Topic 2 - short answer responses Flashcards
Name the model that describes the cell membrane. How did it get this name?
The fluid mosaic model gets its name from its composition. It is composed of a phospholipid bilayer with proteins embedded in it and these molecules can float in the plane of the membrane.
Describe how fossils in stromatolites may be used as evidence that prokaryotic cells existed on earth before eukaryotic cells
The first prokaryotes have been found in fossils called stromatolites which date to about 3.7 billion years ago. However, the first eukaryotic fossils only appeared about 1.5 million years ago. Thus, prokaryotic cells existed on earth before eukaryotic cells
Describe the similarities and differences between prokaryotic cells and eukaryotic cells
Eukaryotic cells are plant and animal cells with a nucleus and membrane-bound organelles. Prokaryotic cells are unicellular organisms without a nucleus or membrane-bound organelles. All cells have a cell membrane, cytoplasm and genetic material. Both plant and animal cells have mitochondria.
State 3 key differences between prokaryotes and eukaryotes
- Size
- Prokaryotes have circular DNA
- DNA in eukaryotes found in nucleus whereas in cytoplasm in bacteria
- Eukaryotes have membrane bound organelles
- Eukaryotes DNA bound to histones
State 3 pieces of evidence for the endosymbiotic theory
- chloroplasts and mitochondria have a double membrane
- the presence of ribosomes in the mitochondria allows them to make their own proteins as do normal cells.
- Mitochondria can divide via binary fission much like prokaryotes do
Describe the main structural features of a mitochondrion
The mitochondrion has a double membrane. the inner membrane is highly folded to form cristae. the outer membrane is smooth. there are circular chromosomes (DNA) within the mitochondrial matrix. there are ribosomes within the matrix. mitochondria have a sausage shape.
Describe the main structural features of chloroplasts
- chloroplasts have a double membrane. the inner membrane is highly folded to form states of thylakoids called grana.
- chloroplasts have circular DNA chromosomes.
- they also have ribosomes in the stroma
State 3 functions of the cytoskeleton
- movement within a cell e.g. movement of chromosomes during mitosis and meiosis; movement of secretory vesicles from Golgi body to the cell membrane
- mechanical support to change a cell’s shape. e.g. to hold organelles in positions to determine the shape of the cell and enable it to change shape.
- cell to cell adhesions: to provide junctions between adjacent cells
State 3 key differences between animal and plant cells
- plant cells have chloroplasts, animal cells do not
- plant cells have a large water-filled central vacuole
- plant cells have a rigid cell wall made of cellulose, animal cells have no cell wall
Describe the factors that limit the rate of photosynthesis
Photosynthesis involves a series of enzymatic reactions, and so factors such as temperature which affect the rate of enzymes will affect photosynthesis. Also, photosynthesis requires light of a particular wavelength and intensity. Concentrations of water and carbon dioxide and the presence of chlorophyll. Thus, anything that affects these factors will affect photosynthesis.
Explain why energy pathways involve many regulated steps
It is usually not possible to convert a substrate directly to a product but requires a number of steps. This has the advantage that, since energy pathways release some energy in the form of heat, this enables the small amounts of energy to be trapped more readily in molecules such as ATP because if it was all released at one, this would denature the enzymes and would kill the cell.
Discuss the advantages of having many regulated steps in biochemical pathways
Small regulated steps produce intermediate compounds that may serve as the starting point of other pathways. Each step of the pathway creates a product with a different shape which requires a different enzyme with a corresponding active site. Having many small steps enables the cell to more tightly control/ regulate this pathway e.g. by feedback/end product inhibition
Discuss one benefit to society of the use of a chemical
Antibiotics are chemicals that can be used to treat bacterial infections. They specifically inhibit the growth of bacteria without harming the eukaryotic host. They are a benefit to society because they greatly reduce morbidity increasing productivity in the workplace reducing economic loss, and reduce the cost of treating the diseases lowering the burden on the healthcare system
Discuss a possible benefit of chemicals that humans use
Fertilisers are chemicals widely used in agriculture. Their benefit is that they increase crop yields to provide more food for the population. However, there is also the possibility that fertiliser in water run-off from the crops could enter waterways and lead to eutrophication adversely affecting aquatic life.
Discuss possible harmful effects of chemicals that humans use
Humans use pesticides in order to eliminate nuisance insects such as locusts or mosquitos and small mammal pests such as rats and mice. If they are not biodegradable, then they may enter the food chain leading to biomagnification. This means that animals higher up the food chain may contain dangerous levels of toxic chemicals if consumed by humans.
Similarities and differences between Binary fission and mitosis
SIMILARITIES
- DNA replication must occur before the process begins
- Both produce genetically identical daughter cells
DIFFERENCES
- The DNA is attached to the membrane in bacteria/prokaryotes whereas attached to spindle fibres in eukaryotes
- Binary Fission is fast whereas mitosis takes longer
What is a homologous pair of chromosomes
A pair of homologous chromosomes is 2 copies of the same chromosome but each one is derived from a different parent. They are genetically identical except for allelic variation.
State the meaning of haploid. Explain why the products of meiosis are haploid cells and contain a single set of chromosomes.
A haploid cell has only one copy of each chromosome and thus only one copy of each gene or one genome (the set of all the different genes of an individual). Meiosis begins with a diploid cell in which the chromosomes are replicated. After meiosis I, the homologous chromosomes have separated producing two haploid cells. No DNA replication occurs before meiosis II where the sister chromatids are then separated. Thus, producing 4 haploid cells. (i.e. they contain a single set of chromosomes)
Explain why all the products of meiosis are genetically different
At prophase I, crossing over between non-sister chromatids of homologous pairs produces new combinations of the parental alleles. In addition, at metaphase I, there is an independent assortment of the homologous chromosomes resulting in new combinations of parental chromosomes. As a result at the end of meiosis, all the gametes formed will be genetically different.
Explain the importance of crossing over and independent assortment in meiosis
Crossing over at prophase I, occurs between non-sister chromatids of homologous pairs to produce new combinations of parental alleles. Independent assortment at metaphase I is the random assortment of homologous chromosomes resulting in new combinations of parental chromosomes. Together these processes ensure that all the gametes produced will be genetically different leading to increased genetic diversity in offspring.
Identify the process which involves the fusion of gametes. State how this process results in a diploid zygote
Fertilisation is the fusion of gametes. This process involves the fusion of 2 haploid gametes, one from the parent, which contain a single set of genes. The resultant zygote is therefore diploid with 2 copies of each chromosome.
Compare the products of mitotic and meiotic cell division
The products of mitosis are 2 genetically identical daughter cells each containing the same full set of genes as the parent cell. The products of meiosis are 4 haploid cells which are all genetically different.
Describe the stages in the cell cycle including checkpoints
G0 cells in a non replicating state are not in the cell cycle. Cell cycle begins with G, which is a growth phase for S. At the end of G, the checkpoint assesses whether there is DNA damage. If so, the cycle steps while the cell tries to repair the damage. Once repaired the cycle continues unless there is too much damage then apoptosis occurs. The next phase is S where DNA replicatio occurs. The cell then moves into G2, the second growth phase. At the end of G2 is another checkpoint where the cell checks that the DNA has been correctly replicated. If not the cell enters apoptosis. If everything is good the cell continues into mitosis and at the M checkpoint, it checks that the chromosomes are aligned correctly on the metaphase plate. If ok then mitosis (meiosis) continues.
Explain how hormones may regulate cell division
Hormones bind to their complementary shaped receptors and trigger single transduction pathways which lead to the production of proteins that stimulate the cell cycle and hence cell division.
