topic 1 Flashcards
Define cell theory
all living organisms are composed of cells (the basic structural, functional and organisational units).
State and explain 4 features common to all cells
- cell membrane- to separate the cell from it’s surroundings
- genetic material that can be passed on (excluding erythrocytes)
- enzymes- to catalyse reactions within the cell
- energy release system (eg respiration)
State 3 atypical examples of cell theory
- striated muscle
- giant algae
- aseptate fungal hyphae
explain how a striated muscle fibre (SMF) does not conform to standard cell theory
muscle cells fuse, forming SMFs that are very long
=> fibres have multiple nuclei despite being surrounded by a single, continuous plasma membrane.
(challenges idea that cells always function as autonomous units)
explain how aseptate fungal hyphae do not conform to standard cell theory
fungi may have hyphae (filamentous structures), which are separated into cells by septa (internal walls)
=> some fungi not partitioned by septa and hence have a continuous cytoplasm along the length of the hyphae
(challenges idea that living structures are composed of discrete cells)
explain how giant algae do not conform to standard cell theory
certain species of unicellular algae may grow to very large sizes
(challenges idea that larger organisms are always made up of many microscopic cells) up to 100 mm in length
give an example of a giant unicellular algae
acetabularia, which may exceed 7cm in length
explain why the ultrastructure of a unicellular organism may be more complex than an individual cell in a multicellular organism
- unicellular must be able to carry out all the functions of life in a single cell.
- multicellular can carry out different functions in different parts of body.
give a eukaryotic and prokaryotic example of a unicellular organism
eukaryotic- amoeba
prokaryotic- s. cervisiae (baker’s yeast)
state the 7 functions of life
metabolism
response
nutrition
excretion
reproduction
growth
homeostasis
homeostasis
the maintenance of a constant, stable environment to keep conditions inside the organism within tolerable limits
growth
irreversible increase in size
reproduction
production of offspring, either sexually or asexually
nutrition
obtaining food to provide energy and materials needed for growth
response
ability to react to changes in the environment
metabolism
chemical reactions inside the cell
excretion
removal of waste products of metabolic reactions
why is size often limited in unicellular organisms?
- surface area affects the rate at which materials enter and leave the cell
- volume affects the rate at which materials are used and produced by the cell
- the surface area to volume ratio affects the rate of metabolism in a cell
- the bigger the cell, the smaller the SA ratio is
- cells having a low SA ratio cannot exchange materials fast enough
what will happen if the SA:VOL ratio of a cell is too small? (3)
- substances will not enter the cell quickly enough
- waste products will accumulate (produced more rapidly than excreted)
- cells may overheat as metabolism produces heat faster than is lost over cell’s surface
what is an emergent property?
a property which a collection or complex system has, but which the individual members do not have, as it arises from the interactions of those individual components
what does an emergent property arise from?
the interaction of cellular components in an organism.
give 3 examples of emergent properties
cells form tissues, tissues form organs, organs form organ systems
what type of organism is paramecium and where does it live
- unicellular eukaryote
- freshwater environments
how does paramecium provide energy for itself?
it is a heterotroph (eats smaller unicellular organisms in order to obtain energy/matter).
main 6 organelles of paramecium
nucleus
cell membrane
cytoplasm
cilia
contractile vacuoles
food vacuoles
paramecium- nucleus
can divide by mitosis to produce the two nuclei that are needed when the cell reproduces (mostly asexually)
paramecium- cell membrane
controls movement of substances/chemicals into and out of cell
paramecium- cytoplasm
contains enzymes that catalyse metabolic reactions, including respiration
paramecium- cilia
- beating of the cilia moves cell through water
- controlled by cell; moves in response to changes in environment
paramecium- food vacuoles
contain smaller organisms that have been consumed (gradually digested + nutrients absorbed into cytoplasm)
paramecium- contractile vacuoles
homeostasis:
- located at each end of cell
- fill up with water from inside the cell then expel excess through plasma membrane.
what type of organism is chlamydomonas and where does it live?
