Photosynthesis is the biochemical process by which green plants, algae and some bacteria synthesise organic food (mainly glucose) using light energy. It occurs primarily in chloroplasts of leaf cells where chlorophyll captures sunlight. The raw materials are carbon dioxide (from air) and water (from soil); the simplified balanced equation is: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. The end products are glucose (used for energy and growth or stored as starch) and oxygen (released to the atmosphere). Photosynthesis is vital because it forms the base of the food chain, provides oxygen for respiration, and helps remove carbon dioxide from the atmosphere, maintaining ecological balance.

Chloroplasts are double-membrane organelles found in green plant cells. Inside, they contain stacked thylakoid membranes (grana) where chlorophyll and other pigments are embedded; the stroma is the fluid surrounding grana and contains enzymes for the synthesis of organic molecules. Light-dependent reactions occur in thylakoid membranes — light energy is used to split water, releasing O₂ and generating ATP and NADPH. The Calvin cycle (light-independent reactions) occurs in the stroma, using ATP and NADPH to fix CO₂ into glucose. Thus, chloroplasts convert light energy into chemical energy stored in sugars.

Major factors include light intensity (increasing light increases the rate until a saturation point where other factors limit it), carbon dioxide concentration (more CO₂ raises the rate up to a point), temperature (optimum temperature range exists; too low or too high reduces enzyme activity), water availability (deficit reduces photosynthesis), and chlorophyll amount (less chlorophyll reduces light absorption). Limiting factor concept: at any moment, the factor in shortest supply restricts the rate of photosynthesis. For example, on a cloudy day light may be limiting; in very hot weather, temperature or water stress may limit the rate.

Take a potted plant and keep it in the dark for 48 hours to remove starch in leaves. Cover part of a leaf with black paper (exclude light) and expose the plant to sunlight for a day. Pluck the leaf, de-starch by boiling in water and treating with alcohol, then test with iodine solution. The exposed part of the leaf will turn blue-black indicating starch presence; the covered part remains pale, showing starch was not produced in absence of light. This proves light is necessary for photosynthesis.

During the light-dependent reactions in chloroplasts, water molecules are split (photolysis) to supply electrons and protons; this releases oxygen as a byproduct. The oxygen diffuses out through stomata into the atmosphere. Plants use oxygen for cellular respiration, a process that breaks down glucose to release energy needed for growth and metabolic activities, especially at night when photosynthesis ceases.

Excess glucose is converted into insoluble starch for storage, typically in leaves, stems or storage organs (roots, tubers). When needed, starch is hydrolysed back to glucose for respiration or converted into other compounds (cellulose for cell walls, sucrose for transport). Plants also use glucose as a carbon source for biosynthesis of proteins and fats.

A typical leaf comprises the cuticle (waxy layer), upper epidermis (transparent), mesophyll (palisade and spongy) and lower epidermis (with stomata). The cuticle reduces water loss. Palisade mesophyll cells are packed with chloroplasts for maximal light absorption and photosynthesis. Spongy mesophyll has air spaces facilitating diffusion of gases (CO₂ and O₂). Veins (xylem and phloem) supply water and minerals and transport food. Stomata, guarded by guard cells, regulate gas exchange and transpiration; their opening in light allows CO₂ in for photosynthesis and O₂ out, while closing prevents excessive water loss.

Guard cells control stomatal aperture by changing turgor pressure. In light, guard cells accumulate potassium ions, lowering their water potential; water enters by osmosis, making them turgid and causing stomatal opening (due to their uneven wall thickness). In darkness or water stress, potassium ions are pumped out, water leaves, guard cells become flaccid and stomata close. This mechanism balances CO₂ uptake for photosynthesis with minimising water loss, allowing plants to adapt to variable environmental conditions.

A common method is to peel the lower epidermis of a leaf (onion or tradescantia) and mount it on a slide. Under a microscope, students observe guard cells surrounding stomatal openings. They note stomata density varies between species and leaf surfaces (usually more on lower surface). Observations often include stomatal shape, presence of guard cells, and sometimes open or closed state depending on plant conditions (turgid in day, closed at night).

