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Chapter 1: Cell Structure

Cell - Fundamental unit of all living things.


Microscope:

  • Light Microscope (LM) → μm 
    • 200 nm or further apart 
  • Electron microscope (EM) → nm
    • Close together as 0.1 nm 

Why use electrons? 

  1. Wavelength is extremely short 
  2. Negatively charged, so it can be focused easily using electromagnets 

Disadvantages of EM: 

  1. Specimen must be dead - viewed in a vacuum, water in the cells boil away so spciment must have all water removed 
  2. Expensive to buy and run
  3. Preparation of material is time consuming and requries expert trainings 

Magnificaton 

I = AM 
  • Max resolution: 200nm, an objects closer than 0.2μm appear as a single item 
  • Magnification ↑ Size of image ↑, but does not always increase the resolution 
Resolution is the ability to distinguish two separate points 
  • Resolution ↑ clearer and precise image is produced 
  • Determined by wavelength of rays that are being used to view the specimen
Max resolution: ½ * 400 = 200 nm

The electromagnetic spectrum

  • Range of different wavelength 
  • Energy ↑, longer the frequency and shorter the wavelength 

Scanning Electron Microscope
Transmission Electron Microscope
Surface structure can be seen
See inside the cell
3D image
2D image
Cut thick section of specimen
Cut thin section of specimen
Developed later
Developed earlier
Placed at the bottom of chamber
Placed at middle of specialised chamber
2 million magnification
50 million magnification


Ultrastructure of an animal fell

  • Nucleus (10~20μm)

    • Nuclear envelope: double membrane that surrounds the nucleus 
      • Controls the movement of substances in and out of the nucleus 
      • Contains reation taking place within it 
    • Nuclear pores: mRNA moves out of nucleus 
    • Chromatin: condenses into chromosomes 
    • Nucleolus: small spherical body within the nucleoplasm 

  • Endoplasmic reticulum and ribosomes 

    • RER 
      • Provides large surface area for synthesis of proteins 
      • Provides a pathway for trnasport of materials 
    • SER 
      • Synthesis, stores and transports lipids and carbohydrates 
      • Contains lytic enzymes (break own all mitochondria) 

  • Riobosomes (25nm) 

    • Small cytoplasmic granules - 80S and 70S 
    • Proteins are made by the ribosomes on RER and enter the sacs and movce through them

RER → GA → Out of the cell (cell surface membrane) 

  • Golgi Body/ Apparatus 

    • Forms glycoprotein 
    • Production of secretory enzymes (pancreatic amylase) 
    • Secretes carbohydrate for cellulose 
    • Forms lysosomes

Role of the Golgi apparatus in seretion 

  • Protein is made in ribosomes on RER 
  • Synthesised protein enters RER cisternae 
  • Vesicles containing protein are budded off the RER 
  • The vesicles move along microtubules 
  • Vesicles are added to the convex face of the Golgi 
  • Protein is chemically modified and concentrated in the cisternae 
  • Vesicles are budded off the concave fae of the Golgi. 
  • The vesicles move along microtubules. 
  • Vesicles fuse with the CSM releasing their contents to the outside 
  • Known as exocytosis. 

Role of the Golgi apparatus in intracellular digestion 

  • Enzymes are made in ribosomes on RER 
  • Synthesised enzymes enter RER cisternae 
  • Vesciels containing concentrated enzymes are budded off the concave face of the Golgi (primary lysosomes.) 
  • The enzymes of the lysosome digest the object. Small molecules produced by digestion are absorbed into the cytoplasm. 
  • Lysosomes can fuse with vesicles conaining objects that need digesting. 
  • Vesicles are formed round objects that need to be digested 
  • Known as endocytosis. 

  • Lysosome (0.1~0.5 μm) 

    •  No internal structure 
    • Contains digestive enzymes which breaks down unwanted structures
      • Mammary glands after lactation (breast feeding)
      • Used to digest bacteria in WBC 
      • Enzymes released in the replacement of cartilage with bone during development 
      • The heads of sperm contain acrosome (lysosome) 
    • No internal structure
    • Contains enzyme such as protease and lipase
    • Autolysis 
      • Completely break down cells after they have died 

  • Mitochondrion (1 μm)

    • Double membrane 
      •  Outer contains porin, which forms wide aqueous channels 
      • Inner membrane is folded to form finger-like cristae 
    • Matrix (inner membrane is folded to form finger-like cristae )
      • Contains protein ribosomes, circular DNA 
    • Site for aerobic respiration (produces energy in the form of ATP)