- unicellular eukaryote
- soil, freshwater, oceans, snow
how does chlamydomonas provide energy for itself?
autotroph- can photosynthesise
main 8 organelles of chlamydomonas
nucleus
cytoplasm
cell wall
cell membrane
chloroplasts
flagella
eyespot
contractile vacuoles
chlamydomonas- nucleus
- asexual reproduction: nucleus can divide by mitosis to produce 2 nuclei
- sexual reproduction: nuclei can fuse + divide to carry out sexual reproduction
chlamydomonas- cytoplasm
contains enzymes that catalyse metabolic reactions, including respiration
chlamydomonas- cell wall/membrane
- wall is freely permeable
- membrane controls what chemicals enter and leave
chlamydomonas- chloroplasts
- where photosynthesis occurs
- contains enzymes
- in dark, carbon compounds can be absorbed from other organisms via the cell membrane
chlamydomonas- flagella
the beating of the two flagella moves the chlamydomonas through the water it lives in
chlamydomonas- eyespot
light sensitive; allows cell to sense where brightest light is and respond by swimming towards it
chlamydomonas- contractile vacuoles
homeostasis:
- located at base of flagella
- fill up with water from inside the cell then expel excess through plasma membrane.
how do specialised tissues develop in multicellular organisms and why is this process important?
by cell differentiation;
allows them to carry out their role more efficiently than if they had many different roles.
define cell differentiation in terms of gene expression
differentiation involves the expression of some genes and not others in a cell’s genome.
give two key properties of stem cells.
self renewal; can continuously divide and replicate.
potency; have the capacity to differentiate into different specialised cell types.
give 3 places where stem cells can be found in the adult body
bone marrow, skin, liver
give 3 different types of stem cells
- embryonic
- cord blood
- adult
describe the growth potential of embryonic stem cells
almost unlimited growth potential; can differentiate into any cell type in the body.
describe the growth potential of cord blood stem cells
limited capacity to differentiate into different cell types (only naturally develop into blood cells)
describe the growth potential of adult stem cells
limited capacity to differentiate into different cell types.
give 1 pro of embryonic stem cells
less chance of genetic damage due to the accumulation of mutations than with adult stem cells.
give 3 cons of embryonic stem cells
- more risk of becoming tumour cells than ASC
- likely to be genetically different from patient
- removal of cells from embryo kills it
give 3 pros of cord blood stem cells
- easily obtained and stored
- fully compatible with tissues of adult that grows from the baby (no rejection probs)
- umbilical cord discarded either way
give 1 con of cord blood stem cells
- limited quantities of stem cells from one baby’s cord
give 3 pros of adult stem cells
- less chance of malignant tumours developing than from ESC
- fully compatible with the adult’s tissues (no rejection probs)
- removal does not kill adult
give 1 con of adult stem cells
- difficult to obtain as there are very few of them/buried deep in tissues. 0
give 2 examples of diseases that can be treated using stem cells
Stargardt’s disease
Leukaemia
explain the use of stem cells to treat Stargardt’s disease
Embryonic cells developed into retina cell and then injected into the eye to improve vision.
explain the use of stem cells to treat leukaemia
- stem cells extracted from bone marrow
- chemotherapy given, bone marrow loses ability to produce blood cells.
- stem cells returned to body and start to reproduce red and white blood cells.
m -> mm
x1000
mm -> μm
x1000
μm -> nm
x1000
give the equation used with microscopes.
size of image = magnification x actual size
what is the maximum resolution of a light microscope?
0.2 micrometers (μm)
what is the maximum resolution of an electron microscope?
