Biology Chapter: Cell - The Fundamental Unit of Life

Cell: The Fundamental Unit of Life

• Cells are the basic structural and functional units of all living organisms.
• All known living organisms are made up of one or more cells.
• The cell theory states that all living cells arise from pre-existing cells by division.

Key Figures in Cell Theory

• 1674: Anton Van Leeuwenhoek observed the first living cells.
• 1665: Robert Hooke discovered the cell.
• 1835: Felix Dujardin discovered fluid content in cells.
• 1839: J.E. Purkinje named the fluid content protoplasm.
• 1838: Matthias Schleiden proposed that all plants consist of cells.
• 1845: Carl Heinrich Braun suggested that the cell is the basic unit of life.
• 1855: Rudolf Virchow proposed that all cells arise from pre-existing cells.

Types of Organisms

• Unicellular organisms consist of a single cell (e.g., Euglena, Paramecium, Yeast).
• Multicellular organisms are composed of multiple cells (e.g., Plants, Animals, Fungi).

Cell Sizes

• Smallest cell: Mycoplasma, measuring 0.1 µm.
• Largest cell: Ostrich egg, measuring 18 cm.
• Human cells vary:
  • Smallest: Sperm cell, 5 µm.
  • Largest: Ovum cell, 120 µm.
  • Longest: Nerve cell, 1 m.

Cell Shape

• Human red blood cells are circular and biconcave for efficient capillary passage.
• Nerve cells have branched shapes to conduct impulses.
• Some cells can alter their shape, while others maintain a fixed structure.

Cell Structure

• Common features in most cells: plasma membrane, nucleus, and cytoplasm.
• Plasma membrane is semi-permeable, maintaining cell homeostasis and protecting contents.
• Cell wall (non-living) provides structure, strength, and rigidity in plants and bacteria.

Nucleus

• Central dense body, usually 10-25 µm in diameter, found in most cells.
• Contains nuclear membrane with pores, nucleoplasm, nucleolus, and chromatin fibers (DNA and proteins).
• Controls cell activities, including metabolism, growth, and replication, through gene expression.

Cytoplasm

• jelly-like substance made of 80% water, containing cytosol and various organelles.
• Organelles include ribosomes, endoplasmic reticulum (smooth and rough), Golgi body, lysosomes, vacuoles, and mitochondria.

Organelles

• Endoplasmic Reticulum (ER): Network linked to the nuclear and cell membranes, includes rough ER (with ribosomes for protein synthesis) and smooth ER (involved in lipid synthesis and detoxification).
• Golgi Body: Stacks of membranes modifying and sorting materials from the ER for transport within or outside the cell.
• Lysosomes: Membrane-bound sacks filled with enzymes for digesting large molecules, pathogens, and worn-out organelles.
• Vacuoles: Storage sacs for liquids; larger in plant cells for turgidity, smaller and temporary in animal cells.
• Mitochondria: Double membrane-bound organelles involved in energy production (ATP synthesis).

Plastids

• Double membrane-bound organelles found in plants and certain algae, involved in storage and photosynthesis.### Plastids
• Plastids are organelles responsible for the synthesis and storage of food.
• Chromoplasts produce pigments affecting the color of cells.
  • Example: Carrots contain carotene; mangoes contain xanthophyll; tomatoes contain lycopene.
• Leucoplasts are colorless plastids that store food such as starch, protein, and oil.
  • Found in organs like tubers and seeds (e.g., potato tubers, maize grains, castor seeds).

Chloroplasts

• Chloroplasts are double membrane-bound organelles predominantly in plant cells.
  • Characterized by a spherical or discoidal shape with two distinct regions: grana and stroma.
  • Grana consist of stacks of thylakoids, which house chlorophyll molecules essential for photosynthesis.
  • Stroma is the dense, colorless fluid where important reactions occur.
• Functions include:
  • Converting light energy into chemical energy stored as food.
  • Providing the green color in leaves, stems, and vegetables.

Centrosome

• The centrosome is a membrane-bound organelle located near the nucleus.
  • Contains two centrioles arranged perpendicularly, made of microtubules.
• Functions:
  • Forming spindle fibers that facilitate chromosome movement during cell division.
  • Assisting in the development of cilia and flagella.

Cytoskeleton

• The cytoskeleton is composed of microtubules and microfilaments.
  • Microtubules are hollow tubes made of tubulin, while microfilaments are thin rods made of actin.
• Functions include:
  • Determining and maintaining cell shape.
  • Providing structural strength.
  • Enabling cellular movements.

Cell Types

• Prokaryotic cells lack a membrane-bound nucleus and organelles.
• Eukaryotic cells possess a membrane-bound nucleus and complex organelles.

Explore the fascinating world of cells, the building blocks of life. This quiz covers key figures in cell theory, types of organisms, and the significance of cells in living organisms. Test your knowledge of cellular structure and function.