IB Bio Leaf Structure and Water Movement
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57 Terms
What are the two main functions of leaves?
Photosynthesis and transpiration
What is the purpose of the upper cuticle (waxy layer)?
Prevents water loss
What type of cells make up the upper epidermis?
Tightly packed transparent cells
Why are upper epidermal cells transparent?
To allow light to pass through to mesophyll
What tissue performs most photosynthesis?
Palisade mesophyll
Why are palisade mesophyll cells elongated?
To increase surface area for photosynthesis
What tissue allows gas exchange within the leaf?
Spongy mesophyll
Why are spongy mesophyll cells loosely packed?
To allow diffusion of gases
What structure allows carbon dioxide to enter the leaf?
Stomata
What tissue transports water in plants?
Xylem
What tissue transports sugars in plants?
Phloem
What are vascular bundles made of?
Xylem and phloem
What type of cells make up xylem?
Dead cells with thick lignified walls
Why are xylem cells hollow?
To allow efficient water transport
What are phloem cells specialized for?
Transport of sugars
What is the transpiration stream?
The movement of water from roots to leaves
What are the two different pathways for water transport?
Symplast and apoplast pathways
What is the apoplast pathway?
Water moves through cell walls and intercellular spaces
What happens at the endodermis in the apoplast pathway?
Water is forced into the symplast due to the Casparian strip
What is the symplast pathway?
Water moves through cytoplasm via plasmodesmata
What structure connects plant cells in the symplast pathway?
Plasmodesmata
What type of transport does the symplast pathway involve?
Passive transport
What is osmosis?
The movement of water from high to low water potential across a semi-permeable membrane
Why does water enter root hair cells by osmosis?
The soil has a higher water potential than the cell
What is active transport?
Movement of substances using energy
Why is active transport important in roots?
To absorb mineral ions against a concentration gradient
How does mineral uptake affect water movement?
It lowers water potential inside roots, increasing water entry
What are root hairs?
Extensions of epidermal cells
What is the function of root hairs?
Increase surface area for water and mineral absorption
Why do roots have a lower water potential than soil?
They actively absorb minerals
What is the root pressure?
The upward push of water caused by osmotic pressure in roots
Where does root pressure originate?
From the root apical meristem
What type of cells divide rapidly at the root tip?
Meristematic cells
What is cohesion?
The attraction between water molecules
Why is cohesion important for water transport?
It keeps the water column intact
What type of bond causes cohesion?
Hydrogen bonds
What is adhesion?
The attraction between water molecules and xylem walls
How does adhesion help water movement?
It helps water climb against gravity
What theory explains water transport in plants?
Cohesion-tension theory
What creates tension in xylem?
Transpiration from leaves
What is transpiration?
The loss of water vapor from leaves
Through which structure does transpiration mainly occur?
Stomata
What drives transpiration?
Evaporation of water from leaf surfaces
How does transpiration affect pressure in xylem?
It creates negative pressure (tension)
What is the role of the Casparian strip?
Forces water into the symplast for selective absorption
What is the Casparian strip made of?
Suberin
What is the function of stomata?
Regulate gas exchange and water loss
What cells control stomatal opening?
Guard cells
How do guard cells open stomata?
By becoming turgid
What happens to guard cells when they lose water?
They become flaccid and close stomata
What factors increase transpiration rate?
High temperature, wind, low humidity, high light
What factors decrease transpiration rate?
Low temperature, high humidity, low light
Why do plants close stomata during drought?
To reduce water loss
What is the main force pulling water up from roots to leaves?
Transpiration pull
Why does water move upward against gravity?
Cohesion, adhesion, and transpiration pull
Why does xylem not need energy for transport?
Water movement is passive
Why are xylem vessels reinforced with lignin?
To prevent collapse under tension