• Phytochrome is a light-sensing pigment that flips between an inactive form (P_R) and an active form (P_FR) depending on the colour of light it absorbs.
• Phytochrome controls seed germination — red light triggers it, far-red light blocks it.
• Phytochrome also controls flowering, by "measuring" night length through the balance of P_R and P_FR.
• P_FR acts as a transcription factor — it enters the nucleus, binds to PIF3, and switches genes on.
• Growth factors (plant hormones) like auxin (IAA), gibberellins, cytokinins, abscisic acid and ethene each control different aspects of growth.
• IAA (auxin) drives phototropism (bending towards light) and geotropism (roots growing down, shoots growing up) — but has opposite effects in shoots vs. roots.
• Gibberellin triggers germination in barley seeds by switching on the gene for amylase in the aleurone layer.
• Core Practical 18 uses a starch-agar assay to measure how gibberellin concentration affects amylase production.
• Animals, unlike plants, coordinate responses using both the nervous system (fast, electrical, short-lived) and the hormonal system (slower, chemical, longer-lasting).

Think about it from the plant's point of view: it can't move, so every decision it makes has to be based on signals from its environment. Should I germinate now, or wait? Should I put energy into flowers this month? These are life-or-death decisions, and the plant needs a reliable way to "read" the light around it. That's exactly what phytochrome does.

Phytochrome is a pigment — a molecule that absorbs specific wavelengths of light — and it exists in two interconvertible forms:

Here's the clever bit: this conversion is reversible, like a switch that flips back and forth depending on which colour of light hits it last.

Early scientists noticed something odd: seeds exposed to red light would germinate, but seeds exposed to far-red light wouldn't. We now know phytochrome explains this completely.

• Even a short burst of red light converts P_R → P_FR, and this triggers germination.
• Far-red light converts P_FR back to P_R, which reverses the effect and prevents germination.

This makes ecological sense too — a seed buried under leaf litter or shaded by a thick plant canopy receives light that's been filtered, richer in far-red than red (because chlorophyll absorbs red light for photosynthesis and lets far-red pass through). High far-red essentially tells the seed "it's shady here, don't bother germinating yet — you won't get enough light to photosynthesise."

Flowering is controlled by night length (not day length, despite the confusing terminology you're about to see!). Nights are short in spring/summer and long in autumn/winter. Depending on the species, a plant will flower when nights are short, or when nights are long.

The plant "measures" night length using the ratio of P_R to P_FR in its leaves:

When night length reaches a certain critical duration, genes controlling flowering are switched on or off — genes that are switched on get transcribed and translated into the polypeptides they code for; genes switched off are not.

"Long-day plants" flower when nights are short (i.e. days are long) — e.g. in summer. Here's the chain of logic, step by step:

Notice the naming trap here: "long-day plant" refers to the plant flowering when days are long — but what the plant is actually detecting and responding to is the short night. Examiners love testing whether you understand this distinction.

So how does a light-absorbing pigment actually end up switching genes on? This is where phytochrome's role as a transcription factor comes in — and it's worth learning this pathway precisely, because it's a classic "explain the mechanism" exam question.

This single mechanism is thought to be responsible for multiple plant responses — not just germination and flowering, but also responses to light and gravity more generally. It's a good example of how one signalling molecule can control many different downstream outcomes just by activating different combinations of genes.

Plants respond to stimuli in various ways, and one of the most important is by altering their growth. For example, a seedling bends toward light because there's more growth on the shaded side than the illuminated side — a directional growth response called a tropism.

Since plants have no nervous system, these growth responses rely entirely on chemical substances called growth factors (also called plant hormones, because they act as chemical messengers just like animal hormones). They're produced in the growing regions of the plant, then move to other tissues to regulate cell growth in response to directional stimuli.

Example: shoots show positive phototropism (grow towards light) but negative geotropism (grow upwards, away from gravity). Roots do the reverse — negative phototropism, positive geotropism.

IAA (indoleacetic acid) is a type of auxin, and it's the hormone examiners focus on most. It influences plant growth by altering the transcription of genes inside plant cells — genes involved in cell growth get switched on or off depending on IAA concentration.

