First 400 words of the document:
Organisms exchange substances with their environment (AQA AS
Biology) PART 5 of 5 TOPICS
Mass transport in plants:
Transpiration is the water loss through the leaves via the stomata. Cohesion forces between water
molecules allows water to be constantly absorbed through the soil. Adhesion forces between the
water molecules and the walls of the xylem causes narrowing of the xylem vessel for faster
As well as water that needs to be transported to all parts of the plant, so does sugars and other
nutrients. Sources are referred to as leaves as this is where the sugars are made and the sinks are
referred to as shoots and root tips as this is where the nutrients need to go to. Translocation is the
process involved to move the nutrients by the mass flow hypothesis in the phloem. Phloem cells are
connected together to make phloem vessels by sieve plates with the pores called sieve tube elements.
Cells known as companion cells aid with the translocation. Phloem loading takes place at the leaves
where the sugars made pass through the companion cells by diffusion down their concentration
gradient into the sieve tube elements. This lowers the water potential and so water from the
neighbouring xylem vessel moves by osmosis down its concentration gradient into the phloem vessel.
As a result of the water and sugars, high pressure is created and so the water and sugars move down
the pressure gradient to the sink end. The sugars then go through various active transport methods
depending on the cell type into the cells at the sink end. As the sugars pass through the companion
cells a high water potential is created in the phloem vessel compared to the xylem vessel and so water
moves down its concentration gradient into the xylem by osmosis but primarily due to transpiration.
NB: A higher water potential in the phloem vessel near the sink end is established compared to the
xylem vessel because the xylem contains nutrients such as nitrates and magnesium that have been
absorbed through the soil making the water potential lower than the water potential in the phloem
vessel as the nutrients are being taken out.
] That's all that you need to know about mass transport in plants [