Biology Chapter 5: The Fundamental Unit of Life

The Fundamental Unit of Life

• The cell is the basic unit of life, discovered by Robert Hooke in 1665.
• Hooke observed the cells in a cork slice with the help of a primitive microscope.
• Later, Antonie van Leeuwenhoek (1674) discovered free-living cells in pond water, and Robert Brown (1831) discovered the nucleus in a cell.

What are Living Organisms Made Up Of?

• All living organisms are made up of cells, which are the basic building units of life.
• Robert Hooke coined the term "cell" to describe these units, and it is still used today in biology.
• Cells can be observed under a microscope, and they are the structural and functional units of life.

Characteristics of Cells

• Cells are the basic units of life, and all living organisms are made up of one or more cells.
• Cells can be single-celled (unicellular) or multi-celled (multicellular) organisms.
• Unicellular organisms, such as Amoeba, Chlamydomonas, and bacteria, are capable of performing all basic functions of life.
• Multicellular organisms, such as plants and animals, have different cells performing different functions.

Cell Structure and Organisation

• A cell has three main features: plasma membrane, nucleus, and cytoplasm.
• The plasma membrane is the outermost covering of the cell, which separates the cell's contents from its external environment.
• The nucleus is the control center of the cell, where genetic information is stored.
• Cytoplasm is the fluid substance inside the cell, where various cell organelles perform specific functions.

Cell Organelles and Functions

• Each cell organelle performs a specific function, such as making new material, clearing waste, and regulating cell activities.
• All cells, regardless of their function or organism, have the same basic organelles.
• Cell organelles work together to constitute the basic unit of life, the cell.

Cell Membrane and Transport

• The plasma membrane is selectively permeable, allowing certain substances to enter or leave the cell.
• Diffusion is the process of movement of substances from a region of high concentration to a region of low concentration.
• Osmosis is a special case of diffusion, where water molecules move across a selectively permeable membrane.
• Cells can gain or lose water through osmosis, depending on the concentration of the surrounding solution.

Cell Wall and Nucleus

• Plant cells have an additional rigid outer covering called the cell wall, which lies outside the plasma membrane.
• The cell wall is mainly composed of cellulose and provides structural strength to plants.
• The nucleus is the control center of the cell, where genetic information is stored.### Cellular Structure and Functions
• Cellular Structure:
  • Cells have a cell wall, plasma membrane, cytoplasm, and nucleus.
  • Prokaryotic cells lack a nuclear membrane and membrane-bound organelles, while eukaryotic cells have these features.

Plasmolysis

• Plasmolysis:
  • Occurs when a living plant cell loses water through osmosis, causing the cell contents to contract away from the cell wall.
  • Can be observed by mounting a Rhoeo leaf in a sugar or salt solution and observing under a microscope.

Cell Organelles

• Nucleus:
  • Contains chromosomes, which are visible as rod-shaped structures only during cell division.
  • Chromosomes contain DNA molecules, which have the information necessary for constructing and organizing cells.
• Endoplasmic Reticulum (ER):
  • A large network of membrane-bound tubes and sheets.
  • Divided into two types: Rough Endoplasmic Reticulum (RER) with ribosomes attached and Smooth Endoplasmic Reticulum (SER) involved in lipid synthesis.
• Golgi Apparatus:
  • A system of membrane-bound vesicles that modify, package, and dispatch products synthesized near the ER.
  • Consists of a complex cellular membrane system.
• Lysosomes:
  • Membrane-bound sacs filled with digestive enzymes that break down foreign materials, worn-out organelles, and cellular waste.
  • Known as the "suicide bags" of the cell, as they can break down the cell itself if needed.
• Mitochondria:
  • Known as the "powerhouses" of the cell, generating energy for the cell through ATP production.
  • Have their own DNA and ribosomes, allowing them to make some of their own proteins.
• Plastids:
  • Present only in plant cells, with two types: chromoplasts (colored) and leucoplasts (white or colorless).
  • Chloroplasts contain chlorophyll and are responsible for photosynthesis.
• Vacuoles:
  • Membrane-bound sacs that store cell sap, providing turgidity and rigidity to plant cells.
  • Involved in storage and digestion of cellular waste and foreign materials.

Cell Division

• Cell Division:
  • The process by which new cells are made, allowing organisms to grow, replace old cells, and form gametes for reproduction.### Cell Structure
• The cell membrane is an active part of the cell, regulating the movement of materials between the cell and its environment.
• In plant cells, a cell wall composed mainly of cellulose is located outside the cell membrane, allowing cells to exist in hypotonic media without bursting.
• The nucleus in eukaryotes is separated from the cytoplasm by a double-layered membrane and directs the life processes of the cell.

Organelles

• The ER functions as a passageway for intracellular transport and as a manufacturing surface.
• The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification, and packaging of substances manufactured in the cell.
• Plastids are membranous organelles found in plant cells, of which there are two types: chromoplasts and leucoplasts.
• Chromoplasts contain chlorophyll and perform photosynthesis, while leucoplasts perform storage functions.

Vacuoles

• Vacuoles are storage sacs for solid or liquid contents.
• Plant cells have very large vacuoles, while animal cells have small-sized vacuoles.
• The central vacuole of some plant cells can occupy 50-90% of the cell volume.

Cell Division

• There are two main types of cell division: mitosis and meiosis.
• Mitosis is the process of cell division that results in two identical daughter cells, each with the same number of chromosomes as the mother cell.
• Meiosis is a process of cell division that involves two consecutive divisions, resulting in four new cells with half the number of chromosomes as the mother cell.

Cell Types

• Prokaryotic cells have no membrane-bound organelles, and their chromosomes are composed of only nucleic acid.
• Eukaryotic cells have membrane-bound organelles and chromosomes composed of nucleic acid and proteins.

Cell Functions

• Cells in organisms divide for growth of body, replacement of dead cells, and formation of gametes for reproduction.
• Cells can obtain food through various means, such as osmosis.
• Osmosis is the movement of water molecules from a high concentration to a low concentration through a selectively permeable membrane.