Brent Cornell

All living things are capable of performing seven key functions (things that cannot perform all seven functions are considered non-living):

• Metabolism, Reproduction, Sensitivity, Homeostasis, Excretion, Nutrition, Growth / Movement  (simple mnemonic = MR SHENG)

In essence, living things undergo complex chemical reactions (metabolism) in order to maintain a stable living condition (homeostasis)

• These chemical reactions require inputs (gained via nutrition) and produce wasteful outputs (removed via excretion)
• To maintain stable conditions, living things must be able to detect changes (sensitivity) and respond accordingly (growth / movement)
• Continued survival also requires living things to be able to repair and replicate themselves (reproduction)

Cell Theory

The fundamental unit that is capable of performing all of the functions of life is the cell – and according to the cell theory:

• The cell is the smallest unit of life (nothing smaller than a cell is considered to be living – e.g. viruses are not alive)
• All living things are composed of cells (or cellular products) – i.e. living organisms may be unicellular or multicellular
• Cells arise from pre-existing cells – life cannot spontaneously generate (as shown by Pasteur’s biogenesis experiment)

Types of cells

Any living thing that performs all the functions of life and can operate as an independent entity is called an organism

• Organisms may be composed of one of two cell types (simpler prokaryotic cells or more complex eukaryotic cells)

All organisms can be classified into different groups (taxonomic ranks) according to certain shared characteristics

• At the highest level of classification are domains, followed by kingdoms (then phyla, class, order, family, genus and species)

Prokaryotes can be classified into two distinct domains:

• Bacteria – A diverse domain that includes all traditional bacterial species (including all pathogenic forms)
• Archaea – Includes most extremophiles (prokaryotes that are found in adverse environments – like high temperatures)

Eukaryotes belong to the domain Eukarya and may be classified into four distinct kingdoms:

• Protists – Include various unicellular and multicellular organisms that lack specialised tissue
• Fungi – Have cell walls made of chitin and obtain nutrition via heterotrophic absorption (i.e. they are decomposers)
• Plants – Have cell walls made of cellulose and synthesise organic nutrients via photosynthesis (i.e. they are producers)
• Animals – Lack a cell wall and obtain nutrition via heterotrophic ingestion (i.e. they are consumers)

Prokaryotic Cells

Prokaryotic cells are the most basic types of cells and contain four key cellular components:

• They are enclosed by a plasma membrane, which separates the internal contents from the external environment
• They contain an internal fluid (called the cytosol) in which various metabolic reactions and biological processes can occur
• The genetic material is composed of a single circular chromosome (called the genophore), located in a region called the nucleoid
• There are ribosomes (70S in size) that function to translate the genetic instructions into cellular activity (by making proteins)

Additionally, prokaryotic cells may contain certain additional cellular components:

• They may also contain autonomous circular DNA molecules called plasmids, which can be transferred via bacterial conjugation
• Hair-like extensions of the plasma membrane (called pili) may mediate surface attachments or facilitate plasmid exchange
• Longer projections called flagella contain microtubules and motor proteins that enable prokaryotic movement (via a whip-like motion)
• All bacterial cells contain a rigid outer cell wall made of peptidoglycan to help maintain the overall shape and structure of the cell
• Some bacteria may possess an additional outer covering called the slime capsule (glycocalyx) to help prevent desiccation

Eukaryotic Cells

Eukaryotic cells have a more complex structure as they contain membrane-bound compartments that perform specific roles:

• Eukaryotic cells have a double-membrane nucleus that stores the genetic material as chromatin (uncondensed linear DNA)
  
• Within the nucleus is a region called the nucleolus, which is the site of ribosome assembly (ribosomes will be 80S in size)
  
• The mitochondrion is the site of aerobic cellular respiration and is responsible for the production of ATP (energy source)
  
• Lysosomes function to break down cellular components, whereas peroxisomes function to break down toxic metabolites
  
• Centrosomes produce microtubule spindle fibres and are involved in the process of cell division (e.g. mitosis or meiosis)
  
• The Golgi complex is a series of membrane stacks and vesicles that act to sort, store, modify and export cellular materials 
• An intracellular membranous network called the endoplasmic reticulum (ER) transports materials between organelles 
  
  • The rough ER is embedded with ribosomes and transports proteins within the cell, while smooth ER transports lipids

Plant cells possess a number of additional cellular components that are no present within animal cells:

• They contain a rigid cell wall made of cellulose to provide mechanical support to the cell and prevent excess water uptake
• They have a large, central vacuole that helps to maintain hydrostatic pressure within the cell and regulate internal pH
• The leaf tissue will contain chloroplasts which are responsible for the process of photosynthesis (not present in root cells)

Fungal cells have a cell wall made of chitin, while protista vary greatly in organisation and do not have distinctive features

Endosymbiosis

The origin of eukaryotic cells can be explained by endosymbiotic theory (they evolved from symbiotic prokaryote interactions)

• Eukaryotic cells are believed to have evolved from the engulfment of a prokaryote by another prokaryote (via phagocytosis) 
• The engulfed prokaryotic cell remained undigested as it contributed new functionality to the engulfing cell (e.g. photosynthesis)
• Over generations, the engulfed cell lost some of its independent utility and became a supplemental support structure (organelle)

Mitochondria and chloroplasts are both eukaryotic organelles suggested to have arisen via endosymbiosis

• Mitochondria were prokaryotes that could undertake aerobic respiration, while chloroplast were photosynthesising cyanobacteria
• Evidence that supports the extracellular origins of these organelles can be seen by the presence of certain prokaryotic features