0.001 micrometers (μm)
why does an electron have a much higher resolution than a light microscope?
beams of electrons have a much shorter wavelength.
explain how light microscopes work
light passes through the specimen, which filters out certain wavelengths of light
explain how an electron microscope works
electrons pass through the specimen and get absorbed.
define resolution
making the separate parts of an object distinguishable
state the two main differences between prokaryotes and eukaryotes
- prokaryotes have a simple cell structure that is not compartmentalised; eukaryotes have a compartmentalised cell structure
- eukaryotes have a nucleus bounded by a nuclear envelope consisting of a double membrane; prokaryotes do not have a nucleus
what do prokaryotes always have that eukaryotes only sometimes have?
a cell wall
give the three functions of a cell wall
- protects cell
- maintains cell shape
- prevents cell from bursting
in prokaryotes, the cell wall contains ——–
peptidoglycan
what is the size of prokaryotic ribosomes?
70S
what does the unit ‘S’ stand for?
Svedberg units
why does the DNA (or nucleoid) in prokaryotes appear lighter on electron micrographs?
it is not associated with proteins
list all the features of a prokaryotic cell that should be drawn.
- cell wall
- cytoplasm
- pili
- flagella
- plasma membrane
- 70S ribosomes
- nucleoid (with naked DNA)
what is the function of the flagella and pili?
- flagella are mainly responsible for motility (sensitive to temperature, chemicals and metals)
- pili are mainly responsible for attachment during conjugation and motility
give 4 advantages of a cell being compartmentalised
- enzymes/substrates for a process can be more concentrated than if they were spread throughout cytoplasm
- substances that could cause damage to cell can be kept inside membrane of an organelle (eg digestive enzymes of a lysosome)
- conditions such as pH can be maintained at ideal for a particular process (may be different to optimum levels for cells)
- organelles with their contents can be moved around within the cell.
list the features of eukaryotic cells that should be drawn
- plasma membrane
- cytoplasm
- 80S ribosomes
- nucleus
- mitochondria
(- rough endoplasmic reticulum)
(- Golgi apparatus)
(- lysosomes)
(- chloroplast)
(- vacuoles and vesicles)
(- microtubules and centrioles)
(- cilia and flagella)
describe the nucleus in eukaryotic cells
- nuclear envelope: a double membrane with pores
- contains nucleolus, where ribosomes are made
- contains chromosomes, consisting of DNA associated with histone proteins, and chromatin (uncoiled chromosomes)
- where transcription occurs (DNA-> mRNA)
describe the rough endoplasmic reticulum (rER) in eukaryotic cells
- consists of cisternae (flattened membrane sacs)
- 80S ribosomes attached to outside of cisternae
- rER synthesises proteins for secretion/cell membrane from the cell
describe the ribosomes in eukaryotic cells
- not surrounded by a membrane
- 80S
- synthesise proteins and release it to work in the cytoplasm
describe the Golgi apparatus in a eukaryotic cell
- consists of cisternae (flattened membrane stacks)
- processes proteins brought in vesicles from the rER
- most are then carried in vesicles to plasma membrane for secretion
describe how the cisternae in the Golgi differ from the cisternae in rER
- not as long
- often curved
- do not have attached ribosomes
- many vesicles nearby
describe the lysosomes in eukaryotic cells
- approximately spherical with a single membrane
- formed from Golgi vesicles
- contain high concentrations of protein and digestive enzymes which are used to break down ingested food/organelles/whole cell
describe the mitochondrion in eukaryotic cells
- surrounded by a double membrane
- inner membrane invaginated to form structures called cristae
- contains matrix (fluid)
- produce ATP by aerobic respiration
- fat digested here if it is being used as an energy source
- contains own DNA
describe the chloroplast in eukaryotic cells
- surrounded by double membrane
- contains stacks of thylakoids (flattened stacks of membrane)
- produce glucose and other organic compounds by photosynthesis, starch grains present if photosynthesising quickly
- contains own DNA
describe vacuoles and vesicles in eukaryotic cells
- consist of single membrane with fluid inside
- vacuoles; store waste products/food/necessary molecules
- vesicles are smaller vacuoles; transport materials
name the three organelles that contain their own DNA
- nucleus
- chloroplasts
- mitochondria
describe microtubules and centrioles in eukaryotic cells
- microtubules are small cylindrical fibres
- move chromosomes during cell division, act as transport routes through cell, hold organelles in place, spindle fibres in mitosis
- centrioles consist of two groups of nine triple microtubules
- form an anchor point for microtubules during cell division
describe cilia and flagella in eukaryotic cells
- both are microtubules covered in membrane
- cilia are shorter, flagella are longer
- cilia move things along the surface of the cell
- flagella move the cell
how do prokaryotes divide?
