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OCR Biology AS F211 May 2012

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Reply 280

skylight17

Original post by aisha302

could someone please explain massflow? & experimental evidence for it..

Mass flow is the theory by which we think solute transport occurs in plants. Any area where sucrose is produced in a plant is known as a source, and any area where it is taken out (usually, used in respiration) is known as a sink.
Sucrose is actively transported into the sieve tubes of the phloem at the source (i.e. source), lowering the water potential inside the sieve and so water enters the tubes via osmosis, creating a higher pressure inside the sieve tubes at the source.

At the sink, sugars leave the phloem to be used up, increasing the water potential inside the sieve tubes, so water leaves via osmosis, lowering the pressure inside the sieve tubes.
The result is a pressure gradient between source to sink, pushing sugars to where they're needed.

Supporting evidence

When the phloem is cut, sap oozes out, showing a pressure gradient.
Suitable water potential gradient between leaves and other plants, in theory.
Phloem sap has a high pH, which is to be expected since hydrogen ions are actively transported out of the cell.
ATP is present in phloem sieve elements in high numbers since it is required for active transport of hydrogen ions.

Evidence against

Sugar travels to many different sinks.
Sieve plates are a barrier to mass flow.
Doesn't require living cells, but phloem cells are alive.

Reply 281

itsmehello

Original post by Joey952

(1) Cells are genetically different in meiosis and 4 cells are produced in meiosis whereas in mitosis only 2 cells are produced.

(2) CO2 reacts with H20 to form carbonic acid H2C03.
H2CO3 splits up to give H+ ions and HC03-
H+ ions have grater affinity to haemoglobin causing oxygen to leave haemoglobin. so when there is more CO2 the dissociation curve shifts to the right because oxygen relased released.
(3) Cl ions move in to maintain the shape of the cell.

(4) The experimental model for mass flow
if you remove bark, and bulge in formed

1) 2/2

2)4/4

3) 0/1 It doesn't maintain the shape it maintains the charge, because HCO3- move out so to balance the charges again Cl- moves in

4)0/2 The question was asking for evidence that ATP is used you gave evidence that phloem is used. Mass flow does not require ATP!

The answer would have been
-the companion cells have many mitochondria
-when a metabolic poison that inhibits the formation of ATP is used translocation stops
or
-The rate of flow of sugars in the phloem is so high that ATP must be used to drive the flow

Reply 282

Jamie66

Original post by Axion

Not on the specification.

Actually it is, it's the very last point but it's on there

Reply 283

mathsclown

Original post by Axion

Not on the specification.

Though it's not in the syllabus, it can crop up, remember mass flow is a build up of pressure at the source so moves to lower hydrostatic pressure at the sink, the mass flow model helps to show this.

Reply 284

arvin_infinity

Spindle fibres break down in telophase ?

Reply 285

Axion

Original post by Jamie66

Actually it is, it's the very last point but it's on there

it's really not.

Reply 286

Axion

Original post by arvin_infinity

Spindle fibres break down in telophase ?

For telophase All I'd put is that chromatids reach poels of the cell
nuclear enveolopes form and chromosomes decondense

Reply 287

Student-Andrew

Questions for people - Explain an advantage of a double circulatory system [2]

How does am electrocardiogram monitor the activity of the heart [4]

On an ECG what does Q,R,S wave indicate [2]

Reply 288

arvin_infinity

Original post by Axion

For telophase All I'd put is that chromatids reach poels of the cell
nuclear enveolopes form and chromosomes decondense

Speaking of which, what does it actually mean when we say chromosomes decondense ..is it the same as saying uncoiling or two different things

Reply 289

Unsworth

My list for what I think is likely to come up (also what I kinda hope) is:

-Mitosis
-Heart; this could be anything from blood flow, to waves of excitation to ECG traces.
-Phospholipid bilayer; I don't think there will be anything on cell signalling though
-Carriage of oxygen/carbon dioxide
-Translocation
-Movement of water in xylem

There was nothing on Mitosis, the heart, the phospholipid bilayer, translocation or carriage of oxygen in the January paper. Although there was content on xylem and phloem, it wasn't specifically about the movement of water/sugar.

That being said, OCR are awful and so unpredictable.

