Cells, prokaryotes and eukaryotes

16/09/2012

1

Cells, Prokaryotes and Eukaryotes

Cells: “Little rooms”

A sense of Scale

Fig. 6.2

Why are large living things made of small cells?Living things (organisms) need to exchange nutrients with the outside world

Surface area to volume ratio is important

Too little surface area for the volumeMeans that exchange cannot happen fast enough to support the whole volume.

Various mechanisms keep the surface area to volume ratio high.

One mechanism is keeping distinct cells in multicellularorganisms.

Fig 6.7

16/09/2012

2

The simplest organisms are of course single cells

And the simplestSingle celled organismsAre Prokaryotes

Fig 6.5

Internal Organization and DNA• Prokaryotic cells usually lack complex compartmentalization– Limited internal organization

• Some prokaryotes do have specialized membranes that perform metabolic functions

• These are usually infoldings of the plasma membrane– Which of course serve to increase the surface area.

© 2011 Pearson Education, Inc.

Fig. 6.5

Prokaryote DNA

• Prokaryotes have relatively few genes

• Relatively little DNA• Usually one circular loop

• May have a few small extra loops called plasmids.

Fig 27.8

16/09/2012

3

Prokaryote reproductionCopy the DNA, then divide in two.

Fig 12.12

Plasmids can be copied from one cell to another, transfering information

Figure 27.13a-3

F plasmidBacterialchromosome

F cell(donor)

F cell(recipient)

Matingbridge

Bacterialchromosome

(a) Conjugation and transfer of an F plasmid

F cell

F cell

Rapid Reproduction and Mutation• Prokaryotes reproduce by binary fission, and offspring

cells are generally identical• Mutation rates during binary fission are low, but because

of rapid reproduction, mutations can accumulate rapidly in a population

• High diversity from mutations allows for rapid evolution• Prokaryotes can also absorb ‘naked’ DNA from their

environment– Usually nothing much happens– Occasionally acquire new capabilities (e.g. Drug resistance)


R Plasmids and Antibiotic Resistance• R plasmids carry genes for antibiotic resistance• Antibiotics kill sensitive bacteria, but not bacteria with specific R plasmids

• Through natural selection, the fraction of bacteria with genes for resistance increases in a population exposed to antibiotics

• Antibiotic‐resistant strains of bacteria are becoming more common


16/09/2012

4

• Their short generation time allows prokaryotes to evolve quickly

– For example, adaptive evolution in a bacterial colony was documented in a lab over 8 years

• Prokaryotes are not “primitive” but are highly evolved


Prokaryotes vs Eukaryotes

Prokaryotes• No Nucleus• Circular DNA• Little internal structure• No organelles

• Always unicellular, binary fission

• Hugely abundant and diverse, but invisible

Eukaryotes• Chromosomes contained in

a nucleus.• Extensive internal structure• Specialized organelles

– Mitochondrion, Chloroplast etc

• All multicellular organisms are eukaryotes.

• Visible, but actually make up only a small portion of life on earth

“Life on Earth is microscopic”

Fig 26.21

What do I mean prokaryotes are diverse?

Genetically• Genes taken from two

random prokaryotes will be much more different than

• Comparable genes taken from two random animals.

Metabolically• All Eukaryotes have pretty

similar metabolism compared to prokaryotes.

• Plants:– Photo,Autotrophs– Generate their energy from

light, carbon from CO2• Animals

– Heterotrophs– Get their energy and carbon

from food.

16/09/2012

5

There are prokaryotes that...

• Get their energy from inorganic chemicals

• “Breath” iron– Use iron as a final electron

acceptor in respiration– What we do with oxygen

• “Burn” iron– Use iron as an original

electron donor in respiration

– What we do with food.

• Aerobic (use Oxygen)• Anaerobic (cannot use

oxygen)• E.g., Methanogens

“exhale” methane– What we do with CO2

Prokaryotes in the living world• “The major biogeochemical

cycles on which we depend were in place three billion years ago, long before the appearance of visible life, and are today maintained by the ‘invisibles’ and their vast range of metabolisms.”

• “The contribution of visible life to biodiversity is very small indeed.”

• “Our view of the natural world [is changing] as radically as did our view of the cosmos when we began looking at it with technologies that allowed us to see more than can be seen with the naked eye.”

• “Neglect of the invisible world is no longer any more acceptable than, say, teaching astronomy but ignoring the existence of galaxies beyond the Milky Way, or teaching physics while refusing to discuss anything smaller than a pin head.”Nee, 2004

Exploring diversity of prokaryotes

Fig 27.15

16/09/2012

6

Until Recently Prokaryote diversity was hard to study

CategorizedBy Shape..

Fig 27.2

Gram Positive vs Gram Negative

More susceptible to those AntibioticsThat target the cell wall More likely to be antibiotic resistant

Fig 27.3

Outer shell involved is in attachment to other cells

Fig 27.4

Exploring Prokaryotes

• “Genetic Prospecting”• Take a sample of (e.g.) Soil, Mud, Water– It will contain billions of prokaryotes

• Purify DNA from the sample

• Sequence the samples• Fit them in among other known samples

Fig 27.15

16/09/2012

7

Extreme environments

• Extremely hot– An autoclave 120C and high pressure sterilizes most bacteria

– “Strain 121” from a sea floor hydrothermal vent is just starting to get comfortable in those conditions.