by binary fission
describe binary fission
- single circular chromosome is replicated
- the two copies of the chromosome move to opposite ends of the cell
- the cytoplasm and cell divide
- each of the daughter cells contains one copy of the chromosome so they are genetically identical
function of exocrine gland cells of the pancreas
secrete digestive enzymes into a duct that carries them to the small intestine where they digest foods
structure of exocrine gland cells of the pancreas
organelles needed to synthesise proteins in large quantities; process them to make them ready for secretion, transport them to the plasma membrane and release them.
- plasma membrane
- mitochondrion
- nucleus
- rER
- Golgi
- vesicles
- lysosomes
function of palisade mesophyll cells of the leaf
photosynthesis- producing organic compounds from CO2 and other inorganic compounds, using light energy.
structure of palisade mesophyll cell of the leaf
- cell wall
- plasma membrane
- chloroplasts
- mitochondrion
- vacuole
- nucleus
why do phospholipids form bilayers in water?
due to the amphipathic properties of phospholipid molecules
what is an amphipathic molecule?
contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions
describe the structure of a phospholipid
- a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
- and two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
describe how phospholipids spontaneously arrange into bilayers.
- hydrophobic tails face inwards and so are shielded from the surrounding polar fluids
- the two hydrophilic heads associate with the cytosolic (intracellular) and extracellular fluids
what is are phospholipids held together by?
weak hydrophobic interactions between the tails
give two properties of a phospholipid bilayer
- hydrophilic / hydrophobic layers restrict the passage of many substances
- fluidity and flexibility: individual phospholipids can move within the bilayer
why is the fluidity of the phospholipid bilayer important?
allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
state the 6 functions of membrane proteins
Junctions
Enzymes
Transport
Recognition
And neurotransmitters
Transduction
Define ‘junctions’ as a membrane protein function
cell adhesion to connect and join groups of cells together in tissues and organs.
Define ‘enzymes’ as a membrane protein function
for immobilised enzymes with the active site on the outside, fixing to membranes localises metabolic pathways (eg small intestine)
Define ‘transport’ as a membrane protein function
act as channels to allow hydrophilic particles across by facilitated diffusion, and as pumps for active transport which use ATP to move particles across the membrane.
Define ‘recognition’ as a membrane protein function
May function as markers for cellular identification
Define ‘and neurotransmitters’ as a membrane protein function
cell-to-cell communication, for example receptors for neurotransmitters at synapses
Define ‘transduction’ as a membrane protein function
hormone binding sites (peptide hormone receptors), for example the insulin receptor.
what two types of membrane proteins are there?
- integral proteins
- peripheral proteins
describe integral proteins
- permanently attached to the membrane
- typically transmembrane (span across the bilayer)
- hydrophobic on at least part of their surface
- embedded in hydrocarbon chains at centre
describe peripheral proteins
- temporarily attached by non-covalent interactions
- associate with one surface of the membrane, not embedded in the membrane
- hydrophilic on surface
- may be attached to surface of integral proteins/have a single hydrocarbon chain in the membrane
the more active a membrane, the —– its protein content
higher
what group of lipid substances does cholesterol belong to?
steroids
why is cholesterol considered amphipathic?
