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BIOL10004: Biology of Cells and Organisms
Lecture 2
Prokaryotic vs Eukaryotic cells: The greatest division amongst organisms is between those cells that have
nuclei (said to be eukaryotic) and those that lack nuclei (or prokaryotic). Of all cells, only bacteria and
Archaea have prokaryotic structure, and these two super Kingdoms are very different from one another.
Origin of Eukaryotic cells from Prokaryotic cells: We have an idea of when eukaryotic cells first arose from
the fossil record, but the origin of eukaryotic cells is not known. Evolution theory states that eukaryotic cells
evolved from prokaryotic cells.
Prokaryotic cells: There are two super Kingdoms of prokaryotes, the bacteria (including the cyanobacteria)
and Archaea, or simplest cells. We have only just begun to understand the Archaea. Prokaryotic cell
structure is very simple, and their mechanism of cell division is also simply when compared to eukaryotic
cells.
There are a large number of bacteria, ubiquitous micro organisms (1-10μm) that are very important in the
environment for the decomposition of organic matter and to recycle nutrients. Bacteria are also the major
cause of disease (e.g. typhoid, cholera, dysentery, TB, plague, gangrene, VD, tetanus, pneumonia,
Legionnaire’s) and vital in food production (e.g. cheese, yogurt, sour cream, sauerkraut, vinegar, citric acid).
Most bacteria are heterotrophic, some are able to photosynthesise (photoautotrophs), some are able to
oxidise inorganic compounds and harvest energy (chemoautotrophs) and some are able to fix nitrogen from
air (important source of green “fertiliser”).
How do Archaea differ from Bacteria?
• Morphologically they don’t
• Over half of Archaean genes were new to science when they were first discovered
• Biochemically, Archaea are nearly as different from Bacteria as they are from Eukaryotes (making
them a super Kingdom)
• The two central processes in molecular biology, genetic transcription and translation are more similar
to those of eukaryotes than bacteria
• Features of the Archaean lipids and their membranes are unusual and Archaea lack a peptidoglycan
wall (except for a few organisms, and in those circumstances, the peptidoglycan is biochemically
distinctive from the bacterial peptidoglycan)
• Most Archaeans are extremophiles (e.g. hot, cold, acidic, alkaline, highly saline, extreme pressure,
etc.), though not all. This story in changing as we learn more about Archaea
• As of 2009, no clear examples of Archaean pathogens are known
• Eukaryotes have almost equal numbers of genes from both Archaea and Bacteria
Prokaryotes are normally unicellular or filamentous. They are ubiquitous with a wall of murein
peptidoglycan (penicillin blocks synthesis). The cell structure is very simple, with a cell membrane,
mesosome, circular chromosome + plasmid and small ribosomes. When present, flagella are simple in
structure when compared to eukaryotes. Bacterial flagella are extracellular and composed of flagellin.
Reproduction is by simply binary fission.
BIOL10004: Biology of Cells and Organisms
Lecture 3
Prokaryotic vs Eukaryotic cells: There are two fundamentally distinct kinds of cells that differ most notably
in the organisation of their genetic material.
Division of labour: The eukaryotes are characterised by their structural and functional complexity, with their
biochemical reactions isolated from one another within distinct cellular compartments. Eukaryotes may be
unicellular or form complex multicellular organisms. Cells contain true nuclei, membrane-bound
mitochondria (+ chloroplasts in plants), an endomembrane system, a cytoskeleton (including microtubules &
actin), flagella, linear chromosomes with histones, large ribosomes and other features.
The great majority of organisms that we are familiar with are eukaryotic. In the 3 super Kingdom system,
there are 2 prokaryotic kingdoms (the bacteria and Archaea), but 1 eukaryotic super Kingdom, which
includes 4 Kingdoms (the protists, fungi, animals and plants).
Nucleus: Aggregation of DNA, control centre for the cell. The nucleus is surrounded by a double membrane,
which is part of the endomembrane system. The major components of the nucleus are the DNA and RNA.
These molecules will be discussed in detail in the genetics section of 600-142.
Mitochondrion (or Mitochondria): Present in almost all eukaryotic cells as the power plant of the cell.
Mitochondria contain enzyme systems for the breakdown of food molecules to release energy. Each
organelle is surrounded by a double membrane, the inner membrane folds inward to produce finger-like
projections called cristae.
