Plant Biology

This topic covers plant structure, transport in plants, reproduction, growth, and plant responses to Stimuli. Plant biology is a core area of the Irish Leaving Certificate Biology syllabus and Underpins understanding of ecosystems, agriculture, and human physiology.

Plant Structure

Plant Tissues (OL/HL)

Plants are multicellular organisms composed of three main tissue systems. Each system has a distinct Structure and function, and understanding their arrangement is essential for explaining transport, Support, and gas exchange.

Dermal tissue (epidermis): outer protective layer. May have a waxy cuticle to reduce water loss. Root hair cells are specialised epidermal cells for absorption. The cuticle is made of cutin, a waxy Lipid polymer secreted by epidermal cells, which is impermeable to water. This adaptation is Particularly important in terrestrial plants that face the constant risk of desiccation.

Ground tissue (parenchyma): fills the interior of the plant. Involved in photosynthesis, Storage, and support. Parenchyma cells are thin-walled, living cells that remain capable of cell Division. In leaves, parenchyma cells in the mesophyll contain chloroplasts for photosynthesis. In Roots and stems, parenchyma cells store starch, oils, or water. Two specialised forms of ground Tissue include collenchyma (unevenly thickened cell walls for flexible support in young stems) and Sclerenchyma (thick lignified walls for rigid support in mature tissues).

Vascular tissue: consists of xylem and phloem, and forms a continuous transport network Throughout the plant.

• Xylem: transports water and dissolved minerals from roots to leaves. Composed of dead, hollow cells strengthened by lignin. Xylem vessels form continuous tubes through which water moves. The lignin provides structural support and prevents the vessels from collapsing under the negative pressure of the transpiration stream. Tracheids, found in all vascular plants, are narrower than vessels and also transport water, but they are the only conducting cells in conifers and ferns.
• Phloem: transports organic substances (mainly sucrose) from leaves to the rest of the plant. Composed of living cells (sieve tube elements and companion cells). Sieve tube elements have perforated end walls called sieve plates that allow sap to flow. Companion cells are metabolically active and provide energy for loading and unloading sugars.

Leaf Structure (OL/HL)

Adaptations for photosynthesis:

• Large surface area — maximises light absorption.
• Thin — short diffusion distance for gases ($O_2$ and $CO_2$).
• Many stomata for gas exchange — on the lower epidermis to reduce water loss.
• Chloroplasts containing chlorophyll — the photosynthetic pigment that absorbs red and blue light.
• Extensive vein network (xylem and phloem) — delivers water and removes products of photosynthesis.

Cross-section of a leaf:

• Upper epidermis: covered by waxy cuticle; transparent to allow light through to the palisade mesophyll.
• Palisade mesophyll: tightly packed columnar cells with many chloroplasts near the upper surface (main photosynthetic tissue). Their position near the top of the leaf ensures maximum light absorption.
• Spongy mesophyll: loosely packed cells with air spaces for gas exchange. The air spaces increase the surface area available for diffusion of $CO_2$ into cells and $O_2$ out of cells.
• Lower epidermis: contains stomata with guard cells. Guard cells control the opening and closing of stomata by changing their turgor pressure.
• Vascular bundles: xylem (top, carries water to mesophyll) and phloem (bottom, carries sugars away).

:::info Guard cells are the only epidermal cells that contain chloroplasts. When light strikes them, Photosynthesis produces ATP, which drives potassium ion ($K^+$) pumps. Potassium ions enter the Guard cells, lowering their water potential. Water follows by osmosis, making the cells turgid and Opening the stomata. :::

Root Structure (OL/HL)

Functions: anchorage, absorption of water and minerals, and storage (in some species such as Carrots and turnips).

Root hair cells: increase surface area for absorption. Each root hair is a tubular extension of An epidermal cell. The large surface area-to-volume ratio maximises the rate of water and mineral Ion uptake by osmosis and active transport respectively. Root hair cells have many mitochondria to Provide ATP for active transport of mineral ions against their concentration gradient.