Cholesterol’s hydroxyl (-OH) group is hydrophilic but the remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic
what does cholesterol’s hydroxyl group align with?
towards the phosphate heads of phospholipids
what do cholesterol’s steroid ring/hydrocarbon tail align with?
with the phospholipid tails
state two functions of cholesterol in mammalian membranes
- controls membrane fluidity
- reduces permeability to some solutes
why does the fluidity of animal cell membranes need to be controlled?
too fluid-> unable to control what substances pass through
not fluid enough-> movement of cell and substances would be restricted
describe the 2ary roles of cholesterol in mammalian membranes
- reduces permeability to hydrophilic particles such as Na+ ions and H+ ions
- helps membranes curve into a concave shape, which helps formations of vesicles during endocytosis
draw the fluid mosaic model
refer elsewhere
describe the Davson-Danielli model
- model whereby two layers of protein flanked a central phospholipid bilayer
- ‘lipo-protein sandwich’: the lipid layer was sandwiched between two protein layers
what were the dark segments under the microscope identified as?
dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers
Give the three main pieces of evidence for the falsification of the Davson-Danielli model
- membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size
- Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place
- Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane
How does the insolubility and variation in size of membrane proteins disprove the DD model?
Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane
how did antibody tagging disprove the DD model?
showed membrane proteins could move and did not form a static layer
how did freeze fracturing disprove the DD model?
rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure
how does cholesterol affect membrane fluidity in high/low temperatures?
at high temperatures, cholesterol reduces membrane fluidity through its interactions with the fatty acid tails, which stabilise the membrane . at low temperatures, cholesterol increases membrane fluidity by preventing the phospholipid tails from packing too close together.
state the 4 ways in which particles move across membranes
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
define diffusion
the net, passive movement of molecules from a region of high concentration to a region of low concentration
explain the different abilities of:
- small non-polar molecules
- small polar molecules
- large polar molecules
to diffuse across the phospholipid bilayer
easiest->hardest
1. small non-polar molecules
2. small polar molecules
3. large polar molecules
polar/charged molecules repel hydrophobic tails of phospholipids
give 3 examples of molecules that will move easily by diffusion
oxygen, CO2, glycerol
give a real-life application of simple diffusion
the cornea has no blood supply so its cells obtain oxygen by simple diffusion from the air (it passes through the fluid/tears and into the cornea)
define facilitated diffusion
the passive movement of molecules across the cell membrane via the aid of a membrane protein
what types of molecules use facilitated diffusion?
molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)
what two types of protein mediate facilitated diffusion?
channel proteins and carrier proteins
what are carrier proteins?
Integral glycoproteins
how do carrier proteins function?
bind a solute and undergo a conformational change to translocate the solute across the membrane
can carrier proteins bind to any molecules?
no; they will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
can carrier proteins bind to any molecules?
no; they will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
can carrier proteins move molecules against the concentration gradient?
may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
are carrier proteins faster or slower than channel proteins?
Carrier proteins have a much slower rate of transport than channel proteins
what are channel proteins?
Integral lipoproteins
how do channel proteins work?
they contain a pore via which ions may cross from one side of the membrane to the other
do channel proteins let any molecules through?
no; they are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
can channel proteins move molecules against the concentration gradient?
no; they can only move molecules along a concentration gradient (i.e. are not used in active transport)
what is an axon? and what is its function?
- a part of a neuron which consists of a tubular membrane with cytoplasm inside
- used to convey messages rapidly in the form of an electrical impulse
what does a nerve impulse consist of?
rapid movements of Na ions into and K ions out of the axon membrane via facilitated diffusion (through Na and K channels)
describe and draw the first three steps of the sodium-potassium pump cycle
- the interior of the pump is open to the inside of the axon; three sodium ions enter the pump and attach to their binding sites
- ATP transfers a phosphate group from itself to the pump; this causes the pump to change shape and the interior is then closed
- the interior of the pump opens to the outside of the axon and the 3 Na ions are released
how is the necessary concentration gradient restored for nerve impulses?
active transport, done by a sodium-potassium pump protein
describe and draw the final three steps of the sodium-potassium pump cycle
- 2 K ions from outside can then enter and attach to their binding sites
- binding of K causes release of the phosphate group; this causes the pump to change shape again
- the interior of the pump opens to the inside of the axon and the 2 K ions are released
define active transport
the use of energy to move molecules against a concentration gradient
how is energy for active transport generated?
