6.2.1 Root hair cell adaptations

Root hair cells: efficient uptake from soil

1. Root hair cell means a specialised epidermal cell with a long projection that absorbs water and mineral ions from the soil.
2. The long projection gives a very large surface area in contact with the thin film of water around soil particles.
3. A thin cell wall and membrane give a short pathway into the cell, which increases the rate of uptake.
4. Many mitochondria release energy by aerobic respiration for active transport of mineral ions.

Water enters by osmosis

1. Osmosis is the net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
2. Soil water is usually more dilute than the cell sap, so water moves into the root hair cell across its cell membrane.
3. This movement is passive and does not require energy from respiration.

Mineral ions enter by active transport

1. Active transport is the movement of particles from a lower concentration to a higher concentration using energy released by respiration.
2. Mineral ions can be less concentrated in the soil than inside the root hair cell, so diffusion cannot move them into the cell.
3. Carrier proteins in the cell membrane move ions such as nitrate and magnesium ions against their concentration gradient.
4. Nitrate ions are needed to make amino acids and proteins, while magnesium ions are needed to make chlorophyll.

6.2.2 Structure of xylem and phloem

Transport tissues: xylem and phloem

1. Xylem is a tissue of dead, hollow, lignified vessels that carries water and dissolved mineral ions from roots to stems and leaves.
2. Phloem is living tissue made of sieve tubes and companion cells that transports dissolved sucrose and amino acids between sources and sinks.
3. The tissues occur together in vascular bundles that run through roots, stems and leaf veins.

Xylem is built for one-way water flow

1. Mature xylem vessel cells are dead and joined end to end, with their end walls lost to form a continuous hollow tube.
2. The vessels contain no cytoplasm, so water meets little resistance as it moves upwards.
3. Lignin is deposited in the walls to make them strong and waterproof, preventing collapse when water is pulled upwards. The deposition of the lignin causes the cells to die.
4. Pits in the walls allow water to move sideways between vessels and into neighbouring tissues.

Phloem is living transport tissue

1. Sieve tube elements are living cells arranged end to end, and their perforated end walls form sieve plates.
2. Sieve tube elements have very little cytoplasm and no nucleus, leaving space for phloem sap to flow.
3. Companion cells contain a nucleus and many mitochondria, providing the energy needed to load and unload sucrose.
4. Phloem can transport substances upwards or downwards because different sources and sinks occur in different parts of the plant.

Comparing the tissues

1. Xylem is dead, hollow, lignified and carries water and mineral ions mainly upwards.
2. Phloem is living, has sieve plates and companion cells, and carries dissolved food in either direction.

6.2.3 Transpiration and stomata

Transpiration: water loss pulls water upwards

1. Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata in leaves.
2. Water evaporates from the moist cell walls of mesophyll cells into the leaf air spaces.
3. Water vapour then diffuses out through open stomata because its concentration is higher inside the leaf than outside.
4. Water lost from mesophyll cells is replaced by water from the xylem, maintaining a continuous transpiration stream.
5. Cohesion between water molecules helps keep an unbroken column of water in the xylem as it is pulled upwards.

Stomata control gas exchange and water loss

1. Stomata are pores in the leaf epidermis through which carbon dioxide, oxygen and water vapour diffuse.
2. Each stoma is surrounded by two guard cells that change shape to open or close the pore.
3. In light and when water is available, guard cells become turgid and curve apart, opening the stoma for carbon dioxide uptake.
4. When water is scarce, guard cells lose turgor and the stoma closes, reducing water loss but also limiting photosynthesis.

6.2.4 Translocation of sucrose

Translocation: moving dissolved food

1. Translocation is the movement of dissolved food, mainly sucrose and amino acids, through phloem from sources to sinks.
2. Glucose made by photosynthesis is often converted to sucrose because sucrose is soluble and suitable for transport.
3. Translocation requires energy for loading and unloading substances, supplied by respiration in companion cells.
4. It is separate from the transpiration stream, which moves water and mineral ions through xylem.

