Plant Transport Notes

Inside the Leaf - Layers of a Leaf

• The leaf structure includes:
  • Cuticle (both upper and lower)
  • Upper epidermis
  • Mesophyll (palisade and spongy)
  • Lower epidermis
  • Guard cells
  • Stoma (plural: stomata)
  • Chloroplast
  • Vacuole
  • Nucleus
  • Cell wall
  • Cytoplasm

Vascular Bundle

• Plants have two cell types for transportation:
  • Xylem: Transports water and minerals from roots to shoots and leaves; one-way transport.
  • Phloem: Transports sugars and amino acids (nutrients) from leaves to growing and storage tissues (roots and stem); two-way transport.
• Vascular bundles arrange these cells, forming continuous systems connecting roots, stems, and leaves.

Xylem

• Xylem vessels:
  • Have thick cellulose cell walls, strengthened by lignin protein.
  • Are hollow and consist of dead cells.
  • Provide support to the plant due to their thick walls.
  • Transport water and minerals from roots to shoots and leaves in one direction.

Phloem

• Phloem consists of columns of living cells called sieve tubes.
• They transport nutrients (sugars and amino acids) via translocation.
• Sugars move from leaves to growing and storage parts (roots and stem) in both directions.
• Cells are joined by small holes in the cell wall at the end of each cell, forming a continuous system.
• End cell walls are called sieve plates.
• Phloem contains companion cells that provide energy to keep the phloem cells alive by sharing their protoplasm.

Cross-Section of a Stem/Root

• Vascular bundles contain both xylem and phloem tissues.
• Roots contain:
  • Root cortex cells
  • Root hair
• Stems contain:
  *Phloem
  *Xylem
• Root:
  • Cortex
  • Phloem
  • Endodermis
  • Pericycle (just inside the endodermis)
• Stems:
  • Phloem
  • Xylem

Movement of Gases In and Out of Leaves

• Plants use carbon dioxide (CO2) during photosynthesis and produce oxygen (O2).
• Gases move in and out through stomata by diffusion.
• When CO_2 concentration is low inside the plant, it diffuses in from the air through stomata.
• When O_2 concentration is high inside the plant, it diffuses out into the air through stomata.

Adaptations of Leaves for Diffusion

• Leaves are thin to decrease the distance gases travel between the air and cells.
• Air spaces between cells increase the speed of diffusion from the air to the cells inside the leaf.
• Lots of stomata (pores) on the undersides of leaves.

Stomata

• Stomata are small holes or pores on the underside of leaves.
• A single hole is called a stoma.
• Each stoma is surrounded by two guard cells controlling its opening and closing.
• During the day, guard cells perform photosynthesis, become rich in glucose, gain water by osmosis, and become turgid, causing the stoma to open.
• Water also evaporates through stomata.

Closing the Stomata

• At night, guard cells become flaccid as they lose water.
• The guard cells move towards one another and close as water moves out by osmosis.
• This happens because at night or under shade plants are unable to produce glucose and become less concentrated.

Wilting

• Wilting occurs when water is scarce or roots are damaged to slow down the transpiration rate, increasing the plant's chance of survival.
• Plants can wilt naturally, or it can be induced artificially (e.g., removing leaves from cuttings).
• When the rate of H2O uptake is much lower than H2O loss, plant cells lose water, reducing pressure and causing leaves to bend.

Transpiration

• Transpiration is the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata.

Transpiration Pull and Stream

• The transpiration stream is a crucial process in plants, facilitating the movement of water from roots to leaves and eventually into the atmosphere.
• Root hair cells absorb water from the soil through osmosis creating high pressure, which then travels via xylem vessels.
• Cohesion and adhesion create a continuous column of water within the xylem.
• As water evaporates low pressure is generated in the leaf producing a suction force also called a transpiration pull, drawing water upward against gravity without using energy.
• This process maintains turgidity, transports nutrients, and cools the plant.

Factors Affecting Transpiration

• The speed at which a plant loses water is called the rate of transpiration.
• Factors:
  • Humidity
  • Light
  • Temperature
  • Wind

Potometer

• A potometer measures transpiration.
• A cut plant stem is sealed into the potometer using a rubber bung.
• An air bubble is introduced to the capillary tube.
• The distance the bubble travels shows how much water the stem has taken up.
• More transpiration results in more bubble movement towards the plant (left direction).
• A potometer can check the effect of environmental factors on air bubble uptake.
• For example, bringing a light bulb near a potometer can increase transpiration rates.
• The plant stem junction is covered by petroleum jelly to avoid water loss.

Testing the Effect of Limiting Factors on the Rate of Transpiration

• Assemble 4 potometers.
  • A. Control: room conditions
• Place each potometer in a different environment:
  • B. Mist
  • C. Wind
  • D. Bright light
• Measure water loss in each potometer every 3 minutes for 30 minutes.

Roots

• Root hairs are single-celled extensions of epidermis cells in the root.
• They grow between soil particles and absorb water and minerals from the soil.
• Water enters the root hair cells by osmosis.
• This happens because soil water has a higher water potential than the cytoplasm of the root hair cell.
• The root hair increases the surface area of the cells significantly.
• This large surface area is important as it increases the rate of the absorption of water by osmosis and mineral ions by active transport.

Roots and Their Adaptations

• Roots consist of microscopic root hair cells.
  • High surface area to volume ratio
  • Vacuole is concentrated in salts to always have a low water potential, so that water moves in by osmosis.
  • Rich in mitochondria to provide energy for active transport
  • Rich in protein carriers to move molecules against the concentration gradient

Pathway of Water from the Soil

• Osmosis causes water to pass into the root hair cells, through the root cortex, and into the xylem vessels.
• Once the water gets into the xylem, it is carried up to the leaves where it enters mesophyll cells.
• Pathway: root hair cell → root cortex cells → xylem → leaf mesophyll cells

Translocation

• The soluble products of photosynthesis are sugars (mainly sucrose) and amino acids.
• These are transported around the plant in the phloem tubes which are made of living cells (as opposed to xylem vessels which are made of dead cells).
• The cells are joined end to end and contain holes in the end cell walls (called sieve plates) which allow easy flow of substances from one cell to the next.
• The transport of sucrose and amino acids in the phloem, from regions of production to regions of storage or use, is called translocation.
• Transport in the phloem goes in many different directions depending on the stage of development of the plant or the time of year; however dissolved sucrose is always transported from the source (where it’s made) to sink (where it’s stored or used).

Translocation and Seasons

• During winter:
  • When many plants have no leaves, the phloem tubes may transport dissolved sucrose and amino acids from the storage organs to other parts of the plant so that respiration can continue.
• During a growth period/spring:
  • The storage organs (e.g., roots) would be the source, and the many growing areas of the plant would be the sinks.
• After the plant has grown usually during the summer:
  • The leaves are photosynthesizing and producing large quantities of sugars; so, they become the source, and the roots become the sinks – storing sucrose as starch until it is needed again.