- The direct hydrolysis of ATP (primary active transport)
- Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)
what proteins does active transport involve?
carrier proteins (called protein pumps due to their use of energy)
describe active transport
- A specific solute binds to the protein pump on one side of the membrane
- The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
- The solute molecule is consequently translocated across the membrane (against the gradient) and released
potassium channels in axons are —-
voltage gated
what causes voltage across membranes?
- an imbalance of positive and negative charges across the membrane
- more +ve outside axon than inside= K channel closed
- more +ve inside axon than outside = K channel open, so ions diffuse through
describe and draw the 3 steps of facilitated diffusion of potassium in axons
- channel closed; there is a net -ve charge inside the axon and a net +ve charge outside
- channel briefly opens; net +ve charge inside and net -ve charge inside; K ions rush outside by translocating ions to create a voltage difference across the membrane
- channel closed by ‘ball and chain’ within milliseconds
define osmosis
the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration/high water potential to a region of high solute concentration/low water potential
why can osmosis happen in all cells?
water molecules, despite being hydrophilic, are small enough to pass through the phospholipid bilayer
some cells have water channels called ——-
aquaporins
describe the function of aquaporins and give examples
they greatly increase membrane permeability to water- eg kidney cells that reabsorb water and root hair cells in plants
what does the fluidity of membranes allow?
materials to be taken into cells by endocytosis or released by exocytosis
define endocytosis
The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane
describe endocytosis
- An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
- The invagination is then sealed off to form an intracellular vesicle containing the material
is endocytosis a passive process?
no, the proteins in the membrane that carry out this process requires energy from ATP
give two main examples of endocytosis
- phagocytosis to ingest/kill pathogens
- in the placenta, proteins from the mother’s blood, including antibodies are absorbed into the foetus
describe the movement of vesicles within a cell
- endocytosis; vesicles can then move through the cytoplasm
- proteins are synthesised by ribosomes
- vesicles enter the rough endoplasmic reticulum and accumulate there
- vesicles bud off from the rER
- the Golgi apparatus modifies the proteins
- vesicles bud off from the Golgi and carry the modified proteins to the plasma membrane
- endocytosis
describe exocytosis
- vesicles fuse with the plasma membrane
- the contents of the vesicle are expelled and the membrane flattens out again
give 2 examples of exocytosis
Digestive enzymes released from gland cells by exocytosis (secretion)
expulsion of waste products or unwanted materials (eg water in unicellular organisms through contractile vacuoles)
define a isotonic solution
a solution that has the same osmolarity as a tissue
define a hypertonic solution
a solution that has a higher osmolarity than a tissue
define a hypotonic solution
a solution that has a lower osmolarity than a tissue
describe what will happen to animal cells in hypertonic/hypotonic solutions
hypertonic solutions; water will leave the cell causing it to shrivel (crenation)
hypotonic solutions; water will enter the cell causing it to swell and potentially burst (lysis)
describe what will happen to plant cells in hypertonic/hypotonic solutions
hypertonic solutions; the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape
hypotonic solutions; the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)
what is a real life application of isotonic solutions
it is important for any human tissues or organs to be bathed in an isotonic solution during medical procedures. usually, NaCl solution is used (normal saline)- osmolarity= 300mOsm= cytoplasm osmolarity
how could we determine the osmolarity of a tissue?
bathe it in a salt solution for a short time and measure the increase/decrease in mass- when mass change=0, that is the same conc
what is the only way that cells can be formed ?
by division of pre-existing cells
how must the first cells have arisen?
from non-living material
what are the 4 main hypotheses for how the main stages of cell production occurred?
