Year 12s: what will be the first way you research your future options? >>

OCR Biology A Exam Thread (Breadth - May 26, 2016 and Depth - June 7, 2016)

Scroll to see replies

Roots- there are two routes: The apoplast pathway where water goes through the cell walls, mostly by diffusion, until it reaches the endodermis cells in root where the path is blocked by a waxy strip called casparian strip, so now it has to go through the Symplast pathway where the water goes through the cytoplasm via osmosis, plasmodesmata connects connects the cytoplasm of plant cells.

Leaves: the water leaves the xylem and moves into the cell mainly by apoplast where it evaporates into the air spaces in the leaf. When the stomata is open, it diffuses out.

Stem: as the water is evaporated, this creates a 'tension' where which pulls more water upwards. Water molecules are cohesive so a whole column of whole water moves too. Also adhesion is partly involved as well since the water molecules are attracted to the walls of xylem vessel, it helps water rise up.

Good luck xo

THANK YOU SO MUCH :smile:

, good luck to you all :smile:

Reply 481

alevelyh

what PAAGs have you all done, we have done heart disection, light microscope , beetroot and benedicts test for sugars :/

Reply 482

plower

Yooooo have you guys learned of the inorganic ions and it's purpose???

Reply 483

alevelyh

has anyone studied inorganic and organic ions?
apart of biollogical molecules

Reply 484

asdfghjkl99

Whats selective pressure

Reply 485

Wonder_land

Can someone please explain cell mediated immunity? 😢

Reply 486

plower

Original post by asdfghjkl99

Whats selective pressure

Selective pressure are like environment factors, eg predation, disease, competition etc which creates a struggle for survival.

Original post by Wonder_land

Can someone please explain cell mediated immunity? 😢

Cell mediated is where T-cells come in, they surface covered with receptors that binds to antigens presented by antigen-presenting cell (phagocyte). Each t-cell has different receptor due to the complimentary antigens. When it binds to one, it activates t-cell where it undergoes clonal explansion- divides producing clones which differentiate into different types: such as T helper cell- release substance to activate B cell, T killer cell that binds to antigen and kills pathogen, T regulatory cell that suppresses immune response from other white blood cell to stop mistakenly attack host body cell and lastly Memory T cells

Reply 487

thompsonnn

Original post by Wonder_land

Can someone please explain cell mediated immunity? 😢

Cell mediated immunity is the immune response to cells that have undergone changes (such as by a virus infection).

Macrophages digest the pathogen by phagocytosis. The antigens from the pathogen combine with glycoproteins in the cytoplasm of the macrophage called the major histocompatibility complex (MHC). The cell then presents these antigens on its own cell-surface membrane and becomes and antigen-presenting cell (APC).

The production of the APC stimulates T-helper cells (due to complementary cell-receptors on its membrane to the antigens) to produce interlukin-1, which in turn stimulates the production of interlukin-2. This induces the rapid division and differentiation of the T-helper cell into T-killer cells or T-memory cells. The T-helper cells produce the protein perforin which causes the formation of pores in pathogens and thus, lysis. T-memory cells give rapid response if the pathogen invades the body again.

Reply 488

alevelyh

i havent learnt inorganic ions

Reply 489

fteam

does everyone know the gas exchange in bony fish and insects?

Reply 490

sugi10

What are the diseases i need to know ? what kind type are they? eg- virus or bacteria

Reply 491

Fabthelion

Original post by fteam

does everyone know the gas exchange in bony fish and insects?

I wont go into too much detail cos im rushing to school butbCounter current system in fish -blood travels opposite way to water coming in and oxygen diffused in water diffuses into fish and concentration gradient of o2 is greater than concentration in blood..pretty much that but not in loads of detail as said..

Insect -o2- spiracles - treacheae - tracheoles whilst co2 moves down its own cg towards spiracles..o2 diffueses inyt fluid in tracheoled ehich is drawn by osmosis later on leaving o2 which directly diffuses into respiring cells

Reply 492

Fabthelion

Original post by sugi10

What are the diseases i need to know ? what kind type are they? eg- virus or bacteria

Bacteria..tb bacterial meningitias ringrot
Virus - hiv influenza toabacoo mosaic
Fungi - athletes foot ring worm black sigatoka
Protoctist - malaria and potato late blight

Reply 493

Rectum_resizer

Cell mediated immunity is to do with antigens processed inside cells. T helpers will bind their receptors to an antigen on an apc, they will then secrete interlukins to stimulate growth of t helpers, t memory and b plasma lymphocytes.
This is from memory so may have some inaccuracies.

Reply 494

adeninesbff

Original post by sugi10

What are the diseases i need to know ? what kind type are they? eg- virus or bacteria

VIRUS:
- Tobacco Mosaic (Plants)
- HIV and Influenza (Humans)

BACTERIA:
- Bacterial Meningitis (Humans)
- Ring Rot (Plants)

PROTOCTISTA:
- Late Blight (Plants)
- Malaria (Humans)

FUNGI:
- Athletes Foot
- Black Sigatoka

Reply 495

Rectum_resizer

Can someone please explain Benedicts test as I don't have a ****ing clue

Reply 496

Black Rose

Original post by Gogregg

Yes, we do need to know it! You'll get the formula, but we need to work out observed - expected and square it then do some other stuff and compare answer to the critical value at p=0.05.Observed value is using your null hypothesis (there will be no difference) so your saying there will be the same number for each testWishing everyone the best of luck today!

Another thing to learn :frown:

Teachers have only told us about Simpson's index, standard deviation and student's t-test. :grumble:

Original post by James3000117

You're thinking of the Hardy-Weinberg principle. Gregg explajns chi squared well, just remember when subbing in that fO is observed values and fE is expected values and you should be able to work from there.

Do you have a link to a good website or something to learn it?

Reply 497

MattLS

Original post by Rectum_resizer

Can someone please explain Benedicts test as I don't have a ****ing clue

This tests for sugars, there are two types of sugars: Reducing (all monosaccaharides) and Non reducing.

Add Benedict's reagent to the solution and heat, If there is a reducing sugar a reddish precipitate will form (Cu2+ [blue] has been reduced to Cu+ [red]).
If there is no change it is probably a non reducing sugar.
To confirm this get a new sample of the solution, add HCl to hydrolyse the polysaccharide to a monosaccharide. The add an alkali to neutralise the solution. Add benedicts reagent and this time it should form a red precipitate.

Reply 498

User1452521

Original post by Dr Adaeze

Oh thank sweet merciful Jesus Christ Jehovah Allah Science Karma The Universe YESSSSSSSSSSSSSSSS. Are you 100% sure?! *Cries from happiness*

100% sure

Reply 499

fteam

yes thats pretty much it, in the bony fish there is 5 gills covered by the operculum, each gill consists of two row of gills called filaments (primary lamalle) which are then folded to form seconday lamallae, providing a large surface area so that when blood flows along gill arch to the filaments there is a large surface area to maximise gaseous exchange during the process known as countercurrent flow where blood flows opposite direction to the water to maximise the absorption of 02.You also need to know the ventilation as the buccal cavity lowers water fills the mouth and the operculum is coordinated so the operculum moves outwards so opercular cavity pressure is lowered helping water move in.