Root tip zones:

1. Zone of cell division (meristem): cells actively divide by mitosis near the root tip. The root cap protects the meristem as the root pushes through soil.
2. Zone of elongation: cells grow longer by absorbing water into their vacuoles. This zone is primarily responsible for pushing the root through the soil.
3. Zone of differentiation: cells begin to specialise into epidermis, cortex, and vascular tissue.
4. Zone of maturation: root hairs develop, and fully differentiated tissues are present. This is where most absorption occurs.

Stem Structure (HL)

Dicot stem cross-section (from outside to inside):

1. Epidermis — single layer of protective cells.
2. Cortex (parenchyma) — for storage and photosynthesis (in green stems).
3. Vascular bundles (arranged in a ring): each contains xylem (inner) and phloem (outer) with cambium between. The cambium is a lateral meristem that produces secondary xylem (wood) and secondary phloem.
4. Pith (central parenchyma) — for storage.

Monocot stem: vascular bundles are scattered throughout the ground tissue rather than arranged In a ring. Monocots generally lack a cambium and do not undergo secondary growth (no thickening of The stem).

Worked Example: Distinguishing Monocot and Dicot Features (HL)

Question: A plant has leaves with parallel veins, flower parts in multiples of three, and Vascular bundles scattered in the stem. Is this a monocot or dicot? Give three reasons.

Answer:

1. Parallel venation in leaves is characteristic of monocots; dicots have net-like (reticulate) venation.
2. Flower parts in multiples of three is a monocot feature; dicots have flower parts in multiples of four or five.
3. Scattered vascular bundles in the stem is a monocot arrangement; dicots have vascular bundles arranged in a ring.

Therefore, this plant is a monocot.

Transport in Plants

Transpiration (OL/HL)

The loss of water vapour from the leaves through the stomata. Transpiration is an inevitable Consequence of gas exchange: stomata must open to allow $CO_2$ to enter for photosynthesis, but this Also allows water vapour to escape.

Transpiration stream:

1. Water is absorbed by root hair cells by osmosis (the soil solution has a higher water potential than the root hair cell cytoplasm).
2. Water moves across the root cortex by osmosis and through the symplastic pathway (through the cytoplasm of cells via plasmodesmata).
3. Water enters the xylem in the stele (central vascular cylinder) of the root.
4. Water moves up the xylem by root pressure and, primarily, the cohesion-tension theory.

Cohesion-Tension Theory (HL)

This is the most widely accepted explanation for the movement of water up xylem vessels. The theory Relies on the physical properties of water molecules.

1. Water evaporates from the spongy mesophyll cell walls into the air spaces (transpiration).
2. This creates a tension (negative pressure) that pulls water up the xylem. The water column is under tension, which can reach pressures as low as $-2$ MPa in tall trees.
3. Water molecules cohere to each other via hydrogen bonding, creating a continuous unbroken column from root to leaf.
4. Water molecules adhere to the walls of the xylem (adhesion), which helps resist the downward pull of gravity.
5. The transpiration pull is the main force driving water up the plant. Root pressure (generated by osmosis pushing water into the xylem) contributes a much smaller force and is mainly important at night when transpiration is low.

Evidence supporting cohesion-tension theory:

• When a xylem vessel is cut, air is drawn in and the water column breaks, stopping flow.
• Measuring devices (potometers) show that water uptake increases with transpiration rate.
• The diameter of tree trunks decreases slightly during the day when transpiration pull is greatest, confirming that the xylem is under tension.

Factors Affecting Transpiration Rate (OL/HL)

Factor	Effect	Reason
Temperature	Higher temperature increases rate	Increases kinetic energy of water molecules; increases evaporation
Humidity	Higher humidity decreases rate	Reduces the water potential gradient between leaf and air
Wind speed	Higher wind speed increases rate	Removes water vapour from leaf surface, maintaining gradient
Light intensity	Higher light increases rate (stomata open)	Stomata open wider in light for photosynthesis, allowing more water loss

Worked Example: Predicting Transpiration Rate (OL/HL)

Question: A potted plant is moved from a cool, dark room to a warm, sunny, windy windowsill. Describe and explain the effect on the transpiration rate.

