cell the unit of life

• Permeable


  Allowing the cell to enter and leave without hindrance
• The plant cell shown in Fig. 26 also shows a portion of the cell walls of six surrounding cells
• Cell walls


  Thick grey lines holding the cells together
• Pits and pores


  Openings in the cell walls
• Cytoplasm


  A semi-liquid substance occupying most of the cell within the cell membrane
• Under a compound microscope, cytoplasm appears colourless, partly transparent and somewhat watery
• Many chemical reactions take place in the cytoplasm
• Living cytoplasm is always in a state of some movement
• Cell organelles embedded in the cytoplasm

  • Endoplasmic reticulum
  • Ribosomes
  • Mitochondria
  • Golgi apparatus
  • Lysosomes
  • Centrosome (only in animal cells)
  • Plastids (only in plant cells)
• Endoplasmic reticulum (ER)


  • An irregular network of double membranes distributed over the entire cytoplasm in a cell
  • Connected to the cell membrane at its outer end
  • Connected to the nuclear membrane at its inner end
  • Appears rough when small granules called ribosomes are attached to it, and appears smooth without them
  • Forms the supporting framework of the cell and also serves as a pathway for the distribution of materials from one part of the cell to the other
• Ribosomes


  • Numerous small granules either scattered freely in the cytoplasm or attached to the membranes of the endoplasmic reticulum
  • The 'factories' for the synthesis of proteins
• Mitochondria


  • Spherical or rod-shaped bodies with double walls and finger-like processes projecting inwards (called cristae)
  • The sites where cell respiration occurs to release energy
  • Store energy in the form of ATP (adenosine triphosphate) and are called "power houses of the cell"
  • Contain their own DNA which can replicate
• Golgi apparatus


  • Occurs in the form of granules, filaments or rods
  • Originated from endoplasmic reticulum
  • Consists of many small groups of hollow tubular structures with membranous walls, called cisternae, and is associated with some minute vesicles and vacuoles
  • Concerned with the secretions of the cell including enzymes, hormones and their packaging and transport to different parts within the cell
• Plastids


  • Special organelles in plant cells in different shapes - oval, spherical and disc-shaped
  • Classified as leucoplasts, chromoplasts and chloroplasts based on their colour
• Leucoplasts


  Colourless plastids that store starch
• Chromoplasts


  • Coloured plastids - yellow, orange and red
  • Contain pigments like xanthophyll and carotene
• Chloroplasts


  • Green-coloured plastids that contain the pigment chlorophyll
  • Function is to trap solar energy and aid in the manufacture of starch and sugar during photosynthesis
  • Contain DNA and the capacity to divide
• Chloroplasts are not the "label" of the cell, as some people describe them
• Green tomatoes and unripe chillies

  Contain chlorophyll, which degenerates during ripening and the masked red (carotene) takes over
• Non-living substances or Cell inclusions

  • Granules
  • Vacuoles
• Granules


  Small particles in the cytoplasm believed to contain food materials like starch, glycogen and fats
• Vacuoles


  • Dead spaces in the cytoplasm filled with water and various substances in solution
  • Larger in plant cells and called cell sap, smaller in animal cells
• Nucleus


  • The most important part of the cell, regulates and coordinates various processes
  • Plays an important part in all cell divisions
  • Contains factors (genes) which determine heredity
• Nuclear membrane


  A double membrane enclosing the nucleus
• Nucleoplasm


  The dense material within the nucleus
• Chromatin fibres


  Thread-like structures that become thick and bead-like during cell division, called chromosomes
• Nucleolus


  Participates in protein synthesis of ribosomes
• The number of chromosomes is definite in each species
• Chromosomes carry the genetic characters from the parents to the offspring</b>
• Chromosomes


  • Made of chromatin, which is composed of hereditary units called genes
  • Genes are made of DNA (deoxyribonucleic acid)
• It is the genes, not the number of chromosomes, that determine the characteristics of a species
• DNA fingerprinting helps in the identification of an individual and can even testify the parentage
• Differences between plant and animal cells

  • Plant cells have a definite cell wall made of cellulose
  • Plant cells lack a centrosome
  • Plant cells have prominent vacuoles, while animal cells have smaller vacuoles or none
  • Plant cells contain plastids, animal cells do not
  • Plant cells are usually large with distinct organelles, animal cells are smaller with less distinct organelles
  • Plant cell cytoplasm is less dense, animal cell cytoplasm is more dense and granular
  • Plant cells have only a thin lining of cytoplasm, mostly pushed to the periphery
• Protoplasm


  • The living substance in an organism, contained in the cells
  • A translucent, somewhat colourless, greyish or brownish fluid
  • Composed of water, proteins, carbohydrates, fats and mineral salts
• It is impossible to make an accurate chemical analysis of protoplasm because it ceases to be protoplasm as soon as it is removed from the organism