Cells: The Working Units of Life - Study Notes

Understanding the fundamental features that make cells the fundamental units of life.
Exploration of prokaryotic cells characteristics.
Exploration of eukaryotic cells characteristics.
Examination of the roles of extracellular structures.
Understanding the origin of eukaryotic cells.

Cell Theory is a unifying theory in biology that states:
- Cells are the fundamental units of life.
- All organisms are composed of cells.
- All cells arise from preexisting cells.
- Quote by E.B. Wilson:
- “Long ago it became evident that the key to every biological problem must finally be sought in the cell; for every living organism is, or at some time has been, a cell.”

Functions of all cells are similar but not identical, allowing for specialization (e.g., cardiomyocytes vs. pyramidal cells).

Cells are generally small, with most cells ranging from 10-20 μm in size.
The largest human cell is the egg (oocyte), measuring approximately 150 μm in diameter, making it barely visible to the naked eye.
Comparative sizes of various specimens:
- Oscillatoria: 7,000 μm
- E. coli: 1,300 x 4,000 μm
- Rickettsia: 475 μm
- Poxvirus: 230 x 320 μm
- Influenza virus: 85 μm
- T2 E. coli bacteriophage: 65 x 95 μm
- Tobacco mosaic virus: 15 x 300 μm
- Poliomyelitis virus: 27 μm

Cells maintain a high surface area-to-volume ratio crucial for efficiency:
- Volume determines the amount of chemical reactions per unit time; larger cells have more metabolic activity.
- Surface area determines the rate at which resources enter and waste exits the cell.
There are advantages to being small which aid in these processes.

1 μm = 1 x $( ext{10}^{-6})$ m

Most cells require a microscope for visibility.
- Maginification: Increase size appearance.
- Resolution: Defines clarity, or the minimum distance at which two points can be distinguished as separate.
- Low resolution examples vs high resolution examples.
Scale of Life:
- Example sizes from atoms to organisms:
- Atoms: 0.1 nm
- Small molecules: 1 nm
- Most bacteria: 1 μm
- Most plant and animal cells observed under light microscope.

Light Microscopes:
- Use glass lenses and visible light.
- Maximum resolution of 0.2 μm.
Electron Microscopes:
- Use electromagnets to focus electron beams.
- Maximum resolution of 0.2 nm.

Light Microscopes:
- Bright-field, phase-contrast, fluorescence microscopy options with various magnification standards.
Scanning Electron Microscope (SEM):
- Provides 3D images of surfaces (e.g., ant's head, eye).

Definition: The plasma membrane serves as the outer boundary of the cell.
Composition:
- Made up of a phospholipid bilayer embedded with proteins and carbohydrates.
Functions:
- Selectively permeable barrier.
- Maintains constant internal environment (homeostasis).
- Involved in communication and signaling.
- Facilitates adhesion between adjacent cells.

Phospholipid Bilayer:
- Composed of hydrophilic heads facing the aqueous environment and hydrophobic tails facing inward.
Embedded Proteins:
- Include receptors, transport proteins, adhesion proteins, etc.

Complex network consisting of proteins like collagen embedded within proteoglycan complexes.
Fibronectin: Attaches the ECM to integrins which are embedded proteins that link the external environment to internal filaments and can transmit signals impacting cell behavior.

Early beliefs about a "bald" cellular surface were corrected by findings of the varying complexity in the components.

Prokaryotic Cells:
- Lack a nucleus and membrane-bound organelles.
- Exhibit simpler structure.
- Characteristics:
- Typically 1-10 micrometers in diameter.
- Contain a plasma membrane enclosing the cell.
- DNA is located in a nucleoid region and not inside a nucleus.
- Cytoplasm includes cytosol plus ribosomes and other structures.
Eukaryotic Cells:
- Contain a nucleus and membrane-bound organelles.
- Exhibit more complexity.
- Characteristics:
- Usually 10-100 micrometers in diameter.
- Organelles perform specialized functions that contribute to cellular operations.
- Examples include the nucleus, chloroplasts, and mitochondria.

Cell Structure:
- Prokaryotes: No nucleus, single circular DNA, smaller ribosomes, and simpler cytoskeleton.
- Eukaryotes: Defined nucleus, multiple linear chromosomes, larger ribosomes, and complex cytoskeleton.
Cell Division:
- Prokaryotes: Binary fission.
- Eukaryotes: Mitosis.

Enclosed by a plasma membrane.
Cytoplasm: Composed of cytosol, ribosomes, and filaments.
Rigidity provided by a cell wall (usually).
Movement facilitated by flagella in some bacteria.
Pili: Hair-like structures that aid in adherence to surfaces and can facilitate genetic exchange.
Fimbriae: Shorter than pili, also help in adherence.

A system of protein filaments maintaining cell structure and assisting in cell division, although simpler than in eukaryotic cells.

Formed with multiple organelles, allowing for larger size and multiple cellular functions.
Organelles include the nucleus, endoplasmic reticulum, mitochondria, and more, each serving specialized roles in cellular metabolism and processes.

Largest organelle within eukaryotic cells, holds genetic material (DNA) with sites for replication and transcription processes.
Surrounded by the nuclear envelope with nuclear pores for molecule transport.

Dense region within the nucleus, site for ribosome assembly, which are then exported to the cytoplasm.