cell structure

• Cell Theory


  • All living organisms are made up of one or more cells
  • Cells are the basic functional unit (i.e. the basic unit of structure and organisation) in living organisms
  • New cells are produced from pre-existing cells
• Eukaryotic cells


  • More complicated compared to prokaryotic cells
• Cell organelles


  • Nucleus
  • Endoplasmic reticulum
  • Golgi apparatus
  • Mitochondria
  • Chloroplasts
  • Lysosomes
  • Ribosomes
  • Cell wall
  • Vacuole
  • Cell-surface membrane
• Nucleus


  • Largest cell organelle
  • Surrounded by two membranes called the nuclear envelope
  • Outer membrane is continuous with a structure called rough endoplasmic reticulum
  • Envelope is perforated by nuclear pores
  • Pores are a gateway where proteins from the cytoplasm enter and where mRNA exits
  • Nucleoplasm is jelly-like material making up most of the nucleus
  • Where DNA replication takes place
  • Site of genetic control of the cell's activities
• Cell surface membrane


  • Plasma membrane is a biological membrane that separates the interior of a cell from its outside environment
  • Fluid mosaic of lipids, proteins and carbohydrates
  • Plasma membrane is impermeable to ions and most water-soluble molecules
  • Cross the membrane through transmembrane channels, carriers and pumps
• Mitochondria


  • Energy generating organelle
  • Surrounded by two membranes
  • Inner layer folds inwards to form the cristae which has a much higher surface area than the outer membrane
  • Cristae are coated in enzymes, which catalyse the reactions of aerobic respirations to produce ATP
  • Cristae project into a liquid called the matrix
• Chloroplasts


  • Use carbon dioxide, water and light energy to build sugars
  • Are present in all green plants
  • Site of photosynthesis
  • Surrounded by a double membrane
  • Is filled with a liquid called the stroma (where some parts of photosynthesis happen)
  • Contains a stack of thylakoid membranes called grana (where some parts of photosynthesis happens)
  • The thylakoid membranes are the site of photosynthesis
• Endoplasmic reticulum


  • Membrane bound networks within the cytoplasm forming flattened sacs
  • Continuous with the outer nuclear membrane
  • Rough endoplasmic reticulum: The surface of flattened sacs are covered with ribosomes, the flattened sacs are called cisternae for compartments separate from cytoplasm, as a compartment, RER isolate newly synthesised proteins from the cytoplasm transporting them as vesicles to other locations in the cell, proteins can also be modified in the RER
  • Smooth endoplasmic reticulum: When the cisternae is smooth lacking ribosomes, site for the synthesis of lipids, cholesterol and hormones, synthesise, store and transport
• Golgi apparatus


  • Are a stack of flattened sacs called cisternae
  • At the end of the cisternae, small vesicles can be seen budding off called golgi vesicles
  • Collects and processes molecules ready for transportations in vesicles to other parts of the cells or out of the cell
  • Also synthesise polysaccharides which are packaged into vesicles to be transported to cellular or extracellular destinations
  • Correctly labels the modified proteins and sorts them in order that they can be sent to the correct destination
  • Transportation occurs in the vesicles
• Lysosomes


  • Spherical sacs surrounded by a single membrane with no internal structure which originates from the golgi apparatus
  • Often contains digestive (hydrolytic) enzymes which would have been synthesised in the RER or golgi apparatus (and lysozymes hydrolyse bacteria)
  • They are important in the breakdown of unwanted structures in the cell as well as imported food vacuoles (i.e polysaccharides, proteins, lipids etc)
  • Also where the cell digests its own material in a process called autolysis
• Ribosomes


  • Very small organelles in the cytoplasm and bound to the rough endoplasmic reticulum
  • Consist of 2 subunits: large and small made from rRNA (ribosomal RNA) and proteins
  • Two types: 80S - found in eukaryotic cells, 70S - found in prokaryotic cells, mitochondria and cytoplasm
  • Site of protein synthesis
• Cell wall


  • Gives support and structure to the plant
  • Made of the polysaccharide cellulose, embedded in a matrix and provides the overall structure of the cell wall
  • Does not seal off a cell completely from its neighbours
  • There are pores within the walls called plasmodesmata which connect two cells together by their cytoplasm, enabling the exchange and transport of substances
  • The middle lamella holds adjacent cells together
  • Mechanical strength
  • Prevents cell bursting from pressure
  • Allows water to pass along it
• Vacuoles


