Mass transport in plants (4.11, 4.12)
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Dicotyledonous plants (dicots) produce seeds with two cotyledons (‘seed leaves’), which store food for the developing embryo and form the first leaves.
Dicots have a vascular system of xylem and phloem running through their stems, roots, and leaves. These transport tissues are arranged in vascular bundles.
In a transverse cross-section of dicot stems, the vascular bundles are located near the outside of the stem in a ring arrangement.
The xylem is on the inner side of each vascular bundle, while the phloem is on the outer side, often with sclerenchyma fibres surrounding the phloem for extra support.
This arrangement provides a scaffolding effect, reducing bending while still allowing some flexibility, so the stem does not snap.
In a longitudinal section of a dicot stem, the xylem and phloem run as continuous strands along the length of the stem.
The xylem is on the inner side of each vascular bundle, while the phloem is on the outer side, often with sclerenchyma fibres surrounding the phloem on the outside of the bundle.
Under the microscope, xylem vessels can be identified as hollow, thick-walled, and lacking cytoplasm. They have a wider lumen than other vessels.
Xylem vessels:
- transport water and mineral ions from roots to leaves through the stem (one-way flow) via the transpiration stream
- provide mechanical support to the plant.
Xylem is a tissue made of cells joined end to end with no end walls between them. This creates a long, hollow tube structure that allows continuous, uninterrupted water transport.
The xylem cells that make up the vessels are dead, meaning they contain no cytoplasm or organelles. This gives the xylem an empty lumen, which reduces resistance to the flow of water.
The vessels are narrow, helping water rise by capillary action.
Xylem vessel walls are impregnated with spirals of lignin around the lumen of the vessel. The spiral pattern provides flexibility, while lignin provides support and additional mechanical strength, preventing the vessels from collapsing inwards under the tension generated during water transport.
Lignin is also waterproof, which minimises water loss from the vessel.
Pits within the walls of the xylem are non-lignified, allowing lateral movement of water between other cells and xylem vessels.
Columns of dead sclerenchyma cells with lignified, thickened cell walls form hollow sclerenchyma fibres.
These fibres are located on the outer side of the vascular bundle and provide mechanical support.
Water is essential in plants for both structural and metabolic reasons:
- It provides support and turgidity, helping cells maintain shape, and is used in photosynthesis to produce glucose and oxygen.
- Water acts as a solvent, dissolving mineral ions for transport in the xylem and assimilates (sugars) for transport in the phloem.
- Evaporation of water helps to keep the plant cool.
- It is a metabolite used in many hydrolysis reactions.
Mineral ions are essential for plant growth and metabolism:
- Nitrate ions () – used to make amino acids, proteins, enzymes, and nucleic acids for growth.
- Magnesium ions () – component of chlorophyll, required for photosynthesis.
- Calcium ions () – important for cell wall stability and cell membrane function.
- Phosphate ions () – used to make ATP, DNA, RNA, and phospholipids.
Mineral ions are actively transported from the soil into root hair cells, against their concentration gradient. This uses carrier proteins / ion pumps, which require ATP from respiration.
Root hair cells are adapted for this by having many mitochondria to supply energy.
Method for investigating the effect of a mineral on plant growth:
- Select plants that are clones / the same variety / the same age to reduce genetic variation.
- Prepare a range of at least five different mineral concentrations in the growth medium.
- Control abiotic variables such as light intensity, temperature, and water availability to ensure they remain constant.
- Plant the seeds or seedlings in the prepared mineral solutions and grow them for a set period of time under controlled conditions.
- Measure a relevant growth parameter, such as increase in height, leaf number, or dry biomass.
- Include repeats at each mineral concentration to calculate mean growth values.
- Record and compare growth across the different mineral concentrations to determine the effect of mineral deficiency.
Stomata are pores mainly on the underside of leaves that allow carbon dioxide to enter and oxygen to leave the plant.
Each stomatal pore is surrounded by kidney-shaped guard cells with a thicker inner wall and thinner outer wall.
When guard cells take in water and become turgid, they swell and bend, opening the stomata. This happens during the day when photosynthesis occurs in the presence of sunlight.
