration.

Example: A potato piece is placed in a sugar solution. If the potato loses 2 grams of mass due to water movement in 30 minutes, calculate the rate of osmosis.

Solution:

• Mass lost = 2 g
• Time = 30 minutes

Rate of osmosis = Mass change / Time = $\frac{2 \text{ g}}{30 \text{ min}} = 0.067 \text{ g/min}$

This simple calculation helps understand how water moves across cell membranes, a key function of the cell membrane.