Sources load food into phloem

1. Source means a plant region that releases or produces more sucrose than it uses, so sucrose enters the phloem.
2. A mature photosynthesising leaf is usually a source because it makes sugars faster than it uses them.
3. A storage organ can become a source when stored carbohydrate is converted into soluble sugar and exported.

Sinks remove food from phloem

1. Sink means a plant region that removes sucrose from phloem for respiration, growth or storage.
2. Growing roots, shoot tips, flowers, fruits and seeds are common sinks because their cells need energy and new biological molecules.
3. A sink can later become a source, so phloem transport can occur upwards or downwards according to the plant's needs.

Phloem structure supports transport

1. Sieve tube elements join end to end, and pores in their sieve plates allow phloem sap to pass between cells.
2. Companion cells have many mitochondria, so they can release energy for active loading and unloading of sucrose.
3. Different sieve tubes can carry sap in different directions at the same time, but movement within one tube is from its source towards its sink.

6.2.5 Leaf structure for photosynthesis

Leaf structure: maximising photosynthesis

1. Photosynthesis is the process in which light energy absorbed by chlorophyll is used to make glucose from carbon dioxide and water, releasing oxygen.
2. The word equation is $carbon\; dioxide+water\to glucose+oxygen$.
3. The balanced symbol equation is $6\mathrm{C}{\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{O}}_2$.
4. A broad leaf provides a large surface area for light absorption, while a thin leaf gives carbon dioxide a short diffusion distance.

Upper layers let light reach chloroplasts

1. The waxy cuticle is transparent so light passes through, and waterproof so it reduces evaporation from the surface.
2. The upper epidermis is thin and transparent, allowing light to reach the palisade mesophyll.
3. Palisade mesophyll is a layer of tall, closely packed cells near the upper leaf surface containing many chloroplasts.
4. Its position and chloroplast density allow it to absorb a high proportion of the incoming light.

Spongy mesophyll speeds gas exchange

1. Spongy mesophyll is a loosely packed layer with many interconnecting air spaces and moist cell surfaces.
2. The air spaces provide a large internal surface area and let carbon dioxide diffuse rapidly from stomata to photosynthesising cells.
3. Oxygen made in photosynthesis diffuses through the same spaces and out of the leaf.
4. The lower epidermis contains many stomata, which allow gas exchange while reducing exposure to direct sunlight and air movement.

Veins supply and remove materials

1. Xylem in each vascular bundle supplies water and mineral ions to leaf cells.
2. Phloem carries sucrose and amino acids away from photosynthesising cells to sources and sinks elsewhere in the plant.
3. A network of veins keeps cells close to transport tissue and also helps support the leaf blade.

6.2.6 Environmental factors on water uptake

Environmental factors change water uptake

1. Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through leaf stomata.
2. As transpiration removes water from leaves, water is pulled through the xylem and replaced by water absorbed from the soil.
3. A factor that increases transpiration therefore usually increases the rate of water uptake by the roots.

Light intensity opens stomata

1. Increasing light intensity usually increases water uptake because stomata open to admit carbon dioxide for photosynthesis.
2. Wider stomatal pores allow more water vapour to diffuse out of the leaf.
3. This increases transpiration and pulls water through the xylem more quickly.
4. In darkness, most stomata close, reducing both water loss and water uptake.

Air movement maintains a steep gradient

1. Moving air removes the moist layer of air beside the leaf surface.
2. The water vapour concentration outside the stomata falls, making the concentration gradient from the leaf to the air steeper.
3. Water vapour diffuses out faster, so transpiration and water uptake increase.
4. In still air, moist air remains near the leaf and the smaller gradient reduces diffusion.

Temperature increases particle movement

1. At a higher temperature, water molecules have more kinetic energy.
2. Water evaporates more rapidly from mesophyll cell surfaces and water vapour diffuses through stomata more quickly.
3. Transpiration and water uptake therefore increase, provided the plant has enough water and the stomata remain open.

