OCR GCSE Biology B (J257) — Ecosystems and Photosynthesis
Every time you look at a towering oak tree, you might wonder how it pumps water 30 metres up into the air without a heart or any moving parts. This relies on a highly adapted transport system and the physical properties of water. Plants use a specialised tissue called xylem to transport water and dissolved mineral ions from the roots up to the leaves.
The xylem carries crucial minerals like nitrates (for protein synthesis) and magnesium (for making chlorophyll). Xylem consists of dead cells that have no end walls, creating a continuous, hollow tube. The cell walls are strengthened with lignin, a tough, waterproof polymer that provides structural support and prevents the vessels from collapsing under negative pressure.
The movement of water from the roots to the leaves is called the transpiration stream. This unidirectional (one-way) flow happens step-by-step:
Understanding how plants "breathe" explains why they don't simply dry out and die on a hot summer day. Stomata are pores found predominantly on the underside of leaves to reduce excessive water loss from direct sunlight. They must open to allow to diffuse into the leaf for photosynthesis, making (water loss) an unavoidable side-effect of gas exchange.
The opening and closing of stomata are controlled by a pair of specialised guard cells. Guard cells have unevenly thickened cell walls, being much thicker on the side facing the stomatal pore.
When water is plentiful, guard cells absorb water by osmosis and become turgid (swollen). Because of their uneven walls, they curve outwards, which opens the pore. When the plant is short of water or in the dark, the guard cells lose water, become flaccid (limp), and close the pore to conserve water.
In spring, a plant's roots feed its leaves, but in summer, the leaves feed the roots. This seasonal shift is made possible by the phloem, a living transport tissue. The phloem moves assimilates—primarily dissolved sugars like sucrose and amino acids—from a source to a sink.
The movement of these substances is called translocation. Unlike the passive flow in the xylem, translocation is an active process requiring ATP energy and is bidirectional, meaning sap can flow up and down the plant simultaneously. Phloem is composed of sieve tube elements, which contain no nucleus to allow easy sap flow, connected by perforated sieve plates.
Translocation happens via a mass flow mechanism:
You dry your laundry much faster on a hot, windy day than on a cold, damp one—plants lose water following the exact same rules. The rate of transpiration is influenced heavily by environmental factors. Higher light intensities increase the rate because stomata open wider to allow more in for photosynthesis.
Higher temperatures also increase the rate by giving water molecules more kinetic energy, leading to faster evaporation and diffusion. Wind increases the rate by sweeping away water vapour from the leaf surface, maintaining a steep concentration gradient. However, higher humidity decreases the rate because the air is already saturated, reducing the concentration gradient.
In the lab, we can estimate transpiration rates using a piece of equipment called a potometer. A potometer actually measures water uptake, which we assume is almost equal to the rate of transpiration.
We can calculate the rate by tracking how far an air bubble moves along a capillary tube in a set amount of time.
Worked Example:
Calculate the rate of transpiration if the air bubble in a potometer moves 24 mm in 15 minutes.
Step 1: Identify the known values.
Step 2: Substitute into the equation.
Step 3: Calculate the final answer with units.
Students often confuse the contents of the phloem. Remember that plants transport carbohydrates as sucrose, not glucose or starch, so always use the exact word 'sucrose' or 'dissolved sugars' in exams.
In 6-mark questions asking you to compare xylem and phloem, examiners expect clear paired statements: e.g., 'Xylem is dead tissue whereas phloem is living tissue' or 'Xylem flow is unidirectional but phloem flow is bidirectional'.
When defining transpiration, always state that water vapour diffuses out of the leaf, not liquid water.
A potometer only measures water uptake, not exactly water lost, because a small fraction of the water is used by the plant for photosynthesis and keeping cells turgid.
Xylem
A plant tissue composed of dead cells that transports water and dissolved mineral ions in a unidirectional flow from roots to leaves.
