CELL ASSOCIATED STRUCTURES

• Every multicellular organism originates from a single cell.
• While observing cells under a microscope, stains like iodine, safranin, or methylene blue are used to clearly visualize different organelles.
• All cells possess three basic components:
  – Cell membrane
  – Cytoplasm
  – Genetic material

DISCOVERY OF CELL (1665)
• Robert Hooke is known as the Father of Cytology.
• While examining a thin slice of cork using a self-designed microscope, he observed honeycomb-like hexagonal compartments.
• He named these compartments “cells”.
• This was the first observation indicating that living organisms are made up of discrete structural units.

DISCOVERY OF LIVING CELL (1674)
• Anton van Leeuwenhoek, known as the Father of Bacteriology, discovered living cells.
• He observed bacteria, protozoa, and other microscopic organisms.

DISCOVERY OF NUCLEUS (1831)
• Robert Brown discovered the nucleus.
• The term “protoplasm” was coined by Purkinje in 1839 for the fluid substance inside the cell.

CELL ORGANELLES

1. ENDOPLASMIC RETICULUM (ER)
• ER functions as a transport system and manufacturing surface.
• It is a network of membrane-bound tubules and vesicles.
• ER membrane resembles plasma membrane.
• Prokaryotes and RBCs lack ER.
• Cells synthesizing large amounts of proteins (liver, pancreas) have abundant ER.

Types of ER:
Rough Endoplasmic Reticulum (RER)
• Ribosomes attached to its surface.
• Site of protein synthesis.
• Appears as flattened sacs.
• Proteins are transported to different parts of the cell.
• RER is attached to the nuclear envelope.

Smooth Endoplasmic Reticulum (SER)
• Lacks ribosomes.
• Involved in lipid and fat synthesis.
• Plays a role in membrane biogenesis.
• Detoxifies drugs and poisons in liver cells.
• Forms an internal transport network.

2. GOLGI APPARATUS
• Discovered by Camillo Golgi.
• Composed of flattened membrane-bound cisternae.
• Modifies, packages, and transports proteins and lipids.
• Converts simple sugars into complex sugars.
• Involved in lysosome formation.
• Packages materials into vesicles for storage or secretion.

3. LYSOSOMES
• Membrane-bound sacs filled with digestive enzymes.
• Enzymes are synthesized in RER.
• Digest worn-out organelles and foreign particles.
• Known as “suicide bags” of the cell.
• Rare in plant cells due to presence of cell wall.

4. MITOCHONDRIA
• Double-membraned organelles.
• Outer membrane is porous; inner membrane forms folds called cristae.
• Site of cellular respiration.
• Produces ATP – energy currency of the cell.
• Possess their own DNA and ribosomes.
• Cells requiring more energy contain more mitochondria.
• Regulate calcium ion concentration.

5. PLASTIDS (ONLY IN PLANT CELLS)
• Similar in structure to mitochondria.
• Contain their own DNA and ribosomes.
• Embedded in stroma.

Types of Plastids:
• Chromoplasts – coloured plastids
• Leucoplasts – colourless, store starch, oil, and proteins

Chloroplasts
• Contain chlorophyll.
• Site of photosynthesis.
• Convert light energy into chemical energy.
• Structure includes:
  – Outer and inner membranes
  – Thylakoids stacked into grana
• Light reactions occur in thylakoids.
• Dark reactions occur in stroma.

6. VACUOLES
• Large central vacuole in plant cells.
• Occupies 50–90% of plant cell volume.
• Stores water, sugars, amino acids, proteins, wastes.
• Maintains turgidity.
• Food vacuoles present in unicellular organisms.
• Contractile vacuoles expel excess water.

OTHER IMPORTANT STRUCTURES (NOT ORGANELLES)

CYTOSKELETON
• Network of protein filaments and tubules.
• Provides shape, structure, and movement.

RIBOSOMES
• Non-membranous structures.
• Composed of RNA and proteins.
• Site of protein synthesis.
• Found in both prokaryotic and eukaryotic cells.
• Consist of two subunits.

UNIQUE STRUCTURES IN SOME CELLS

CILIA
• Hair-like structures.
• Found in respiratory tract.
• Help trap dust and expel mucus.

FLAGELLA
• Tail-like structure for movement.
• Found in bacteria.
• Human sperm cell has flagellum.

DIFFERENCES BETWEEN ANIMAL AND PLANT CELLS

Animal Cell
• No cell wall
• Irregular or round shape
• Centrosomes present
• Plastids absent
• Small or absent vacuoles
• Cilia present
• Numerous mitochondria
• Heterotrophic nutrition
• Single complex Golgi apparatus

Plant Cell
• Cell wall present
• Rectangular shape
• Centrosomes absent
• Plastids present
• Large central vacuole
• Cilia absent
• Fewer mitochondria
• Autotrophic nutrition
• Golgi present as dictyosomes

SIMILARITIES BETWEEN PLANT AND ANIMAL CELLS
• Plasma membrane present
• Ribosomes present
• Nucleus present
• Endoplasmic reticulum present
• Golgi apparatus present
• Cytoplasm present

TYPES OF PLANT CELLS
• Phloem cells – transport food
• Xylem cells – transport water
• Sclerenchyma – provide rigidity
• Collenchyma – provide support
• Parenchyma – storage

TYPES OF ANIMAL CELLS
• Nerve cells – transmit impulses
• Skin cells – protection
• Blood cells – transport gases and nutrients
• Muscle cells – movement
• Bone cells – support and structure

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Subject: Science

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