cell structures part 1
Archaean Cells
The lineage called Archaea was discovered less than 50 years ago.
Remains poorly understood, with major new groups being described regularly.
All archaea described to date are single-celled and relatively small compared to eukaryotes.
Most archaea inhabit extreme environments:
Deep sea
Hot springs
Frozen areas
Areas lacking oxygen
Highly basic, acidic, or salty conditions.
The study of archaea is challenging due to:
Difficulty in replicating their extreme habitats in laboratory settings.
Lack of known archaea causing disease in humans or having economic importance, reducing research motivation.
Two distinctive aspects of archaea cell structure:
Unique hydrocarbon chains in membrane lipids.
Unique types of carbohydrates in cell walls.
Connection between unique membranes/walls and habitat:
Their unique membranes and walls help archaea cope with extreme physical challenges (e.g., temperature, pressure, pH).
Introduction to Cells
The cell is the fundamental unit of life; it is the smallest entity that functions as an independent organism.
Individual cells are the building blocks of larger multicellular organisms.
Key points before exploring cell structures:
Most species on Earth are single-celled organisms, found in diverse habitats:
Inside glacial ice.
In seawater at over 100°C, 3000 meters deep.
In rock crevices up to 5 km deep and 10 km below the ocean floor.
Floating in the atmosphere 11 km above Earth.
Within and on multicellular organisms' bodies.
Three main lineages (or domains) of life (tree of life):
Archaea (ar-KEE-ya)
Bacteria
Eukarya (you-CARE-ya)
Distinctive and functionally important structural differences among these lineages.
Bacterial Cells
Like archaea, most bacterial cells are smaller than the average eukaryotic cell.
Most bacteria possess:
Cell walls
One or more flagella
Differences between bacteria and archaea:
Bacteria discovered over 450 years ago; studied due to their impact on human health and significance in commercial applications (food/drugs).
A few species of bacteria are multicellular, unlike archaea.
Bacteria have organelles and cytoskeletal structures, while evidence of these in archaea is emerging.
Bacterial chromosome characteristics:
Most bacteria have a single, circular chromosome.
This chromosome is viewed as an organelle due to its circular form, association with proteins, and compact structure.
Shared Cell Structures
Four structures found in every cell:
Cell membrane: A lipid bilayer with membrane proteins and carbohydrates, regulates entry/exit of molecules.
Genetic material: Heritable information in the form of DNA molecules (chromosomes), can be linear or circular.
Ribosomes: Complexes of RNA and proteins; primary site for protein synthesis.
Cytoskeletal elements: Provide structural support and assist in material transportation within the cell.
Three additional common elements:
Organelles: Membrane-bound internal compartments performing specialized tasks.
Cell wall: Provides rigidity and protection, usually composed of carbohydrates.
Flagella: Long projections used for propulsion through water.
Size Comparison of Prokaryotic and Eukaryotic Cells
An average eukaryotic cell is 15,000 times larger than an average prokaryotic cell.
The number of average-sized prokaryotic cells that could fit inside a single average-sized eukaryotic cell would depend on cell dimensions, not volumes.
An example of antibiotic function shows the importance of ribosome structure:
Some antibiotics inhibit bacterial ribosomes, ceasing protein production and killing the bacteria.
Prokaryotic vs Eukaryotic Cells
Prokaryotes include archaea and bacteria (cells lacking a nucleus); eukaryotes possess a nucleus.
Key observations about shared structures:
All cells have key features like a cell membrane, chromosomes, ribosomes, and cytoskeletal elements.
Structure of shared components varies by lineage.
Components unique to each lineage include lipids in membranes and carbohydrates in cell walls, as well as distinct flagella structures.
Overview of organelles:
Organelle structures vary between prokaryotes and eukaryotes, with no organelles shared between the two.
Diversity and number of organelles in eukaryotes exceed those in bacteria.
Eukaryotic cells tend to be larger than prokaryotic cells due to efficiency in size and function.
Definitions
Chromosome: A DNA molecule, circular or linear depending on the species, typically associated with specific proteins.
Ribosome: A molecular machine of RNA and proteins, site of protein synthesis.
Organelle: Any membrane-bound compartment within a cell.
Cytoskeleton: Rod-like proteins providing structural support and facilitating transport and movement.
Cell wall: A rigid exterior composed of carbohydrates and other molecules, outside the cell membrane.
Flagellum: A long, flexible structure that propels the cell through fluid.
Nucleus: A double-membrane organelle containing linear chromosomes in eukaryotes.
Mitochondrion: An organelle in eukaryotes that produces energy by oxidizing sugars.
Eukaryote: Organisms whose cells contain a nucleus and mitochondria.
Prokaryote: Organisms in the Archaea and Bacteria lineages lacking a nucleus.
DNA and Nucleus Size Comparison
The total DNA in a human cell stretches up to 2 meters, while an average nucleus is just 10 micrometers (10 μm or 10 × 10^-6 m) long.
To compact DNA to fit in the nucleus:
Compaction factor = total DNA length / nucleus length, yielding 200,000 times compaction.
Unique Eukaryotic Organelle Structure
Mitochondrion: Surrounded by two membranes where the inner membrane forms tubes and sacs. Essential for ATP production during cellular metabolism.
Eukaryotic cells contain numerous membrane-bound organelles, with nucleus and mitochondrion being highly prevalent across the domain.