Endosymbiont theory (Endo - inside, Symbiont -living in a mutually beneficial relationship)

  • Mitochondrion and chloroplast contains 70S ribosomes and small circular DNA 
  • Mitochondrion and chloroplast are ancient bacteria now living inside large animal and plant cells 
  • DNA + ribosomes of chloroplast and mitochondrion are still active but not independent 
Vesicle
Vacuole
Membrane enclosed inside the cell, containing different types of fluid
A type of vesicle, containing mostly water
Small in size
Large in size
Found in eukaryotic cells
Found in eukaryotic and prokaryotic cells
Compound of water, nutrients, enzymes, waste and ions
Mostly water
Involved in metabolism, temporary storage and transport molecules
Involved in storing substances, mostly water contribute to structural support to the cell
Lysosomes transport vesicles, secretory vesicles
Bacteria, plant, fungi and animal cells contain vesicles

Cell surface membrane (7nm)

  • Boundary between cell cytoplasm and environment 
  • Controls the movement of substances into and out of cell 
  • Contains 
    • 45% proteins 
    • 45% phospholipid 
    • 10% cholesterol, glycoprotein, glycolipid 
  • Fluid Mosaic Model (phospholipids move sideways, and the pattern produced by the scattered protein molecules) 

Microvilli

  • Finger-like extensions of the cell surface membrane 
  • Increases surface area for reabsorption of glucose 

Microtubules and microtubule organizing centers (MTOCs) (25nm)

  • Maintenance of cell shape 
  • Cell motility (cilia, flagellum)
  • Chromosome movement in cell division (spindle fibre)
  • Organelle/vesicle movement
    • Unbranched hollow arranged as protein tubulin 
      • α-tubulin and  β-tubulin which forms dimers 
      • Joined to form protofilaments (POLYMERISATION) 
      • 13 protofilaments line up 

Centrioles (500nm)

  • Hollow cylinders, two centrioles in a cell and they lie at right angles, 9 sets of 3 microtubules 
  • Occurs in animal cells only 
  • During prophase, centrioles replicate themselves and two pairs migrate to opposite poles of the cell 
  • Centrosome: involved in the formation of spindle fibres.
  • Cillia/cilium: short projections made from microtubules - for movement. 
    • Rhythmic waving, beating motion
    • Keeps the airways clear from mucus and dirt
  • Flagella/flagellum: long projections made from microtubules - for movement.
    • Flagellum of sperm cell propels itself through female reproductive tract

Ultrastructure of a plant cell 

Chloroplast (3~10 μm)

  • During the first stage of photosynthesis, light energy is absorbed by the chlorophyll. Some are used to manufacture ATP from ADP. 
  • Water is splitted into hydrogen (for fuel) and oxygen
  • Thylakoids stack up like piles of coins forming grana 
  • Chlorophyll and ATP synthase are situated in the thylakoid membranes 
  • Stroma: Colorless and light indepdent reaction (dark reaction) of photosynthesis occurs 
Feature
Prokaryotic Cells
Animal Cells
Plant Cells
CSM
✓
✓
✓
Cell wall
✓
Made up of peptidoglycans/ Murein cell wall
X
✓
Nucleus and envelope
X
✓
✓
Chromosomes
✓ (circular DNA, plasmids)
✓ (linear DNA associated with histones)
✓ (linear DNA)
Mitochondria
X
✓
✓
Chloroplast
X
X
✓
RER, SER and GA
X
✓
✓
Ribosomes
✓ (70S)
✓ (80S in cytoplasm, 70S in mitochondria)
✓ (80S in cytoplasm, 70S in mitochondria and chloroplast)
Centrioles
X
✓
X


Virus (20~300 nm) 

  • Contains: a self-replicating molecule of DNA or RNA 
  • Protective coat (capside) of protein molecules (capsomere)
  • All viruses are parasitic because they can only reproduce by infecting and taking over living cells 

Differential centrifugation - may be used to isolate cell components 

  • Cell extracts are centrifuged at great speeds to separate the components 
  • Factors affecting: 
    • Magnitude of centrifugal force, which depends on the speed 
    • Size of the organelle 
    • Density 
  • Low speed - 1000 g for 10 minutes 
  • Medium speed - 20 000g for 20 minutes 
  • High speed - 80 000g for 60 minutes 

Buffer solution - prevents pH change which might denature the enzymes 
Isotonic solution - prevents osmotic damage to cells 

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