Cell : The Unit of Life

FEATURE

PROKARYOTIC CELL

EUKARYOTIC CELL

Nucleus

Absence of well-defined nucleus

Presence of well-defined nucleus with a nuclear membrane

Nucleolus

Absent

Present

Genetic material

Presence of a single length of only DNA

Presence of several lengths of DNA wound around certain proteins

Ribosomes

Presence of smaller ribosomes

Presence of larger ribosomes

CHARACTERISTICS

FUNCTIONS

Plasma membrane

• Very thin, flexible and delicate living semi-permeable membrane
• Lipid bilayer, integral and peripheral proteins

• Acts as an effective barrier
• Regulates the entry of certain solutes and ions

Cell wall (only in plant cells)

• Freely permeable, mainly composed of cellulose
• Consists of primary and secondary walls and middle lamella

• Gives rigidity and shape to plant cells
• Provides protection

Cytoplasm

• Contains a mixture of water and soluble organic and inorganic compounds and various cell organelles

•  Seat of occurrence of glycolysis (production of pyruvic acid)

Endoplasmic reticulum

• May be smooth (SER) or rough (RER)

• Acts as a supportive framework of the cell
• RER involved in protein synthesis and secretion
• SER involved in synthesis of glycogen, cholesterol, fat and oil

Mitochondria

• Double-walled, inner wall thrown into folds called cristae
• Known as powerhouses of the cell

• Seat of aerobic respiration
• Synthesise respiratory enzymes and energy-rich compounds

Golgi apparatus (in animal cells)

Dictyosomes (in plant cells)

• Consists of a set of membrane-bound, fluid-filled vesicles and vacuoles
  
  

• Synthesis of plasma membrane, cell wall
• Synthesis and secretion of enzymes and hormones

Ribosomes

• Single-walled, dense, spherical bodies composed mainly of RNA and proteins
• Known as protein factories of the cell

•  Synthesis of proteins

Lysosomes

• Membrane-bound vesicles containing 40 different types of hydrolytic enzymes
  
  

•  Intracellular digestion

Centrosomes

• Contains one or two centrioles which are surrounded by radiating microtubules to form an aster shape
  
  

•  Initiates and regulates cell division

Plastids

• Double membrane, proteinaceous matrix containing DNA and disc-like structures called thylakoids containing chlorophyll
  
  

• Chromoplasts: Impart colour to flowers and fruits
• Chloroplasts: Trap solar energy for photosynthesis
• Leucoplasts: Store starch

Nucleus

• Mostly spherical and dense, surrounded by nuclear membrane with pores

• Nuclear matrix contains nucleolus and chromatin

• Nucleoli are spherical structures, one or more in number

• Chromatin fibres are a network of thread-like structures made of DNA
  
  

• Regulates cell cycle and cell functions
• Nucleolus participates in protein synthesis by forming and storing RNA
• Chromosomes carry hereditary information or genes

Vacuoles

• Non-living, membrane-bound structures

• Storage of water and other substances, food, pigments and waste products

Granules

• Small particles, crystals or droplets
  
  

• Starch (in plant cells), glycogen (in animal cells) and fat-containing granules serve as food for the cell

Cytoskeleton

• Filamentous proteinaceous structures present in the cytoplasm
  
  

• Mechanical support, motility and maintenance of shape of the cell

Cilia and Flagella

• Hair-like outgrowths of the cell membrane
  
  

•  Work like oars causing movement

Microbodies

• Membrane-bound minute vesicles
  
  

•  Contain various enzymes