IAA is produced in the growing regions of a plant, then redistributed to other tissues:

• Cell-to-cell transport happens by diffusion and active transport
• Longer-distance transport happens via the phloem

Crucially, environmental stimuli like light and gravity cause IAA to become unevenly distributed across the plant — and it's this uneven distribution that produces uneven, directional growth.

When light shines on a stem from one side:

Roots respond to gravity through geotropism. Remember: this is the opposite effect to shoots.

Note: you don't need to know about amyloplasts detecting the direction of gravity for this specification — just the IAA redistribution and its effect on cell elongation.

When a barley seed is shed from the parent plant, it's in a state of dormancy — very little water, metabolically inactive. This lets it survive harsh conditions (like a cold winter) until conditions are right for germination.

A barley seed has three key structures:

Here's the full chain of events once conditions become favourable and the seed absorbs water:

To investigate the effect of gibberellin concentration on the production of amylase in germinating barley seeds, using a technique called a starch agar assay.

This is the single most commonly-tested logic point in this practical, so let's walk through why it's necessary. In a normal germinating seed, the embryo itself produces gibberellin. If you left the embryo intact, you'd have no way of knowing whether any amylase production was due to the gibberellin concentration you added, or gibberellin the embryo was making on its own — an uncontrolled variable that would ruin the experiment.

The solution: cut the seed in half using a scalpel, and discard the half containing the embryo (found at the more pointed end of the seed). What's left is a seed half containing only endosperm and aleurone layer — with zero internal gibberellin production. Now, any amylase produced is entirely due to the gibberellin concentration the investigator applies.

Starch stains blue-black with iodine. Wherever amylase has diffused out of the cut seed and broken down starch, there will be no starch left to stain — leaving a clear zone around the seed.

Both plants and animals must respond to changes in their internal and external environment to survive — finding food, avoiding harm, staying at a favourable temperature. But while plants rely solely on chemical signals, animals use both nervous AND hormonal coordination — because different jobs need different speeds.

Stimuli are detected by specialised receptor cells, found in sense organs (eyes, ears, skin) or internally (e.g. pressure receptors in blood vessels). These send signals via the nervous system or hormonal system to a co-ordination centre (brain or spinal cord), which sends signals onward to effectors — muscles or glands — which carry out the actual response.

The nervous system is split into two parts:

• Central Nervous System (CNS) — the brain and spinal cord; acts as the central coordinating centre.
• Peripheral Nervous System (PNS) — all the nerves in the body, each a bundle of neurones, connecting receptors to the CNS and the CNS to effectors.

Information travels as electrical impulses along neurones (sensory, relay, and motor neurones). The full nerve pathway is:

Worked example:

The arm muscle then contracts, pulling the hand away from the hot surface.

Hormones are chemical substances produced by endocrine glands and carried in the blood. Endocrine glands are ductless — they secrete hormones directly into the bloodstream, rather than through a tube like exocrine glands do.

Hormones bind to specific receptors on the cell surface membranes of target organs, changing that organ's activity. Because they must travel through the blood, hormones act more slowly than nerve impulses — which is exactly why they're used for processes that don't need an instant response (growth, metabolism, reproduction, blood glucose regulation).

Worked example:

The liver cells respond to insulin by converting glucose into glycogen, lowering blood glucose levels.

• I can explain the difference between P_R and P_FR, including which wavelength converts one into the other.
• I can explain why red light triggers germination and far-red light prevents it.
• I can explain how phytochrome levels change between day and night, and how this controls flowering in long-day plants.
• I can describe the mechanism by which P_FR acts as a transcription factor (nucleus, PIF3).
• I can define tropism, phototropism, geotropism, and positive/negative tropisms with examples.
• I can explain how uneven IAA distribution causes a shoot to bend towards light.
• I can explain why the SAME increase in IAA speeds up growth in shoots but slows it down in roots.
• I can describe, step by step, how gibberellin triggers amylase production in a germinating barley seed.
• I can explain why the embryo half of the seed must be removed in the starch agar assay.
• I can describe the full method and results interpretation for Core Practical 18 (starch agar assay).
• I can state and use the pathway: stimulus → receptor → sensory neurone → CNS → motor neurone → effector.
• I can state and use the pathway: stimulus → receptor → hormone → effector.
• I can compare nervous and hormonal coordination by speed, message type, and duration of effect.