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Palisade cells contain many chloroplasts, while spongy mesophyll has air spaces that shorten gas diffusion paths. Stomata are pores controlled by guard cells, allowing gas exchange and influencing water loss. The waxy cuticle reduces evaporation from surfaces. These adaptations balance carbon dioxide uptake for photosynthesis with the need to retain water; closing stomata can reduce both water loss and carbon dioxide supply.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Relate leaf and vascular structures to function
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which tissue transports sugars?
Phloem. This movement is called translocation.
After watching 2: Which condition usually reduces transpiration?
Higher humidity. A smaller vapour gradient reduces diffusion.
What you'll be able to do
Palisade cells contain many chloroplasts, while spongy mesophyll has air spaces that shorten gas diffusion paths. Stomata are pores controlled by guard cells, allowing gas exchange and influencing water loss. The waxy cuticle reduces evaporation from surfaces. These adaptations balance carbon dioxide uptake for photosynthesis with the need to retain water; closing stomata can reduce both water loss and carbon dioxide supply.
Water enters root cells by osmosis and moves into xylem. Evaporation from leaf surfaces and diffusion out through stomata create the transpiration stream. Xylem vessels are hollow and reinforced with lignin, supporting transport and the plant. Higher temperature, greater air movement and lower humidity often increase transpiration, while light can affect stomatal opening. A potometer estimates water uptake, which is a useful proxy but is not exactly the same as total water loss.
Phloem transports dissolved sugars from sources to sinks through translocation. A source exports sugar, such as a photosynthesising leaf; a sink uses or stores it, such as a growing root or fruit. Phloem can transport in different directions in different tubes, unlike a simplified always-upwards model. Minerals enter roots through mechanisms that may require active transport. Keep sugar transport distinct from the water-and-mineral stream in xylem.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: What limitation should be stated for a potometer?
Water uptake approximates, but is not identical to, transpiration.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.