Reply 290

Axion

Original post by Student-Andrew

Questions for people - Explain an advantage of a double circulatory system [2]

How does am electrocardiogram monitor the activity of the heart [4]

On an ECG what does Q,R,S wave indicate [2]

double - extra pressure generated by heart on second pump
ECG - two pads either side of the heart. It measures the potential difference (no diea what would get you the 4 marks)
QRS - ventricular systole the electrical signal

Reply 291

itsmehello

Original post by Unsworth

1. - Meiosis produces genetically different different cells, whereas mitosis produces identical cells.
- Meiosis produces cells with half the number of chromosones, whereas mitosis produces cells with full number of chromosomes.

2. Bohr effect is the dissociation of oxygen from oxyheamoglobin due to hydrogen ions. As carbon dioxide levels in the blood increase, there is a higher concentration of hydrogen ions. These cause oxygen to dissociate from oxyheamoglobin. When carbon dioxide is present, heamoglobin is less saturated with oxygen. Therefore as the carbon dioxide level increase, more oxygen is readily released from heamoglobin, which is essential for aerobic respiration to continue.

3. They move into red blood cells to maintain their charge.

4. -Companion cells have mitochondria
- There are poisons available which inhibit the formation of ATP, as a result translocation doesn't happen.

1) 2/2
2) 4/4 Good answer!!
3) 1/1
4) 2/2

Original post by mathsclown

4 daughter cells that are genetically differen from each other and to the parent cell and have the haploid number of chromosomes

Bohr effect: respiring tissues produce carbon dioxide this means there is a low pO2, as carbon dioxide reacts with water to form carbonic acid this is broken down, the ion that plays a big role in the bohr effect is the h+ ion, as the oxyhameoglobin dissociates, the h+ ions take up their space on the hameoglobin, in respiring tissues this happens at a higher rate as more co2 is produced, as haemoglobin loses its affinity for oxygen so dissociates more as more h+ ions are produced. Hate wording it -.-

Hmm...use of a metabolic inhibitor in a phloem stops translocation and sieve tubes can only survive by help from companion cells which are pakced with mitochondria to provide atp to actively transport sucrose into the sieve tubes? Not sure guessed :/

1) 2/2
2) 4/4
4) 2/2

:smile:

Reply 292

skylight17

Original post by Axion

Not on the specification.

Clearly haven't done your revision throughly enough! :wink:

Very last point under Transport in Plants...

(m) describe, with the aid of diagrams, the
mechanism of transport in phloem involving
active loading at the source and removal at
the sink, and the evidence for and against
this mechanism (HSW1, 7a).

Reply 293

Axion

I hope the partial pressuer and dissociation curve doesn't come up. Its a real mouthful.

Reply 294

Axion

Original post by skylight17

Clearly haven't done your revision throughly enough! :wink:

yes - this is referring to mass flow. No experiment is needed. I have learnt the spec, hardly anything else as not really necessary.

Reply 295

skylight17

Original post by Jamie66

Actually it is, it's the very last point but it's on there

Just to reiterate what you said - see above!

Reply 296

mathsclown

Have to admit, the CGP revision guide is 10x better than the examiners text book!

Reply 297

skylight17

Original post by Axion

yes - this is referring to mass flow. No experiment is needed. I have learnt the spec, hardly anything else as not really necessary.

It's EXPERIMENTAL EVIDENCE, which is the points I made above about the for and against the mass flow mechanism - please read more carefully.

Reply 298

itsmehello

Original post by Student-Andrew

Questions for people - Explain an advantage of a double circulatory system [2]

How does am electrocardiogram monitor the activity of the heart [4]

On an ECG what does Q,R,S wave indicate [2]

1) As blood flows back through the heart after the pulmonary circulation it can then pump blood at a higher pressure so the blood flows to the tissue faster and blood also travels further.

2) The hearts contractions and waves of excitations can be detected through the skin, so sensors are attached to the skin and it is connected to a monitor, this is converted to a trace

3)Contraction of ventricles- not sure if I'll get two marks for that :colondollar:

Reply 299

scorpio22

Original post by Student-Andrew

Questions for people - Explain an advantage of a double circulatory system [2]

How does am electrocardiogram monitor the activity of the heart [4]

On an ECG what does Q,R,S wave indicate [2]

double : blood is pumped around the body twice in a full circulation,it keeps oxygenated to body and deoxygenated blood to lungs seperated to maintain the pressure.

ecg monitors the activity of the heart by then sensoring in the skin picking up traces due to heart contracting.

q,r,s indicates the excitation of ventricles.

(edited 11 years ago)