explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins

Transport Mechanisms – Companion Cells and Phloem Loading

Learning Objective

Explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins.

1. Overview of Phloem Structure

In angiosperm leaves, the phloem consists of two main cell types:

• Sieve‑tube elements (SEs) – long, enucleate conduits for bulk flow of photosynthates.
• Companion cells (CCs) – metabolically active cells that remain attached to each SE and regulate loading and unloading of assimilates.

2. Role of Companion Cells in Assimilate Loading

Companion cells are the “gatekeepers” that move sugars (mainly sucrose) and amino acids from the mesophyll into the sieve‑tube lumen. This process is called phloem loading and can occur via two pathways:

1. Apoplastic loading – assimilates cross the cell wall space (apoplast) and are taken up by carrier proteins in the CC plasma membrane.
2. Symplastic loading – assimilates move cell‑to‑cell through plasmodesmata directly into the SE‑CC complex.

Most A‑level curricula focus on the apoplastic route because it explicitly involves proton pumps and cotransporters.

3. The Proton Gradient – The Driving Force

At the heart of apoplastic loading is an electrochemical gradient of protons (H⁺) generated by the plasma‑membrane H⁺‑ATPase (a proton pump) in the companion‑cell membrane.

• The pump hydrolyses ATP to export H⁺ from the cytosol to the apoplast.
• This creates a low‑pH, high‑positive‑charge environment outside the cell and a more negative interior.

The resulting proton‑motive force (PMF) is used by secondary active transporters to move sucrose and other solutes against their concentration gradients.

4. Cotransporter Proteins Involved in Loading

Two major families of secondary transporters use the PMF:

• Sucrose‑H⁺ symporters (SUTs) – transport sucrose into the CC together with H⁺.
• Amino‑acid‑H⁺ antiporters (e.g., AAPs) – exchange external H⁺ for internal amino acids.

5. Step‑by‑Step Mechanism of Apoplastic Loading

1. Photosynthesis in mesophyll cells produces sucrose, which diffuses into the apoplast.
2. The H⁺‑ATPase in the CC plasma membrane pumps protons out, lowering apoplastic pH (\overline{5}.5) and generating an electrochemical gradient.
3. Sucrose‑H⁺ symporters bind one sucrose molecule and one (or two) protons from the apoplast.
4. Binding triggers a conformational change that releases sucrose and H⁺ into the cytosol of the companion cell.
5. High cytosolic sucrose concentration drives diffusion of sucrose into the adjacent sieve‑tube element through plasmodesmata.
6. Protons that entered the CC are recycled back to the apoplast by the H⁺‑ATPase, completing the cycle.

6. Energetics

The overall reaction for one sucrose molecule transported by a SUT can be expressed as:

\$\text{Sucrose}{\text{apoplast}} + n\text{H}^+{\text{apoplast}} + \text{ATP} \rightarrow \text{Sucrose}{\text{cytosol}} + n\text{H}^+{\text{cytosol}} + \text{ADP} + P_i\$

where n is typically 1–2, depending on the transporter isoform.

7. Comparison of Loading Strategies

8. Key Points to Remember

• Companion cells are metabolically active and contain abundant mitochondria to supply ATP for the H⁺‑ATPase.
• The proton gradient is the central energy source for secondary active transport of sucrose and amino acids.
• SUT proteins are examples of cotransporters that couple sucrose uptake to proton influx.
• After loading, the high osmotic pressure in the SE‑CC complex draws water from surrounding tissues, generating the pressure‑flow that drives long‑distance transport.