Cell

• Types of cells based on number of cells


  • Single-celled
  • Few-celled
  • Multi-celled
• Single-celled organisms


  • Bacteria
  • Yeast
  • Amoeba
• Few-celled organisms


  • Spirogyra
  • Volvox
• Multi-celled organisms


  • Human beings
  • Mango trees
• An average-sized adult human constitutes approximately:
• Cells


  Very small and seen only with a microscope
• Smallest cells


  • Bacteria (0.3-5.0 micrometre)
  • Red blood cells (about 7 micrometre)
• Longest cells


  • Nerve cells (extending from finger tip up to the spinal cord)
• Largest cells


  • Ostrich egg (before development begins)
• Reasons cells generally remain small in size


  • Different regions of a cell can communicate with each other rapidly for the cell to function effectively
  • Cells have a large surface area / volume ratio
• Cell shapes


  Often related to the different functions they perform
• Cell shapes


  • Circular (human red blood cells)
  • Amoeboid (white blood cells)
  • Long and contractile (muscle cells)
  • Bean-shaped (guard cells of stomatal pore)

  
• White blood cells are amoeboid (amoeba-like movement, with pseudopodia) that can squeeze out through capillary walls

  
• Nerve cells are long to conduct "impulses" from distant parts of the body to the brain and vice-versa
• Muscle cells are long and contractile to pull or squeeze the parts
• Guard cells of stomatal pore in the leaves are bean-shaped to open and close the pore
• Generalised cell


  Consists of three essential parts: cell membrane, nucleus, and cytoplasm

  
• Cell organelles


  Parts of a cell that have a definite shape, structure and function, like organs in the body

  
• Cell membrane


  Living and has fine pores through which substances may enter or leave the cell
• Cell membrane composition


  Lipoproteins
• Cell membrane permeability


  Selective, allowing only certain substances to pass through while preventing others
• Cell wall


  Surrounds the cell membrane in plant cells, made of cellulose, gives shape and rigidity without interfering with cell membrane functions
• Cell wall permeability


  Freely permeable, allowing substances in solution to enter and leave the cell without hindrance
• Cotton, jute and coconut fibres are the cell walls of their dead cells
• Cytoplasm


  Semi-liquid substance that occupies most of the cell within the cell membrane
• Many chemical reactions take place in the cytoplasm
• Living cytoplasm is always in a state of some movement
• Endoplasmic reticulum (ER)


  An irregular network of double membranes distributed over the entire cytoplasm, connected to the cell membrane and nuclear membrane, provides a framework and pathway for distribution of materials
• Ribosomes


  Small granules either scattered freely in the cytoplasm or attached to the ER membranes, sites of protein synthesis
• Mitochondria


  Double-walled bags with inner walls produced into finger-like processes (cristae), sites of cell respiration to release energy in the form of ATP, the "power houses of the cell"
• Golgi apparatus


  Granules, filaments or rods originated from ER, involved in secretion, packaging and transport of enzymes, hormones, etc. within the cell
• Lysosomes


  Small vesicles containing digestive enzymes, destroy and digest foreign substances, digest stored food during unfavourable conditions, also called "suicide bags"
• Centrosome and centrioles


  Found only in animal cells, develop spindle fibres during cell division, contain two centrioles arranged at right angles
• Plastids


  Special organelles found only in plant cells, classified as leucoplasts, chromoplasts, and chloroplasts based on colour and function
• Leucoplasts


  Colourless plastids that store starch
• Chromoplasts


  Coloured plastids (yellow, orange, red) containing pigments like xanthophyll and carotene
• Chloroplasts


  Green-coloured plastids containing chlorophyll, responsible for photosynthesis
• Some colouring pigments like anthocyanins are not associated with plastids, instead dissolved in the cell sap of vacuoles
• Chloroplasts contain DNA and have the capacity to divide
• Green turns into red as chlorophyll degenerates and the masked red carotene takes over during ripening of tomatoes and chillies