2.1 Cell Structure

• Magnification

  how many times larger the image produced by the microscope is than the real object
• Resolution


  is ability to distinguish between objects that are close together
• Optical light microscopes

  • uses light to form images
  • limited resolution and magnification
  • max magnification of X1500
  • cannot be used to observe small organelles in cells
• Electron microscope

  • uses electrons to form images
  • more resolution and magnification compared to light microscope
  • more detailed image
  • max magnification of X1,500,000
  • Two types: transmission electron microscope and scanning electron microscope
• Transmission electron microscope

  • use electromagnets to focus a beam of electrons which is transmitted through the specimen
  • denser parts appear more darker of the final image produced
  • advantages: high-resolution images and can identify internal structures within the cells
  • disadvantages: only thin specimens can be observed, cannot be used to observe live specimens, does not produce a colour image
• Scanning electron microscopes

  • scans a beam of electrons across a specimen
  • beam bounces off the surface of the specimen meaning that they can produce 3D images
  • advantages: creates 3D images and allows the external 3D structure of specimen to be observed
  • disadvantages: lower resolution compared to TEM, cannot be used to observe live specimens and not a colour image
• Laser scanning confocal microscope

  • relatively new
  • specimen must be stained with fluorescent dyes
  • multiple depths of specimen are scanned to produce an image
  • advantages: can be used on thick specimens, allows external 3D structures to be observed and very clear images are produced
  • disadvantages: slow process and the laser can sometimes cause photodamage to the cells
• Preparing a microscope slide of liquid specimen

  1. Add few drops of sample to the slide using a pipette
  2. Cover the smear with a coverslip and gently press down to remove any air bubbles
• Preparing a microscope slide of solid specimen

  1. Use scissors to cut a small sample of the tissue
  2. Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide
  3. Apply a stain
  4. Gently place a coverslip on top and press down to remove any air bubbles
• Nucleus

  • found in all eukaryotic cells
  • separated from the cytoplasm by a double membrane called the nuclear envelope with has many nuclear pores
  • nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus and also allowing enzymes like DNA polymerase to travel in
  • contains chromatin
  • nucleolus is the site of ribosome production
• Rough endoplasmic reticulum

  • found in both animal and plant cells
  • surface is covered in many ribosomes
  • formed from continuous folds of membrane continuous with the nuclear envelope
  • processes proteins made by the ribosomes
• Smooth endoplasmic reticulum

  • found in both animal and plant cells
  • does not have ribosomes of the surface
  • involved in the production, processing and storage of lipids, carbohydrates and steroids
• Golgi apparatus

  • found in both animal and plant cells
  • flattened sacs of membrane similar to the SER
  • modifies proteins and lipids and packages them in golgi vesicles
  • vesicles then transport the proteins and lipids to the destination
• Ribosomes

  • found in all cells
  • found freely in the cytoplasm of all cells and also a part of the RER in eukaryotic cells
  • each ribosome is a complex of ribosomal RNA and proteins
  • 80S ribosomes are found in eukaryotic cells
  • 70S ribosomes are found in prokaryotes, mitochondria and chloroplasts
  • site of translation
• Mitochondria

  • site of aerobic respiration within all eukaryotic cells
  • can be seen with a light microscope
  • surrounded by a double membrane with inner membrane folded to form cristae
  • matrix formed contains enzymes
  • small circular pieces of DNA and ribosomes are also found in the matrix
• Lysosomes

  • specialist forms of vesicles which contain hydrolytic enzymes
  • break down nuclear waste materials like worn out organelles
  • used by the immune system and apoptosis
• Chloroplasts

  • only found in plant cells
  • larger than mitochondria and is surrounded by double membranes
  • thylakoids contain chlorophyll
  • chloroplasts are the site of photosynthesis
  • also contains small circular pieces of DNA and ribosomes used to synthesise proteins
• Plasma membrane

  • all cells are surrounded by a plasma membrane which controls the exchange of materials between the internal cell environment and the external environment
  • formed from a phospholipid bilayer
• Centrioles

  • hollow fibres made of microtubules
  • two centrioles at right angles to each other from a centrosome which organises spindle fibres in cell division
  • not found in flowering plants and fungi
• Cell wall

  • found in plant cells but not animal cells
  • formed outside of the cell membrane
  • offers structural support
  • contains cellulose in plants and peptidoglycan in bacterial cells
  • narrow threads on cytoplasm called plasmodesmata connect the cytoplasm of neighbouring plant cells
• Flagella

  • found in specialised cells
  • similar structure to cilia
  • contract to provide cell movement like in sperm cells
• Cilia

  • hair like projections made of microtubules
  • allows for movement of substances over the cell surface
• Organelles involved in protein synthesis

  • nucleus
  • ribosomes
  • rough endoplasmic reticulum
  • golgi apparatus
  • cell surface membrane
• Protein synthesis

  1. DNA from the nucleus is copied into a molecule of mRNA (transcription)
  2. mRNA strand leaves the nucleus through nuclear pores and attaches to a ribosomes on RER
  3. ribosome reads the genetic instructions and uses this code to synthesise a protein (translation)
  4. proteins are processed in RER
  5. processed proteins are then transported to the golgi apparatus in vesicles which fuse
  6. golgi apparatus modifies the proteins preparing them for secretion
  7. modified proteins then leave the golgi apparatus in secretory vesicles which fuse with the cells membrane and are released via exocytosis
• Cytoplasm

  • extensive network of protein fibres
  • made up of 2 types of fibres: microfilaments and microtubules
  • microfilaments: solid strands which cause some cell movement and movement of some organelles within cells
  • microtubules are hollow strands which all organelles and other cell contents along these fibres using ATP
  • intermediate filaments are also found within the cytoskeleton
• Importance of cytoskeleton

  • provides the cell with mechanical strength that helps to maintain the shape of the cell. It also supports the organelles keeping them in position
  • it aids transport within cells by forming tracks along which organelles can move
  • enables cell movement via the cilia and flagella
• Prokaryotes

  • 0.5-5 micrometres diameter
  • DNA circular with no proteins
  • cell division occurs by binary fission
  • 70S ribosomes
  • very few organelles which are not membrane bound
  • cell walls made of peptidoglycan
• Eukaryotes

  • up to 100 micrometres diameter
  • DNA is associated with histone proteins formed into chromosomes
  • cell division occurs via mitosis or meiosis
  • 80S ribosomes
  • numerous types of organelles which are membrane bound
  • cell walls are present in only plant cells (cellulose) and fungi (chitin)