AQA GCSE Biology 4.2.3 Plant Tissues, Organs and Systems Exam Questions

About AQA GCSE Biology 4.2.3 Plant Tissues, Organs and Systems

This section completes the AQA Organisation topic on Paper 1. It explains how the tissues of a leaf are adapted for photosynthesis and gas exchange, and how roots, stems and leaves form a plant organ system for transporting water, mineral ions and sugars. It links to osmosis and active transport in 4.1.3 and to photosynthesis in 4.4.1. Examiners often ask you to label leaf cross-sections, explain how temperature, humidity, air movement and light affect transpiration, compare xylem and phloem, and calculate rates of water uptake from potometer data.

What you need to know

• 4.2.3.1 Plant tissues Leaf tissues include epidermal tissue (covered by a waxy cuticle), palisade mesophyll (many chloroplasts for photosynthesis), spongy mesophyll (air spaces for diffusion of gases), xylem and phloem, and meristem tissue at growing tips.
• Stomata and guard cells Stomata are mostly on the lower surface of the leaf. Guard cells open and close them to control gas exchange and water loss.
• Xylem Xylem tissue transports water and mineral ions from the roots to the stems and leaves. It is made of hollow tubes of dead cells strengthened by lignin.
• Phloem and translocation Phloem is made of elongated cells with pores in the end walls (sieve plates) that let cell sap move from one cell to the next. Translocation is the movement of dissolved sugars from the leaves to the rest of the plant for immediate use or storage.
• Transpiration Transpiration is the loss of water vapour from the leaves through stomata, which draws water up the xylem in the transpiration stream. The rate increases with higher temperature, lower humidity, more air movement and higher light intensity (more stomata open).
• Measuring transpiration A potometer measures water uptake by a shoot, which is used as an estimate of transpiration. Rate = distance moved by the bubble (or volume) / time. Be able to process data and calculate the mean rate.

Common mistakes in 4.2.3 Plant Tissues, Organs and Systems

• Saying xylem carries sugars or phloem carries water upwards only. Xylem carries water and mineral ions up; phloem carries dissolved sugars both up and down.
• Saying transpiration is the movement of water through the plant. Strictly it is the evaporation and loss of water vapour from the leaves, which causes the transpiration stream.
• Saying humid air increases transpiration. High humidity reduces the concentration gradient of water vapour, so transpiration slows.
• Saying a potometer measures transpiration directly. It measures water uptake, some of which is used in photosynthesis or kept in cells.
• Saying xylem cells are living. Xylem vessels are made of dead cells with no end walls.

Try three 4.2.3 Plant Tissues, Organs and Systems exam-style questions