  • Permanent vacuoles only exists in plant cells, animal cells can contain temporary vacuoles
  • Consists of a single membrane called the tonoplast filled with cell sap, a watery solution of different substances, including sugars, mineral salts, enzymes, wastes, pigments
  • Important in keeping the cell firm, when it is full of sap the cell is said to be turgid
  • Sugars and amino acids may act as a temporary food store
• Prokaryotic cells


  • Smaller than eukaryotic cells
  • Lack the organisation and structure of eukaryotes
  • Do not have a nucleus - a single circular DNA molecule, free floating, no histones
  • Lack other membrane bound organelles
  • A cell wall that contains murein (peptidoglycan), a glycoprotein
• Prokaryotic cells


  • Bacteria
  • Blue-green algae
• Components of prokaryotic cells

  • Ribosomes
  • Plasmids
  • Mesosomes
  • Capsule / slime layer
  • Flagellum
  • Nucleoid
• Ribosomes


  Protein synthesis
• Mesosome


  Contains proteins required for respiration and photosynthesis
• Capsule / slime layer


  Layer which surrounds the cell, used for attachment, as a food reserve, and as protection against dehydration, enzyme and phagocytosis
• Viruses


  • Segments of nucleic acid enclosed in a protein coat
  • Acellular - they are not cells
  • Capsid - protein coat
  • Attachment proteins are used to cling to suitable hosts
• Viral genome


  DNA or RNA
• Viral morphologies


  • Polyhedral
  • Helical
  • Spherical
  • Filamentous
  • Complex
• Bacteriophage


  A virus which uses a bacteria cell as host
• Magnification


  How much bigger the image is than the specimen
• Resolution


  How detailed the image is, how well a microscope distinguishes between two points that are close together
• Light microscopes


  • Use light to form an image
  • Have a maximum resolution of about 0.2 micrometres
  • Cannot be used to view organelles smaller than 0.2 micrometres
  • Maximum useful magnification is about x1500
• Using the optical microscope

  1. Ensure the objective lens is on the lowest magnifying power

  2. Place the slide on the microscope stage

  3. Use stage clips

  4. Look into the eyepiece

  5. Use the coarse focus

  6. Turn the objective lens onto a higher magnifying power

  7. Use the fine focus
• Electron microscopes


  • Use beams of electrons, rather than light, to produce images
  • Can view objects as small as the diameter of an atom
  • Have a higher resolution, meaning they give more detailed images
  • Have a maximum resolution of about 0.0002 micrometres
  • Maximum useful magnification is about x1500000
  • Produce black and white images, but these are often coloured by a computer
• Types of electron microscopes

  • Transmission electron microscopes (TEMs)
  • Scanning electron microscopes (SEMs)
• Transmission electron microscopes (TEMs)

  • Use electromagnets to focus on a beam of electrons, which is then transmitted through the specimen
  • Denser parts of the specimen absorb more electrons, making them look darker on the image
  • They give a high resolution image, so internal structures of organelles can be seen
  • Must view specimen in a vacuum, so not good for looking at live organisms
  • Can be used on thin specimens
• Scanning electron microscopes (SEMs)

  • Scan beam of electrons across the specimen
  • Knocks off electrons from specimen, which are gathered in a cathode ray tube to form an image
  • Images show the surface of the specimen and can be 3D
  • Can be used on thick specimens
  • Give lower resolution images
• Cell fractionation


  1. Homogenisation - breaking up the cells

  2. Filtration - getting rid of the big bits

  3. Ultracentrifugation - separating the organelles
• Specimens from electron microscopy must be preserved and dehydrated, so living cells cannot be viewed
• Homogenised cell filtration


  1. Cell debris filtered through gauze

  2. Organelles pass through gauze
• Ultracentrifugation


  1. Cell fragments poured into tube

  2. Tube spun at low speed

  3. Heaviest organelles form pellet

  4. Lighter organelles stay in supernatant

  5. Supernatant drained and spun at higher speed

  6. Process repeated at higher speeds
• Organelles are separated in order of mass, with chloroplasts coming after nuclei and before mitochondria in plant cells