Chloroplasts: Present in Kingdoms Plantae and Protista, catching light energy and synthesising food
molecules by the process of photosynthesis. Organisms with chloroplasts are therefore all the base of all
food chains. Chloroplasts contain chlorophyll, a green pigment that traps light energy. Each organelle is
surrounded by a double membrane envelope. The inner membrane has complex folds that form enclosed
sacks of membrane called thylakoids. A stack of thylakoids is called a granum, while lamellae are thylakoids
that connect two granae. The matrix that surrounds the membrane components is called the stroma.
Chloroplast structure and pigmentation varies considerably between different kinds of organisms.
Endosymbiosis and the origin of organelles: There is no real evidence to explain the origin of the nucleus,
but it probably arose by the aggregation of flattened membrane spheres around a nucleoid containing DNA
and RNA. The nuclear pores remain to allow communication between the nucleus and cytoplasm (for
example, to allow for the passage of RNA out of the nucleus to the cytoplasm). The membrane probably
came from invaginations of the cell membrane.
The eukaryotic cell may actually be a chimera of other cells. Double membrane bound organelles
(mitochondrion & chloroplast) are probably prokaryotes living permanently inside a eukaryotic cell.
Evidence for an endosymbiotic origin of organelles is interesting, and is still accumulating in the scientific
literature.
Mitochondrion: Thought to have originated as symbiotic, aerobic bacteria (purple bacterium)
Chloroplast: Believed to be derived from a symbiotic, photosynthetic bacteria (or cyanobacterium)
Secondary or Eukaryotic endosymbiosis: There are organisms today that appear to be in the process of
obtaining organelles, particularly chloroplasts, from other organisms. If a eukaryotic cell devours another
eukaryotic cell and obtains a chloroplast from it, the process is referred to as secondary or eukaryotic
endosymbiosis. An example is a cryptomonad cell.
BIOL10004: Biology of Cells and Organisms
The Cellular Basis of Life
All cells:
• are enclosed by the plasma membrane
• contain a semifluid matrix called cytoplasm
• contain ribosomes (sites of protein synthesis)
The first cells to evolve were bacteria:
• simple structure
• lack internal compartments
• called prokaryotes (pro ! before, karyon ! kernel or nucleus)
• double-helical strand in an area called the nucleoid
More complex cells are called eukaryotes:
• DNA is enclosed within a membrane-bound nucleus
• range from single celled organisms to complex multi-cellular plants and animals
• cells have to be able to communicate and cooperate
Prokaryotes
• smaller (about 1μm) than eukaryotes, in general
• semi-rigid cell wall surrounds plasma membrane
• a few hundred – a few thousand ribosomes (about 15nm in diameter)
• DNA is a template for mRNA synthesis
• mRNA bind to ribosomes and use the nucleotide sequence for protein synthesis
• lack cytoskeletal structures e.g. microfilaments and microtubules
• often have flagella (different structurally from eukaryotic flagella)
• proteinaceous fibres (flagellin)
• form a stiff coiled filament
• projects from a special structure in the membrane that serves as a motor
• movement generated by rotation of curved filament (like a propeller)
• direction can be changed by reversing the direction of rotation
Eukaryotes
• possess internal, membrane-bound components, organelles
• each organelle has a specific function
• the cytosol is an aqueous solution of molecules with a gel-like consistency
• the cytosol and subcellular components (excluding the nucleus) form the cytoplasm
• the cytoplasm and the nucleus form the protoplasm
N.B. See Fig 4.3, page 77 for a good diagram
The Nucleus
• surrounded by a double membrane called the nuclear envelope
• nuclear envelope usually contains one or several nuclei
• nucleoli are darkly staining regions which contain high concentrations of RNA, proteins and DNA
• the two membranes of the nuclear envelope are separated by 50nm
• they are perforated by nuclear pores
• nuclear pores allow the movement of certain molecules between the cytoplasm and the nucleoplasm
• the inner and outer membrane have distinct chemical compositions
BIOL10004: Biology of Cells and Organisms
Nuclear Pores
• each pore is a precisely organised complex containing multiple copies of about 30 different proteins
• some of these proteins form a meshwork of fibrils that projects from the walls of the pore into the
channel, cytosol and nucleus
• small molecules (up to 5 kDa) can diffuse freely
• passive transport of larger molecules is prevented
• large DNA and RNA molecules (100-200 kDa) are actively moved through the pores
• nuclear proteins contain short amino acid sequences called a nuclear localisation signal
• this binds to cytosolic nuclear import receptors which then facilitate their passage into the nucleus
• proteins must also have nuclear export signals which bind to nuclear export receptors
Chromosomes