Answer: The transpiration rate will increase significantly. Three factors change Simultaneously:

1. Temperature increases: higher temperature gives water molecules more kinetic energy, increasing the rate of evaporation from the spongy mesophyll.
2. Light intensity increases: guard cells become turgid in the light, opening stomata wider and increasing the area through which water vapour can escape.
3. Wind speed increases: moving air carries away water vapour that accumulates near the leaf surface, maintaining a steep water potential gradient between the inside of the leaf and the outside air.

Translocation (HL)

The transport of organic substances (mainly sucrose) in the phloem. Unlike xylem transport, which is A passive process driven by physical forces, translocation requires energy (active transport at the Source).

Mass flow hypothesis (pressure flow hypothesis):

1. Sucrose is actively loaded into the phloem sieve tube elements at the source (e.g., photosynthesising leaves) by companion cells using ATP.
2. This lowers the water potential inside the sieve tube elements.
3. Water enters by osmosis from the adjacent xylem, creating high hydrostatic pressure at the source.
4. At the sink (e.g., roots, growing tips, fruits), sucrose is unloaded (either actively or by diffusion if the concentration gradient allows).
5. Water leaves the phloem by osmosis, lowering the hydrostatic pressure at the sink.
6. This creates a pressure gradient from source to sink, driving bulk flow of phloem sap through the sieve tubes.

Evidence for translocation:

• Aphids feed on phloem sap using their stylet (mouthpart), which penetrates sieve tubes. The sap collected has a high sucrose concentration (up to 30%).
• Ringing a stem (removing a ring of bark containing phloem) causes swelling above the ring as sugars accumulate. The tissue below the ring dies as it is cut off from its sugar supply.
• Radioactive tracers (^{14}\mathrm{C in $CO_2$) fed to leaves show movement of labelled sugars to other parts of the plant through the phloem.
• Phloem sap always flows from source to sink, not in the reverse direction.

Limitations of the mass flow hypothesis:

• Sugar concentration is not always higher at the source than at the sink.
• The model does not fully explain how sugars are loaded and unloaded against concentration gradients at certain sinks.

Worked Example: Interpreting Ringing Experiment Results (HL)

Question: A gardener removes a complete ring of bark from around the trunk of a tree but leaves The xylem intact. After several weeks, the bark above the ring swells. Explain this observation.

Answer: The ring of bark removal removes the phloem tissue, which is located in the bark. The Phloem is responsible for translocating sucrose from the leaves (source) to the roots (sink). With The phloem pathway severed:

1. Sucrose produced by photosynthesis in the leaves can still reach the region above the ring but cannot pass below it.
2. Sugars accumulate in the phloem above the ring, increasing the osmotic potential and causing water to enter by osmosis.
3. This causes the tissue above the ring to swell. Meanwhile, the roots below the ring are deprived of sugars and may eventually die.

This experiment provides strong evidence that the phloem, not the xylem, is the tissue responsible For translocation of organic substances.

Plant Reproduction

Sexual Reproduction in Flowering Plants (OL/HL)

Flower Structure

• Sepal: protects the flower bud. Collectively called the calyx.
• Petal: attracts pollinators through colour and scent. Collectively called the corolla.
• Stamen (male): anther (produces pollen grains by meiosis, each containing a male gamete) + filament (supports the anther).
• Carpel/Pistil (female): stigma (receives pollen, sticky surface) + style (connects stigma to ovary) + ovary (contains ovules, each containing an egg cell and two polar nuclei).

A flower containing both stamens and carpels is called bisexual (hermaphrodite). Flowers Containing only one type are unisexual (either male or female).

Pollination (OL/HL)

The transfer of pollen from anther to stigma.

Self-pollination: pollen from the same plant lands on its own stigma. Advantages: reliable, does Not require a pollinator. Disadvantages: no genetic variation, inbreeding depression.

Cross-pollination: pollen from a different plant of the same species. Advantages: genetic Variation, potentially stronger offspring. Disadvantages: requires a pollination agent, less Reliable.

