Nutrition and transport in plants, at a glance

Stomata and mesophyll air spaces shorten the diffusion route to photosynthesising cells; chloroplast-rich palisade tissue captures light.

• Palisade: many chloroplasts near the upper surface.
• Spongy mesophyll: air spaces and moist exchange surfaces.
• CO2: stoma -> air space -> moist cell wall -> cell -> chloroplast.

Keep in mind: Carbon dioxide reaches mesophyll cells mainly by diffusion through stomata and air spaces. Xylem transports water and mineral ions.

Xylem carries water and mineral ions; phloem carries food, mainly sucrose. Their positions help identify them in a dicot section.

• Dicot stem bundle: xylem inside, phloem outside.
• Leaf vein: xylem above, phloem below.
• Phloem contains sieve-tube elements and companion cells.

Keep in mind: Mature xylem vessels are dead and hollow. Phloem sieve-tube elements are living and depend on companion cells.

A root hair provides a large surface for uptake. Water enters by osmosis down a water-potential gradient; mineral ions can be actively transported against their concentration gradients.

• Long extension -> large surface area.
• Water uptake uses osmosis and water potential.
• Ion uptake against a gradient needs energy from respiration.

Keep in mind: Osmosis describes water movement only. Nitrate uptake can use active transport when it is against a concentration gradient.

Chlorophyll absorbs light energy, which is converted to chemical energy as carbon dioxide and water form carbohydrates and oxygen.

• Carbon in glucose comes from carbon dioxide.
• Light energy becomes chemical energy in food.
• Glucose: respiration, starch, cellulose, sucrose and other organic compounds.

Keep in mind: Carbon dioxide and water supply matter for photosynthesis. Light supplies energy. Roots mainly absorb water and mineral ions; the plant makes organic food.

Change one factor, control the others and measure oxygen output over a fixed time. Repeated measurements make the comparison more reliable.

• Independent variable: change one factor.
• Dependent variable: oxygen output per unit time.
• Control plant amount, other factors, acclimatisation and timing.

Keep in mind: It can also increase temperature. A fair test must control or monitor this additional change.

A limiting factor restricts the rate. Increasing it raises the rate only until another factor becomes limiting.

• A rising region: increasing that factor increases the rate.
• A plateau: another factor may now limit the rate.
• A plateau alone cannot identify the other limiting factor.

Keep in mind: A plateau shows a steady rate, which can still be high. It means additional light is no longer increasing that rate under those conditions.

Water evaporates from mesophyll cell surfaces and diffuses out as vapour. This loss creates a pull that draws water up continuous columns in xylem.

• Evaporation: mesophyll cell walls -> air spaces.
• Diffusion: water vapour -> outside through stomata.
• Transpiration pull: xylem water moves from roots towards leaves.

Keep in mind: Mature xylem vessels are hollow and dead, without functioning cell membranes. Water moves along them by bulk flow; osmosis occurs where water crosses partially permeable membranes elsewhere.

A potometer measures water uptake as an estimate of transpiration. Environmental factors change evaporation, the vapour gradient or stomatal opening.

• Potometer = water uptake, used to estimate transpiration.
• Wind and lower humidity steepen the vapour gradient.
• Temperature affects evaporation; light often affects stomatal opening.

Keep in mind: It measures uptake through the apparatus. Uptake is an estimate of transpiration because some water is retained or used in plant processes.

If water loss exceeds uptake, cells lose water and turgor. Soft tissues become flaccid and the plant wilts.

• Compare rates: loss greater than uptake causes a deficit.
• Water loss -> smaller vacuoles -> lower turgor -> drooping.
• Closed stomata conserve water but restrict CO2 entry.

Keep in mind: Wilting can occur whenever water loss exceeds uptake sufficiently. Both processes can still be occurring.

Translocation moves food, mainly sucrose, through phloem from sources to sinks. Its direction depends on where food is supplied and used or stored.

• Translocation: food, mainly sucrose, in phloem.
• Source supplies; sink uses or stores.
• Ringing and tracers provide evidence for the pathway.

Keep in mind: Phloem carries food from sources to sinks, which may be above or below. Xylem chiefly carries water and mineral ions from roots towards leaves.