本期的內(nèi)容是細胞的特征。本文集的這一部分是遺傳、進化與生態(tài)學 Genetics, Evolution, and Ecology. 這門課理論上建議在閱讀完文集的第一部分的內(nèi)容之后再開始學習，但基礎(chǔ)不足的朋友也可以嘗試閱讀喔~

這一部分的主要內(nèi)容均來自 Prof. Angela J. Roles 的課程，因此本文集的這一部分均不會標記為原創(chuàng)。但由于文本來源不清晰，UP主還是一個字一個字碼出來的文章，本文禁止非授權(quán)的轉(zhuǎn)載，謝謝！

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Lesson 3:?Cells?- Cellularity

Cellularity (就是cellular的名詞) unifies life.

- Unity: what do the three domains have in common?

[1] Membranes: Cells have membranes

?Membranes create insides and outsides (i.e., internal microenvironments)

- Enables regulation of the local chemical environment (e.g., homeostasis), regulated movement across membrane;

- Achieved via a plasMa membrane and sometimes cell walls;

Diversity of membranes and cell walls

While membranes are a unifying feature, there is some variation in composition. And different types of organisMs build cell walls out of different materials.

?Eukaryotic cell membranes are a lipid bilayer (LB)

????- Animal cells lack a cell wall.

????- Plant cells have a cellulose-based cell wall.

????- Fungal cells (usually) have a chitin-and-glucan-based cell wall.

?Bacterial cells have a LB membrane and (usually) a peptidoglycan cell wall.

?Archaean cells have a lipid mono- or bi-layer membrane and a cell wall composed of surface layer proteins (S-layers).

????Thinking Question: What might be the function(s) of a cell wall? Why might organisMs differ in the materials they use to build one?

[2] Reproduction: Cells replicate themselves (reproduce)... but not all in the same way!

?Cells replicate themselves, using information stored in DNA (or sometimes RNA in viruses)

????- Eukaryotes use mitosis (including vegetative methods like budding) or meiosis, followed by cytokinesis;

????- Bacteria use binary fission, and a few variations of this method (like endospores)

????- Archaea reproduce by methods that resemble bacteria in some ways and eukaryotes in others.

[3] MetabolisM: Cells perform metabolic reactions

?Life depends on the assimilation of energy from an external source (ex: the sun, food)

?Cells carry out the chemical reactions of life (they have metabolisM)

????- MetabolisM = the set of chemical reactions that enable life’s essential functions;

????- Enables building proteins (requiring ribosomes) in the cytoplasM;

????- Cells may contain other specialized structures (e.g., organelles, endomembrane system).

Metabolic capabilities constrain habitable environments

?OrganisMs vary in the energy sources they can use, depending on the metabolic abilities encoded in their genomes.

????- Photosynthetic organisMs (plants, algae, some archaea) use the sun’s energy to fix carbon (get organic carbon from inorganic compound) from the atmosphere.

????- Some bacteria and archaea can fix atmospheric nitrogen into ammonia.

????- Fungi possess a wide range of metabolic pathways. Some taxa can decompose cellulose and lignin while others manufacture antibiotics.

????- Some archaea are chemotrophs, able to use energy from sulfur or ammonia to fix carbon.

????- Animals are heterotrophs: obtaining their energy from the consumption of other organisMs.

[4] Viruses

?Biologists do not agree on whether or not viruses are alive.

????- Membranes: viruses have a capsid (protein coating) protecting their genetic material and some have an outer envelope (made from host materials) that acts like a membrane;

????- Reproduction: viruses have their own genomes (DNA or RNA) and replicate themselves using the host’s existing machinery;

????- Size: generally much, much sMaller than cellular life! (But check out giant viruses)

?Most viruses lack their own metabolisM, hijacking their host’s resources instead. Viruses typically manipulate the host cell’s metabolic activity.

[5] Cell Size: Cells are constrained to be sMall

?Typical cell sizes: Diameter < 500 microns;

?Plant/animal cells ~10-100 microns; bacterial cells ～1-10 microns.

Cell size is constrained by rate of diffusion tradeoffs:

Volume (V)?increases faster than surface area (SA); both increase with size.

?More SA means greater ability to diffuse resources into cell.

?Greater V reduces the rate of diffusion within the cell and means higher resource needs.

?Solutions to size constraints might include:

????- Multi-cellularity or cooperation among cells (e.g., biofilms in bacteria).

????- Compartmentalization within the cell (e.g., organelles or multinucleation).

????- Elongated rather than spherical cell shape.