Agents of pollination:

Agent	Flower adaptations
Wind	Small, dull flowers; large feathery stigmas; light, smooth pollen; exposed anthers
Insects	Brightly coloured petals; scent; nectar; sticky pollen; landing platform
Birds	Large, brightly coloured (often red); copious nectar; sturdy perch
Water	Long, floating pollen; small, inconspicuous flowers (rare)

:::info Wind-pollinated flowers (e.g., grasses) produce enormous quantities of lightweight pollen. Only a tiny fraction reaches a stigma, but the sheer volume ensures successful reproduction. Insect-pollinated flowers produce less pollen but it is often larger and stickier to adhere to Pollinators. :::

Fertilisation (OL/HL)

1. Pollen grain lands on the stigma and germinates, absorbing sugars and water.
2. A pollen tube grows down the style to the ovule, directed by chemical signals from the ovule.
3. The pollen tube nucleus (generative nucleus) travels down the tube, dividing by mitosis to produce two male gamete nuclei.
4. Double fertilisation occurs:

• One male gamete fuses with the egg cell $\to$ zygote (2n).
• The other male gamete fuses with two polar nuclei $\to$ endosperm (3n), a nutritive tissue.

Double fertilisation is unique to flowering plants (angiosperms) and ensures that the endosperm only Develops if fertilisation has occurred, avoiding waste of resources.

Seed and Fruit Development (OL/HL)

• The zygote develops into the embryo.
• The endosperm provides nutrition for the developing embryo.
• The ovule develops into the seed (the integuments become the seed coat/testa).
• The ovary develops into the fruit (the ovary wall becomes the pericarp).
• Other flower parts (sepals, petals, stamens) wither and fall off.

Seed Structure (OL/HL)

• Seed coat (testa): protective outer layer derived from the integuments of the ovule. Prevents desiccation and physical damage. May be hard and impermeable in some species (dormancy mechanism).
• Embryo: consists of the radicle (future root), plumule (future shoot), and cotyledon(s) (seed leaves). The radicle is always the first structure to emerge during germination.
• Endosperm: food store (in monocots; in dicots, food is stored in the cotyledons).
• Micropyle: a small pore in the testa through which water enters during germination.
• Scar (hilum): point where the seed was attached to the ovary wall.

Monocot vs dicot seeds:

Feature	Monocot seed	Dicot seed
Cotyledons	One	Two
Endosperm	Present (main food store)	Often absent (food in cotyledons)
Example	Maize, wheat	Bean, pea

Germination (OL/HL)

Conditions for germination:

• Water: for enzyme activation (hydrolytic enzymes break down stored food), metabolic processes, and to swell and burst the seed coat (imbibition).
• Oxygen: for aerobic respiration to provide ATP for cell division and growth.
• Suitable temperature: for optimal enzyme activity ( $20$—$30\degree C$ for temperate species).
• Some seeds also require light or specific temperature treatments (stratification, scarification) to break dormancy.

Process:

1. Water is absorbed (imbibition) — the seed swells and the testa may rupture.
2. Enzymes (amylase, protease, lipase) are activated and break down stored food:

• Starch $\to$ maltose $\to$ glucose
• Proteins $\to$ amino acids
• Lipids $\to$ fatty acids + glycerol

3. The soluble products are transported to the embryo.
4. The radicle emerges first, growing downwards (positive geotropism) to anchor the plant and absorb water.
5. The plumule emerges, growing upwards (negative geotropism, positive phototropism) towards light.
6. Photosynthesis begins once the plumule reaches light and the first leaves expand.
7. The seedling becomes nutritionally independent once its own photosynthesis exceeds its reserves.

Asexual Reproduction (Vegetative Propagation) (HL)

Methods include:

• Runners (strawberries): horizontal stems that grow along the ground surface, producing new plants at nodes.
• Tubers (potatoes): swollen underground stems (not roots) containing buds (eyes) that can grow into new plants.
• Bulbs (onions, daffodils): underground storage organs consisting of fleshy leaf bases surrounding a short stem. Each bulb can divide to produce new bulbs.
• Cuttings: a piece of stem is cut and placed in moist soil or water, where it develops roots. Auxin rooting powder is often applied to stimulate root formation.

Advantages: rapid, no pollinator needed, produces genetically identical offspring (clones) that Are well-adapted to the current environment.

Disadvantages: no genetic variation (all offspring are vulnerable to the same diseases and Environmental changes), can lead to overcrowding and competition for resources.

