Topic 1 : Cells – 1.2 Ultrastructure of cells

Topic 1 : Cells
1.2 Ultrastructure of cells

Cell organelles

Prokaryotic Cells

  • Cell wall
    • Gives cell rigid structure
  • Plasma membrane
    • Controls what gets in and out of the cell
  • Cytoplasm
    • Holds all organelles together, gives cell structure
  • Nucleoid
    • Unbound clump of genetic matter
  • Plasmid
    • Floating chunks of genetic matter
  • Flagella
  • Pili

Eukaryotic Cell

  • Nucleus
    • Control center of cell
    • Replicates DNA
    • Transcription and modification of RNA
    • First is nuclear envelope which contains nuclear pores which send materials in and out
    • Then is chromatic which contains genetic matter (blobs)
    • Then is center which is nucleolus (the only thing that matters)
    • Chromatin = DNA
      • Stored in chromosomes
        • Chromosome: Long and thin, hard to see even with electron microscope. However, when it is cut or multiplies, it becomes short and thin, this is visible with a light microscope
    • the nucleolus is inside of the nucleus and produces ribosomes

  • Endoplasmic Reticulum
    • Rough
      • Many interconnected sacs with ribosomes attached to its membrane (it’s what makes it rough)
      • synthesizes and packages proteins
      • folds protein molecules in sacs called cisternae
      • transport of synthesized proteins in vesicles to the Golgi apparatus
    • Smooth
      • contains enzymes that help synthesize lipids, phospholipids and steroids
      • detoxification
      • contains ions that the cell may need later such as sodium and calcium
  • Golgi Apparatus
    • transports, modifies, and packages proteins and lipids into vesicles
    • takes products from rough ER (bulk flow), then the cisternae process, sort and label the chemicals
    • they are then dispatched to other parts of the cell or to the extracellular space
  • Mitochondria
    • powerhouse of the cell
    • Production of energy via synthesis of ATP molecules
    • Important in cellular respiration
      • also contains strands of glycerol as reserve power
    • regulates cellular metabolism
  • Lysosomes
    • waste treatment plants of the cell
    • breaks down cellular waste
    • transports undigested material to the cell membrane for removal
    • cell breaks down if the lysosome explodes
  • Vesicles
    • Transports materials in or out of cells
  • Vacuole
    • isolates materials that may be harmful/waste products
    • contains water
    • maintains turgor/hydrostatic pressure
    • maintains pH
    • exports unwanted substances
  • Cell Membrane
    • Semi-permeable membrane, made of proteins and lipids
    • Semipermeable: Allows certain substances to enter
    • All Eukaryotic Cells have a cell membrane
    • Made of protein and lipids
  • Cytoplasm
    • Site for all metabolic reactions & holds organelles
    • Metabolism: Sum total of all bodily reactions
    • Nearly all water
      • Some dissolved substances such as minerals
  • Ribosomes
    • Site for protein synthesis
      • Amino acids used as material, and RNA as instructions
    • Attached to a network called the RER or scattered freely throughout the cytoplasm
    • Protein is needed for repairing damage or directing chemical processes
  • Centrosomes
    • give structure to the microtubule
    • assemble microtubules
    • centrioles
  • Cytoskeleton – gives structure
  • Cell membrane
    • protects the cell and encloses its contents
    • monitors what enters and exits the cell
  • Cell wall (plants only)
    • provides protection, structure and support
    • prevents water loss
    • protection from the environment
  • Chloroplast (plants only)
    • sites of photosynthesis
    • contain chlorophyll (granum)

Prokaryotic cell structure

  •  All prokaryotes have a cell membrane and a cell wall surrounding the outside membrane. The cell wall is made from peptidoglycan. The entire interior of the cell is filled with cytoplasm (not compartmentalized) as no membrane-bound nucleus is present. (bacteria cells)
  • The cell wall
  • The plasma membrane
  • Flagella
  • Pili
  • Ribosomes
  • The nucleoid (a region containing free DNA)

The cell wall and plasma membrane

  •  The prokaryotic cell wall protects and maintains the shape of the cell
  • To large extent the plasma membrane controls the movement of materials into and out of the cell, and it plays a role in binary fission of the prokaryotic cell. All cellular processes within prokaryotic cells occur within the cytoplasm.
  • Cell wall is made from peptidoglycan not from celluos

Pili and flagella

  • Hair-like growths on the outside of the cell wall; Used for attachment, joining bacteria cells in preparation for the transfer of DNA (sexual reproduction)

Ribosomes(70s)

  • Occur in all prokaryotic cells (granular appearance in an electron micrograph of prokaryotic cell)
  • Sites of protein synthes

The nucleoid region

  •  Non-compartmentalized
  • Contains a single, long, continuous, circular thread of DNA, the bacterial chromosome
  • Involves cell control and reproduction

Binary fission

  •  Replicated semi-conservatively
  • Two DNA loops attached to membrane
  • Form two separate cells
  • Genetically identica

Summary

  •  DNA is not enclosed within a membrane and forms one circular chromosome
  • DNA is free; not attached to proteins
  • Lack membrane-bound organelles
  • Cell wall is made up of peptidoglycan
  • Divide by binary fission
  • Characteristically mall in size

Eukaryotic cell structure

Eukaryotes have a much more complicated cellular structure. The inside of the cell also contains cytoplasm but it is separated by compartments that allow for specialization. The compartments are membrane bond organelles such as the nucleus and the mitochondria. Compartmentalization enables different chemical reactions to be separated and chemicals to be isolated (increase efficiency.)

