Plant Biology | Leaving Cert - Wyatt's Notes

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.

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.

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).

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.

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).

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.

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.

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.

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.

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.

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.

• 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).

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:

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.

• 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 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:

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

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

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.

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.

• 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 (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.

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.

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.

Biology is the study of living systems at every scale, from molecules to ecosystems. Cells are the fundamental units of life, each one a factory of chemical reactions powered by energy from food or sunlight. Understanding how structures relate to functions, like how the shape of an enzyme determines what it can do, helps you see why organisms work the way they do. The key insight is that all life shares common molecular machinery, from DNA to ATP, which is why studying one organism often reveals principles that apply across all life.

• 1 Cell
• 2 Ecology
• 1 Atomic Structure
• 2 Bonding