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2. How was the experiment of Miller and Urey important in illustrating the nature and practice of
science?
(A) Urey and Miller’s experiment was based upon several different hypothesises about the
origin of life.
(B) The experiments were designed so that other scientists could not replicate them.
(C) Urey and Miller’s experiments were designed to test one hypothesis about the origin of
life.
(D) Urey and Miller proved they could do an experiment that no one else wanted to do.
Hi, sssona09!
In these multiple choice questions, a good tactic is to disregard two completely incorrect options. This will make it easier to distinguish which answers you're looking for.
As for the Urey-Miller experiment, the significant feature of the Urey-Miller experiment was that it confirmed Haldane and Oparin's theory about the origin of life. By simulating the conditions that they found on the primitive earth, they decided to test the theory in the 1950's using hypothesis that complex organic molecules could have been created using inorganic molecules through slow reactions.
Thus, we can conclude that the correct answer is (C) as they only performed the experiment to prove Haldane and Oparin's hypothesis.
Is anyone able to help me in differentiating xylem and phloem? just like key points or any links to good diagrams? for some reason i have so much trouble with this
Hi, leighshapiro!
The defining features of the xylem and phloem are in its structure and mechanisms. I recommend drawing up a table; it should include the structure (living v dead cells, defining features), mechanism (and what they are called), as well as what they transport. This will give you an idea as to what you need to include when differentiating the two.
Xylem:
The xylem is composed of dead cells that lie end to end. A good way to remember this is the connotation of the 'x' eyes (x_x) as dead.
Xylem = dead cells.
The structure of the xylem involves the dead cells disintegrating and producing
continuous,
hollow, fluid filled tbues.
Another defining feature about the xylem is what it transports: water and mineral ions, which travel only in an upward motion.
The best model that scientists have hypothesised about the movement of water and mineral ions around the xylem is the
Transpiration-Adhesion-Cohesion-Tension model. You can remember the acronym (TACT)!!
The movement of water and mineral ions start from the soil. Water and mineral ions diffuse from the soil into the plants via the plant roots. Water movement is referred to as
osmosis, which is passive transport, whereas the mineral ions are both
active and
passive.
Since the transport in the xylem is uni-directional, meaning the flow of materials occurs in one direction only, the only way to get to the leaves is upwards. This process covers both,
adhesion and
cohesion.
Adhesion: refers to the process where water molecules are bound to the
walls of the xylem.
Cohesion: refers to the process where water molecules are bound to one another.
The way I'd like to remember this concept is the whole notion of cohesion. When we have cohesion in a classroom, everything makes sense. We work together to understand. This means that they are 'helping' each other (ie binding to one other).
The stream of movement then transpires in the leaves and into the open air. The notion of water and mineral ions moving up to the leaves is referred to as:
transpiration pull. I'd like to remember this as: our end result is the transpiration into the leaves, but in order to do this, we need to be pulled up (ie. transpiration pull).
Finally, water is lost due to evaporation through tiny pores, called stomates.
That's the process of the transportation of the materials inside the xylem!!
Phloem:
Unlike the xylem, the phloem consists of living and connected cells that run from leaves to roots. Their defining structures are: sieve tubes and companion cells.
Sieve tubes: series of cells joined end to end with perforated cross walls between cells (sieve plates) that transport the sugars throughout the plant.
Companion cells: specialised cells with a nucleus that always appear with the sieve tube element and serve to assist in the active loading and unloading of nutrients into and out of the sieve elements.
The mechanism used is:
pressure-flow hypothesis. Some schools like to use 'source-to-sink theory'. The HSC, afaik, don't mind which you use, so long as you describe their mechanisms.
This describes the processes responsible for the movement of sugars through the phloem tissues.
Glucose that is produced in the leaves need to be converted into sucrose. Glucose is a monosaccharide (single sugar), whereas sucrose is a disaccharide (double sugar), which is composed of glucose and fructose.
Sucrose is then loaded from the leaves (the 'source', so to speak) into sieve tubes via active transport. The energy used to carry sucrose down the phloem tube comes from the companion cells. Water then follows the sucrose, moving into the phloem tissue via osmosis via passive transport. This water comes from the xylem tissue which lies adjacent to the phloem.
Now, the presence of water increases the pressure inside the sieve tubes, forcing sucrose to flow from one sieve tube cell to another. In this way, sucrose flows down the phloem. When sucrose reaches the tissues that require it (like the roots), it is 'unloaded' from the sieve tubes via active transport. At this point, water molecules leave the phloem via osmosis.
I personally think the pressure-flow is harder than the TACT model, just because there's a lot more going on. But these are the distinctive elements of the two. Another thing you need to note is that: xylem transports materials via
transpiration, while the phloem transports nutrients via
translocation.
If you're still confused, watch these videos to understand more about the models and tissues.
Cohesion, Adhesion & Surface Tensionl[1]Pressure Flow Hypothesis[2]Sources:
[1] Cohesion, Adhesion & Surface Tension. (2014). [video] bionerdery.
[2] Pressure Flow Hypothesis. (2017). [video] Directed by D. Weber. Biogene.
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