遗传 进化与生态学 3 - Cells / Cellularity

本期的内容是细胞的特征。本文集的这一部分是遗传、进化与生态学 Genetics, Evolution, and Ecology. 这门课理论上建议在阅读完文集的第一部分的内容之后再开始学习，但基础不足的朋友也可以尝试阅读喔~

这一部分的主要内容均来自 Prof. Angela J. Roles 的课程，因此本文集的这一部分均不会标记为原创。但由于文本来源不清晰，UP主还是一个字一个字码出来的文章，本文禁止非授权的转载，谢谢！

封面/头图：https://images.squarespace-cdn.com/content/v1/5ace248ea2772c484f925def/1553009634641-DKORVRM74V5WOUW70VNV/ke17ZwdGBToddI8pDm48kNvT88LknE-K9M4pGNO0Iqd7gQa3H78H3Y0txjaiv_0fDoOvxcdMmMKkDsyUqMSsMWxHk725yiiHCCLfrh8O1z5QPOohDIaIeljMHgDF5CVlOqpeNLcJ80NK65_fV7S1UbeDbaZv1s3QfpIA4TYnL5Qao8BosUKjCVjCf8TKewJIH3bqxw7fF48mhrq5Ulr0Hg/image-asset.png

Lesson 3: Cells - Cellularity

Cellularity (就是cellular的名词) 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.