In plants, the leaf is the main photosynthetic organ.
List four ways in which the structure of a dicotyledonous leaf is adapted for gas
exchange. (4)
- stomata for gaseous exchange
- thin cell walls allow gas dissolve and enter
- thin leaf for short diffusion distance of CO2
- large surface area of, palisade / (spongy) mesophyll, cells / tissue
- air spaces allow gas diffuse through leaf
Explain why the discs float after being illuminated for a length of time. (3)
- photosynthesis takes place
- oxygen is produced
- oxygen collects in air spaces
- discs become more buoyant
State the general name given to changes in the environment that can be detected by
receptors. (1)
Stimuli
Receptors are often described as biological transducers, structures which convert
energy from one form into another.
Explain how receptors in mammals convert energy into action potentials. Use named
examples of receptors in your answers. (6)
- rods & cones are photoreceptors which detect light;
- olfactory cells have chemoreceptors that detect chemicals;
- Pacinian corpuscle detects pressure changes in skin
- baroreceptors detect blood pressure changes;
- osmoreceptors detect changes in blood water potential;
- Ruffini’s endings in skin have thermoreceptors that detect temperature changes
- hair cells in semicircular canals detect movement;
- hair cells in cochlea detect sound (4P)
- stimulus causes sodium channels to open
- sodium ions enter cell
- if stimulus is greater than threshold due to all or nothing principle causes depolarisation
- increased stimulus leads to increased frequency of action potentials
- causing e.g. hyperpolarisation in rod cell
- but if weak stimulus then generator potential (4P)
February 2001, the BBC reported that scientists had discovered a ‘new species’ of camel
in a remote part of Asia. These camels differ from domesticated Bactrian camels in the
following ways:
3% of their DNA base sequences are different
their humps are further apart
they have hairier knees
there is no freshwater in the area and they survive by drinking salty water.
One possibility is that domesticated camels were bred from this wild stock many generations
ago.
Describe how changes in DNA could occur. (3)
- due to random mutation
- (named) mutagen causing mutation e.g. biololgical agent such as contaminated food with myotoxin
- due to deletion / subsitution (named) mechanism;
- change in structure and function of DNA base sequence
Describe what further evidence is required to show that this ‘new species’ is a different
species from the domesticated camels.
[3]
- interbreed ( with domesticated camels <—context of question )
- to produce fertile offspring ( 2P in 1 sentence - ‘fertile’ and ‘offspring’ )
The BBC also reported: ‘The salt water is not ideal for the camels and they have had to
adapt to drink it. Some young animals are unable to adapt and they die as a result’.
Explain the effect on body tissues of drinking only salty water.
[2]
- body tissues lose water via osmosis
- due to cells become plasmolysed as water potential decreases and solute potential increases
Explain how genetic variation in the ancestral wild camel population enabled the
development of this ‘new species’.
[2]
- genetic variation in camel’s tolerance to salt causes mutation in DNA to produce an advantageous allele
- camel survives and reproduces to pass adv allele to offspring
- over many generations, the allele frequency increases
A dog breeder wishes to know whether a dog with sable trim is either homozygous or
heterozygous for this particular trait.
State the cross needed to determine the dog’s genotype.
[1]
Test cross / back cross
Explain why the offspring of this cross will reveal the genotype of the dog.
[3]
- if homozygous, all offspring are sable
- but if any offspring are copper then
- must be heterozygous therefore it must carry the ac allele
The inheritance of coat pattern in Australian Shepherd dogs illustrates the principle of multiple alleles at a single locus.
Explain the meaning of multiple alleles
[2]
- More than two forms of a gene
Explain the meaning of locus
[2]
- position of an allele on a chromosome
