Ch 1Transport in Plants — Class 12 Botany, concept-first.

A plant has no heart and no blood vessels, yet it must move water, mineral ions, the sugars it makes in photosynthesis, and chemical messengers (hormones) to every living cell in its body.

Diffusion is the simplest way a substance moves inside a plant: molecules in constant, random motion spread out from a region where they are concentrated to a region where they are scarce, until the c…

Facilitated diffusion is still diffusion in the sense that it only ever moves a substance downhill, from high concentration to low — it cannot create a gradient, only speed up movement along one that…

Some carrier proteins move only a single kind of molecule, independent of anything else crossing the membrane — this is a uniport.

Where diffusion (facilitated or not) only ever carries a substance downhill, active transport does the opposite — it pumps molecules from a region of lower concentration to a region of higher concentr…

Putting simple diffusion, facilitated diffusion and active transport side by side makes their similarities and differences easy to see.

Water is not just one input among many for a plant — it is central to essentially every physiological process, because the protoplasm of a living cell is, at its core, water with countless molecules d…

Water potential (symbol Ψw, the Greek letter psi) measures the free energy of water in a system, and it is the master concept for predicting which way water will move.

A plant cell is bounded by a plasma membrane and, outside that, a cell wall; the wall itself is freely permeable and offers no real barrier to water or dissolved substances, so the plasma membrane and…

How a plant cell behaves in a given solution depends on comparing the solution's osmotic pressure to that of the cell's own cytoplasm:

Imbibition is a special case of diffusion in which water is absorbed by solid, specifically colloidal, materials, causing them to swell and increase in volume.

A simple classroom demonstration makes the transport route visible: standing a cut twig bearing white flowers in coloured water, then later cutting across the stem, shows the dye has travelled specifi…

Roots absorb the great majority of a plant's water — exactly why watering is applied to the soil rather than the leaves — and the actual absorbing structures are root hairs: thin-walled, slender exten…

Once water has been absorbed by the roots and has entered the vascular tissue, a further question arises: how does that water then travel all the way up to the rest of the plant, against the pull of g…

As mineral ions from the soil are actively pumped into the root's vascular tissue, water follows along its own water-potential gradient, and this raises the internal pressure of the xylem — a positive…

Despite having no heart, plants move water upward through the xylem at surprisingly high rates — up to roughly 15 metres per hour.

Transpiration is the evaporative loss of water from a plant, occurring mainly through the stomata (singular stoma) of the leaves — the same pores through which oxygen and carbon dioxide are also excha…

Far from being a wasteful side-effect, transpiration serves several genuinely useful purposes at once: - Generates the transpirational pull that drives water absorption and long-distance transport.

A plant obtains its carbon, and most of its oxygen, directly from carbon dioxide in the atmosphere. Every other nutritional requirement — the various mineral nutrients a plant needs to grow and functi…

Unlike water, mineral nutrients cannot simply be absorbed passively by roots, for two reasons:

Once mineral ions reach the xylem — by active uptake, passive uptake, or a combination — they travel further up through the stem and out to every part of the plant, riding along in the transpiration s…

Food, mostly in the form of the sugar sucrose, travels through the plant via the vascular tissue phloem, moving from a source to a sink.

The generally accepted explanation for how sugars travel from source to sink through the phloem is the pressure flow hypothesis (also called the mass flow hypothesis). It proceeds in a clear cycle: