Cell structure

• Cell


  The basic unit of structure and function in living organisms
• Cell theory


  • Cells are the building blocks of structure in living organisms
  • Cells are the smallest unit of life
  • Cells are made from other cells by division
  • Cells are the site of all chemical reactions of life
  • Cells contain a blueprint (information) about their growth, development and behaviour
• Organelle


  A distinct part of a cell which has a particular structure and function
• Features of organisms


  • Cytoplasm
  • Cell membrane
  • DNA
  • Ribosomes
  • Enzymes
• Cytoplasm


  Where most chemical reactions take place
• Cell membrane


  Keeps cell contents in place and controls what enters/leaves the cell
• DNA


  The genetic material that contains units of inheritance passed from one generation to the next
• Ribosomes


  Cell structures whose function is to form proteins
• Enzymes


  Speed up the rate of chemical reactions in cells
• Nucleus


  • Visible under light microscope
  • Largest cell organelle, approx 10 μm in diameter
  • Surrounded by a double membrane (envelope)
  • Controls all cellular activities by coding for the synthesis of proteins and enzymes
• Nucleolus


  • Visible under light microscope
  • Function is to form ribosomes
• Cytoplasm


  The living contents of eukaryotic cell divided into the nucleus and the cytoplasm, the two forming the protoplasm
• Endoplasmic Reticulum (ER)


  • Visible under electron microscope
  • Consists of membrane bound sacs called cisternae
  • Two types: Rough ER (covered by ribosomes) and Smooth ER (synthesises lipids and steroids)
• Ribosomes


  • Visible under electron microscope
  • Two types: 70S in prokaryotes and eukaryotes, 80S in eukaryotes only
  • Diameter of approx 20nm, made of protein and ribosomal RNA
  • Site of protein synthesis, attached to RER for secreted proteins, free for proteins used within the cell
• Golgi Apparatus


  • Visible under light microscope
  • Consists of flattened membrane bound sacs called cisternae
  • Dynamic structure, formed by fusion of vesicles from RER and vesicles budding off from it
  • Functions to transport and modify materials, e.g. add carbohydrates to proteins, secrete carbohydrates, form lysosomes
• Lysosomes


  • Occur in most eukaryotic cells
  • Surrounded by a single membrane and contain hydrolytic (digestive) enzymes
  • Involved in digestion of material in food vacuoles and worn out organelles
  • Diameter of 0.2-0.5 μm
  • In plant cells, the large permanent vacuole may act as a lysosome
• Mitochondrion


  • Rod shaped organelles that occur in all eukaryotes
  • Double membrane (envelope) enclosing the matrix
  • Inner membrane is folded to form cristae
  • Has 70S ribosomes and a ring of DNA to synthesise some of its own proteins
  • Contains ATP synthase molecules on the inner membrane to synthesise ATP
  • Function is to carry out aerobic respiration
• Chloroplasts


  • Occur in eukaryotic cells (plants)
  • Biconvex discs, 3-5 μm in diameter, bound by a double membrane (envelope)
  • Ground substance is the stroma, with a system of thylakoid membranes containing chlorophyll
  • Thylakoid membranes form stacks called grana
  • Has 70S ribosomes and a ring of DNA to synthesise some of its own proteins
  • Site of photosynthesis, with light dependent reactions on the thylakoids and light independent reactions in the stroma
• Microtubules


  • Occur in eukaryotes, visible under electron microscope
  • Hollow cylinders made up of helically arranged tubulin subunits, diameter of 24nm
  • Functions: form centrioles, cilia and flagella, involved in movement of organelles, form cytoskeleton
• Centrioles


  • Hollow structures that occur in pairs in animal cells (absent in plant cells)
  • Each centriole has nine triplets of microtubules
  • Function is to form cilia and flagella
• Cell wall


  In addition to the cell membrane, plant cells have a cellulose cell wall with high tensile strength
• Functions of cell walls


  • Provide mechanical support for the cell and plant
  • Allow plant cells to become turgid when they absorb water
  • Form a pathway (apoplast) for water flow
  • Secrete the cuticle that reduces water loss
• Plasmodesmata


  Cytoplasmic strands linking two plant cells, allowing substance exchange
• Vacuoles


  • Fluid filled sacs bounded by a single membrane
  • Animal cells have small temporary vacuoles, plant cells have a large permanent vacuole containing cell sap
  • Functions: enable cell turgidity, store pigments, digest old cells, store waste, store food
• Microvilli


  Microscopic extensions of some animal cells that increase surface area for absorption
• Differences between animal and plant cells

  • Smaller size in animal cells
  • Centrioles present in animal cells, absent in plant cells
  • No cell wall in animal cells, cell wall present in plant cells
  • Food stored as glycogen in animal cells, starch in plant cells
  • Small temporary vacuoles in animal cells, large permanent vacuole in plant cells
  • No flagella/cilia in plant cells, sometimes present in animal cells
  • No plasmodesmata in animal cells, present in plant cells
  • No microvilli in plant cells, sometimes present in animal cells
  • No chloroplasts in animal cells, present in plant cells
  • Secretory vesicles present in animal cells, absent in plant cells
  • No middle lamellae in animal cells, present in plant cells
• Eukaryotes


  Organisms whose cells have a nucleus
• Prokaryotes


  Organisms whose cells lack nuclei
• Structures in a prokaryotic cell

  • Cell wall made of peptidoglycans
  • Circular DNA as genetic material
  • 70S ribosomes for protein synthesis
  • Flagellum for locomotion
  • Mesosomes for increased surface area for respiration
  • Plasmids for antibiotic resistance
  • Capsule for protection
  • Pili for attachment and conjugation
• Viruses


  Infective agents consisting of nucleic acid (DNA or RNA) in a protein coat, able to multiply only within living host cells
• Fluid mosaic model of cell membrane

  • Fluid because phospholipids and some proteins can move within the membrane
  • Mosaic because proteins give a dotted appearance when viewed from above
  • Width of about 7.5nm
  • Largely made of phospholipids in a bilayer, with proteins that traverse the whole width
• Surface membrane


  • Increase surface area for respiration
• Plasmids


  Small rings of DNA that enable bacteria to resist certain antibiotics
• Capsule


  • Has a protective function, limits the ability of phagocytes to engulf bacteria
• Pili


  • Enable bacteria to stick to surfaces and are involved in conjugation (sexual reproduction)
• Viruses


  Infective agents that typically consist of a nucleic acid (DNA or RNA) molecule in a protein coat, are too small to be seen by light microscopy, and are able to multiply only within the living cells of a host
• Virus DNA or RNA


  Takes over the protein synthesising machinery of the host cell to make new virus particles
• Cell surface membrane


  • Fluid because the phospholipids and some proteins can move within the membrane
  • Mosaic because when the membrane is viewed from above, the proteins give a dotted appearance
  • Width of the membrane is about 7.5 nm
  • Largely made up of phospholipids that form a bilayer
• Intrinsic (integral) proteins


  • Traverse the whole width of the membrane
• Extrinsic proteins


  • On the outer/inner part of the membrane