Principles of Cell Biology (BIOL2060)

Department of Biology
Memorial University of Newfoundland

Cells and Organelles

Prokaryotes and eukaryotes are very different (see Table 4-1 )

Prokaryotes: bacteria
plasma membrane
circular DNA
ribosomes (70S)
binary fission

Eukaryotes: Animal and Plant
plasma membrane and internal membrane-bound compartments / organelles
DNA  plus proteins ­ linear chromosomes
larger ribosomes (80S)
mitosis & meiosis
endocytosis & exocytosis

Evolution of the Eukaryotic Cell and the origin of chloroplasts and mitochondria
Two theories:
1) Autogenous theory:
The eukarotic cell evolved directly from a single prokaryote ancestor through compartmentalization of functions arising from invaginations of the prokaryote’s plasma membrane.
Eg. the endoplasmic reticulum (ER), Golgi, and the nuclear membrane, and of organelles enclosed by a single membrane
According to this theory, mitochondria and chloroplasts have evolved within by compartmentalizing plasmids within vesicles from the plasma membrane.

2) Serial Endosymbiotic Theory (SET) of the Evolution of Eukaryotic cells
Prokaryotic cell engulfs aerobic bacteria
Rather than digesting them, the bacteria remain, as symbionts, benefiting the host cell by removing harmful O2 and helping in the production of ATP.
As interdependence between the aerobic bacterium and the host cell grows, the bacterium becomes the mitochondrion.
Some of these cells also engulf and keep blue-green algal cells which become chloroplasts.

What evidence do we have for SET and the origin of mitochondria and chloroplasts from free-living bacteria?
1) size
2) appearance
3) double membrane
4) binary fission
5) common enzymes for respiration, photosynthesis
6) Genetic material:
single circular DNA molecule
no histone proteins; few proteins
continuous  S phase
similarity in gene sequences eg. rDNA, tDNA
7) Similarity in ribosomes of organelles and prokaryotes (compared to cytoplasm of eukaryotes):
70S ribosomes in organelles and prokaryotes (vs 80S in cytoplasm)
sensitivity to antibiotics
start codons for the start of protein synsthesis

Mitochondria and chloroplasts are "semi-autonomous organelles"

1) Cell membranes
cell boundary (plasma membrane: PM)
internal compartments
selectively permeable
lipids and proteins
~8 nm

2) Extracellular boundaries:
1) plant cell walls
2) fungal cell walls
3) animal extracellular matrix (ECM)
reinforcement,  protection and communication

3) Ribosomes
site of protein synthesis, translation (mRNA to protein)
Free or associated with membranes (Rough endoplasmic reticulum: rER)
eukaryotes: 30 nm diameter
(and in the mitochondria and cytoplasm)

4) Nucleus
compartmentalises genetic material
site of DNA synthesis (S)
site of RNA synthesis (transcription)
double membrane, nuclear pores, chromatin, nucleolus

5) Endomembrane system
a series of internal membranes
endoplamsimic reticulum (ER), Golgi apparatus & vesicles
structurally and functionally related
for synthesis and secretion of proteins, carbohydrates and lipids (Fig 4-17)

6. Lysosomes (Fig 4.18)

7. Peroxisomes (Fig 4.19)
variety of processes
generate hydrogen peroxide as a by-product

Semi-autonomous organelles:
DNA - circular molecule, ribosomes, fission
Energy Flow

8. Chloroplasts
energy of sunlight to fuel molecules

9. Mitochondria
fuel molecules to ATP
ACR (heat)

10. Cytoskeleton
3D interconnected array of protein-based filaments
microtubules, microfilaments, intermediate filaments
cell shape, movement, cell signalling, endocytosis, mitosis

Notes prepared from Becker's World of the Cell, 8th edition
Hardin, Bertoni & Kleinsmith, 2012
Figures copyright of Pearson Education Inc.
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