Cell Bio E1 Ch1

4 features shared by all living cells?

1. outer plasma membrane

2. Heritable genetic info (DNA)

3. Presence of universal molecular language to transcribe and translate genetic info (expressed using RNA and proteins)

4. Expenditure of free energy to maintain a living state

LUCA

Last Universal Common Ancestor

Prokaryotes

bacteria + archaea, 1-10 um in size, does NOT have organelles, genome size of ~1,000-4,000 genes, circular topology

Eukaryotes

plants, fungi, animals, 10-100 um in size, contains organelles, genome size made of over 10,000 genes, linear topology

What are smaller? Prokaryotes or Eukaryotes

Prokaryotes (smaller, no organelles, circular genome)

Eukaryotes (larger, organelles, linear genome)

Endosymbiotic Theory

Eukaryotes arose when prokaryotic archaea engulfed bacterial prokaryotes and they became symbiotically dependent on one another.

Process of Endosymbiotic theory; Draw it

archeal cell —> engulfs bacterium —> bacterium remains inside the archeal cell —> both organisms become dependent on each other (symbiotic relationship develops) —> mitochondria like organelle —> heterotrophic eukaryotic cell develops

Importance of endosymbiotic theory

Some structures inside modern eukaryotic cells originated from bacteria that became permanent residents inside another cell

5 pieces of Evidence for Endosymbiotic theory

1. Eukaryotes genetically related to archaea, but have own characteristics associated with bacteria

2. Mitochondria and chloroplasts have their own genes/genomes consistent with formerly independent organism

3. Independent fission of mitochondria and chloroplasts, similar to prokaryotes resembles bacterial reproduction

4. Mitochondria and chloroplast have double membrane consistent with engulfment

5. Express unique transport proteins reminiscent of prokaryotic proteins

A plant cell chloroplast appears to have an inner and outer membrane separated by an intermembrane space. What theory does this support? What microscope was likely used?

The endosymbiotic theory states that billions of years ago, a large ancestral eukaryotic cell engulfed a photosynthetic prokaryote. Instead of digesting it, the host cell kept it alive in a symbiotic relationship. Over time, this captured organism evolved into the modern chloroplast.

TEM (transmission electron microscope) to view the internal structures like the inner and outer membrane.

Volvox carteri

a multicellular green alga that includes somatic (support organism’s functions and handle locomotion) and reproductive cells (contribute to reproduction) (eg of different cells —>different functions —> cooperation —> multicellular life)

Synergy

somatic cells keep the organism afloat in light-rich environments, allowing reproductive cells to efficiently allocate energy toward reproduction

What happens if somatic cells are killed in the colony?

the colony loses motility and ability to navigate toward light, sinking and starving the reproductive cells

What happens if reproductive cells are killed in the colony?

the colony can swim and perform photosynthesis but can’t pass on genetic material or create offspring, resulting in an evolutionary dead end

Colonial Theory

Multicellularity arose through cooperation between unicellular organisms led to evolution of multicellular organisms.

• Single-celled flagellates formed colonies, eventually undergoing cellular differentiation and functional specialization (e.g., transition from Chlamydomonas —> Gonium —> Pandorina —>` Yamagishiella, eudorina, Pleodorina, Volvox)

4 evidence supporting colonial theory

1. gradient of species exhibiting intermediate steps between unicellularity and true multicellularity

2. high genetic homology between closely related single-celled and colonial species

3. cellular differentiation into distinct germline (reproductive) vs. somatic lineages

4. intercellular communication and cell-adhesion molecules linking individual cells

TEM use case

Ideal for visualizing ultrathin cross-sections of internal cellular structures and organelle membranes at sub-nanometer resolution (e.g. thylakoid stacks)

Confocal fluorescence use case

ideal for imaging live or fixed cells tagged with fluorescent markers to study 3D multicellular architecture, protein localization, or cell-cell adhesion

Negative control

group where no treatment/effect is expected (baseline background level)

Positive control

group given a treatment known to produce a positive result (validates experimental assay)

What piece(s) of evidence would you expect to observe if the Colonial Theory is true?

Design an experiment to test one of these predictions. State your hypothesis, choose some experimental tools (including a microscope you might use), and predict the outcome of your experiment if your prediction is true.

Cell adhesion molecules (glycoproteins) keep colonies together and acts as a adhesion.

Hypothesis: If we inhibit or knock out cell-adhesion proteins in Volvox then the colonial structure will break into unicellular organisms like Chlamydomonas.

Experimental tools:

• Model: Volvox carteri

• Genetic Tool: CRISPR-Cs

• Control group: untreated wild-type carteri in standard growth media

• Experimental group: genetically modified Volvox carteri with the adhesion gene knocked out

• Confocal fluorescence microscope with fluorescent cell-membrane staining

Predicted Outcomes if hypothesis is true: For control group, cells remain physically attached as a organized, swimming sphere with clear separation between outer somatic and inner reproductive cells. For experimental group, the sphere collapses and dissociates into isolated, individual cells that swim independently (like single-celled Chlamydomonas)

Conclusion: proves that cell-cell adhesion mechanisms are a fundamental evolutionary requirement for single cells to form stable multicellular colonies.