Worked Example: Germination Experiment Analysis (OL/HL)

Question: A student sets up four test tubes, each containing 10 cress seeds on moist cotton Wool. Tube A is kept at $20\degree C$. Tube B is kept at $20\degree C$ but the cotton wool is dry. Tube C is kept at $4\degree C$. Tube D is kept at $20\degree C$ with no oxygen. After 5 days, only Tube A shows germination. Explain the results.

Answer:

• Tube A: All conditions for germination are met (water, oxygen, suitable temperature), so the seeds germinate.
• Tube B: No water is available, so enzymes cannot be activated and metabolic processes cannot proceed. No germination occurs.
• Tube C: The temperature ($4\degree C$) is too low for optimal enzyme activity. Enzymes catalysing the breakdown of stored food operate too slowly, preventing germination.
• Tube D: No oxygen is available for aerobic respiration. Without ATP from respiration, the embryo cannot carry out cell division and growth.

This experiment demonstrates that water, a suitable temperature, and oxygen are all necessary Conditions for germination.

Plant Growth

Growth Regions (OL/HL)

• Apical meristems: at the tips of roots and shoots (primary growth, increase in length). Apical meristems contain undifferentiated cells that divide rapidly by mitosis. Some daughter cells remain meristematic while others elongate and differentiate.
• Lateral meristems (cambium): in the vascular bundles (secondary growth, increase in girth). The vascular cambium produces secondary xylem (wood) towards the inside and secondary phloem towards the outside. Cork cambium produces cork (bark) on the outside, replacing the epidermis in older stems.

Plant Hormones (HL)

Plant hormones (phytohormones) are chemical messengers produced in small quantities that regulate Growth, development, and responses to stimuli. Unlike animal hormones, plant hormones can act on the Cells that produce them (autocrine) as well as on distant cells.

Hormone	Site of production	Effect
Auxin (IAA)	Apical bud, young leaves	Cell elongation, apical dominance, root initiation
Gibberellin	Young leaves, roots, embryos	Stem elongation, seed germination
Cytokinin	Root tips	Cell division, delays leaf senescence
Ethylene	Ripening fruits, ageing leaves	Fruit ripening, leaf fall (abscission)
Abscisic acid (ABA)	Leaves, root caps	Inhibits growth, closes stomata (stress response)

Apical Dominance (HL)

Auxin produced by the apical bud inhibits the growth of lateral buds. The mechanism works as Follows:

1. Auxin is synthesised in the apical bud and transported downwards through the stem.
2. High auxin concentrations in the lateral buds prevent them from growing.
3. The plant therefore grows taller rather than bushier, which is advantageous for competing for light.
4. Removing the apical bud (pruning) reduces auxin levels, allowing lateral buds to grow and producing a bushier plant.

This principle is widely used in horticulture and agriculture. For example, pinching out the growing Tips of tomato plants encourages lateral shoot growth and increases fruit yield.

Tropisms (OL/HL)

A tropism is a growth response to a directional stimulus. The response can be towards the stimulus (positive tropism) or away from it (negative tropism).

• Phototropism: response to light. Shoots grow towards light (positive phototropism); roots grow away from light (negative phototropism). Positive phototropism in shoots maximises light absorption for photosynthesis.
• Geotropism (gravitropism): response to gravity. Roots grow towards gravity (positive geotropism) to anchor the plant and access water and minerals. Shoots grow away from gravity (negative geotropism) to reach light.

Mechanism (HL): Auxin redistributes unevenly in response to the stimulus:

• In shoots: more auxin accumulates on the shaded side, causing cells to elongate more on that side, bending the shoot towards light. Auxin promotes cell elongation in shoots by activating proton pumps that loosen the cell wall (acid growth hypothesis).
• In roots: more auxin accumulates on the lower side, but unlike in shoots, high auxin concentration inhibits cell elongation in roots. The upper side elongates more, causing the root to bend downward.

:::caution A common source of confusion: auxin stimulates elongation in shoots but inhibits Elongation in roots. Roots are far more sensitive to auxin than shoots, so the same concentration That promotes growth in shoots inhibits growth in roots.

Worked Example: Explaining Phototropism (HL)

Question: A potted plant is placed near a window so that light comes from one side only. After Several days, the stem bends towards the window. Explain the mechanism responsible for this Response.

