🌿 Plant Biology

Plant Kingdom

Multicellular, mostly photosynthetic eukaryotes with cellulose walls, from mosses to flowering plants.

Photosynthesis

Plants use light, water and carbon dioxide to make glucose and oxygen.

Chlorophyll

The green pigment in chloroplasts that absorbs mainly red and blue light.

Roots

Organs that anchor the plant and absorb water and minerals from the soil.

Stems

Organs that support leaves and flowers and carry water and sugars through the plant.

Leaves

Flat, thin organs adapted to capture light and exchange gases for photosynthesis.

Flowers

Reproductive structures of angiosperms containing sepals, petals, stamens and carpels.

Seeds

Protective packages containing an embryo and food store that can grow into a new plant.

Germination

The sprouting of a seed into a seedling when water, oxygen and suitable temperature are available.

Pollination

Transfer of pollen from anther to stigma by wind, insects or other agents.

Fruits

Mature ovaries that protect seeds and help disperse them.

Seed Dispersal

Spreading of seeds by wind, water, animals or explosive release to reduce competition.

Stomata

Pores on leaves flanked by guard cells that open and close to regulate gas exchange and water loss.

Xylem

Vascular tissue made of dead cells that carries water and minerals upward from roots.

Phloem

Vascular tissue made of living cells that transports sugars from sources to sinks.

Intermediate topics (18)

Transpiration

Evaporation of water from leaves that pulls water up the plant through the xylem.

Cohesion-Tension Theory

Water molecules stick together and to xylem walls, so evaporation tension pulls a continuous water column up.

Translocation

The pressure-flow movement of sucrose through phloem from sources to sinks.

Light-Dependent Reactions

In the thylakoid membranes, light splits water, releasing oxygen and making ATP and NADPH.

Calvin Cycle

Light-independent reactions in the stroma that use ATP and NADPH to fix carbon dioxide into sugar.

Rubisco

The enzyme that fixes carbon dioxide in the Calvin cycle and is likely the most abundant protein on Earth.

Photosynthetic Pigments

Chlorophylls, carotenoids and xanthophylls absorb different wavelengths and widen the usable light spectrum.

Limiting Factors of Photosynthesis

Light intensity, carbon dioxide concentration and temperature can each limit the rate at any time.

Plant Hormones

Auxin, gibberellin, cytokinin, abscisic acid and ethylene regulate growth, development and responses.

Auxin and Phototropism

Auxin moves to the shaded side of a shoot, causing cells there to elongate so it bends toward light.

Gravitropism

Directional growth in response to gravity, with roots growing down and shoots growing up.

Photoperiodism

Responses such as flowering triggered by the length of day and night, mediated by phytochrome.

Plant Tissues

Meristematic, dermal, ground and vascular tissues organise the plant body.

Primary and Secondary Growth

Primary growth lengthens shoots and roots; secondary growth thickens stems into wood.

Monocots vs Dicots

Monocots have one cotyledon, parallel leaf veins and scattered bundles; dicots have two, netted veins and ringed bundles.

Alternation of Generations

The plant life cycle alternates between a haploid gametophyte and a diploid sporophyte.

Double Fertilisation

In angiosperms one sperm fertilises the egg and the other forms the triploid endosperm.

Plant Nutrition and Mineral Deficiency

Nitrogen, phosphorus and potassium are key; deficiencies cause stunted growth or yellow leaves.

Advanced topics (23)

C4 Photosynthesis

A pathway that concentrates CO2 in bundle sheath cells to reduce photorespiration in hot climates, as in maize.

CAM Photosynthesis

Desert plants open stomata at night to fix CO2 into organic acids for daytime use.

Photorespiration

Rubisco binds oxygen instead of carbon dioxide, wasting energy and releasing CO2.

Z-Scheme of Electron Transport

Electrons flow from photosystem II to photosystem I through carriers, pumping protons for ATP synthesis.

Cyclic Photophosphorylation

Electrons cycle around photosystem I to make extra ATP without producing NADPH or oxygen.

Plant Defence Responses

Plants use thorns, toxins, jasmonate signalling and systemic acquired resistance against herbivores and pathogens.

Plant Immunity Receptors

Surface and intracellular NLR receptors detect pathogen molecules and trigger the hypersensitive response.

Phytochrome Signalling

Red/far-red photoreceptors toggle between forms to control germination, shade avoidance and flowering.

Plant Cell Wall Biosynthesis

Cellulose synthase complexes at the membrane assemble microfibrils, reinforced by hemicellulose and lignin.

Abscisic Acid and Drought

ABA triggers guard cell closure and gene responses that help plants survive water stress.

Plant Genetic Engineering

Agrobacterium and gene guns introduce genes to create crops resistant to pests or tolerant to herbicides.

Plant Tissue Culture

Growing whole plants from small explants on sterile media with defined hormones, enabling cloning.

Root Nodules and Symbiosis

Legume roots form nodules housing nitrogen-fixing rhizobia after exchanging signalling molecules.

Plant Phylogeny

Mosses, ferns, gymnosperms and angiosperms form lineages tracing the conquest of land.

Floral Development ABC Model

Combinations of A, B and C class genes specify sepals, petals, stamens and carpels.

Vernalisation

Exposure to prolonged cold that makes some plants competent to flower later.

Mycorrhizal Networks

Fungal hyphae link plant roots, transferring nutrients and signals between individuals.

Plant Secondary Metabolites

Alkaloids, terpenoids and phenolics mediate defence and are the source of many drugs.

Apical Dominance

The terminal bud suppresses lateral buds through auxin, shaping plant architecture.

Plant Water Potential

Combines solute and pressure potential to predict water movement into and between plant cells.

Algal Blooms and Plant Evolution

Charophyte green algae are closest relatives of land plants, sharing key cell division and wall traits.

Crop Domestication

Selection for traits like non-shattering seeds transformed wild grasses into wheat, rice and maize.

Photosynthesis Efficiency Engineering

Strategies include improving rubisco, adding carbon concentrating mechanisms and optimising canopy light use.