• Extremely salty– “The Dead Sea isn’t dead – it just doesn’t have any fish.”

• Extremely acidic• Solid Rock.

– Yes, you read that right– Some microbes live off chemical energy in the pores of solid rock.

Eukaryote cells are a lot bigger

Fig 27.17

A single‐celled eukaryote related to plants

Fig. 6.8

A single‐celled Eukaryote related to animals

Fig. 6.8

16/09/2012

8

A typical cell in an Animal

• Bounded by a membrane• No Cell Wall• Linear Chromosomes in a

Nucleus bounded by nuclear membrane

• Nucleus surrounded by Endoplasmic reticulum

• Ribosomes• Cytoskeleton• Various other membrane

bound organelles, esp:‐Mitochondrion‐Golgi Apparatus‐Peroxisome/Lysosome

Fig. 6.8

A typical Plant Cell

Similarities to Animal Cell:Nucleus, endoplasmic reticulum, ribosomesMitochondria

Differences:

Plasma membrane surrounded by a cell wall

‐ Rigid, made of cellulose

Big central vacuoleChloroplast

Fig. 6.8

The NucleusSort of the defining feature of Eukaryote cells.

Contains the DNA, which is packaged into Chromosomes Keeps large amounts of DNA organized

Bounded by a double membrane(sort of like a cell within a cell)But with pores.

Outer membrane contiguous with endoplasmic reticulum

Fig. 6.9

Endoplasmic Reticulum

Endo – withinPlasmic – the cytoplasmReticulum – network.

A highly folded membrane, contiguous in places with nuclear envelope

Rough ER had many bounded ribosomes

Outer edges ‘bleb’ off into transport vesicles

Fig 6.11

16/09/2012

9

Fig 6.11

Ribosomes

Made primarily of RNA

Central in protein synthesis“Read” a transcript of the DNA code(Details in genetics section)

Very numerous in the cell

(NB Ribosomes are also present in prokaryotes, but not typically bound to membranes).

Fig 6.10

Golgi ApparatusAppears to function in a sorting and transport capacity.Proteins synthesized by the ribosomes in Rough ERContained within vessicleswhich merge with Golgi,May be some additional processing (e.g. Folding into correct shape)Eventually ‘delivered’ to area where they are needed‐Made into lysosomes/peroxisomes‐Merge with cell membrane‐Etc. Fig 6.15

E.g. Lysosomes

Lysosomes contain digestive enzymes

Therefore contained in a vessicle where they can’t digest the cell itself.

But allow digestion of food particlesPhagocytosis food vacuoleLysosome merges with vacuoleDigestion takes place(single celled eukaryotes) Fig 6.13

16/09/2012

10

Two Really Important Organelles

Mitochondria – respiration

Chloroplasts –Photosynthesis.

Fig 9.2

Mitochondrion

Outer membraneIntermembrane spaceInner membrane“Matrix” (the inside)

There is some DNA in mitochondriaCodes for a few proteins particularly importantIn respirationSome ribosomes

Inner membraneHighly foldedIncreased surface area

Fig. 6.17

Chloroplast somewhat similar

Outer membraneIntermembrane spaceInner membraneStromaThylakoid membraneThylakoid space.

DNA and Ribosomes as in Mito’s

Thylakoid highly folded – surface areaFig 6.18

Mito’s and Chloro’s as Endosymbionts.

Surprisingly, the DNA of mitochondria and chloroplasts is more similar To the DNA of prokaryotes than it is to Eukaryotes

Evidence that mitochondria and chloroplasts areProkaryotic symbionts

Ancient symbiosis – many mitoand chloro genes have “migrated” to the nucleus

Original symbiosis 2‐3 Billion years ago.

Fig 6.16

16/09/2012

11

Prokaryotes with highly infolded plasma membranes.

Fig 27.7

Cytoskeleton.

Actin

Structural protein

Polymer of actin Subunits(a polymer of polymers)

Tensive

Table 6.1

Recall MuscleMyson pulls on strands of Actin

Fig 6.27

Actin is important in cellular movementphagocytosis, cytoplasmic streaming

Fig 6.27

16/09/2012

12

Cytoskeleton.

Keratin

Also tensive

Fibrous polypeptidesCoiled together

Major anchoring

External protein structurese.g. Hair

Table 6.1

Cytoskeletin

Microtubules

Rigid, resist compression

Tubulin dimers

Table 6.1

Motor proteins

One way organelles move things to the right part of the cell

Use Atp energy

Pull the vesicle along microtubule

Fig 6.21

Cellular Movement

A series of motor proteins connecting two microtubules

Move the doublets laterally

Push cellular structures into positione.g. chromosomes during cell division.

Fig 6.25

16/09/2012

13

If the two proteins are anchored:waving motion of cilia.

Fig 6.25 Fig 6.23

A flagellum rotates driven by a protein ‘motor’ at the base

Intercellular connectionsrecall membrane proteins bind to things outside the cell

Fig 6.6

Fig 6.30

16/09/2012

14

Cell Connections ‐ Animals

Tight junctions: a series of closely connected proteins (sort of like a sewn seam)

Channels between cells

Increased surface area where absorption is important.

Fig 6.32

Cell Connections ‐ PlantsCell walls are connected

Cells are therefore held in place

Series of pores through cell walls (technically called plasmodesmata)

Fig 6.33