Aquatic plants have stomata on the upper surface to facilitate gas exchange with the air above the water. Since the lower surface is submerged and gas diffusion in water is slower, having stomata on upper surface helps obtain CO₂ and release O₂ efficiently for photosynthesis and respiration.

Sun leaves are typically thicker with more developed palisade layers and higher chloroplast density, allowing efficient light capture and higher photosynthetic rates. Shade leaves are thinner, broader and have a larger surface area relative to thickness to capture diffuse light; they may have fewer palisade layers. These structural differences optimize photosynthesis under differing light intensities.

Leaf veins contain xylem and phloem — xylem transports water and minerals to the leaf tissues, phloem carries synthesized food away. The network of veins provides mechanical support, preventing tearing and helping to maintain leaf shape. Parallel venation (monocots) and reticulate venation (dicots) reflect adaptations to different load and transport patterns.

Roots absorb water and minerals primarily through root hairs, which increase surface area. Water enters root hair cells by osmosis; mineral ions are taken up by active transport. From root hairs, water moves across cortex to xylem via apoplast (through cell walls) and symplast (through cytoplasm via plasmodesmata) pathways. Xylem vessels (dead, lignified tubes) conduct water upward to stems and leaves. The movement is driven by transpiration pull (major), cohesion and adhesion of water molecules, and root pressure (minor). Root structure — epidermis with root hairs, cortex, endodermis (with Casparian strip regulating flow), and vascular cylinder — facilitates selective uptake and transport.

Transpiration is the loss of water vapour from aerial plant parts, mainly leaves, through stomata. Advantages: creates transpiration pull aiding water and mineral transport, cools leaves by evaporation, and helps maintain turgor for structure. Disadvantages: excessive water loss can lead to wilting and reduced photosynthesis under drought conditions. Plants balance transpiration by regulating stomatal opening and structural adaptations (waxy cuticle, reduced leaves) depending on habitat.

The cohesion-tension theory states that transpiration at leaf surfaces creates a negative pressure (tension) in xylem, pulling up a continuous column of water from roots to leaves. Cohesion (hydrogen bonding) between water molecules maintains column integrity, while adhesion to xylem walls helps water climb. Tension generated by transpiration provides the driving force; cohesion and adhesion ensure unbroken flow even against gravity.

Root pressure arises when active uptake of ions by root cells into xylem lowers water potential, causing water to move into xylem by osmosis, generating a positive hydrostatic pressure that pushes water upward. It is most noticeable at night when transpiration is low, leading to exudation of sap from cut stems (guttation) in some plants. Root pressure alone cannot explain water ascent to tall trees but contributes near the roots.

A potometer measures the rate of water uptake by a leafy shoot, which is closely related to transpiration rate. The apparatus consists of a graduated capillary tube connected to a plant shoot; as the plant transpires, water is drawn from the tube, moving an air bubble along the scale. By measuring bubble movement over time, students estimate water uptake. Careful sealing and avoiding air bubbles initially are essential for accurate measurement. Potometer readings provide comparative data on factors affecting transpiration (light, humidity, wind).

Xerophytes (plants of arid regions) show adaptations: thick waxy cuticle reduces evaporation; reduced or rolled leaves minimize surface area; sunken stomata lower transpiration by creating humid microenvironments; succulent stems store water; extensive root systems increase water uptake. Examples: cactus (succulent stem, spines instead of leaves), oleander (thick cuticle), grasses with rolled leaves during drought.

Translocation is the transport of soluble organic compounds (mainly sucrose) through phloem from source (where sugars are produced or released, e.g., mature leaves) to sink (where they are used or stored, e.g., roots, fruits, growing tips). The pressure-flow hypothesis explains movement: sugars are actively loaded into phloem at source, lowering water potential; water enters by osmosis generating high turgor pressure that pushes sap toward sinks; at sinks sugars are unloaded, water exits, lowering pressure. This bidirectional system efficiently distributes energy substances to growing or storage tissues.

Phloem consists of sieve tube elements (living cells connected end-to-end with sieve plates), companion cells (metabolically active cells closely associated with sieve tubes), phloem fibres and phloem parenchyma. Companion cells maintain the sieve tubes, actively load and unload sugars, and provide ATP and metabolic support. Sieve plates have pores allowing sap flow between cells. The living nature of phloem cells is key to the active processes of translocation.