• long DNA molecules wind around histone molecules
• this forms a nucleosome
• this arrangement allows DNA to twist into a helix, forming a chromatin strand
• when a cell is not actively dividing
• some chromatin strands aggregate, forming densely staining regions called heterochromatin
• some disperse, forming lightly staining regions called euchromatin
• euchromatic regions are sites of active gene transcription
• when a cell is dividing, the chromatin strands bunch up and thicken to form chromosomes
Nucleolus
• subcompartment of the nucleus
• darkly staining ! easy to see under a microscope
• the site of synthesis and maturation of rRNA
• the site of assembly of ribosomal subunits for export to the cytoplasm
• cells generally make 10,000 ribosomes per minute
• nucleolus contains hundreds of copies of rRNA genes
• the nucleolus disappears during cell division
Mitochondria
• very different from most organelles
• have their own DNA
• make their own proteins
• can grow and divide (with help from other cell components)
• believed to have evolved from bacteria and were engulfed by eukaryotes
• site of cellular respiration (release of energy during the oxidation of sugars and fats)
• hard-working cells have more mitochondria, and vice versa
• large enough to be seen through a light microscope
• the appearance of mitochondria varies (can be spherical or elongated, can fuse or fragment)
• some are fixed, while others are part of cytoplasmic flow
• surrounded by a double membrane
• the outer membrane is highly permeable
• allows ions and small molecules (up to 10 kDa) to pass through freely
• inner membrane is folded (cristae)
• inner membrane is highly impermeable, especially to ions
• this property allows the generation of an electrochemical gradient (unequal distribution of ions)
BIOL10004: Biology of Cells and Organisms
Lecture 4
Division of labour in the cytoplasm: Organelles such as mitochondria and chloroplasts are fully integrated
into the cytoplasm of eukaryotic cells, yet clearly carry out specific functions. These organelles are believed
to be derived from ancient endosymbionts. This division of labour within cells is characteristic of
eukaryotes. In addition, there are many other membrane-bound organelles of the cytoplasm that carry out
specific functions, and these components are generally said to be part of the endomembrane system.
The endomembrane system: A system of compartments that generally includes all of the membrane-bound
components of the cell except mitochondria and chloroplasts, but including the nuclear envelope. Many
components of the cytoplasm appear to be separate in electron micrographs, but they are in fact part of a
continuum and are physically joined. The relationship of these membrane-bound components to one another
is continually changing. Cells and cellular components are not static, as suggested in electron micrographs,
but dynamic. The heart of the system is the endoplasmic reticulum (or ER), but also includes the Golgi
apparatus, vacuoles, lysosomes and other membrane-bound vesicle microbodies.
Endoplasmic reticulum: The heart of the endomembrane system, consisting of membrane cisternae that
ramify through the cytoplasm in the form of internal compartments and channels.
Golgi apparatus: Consists of flattened stacks of membranes or cisternae, called Golgi stacks, that function in
the collection, packaging and distribution of molecules synthesised elsewhere in the cell.
Vacuoles: Most common in plant cells where they may occupy a large portion of the whole cell. Vacuoles
are surrounded by a membrane, called the tonoplast, that is similar in structure and function to the cell
membrane. Vacuoles have several functions, including the maintenance of osmotic equilibrium, food
storage, and use as a waste disposal dump.
Lysosomes: Contain hydrolytic enzymes for intracellular disgestion, digesting either food taken into the cell
in food vacuoles, or worn out cellular components. The enzymes are isolated in vesicles to prevent their
general release into the cytoplasm.
Components of the Cytoskeleton: If you remove all the organelles and the components of the endomembrane
system from a cell, the cell would still retain its shape and be capable of certain movements if energy was
available. The non-membrane components of cells responsible for these features of eukaryotic cells are
referred to as the cytoskeleton.
Major element of the cytoskeleton, including cytoskeletal motors:
• Actin filaments (termed microfilaments) – composed of actin protein that forms long filaments.
Common motor is myosin
• Microtubules – composed of a protein called tubulin, which is about 7-8 nm in diameter. Tubulin
molecules are arranged in 13 protofilaments to form a cylinder that is 25 nm in diameter, but varies
considerably in length. Common motors are kinesin and dynein
• Intermediate filaments – composed of proteins about 10 nm in diameter
Differences between plant and animal cells: Simply put, plants have chloroplasts, vacuoles and cell walls,
and animal cells don’t.