Plant organ system — AQA GCSE Biology

Transpiration is the loss of water vapour from leaves, mainly through stomata. The rate depends on water vapour diffusing out, driven by the concentration gradient between the leaf's interior and the surrounding air. Temperature raises the kinetic energy of water molecules, increasing evaporation and steepening the gradient. Humidity reduces the gradient because moist external air slows diffusion. Air movement removes humid air from around the leaf, maintaining a steep gradient and increasing the rate. Light intensity stimulates stomatal opening for photosynthesis, so more stomata are open and the rate increases. A useful method is to compare water uptake of a leafy shoot using a potometer, or mass loss using a balance.

A compound measure combines two different quantities, for example distance per unit time or volume per unit time. The rate of transpiration is a compound measure because it compares an amount of water lost with the time taken. It can be expressed as volume of water lost per minute, for example cm³ min⁻¹, or as mass lost per minute, for example g min⁻¹, or as distance moved by an air bubble per minute, for example mm min⁻¹. To calculate it, divide the change in water lost or bubble distance by the time taken. To compare conditions fairly, use the same time interval or calculate a rate for each condition. A potometer measures water uptake, which is a close approximation of transpiration under the experimental conditions.

This skill underpins practical work on plant transport. Suppose a student measures the mass of a leafy shoot on a balance every 5 minutes as it transpires. They record a table of time in minutes and mass in grams, then translate that numerical data into a line graph: time on the x-axis, mass on the y-axis. Choosing a scale means making each axis span the data range with equal, labelled intervals, so the plotted points are spread out and easy to read. Extracting information means reading a value from the graph, such as the mass at 12 minutes, by interpolating between plotted points. Interpreting means explaining the trend, for example that mass falls fastest in warm, moving air because transpiration removes water vapour quickly.

A plant organ system is a group of organs working together to transport substances. Roots absorb water and mineral ions from the soil. The stem contains xylem and phloem and holds the leaves in a position to absorb light. Leaves absorb light and carbon dioxide and carry out photosynthesis, producing sugars that are transported to other parts of the plant. Xylem transports water and dissolved mineral ions from the roots upwards to the stem and leaves. Phloem transports dissolved sugars and other products of photosynthesis from the leaves to growing regions and storage organs. Together these organs form a continuous transport system that connects sources and sinks throughout the plant.

Transpiration is the loss of water vapour from the leaves and stems of a plant, mainly through stomata. Water evaporates from the surfaces of cells inside the leaf, diffuses through the air spaces and exits through stomata. This loss creates a pull that draws water up the xylem from the roots. Stomata are small pores, mostly on the lower surface of a leaf, each surrounded by two guard cells. When guard cells take in water they become turgid and the pore opens; when they lose water they become flaccid and the pore closes. This controls gas exchange and water loss. Translocation is the movement of dissolved sugars and other substances through the phloem from sources, such as leaves, to sinks, such as growing regions and storage organs.

Root hair cells are specialised cells on the surface of roots. Each has a long, thin extension that increases the surface area for absorption. Water enters by osmosis because the water potential inside the root hair cell is lower than in the soil solution, so water moves across the partially permeable membrane from a dilute solution to a more concentrated solution. Mineral ions are absorbed by active transport, which uses energy from respiration to move ions against their concentration gradient, from a lower concentration in the soil to a higher concentration inside the cell. The large number of mitochondria in root hair cells supplies the energy needed for active transport.

Xylem is a transport tissue forming a continuous pipeline from root tips through stems to leaves. Root hair cells absorb water by osmosis and mineral ions by active transport, so the sap entering xylem is a dilute solution of ions such as nitrate, phosphate, magnesium and potassium. Once inside, water and dissolved ions move upwards as a column through hollow dead cells. This flow supplies photosynthesis in leaves with water and supplies growing regions with ions for making proteins, chlorophyll and other molecules. A useful method is tracing a labelled pathway: soil → root hair cell → root cortex → xylem → stem → leaf. Transpiration pull, driven by evaporation from leaves, draws the column upwards, while root pressure also contributes. Understanding this pathway explains why wilting, mineral deficiency and ringing experiments affect shoots.

Xylem tissue is built from dead, elongated cells joined end to end to form hollow tubes. Because the end walls break down, there is an uninterrupted lumen through which water and dissolved ions travel. The cell walls are thickened with lignin, a tough waterproof polymer that prevents collapse under the tension created when water is pulled upwards. Lignin also makes the walls impermeable, so water stays inside the lumen rather than leaking sideways. Pits in the walls allow some lateral movement between neighbouring xylem vessels. These features are adaptations: hollow tubes reduce resistance to flow, lignin gives strength, and dead cells remove cytoplasmic obstacles. In the transpiration stream, evaporation from leaves creates tension that pulls the water column up the stem. This cohesion-tension mechanism depends on the continuous, reinforced plumbing that xylem provides.

Stomata are microscopic pores found mainly in the lower epidermis of leaves. Each pore is bordered by two guard cells. When guard cells take in water by osmosis they become turgid and curved, opening the pore; when they lose water they become flaccid and the pore closes. Open stomata allow carbon dioxide to diffuse in for photosynthesis and oxygen to diffuse out as a by-product. At the same time, water vapour diffuses out, contributing to transpiration and water loss. Guard cells therefore balance the need for gas exchange against the risk of dehydration. Environmental factors such as light, temperature, humidity and water availability influence stomatal opening. A practical method is using a potometer to estimate water uptake, or observing stomata under a microscope after peeling the lower epidermis.

Carrot roots store mainly sucrose, so use potato tubers or seeds for starch storage examples. Phloem is the plant tissue that carries dissolved sugars, mainly sucrose, made in photosynthesis. Leaves are the source where sugar is loaded into phloem; growing regions, roots, fruits, and storage organs are sinks where sugar is unloaded. Transport in phloem is called translocation and occurs in different directions depending on need. Phloem tissue contains sieve tube elements joined end to end with perforated sieve plates, and companion cells that support the sieve tubes with energy. Sugars are used immediately for respiration and growth, or converted to starch and stored in organs such as potato tubers and seeds.

Translocation is the transport of dissolved food molecules, mainly sucrose and amino acids, through phloem tissue from sources to sinks. Sources are regions that produce or release sugars, such as mature leaves carrying out photosynthesis. Sinks are regions that use or store them, such as growing tips, roots, fruits and storage organs. The food molecules are dissolved in cell sap and travel through living phloem cells, so translocation requires energy from respiration. This distinguishes it from transpiration, which is the loss of water vapour and the movement of water and mineral ions through xylem. A useful method is to trace a sugar made in a leaf to a growing bud or root, naming the source, the phloem pathway and the sink.

Phloem tissue forms tubes made of elongated living cells joined end to end. The end walls between neighbouring phloem cells contain pores, so cell sap can pass from one cell to the next along the tube. This continuous pathway allows dissolved food molecules to be translocated from sources to sinks. The cells remain alive, which is essential because loading and unloading of sugars depends on energy from respiration. A useful method is to imagine a line of elongated cells with perforated end walls: sap enters at a source, flows through the pores, and leaves at a sink. Comparing this with xylem, whose cells are dead and hollow, helps clarify why phloem is described as living transport tissue.