- production of carbon compounds such as sugars and amino acids
- assembly of carbon compounds into polymers
- formation of membranes
- development of a mechanism for inheritance
production of carbon compounds such as sugars and amino acids as a hypothesis for cell production
Miller and Urey passed steam through a mixture of methane, hydrogen and ammonia- a mixture representative of the atmosphere of the early earth. when electrical discharges (= lightning) were used, amino acids/other C carbons were formed
assembly of carbon compounds into polymers as a hypothesis for cell production
deep sea vents:
- cracks characterised by gushing hot water carrying reduced inorganic chemicals such as iron sulphide
- these could be sources of energy for the assembly of C compounds into polymers
formation of membranes as a hypothesis for cell production
if phospholipids/other amphipathic C compounds existed, they would have naturally assembled into bilayers. experiments have sown these bilayers readily formed vesicles, resembling membranes which would have allowed different internal chemistry from the surroundings to develop
development of a mechanism for inheritance as a hypothesis for cell production
enzymes needed to replicate DNA- but genes needed for enzymes to be made
- RNA was original genetic material
why could RNA have been the original genetic material
as it stores information the same as DNA but is self replicating and can act as a catalyst
what is endosymbiotic theory?
the idea that some organelles in eukaryotes were formed by the taking in of prokaryotes
describe endosymbiotic theory
- prokaryotic organism that had developed the process of aerobic cell respiration were taken in by larger prokaryotes that could only respire anaerobically via endocytosis
- smaller prokaryotes were not killed but continued to live in the cytoplasm, growing and dividing as fast as the larger ones
- smaller prokaryotes lost unnecessary functions and persisted over millions of years of evolution to become mitochondria
why were larger and smaller prokaryotes able to live together?
as they were in a symbiotic/mutualistic relationship in which both of them benefited- larger supplied with energy, smaller supplied with food
why does endosymbiotic theory support the origin of chloroplasts?
prokaryotes that had developed photosynthesis taken in my larger cell
give 4 pieces of evidence for endosymbiotic theory
- mitochondria and chloroplasts have their own genes, on a circular DNA molecule (=prokaryotes)
- they have their own 70S ribosomes of a size and shape typical of some prokaryotes
- they transcribe their DNA and use the mRNA to synthesise some of their own proteins
- they can only be produced by division of pre-existing m/c.
define mitosis
the division of the nucleus into two genetically identical daughter nuclei
what must happen before mitosis can occur?
all of the DNA in the nucleus must be replicated
when is the DNA in the nucleus replicated
during interphase
what is interphase
the period before mitosis
what happens during interphase?
each chromosome is converted from a single DNA molecule into two identical DNA molecules (chromatids)
what are the functions of mitosis?
- embryonic development
- growth
- tissue repair
- asexual reproduction
state the 4 phases of mitosis
- prophase
- metaphase
- anaphase
- telophase
mitosis is a ——– process
continuous
interphase is a…
very active phase in the life of the cell when many metabolic reactions occur
what only occurs during interphase?
DNA replication in the nucleus and protein synthesis in the cytoplasm
why does the no of mitochondria/chloroplasts in the cytoplasm increase during interphase?
due to the growth and division of these organelles
what do plants also do during interphase?
they synthesise cellulose and use vesicles to add it to their cell walls
what 3 phases does interphase consist of?
- G1 phase
- S phase
- G2 phase
G1 phase
cell grows physically larger and copies organelles/cellular components except the chromosomes
S phase
cell replicates all genetic material in its nucleus
G2 phase
cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis
what happens to cells that are never going to divide?
they enter a phase called G0
how do chromosomes condense
by supercoiling during mitosis
what makes up each chromosome during mitosis?
a chromatid
how does condensation occur for chromosomes?
by repeatedly coiling the DNA molecule to make the chromosome shorter and wider
what is involved in supercoiling?
histones (proteins) associated with DNA and enzymes
prophase
- chromosomes become shorter and fatter by supercoiling
- nucleolus breaks down
- microtubules grow from MTOCs to form spindle-shaped array that links poles of the cell
- end; nuclear membrane breaks down
function of the nucleolus
area inside the nucleus of a cell that is made up of RNA and proteins and is where ribosomes are made.
