Chapter: Co-ordination and Control in Plants
Welcome to this study guide for CCEA A2 Biology Unit 1 (Physiology, Co-ordination and Control, and Ecosystems). While animals rely on complex nervous systems and electrical impulses to respond to their environment, plants achieve remarkable feats of communication, growth, and timing entirely through chemical signals and photoreceptors. Don't worry if this chapter seems detailed at first; we will break down each mechanism step-by-step so you can approach your A2 exam with complete confidence!
---1. Plant Growth, Meristems, and Auxins
Plant Meristems: Where Growth Happens
Unlike animals, which grow throughout their bodies, plants restrict their cell division to specific regions called meristems:
• Apical Meristems: Found at the tips of growing shoots and roots, responsible for primary growth (increasing the length of the plant).
• Lateral Meristems (Cambium): Found along the sides of stems and roots, responsible for secondary growth (increasing the thickness/girth of the plant).
Indole-3-Acetic Acid (IAA) / Auxin
Auxins are plant growth regulators. The most common naturally occurring auxin is Indole-3-Acetic Acid (IAA). IAA is synthesised predominantly in the shoot apical meristem and is transported basipetally (downwards away from the tip into the zone of elongation).
Mechanism of Action: The Acid Growth Hypothesis
How does IAA actually cause a plant stem to grow? It promotes cell elongation through a precise biochemical pathway:
1. IAA binds to receptor proteins on the cell surface membrane of target cells in the elongation zone.
2. This stimulates the active transport of hydrogen ions (\(\text{H}^+\)) from the cytoplasm into the primary cell wall using proton pumps.
3. The accumulation of \(\text{H}^+\) lowers the \(\text{pH}\) of the cell wall (making it more acidic).
4. The low \(\text{pH}\) activates specific enzymes that break bonds between cellulose microfibrils, loosening the cell wall matrix.
5. Water enters the cell by osmosis, and the resulting internal turgor pressure pushes against the relaxed, flexible cell wall, causing the cell to expand and elongate permanently.
Differential Sensitivity: Shoots vs. Roots
A crucial rule to remember for your exams is that shoot cells and root cells respond differently to the concentration of IAA:
• In Shoots: Higher concentrations of IAA stimulate cell elongation.
• In Roots: Root cells are far more sensitive; high concentrations of IAA inhibit cell elongation.
Apical Dominance
Have you ever noticed that a pine tree grows tall and conical with a dominant main trunk? This is due to apical dominance. High concentrations of IAA produced by the growing shoot tip diffuse downwards and suppress the growth of lateral (axillary) buds. If the shoot tip is removed (pruned), IAA levels drop, and the lateral buds begin to grow into side branches.
Section Takeaway: IAA is made in shoot tips, moves downwards, and causes cell elongation in shoots by pumping \(\text{H}^+\) ions into cell walls to loosen cellulose microfibrils. High IAA stimulates shoot elongation but inhibits root elongation.
---2. Directional Growth Responses: Tropisms
A tropism is a directional growth response of a plant towards or away from an external directional stimulus. Growth towards a stimulus is positive, while growth away from a stimulus is negative.
A. Phototropism (Response to Unilateral Light)
Phototropism ensures that leaves receive maximum sunlight for photosynthesis.
• Step 1: When a shoot receives unilateral light (light from one direction), IAA produced in the shoot tip is redistributed laterally from the illuminated side to the shaded side.
• Step 2: A higher concentration of IAA builds up on the shaded side compared to the illuminated side.
• Step 3: The higher IAA concentration on the shaded side causes greater cell elongation there via the acid growth mechanism.
• Step 4: Because cells on the shaded side elongate faster than cells on the light side (differential cell elongation), the shoot bends towards the light. This is positive phototropism.
B. Geotropism / Gravitropism (Response to Gravity)
Geotropism ensures that roots anchor firmly into the soil and access water and mineral ions, while shoots emerge into the air.
• Step 1: When a seedling root lies horizontally, gravity causes IAA to accumulate along the lower side of the root.
• Step 2: In roots, high concentrations of IAA inhibit cell elongation.
• Step 3: Therefore, cells on the upper side (with less IAA) elongate much faster than cells on the lower side.
• Step 4: This differential growth causes the root to bend downwards in the direction of gravity. This is positive geotropism.
Summary Comparison Table
• Shoot in Unilateral Light: IAA accumulates on shaded side \(\rightarrow\) Cells elongate faster on shaded side \(\rightarrow\) Bends towards light (Positive Phototropism).
• Root placed Horizontally: IAA accumulates on lower side \(\rightarrow\) Cells elongate slower on lower side (inhibited) \(\rightarrow\) Bends downwards (Positive Geotropism).
Section Takeaway: Tropisms are driven by the lateral redistribution of IAA, resulting in differential cell elongation (never write "cell division" when explaining tropisms!).
---3. Photoperiodism and the Phytochrome System
What is Photoperiodism?
Photoperiodism is the physiological response of a plant to the relative lengths of daylight and darkness, which controls seasonal events such as flowering.
Phytochrome: The Blue-Green Photoreceptor
Plants detect light using a blue-green pigment called phytochrome, which exists in two interconvertible forms:
1. \(\mathbf{P_R}\) (Phytochrome Red):
• The inactive form of phytochrome.
• Absorbs red light (wavelength approximately \(660\text{ nm}\)).
• When it absorbs red light (during daylight), it is rapidly converted into \(\text{P}_{\text{FR}}\).
2. \(\mathbf{P_{FR}}\) (Phytochrome Far-Red):
• The physiologically active form of phytochrome.
• Absorbs far-red light (wavelength approximately \(730\text{ nm}\)).
