# Make the internet a better place to learn

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## Who were the first people known to produce food through agriculture?

Mandira P.
Featured 2 months ago

Agriculture started independently, in different locations of the world. At least eleven centres of origin are recognised. People of each centre domesticated only certain species of plants and animals.

#### Explanation:

Ancient people of China, Peru, Mesopotamia, Indus valley and Nile valley became engaged in farming at least 10,000 years ago. Pigs were domesticated in Mesopotamia 13,000 years ago followed by domestication of sheep and cattle in nearby areas. This is why, fertile crescent of near east is regarded as the birthplace of agriculture, and it is here, man first cultivated wheat.

That does not mean that contributions from other centres are of less importance. Maize, papaya and potato were domesticated in the New World; edible mango, orange, tamarind and spices originated in south east Asia; while litchi and onions were first cultivated in China.

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## How does divergent evolution differ from convergent evolution?

Mandira P.
Featured 2 months ago

Generally evolution is divergent: I say so because that is how this earth has today become home to immensely diverse kinds of life.

• Divergent evolution simply means appearance of more than one descendant species from an ancestral species population. Mammalian forelimbs, for example, follow an ancestral pentadactyl limb plan but work very differently in different orders.
• Divergent evolution may lead to appearance of homologous organs. Divergence appears due to adaptation of related organisms in different environmental conditions and habit.

• We also find organisms which evolved superficial/plastic similarity in appearance or similar adaptations in habit to other organisms, with which they are not closely related. This is convergent evolution as exhibited by shark and dolphin: they belong to different vertebrate class but both are adapted to aquatic life.
• Convergent evolution leads to appearance of analogous organs where infrastructure of such organs may differ but the functional achievement remains same. This is because of the fact that similarity in appearance and behaviour evolves due to similar environmental pressure.

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Kate M.
Featured 4 weeks ago

See explanation below.

#### Explanation:

There are several ways this question could be answered depending on what is meany by spread.

Agricultural ideas and practices are spread when people move to new areas, when one group trades with another group, and generally speaking when groups come into contact with one another. Purposefully or not, ideas are shared and technologies and new species may accidentally be introduced.

Origin and spread of different crops:

For example, the Pilgrims in the United States learned to plant corn, squash, and beans together to maintain soil fertility from the Wampanoag. The use of animal drawn plows spread from Egypt south to Sudan.

The image below shows the spread of potato cultivation across the globe:

Ancient forms of drip irrigation have been found in Egypt and Afghanistan. In the 1920s Germans were using perforated pipes for irrigation, the first efficient drip irrigation system was developed in Israel and spread to the US and Australia in the 1960s. You can read more about how drip irrigation has spread here.

Today, agricultural ideas spread through the internet, TV, travel and tourism, international conferences on agriculture that are held to attract people from all over the world, and when companies purposefully travel to sell their products and technologies to new areas.

You may want to read about the Neolithic Revolution, history of agriculture, how coffee cultivation spread, or how drip irrigation usage has expanded globally.

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## What role does transpiration play in the water cycle?

G_Ozdilek
Featured 1 month ago

Transpiration is the evaporation of water from internal surfaces of living parts of plants (leaves, stems, etc.).

#### Explanation:

In water cycle, plants have critical roles. We know that forest areas guarantee continuation of streamflow due to water regulation in such areas compared to that in open (plant poor) areas.

Transpiration depends on some parameters, such as atmospheric humidity and temperature, area of plant leaves, stems, etc. Opening and closing the stomata is possibly the plant's most important means of regulating water loss via transpiration.

Transpiration accounts for approximately 10% of all evaporating water. Evaporation occurs from streams, lakes, seas, etc. When you think almost 70% of the Earth is covered by seas, you can understand the magnitude of transpiration.

Let me assume that 30% of evaporation occurs from land area. And one-third of this evaporation is called transpiration.

Plants get water via their root system. While they use water (photosynthesis), they also lose this water by transpiration. However, transpiration is a slower (regulated) process compared to that in evaporation from open water bodies. Soil, soil water, plant activity, meteorological parameters, etc. all have roles in transpiration.