Answer: The bending is due to positive phototropism, mediated by the plant hormone auxin (IAA):

1. Light causes auxin to redistribute to the shaded side of the stem.
2. The higher concentration of auxin on the shaded side stimulates cell elongation (by activating proton pumps that lower the pH, loosening cellulose fibres in the cell wall and allowing turgor-driven expansion).
3. Cells on the shaded side elongate more than cells on the illuminated side.
4. This differential growth causes the stem to bend towards the light source.

If a light-proof cap is placed over the tip of the shoot, no bending occurs, demonstrating that the Tip is the site of auxin production and light perception.

Worked Example: Geotropism in Roots (HL)

Question: A germinating seedling is placed horizontally. After two days, the root curves Downwards and the shoot curves upwards. Explain this in terms of auxin.

Answer:

• In the root (positive geotropism): Gravity causes auxin to accumulate on the lower side of the root. In roots, high auxin concentration inhibits cell elongation. Cells on the upper side elongate more, causing the root to bend downwards.
• In the shoot (negative geotropism): Gravity causes auxin to accumulate on the lower side of the shoot. In shoots, auxin promotes cell elongation. Cells on the lower side elongate more, causing the shoot to bend upwards.

The opposite effects of auxin on roots and shoots explain why the two organs grow in opposite Directions in response to the same gravitational stimulus.

Common Pitfalls

1. Xylem vs phloem — xylem carries water and minerals upwards; phloem carries organic substances both up and down. Xylem is composed of dead cells; phloem is composed of living cells.
2. Transpiration vs translocation — transpiration is the loss of water vapour from leaves (passive, through xylem); translocation is the transport of sugars through phloem (requires energy).
3. Double fertilisation — one gamete fertilises the egg (2n zygote), the other fertilises the polar nuclei (3n endosperm). This process is unique to flowering plants.
4. Tropisms — know the direction of response for both shoots and roots for both light and gravity. Remember that shoots are positively phototropic and negatively geotropic; roots are negatively phototropic and positively geotropic.
5. Seed structure — monocots have one cotyledon with endosperm as the food store; dicots have two cotyledons that store the food.
6. Guard cells and stomata — guard cells gain and lose potassium ions ($K^+$), not just water. The movement of potassium ions drives the osmotic changes that open and close stomata.
7. Cohesion-tension vs mass flow — the cohesion-tension theory explains water movement in xylem (passive); the mass flow hypothesis explains sugar movement in phloem (requires active transport).
8. Root hair cells vs root hairs — a root hair is a single extension of one epidermal cell. Root hairs are found in the zone of maturation, not the zone of cell division.

Summary Table: Plant Hormones and Their Effects (HL)

Hormone	Promotes	Inhibits	Practical use
Auxin	Cell elongation in shoots; root initiation in cuttings	Lateral bud growth (apical dominance)	Rooting powder; weedkillers (synthetic auxins cause uncontrolled growth)
Gibberellin	Stem elongation; germination (breaks seed dormancy)	—	Spraying on dwarf varieties of crops to increase stem length; brewing industry
Cytokinin	Cell division (cytokinesis); delay leaf ageing	—	Tissue culture; extending shelf life of cut flowers
Ethylene	Fruit ripening; leaf fall	—	Bananas shipped green and ripened with ethylene gas; promoting uniform ripening
Abscisic acid	Stomatal closure during water stress	Growth; germination	Antitranspirant sprays; maintaining seed dormancy

Practice Questions

Ordinary Level

1. Describe the structure of a leaf and explain how it is adapted for photosynthesis.
2. Explain the process of transpiration and describe two factors that affect its rate.
3. Describe the structure of a seed and the conditions necessary for germination.
4. Explain the difference between self-pollination and cross-pollination.
5. Describe how water moves from the soil into the xylem of a plant root.
6. Name the four zones of a root tip and state the function of each.

