Unit 1 Lesson 5 Notes: Cells

Biological Foundations: Cells and Life

Cell Theory: All living organisms are made up of one or more cells; all cells arise from preexisting cells.
Biological definition of life: life involves growth and development, response to the environment, cellular structure and composition, reproduction, heredity, metabolism, and homeostasis. All living things on Earth are composed of at least one cell surrounded by a plasma membrane.

Nucleic acids: DNA and RNA. RNA is single-stranded, uses ribose sugar, and uracil instead of thymine.
DNA provides a universal language for protein synthesis across all living things.
Transcription: the sequence of DNA is copied into messenger RNA (mRNA). Mapping example: A\to U,\quad T\to A,\quad C\to G,\quad G\to C\, in the mRNA.
Translation: mRNA is read by tRNA to assemble a specific polypeptide (protein).

Flow of genetic information: \text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Polypeptide} (transcription and translation).
Ribosomes translate mRNA into proteins; the rough endoplasmic reticulum (ER) has ribosomes; the smooth ER lacks ribosomes and performs varied functions including detoxification.

Amphipathic phospholipids: a hydrophilic, polar phosphate head and hydrophobic tails.
Fluid Mosaic Model: membranes consist of phospholipids, cholesterol, and membrane proteins; carbohydrates extend outside as cell surface markers.

A bilayer forms a barrier between inside and outside of the cell; water and small molecules move across via channels or transport mechanisms.
Vesicles bud from or fuse with membranes during transport and trafficking.

Cells are optimized for surface area-to-volume for environmental interactions and nutrient exchange.
No organism is a single giant cell due to limitations in moving wastes and materials across large membranes.

Some single-celled organisms are larger, but most are small; prokaryotic cells (bacteria) are typically at least ~10x smaller than many eukaryotic cells.
Human cells vary in size; overall, eukaryotic cells have greater size due to internal compartmentalization.

All cells are enclosed by a plasma membrane that separates internal environments from the surroundings.
Eukaryotic cells contain internal compartments (organelles) with their own membranes, enabling different environments and specialized functions.

Cytoplasm: the main internal space outside the organelles.
Nucleus: stores DNA, copies information into RNA, and makes ribosomes.
Ribosomes: translate mRNA into polypeptides (proteins).
Golgi apparatus: packages proteins in membrane-bound vesicles for export.
Lysosomes: contain enzymes that break down wastes.
Mitochondria: powerhouse of the cell; generate ATP via glucose and oxygen; have their own DNA; double membrane; evidence for endosymbiotic origin.

Pathway: ribosomes synthesize proteins → rough ER packages proteins → Golgi apparatus processes and ships them → vesicles transport to destinations (outside cell or within).
Mitochondria characteristic: double membrane, own circular DNA, similar size to bacteria, vital for energy production.

Mitochondria (and chloroplasts in plants) likely originated from bacteria that entered a larger host cell and formed a symbiotic relationship. Evidence includes their own DNA and ribosomes, and double membranes.

Viruses outside a host cell do not acquire or use energy or nutrients and are not considered living in the absence of a host.
Inside host cells, viruses hijack cellular machinery to reproduce and may be enclosed by host-derived membranes; they are capsules of nucleic acids and proteins.

Life is characterized by cellular organization, growth, response, metabolism, homeostasis, reproduction, and heredity; cells form the basis of all living things.
The cell is organized into compartments and membranes that enable specialization and efficient transport and metabolism.
DNA is the universal genetic language; transcription and translation convert genetic information into functional proteins.
The mitochondrial endosymbiotic origin explains the presence of semi-autonomous organelles with their own DNA.
The plasma membrane is best understood through the fluid mosaic model with amphipathic lipids, proteins, cholesterol, and external carbohydrate chains.