Another important feature of plants in water cycle is that they minimize erosion. Rainfall droplets hit the ground with more than 9 meters per second velocity. If it is an open, bare soil, erosion is a big problem. However, rainfall droplets hit plants first and then hit the ground if surface is covered by green plants. Therefore, plant cover minimized erosion rates.

1

## What is the name of the macromolecule in which phosphorus is found?

Martin M.
Featured 1 month ago

Phosphorus if found in DNA, cell membranes and ATP.

#### Explanation:

In living organisms phosphorus is largely observed as a phosphate group, see the picture below.

As can be seen, the phosphate group has a negative charge. This makes this part of a larger molecule hydrophilic (it can form hydrogen bonds and can dissolve in ${H}_{\text{2}} O$, water)

DNA and other information carriers
DNA are large molecules that have a certain sequence, which codes for the make of proteins in the cell. These DNA strands consist out of 4 different bases placed with a certain sequence along a line.

In the picture above a very schematical single DNA strand is showed. The 4 different bases are coloured and connected to the backbone (the black horizontal line) in a certain way. This backbone is build up out of a lot of phosphate groups! This makes DNA negative and hydrophilic on the outside.

Cell membranes
Every cell has a membrane around it. You can see this like a border to enter or leave the cell. This border consists out of many phospholipids. These are showed below.

The hydrophilic head of a phospholipid is made up from the negatively charged phosphate group.

ATP, the energy storage
Certain molecules in the cells are used for the storage of energy. This energy can be used to activate other processes in the cell. An example of an energy storage molecule is ATP (adenosine triphosphate), which consists out of 3 phosphate groups!

Image Source

2

## What factors can affect river species diversity?

Kate M.
Featured 4 weeks ago

Many factors influence the alpha diversity found in a riverine ecosystem including the age of the river, dissolved oxygen, organic matter in the river, total suspended solids, pollution, introduction of invasive species, dams and changes to the river flow all affect biodiversity.

#### Explanation:

Many factors influence the alpha diversity found in a riverine ecosystem including age of the river, dissolved oxygen, organic matter, total suspended solids, pollution, introduction of invasive species, dams and changes to the river flow all affect biodiversity.

Generally speaking, greater heterogeneity leads to more niches available for exploration and more species exploiting those niches. Hence, a stream that varies in width, depth, organic matter, available sunlight, and other biophysical characteristics is going to be more diverse that a stream that is consistent in those same factors.

More organic matter (dead leaves and such which act as a source of food) generally leads to increased diversity(see here).

Higher levels of total suspended solids in the water is linked to lower diversity (see here).

Daming rivers or changing their flow is typically harmful and leads to lower diversity, as does pollution and the introduction of invasive species.

Stressors affecting San Francisco Bay Delta water quality, habitat condition, and ecosystem structure and functioning:

2

## How many resonance structures can be drawn for ozone?

Martin M.
Featured 1 month ago

There are 2 major resonance structures for ${O}_{3}$

#### Explanation:

We will first determine the lewis structure of ${O}_{3}$, with these rules in mind.

1. ${O}_{3}$ has 3 times the O atom (6 valence ${e}^{-}$). Therefore it has a total of $3 \cdot 6 = 18 \cdot$ valence electrons. $\frac{18}{2} = 9$ electron pairs
2. $3 \times 8 = 24 {e}^{\text{-}}$ needed for octet rule. This is $\frac{24}{2} = 12$ electron pairs.
3. $12 - 9 = 3$ bonding electron pairs
4. $9 - 3 = 6$ nonbonding electron pairs
5. To easily determine the formal charges on the atoms, we first propose a structure. The structure is drawn below. To propose a structure, draw one atom in the middle and the others around it. A bit of practice helps at this stage. there are 3 bonding electron pairs and 6 nonbonding. Be sure to give every atom 8 electrons!

In the lewis structure, every line represents 2 electrons.
Now have a look at the middle S atom. We may determine the formal charges with the formula:
Formal charge = $6 - 0.5 \times 6 - 2 = + 1$

Now compare this number 5 with the number of valence electrons that an O atom on its own has (use periodic table), which is 6 (see step 1). Now we see that 5<6, therefore it has a formal charge of $+ 1$.

