Cell: Structure and Functions - Complete Guide for Class 11 Biology

Overview of Cell: Structure and Functions

Cells form the structural and functional units of all living organisms. Class 11 NCERT Biology introduces the cell as the smallest unit capable of independent life. Cells can be broadly classified into prokaryotic and eukaryotic types. Eukaryotic cells, which include plant and animal cells, have membrane-bound organelles that perform specific functions. Understanding cell structure helps explain how life processes like growth, metabolism, and reproduction occur.

Key features of cells include:

• Presence of a plasma membrane
• Cytoplasm containing organelles
• Genetic material housed in the nucleus (in eukaryotes)

This chapter lays the foundation for advanced topics in biology by detailing the cell’s architecture and functions.

Nucleus: The Control Centre of the Cell

The nucleus is a prominent, membrane-bound organelle found in eukaryotic cells. It was first described by Robert Brown in 1831. The nucleus regulates cellular activities by controlling gene expression and DNA replication.

Structure of Nucleus:

• Surrounded by a nuclear envelope with two parallel membranes separated by a perinuclear space of 10–50 nm
• Nuclear pores allow selective exchange of RNA and proteins between nucleus and cytoplasm
• Contains nucleoplasm, chromatin (DNA + proteins), and nucleolus

Functions:

• Stores genetic material (DNA)
• Controls protein synthesis via mRNA
• Nucleolus synthesizes ribosomal RNA

During cell division, chromatin condenses to form visible chromosomes. The centromere is a primary constriction on chromosomes that holds chromatids together and attaches spindle fibers.

Cell Membrane and Cytoplasm: Gatekeepers and Workspaces

The cell membrane, also called the plasma membrane, encloses the cell and regulates the movement of substances in and out. It is selectively permeable, maintaining the internal environment.

Features of Cell Membrane:

• Composed of a phospholipid bilayer with embedded proteins
• Fluid mosaic model describes its dynamic structure

Cytoplasm:

• Jelly-like fluid called cytosol
• Contains organelles and cytoskeleton
• Site of many metabolic reactions

Together, the membrane and cytoplasm support cellular integrity and function.

Mitochondria: The Powerhouses of the Cell

Mitochondria are double membrane-bound organelles responsible for producing cellular energy.

Structure:

• Outer membrane is smooth
• Inner membrane folds into cristae to increase surface area
• Contains mitochondrial matrix with enzymes, DNA, and ribosomes

Function:

• Site of aerobic respiration
• Produces ATP, the energy currency of the cell

Worked Example:

If 1 glucose molecule yields 36 ATP molecules during cellular respiration, calculate ATP yield from 5 glucose molecules.

$$5 \times 36 = 180 \text{ ATP molecules}$$

This energy supports all cellular activities.

Plastids: Types and Functions in Plant Cells

Plastids are double membrane-bound organelles found mainly in plant cells and some protists. They contain pigments and perform functions such as photosynthesis and storage.

Types of Plastids:

Chloroplast Structure:

• Lens-shaped, 5–10 µm long
• Inner membrane encloses stroma
• Thylakoids arranged in grana
• Site of light reactions in photosynthesis

Plastids are crucial for plant survival and energy production.

Ribosomes and Endoplasmic Reticulum: Protein Factories and Transporters

Ribosomes are small particles either free in the cytoplasm or attached to the endoplasmic reticulum (ER). They are the sites of protein synthesis.

Ribosomes:

• Made of rRNA and proteins
• Size: 70S in prokaryotes, 80S in eukaryotes

Endoplasmic Reticulum:

• Rough ER has ribosomes; synthesizes and transports proteins
• Smooth ER lacks ribosomes; involved in lipid synthesis and detoxification

Together, these organelles ensure proteins are synthesized and transported efficiently within the cell.

Comparison of Plant and Animal Cells

Understanding differences between plant and animal cells is essential for Class 11 students.

This comparison helps identify cell types under a microscope and understand their functions.