Cell Theory (SCR) Flashcards
What is cell theory?
Living organisms are composed of cells. cells are the smallest unit of life. cells can only arise from pre-existing cells. organisms that are only one cell carry out all functions of life in that cell.
Why is DNA as a genetic component required in cells?
for protein synthesis. (contain information needed for a cell to carry out its functions. it can be copied and passed on to other cells)
why is cytoplasm that is made out of mainly water and cells enzymes required in cells?
- water allows for substances to be dissolved
- enzymes catalyse different reactions- metabolism of the cells
why is a plasma membrane which is mainly made out of lipids required in cells?
controls entry and exit of substances
what makes a living thing living?
1) living things use energy in order to keep themselves in a highly ordered state
2) living things can pass on the ability to maintain a highly ordered state to their offspring
How did Pasteur disprove the theory of spontaneous generation?
Scientists believed in ‘spontaneous generation’ of the formation of living organisms; this was the case because there was no knowledge of microbes or cells.
- made nutrient broth by boiling water containing yeast and sugar. ( to kill any existing micro-organisms)
- poured into swan necked flasks
- due to the swan neck, very little air was able to reach the nutrient broth and so very little bacteria grew
What makes something living?
movement- the ability to go from A to B
reproduction- the ability to procreate and increase the number of your species
sensitivity/stimuli- the ability to react to changes in the environment
control (homeostasis)- the ability to maintain their internal environment
growth- an irreversible increase in size
respiration- to get energy for use in cells
excretion- to be able to remove toxic metabolic waste
Nutrition- obtaining food, to provide energy and the materials needed for growth
what was the pre-biotic atmosphere like?
- only traces of O2 as it reacted with other elements
- methane concentration was very high due to intense volcanic activity and many meteors
- Carbon dioxide concentrations were also higher due to volcanic activity
(temperature was also higher due to CO2 and CH4 trapping Greenhouse Gasses; this happened even though the sun was emitting 20% less energy
The ozone layer (in the stratosphere) that absorbs UV, protecting us from skin cancer, would not have existed. UV is high energy, so without the ozone there would have been more chemical reactions.
Evidence for the origin of carbon compounds
- this was tested in 1950 by Stanley Miller and Harold Urey
- they mixed CO2, NH3, CH4 and H2O then used electrical discharges to stimulate lightning
- after the experiment had been running for a day, the water turned pink; after a week it turned dark red. Analysis showed that a variety of carbon compounds had been produced including more than 20 different amino acids. —> this showed that it was possible for carbon compounds to form spontaneously on earth before life had evolved
Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers
Membranes play an important role in cells because they separate the genetic material and biochemical processes inside the cell from its outside environment; this is known as compartmentalisation
Membrane formation would have been a crucial step in the origin of cells
It is likely that the membranes of the first cells were composed of fatty acids because of the amphipathic nature of these molecules
Fatty acids are major components of lipids
When a few lipid molecules are placed in water, they will naturally form a monolayer on the surface; the polar parts of the lipid will be in water, while the non-polar parts will stick out of the water surface
If more lipid molecules are added, they form bilayers with the polar parts facing outward towards the watery environment while the non-polar parts will point towards each other
These bilayers will spontaneously form microspheres, or small vesicles, which could possibly have formed the membranes of early cells
These early membranes would have separated the internal chemistry of the cells from their outside environment
It is theorised that the fatty acids could have combined with glycerol during condensation reactions to form triglycerides as membranes evolved
Finally, these triglycerides could have undergone phosphorylation to form simple phospholipids which make up the main component of modern cell membranes
Eukaryotic cells evolved to contain multiple internal compartments, allowing further division of activity within cells
RNA as presumed first genetic material
- to replicate DNA and pass genes to offspring, living organisms need enzymes
- to make enzymes they need genes. at an earlier phase of evolution, RNA may have been the genetic material: RNA can store information in the same way as DNA but it is self replicating and it can act as a catalyst
- some viruses use RNA as their genetic material, supporting the theory that RNA could have been used before genes made of DNA evolved
- viruses with RNA involved tend to have a high mutation rate—> because the polymerase enzyme that is used to copy RNA is much less accurate than the one used for DNA
- a small number of reactions are catalysed by RNA (even though it does not have a complex three dimensional structure). RNA can catalyse the formation of peptide bonds.