Lignin
A tough, waterproof substance that strengthens and thickens the walls of xylem vessels, preventing them from collapsing.
Transpiration stream
The continuous movement of water and mineral ions through the xylem from the roots to the leaves.
Osmosis
The movement of water molecules from a region of higher water concentration (or higher water potential) to a region of lower water concentration through a partially permeable membrane.
Active transport
The movement of substances across a cell membrane against a concentration gradient, requiring metabolic energy (ATP).
Cohesion
The sticking together of water molecules due to weak intermolecular forces, helping to maintain a continuous column of water in the xylem.
Stomata
Tiny pores on the leaf surface (mostly the underside) that open to allow gas exchange and close to control water loss.
Transpiration
The loss of water vapour from the aerial parts of a plant, caused by evaporation and diffusion through the stomata.
Guard cells
Specialised cells that surround each stoma and control its opening and closing by changing shape.
Turgid
Swollen with water; the state of guard cells when they have absorbed water by osmosis, causing the stomatal pore to open.
Flaccid
Limp due to water loss; the state of guard cells when they lose water, causing the stomatal pore to close to conserve water.
Phloem
A living plant tissue that transports dissolved sugars (sucrose) and amino acids bidirectionally from sources to sinks.
Assimilates
The substances synthesised by a plant and transported in the phloem, primarily sucrose and amino acids.
Sucrose
The main form of dissolved sugar transported in the phloem.
Source
The part of a plant where assimilates (like sucrose) are produced or released from storage (e.g., mature leaves or germinating tubers).
Sink
The part of a plant where assimilates are consumed for growth or stored (e.g., growing roots, fruits, or tubers).
Translocation
The active process of moving dissolved sugars and amino acids through the phloem from sources to sinks.
Companion cells
Cells adjacent to phloem sieve tubes that contain many mitochondria to produce the ATP needed for active loading of sucrose.
Put your knowledge into practice — try past paper questions for Biology B
Xylem
A plant tissue composed of dead cells that transports water and dissolved mineral ions in a unidirectional flow from roots to leaves.
Lignin
A tough, waterproof substance that strengthens and thickens the walls of xylem vessels, preventing them from collapsing.
Transpiration stream
The continuous movement of water and mineral ions through the xylem from the roots to the leaves.
Osmosis
The movement of water molecules from a region of higher water concentration (or higher water potential) to a region of lower water concentration through a partially permeable membrane.
Active transport
The movement of substances across a cell membrane against a concentration gradient, requiring metabolic energy (ATP).
Cohesion
The sticking together of water molecules due to weak intermolecular forces, helping to maintain a continuous column of water in the xylem.
Stomata
Tiny pores on the leaf surface (mostly the underside) that open to allow gas exchange and close to control water loss.
Transpiration
The loss of water vapour from the aerial parts of a plant, caused by evaporation and diffusion through the stomata.
Guard cells
Specialised cells that surround each stoma and control its opening and closing by changing shape.
Turgid
Swollen with water; the state of guard cells when they have absorbed water by osmosis, causing the stomatal pore to open.
Flaccid
Limp due to water loss; the state of guard cells when they lose water, causing the stomatal pore to close to conserve water.
Phloem
A living plant tissue that transports dissolved sugars (sucrose) and amino acids bidirectionally from sources to sinks.
Assimilates
The substances synthesised by a plant and transported in the phloem, primarily sucrose and amino acids.
Sucrose
The main form of dissolved sugar transported in the phloem.
Source
The part of a plant where assimilates (like sucrose) are produced or released from storage (e.g., mature leaves or germinating tubers).
Sink
The part of a plant where assimilates are consumed for growth or stored (e.g., growing roots, fruits, or tubers).
Translocation
The active process of moving dissolved sugars and amino acids through the phloem from sources to sinks.
Companion cells
Cells adjacent to phloem sieve tubes that contain many mitochondria to produce the ATP needed for active loading of sucrose.