Girdling removes a ring of bark including phloem but leaves xylem intact. After girdling, sugars produced in leaves accumulate above the girdle because phloem transport is interrupted, causing swelling; below the girdle, roots become starved and may weaken, as xylem still transports water but not food. This demonstrates phloem’s role in downward translocation of organic nutrients.

Phloem transport is bidirectional because it moves substances from multiple sources to multiple sinks depending on plant needs. For example, during early spring, storage organs (roots/tubers) act as sources exporting sugars to growing buds (sinks); later, mature leaves become sources exporting sugars to roots and developing fruits (sinks).

A classic method uses radioactively labelled carbon (C-14) supplied as CO₂; after photosynthesis, autoradiography shows labelled compounds moving through phloem to sinks. At school level, ring-barking and use of dyes (e.g., placing crushed sugar beet in water and tracing with staining) or observing accumulation above a girdle can demonstrate movement qualitatively.

Respiration in plants involves enzymatic breakdown of glucose to release energy for cellular activities. Aerobic respiration uses oxygen to fully oxidise glucose into CO₂ and water, releasing more energy (ATP); it occurs continuously in plant cells (e.g., mitochondria during growth). Anaerobic respiration occurs under oxygen-limited conditions (waterlogged soils); in some organisms like yeast it produces ethanol and CO₂ (fermentation) and releases less energy. Plants may undergo anaerobic pathways in roots during flooding leading to metabolic stress.

Plants remove wastes through several means: volatile wastes (e.g., O₂) diffuse through stomata; some wastes are stored in vacuoles or tissues (e.g., tannins) and later shed as leaves or bark; excess water is lost by transpiration or guttation; secretion through resin ducts or exudation can eliminate secondary metabolites. Seasonal leaf fall is an adaptation to discard accumulated wastes. Unlike animals, plants lack specialized excretory organs but use structural and physiological routes for disposal.

As in earlier experiment, de-starch a plant by keeping it in dark, then expose a leaf to sunlight; remove the leaf, boil in water to kill and soften tissue, soak in alcohol to remove chlorophyll (decolorise), rinse and treat with iodine solution. Blue-black colour indicates starch. If part of the leaf was covered during exposure, that part will not show blue-black colour, confirming photosynthesis (starch production) requires light. This also demonstrates sites of storage and photosynthetic activity.

Insectivorous plants (e.g., Venus flytrap, pitcher plants) trap and digest insects to supplement nitrogen and mineral nutrients deficient in their native, nutrient-poor soils (bogs). They possess specialised leaves forming traps with digestive enzymes or symbiotic bacteria to break down prey. These adaptations allow survival and growth in habitats where soil nutrients are inadequate for normal uptake.

Photosynthesis is the primary source of organic matter and energy for ecosystems, forming food webs and enabling biomass accumulation. It also sequesters CO₂, moderating atmospheric greenhouse gas levels. Transpiration contributes to the water cycle by releasing water vapour, influencing local climate and humidity; collectively, vegetation transpiration affects rainfall patterns and microclimates. Thus, plant processes play crucial roles in biogeochemical cycles and ecological stability.

Agriculture benefits from plant life-process knowledge: (1) Irrigation scheduling uses transpiration and water uptake principles to supply water efficiently. (2) Fertiliser application is informed by translocation and nutrient uptake to target growth stages. (3) Crop spacing and pruning optimise light interception and photosynthesis. Understanding plant physiology improves yield, resource use efficiency and stress management.

Leaves are structurally adapted for efficient photosynthesis and gas exchange: a thin, broad lamina provides large surface area for light capture; cuticle minimises water loss; upper epidermis is transparent for light penetration; palisade mesophyll has numerous chloroplasts for high photosynthetic activity; spongy mesophyll’s air spaces facilitate gas diffusion; veins with xylem and phloem supply water/minerals and distribute synthesized food; stomata (with guard cells) regulate gas exchange and water loss. Examples: palisade adaptation for light capture, stomatal control balancing CO₂ uptake with transpiration, and vein network supporting transport and mechanical strength. These structure–function relationships ensure leaves perform photosynthesis efficiently while managing water balance and nutrient distribution.