A-Level Biology OCR Notes

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Plant Responses & Hormones

All multicellular organisms need to respond to changes in their environment (stimuli) in order to survive
Tropisms are a directional growth response in plants, in which the direction of the response is determined by the direction of the external stimulus e.g. phototropism, geotropism, chemotropism & thigomotropism
Growth only occurs at the meristems.
- The apical meristem is at the tip of the roots and shoots
- The lateral bud meristem is present in buds and gives rise to side shoots
- The lateral meristem forms a cylinder outside the roots and shoots and are responsible for the widening of the roots and shoots
- Intercalary meristems are between nodes and cause shoot elongation
Plants respond to directional stimuli using specific hormones, which move to regions where they are needed from growing regions
Auxins causes elongation of shoot cells, while it also inhibits root cell elongation in order to cause positive geotropism & phototropism.

Auxins cause protons to be actively transported into spaces in the cell wall, activating expansins which loosen cellulose in the cell wall. Increasing plasticity of the cell wall for elongation.
Cytokinins delay leaf senescence. Can also be used commercially in tissue culture to promote bud and shoot growth.
Abscisic acid causes stomatal closure when there is low water availability. It also inhibits seed germination and growth
Gibberellins cause stem elongation and seed germination. Commercially it can be used in fruit production by elongating the stem, also used to induce seed formation and barley seed germination
Ethene is used commercially to promote fruit drop and ripening
Auxins also promote elongation, inhibit side-shoot growth (apical dominance) and inhibit leaf abscission. Can be used commercially to prevent fruit drop, produce seedless fruits, as a herbicide and encourage root growth.

The Nervous System

The central nervous system is the central part of the nervous system is composed of the brain and spinal cord.
The peripheral nervous system is made up of sensory and motor nerves connecting the sensory receptors and effectors to the CNS
The somatic nervous system voluntarily controls skeletal muscle
The autonomic nervous system controls unconscious activities including heart rate, smooth muscle in the digestive system, airways and glands.
The sympathetic system is active when the body is under physical or psychological stress. Many sympathetic nerves leave the spinal cord and they don't have to split as much
The parasympathetic system is active under normal conditions of rest and conserves energy. Only a few parasympathetic nerves leave the spinal cord. These split so that they contact multiple effectors

	Sympathetic Syste	Parasympathetic System
Neurone Structure	Short pre-ganglionic neurones Long post-ganglionic neurones	Long pre-ganglionic neurones Short post-ganglionic neurones in the effector
Neurotransmitter	Noradrenalin	Acetylcholine
Heart Rate	Increases	Decrease
Pupil Diameter	Increases	Decreases
Digestive System	Decreases activity	Increases activity

The medulla oblongata controls cardiac and smooth muscles via the autonomic nervous system. E.g. respiratory, cardiac and vasomotor centres.
The hypothalamus and pituitary gland control various bodily functions and homeostatic mechanisms. E.g. it controls water potential and temperature
The cerebrum carries out the higher brain functions such as thought, language, vision, emotional responses and factual memory.
The cerebellum control balance and fine motor movements. It stores the detailed information about how to carry out the movement which is updated by practice and learning.

Neuromuscular Junction

When an action potential reaches the junction, voltage-gated calcium channels open, causing calcium ions to diffuse into the neurone. Synaptic vessels to fuse with the presynaptic membrane and release acetylcholine into the synapse. It diffuses across the synapse and binds with receptors on the muscle cell surface membrane, opening sodium channels. The muscle fibre depolarisation causes an action potential and muscle contraction.
Acetylcholinesterase breaks down acetyl choline
Neuromuscular junction & cholinergic synapse differences

Neuromuscular Junction	Cholinergic Synapse
Only excitatory	Can be excitatory or inhibitory
Links neurones to muscle	Links either neurones to neurones or neurones to other effectors
The action potential ends here	Another action potential may be generated along the post-synaptic neurones
Only motor neurones are involved	Intermediate, motor and sensory neurones may be involved
Acetylcholine binds to receptors on the membrane of the muscle fibre	Acetylcholine binds to receptors on membrane of post-synaptic neurone

Muscles

Cardiac muscle cells are myogenic, joined by intercalated discs and form branches to allow electrical stimulation to spread evenly. It can contract powerfully without fatiguing.
Smooth muscle contracts slowly and regularly and is controlled by the autonomic nervous system.

Skeletal muscles act in antagonistic pairs against an incompressible skeleton to allow movement
Skeletal muscle is made up of fibres called myofibrils, which in turn are made up of many repeating units, called sarcomeres
Myofibrils are made up of two types of protein filaments, the thinner actin and the thicker myosin

Muscle Contraction

The sliding filament theory describes how muscle contraction occurs
An action potential travels into the muscle fibre via T tubules, causing release of calcium ions from the sarcoplasmic reticulum. The calcium ions bind to the tropomyosin molecules and cause them to move, exposing the myosin binding site on the actin filament. Myosin attaches to actin forming a actin-myosin cross-bridge. ATPases hydrolyse ATP to detach the myosin head, allowing reattachment at a further site. This cycle continues, causing sarcomeres to shorten

When nervous stimulation stops, Ca2+ ions are actively transported back into the sarcoplasmic reticulum using energy from ATP hydrolysis. This allows tropomyosin to block the actin filament from binding to myosin and muscle contraction stops.
ATP can be generation via aerobic or anaerobic respiration
Phosphocreatine generates ATP quickly by adding phosphate to a molecule of ADP released by the contracting muscle

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