Plant Tissues, Organs and Systems

OCR

GCSE

Multicellular plants are organized into specialized tissues, organs, and systems to facilitate survival, primarily through the transport of substances and photosynthesis. Key tissues including epidermal, palisade mesophyll, spongy mesophyll, xylem, and phloem exhibit specific structural adaptations for gas exchange, water uptake, and sugar transport. The transpiration stream moves water and mineral ions via physical forces driven by evaporation, whereas translocation actively moves dissolved sugars from sources to sinks, requiring energy from respiration.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating that xylem vessels transport water and mineral ions, while phloem tissue transports dissolved sugars (sucrose)
Credit responses that explain transpiration as the evaporation of water from cell surfaces followed by diffusion out through open stomata
Award 1 mark for linking increased light intensity to wider stomatal opening, resulting in a higher rate of transpiration
Candidates must describe translocation as the movement of sugars from 'source' (e.g., leaves) to 'sink' (e.g., roots/storage organs)
Award 1 mark for identifying that the movement of water in the xylem is driven by the transpiration pull and cohesion between water molecules

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You have correctly identified the tissue, but you need to specify *what* it transports to get the mark"
"Be careful with your terminology: water evaporates from cells, then diffuses out of the leaf — ensure you have the order right"
"You mentioned the potometer measures water loss. Remember to clarify that it actually measures uptake, which we *assume* equals loss"
"Excellent use of the term 'translocation'. To improve, specify that this requires energy (active transport) unlike transpiration"

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating that xylem vessels transport water and mineral ions, while phloem tissue transports dissolved sugars (sucrose)
Credit responses that explain transpiration as the evaporation of water from cell surfaces followed by diffusion out through open stomata
Award 1 mark for linking increased light intensity to wider stomatal opening, resulting in a higher rate of transpiration
Candidates must describe translocation as the movement of sugars from 'source' (e.g., leaves) to 'sink' (e.g., roots/storage organs)
Award 1 mark for identifying that the movement of water in the xylem is driven by the transpiration pull and cohesion between water molecules

Examiner Tips

Expert advice for maximising your marks

💡When describing the transpiration stream, explicitly use the term 'cohesion' regarding water molecules; this is frequently required for full marks in OCR explanations
💡Distinguish clearly between 'transpiration' (water loss) and 'translocation' (sugar movement) — mixing these terms is a major cause of lost marks
💡In potometer questions involving calculations, check units carefully (e.g., converting minutes to hours) before calculating the rate
💡For 'Suggest' questions regarding potometers, focus on environmental control (e.g., 'maintain constant temperature') to ensure validity

Common Mistakes

Pitfalls to avoid in your exam answers

Stating that water moves up the xylem by osmosis (incorrect; it is driven by physical transpiration pull and cohesion)
Confusing the direction of flow: failing to recall that xylem flow is one-way (upwards) while phloem flow is bidirectional
Believing a potometer measures water loss directly; it strictly measures water uptake, which is assumed to equal loss
Describing the cuticle as a cell layer rather than a waxy, non-cellular coating

Key Terminology

Essential terms to know

Structural adaptations of leaf tissues (epidermal, mesophyll, stomata)

Xylem structure and the mechanism of the transpiration stream

Phloem structure and the mechanism of translocation

Factors affecting the rate of transpiration (temperature, humidity, air movement, light intensity)

Meristematic tissue and cell differentiation

Likely Command Words

How questions on this topic are typically asked

State

Describe

Explain

Calculate

Suggest

Compare

Practical Links

Related required practicals

•{"code":"PAG 6","title":"Investigating the rate of transpiration using a potometer","relevance":"Direct application of transport principles and environmental variables"}
•{"code":"Microscopy","title":"Observation of stomata and guard cells","relevance":"Visualizing the structures controlling gas exchange and water loss"}

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