Aqa Biology AS unit 2 five/six markers

Explain how water enters xylem from the endodermis

• 6 MARKER
• Casparian ***** blocks the apoplastic pathway and only allows water to move through the symplastic pathway (1)
• Active transport by endodermis of ions/salt into the xylem (2)
• Lower water potential in the xylem so water moves by osmosis down a water potential gradient into the xylem (1)
• Evaporation of water from the leaves results in a transpiration pull (2)
• Water molecules bind to the xylem and create a continuous water column travelling up to the leaves (2)

Describe how DNA is replicated.

• 6 MARKER.
• DNA strands separate and the H-bonds break (1)
• DNA helicase is involved in this process (1)
• Each strand acts as a template and free nucleotide bases attach (2)
• Complementary base pairing occurs, e.g. AT, GC (1)
• DNA polymerase joins the nucleotides onto the new strands (1)
• H-bonds reform (1)
• Semi-conservative replication (1)

Describe how tissue fluid is formed and how it is returned

• 6 MARKER.
• Formation:
• High hydrostatic pressures force water and fluid out (2)
• Large proteins remain in the capillary (1)
• Return:
• Low water potential in capillary due to plasma proteins (2)
• Water enters capillary by osmosis (2)
• Some is returned via the lymphatic system (1)

Explain how oxygen is loaded, transported and unloaded in the body.

• 6 MARKER.
• Haemoglobin has a high affinity for oxygen so carries it in red blood cells (2)
• Loading/uptake in the lungs (1)
• At high partial pressure (1)
• Unloads/dissociates to respiring tissues (1)
• At low partial pressure (1)
• Unloading is linked to a higher carbon dioxide concentration (1)

Describe how gene transmission and selection have led to antibiotics being less effective.

• 6 MARKER.
• Antibiotic resistant gene/allele is shared/spread through vertical gene transmission (2)
• Resistant bacteria survive and reproduce (1)
• Increase in frequency of resistant gene/allele in future generations (1)
• Horizontal gene transmission (1)
• Plasmid DNA cloned/spread through conjugation (2)
• Can occur between bacteria of different species (1)

Arteries and arterioles take blood away from the heart; how are their walls adapted to this function?

• 6 MARKER.
• Elastic tissue stretches under pressure when the heart beats (1)
• Recoils after (1)
• Evens out/maintains a smooth pressure flow (1)
• Muscle contracts (1)
• Reduces diameter of the lumen (1)
• Changes flow/pressure (1)
• Epithelium is smooth (1)
• Reduces friction (1)

Describe the processes that lead to a reduction in genetic variation.

• 6 MARKER.
• (General principle) - reduced variety/number of alleles in new population (1)
• Founder effect: a few individuals from a population become isolated/form colonies (2)
• Genetic bottlenecks: significant fall in the size of the population (2)
• Selective breeding/artificial selection (1)
• Using organisms with particular alleles/traits (1)

Scientists studied two species of North American seahorses; how could they distinguish how closely related they are?

• 6 MARKER.
• Compare DNA/sequence of bases and nucleotides (2)
• DNA hybridisation (1)
• Separate DNA strands (1)
• Mix DNA strands of different species (1)
• Temperature requires to separate the strands indicates relationship (1)
• Compare same/named protein (1)
• Sequence of amino acids/primary structure (1)