Explaining experimental results

1. Use repeat readings to identify anomalies and calculate a mean, which makes the comparison more reliable.
2. Describe the pattern in the data before explaining it with stomatal opening, kinetic energy or the water vapour gradient.

6.2.7 Rate calculations for transpiration

Transpiration rates: distance divided by time

1. Potometer means apparatus that estimates transpiration rate by measuring water uptake by a leafy shoot.
2. The air bubble moves as the shoot takes up water, so its distance travelled can be measured against time.
3. A potometer does not measure transpiration directly because some absorbed water is used in photosynthesis, growth and maintaining cell turgor.

Calculate distance per unit time

1. Use $rate=\frac{distance\; moved\; by\; bubble}{time\; taken}$.
2. If distance is measured in millimetres and time in minutes, the unit is $\mathrm{mm}\mathrm{mi}{\mathrm{n}}^{-1}$.
3. Convert quantities before calculating when the question gives different time or distance units.

Calculate water volume per unit time

1. For a cylindrical capillary tube, its cross-sectional area is $A=\pi r^2$.
2. The volume of water taken up is $V=Al=\pi r^{2}l$, where $l$ is the bubble distance.
3. The volume uptake rate is $rate=\frac{\pi r^{2}l}{t}$.
4. Use matching length units throughout so the volume unit is correct, such as $\mathrm{m}{\mathrm{m}}^3$.

Calculate means and percentage change

1. For repeats, calculate $mean=\frac{sum\; of\; valid\; readings}{number\; of\; valid\; readings}$.
2. Exclude an anomalous reading only when there is evidence that it does not fit the repeat pattern.
3. Calculate percentage change using $percentage\; change=\frac{new\; rate-original\; rate}{original\; rate}\times 100$.

Use graphs and tables correctly

1. Put the independent variable on the horizontal axis and transpiration or water-uptake rate on the vertical axis.
2. Include units in each axis label and use a suitable linear scale that fills most of the graph.
3. A straight or smooth line of best fit should represent the overall pattern rather than joining every point automatically.

6.2.8 Plant adaptations to extreme environments

Dry-environment plants reduce water loss

1. Xerophyte means a plant adapted to live where water is scarce, with features that reduce transpiration.
2. Water loss threatens cell turgor, transport and photosynthesis, so xerophyte leaves reduce evaporation or trap moist air near stomata.
3. Each useful adaptation changes leaf size or shape, the cuticle, or the number and position of stomata.

Smaller leaves reduce surface area

1. Small leaves or spines provide less surface area from which water can evaporate.
2. A low surface-area-to-volume ratio reduces the exposed surface compared with the amount of tissue containing stored water.
3. Some plants shed their leaves during dry periods, removing a major surface for transpiration.

Leaf shape traps humid air

1. Rolled leaves enclose stomata inside a sheltered space where water vapour accumulates.
2. Leaf hairs trap still, moist air and reduce the movement of air across stomata.
3. The smaller water vapour concentration gradient between the leaf and the trapped air slows diffusion out of the leaf.
4. Marram grass combines rolled leaves, hairs and stomata on the inner surface, so all three features act on the same gradient.

A thick cuticle reduces evaporation

1. A thick, waxy cuticle forms a waterproof barrier over the epidermis.
2. This reduces evaporation through the leaf surface and limits water loss when stomata are closed.
3. The cuticle remains transparent, allowing light to reach photosynthesising tissues.

Stomata limit vapour loss

1. Having fewer stomata reduces the total pore area through which water vapour can diffuse.
2. Sunken stomata sit in pits that shelter them from air movement and trap humid air.
3. Closing stomata during the hottest or driest conditions conserves water, although it also reduces carbon dioxide uptake and photosynthesis.

Explaining an unfamiliar adaptation

1. Identify how the feature changes exposed surface area, evaporation, air movement, stomatal opening or the water vapour gradient.
2. Then link that change to slower diffusion of water vapour and a lower rate of transpiration.