Basically what you do with formal charge is to compare the electrons directly around the atom to the number of valence electrons it normal has. Since we know it should have 6 electrons, and we count 5 (see below), we can say that it has 1 electron less, and therefore it must have a $+ 1$ charge. When you get the hang of it, it goes faster.

Let's do the same for the oxygen on the right. According to the periodic table, it normally has 6 valence electrons. In the structure, it contains 6. So there is no formal charge.

The atom on the right has 7 electrons, therefore it has a formal charge of $- 1$.

Now to draw resonance structures, we move electron pairs around.

In above image, we indicate the movement of an electron pair with a double headed arrow. The charges do also change accordingly. To indicate that these structures are resonance structures, we place square brackets around them, with a two-sided arrow in between.

These two resonance structures are the major contributors to the resonance hybrid.

Two other structures can be drawn as well:

But these two structures contain both an oxygen atom that doesn't obey the octet rule (doesn't have 8 electrons around it). Therefore these structures are the minor contributors to the resonance hybrid.

The resonance hybrid is an approximation between the resonance structures. The major contributors are the most improtant, therefore the resonance hybrid of ozone will look like:

1

## What are the benefits of using hydrogen fuel-cells?

EET-AP
Featured 1 month ago

Fuel cells can be used to generate electricity with an increasing level of efficiency as better electrolytes are discovered. The benefits are described below:

#### Explanation:

1) Fuel cells can produce electric power from sustainable sources.
Hydrogen used to fuel the cells can itself be produced using electrolysis systems powered by solar panels or wind turbines.

2) The hydrogen can be produced and stored at any time so now there is a method of storing solar and wind energy as hydrogen fuel.

3) The products of fuel cells are water (${H}_{2} O$), and heat. There are no carbon emissions from most fuel cell operations.

4) The water produced can be re-used to produce more hydrogen ${H}_{2}$ fuel as stated above. The heat can be used to heat the power plant or captured to be re-used in other processes.

5) Fuel cells are quickly becoming more efficient in operation than internal combustion engines for powering cars.

6) Fuel cell powered vehicles can be used more safely indoors than any fossil fuel powered vehicles such as construction lifts and fork lifts.

7) Fuel cells can also be run on escaping methane gas $C {H}_{4}$ as from swamps or landfills. Carbon dioxide $C {O}_{2}$ will be produced, but this is not as undesirable as escaping methane.

https://en.wikipedia.org/wiki/Fuel_cell

1

## What are sources of arsenic in groundwater?

Mic
Featured 1 month ago

Arsenic can be found in naturally occurring bedrock and contaminate ground water or can infiltrate groundwater from weathering minerals and ores in addition to anthropocentric sources.

#### Explanation:

Arsenic can be found naturally in high concentrations in localized areas when drilling wells for drinking water can intersect bedrock with arsenic. Additionally, elevated levels of arsenic in groundwater can be near areas of weathering of arsenic containing minerals and ores. Arsenic can also pollute groundwater from various industrial activities such as abandoned mines that runoff or infiltrate groundwater aquifers.

Arsenic can travel in many other ways as described in the picture below:

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## How is ozone layer a pollutant?

Anonymous
Featured 1 week ago

Ozone is a pollutant only if it is present near the earth's surface such that we inhale it. It protects us from the sun's harmful rays when in the stratosphere.

#### Explanation:

Near the earth's surface, if inhaled, it causes eye irritation. People with certain genetic characteristics, and people with reduced intake of certain nutrients, such as vitamins C and E, are at greater risk from ozone exposure.

Ozone can:

1. Make it more difficult to breathe deeply and vigorously.
2. Cause shortness of breath, and pain when taking a deep breath.
3. Cause coughing and sore or scratchy throat.
4. Inflame and damage the airways.
5. Aggravate lung diseases such as asthma, emphysema, and chronic bronchitis.
6. Increase the frequency of asthma attacks.
7. Make the lungs more susceptible to infection.
8. Continue to damage the lungs even when the symptoms have disappeared.
9. Cause chronic obstructive pulmonary disease (COPD).

To learn more, you may want to check out these related questions on Socratic:
What are the positive and negative effects of the ozone layer?
What is the concern over stratospheric ozone?
How has a thinning zone layer affected human health?