AS OCR Biology Unit 1

Sure!

First of all you have to know that the Xylem and the Phloem are Tissues (Tissues are a combination of cells that work for a specific function).

Xylem, the main things you need to know are:
-It transports water from the roots up the stem to the aerial(leaf) parts of the plant, however it can be transported down to the roots containing solutes to create a water potential gradient for water to come up to the stem.
-It has no cell contents, this is why it isn't considered as a cell.
-It has no nucleus.
-It has been lignified (impregnated with lignin) - this means xylem vessel is virtually dead.
-It is waterproof (by the lignin).
-Sometimes this lignifcation doesn't become complete and that's where you get the bordered pits from, so water can travel from column to column (adjacent vessels).

Phloem, the main things you need to know are:
-It transports sucrose and assimilates(sucrose-like substances).
-It is made out of two parts, the sieve tube elements and the companion cells.
((You can go over the sieve-tube elements and companion cells roles after))

Mass flow is just the movement of assimilates up or down the phloem.
Hydrostatic pressure is the pressure created by a fluid pushing against the sides of a container, this basically means the pressure that water(usually) has on a blood vessel for example, this hydrostatic pressure can push out fluids or it can be very weak that it doesn't affect the surrounding cells.

Hope this has helped you, if you still have any questions I will be delighted to help.
Student07.

Okay, mass flow is basically the movement of assimilates from source to sink, where you load the assimilates and then you transport it and then you unload it. Source -> Transportation via Sieve Tubes -> Sink (Usually at the roots to create a steep water potential gradient).

Remember Mass Flow can go either direction. Up or down the stem.
You're welcome.

Student07.

Thank you again!!! This has also helped a lot!

Out of all my AS-Levels, I tend to find Biology (and Maths haha) the hardest just because I don't know what the question is asking for!

So thank you for all your help

No Problem! If you have anymore questions feel free to PM me or ask via my ask.fm page (links on my profile page).