metaphase
- microtubules continue to grow and attach to the centromeres on each chromosome
- microtubules put under tension to test whether attachment is correct (by shortening of the microtubules at the centromere)
- attachment=correct, microtubules remain on equator of cell
what allows the chromatids of a chromosome to attach to microtubules?
the two attachment points on opposite sides of each centromere
anaphase
- each centromere divides, allowing the pairs of sister chromatids to separate
- spindle microtubules pull them rapidly towards the poles of the cell
telophase
- at each pole the chromosomes are pulled into a tight group by the MTOC and a nuclear membrane reforms around them
- chromosomes uncoil and nucleolus is formed
telomere
structures found at the ends of chromosomes. They cap and protect the end of a chromosome
centromere
links a pair of sister chromatids together during cell division.
equation for mitotic index
no of cells in mitosis/total no of cells
MTOC
microtubule-organizing center; a structure found in eukaryotic cells from which microtubules emerge.
when does cytokinesis occur
after mitosis
what is cytokinesis
process of cell division
describe cytokinesis in plants
- vesicles are moved to the equator, where they fuse to form tubular structures
- these merge into two layers of membrane across the whole of the equator
how are cell walls formed after mitosis?
- pectins and other substances brought by vesicles and deposited by exocytosis between the two new membranes, forming the middle lamella that links new cell walls.
- both dcs then bring cellulose to the equator and deposit it adjacent to the middle lamella via exocytosis. this forms a cell wall adjacent to the equator
describe cytokinesis in animal cells
- cleavage furrow formed (plasma membrane pulled inwards around equator).
- this is accomplished by using a ring of contractile protein (actin and myosin) - when cleavage furrow reaches centre, the cell is pinched apart into 2 cells
what are cyclins involved in?
the control of the cell cycle
description/functions of cyclins
proteins:
- control the cell cycle and ensure that cells divide only when new cells are needed
how do cyclins work
- bind to enzymes called cyclin-dependent kinases, causing them to become active and attach phosphate groups to other proteins in the cell
- this triggers the other proteins to become active and carry out tasks specific to one of the cell cycle phases
cyclin D
triggers cells to move from G0 to G1 and from G1 to S
cyclin E
prepares cell for DNA replication in the S phase
cyclin A
actives DNA replication inside the nucleus in the S phase
cyclin B
promotes the assembly of the mitotic spindle and other tasks in the cytoplasm to prepare for mitosis
when will cyclin levels peak
when their target protein is required for function
state three things involved in the formation of primary and secondary tumours
- mutagens
- oncogenes
- metastasis
what are tumours
abnormal groups of cells that develop at any stage of life in any part of the body
when are tumours benign
when the cells adhere to each other and do not invade nearby tissues or move to other parts of the body
when are tumours malignant
when the cells become detached and move elsewhere in the body and develop secondary tutors
what are cancers?
diseases due to malignant tumours
what are carcinomas
malignant tumours
define a mutagen
an agent that changes the genetic material of an organism (either acts on the DNA or the replicative machinery)
describe the 3 types of mutagens
- Physical – Sources of radiation including X-rays (ionising), ultraviolet (UV) light and radioactive decay
- Chemical – DNA interacting substances including reactive oxygen species (ROS) and metals (e.g. arsenic)
- Biological – Viruses, certain bacteria and mobile genetic elements (transposons)
what are carcinogens
Mutagens that lead to the formation of cancer
define a mutation
random changes to the base sequences of genes
most genes ——- if they mutate
do not cause cancer
what are oncogenes
genes that become cancer-causing after mutating
why are oncogenes cancer-causing
in a normal cell, oncogenes are involved in the control of the cell cycle and cell division, so mutations in them will result in uncontrolled cell division = tumour formation
metastasis
the movement of cells from a primary tumour to set up secondary tumours in other parts of the body
smoking and cancer
there is a positive correlation between smoking and incidence of cancer;
- cancer of mouth, pharynx, larynx and lungs
- esophagus, stomach, kidney, bladder, pancreas cervix
- Cigarette smoke contains over 4,000 chemical compounds, over 60 of which are known to be carcinogenic