When guard cells lose water and become flaccid, the stomata close. This happens at night when photosynthesis cannot occur.
Transpiration is the loss of water vapour from a plant by evaporation from the leaf and diffusion into the atmosphere through the stomata.
It occurs as a consequence of gas exchange through the stomata.
Water evaporates from the surfaces of the spongy mesophyll cells into the air spaces within the leaf and then diffuses out of the leaf through the open stomata down a water vapour potential gradient.
The transpiration stream is the movement of water from the roots to the leaves through the xylem in the stem. This process is explained by the cohesion–tension theory.
In the cohesion–tension theory, transpiration reduces the water potential in the mesophyll cells, causing water to move by osmosis from the xylem into these cells. This creates tension (negative pressure) in the xylem, causing water to move down a pressure gradient by mass flow from roots to leaves.
In the cohesion–tension theory, the cohesion between water molecules, forms a continuous column of water in the xylem, which is pulled upwards as a result of the tension created by transpiration.
Adhesion occurs between water molecules and the walls of the xylem vessels, helping the water column move up the vessel by capillary action and resist the effects of gravity.
The function of the phloem is the movement of assimilates (such as sucrose, amino acids, hormones and salts), which are dissolved in water from source (site of production) to sink (site of usage). This process is called translocation and it requires energy.
Translocation is a two-way flow, moving assimilates both up and down the plant by the process of mass flow, down a pressure gradient.
Phloem is a tissue made of two types of cells – sieve tube elements and companion cells.
Sieve tube elements are cells stacked end to end to form long, hollow, continuous tubes, which allow mass flow of assimilates with minimal resistance. Although sieve tube elements are living cells, they contain very little cytoplasm, no nucleus, and few organelles, further reducing resistance and facilitating the flow of substances.
The end walls of sieve tube elements, called sieve plates, contain pores that allow assimilates to pass through and connect the cytoplasm of adjacent cells. The sieve plates also limit the size of molecules that can pass and slightly reduce the rate of flow.
The cell walls of sieve tube elements are thin, not lignified (cellulose only), and do not contain pits.
Companion cells act as the “life support” for sieve tube elements, carrying out the living functions. They contain dense cytoplasm, a large nucleus, and many mitochondria, and are connected to the sieve tube elements via plasmodesmata.
The mitochondria and other organelles provide energy for active transport, allowing the companion cells to load sucrose from the source (where assimilates originate) into the sieve tube elements through active loading.
A source is a part of the plant where assimilates are produced.
Examples include green leaves, storage organs such as tubers that are unloading their stores, and food reserves in germinating seeds.
Glucose produced in photosynthesising cells is converted into an assimilate, such as sucrose, for translocation in the phloem.
Sucrose is soluble, allowing it to move in sap, and relatively metabolically inactive, so it is not used in respiration. Transportation as sucrose also reduces diffusion out of the sieve tubes, as entry and exit are controlled by transport proteins.
For the active loading of sucrose into the companion cells from the surrounding tissue, protons
() are actively pumped out using ATP and a proton pump. This creates a proton gradient.
Protons then re-enter the cell via co-transport proteins, carrying sucrose molecules with them. This process actively loads sucrose into companion cells, from where it diffuses into sieve tube elements for translocation.
The entry of sucrose into sieve tube elements at the source decreases the water potential in the sieve tube elements. This causes water to enter the phloem by osmosis from the surrounding cells and xylem vessels. This results in higher hydrostatic pressure within the sieve tube elements.
When the sucrose arrives at the sink, it diffuses out of the sieve tube elements into the surrounding cells. This causes the water potential inside the sieve tube elements to increase so water leaves by osmosis down a water potential gradient. This lowers the hydrostatic pressure.
A hydrostatic pressure gradient drives the mass flow of sucrose dissolved in water from source to sink. The high hydrostatic pressure in the phloem at the source enables rapid transport over long distances.
A sink is a part of a plant where assimilates are used or stored. Assimilates are translocated from source to sink down a concentration gradient in the phloem. At the sink, assimilates are converted into glucose for respiration or starch for storage, maintaining a concentration gradient between the phloem and cells.
Examples include growing roots, dividing meristems, developing seeds, fruits, or storage organs.