• In far-red light, it is rapidly converted back into \(\text{P}_{\text{R}}\).
• In the dark (during nighttime), \(\text{P}_{\text{FR}}\) slowly reverts back to \(\text{P}_{\text{R}}\) (known as dark reversion).
Quick Conversion Summary:
\(\text{P}_{\text{R}} \xrightarrow{\text{Red light } (660\text{ nm}) \text{ / Daylight (Rapid)}} \text{P}_{\text{FR}}\)
\(\text{P}_{\text{FR}} \xrightarrow{\text{Far-red light } (730\text{ nm}) \text{ (Rapid)}} \text{P}_{\text{R}}\)
\(\text{P}_{\text{FR}} \xrightarrow{\text{Darkness / Night (Slow reversion)}} \text{P}_{\text{R}}\)
Measuring Night Length: Short-Day vs. Long-Day Plants
Crucial Exam Fact: Plants do not measure the length of daylight; they measure the duration of the uninterrupted dark period (night length).
• Short-Day Plants (SDPs) / Long-Night Plants:
These plants flower in late summer, autumn, or winter when nights are long. They require a dark period that is longer than a critical length.
Role of Phytochrome: In SDPs, \(\text{P}_{\text{FR}}\) acts as an inhibitor of flowering. During a sufficiently long, uninterrupted night, all the \(\text{P}_{\text{FR}}\) has time to slowly revert to \(\text{P}_{\text{R}}\). The inhibition is removed, and flowering occurs.
• Long-Day Plants (LDPs) / Short-Night Plants:
These plants flower in late spring and summer when nights are short. They require a dark period that is shorter than a critical length.
Role of Phytochrome: In LDPs, \(\text{P}_{\text{FR}}\) acts as a promoter of flowering. Because summer nights are short, not all \(\text{P}_{\text{FR}}\) has time to revert to \(\text{P}_{\text{R}}\). The remaining high level of active \(\text{P}_{\text{FR}}\) triggers flowering.
Night-Break Experiments
If a short-day plant is exposed to a brief flash of red light in the middle of its long dark period (a "night break"):
• The red light instantly converts \(\text{P}_{\text{R}}\) back into active \(\text{P}_{\text{FR}}\).
• The presence of \(\text{P}_{\text{FR}}\) immediately inhibits flowering in SDPs.
• Conversely, a night break promotes flowering in LDPs because it raises \(\text{P}_{\text{FR}}\) levels.
Section Takeaway: Daylight converts \(\text{P}_{\text{R}} \rightarrow \text{P}_{\text{FR}}\). Darkness allows slow reversion \(\text{P}_{\text{FR}} \rightarrow \text{P}_{\text{R}}\). \(\text{P}_{\text{FR}}\) promotes flowering in Long-Day Plants and inhibits flowering in Short-Day Plants.
---4. Additional Plant Growth Regulators
Auxin is not the only plant hormone you need to know for CCEA A2 Biology. Three other key growth regulators are tested:
1. Gibberellins (GA)
• Stem Elongation: Gibberellins stimulate rapid cell elongation in internodes (the regions of stem between leaves). Plants with mutant dwarf genes often lack gibberellins; applying GA causes them to grow to normal heights.
• Breaking Seed Dormancy & Germination: In cereal seeds (such as barley), the embryo releases gibberellins when water is absorbed. GA diffuses to the aleurone layer, where it activates the transcription and synthesis of \(\alpha\)-amylase. This enzyme hydrolyses insoluble starch in the endosperm into soluble maltose, providing glucose for respiration and seedling growth.
2. Abscisic Acid (ABA)
• Stress Hormone: ABA is released during environmental stress, particularly drought and water deficiency.
• Stomatal Closure: When roots detect low soil water, ABA travels to the leaves and triggers the rapid loss of potassium and water from guard cells, causing stomatal closure to reduce transpiration.
• Dormancy: ABA maintains seed and bud dormancy throughout cold or dry conditions, preventing premature germination until favourable conditions return.
3. Ethene (Ethylene)
• Gaseous Regulator: Ethene is a hydrocarbon gas produced by aging plant tissues.
• Fruit Ripening: Promotes the breakdown of cell walls, conversion of starches to sugars, and softening of fruits (e.g., in bananas and apples).
• Abscission: Stimulates the shedding of leaves, flowers, and fruits by promoting the formation of an abscission layer at the base of the petiole.
Section Takeaway: Gibberellins promote stem growth and seed germination (\(\alpha\)-amylase activation in barley). ABA is the stress hormone causing stomatal closure and dormancy. Ethene is a gas driving fruit ripening and abscission.
---5. Common Pitfalls and Exam Tips
• Pitfall 1: Day Length vs. Night Length: Always explain flowering in terms of the uninterrupted length of the dark period. A short-day plant is strictly a long-night plant.
• Pitfall 2: Confusing Auxin Effects: Remember that shoot cells and root cells have opposite responses to high IAA. High IAA = longer shoot cells, but shorter/inhibited root cells!
• Pitfall 3: Cell Division vs. Cell Elongation: In tropisms, shoots and roots bend because cells elongate at different rates on opposite sides, not because they divide faster on one side.
• Pitfall 4: Phytochrome Forms: Remember the rule: \(\text{P}_{\text{R}}\) absorbs Red light (\(660\text{ nm}\)) and becomes \(\text{P}_{\text{FR}}\). \(\text{P}_{\text{FR}}\) is the active form that reverts slowly in the dark.
Quick Concept Checklist
Can you describe the Acid Growth Hypothesis step-by-step?
Can you explain why a flash of red light in the middle of the night prevents a chrysanthemum (SDP) from flowering?
Can you state the role of GA in the aleurone layer of barley grains?