A Tour of the Cell

2 Types of Cells

• Prokaryotes: Domain Bacteria & Archaea
  • No nucleus
  • DNA in a nucleoid
  • Cytosol
  • No organelles other than ribosomes
  • Small size
  • Primitive
  • i.e. Bacteria & Archaea
• Eukaryotes (Domain Eukarya): Protists, Fungi, Plants, Animals
  • Has nucleus and nuclear envelope
  • Cytosol
  • Membrane-bound organelles with specialized structure/function
  • Much larger in size
  • More complex
  • i.e. plant/animal cell

Cell Size and Scale

• Cells must be small to maintain a large surface area to volume ratio
• Large surface area allows increased rates of chemical exchange between cell and environment
• The diffusion rate increases as the area to volume ratio increases.
  • The two are directly proportional.

Nucleus

• Nucleus Function: control center of cell
• Contains DNA
• Surrounded by double membrane (nuclear envelope)
  • Continuous with the rough ER
• Nuclear pores: control what enters/leaves nucleus
• Chromatin: complex of DNA + proteins; makes up chromosomes
• Nucleolus: region where ribosomal subunits are formed

Ribosomes

• Ribosomes Function: protein synthesis

• Composed of rRNA + protein

• Large subunit + small subunit

• Types:

  1. Free ribosomes: float in cytosol, produce proteins used within cell
  2. Bound ribosomes: attached to ER, make proteins for export from cell

Endoplasmic Reticulum

• Endoplasmic Reticulum: network of membranes and sacs

• Types:

  1. Rough ER: ribosomes on surface
  • Rough ER Function: package proteins for secretion, send transport vesicles to Golgi, make replacement membrane
  1. Smooth ER: no ribosomes on surface
  • Smooth ER Function: synthesize lipids, metabolize carbs, detox drugs & poisons, store Ca2+

Golgi Apparatus

• Golgi Apparatus Function: synthesis & packaging of materials (small molecules) for transport (in vesicles); produce lysosomes
• Series of flattened membrane sacs (cisternae)
  • Cis face: receives vesicles
  • Trans face: ships vesicles

Lysosomes

• Lysosomes Function: intracellular digestion; recycle cell’s materials; programmed cell death (apoptosis)
• Contains hydrolytic enzymes

Vacuoles

• Vacuoles Function: storage of materials (food, water, minerals, pigments, poisons)
• Membrane-bound vesicles
• Eg. food vacuoles, contractile vacuoles
• Plants: large central vacuole -- stores water, ions

Mitochondria

• Mitochondria Function: site of cellular respiration
• Double membrane: outer and inner membrane
• Cristae: folds of inner membrane; contains enzymes for ATP production; increased surface area to ↑ ATP made
• Matrix: fluid-filled inner compartment

Chloroplasts

• Chloroplasts Function: site of photosynthesis
• Double membrane
• Thylakoid disks in stacks (grana); stroma (fluid)
• Contains chlorophylls (pigments) for capturing sunlight energy

Endosymbiont Theory

• Mitochondria & chloroplasts share similar origin
• Prokaryotic cells engulfed by ancestors of eukaryotic cells
• Evidence: 
  • Double-membrane structure
  • Have own ribosomes & DNA
  • Reproduce independently within cell

Peroxisomes

• Peroxisomes Functions: break down fatty acids; detox alcohol
• Involves production of hydrogen peroxide (H2O2)

Cytoskeleton

• Cytoskeleton: network of protein fibers
• Cytoskeleton Function: support, motility, regulate biochemical activities

Intracellular Junctions

• Tight junctions: 2 cells are fused to form watertight seal
• Desmosomes: “rivets” that fasten cells into strong sheets
• Gap junctions: channels through which ions, sugar, small molecules can pass

Plant Cells

• Cell wall: protect plant, maintain shape
  • Composed of cellulose
• Plasmodesmata: channels between cells to allow passage of molecules

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