Cell Structure

• Types of microscopes

  • Light
  • Electron
• Magnification

  How much bigger a sample appears to be under a microscope than it is in real life
• Resolution

  The ability to distinguish between 2 separate points
• Advantages of a light microscope over an electron microscope

  • Can observe living tissue
  • More portable
  • Easier to use - no technical training required
  • Possible to see real/natural colours and a live specimen
  • Can stain particular types of tissue for better visibility
• Cells function to

  1. Transfer energy

  2. Produce biological molecules including proteins

  3. Exchange substances with their surroundings
• Cell surface membrane

  • Selectively permeable membrane that allows for the exchange of certain substances
  • Barrier between cytoplasm and external environment
  • Cell recognition (surface antigens)
  • Selection of substances that enter/leave cells
• Nucleus

  • Controls cell’s activities
  • Very dense, takes up colour the most when stained
  • Divides first during cell division
  • Surrounded by 2 membranes, known as the nuclear envelope which is continuous with the RER
  • Contains: a) nuclear pores: allow and control substances entering in (protein to make ribosomes, ATP, some hormones, nucleotides) and leaving (mRNA, ribosomes for protein synthesis) of nucleus
• Nucleus components

  • Nuclear pores: allow and control substances entering in (protein to make ribosomes, ATP, some hormones, nucleotides) and leaving (mRNA, ribosomes for protein synthesis) of nucleus
  • Nucleolus (2.5 µm): contains loops DNA from several chromosomes and synthesises ribosomes
• Ribosomes are composed of 2 subunits
• Ribosomes carry out protein synthesis
• Ribosomes in cytoplasm are 80S, in chloroplasts & mitochondria are 70S
• Rough endoplasmic reticulum (RER) is a single membraned organelle with attached ribosomes, the site of protein synthesis, and where proteins are often modified
• Small sacs (vesicles) break off from the RER and join to form the Golgi body
• Golgi body is a stack of flattened sacs formed by vesicles from the RER, packages substances into vesicles for transport, and is involved in glycosylation, phosphorylating proteins, assembly of polypeptides into proteins (4º structure), folding proteins, and removing the 1st amino acid methionine to activate proteins
• Smooth endoplasmic reticulum (SER) synthesizes lipids and steroids such as cholesterol and reproductive hormones
• Lysosomes are spherical sacs surrounded by a single membrane, contain hydrolytic enzymes, responsible for digestion/breakdown of unwanted structures, and can even digest whole cells
• Mitochondria carry out aerobic respiration, synthesize ATP, have more in cells with higher energy demand, and release energy from energy-rich molecules to ATP
• ATP is the energy-carrying molecule in all living cells, released by breaking down ATP to ADP in a hydrolysis reaction
• Microtubules are long, rigid, hollow tubes made of tubulin protein, with ⍺ & β tubulin forms combining to form dimers, and 13 protofilaments in a ring to form a cylinder
• Tubulin

  • Long, rigid, hollow tubes found in the cytoplasm
  • Made of a protein called tubulin
  • Tubulin has 2 forms – ⍺ & β tubulin
  • ⍺ & β tubulin molecules combine to form dimers
  • Many dimers are joined end to end to form protofilaments
  • 13 protofilaments are in a ring to form a cylinder with a hollow center
  • This cylinder is the microtubule
• Microtubules

  • Supports and gives shapes to the cell
  • The assembly of microtubules from tubulin molecules is controlled by special locations in cells called microtubule organizing centers (MTOCs)
• Centrioles (and centrosomes)

  • Outside the nucleus of animal cells
  • 2 centrioles are present close together at right angles in a region called the centrosome
  • Centrioles are hollow cylinders about 500 nm long
  • Produces spindle fibers
  • Organizes microtubules
• Chloroplasts

  • Diameter 3-10 um
  • Carries out photosynthesis
  • Contains starch grains, circular DNA, and 70S ribosomes
  • ATP is also produced here
• Cell wall

  • Gives cell definite shape
  • Rigid as made of cellulose
  • Freely permeable
  • Prevents cell from bursting
• Plasmodesmata


  • Plant cells are linked to neighboring cells by means of fine strands of cytoplasm called plasmodesmata which pass through pore-like structures in their walls
  • Allows the transport of water, sucrose, amino acids, ions, etc., between cells without crossing membranes
  • This is called movement through the symplastic pathway
  • Allows communication/signaling between cells
• Vacuoles

  • Surrounded by a partially permeable tonoplast which controls exchange between the vacuole and cytoplasm
  • Helps regulate osmotic properties of cells
  • Fluid present in the vacuole consists of:
• Prokaryotic cells

  • Organisms that lack nuclei or proper nuclear membranes
  • Unicellular
  • 1-5um
  • Cell wall made of murein (peptidoglycan = protein + polysaccharides)
  • No membranes around organelles
  • 70S (smaller) ribosomes
  • Genetic material in the form of circular DNA
  • Have no ER