Biology 1020 lecture 6 Cell Structure 1
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- Basic structural and functional unit of every organism.
- First observed by Hooke and Leeuwenhoek in the late 1600s with the first microscopes
Chemical components of cells
All cells have to...
- Separated from external enviroment. a.k.a, plasma membrane
- Undergo chemical reactions in aqueous environment ---> Semi-fluid substance called cytosol
- Make proteins --> ribosomes
- Trasmit genetic materials --> chromosomes
- Acquire nutrients and expel wastes
Selective barrier: allows passage of some materials (oxygen, nutrients, and waste) and blocks others
General structure- a double layer of phospholipids
Hydrophilic region - outside of the cell and inside of the cell.
Hydrophobic region in between the two hydrophilic regions
Cells exist in a narrow size window
- Cells: 1 - 100 micrometres
- Organelles: ~1 micrometre
Cell Size: Why so narrow a size range?
- Large enough for all things needed for life and reproduction.
- - Enzymes, DNA, etc.
- => This sets lower size limit
- Small enough so that that the plasma membrane area allows for nourishment of needed materials in, and wastes out.
- => This sets the upper size limit
Small cell - large surface area relative to volume
- 1-5 micrometers.
- Much smaller than eukaryotic cells
The first cells on earth were prokaryotic
- Small, did not use oxygen in metabolism
- 3.5 billion YBP
- - Layered rock containing fossilized prokaryotes - stromatolites
- First life - 3.9 billion YBP
What was it like on early Earth?
Lightning, volcanic activity
- -water vapor which condensed to form oceans
- - nitrogen, CO2, methane, NH3, H2 and hydrogen sulfide from volcanic eruptions
Formation of first cells
Membrane requires organic macromolecules
Hypothesized stages in the formation of the first cells
- 1. Non- living sythesis of small organic molecules. ex. amino acids, phospholipids
- 2. Polymerization into macromolecules. Ex. amino acids --> proteins
- 3. Enclose macromolecules within membranes
- * internal chemistry different from surroundings
- 4. Origin of self replicating molecules --> RNA world
Abiotic sythesis of organic molecules:
Miller's (1953) experiments
- Attempt to recreate the "primitive soup"of early Earth
- Flask with H2, CH4, NH3
- Electricity, condensation
- Amino acids formed after a week
Evidence for abiotic membrane formation
- Phospholipids that make up cell membranes self assemble in aqueous environments
- - Forms a lipid bi-layer
- PROTOCELLS - Abiotically created molecules within a membrane; created by adding lipid to water
- * Swell and shrink, "give birth" to daughters
- * Simple metabolic reactions ocur within if reactants and enzymes included
Evidence for RNA as the first self replicating molecules: ribozymes
- Some RNAs (ribozymes) can carry out enzyme like catalytic functions
- * Can make complimentary segments of RNA if nucleotides are present
Selection for RNA molecules that were more stable or replicated more quickly
Metabolism of the first cells?
- Low oxygen levels, oxygen not used in metabolism
- ATP generated without oxygen - another molecule (ex. sulfate)
Photosythesis by Early Prokaryotes
- Produced oxygen as a byproduct
- Oxygen accumalated: Oxygen revolution
- Extinction of groups that could not tolerate oxygen
- Some prokaryotes survived and adapted using cellular respiration (using oxygen) to harvest energy
- * produces more ATP
Prokaryotic cell structure
- No membrane-bound organelles within the cell
- DNA in an unbound region called the nucleoid
- Move with a flagella
- They attach to surfaces with fimbriae and/or the capsule
- Protected against dehydration by the capsule (jelly like outer coating)
- Rigid cell wall maintains shape and protects cell. Consists of peptidoglycan.
- Plasma membrane encloses cytoplasm
- Infoldings of plasma membranes in some
Polymer of sugars crossed linked by peptides
Prokaryotes exist in communities
Prokaryotes grow quickly
- Adapt to changing conditions quickly
- low levels of nutrients
- beneficial mutations
Unicellular eukaryotes (protists) appeared first after prokaryotes.
Structure of Eukaryotic Cells
- Internal membranes partition the cell into organells.
- Cytoplasm: materials inside the cell membrane excluding nucleus (cytosol is the jelly-like portion of the cytoplasm)
- Contains the genes (DNA) ~5 μm in diameter
- Enclosed by double membrane - nuclear envelope
- Shape maintained by a network of proteins of the inside face - nuclear lamia
- Material (eg. proteins, RNA) enters and leaves through nuclear pores
- DNA (in chromosomes) is bundled with proteins in a compact form - chromatin
- Ribosomal RNA, rRNA, and proteins form dense granules adjoining chromatin - Nucleolus/nucleoli
- Proteins imported into nucleus and added to rRNA to make RIBOSOMES
Some red blood lack a ...
Nucleus. They can carry more hemoglobin that way.
- Free -->
- - Suspended in the cytosol
- - Making cytosolic proteins (glycolysis enzymes)
- Bound -->
- - Attached to endoplasmic or nuclear envelope
- - Make proteins for secretion or for membranes
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