Plant organ system — AQA GCSE Combined Science

Transpiration is the loss of water vapour from leaf surfaces, mainly through stomata. Water evaporates from moist cell walls into air spaces and diffuses out when the stomata are open. The rate depends on the diffusion gradient between the humid air inside the leaf and the drier air outside. Raising temperature increases kinetic energy and evaporation, so the gradient steepens and the rate rises. High humidity reduces the gradient, slowing diffusion. Air movement sweeps away humid air, maintaining a steep gradient and increasing the rate. Light intensity controls stomatal opening: in most plants stomata open in bright light, increasing the rate, while darkness closes them and reduces it. Explaining means linking each change to the gradient or to stomatal aperture, not merely naming a direction.

A compound measure combines two different quantities, so its unit is a combination of two units. Rate of transpiration is a compound measure because it compares a volume of water lost with a time taken. It is calculated as rate = volume of water lost ÷ time taken, giving units such as cm³ per minute or cm³ min⁻¹. A potometer can estimate water uptake by a leafy shoot, and the distance moved by an air bubble along a capillary tube can be converted into a volume using the tube's cross-sectional area. Students should choose the correct values, divide volume by time, and interpret the result: a larger value means a faster rate. They should also be able to compare rates, convert units such as minutes to seconds, and use a rate to find a volume or a time.

This skill means moving confidently between a graph and the numbers it represents. In the plant organ system topic you might be given a graph of transpiration rate against light intensity, temperature, humidity or wind speed, and asked to read values, calculate a rate, or plot points from a table. To translate graphical to numerical form, read the axes and units, locate the required point, and state the value with its unit. To translate numerical to graphical form, choose a sensible scale, plot each pair of values accurately, and draw a line or curve of best fit. You also need to describe patterns: for example, as light intensity rises, transpiration rate rises until stomata are fully open, then levels off. Accuracy in reading scales and labelling axes is assessed through practical and data-handling questions.

Graph plotting is a core practical skill tested across biology, chemistry and physics. You must choose a sensible scale for each axis, label both axes with quantity and unit, and plot points accurately before drawing a line or curve of best fit. For example, in a plant transport investigation measuring water uptake at different light intensities, put light intensity in lux on the x-axis and water uptake in cm³ on the y-axis. Choose a scale so the data fills most of the grid, using equal steps such as 1, 2, 5 or 10 units per major square. Plot each point with a small cross, then draw a smooth line or curve that follows the trend rather than joining every point dot-to-dot. Assessed through practical questions and required practicals where you interpret and present data.

Interpreting data means reading values from graphs, charts and tables and explaining what they show about a biological process. In the plant organ system topic, you might read a graph of transpiration rate against temperature or a table of stomatal density on upper and lower leaf surfaces. To extract information, locate the correct row or column, read across or down carefully, and quote values with units. To interpret, describe the trend, compare sets of data, and link the pattern to scientific ideas such as evaporation, diffusion or water loss. For example, a graph showing transpiration rising as temperature increases can be explained by faster evaporation and diffusion of water vapour. Assessed through structured questions requiring values, trends and explanations.

A plant is not just a collection of separate parts: its roots, stem and leaves work together as an organ system that moves substances from where they are taken in or made to where they are needed. Roots absorb water and mineral ions from the soil. The stem contains vascular tissue and holds the leaves up, providing a transport route between roots and leaves. Leaves absorb light and carbon dioxide and make glucose by photosynthesis. Water and dissolved mineral ions travel up from the roots through the stem to the leaves, while sugars made in the leaves are transported to growing regions, storage organs and roots. This coordination allows the whole plant to survive, grow and reproduce.

Transpiration is the loss of water vapour from plant leaves, mainly through stomata. Water evaporates from the surfaces of mesophyll cells, diffuses through air spaces and exits through stomatal pores. This creates a pull that draws water up the xylem from the roots in a continuous column. Stomata are tiny pores, each surrounded by two guard cells; they open to allow carbon dioxide in for photosynthesis and close to reduce water loss. Translocation is the movement of dissolved sugars and other organic substances through the phloem from sources such as leaves to sinks such as roots, fruits and growing tips. It requires energy and can occur in either direction.

Root hair cells are epidermal cells on young roots. Each has a long, thin extension that pushes between soil particles, greatly increasing the surface area in contact with soil water. This large surface area speeds up diffusion and osmosis, so water moves efficiently from the soil solution into the cell. Water enters by osmosis because the soil solution is usually more dilute than the cell contents, so water moves down a water potential gradient across the partially permeable membrane. Mineral ions are often at a lower concentration in the soil than inside the cell, so they cannot enter by diffusion alone. The cell uses energy from respiration to move them in by active transport, against the concentration gradient, using carrier proteins in the membrane. Root hair cells therefore contain many mitochondria to release the energy needed.

Xylem is a plant transport tissue that carries water and dissolved mineral ions upwards from the roots through the stem to the leaves. Xylem cells are dead at maturity and form hollow tubes, so there is an uninterrupted column of water with little resistance to flow. Their walls are strengthened with lignin, a rigid material that prevents the tubes collapsing under tension and helps support the plant. Water moves through xylem in the transpiration stream. Transpiration is the loss of water vapour from leaves, mainly through stomata. As water evaporates from leaf cells, water is pulled up the xylem to replace it, and this pull, together with cohesion between water molecules, maintains a continuous column. Mineral ions dissolved in the water are carried along with it.

Leaves are thin and flat to give a large surface area for diffusion, but that same surface lets water evaporate. Stomata are microscopic pores, mostly in the lower epidermis, each bordered by two guard cells. When guard cells take in water they swell and become turgid, bending apart so the pore opens; 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, while water vapour also escapes by transpiration. Closing them reduces water loss but also restricts gas exchange, so the plant balances photosynthesis against dehydration, for example shutting stomata in hot, dry, windy conditions.

Phloem is a transport tissue made of living cells, including sieve tubes and companion cells, that carries dissolved sugars such as sucrose. Sugars are made by photosynthesis in the leaves, the source, and are loaded into the phloem. They are then moved to sinks, which are parts of the plant that use or store them, such as growing shoots, roots, fruits and storage organs. This movement of food molecules through phloem is called translocation. Sugars may be used immediately in respiration or growth, or converted to starch for storage. Unlike xylem, which carries water and mineral ions upwards in dead hollow cells, phloem transport is bidirectional and requires living cells.

Phloem is a transport tissue in plants. It is made of tubes formed from elongated cells joined end to end, running through stems, roots and leaves. The end walls between neighbouring phloem cells are not solid; they contain pores, so they are perforated. These pores allow cell sap, a sugar-rich fluid, to move from one phloem cell to the next along the tube. This movement transports dissolved sugars and other solutes around the plant, for example from a leaf where photosynthesis makes sugars to a growing bud or storage organ. In the required practical context, you may observe phloem in stained transverse sections of a stem, where it appears as small cells near the outside. You do not need to name every cell type or explain the transport mechanism in detail.