3.3 ocr alevel bio

Why do plants need exchange surfaces?

• small SA:V ratio → need specialised surfaces - take in CO₂ for photosynthesis, release O₂ - take in O₂ for respiration, release CO₂ - need water and mineral ions from soil

How are leaves adapted for gas exchange?

• thin (short diffusion path) - large surface area - air spaces in spongy mesophyll (gas diffusion) - stomata (open/close for gas exchange)

Role of stomata

Control gas exchange and water loss

When are stomata usually open?

Daytime (light triggers opening for CO₂ uptake for photosynthesis)

When are stomata usually closed?

Night (no photosynthesis, conserve water)

What controls opening and closing of stomata?

Guard cells → change turgor pressure

How do guard cells open stomata?

• light triggers active transport of K⁺ ions into guard cells - lowers water potential - water enters by osmosis - guard cells become turgid, bend apart → pore opens

How do guard cells close stomata?

• ions leave guard cells → water potential rises - water leaves by osmosis - cells become flaccid → pore closes

What is transpiration?

Loss of water vapour from aerial parts of plant (mainly through stomata)

What is the compromise of stomatal opening?

Plant needs CO₂ for photosynthesis but loses water → balance between photosynthesis and water conservation

What are xerophytes?

Plants adapted to dry environments (e.g. cacti, marram grass)

Xerophytic adaptations to reduce water loss

• thick waxy cuticle (reduces evaporation) - sunken stomata (trap moist air) - reduced stomata density - hairy leaves (trap moist air) - curled leaves (reduce SA exposed) - succulent tissues (store water) - CAM metabolism (open stomata at night)

What are hydrophytes?

Plants adapted to living in water (e.g. water lilies)

Hydrophytic adaptations

• stomata only on upper surface (exposed to air) - large air spaces (buoyancy, store O₂) - reduced roots (water uptake not needed) - thin or no cuticle - flexible stems/leaves (water support)

What is translocation?

Movement of assimilates (e.g. sucrose, amino acids) in phloem from source to sink

Source in a plant

Where assimilates are produced (e.g. leaves in photosynthesis)

Sink in a plant

Where assimilates are used or stored (e.g. roots, fruits, meristems)