0610 · 8.4Translocation

Translocation, sources and sinks

0:00A tomato grows bigger and sweeter, even though it cannot make enough food for itself. Underground, roots keep growing without receiving any light. So how do these parts get the materials they need? The answer is a transport process called translocation. But this is not simply a journey from the top of a plant to the bottom. Sometimes materials travel towards the roots. Sometimes they travel towards growing shoots. And the same storage organ can receive materials at one time of year and supply them at another.

0:26By the end of this video, you will understand what translocation moves, how to identify where that journey starts and ends, and why those starting points and destinations can change. First, we need just a little background. Photosynthesis produces glucose. Plants can convert this glucose into sucrose, a sugar transported around the plant. Plants also need amino acids, which are used to make proteins. Growing cells need these proteins to build new structures.

0:52The transport tissue involved here is phloem. Where phloem is found in roots, stems and leaves is covered in the video on xylem and phloem. For this topic, keep one connection firmly in mind. Phloem transports sucrose and amino acids between parts of the plant. Here is the precise definition. Translocation is the movement of sucrose and amino acids in phloem from sources to sinks. There are three important pieces in that definition.

1:16Sucrose and amino acids are the substances being transported. Phloem is the tissue carrying them. And sources and sinks describe where the substances come from and where they go. Sources are the parts of plants that release sucrose or amino acids. Sinks are the parts of plants that use or store sucrose or amino acids. Think of a source as a supplier and a sink as a receiver. A delivery starts with a supplier releasing materials and ends with a receiver taking them in.

1:43But unlike a shop with a permanent sign outside, a plant organ does not necessarily keep the same role throughout its life. The key question is not just which organ you are looking at. It is what that organ is doing at that particular time. Is it supplying substances to other parts, or receiving substances to use or store?

Sources and sinks in plants

2:02Let us start with a mature leaf in the light. It carries out photosynthesis and produces glucose. Some of that glucose is used within the leaf. Some can be converted into sucrose and released into the phloem for transport elsewhere. When the leaf supplies sucrose to other parts of the plant, it acts as a source. Notice that a source does not have to give away everything it makes. The leaf still needs materials for its own cells.

2:25What matters is that it supplies some to other parts. Now follow that sucrose towards a growing root tip. The cells there are dividing and growing. They need a supply of materials even though they cannot photosynthesise underground. Sucrose arriving at the root can be converted into glucose, which can be used in respiration to release energy. Amino acids arriving there can be used to make proteins. Because this growing region uses the substances it receives, it acts as a sink.

2:51Other growing regions can also be sinks. A developing fruit needs materials as it grows. A young shoot needs materials to form new cells. A young leaf that cannot yet supply enough of its own needs may receive sucrose from older leaves. That young leaf is a useful reminder. Being a leaf does not automatically make an organ a source. A developing leaf may initially be a sink. Later, when it is mature and supplying sucrose to other parts, it can become a source.

3:18A sink does not always use its incoming materials immediately. It may store them instead. Consider a potato tuber. This is a swollen underground stem that stores food reserves. While the plant has mature, photosynthesising leaves, sucrose can travel from those leaves to the developing tuber. In the tuber, the arriving sugar can be converted into starch for storage. The tuber is therefore a sink during this stage. Receiving sucrose and converting it into a stored substance still makes it a sink.

3:47The transported material does not have to remain unchanged after arrival. Why store these reserves? Because the plant may later need to grow when its leaves are not yet able to provide enough food. Storage separates the time when materials are available from the time when they are needed. This gives us the central reason that source and sink roles can change. A part that is building up reserves receives materials.

4:08A part that is releasing those reserves supplies materials.

Potato tuber source and sink

4:12Let us work through a complete example. Imagine a potato tuber growing on a leafy plant during summer. Later, that tuber begins to sprout before the new leaves can photosynthesise enough to support the growing shoot. The question is this. What role does the tuber play at each stage, and why? Start with summer. We are told that the plant has leaves supplying sucrose and that the tuber is building up reserves.

4:35That tells us where the sucrose comes from and what happens when it arrives. The mature leaves release sucrose, so they are sources. The tuber receives sucrose and stores reserves, so it is a sink. The direction of translocation is from the leaves to the tuber through the phloem. Now consider sprouting. The young shoot needs materials for growth, but its leaves cannot yet provide enough. So the plant must draw on its stored reserves.

5:00Stored starch in the tuber is broken down, and sucrose is made available for transport to the growing shoot. The tuber now releases sucrose, so it acts as a source. The growing shoot uses the incoming substances, so it acts as a sink. The tuber has not moved or become a different organ. Its activity has changed. Earlier, it received materials for storage. Later, it released materials to support growth.

5:22That is why its role changes. Notice the method we used. First, identify the time or stage of growth. Next, identify which part releases the transported substances. Then identify which part uses or stores them. Finally, trace the journey from source to sink. This also explains a common misunderstanding about direction. Translocation is not always downwards. A mature leaf can supply a root below it and a growing shoot above it.

5:51Across the plant, transport can therefore occur upwards and downwards along different phloem pathways. The useful rule is from source to sink, not from top to bottom. Another mistake is saying that phloem transports glucose. Photosynthesis produces glucose, but the sugar named in the definition of translocation is sucrose. Keep the substance being made in photosynthesis separate from the sugar being transported. Also, do not forget amino acids. Translocation is not just sugar transport.

6:19Growing tissues need amino acids to make proteins, so amino acids belong in the definition too. And a sink is not a place where useful materials are wasted or lost. It is a place where they are used or stored. A growing root and a storage tuber are both sinks, but for different reasons. Finally, avoid treating source and sink as permanent labels. Saying that all leaves are sources, or that all storage organs are sinks, misses the important idea.

6:44Always look at what the organ is doing now.

Exam answer and recap

6:47Let us connect this to an exam-style question. A carrot plant stores reserves in its root during its first growing season. At the start of its next growing season, reserves in the root support a new shoot. Describe translocation and explain why the storage root acts as a sink at one time and a source at another. There are two jobs here. Describe asks you to give the relevant features of translocation.

7:09Explain asks you to connect the changing role of the root to what happens to its reserves. Build your answer from separate, precise points. Translocation is the movement of sucrose and amino acids in phloem from sources to sinks. During the first growing season, the storage root receives sucrose for storage. The storage root therefore acts as a sink. At the start of the next growing season, stored reserves are broken down and sucrose is released from the root.

7:38The storage root therefore acts as a source. Sucrose is transported to the new shoot, where it is used for growth. That explanation works because it links each role to a process. Simply saying that the root changes with the seasons would not explain why it changes from sink to source. Let us finish with the whole picture. Translocation moves sucrose and amino acids through phloem from sources to sinks.

8:01Sources release these substances. Sinks use or store them. A mature photosynthesising leaf can supply other parts. Growing tissues receive materials for growth. Storage organs receive materials when building reserves, but can supply materials when those reserves are mobilised. So whenever you meet a translocation question, ask two things. Who is supplying, and who is receiving? Once you identify those roles at that particular time, the direction of transport and the reason for it become much easier to understand.