Tutor profile: Raymond B.
Subject: Natural Sciences
What is evolution and how do we know its real?
Evolution is an extraordinarily simple process, the primary driver of which is natural selection. While natural selection is simple, the genetic structures it effects are extremely complex. Natural selection simply that nature, just by being tough to live in, will only allow the "most fit" creatures to survive. "Most fit" could mean the strongest (elephants), the smartest (humans), the biggest (blue whales), the smallest (certain insects), the fastest (falcon, cheetah, gazelles), the toughest (turtles), the ones that can drink the least water and live (camels), or any combination of traits you can imagine. Sometimes, criteria for "best fit" change, and the mode of survival that has been working for tens of thousands of years suddenly doesn't work anymore. This can cause the population to change drastically or (sadly) go extinct. A great example of this is mouse populations and their coloration. Imagine some mice that live in a grassland. For the past 20 years, their has been lots of rain. The soil is dark brown and fertile, so the hardest mice to see for the predators are the dark brown ones. Therefore, the majority of living mice are dark brown, because mice that are born light brown die more often. Over generations, of which there will be over 5 per year, the mice that reproduce the most are dark brown, while the ones that die the most often and don't have babies are light brown. However, suddenly there is a drought, for several years. Now, the grasses and soil have turned light brown, so the mice that are hunted become the dark brown ones. The population will become predominantly light brown, with dark brown mice being less fit. The key to remember is that natural selection acts on populations, not on individuals. If light brown mice had stopped being born at all, there would have been no "light brown" gene to express, so the mice population would probably be reduced further and may even go locally extinct due to the change in the environment. Now, imagine that rather than a single variable, all the difficulties of life are heaped on the mouse population, and adaptation occurs based on many factors.
Subject: Earth Science
How do tides work?
The tides are based on the gravitational forces acting on the Earth, primarily the Sun's gravity and the Moon's gravity. Water in Earth's oceans is pulled away from the surface of the Earth by both the gravity of the sun and moon. As the Earth spins, water on the surface spins a little less quickly due to its inertia working to keep it where it is. Think about how the water moves inside a jug when it is pushed across a table. Liquids do not have a rigid structure, so individual particles are acted upon by forces as individual units. Gravity pulls on them just like iron filings are pulled upwards by a magnet, with the ones closest to the magnet being pulled the most strongly, while the filings on the edge or at the bottom of the pile are pulled up less. The moon acts just like this magnet, so it will pull water up directly towards it, causing higher tides under it. The sun acts the same way, pulling water directly towards it at all times. When the moon is full it pulls water directly away from the direction the sun pulls, causing higher than normal water levels on both the moonward and sunward sides of Earth, and lower than normal levels on the other 2 sides. When there is a new moon, the sun and moon pull together in the same direction, causing the same effect. This is known as the spring tide. When the sun and moon pull at a perpendicular angle to each other, the moon pulls some water towards it while the sun pulls water toward it as well. This causes the water to be not particularly high or low at any point on Earth's surface, and is known as a neap tide. This is easier to explain with diagrams and I promise we can use them in class!
How does photosynthesis work?
Firstly, we must break photosynthesis into "light-dependent reactions" and the non-light dependent reactions also known as the "Calvin cycle". Light dependent reactions work a little bit like an electrical current. Appropriately, the initiator of the reaction is a photon of just the right frequency, which strikes an electron in the outer orbital of the magnesium ion at the end of a carbon chain in a chlorophyll (or other photosensitive pigment) molecule. This electron is knocked off the magnesium ion, and its energy is transferred (think pool balls striking each other) down the chain to the "reaction center" in the thykaloid membrane. Confusingly named, according to the order of discovery, the electron's energy initiates photosystem II (PSII) first. In PSII, the electron splits water (hydrolysis) making O2 and H+, which is stored in the intermembrane space, and will be used again later. Then the electron leaves PSII, and some of its energy is used to pump more hydrogen into the intermembrane space, before entering PSI. In PSI, the energy of another photon is used to re-excite the electron, the energy of which is used to act on chemicals NADP+ and ADP to make NADPH and ATP. Some of this is used to fuel the cell's activities immediately, but most is used in the Calvin cycle, where sugar is made. Additional ATP is made when all that extra H+ we stored in the intermembrane space is allowed to pass through a special little turbine (think hydroelectric dam) called ATP synthase.
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