Higher Level

1. Explain the cohesion-tension theory of water transport in plants.
2. Describe the mass flow hypothesis for translocation in the phloem, including evidence that supports it.
3. Explain how auxin controls phototropism in shoots.
4. Describe the process of double fertilisation in flowering plants.
5. Explain apical dominance and how pruning works at a hormonal level.
6. Explain why auxin has opposite effects on cell elongation in shoots and roots.
7. Describe the adaptations of insect-pollinated flowers and explain how each adaptation increases the chances of successful pollination.
8. A potometer is used to measure the rate of water uptake by a plant shoot. Describe how you would use a potometer to investigate the effect of wind speed on transpiration rate. State the variables that should be controlled.

Applied Plant Biology (HL)

Agriculture and Plant Hormones

Plant hormones have significant applications in agriculture and horticulture. Understanding these Applications is important for the Leaving Certificate.

Synthetic auxins (e.g., 2,4-D): used as selective weedkillers. Broad-leaved plants (dicots) are More sensitive to auxins than narrow-leaved plants (monocots like grasses and cereals). Spraying a Field of wheat with 2,4-D kills dicot weeds without harming the crop. Synthetic auxins cause Uncontrolled, unsustainable growth in susceptible plants, effectively killing them.

Ethylene and fruit ripening: Ethylene gas is used commercially to control fruit ripening. Bananas are harvested while green and unripe, shipped to reduce damage and spoilage, then exposed to Ethylene gas upon arrival to trigger uniform ripening. Removing ethylene (e.g., by ventilation or Using ethylene absorbers) delays ripening and extends shelf life.

Gibberellins in brewing: Gibberellic acid ($GA_3$) is sprayed on barley grains during the Malting process. It stimulates the production of amylase enzymes in the aleurone layer of the seed, Which breaks down starch into maltose sugar. The maltose is then fermented by yeast to produce Alcohol.

Gibberellins and dwarf varieties: Some crop varieties are dwarf due to a genetic Mutation that reduces gibberellin production. Spraying these varieties with gibberellins causes them To grow to normal height, or applying gibberellin inhibitors to normal varieties produces compact Dwarf plants that are less prone to wind damage.

Water Uptake Experiments

Using a potometer to measure transpiration rate:

A potometer measures water uptake by a cut shoot, which is assumed to approximate the transpiration Rate (since most water taken up is lost through transpiration).

Procedure:

1. Cut a leafy shoot underwater (to prevent air bubbles entering the xylem).
2. Assemble the potometer and fill it with water, ensuring no air bubbles are present.
3. Seal all joints with petroleum jelly to ensure the system is watertight.
4. Record the position of the air bubble on the capillary tube at regular intervals (e.g., every minute for 10 minutes).
5. Calculate the rate of water uptake as volume per unit time (e.g., $cm^3$/min).
6. Change one variable (e.g., use a fan for wind, a lamp for light, a plastic bag for humidity) and repeat the measurements.

Controlled variables: temperature, light intensity, humidity, plant species, shoot size, time of Day.

Limitations: The potometer measures water uptake, not transpiration directly. Some water is used In photosynthesis and other metabolic processes. Cutting the shoot may alter its normal behaviour.

Worked Example: Potometer Data Analysis (HL)

Question: A student using a potometer records the following movement of an air bubble along a Capillary tube of diameter 1 mm:

Time (min)	Distance moved (mm)
0—1	12
1—2	12
2—3	11
3—4	13
4—5	12

Calculate the mean rate of water uptake in $mm^3$/min.

Answer:

Mean distance per minute $= \dfrac{12 + 12 + 11 + 13 + 12}{5} = \dfrac{60}{5} = 12$ mm/min.

Cross-sectional area of capillary tube $= \pi r^2 = \pi \times (0.5)^2 = 0.25\pi$ $mm^2$.

Volume per minute $= 12 \times 0.25\pi = 3\pi \approx 9.42$ $mm^3$/min.

Therefore, the mean rate of water uptake is approximately 9.42 $mm^3$/min.

Plant Defence Mechanisms (HL)

Plants cannot move away from threats, so they have evolved a range of physical and chemical defence Mechanisms.

Physical defences:

• Waxy cuticle: prevents pathogen entry and reduces water loss.
• Cellulose cell wall: acts as a physical barrier to pathogens. Some plants thicken their cell walls with lignin or suberin when attacked.
• Bark: thick, dead tissue on stems and roots that provides a barrier against pathogens and herbivores.
• Thorns and spines: deter herbivores (e.g., roses, hawthorn).
• Stinging hairs: inject irritants into herbivores (e.g., nettles).

