Edexcel Biology Unit 6 Key Word Glossary

Phototropism - the growth of a plant in response to light, shoots are positively phototropic (grow towards light), roots are negatively phototropic (grow away from light).

Auxin - the plant growth substance involved in phototropism, the first discovered was IAA. 

Phytochrome - a molecule consisting of a protein component bonded to a non-protein light-absorbing pigment molecule, phytochromes trigger of inhibit germination. 

Photoperiod - the relative day length of day and night, it is these environmental cues that determine flowering time. 

Long-Day Plants - only flower when day length exceeds a critical value, need P_fr to stimulate flowering.

Short-Day Plants - need long periods of uninterrupted darkness to flower, P_fr inhibits flowering. 

Etiolation - plants which are grown in the dark become tall and thin, with fragile stems and long internodes and small, plate, yellowish leaves. 

Sensory Neurones - carry impulses from sensory cells to the CNS, cell body in the middle of the cell.

Relay Neurones - transmit impulses between sensory neurones and motor neurones, cell body at the end of the cell. 

Motor Neurones - carry impulses from the CNS to effectors, cell body at the end of the cell. 

Axons - fibres which carry impulses away from the cell body.

Dendrons - carry impulses towards the cell body. 

Myelin Sheath - made of Schwann Cells wrapped around the axon, no depolarisation can take place here as the fatty material insulates the impulse.

Resting Potential - a neurones state when it is not being stimulated, the outside is positively charged compared to the inside. 

Action Potential - the sequence of events after a stimulus big enough to trigger a rapid change in potential difference.

Nodes of Ranvier - the spaces between the Schwann Cells where depolarisation can take place, localised circuits are set up here. 

Saltatory Conduction - impulses jump from node to node which speeds up transmission as the ionic movements associated with the action potential occur much…