Ravela Da Cruz

CBSE Class 11 Biology Chapter 8 Revision Notes

A cell is the most basic unit of life. It consists of the cell membrane, which encloses the cytoplasm and other organelles.

Chapter 8: Cell: The Unit of Life Revision Notes

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Multiple Choice Questions

  1. Ribosomal RNA is actively synthesized within the _______________

  2. The structures that are formed by stacking of organized flattened membranous sacs in the chloroplasts are called ________________

  3. The osmotic expansion of a cell kept in water is mainly regulated by _______________

  4. Which one of these is not a constituent of cell membrane? ___________

  5. Peptide synthesis inside a cell takes place within the ______________

  • The term cell was obtained from the Latin word “cellula,” which means a hollow space.
  • It was discovered in 1665 by a British Scientist named Robert Hooke.
  • He observed a piece of cork under a microscope and found compartments that looked like honeycombs which he called cells.
  • The study of cells is called cytology.

What is the basic unit of life?

  • All living organisms are made of cells. It can be a single cell for unicellular organisms or many cells for multicellular organisms.
  • A cell is composed of a cell membrane surrounding various organelles such as a nucleus, endoplasmic reticulum, Golgi apparatus, chloroplasts, etc.
  • Some cells are very specific and have certain functions and characteristics.

Why are cells called the building blocks of an organism?

  • All cells are broadly grouped as prokaryotic or eukaryotic cells.
  • Prokaryotic cells neither possess a membrane-bound nucleus nor any membrane-bound organelles.
  • Prokaryotic cells include different forms of Bacteria and Archaea.
  • Eukaryotic cells include a membrane-bound nucleus and other membrane-bound cell organelles.
  • Examples of eukaryotic cells are animal cells, plant cells, fungi, protists, etc.
  • Cells make up tissues which then form organs.
  • Many organs together make up an organ system like the digestive, nervous, reproductive, etc.
  • Many systems put together make up an organism.

The basic function of a cell

  • Any individual cell can synthesize the nutrients and energy needed to survive.
  • It can replicate and produce more cells and thus have its progeny.
  • There are a variety of chemical reactions which take place within a cell.
  • Multicellular organisms have differentiated cells that perform different functions such as digestion, respiration, photosynthesis, reproduction, etc.
  • A cell is designed to metabolize its food and expel its waste products.
  • There is a cooperation between different cells as various reactions occur to sustain the living organism.

Structure of a cell

  • The cell is divided into the nucleus and the cytoplasm.
  • The nucleus is at the center of the cell and is circular or oval-shaped.
  • The cytoplasm surrounds the nucleus, called the cytoplasmic solution or the cytosol.
  • It contains the cell organelles.
  • The cell membrane or the plasma membrane acts as the barrier between other cells and intra and extracellular fluids.
  • It also holds adjacent cells within a tissue.
  • A cell membrane is composed of a double layer of lipids and phospholipids with proteins within them.
  • The proteins help to move other molecules across the membrane.

Cell membrane

Source: Cell membrane parts

  • The membrane is divided into a bimolecular layer of polar and nonpolar parts.
  • The nonpolar part is inward and away from the water, and the polar parts are situated outwards towards the extracellular fluid.
  • The phospholipid molecules are not attached chemically and are therefore free to move independently.
  • It makes the bilayer flexibly fluid.
  • It can be envisioned as a double layer of pins where the pinheads are outside and the needle parts point to the inside.
  • It is called the fluid mosaic model of a cell membrane.
  • The cholesterol molecules present in the membrane help transport particles to various cell organelles by forming vesicles.

There are two types of proteins in the cell membrane, which are:

  1. Peripheral membrane proteins are present on the surface of the membrane, which associates with other cytoskeletal elements and thus affects the motility and shape of the cell.
  2. Integral membrane proteins are present throughout the width of the membrane and cross through the polar and nonpolar parts of the cell membrane.
  • A layer of monosaccharides is present on the surface of the membrane and is connected to the proteins and the membrane lipids.
  • It is called the “glycocalyx” and is crucial for recognizing cells intracellularly.
  • Membrane junctions are integrins which are proteins within membranes, and they connect to specific proteins on the other cells placed adjacently or to proteins present extracellularly.
  • They differentiate between cells of different tissues.

Cell organelles

Cell organelles

Source: Cell organelles

  • Nucleus is the largest membrane-bound organelle and contains genetic material such as DNA and RNA.
  • The nucleolus is a filamentous part containing RNA and protein constituents of the ribosomes, and it is present inside the nucleus.
  • The endoplasmic reticulum(ER) is a network of membranes that packs proteins for the Golgi apparatus.
  • Golgi apparatus is a sack that sorts proteins into vesicles.
  • Ribosomes consist of proteins and RNA and are sites of the synthesis of proteins.
  • Mitochondria are sites of energy synthesis and other chemical processes.
  • Endosomes direct vesicles by either fusing with them or pinching them.
  • Lysosomes digest cell debris and pathogenic bacteria and help the immune system.
  • Peroxisomes help in chemical reactions by removing the hydrogen from many molecules like hydrogen peroxide.

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