CBSE Class 11 Biology Notes: Chapter 8 - Cell: The Unit of Life

Key Terms and Definitions

Cell

The fundamental, structural, and functional unit of all living organisms.

Protoplasm

The living content of a cell that is surrounded by a plasma membrane. It comprises the cytoplasm and the nucleus.

Cytoplasm

The main arena of cellular activities in both plant and animal cells. It is the jelly-like substance filling the cell, enclosed by the cell membrane, excluding the nucleus.

Fluid Mosaic Model

The most accepted model for the structure of the plasma membrane, proposed by Singer and Nicolson. It describes the membrane as a quasi-fluid lipid bilayer with proteins embedded in it.

Plasmid

Small, circular, double-stranded DNA molecule that is distinct from a cell's chromosomal DNA. Commonly found in bacteria, they often carry genes for antibiotic resistance.

Glycocalyx

The outermost layer of a prokaryotic cell envelope, which can be a loose sheath (slime layer) or a thick, tough capsule. It aids in protection and adhesion.

Centromere

The primary constriction on a chromosome that holds the two sister chromatids together. Kinetochores are disc-shaped structures present on the sides of the centromere.

Endomembrane System

A group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.

The Cell Theory: A Unifying Concept

The Cell Theory is a cornerstone of modern biology. Initially proposed by Matthias Schleiden (a botanist) and Theodor Schwann (a zoologist) in 1838-39, it has been refined over time. The original theory stated that all plants and animals are composed of cells and products of cells. However, it failed to explain how new cells were formed. This gap was filled by Rudolf Virchow in 1855, who famously stated "Omnis cellula-e-cellula", meaning that all cells arise from pre-existing cells.

The modern cell theory can be summarized in three main points:

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells.

This theory unified the study of botany and zoology by identifying the cell as the common building block of all life. It highlights that no matter how diverse organisms are, their fundamental activities are carried out at the cellular level. Viruses, however, are an exception to the cell theory as they are acellular and are considered to be on the boundary of living and non-living.

Prokaryotic vs. Eukaryotic Cells

Must-Remember Facts About Cell Organelles

• {"point":"Mitochondria and chloroplasts are semi-autonomous organelles; they have their own DNA (circular), 70S ribosomes, and can replicate independently of the cell."}
• {"point":"Ribosomes are the only organelles without a membrane. They are found in both prokaryotes and eukaryotes."}
• {"point":"The Endomembrane System consists of the Endoplasmic Reticulum (ER), Golgi apparatus, Lysosomes, and Vacuoles. Their functions are coordinated."}
• {"point":"Rough Endoplasmic Reticulum (RER) has ribosomes on its surface and is involved in protein synthesis and secretion."}
• {"point":"Smooth Endoplasmic Reticulum (SER) lacks ribosomes and is the major site for synthesis of lipids and detoxification of drugs."}
• {"point":"The Golgi apparatus is the 'post office' of the cell. It modifies, sorts, and packages materials for transport. Its faces are called cis (forming) and trans (maturing)."}
• {"point":"Lysosomes are 'suicidal bags' containing hydrolytic enzymes that digest waste materials and old organelles."}
• {"point":"The Nucleus contains the cell's genetic material (chromatin) and controls all cellular activities. The nucleolus within it is the site of ribosomal RNA (rRNA) synthesis."}
• {"point":"Centrosomes, containing centrioles, are found in animal cells and help in cell division by forming the spindle apparatus. They are absent in higher plants."}

Chromosome Types Based on Centromere Position

• {"name":"Metacentric","description":"Centromere is in the middle, forming two equal arms of the chromosome. The chromosome appears 'V-shaped' during anaphase."}
• {"name":"Sub-metacentric","description":"Centromere is slightly away from the middle, resulting in one shorter arm (p arm) and one longer arm (q arm). The chromosome appears 'L-shaped' during anaphase."}
• {"name":"Acrocentric","description":"Centromere is situated close to one end, forming one extremely short arm and one very long arm. The chromosome appears 'J-shaped' during anaphase."}
• {"name":"Telocentric","description":"Centromere is at the terminal end of the chromosome, resulting in only one arm. This type is not found in humans."}

Common Board Exam Traps

Don't confuse Centromere, Centrosome, and Centriole.

• Centromere: The primary constriction on a chromosome that joins sister chromatids.
• Centrosome: An organelle in animal cells, located near the nucleus, which contains the centrioles. It organizes microtubules and provides structure for the cell.
• Centriole: A pair of cylindrical structures found inside the centrosome. They are key to forming the spindle fibers during cell division.

Remember: Centromere is on a chromosome, while Centrioles are inside a Centrosome.

Quick Revision Check

• Which organelle is known as the 'powerhouse of the cell' and why? The mitochondrion. It is the site of aerobic respiration where ATP (the energy currency of the cell) is generated.
• Name two cell organelles that are double-membraned. Mitochondria and Chloroplasts. (The Nucleus is also a double-membraned organelle).
• What is the main function of the Golgi apparatus? Modification, sorting, packaging, and transport of proteins and lipids for secretion or delivery to other organelles.
• What are the ribosomes of prokaryotes and eukaryotes, respectively? Prokaryotic ribosomes are 70S type. Eukaryotic cytoplasmic ribosomes are 80S type.

Frequently Asked Questions