Simple schematic: Single-celled prokaryotes vs multicellular eukaryotes (Us!).
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Prokaryotes vs. Eukaryotes:
Prokaryotes (bacteria and archaea) are the earliest form of life.
Eukaryotes came later and include fungi, plants, and animals.
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Primary distinctions:
Eukaryotes have a membrane-bound nucleus; prokaryotes have a free nucleoid.
Prokaryotes lack most of the organelles that eukaryotes possess.
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Eukaryotic cells (plants and animals):
Animal cells tend to be larger and more rounded.
Plant cells tend to be smaller and more rectangular (square-like).
Plant cells have two major components not typically found in animal cells: cell walls and chloroplasts.
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Typical cell sizes:
Most plant and animal cells: between 10-100μm.
Bacteria are smaller.
How many cells in the body? Approximately 3.72×1013 cells (often quoted as 37.2 trillion).
Why so many, and why small? Small cell size helps conserve cell metabolism because a higher surface area-to-volume ratio facilitates exchange of nutrients and wastes; metabolism is the process by which food sources are converted to energy.
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Organelle cards:
Located in this module on eCampus under outcomes/activities and end-of-module checklist.
Not all organelles will be discussed in depth in lecture; emphasis is on how organelles work together.
Students are responsible for studying the cards and knowing the structure and function of the listed organelles and the cell types they’re found in.
You’ll be tested on this information in the future.
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Quick check: Unknown cell type contains a membrane-bound nucleus, ribosomes, and mitochondria.
A: prokaryote
B: eukaryote
C: could be either
Conclusion: Presence of a membrane-bound nucleus indicates a eukaryote (answer: B).
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Quick check: If a cell has a cell wall, what can you conclude?
A: Plant cell
B: Animal cell
C: Could be either plant or animal cell
Conclusion: A cell wall strongly suggests a plant cell (answer: A).
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How do organelles work together? The endomembrane system (endo = within) is a coordinated system of organelles in eukaryotes.
Re-packages proteins and prepares them for final destinations.
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ER signal sequence map (conceptual):
Presence of a transmembrane signal sequence acts as a “zip code.”
If a signal sequence is present, ribosome docking at the rough ER occurs, leading to protein deposition into the rough ER via the ribosome.
Proteins are transported to the Golgi via vesicles.
If no signal sequence is present, the protein is synthesized in the cytoplasm.
The diagram contrasts the options for a protein as it travels through the endomembrane system.
Note: This page emphasizes how the presence or absence of signal sequences guides the trafficking path of newly synthesized proteins.
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Endomembrane system at work (stepwise):
Proteins begin their journey at the rough ER.
Ribosomes deposit proteins inside the rough ER.
A signal sequence guides targeting; the Signal Recognition Particle (SRP) binds this sequence.
SRP then binds its receptor on the rough ER membrane.
If there is no signal sequence, the ribosome remains in the cytoplasm and the protein is not targeted to the ER.
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If there is a mutation in the signal sequence or in SRP or the SRP receptor: the protein would remain in the cytoplasm (i.e., would not be directed to the ER).
Question exercise: A mutation in any of these components would prevent proper docking and translocation.
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Golgi apparatus structure within endomembrane system:
cis-face: the receiving side of the Golgi (cis = same).
trans-face: the shipping side of the Golgi (trans = opposite).
Golgi role: final modifications of proteins.
Exit from Golgi occurs at the trans-face.
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Vesicle movement within the cell:
Vesicles travel through the endomembrane system with help from motor proteins like kinesin.
Kinesin carries vesicles along microtubule tracks; kinesins are motor proteins composed of a head, stalk, and tail.
ATP is used by kinesin to function; chemical basis: \mathrm{ATP} + \mathrm{H2O} \rightarrow \mathrm{ADP} + \mathrm{Pi} + \Delta G_{hyd}}.
Microtubules form long tracks that provide structural support and rigidity for transport.
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Vesicle trafficking from Golgi to lysosome or other destinations:
If proteins exit the Golgi in a lysosome, they are used for digestion.
Lysosomes have a very low (acidic) pH that enables digestion of materials.
Autophagy: internal digestion of cellular components for reuse.
Phagocytosis: uptake of external cellular material (external food) by the cell.
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Not all proteins go to lysosomes; some are destined for other organelles provided they contain a special “zip code” sequence.
Three alternate jobs for proteins with targeting signals:
1) Transfer to other organelles.
2) Become transmembrane proteins embedded in the phospholipid bilayer; their polar/non-polar properties determine how they sit in the bilayer.
3) Secretion/export out of the cell.
Targeting signals allow recognition between proteins and organelles.
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If a protein is not a transmembrane protein, it can be secreted outside the cell.
Secreted proteins can interact with other cells and participate in cell-cell communication.
Endomembrane system at work: further trafficking can lead to secretion.
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Recap of not-all proteins remaining in lysosomes: some are exported out of the cell for intercellular communication.
Emphasis on the breadth of protein destinations within the cell and beyond.
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Why should you care?
Proteins are the functional entities within cells.
Without the endomembrane system, the fate and proper localization of proteins would be compromised.
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ER signal sequence map (revisited):
Present? Ribosome docking at rough ER occurs when a signal sequence is present.
Trans-membrane protein? Signal sequence presence influences integration into membranes.
Zip code signal sequence present? YES/NO determines routing.
Protein deposited into rough ER by ribosome; transported to Golgi via vesicles; final destinations determined by sorting signals.
This page is a prompt to fill in a chart (blue boxes) detailing choices proteins make as they travel through the endomembrane system.
Review the organelle cards (linked in Module 3 outcomes/objectives tab); essential study resource.
Read selected sections of Chapters 7 and 6B in Mastering Biology; readings supplement course material and chapter questions.
Complete relevant Chapter questions in Mastering Biology; questions are numbered the same as the readings.
Optional practice materials: Practice questions 4 (Cells & organelles), Practice questions 5 (Membranes), Practice worksheet 3 (Cells & organelles), Practice worksheet 4 (Transport & osmosis).
Reminder: Course schedule provides structure for quizzes and readings; assignments are due the night before the exam for that unit, though completing tasks per the schedule helps manage workload.