Whole-organism survival depends on coordinated transport: cells must be supplied with water, gases, nutrients, ions, and signals while removing wastes. Multicellular organisms integrate membrane transport with specialised tissues, bulk flow, and regulated exchange surfaces.

Core principle: linking membranes to body-wide transport

At the cellular level, selective membranes control what enters and leaves cells. At the organism level, transport networks connect many exchange sites to all tissues, allowing rapid distribution despite long diffusion distances.

Key integration challenges include:

• Maintaining steep concentration gradients at exchange surfaces

• Moving materials quickly over long distances

• Adjusting transport rates as temperature, salinity, oxygen availability, hydration, and activity change

Bulk flow: long-distance transport by pressure

Diffusion alone is too slow across large organisms, so many systems use bulk flow to move fluids and dissolved solutes together.

Bulk flow is effective because organisms can:

• Generate pressure differences (pumps, muscular contraction, or water potential differences)

• Use branching networks to deliver materials close to cells, where diffusion completes the final short step

Animals: circulation couples delivery, exchange, and removal

In animals, circulatory systems integrate:

This figure summarizes capillary exchange by contrasting net filtration at the arterial end with net reabsorption at the venous end. It visualizes how hydrostatic pressure pushing fluid out and colloid osmotic pressure pulling fluid in shift along the capillary, producing directional bulk flow between blood and interstitial fluid. This provides a mechanistic bridge between “bulk flow” and “capillary exchange” in animal transport systems. Source

• Convective transport (bulk flow of blood) for rapid distribution

• Capillary exchange where thin walls enable diffusion of gases, nutrients, and wastes

• Membrane transport in specific tissues (e.g., epithelia) to move ions and solutes directionally, shaping fluid composition

Important design features that improve efficiency:

• Extensive capillary surface area near metabolically active tissues

• Short diffusion distances from capillary to cell

• Variable blood flow controlled by vessel diameter (redistributing resources during exercise, thermoregulation, or stress)

Plants: vascular transport links roots and leaves

In plants, transport must connect organs with very different functions:

• Roots absorb water and mineral ions from soil

• Leaves exchange gases and drive water movement while making sugars

• Stems distribute resources between sources (photosynthetic tissues) and sinks (growing/storage tissues)

Integration across organs typically involves:

• Directional uptake across root cell membranes (often requiring selective channels and transporters)

• Long-distance bulk flow through vascular tissues

• Regulated exchange at leaves (gas exchange while limiting water loss)

Exchange surfaces: maintaining gradients while meeting demand

Efficient organism-level transport depends on exchange surfaces that keep gradients steep:

• Large surface area (folding/branching)

• Thin barriers between fluid compartments and cells

• Continuous renewal of external medium and internal transport fluid (ventilation and circulation)

Countercurrent systems maximise transfer

Some organisms arrange flows in opposite directions to maintain a gradient along the entire exchange surface.

Countercurrent exchange in fish gills is illustrated by showing water flow over lamellae opposite to blood flow in capillaries. Because the two fluids move in opposite directions, a diffusion gradient for $O_2$ is maintained across the full length of the lamella, maximizing net oxygen uptake. The diagram also emphasizes how exchange surfaces (thin barriers + large surface area) integrate with bulk flow to support whole-organism transport. Source

A countercurrent design supports efficient uptake or retention because:

• The gradient does not rapidly reach equilibrium at one end

• Transfer can continue along the full length of the exchange region

• Whole-organism delivery (bulk flow) and cellular uptake (membrane transport) work together to sustain performance under changing conditions

Coordinating transport in response to environmental change

To meet the syllabus emphasis on responding to environmental change, organisms regulate transport by altering:

• Pump activity and transporter abundance in specific epithelia (adjusting ion and solute movement)

• Permeability of membranes at key interfaces (changing how readily water or solutes cross)

• Flow rates through organs (changing delivery and removal capacity)

• Surface exposure at exchange sites (opening/closing structures to balance competing demands)

Examples of coordinated responses include:

• Changing ventilation and perfusion to match oxygen demand

• Redirecting blood flow to muscles during activity or to skin during overheating

• Adjusting root uptake, vascular flow, and leaf exchange to balance carbon gain with water conservation during drought or heat