AQA Biology AS Level Unit 2 thread

Amazing thanks, a lot, perfect explanation

What are the standard raw marks out of 85 needed for an A?
Is the best way of getting an A practicing applications q's considering you know the content?
Are there more application questions on Unit 2 compared to Unit 1?

I think probably 63 out of 85. I actually enjoy unit 2 more unit 1

I need to revise unit 2 anyway, so I may as well explain

Leaf adaptations and structure

Leaves are adapted to gas exchange in multiple ways-
No living cell is far from external air
diffusion occurs in the gas phase
large surface area due to thin, flat shape
small pores called stomata in abundance for gas exchange
interconnecting air spaces through mesophyll

There is a diagram of the structure of a leaf in your book, so I won't explain that, apart from a few key points. The palisade mesophyll layer is where the majority of photosynthesis takes place, so the cells must have adaptations to optimise this. These include the regular arrangement of the cells, so that every cell is in the optimum position for absorption of sunlight, a large vacuole so that all chloroplasts in the palisade cells are forced to the surface of the cell so to be exposed to more sunlight, and an abundance of chloroplasts. The air space in the spongy mesophyll layer is where all of the oxygen and carbon dioxide diffuse through, and gas enters and leaves the leaf through many stomata, usually on the lower epidermis, which can be closed and opened to certain degrees by the guard cells that control them. In order to prevent water loss plants can shut stomata, for example.

Movement of water through roots

Root hairs are the exchange surfaces on the roots that are responsible for the absorption of water and mineral ions. Root hairs are long, thin extensions, so provide a large surface area through which gases and ions can diffuse , and have a very thin surface layer so the diffusion pathway is short. They stick out into the soil and are surrounded by soil solution, although that is mostly water, so the water potential of the soil solution is higher than the root hair cell contents because it contains many minerals and ions, so water moves by osmosis into the roots along the water potential gradient. Water can then travel by two means, the first being:-

The apoplastic pathway

The simple one I think, this is where as water is drawn into the endodermal cells, more water is pulled in due to the cohesive properties of water, creating a tension that pulls water along the cell walls of the root cortex. The mesh like structure of the cortex cell walls allows for an easy movement with little to no resistance

the symplastic pathway

the slightly harder one, I have "sympathy for those who have to learn it". This is where osmosis is involved, and is a little more complex:-
-> Water that has entered by osmosis increases the water potential of the root hair cells. These now have a higher water potential than first neighbouring cortex cell, so water moves by osmosis into the cortex cell, along the concentration gradient. The first cell now has a higher water potential than the next, so the process repeats itself. The loss of water from the first cell in the cortex to the next lowers its water potential so it draws in even more water by osmosis, and the process continues, think of it like a wave of water potential. It moves this way along the cytoplasm of the cells, and moves between cells along a thin connected bit of cytoplasm called the plasmodesmata.

Water into the xylem

When water arrives at the endodermis by the apoplastic pathway, a ring of waterproof tissue called the casparian strip stops the water moving across the cell wall, meaning it is forced into the symplastic pathway, and it is because of this that all water arrives at the xylem in the symplastic pathway. However, the water potential of the xylem is higher than the surrounding cells, so water cannot enter passively by osmosis. This is remedied by the fact that carrier proteins in the endodermal cells actively transport mineral ions into the xylem, lowering its water potential to less than the surrounding cells and allowing water to move into the xylem by osmosis. Because it involved active transport, this process is not passive and requires energy. This process as a whole is called root pressure. Root pressure can be reduced by metabolic inhibitors to prevent energy release required for active transport, or decrease in respiratory substrate availability.

Movement of water up the stem

Because water is cohesive, it forms a long, unbroken column of water running the length of the xylem, and adheres/sticks to the xylem walls. Tension is created in this column because water leaves the leaf by transpiration, and since all water molecules are to an extent attracted to each other, the loss of water molecules into the atmosphere drags other water molecules forward, meaning the column of water in the xylem is pulled by the created tension up the xylem constantly, then transported across the leaves via the apoplastic or symplastic pathway. This is referred to as the transpiration stream, or transpiration pull.

Cohesive properties- Water is a polar molecule, so has poles of different charges, the hydrogen and a partial positive charge, and the oxygen is a partial negative charge. Due to this, nearby water molecules are attracted to the water molecule, with the positive hydrogen being attracted to the negative oxygen and vice versa, so all molecules of water attract each other. (not on the syllabus but may help you understand!)

I hope you understand when i say i can't be bothered to type up all the factors affecting transpiration and xerophytic adaptations as well, but what i've said here is basically a crash course in movement of water all the way through the plant

thanks for posting, and good luck in your exam

hey! thanks for posting! 😛

one thing, though 😕 what do you mean by 'respiratory substrate'?

Hi is this what you mean because even at that I am getting 2 cycles because if the first cell replicates, it forms 2 cells. There are now 3 cells. All 3 cells replicate to form a total of 6 cells, there is now 9 cells all together hence 2 cycles Like I have shown in my diagram below

What do u guys think is gnna come up on this paper seeing as the unit 1 one was not really about 'actual' biology

Posted from TSR Mobile

Could someone go over tissue fluid? As I have no clue when it comes to that.
Thank you.

Tissue fluid is the fluid which contains all the necessary cellular respiratory substances such as respiratory gases, glucose, proteins and salts and is therefor the means by which these materials are exchanged between the blood and the cells.

Tissues fluid is formed from blood plasma and so provides a constant environment for these cells to exchange substances with.

The pumping of the heart created a pressure called hydrostatic pressure at the arterial end of the capillaries and this forces blood plasma out of the capillaries and once this plasma has left the capillary, it is now called tissue fluid.

Large molecules remain in the capillary such as the proteins and this lowers the internal water potential of the capillary so that the wp of the surrounding tissue fluid is higher.

As the fluid has left the capillary, the pressure within the capillary reduces as a result so that as the blood within the capillary travels to the venal end, it's pressure is significantly lower.

This reduction in pressure as well as the increased internal water potential of the capillary means that water now re-enters the capillary from the tissue fluid via osmosis and any remaining tissue fluid outside the capillary is removed by the lymph nodes of the lymphatic system which transport this excess fluid back into the circulatory system.

Hope this helps

Posted from TSR Mobile

Thank you, one part that confused me, you said that the internal water potential in the capillary is reduced, then in the last part said it is increased, is it possible for some clarification on this?
Thank you!!

Could someone go over cohesion tension theory please?
Thanks.

Anyone else find it hard to do past papers as they just take so damn long??? its like a marathon.

Do one, then have a break and then do another one, pace yourself