Chemical defences:

• Tannins: bitter-tasting compounds that deter herbivores. They also bind to proteins in the gut of insects, reducing the nutritional value of the plant tissue.
• Alkaloids: nitrogen-containing compounds such as nicotine, caffeine, and morphine that are toxic to herbivores and pathogens.
• Phytoalexins: antimicrobial compounds produced in response to pathogen attack. They are not normally present but are synthesised rapidly when the plant detects a pathogen.
• Canavanine: an amino acid analogue found in some legumes. When ingested by insects, it is incorporated into their proteins in place of arginine, producing defective proteins that are toxic.

Osmosis in Plant Cells

Understanding osmosis is critical for explaining water movement in plants.

Key definitions:

• Water potential ($\psi$): the tendency of water to move from one area to another. Pure water has a water potential of 0. Adding solutes lowers (makes more negative) the water potential.
• Osmosis: the net movement of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane.

Plant cells in different solutions:

Solution type	Effect on plant cell
Hypotonic (high $\psi$)	Water enters; cell becomes turgid; cell wall prevents bursting
Isotonic (equal $\psi$)	No net water movement; cell is flaccid
Hypertonic (low $\psi$)	Water leaves; cytoplasm shrinks (plasmolysis); cell becomes flaccid

Turgor pressure is essential for plant support. When cells lose turgor (e.g., during water stress), The plant wilts. This is why watering a wilted plant restores its upright posture.

Worked Example: Water Potential Calculations (HL)

Question: The root hair cell cytoplasm has a water potential of $-700$ kPa. The soil solution Has a water potential of $-100$ kPa. In which direction will water move? Explain your answer.

Answer: Water will move from the soil into the root hair cell. This is because the soil Solution has a higher (less negative) water potential ($-100$ kPa) compared to the root hair cell Cytoplasm ($-700$ kPa). Water always moves from a region of higher water potential to a region of Lower water potential by osmosis across the selectively permeable cell membrane.

Xylem and Phloem: Detailed Comparison (HL)

Feature	Xylem	Phloem
Direction of flow	One way (upwards)	Two way (source to sink)
Substance transported	Water and dissolved minerals	Organic substances (mainly sucrose)
Cell status	Dead at maturity	Living at maturity
Cell wall	Thickened with lignin	Thin, cellulose only
End walls	Absent (continuous tube)	Sieve plates with pores
Associated cells	None (vessels and tracheids)	Companion cells (metabolically active)
Mechanism	Passive (cohesion-tension)	Active (mass flow, requires ATP)
Role in support	Yes (lignin provides rigidity)	No

Common Exam Errors in Plant Biology

1. Confusing xylem and phloem direction of transport: Remember that xylem only carries water upwards, while phloem can carry sugars in either direction depending on where the source and sink are located.
2. Stating that plants “breathe” through stomata: Stomata allow gas exchange for photosynthesis and respiration, but the process is diffusion, not breathing. Use precise terminology.
3. Confusing tubers with roots: Potatoes are swollen underground stems (tubers), not roots. They have buds (eyes) and nodes, which are stem features.
4. Writing “oxygen is needed for photosynthesis”: Oxygen is a product of photosynthesis, not a reactant. Carbon dioxide and water are the reactants.
5. Stating that auxin is “destroyed” on the illuminated side: Auxin is redistributed (moves to the shaded side), not destroyed. The concentration becomes higher on the shaded side.
6. Forgetting that endosperm is triploid (3n): It results from the fusion of one male gamete with two polar nuclei, so it has three sets of chromosomes.

Worked Examples

Example 1: Comparing mitosis and meiosis

Compare mitosis and meiosis in terms of the number of divisions, daughter cells produced, and genetic variation.

Solution:

Feature	Mitosis	Meiosis
Divisions	1	2
Daughter cells	2	4
Chromosome number	Same as parent	Half of parent
Genetic variation	No	Yes (crossing over, independent assortment)
Function	Growth, repair, asexual reproduction	Production of gametes

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