StemUp: OCR A A level Biology 3.1.3 Transport in plants

Why do multicellular plants need transport systems? (3)

- Plants need to transport water, minerals, and sugars and return waste to the environment

- They have a small surface area to volume ratio and a high metabolic rate

- So direct diffusion would be too slow to meet their metabolic needs

What are the functions of xylem and phloem in plants? (2)

- Xylem: Transports water and mineral ions upward from roots to leaves

- Phloem: Transports sugars in solution, both upwards and downwards throughout the plant

What is the structure and function of xylem in the roots? (2)

- Xylem is arranged in an X-shape in the centre, surrounded by phloem

- This arrangement provides structural support for the root as it pushes through soil

How is the xylem structured in stems? (2)

- Xylem is found on the inside of vascular bundles

- Which are arranged in a ring around the stem, with phloem on the outside and separated by cambium

How are xylem and phloem arranged in leaves?

1. Xylem and phloem form a network of veins that support the leaves

2. With xylem on top and phloem on the bottom

What is the structure of xylem vessels? (3)

- Xylem vessels are long, tube-like structures made from dead cells with no end walls

- Forming an uninterrupted tube for water transport

- The walls are thickened with lignin, which strengthens, waterproofs, and allows flexibility

What are pits in xylem vessels, and what is their function? (2)

- Pits are areas in the xylem walls with no lignin

- Allowing water and ions to move in and out of the vessels

What is the structure of phloem vessels? (2)

- Phloem is composed of sieve tube elements and companion cells

- Sieve tube elements have no nucleus and few organelles, and they are connected end-to-end with sieve plates

- Allowing solutes to pass through

What is the function of companion cells in the phloem? (2)

- Companion cells have a dense cytoplasm, large nucleus, and many mitochondria

- They provide energy for active transport and other functions of the sieve tube elements

What is transpiration? (2)

- Transpiration is the loss of water from the aerial parts of a plant

- Mainly through the stomata

Why is transpiration unavoidable for plants? (2)

- Stomata must open for gas exchange (CO₂ in, O₂ out) for photosynthesis

- Leading to water loss through the open stomata

What are the steps in water movement during transpiration? (3)

1. Osmosis from xylem to mesophyll cells

2. Evaporation from mesophyll cells to form water vapour in air spaces

3. Diffusion of water vapour through stomata

What factors affect the rate of transpiration? (8)

- Number of leaves

- Number, size, and position of stomata

- Presence of a cuticle

- Light

- Temperature

- Humidity

- Air movement

- Water availability

What does a potometer measure? (1)

Measures water uptake, which gives an estimate of transpiration

Why are potometer measurements only an estimate of transpiration rate? (2)

- Not all water taken up is lost via transpiration

- Some is used for photosynthesis and to maintain turgidity

How do you use a potometer to estimate transpiration rates? (6)

1. Cut a healthy shoot underwater

2. Insert the shoot into the apparatus underwater

3. Ensure the apparatus is airtight

4. Dry the leaves and allow the shoot to acclimatise

5. Keep conditions constant except for the variable being tested

6. Measure the distance the air bubble moves per unit time to estimate water uptake

How does water enter a plant? (2)

1. Water enters the plant through root hair cells by osmosis

2. Moving from areas of higher water potential (soil) to areas of lower water potential (inside the plant)

How are root hair cells adapted for water absorption? (4)

1. Large surface area for absorption

2. Thin walls for a short diffusion pathway

3. Unlignified/permeable to allow entry of water and minerals

4. Low water potential to facilitate water uptake by osmosis.

What are the two main pathways for water transport through plants? (2)

- Symplast pathway: Water moves through the cytoplasm and plasmodesmata by osmosis

- Apoplast pathway: Water moves through the cell walls by diffusion and mass flow until it reaches the Casparian strip, where it is forced into the symplast pathway

What is the role of the Casparian strip in the root? (2)

- Blocks the apoplast pathway and forces water to enter the symplast pathway

- Before reaching the xylem

How is water transported through the plant in the xylem? (3)

- Water is pulled up the xylem by:

- Cohesion (water molecules stick together)

- Adhesion (water sticks to xylem walls)

- Tension created by water evaporating from the leaves

What is the transpiration stream? (2)

- The movement of water from roots to leaves through the xylem

- Driven by transpiration and involving cohesion, adhesion, and tension forces

How are xerophytic plants adapted to reduce water loss? (6)

1. Sunken stomata to trap water vapour and reduce the water potential gradient

2. Hairs around stomata to trap moist air

3. Rolled leaves to trap moist air in windy conditions

4. Thick waxy cuticle to reduce evaporation

5. Spines to reduce surface area for water loss

6. Stomata close during hot conditions.

How are hydrophytic plants adapted to survive in water?

- Air spaces in tissues to help float and store oxygen

- Stomata on the upper surface of floating leaves for gas exchange

- Flexible leaves and stems to prevent damage from water currents

What is translocation in plants? (2)

- The movement of dissolved substances (assimilates) like sucrose

- From a source (where sugars are made) to a sink (where sugars are used or stored) via the phloem

What are the source and sink in plant translocation? (2)

- Source: Where sugars are produced (e.g., leaves)

- Sink: Where sugars are used or stored (e.g., roots or growing leaves)

How are hydrogen ions involved in translocation? (2)

1. ATP is used to actively transport hydrogen ions out of companion cells

2. Creating a concentration gradient for sucrose transport into the phloem

How is sucrose loaded into the phloem? (3)

1. Hydrogen ions move back into companion cells via co-transport proteins

2. Bringing sucrose with them

3. Which then diffuses into the sieve tubes

How does water move into the phloem during translocation? (3)

1. The increased sucrose concentration in the sieve tubes lowers the water potential

2. Causing water to enter from the xylem by osmosis

3. Increasing the pressure inside the sieve tubes

How do solutes move from source to sink? (2)

1. Water carrying solutes moves down the sieve tubes from higher pressure at the source to lower pressure at the sink

2. Where solutes are unloaded and used

What evidence supports the mass flow hypothesis? (3)

1. Radioactive tracers show labelled carbon dioxide from photosynthesis in the phloem

2. Ringing experiments (removing phloem) cause sugars to accumulate above the ring

3. Aphids leave mouthparts in plant stems, and sap flows faster near the leaves, showing a pressure gradient

What evidence contradicts the mass flow hypothesis? (3)

1. Not all solutes move at the same rate

2. Sucrose moves to all parts of the plant equally, not faster to areas with low concentrations

3. The role of sieve plates is unclear as they could impede flow