Evidence for a last universal common ancestor
There are many different possible genetic codes a species can use; however upon investigation the genetic code was found to be universal. the meanings of the 64 different codons could be assigned in a limitless number of ways, having different meanings for different species—> making it highly unlikely that two species would use the same genetic code by chance. Instead, the obvious explanation would be that they inherited it from a common ancestor (LUCA))
Approaches used to estimate the dates of the first living cells and the last universal common ancestor
- we can deduce that the earliest cells must have existed before 3.42 Gya (as stromatolites came into existence at around 3.42 Gya)
- the oldest rocks on earth have all been metamorphosed by heat and pressure, so they do not contain clearly recognisable fossils; therefore the only evidence of life is from isotope rations. carbon originating from living organisms has a low 13C/12C ratio
Evidence for the evolution of the evolution of the last common ancestor in the vicinity of hydrothermal vents.
- researchers have identified genes that occur widely among groups of organisms that originated early in life- bacteria and archea- and if LUCA was a common ancestor it probably had the gene.
- researchers have identified 355 protein families that are likely to have been in LUCAs genome. These are genes required for anaerobic metabolism and for fixing CO2 and N2 —> from this we can deduce LUCA lived in an environment rich on H2, CO2 and Fe (hydrothermal vents)
- hydrothermal vents are cracks in the earths surface
- they have high temps 60-90 degrees celsius
- H2, CH4, NH3, S2 (readily accesable sources of energy)
Features in Eukaryotic Cells (Plastids)
A family of organelles with two outer membranes and internal membrane sacs
Animals- none
Fungi- none
Plants- chloroplasts for photosynthesis and amylopasts to store starch
Features in Eukaryotic Cells (Cell Wall)
Animals- none
Fungi- yes; made of chitin
Plants- yes; made of cellulose
Features in Eukaryotic cells (vacuole)
Animal- Small temporary vacuole to expel water or digest food or pathogen taken in by phagocytes
Fungi and Plants- Large permanent vacuole used for storage of substances and pressuring cells
Features in Eukaryotic cells (centrioles)
Cylindrical organelles that organise the assembly of structures that are composed of microtubules
Animal- used to construct the spindle that moves chromosomes in mitosis and the microtubules in cilia and flagella
Fungi and Plants- Absent. Only exception is in those that have swimming male gametes with flagella
Features in Eukaryotic Cells (Undulipodia)
Cilia and flagella used to generate movement of cell of the fluid adjacent to a cell
Animal- Present in many e.g. the tails of male gametes (sperm)
Fungi and Plants- Absent. Only exception is in those that have swimming male gametes with flagella
Eukaryotic vs Prokaryotic cells
Eukaryotes have a distinct nucleus divided into compartments and has membrane bound organelles with 80s ribosomes (e.g. plant, animal and fungi cells)
Prokaryotes don’t have a distinct nucleus divided into compartments and they don’t have membrane bound organelles. they have 70s ribosomes
What is the advantage of having compartmentalised nucleus and cytoplasm separately?
In protein synthesis- the mRNA can be modified in the nucleus before being attached to the ribosome in the cytoplasm during the translation
Eukaryotic Cell components (Nucleus)
Function
this is where DNA is replicated and transcribed to form mRNA. the nuclear membrane has pores through it and the mRNA is transported out through these. Contains chromosomes made up of DNA which is coiled around histone proteins. This is called chromatin when the chromosomes uncoil
Eukaryotic Cell components (Rough endoplasmic reticulum)
Sheets of membranes called cisternea which have 80s ribosomes attached onto the outside. The ribosomes synthesise protein which is then transported out through the cisternea in vesicles and then move to the Golgi apparatus
Eukaryotic Cell components (Golgi apparatus)
Made up of shorter curved cisternea. it processes the proteins made by the rough endoplasmic reticulum and then these are carried to the membrane in vesicles to be secreted.
Eukaryotic Cell components (Lysosomes)
These are spherical and formed from the Golgi vesicles. they contain high levels of proteins including enzymes which can break down organelles, food or whole cells.
Eukaryotic Cell components (Mitochondria)
Site of respiration to release energy in the form of ATP. Has a double membrane with invaginations called cristae and a fluid inside called the matrix
Eukaryotic Cell components (Free ribosomes
These are formed in the nucleolus part of the nucleus and are known as 80s ribosomes. they synthesise proteins to work in the cytoplasm.
Eukaryotic Cell components(Chloroplasts)
Produce glucose and other compounds via the process of photosynthesis. they have stacks of thylakoids inside (flattened stacks of membrane) and sometimes contain starch
Eukaryotic Cell components (vacuoles and vesicles)
Have a single membrane and fluid inside. can be large in plants and animal cells can use them to:
- absorb and digest food inside
-used to expel excess water
vesicles are used to transport material inside cells