Organelles of eukaryotic cells

  •  Rough/ Smooth Endoplasmic reticulum
  • Ribosomes
  • Lysosomes
  • Golgi apparatus
  • Mitochondria
  • Nucleus
  • Chloroplasts
  • Vacuole

Cytoplasm

  •  Occurs inside the plasma membrane or the outer boundary of the cell, where organelles are found
  • The fluid portion of the cytoplasm around the organelles is called the cytosol

Endoplasmic reticulum

  •  Extends from the nucleus to the plasma membrane
  • Transports materials throughout the internal region of the cell
  • Smooth ER (no ribosomes on its exterior) and Rough ER (with ribosomes on its exterior)
    ⇒ Rough endoplasmic reticulum
  • Consist flattened membrane sacs, called cristernae
  • Contains ribosome for secreted protein (transportation)

Ribosomes(80s)

  •  Composed of RNA and protein
  • Proteins Synthesis

Lysomsomes

  •  Spherical with a single membrane
  • Formed from Golgi Vesicles
  • Contain digestive enzymes for breakdown of
    1. Ingested food in vesicles
    2. Damaged/unwanted organelles
    3. The cell itself
  • Stain heavily – appear dark on micrographs

Golgi apparatus

  •  Flattened membrane sacs, called cisternae
  • No attached ribosmes
  • Sited close to the plasma membrane
  • Shorter and more curved cisternae
  • Processes proteins from the rER. The proteins are then repackaged in vesicles for secretion outside the cell.

Mitochondria

  •  Double membrane
  • Smooth outside, folded inside
  • Folds—-cristae
  • Variable in shape
  • Site of ATP production by aerobic respiration
    Endosymbiotic theory: mitochondria and chloroplast, long before, are individual
    bacteria who decide to join larger eukaryotic cells.

Nucleus

  •  Generally spherical with a double membrane
  • Pores (holes) are present in the membrane
  • Contains genetic information in the form of chromosomes (DNA and associated
    histone proteins)
  • Uncoiled chromosomes are referred to as chromatin – they stain a dark colour and are
    concentrated at the edges of the nucleus
  • mRNA is transcribed in the nucleus (prior to use in protein synthesis in the cytoplasm)
  • mRNA leaves the nucleus via the pores (DNA is too large to move through the pores)

Chloroplasts(plants only)

  •  Many, but not all, plant cells contain chloroplasts
  • A double membrane surrounds the chloroplast
  • Inside are stacks of thylakoids
  • Each thylakoid is a disc composed of a flattened membrane
  • The shape of chloroplasts is variable but is usually ovoid
  • The site of photosynthesis and hence where glucose is produced.
  • Starch grains maybe present if photosynthesis is happening quickly

Cell wall(plants

  •  An extracellular component not an organelle.
  • Secreted by all plant cells (fungi and some protists also secrete cell walls).
  • Plant cell walls consist mainly of cellulose which is:
    1. Permeable – does not affect transport in and out of the cell
    2. Strong – gives support to the cell and prevent the plasma membrane bursting when under pressure
    3. Hard to digest –resistant to being broken down, therefore lasts a long time without the need for replacement/maintenance

Vacuoles

  •  Single membrane with fluid inside
  • In Plant cells vacuoles are large and permanent, often occupying the majority of the cell volume
  • In animal cells vacuoles are smaller, temporary, and used for various reasons.
    e.g. to absorb food and digest it.
ProkaryoticEukaryotic
DNA in a ring without proteinDNA with proteins as chromosomes
DNA free in the cytoplasmDNA enclosed within nucleus
No mitochondriaMitochondria present
70s ribosomes80s ribosomes
No internal compartmentalizationinternal compartmentalization present to form many organelles
Size less than 10μmSize more than 10 μm

A comparison of plant and animal cells and their extracellular components

Plant Animal
The exterior of the cell only includes a cell wall with a
plasma membrane inside
The exterior of the cell only includes a plasma
membrane; no cell wall
Chloroplasts presentNo chloroplasts
Vacuoles presentNo vacuoles
70s ribosomes80s ribosomes
No internal compartmentalizationinternal compartmentalization present to form many organelles
Size less than 10μmSize more than 10μm

 

Electron microsope

  •  The limit of resolution is the minimum distance that can be observed before two objects merge together to form one object. The smaller the limit of
    resolution the higher the resolving power.
  •  Electron microscopes have a greater resolution (about .001 μm) when compared to a light microscope (about 0.2 μm)
  •  The resolution of light microscopes is limited by the wavelength of light (400-700 nm). If the magnification becomes too great the image becomes
    blurry
  • Electrons have a much shorter wavelength so they have much greater resolution (about 200x greater than a light microscope)
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