We find out why there are no elephant sized Amoebas in Episode 7. Melanie begins with recapping the difference between the definitions of surface area and volume.
We find out why there are no elephant sized Amoebas in Episode 7. Melanie begins with recapping the difference between the definitions of surface area and volume (1:27). The first APsolute RecAP CHALLENGE is announced! (2:30). Visit a Starbucks and be sure to email your submissions by April 15, 2020 for a chance to win a personalized exam prep podcast. Get a rundown of calculation variables and equations (3:18). Don’t forget that cells function best with the greatest surface area to volume ratio ( 4:28). Melanie makes us get caught in concert traffic (4:47) before sharing some efficient human examples of large surface area adaptations (5:42).
The Question of the Day (6:50) asks “Which has a greater surface area to volume ratio - a spherical cell with a radius of 50 um or 20 um?”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap surface area to volume ratio.
Lets Zoom out:
Unit 2 - Cell Structure and Function
Topic - 2.3 Cell Size
Big idea - Energy
Why are there no elephant sized amoebas? Today's podcast will attempt to answer this question. The college board has two primary learning objectives for this topic. Students should be able to: First - Explain the effect of surface area to volume ratios on the exchange of materials between cells or organisms and the environment. Meaning, you should be prepared to discuss microscopic and macroscopic examples. Second – students should be able to explain how specialized structures and strategies are used for efficient exchange of molecules to the environment
Lets Zoom in:
First, some definitions.
Surface area. Capital S, Capital A– a measurement of how much exposed substance an object has. This is 2D space and will have a number expressed in units squared
Volume – capital V - is a measurement of the amount of 3D space an object has. This number can be expressed in units cubed, but has other units specific to the state of matter of solid, liquid, or gas.
As a point of comparison – you go to Starbucks and order a Venti pumpkin spice latte because you just can’t get enough. The exterior portion of the cup that your hand is touching is the surface area. The latte in liquid form occupies the 3D space in the cup and represents volume. Because the cup has height and a radius – you could get fancy and calculate the surface area to volume ratio of your PSL. Actually – here’s an Apsolute Recap challenge – Students – it's time to practice. Calculate the surface area to volume ratios of the tall, grande and venti Starbucks cup. Make a claim about which cup has the least amount of heat loss to the environment, provide evidence for your claim with calculations and justify your reasoning. Send your responses to the apsoluterecap@gmail.com by April 15th for a chance to win a personalized exam prep podcast. Remember – the 2020 test is on May 11th.
Ok so about those calculations I mentioned – We can’t compare a ratio without figuring out our numerator (top fraction number) and denominator (bottom fraction number) individually. The equations will be provided for you on the exam math sheet on the day of the test, but it is still your role to apply them correctly.
Here is a run down: Capital V is volume (lowercase v is velocity…which is physics). The pi symbol looks like a cursive r and is italicized. This is often truncated at 3.14 but it is always more accurate to use the pi symbol in your calculator Lowercase r is radius (or half the distance across a circle). l is length, h, height, w width and s the length of a cube side. You will also see superscripts, or numbers above variables that tell you to square (2) or cube (3) the value. Squaring is to multiply a number by itself and cubing is to multiply a number by itself twice.
All this math, but what’s the main idea? – cells function best with the greatest surface area to volume ratio. All of the action, or metabolism of the cell, is happening within the volume. And so, they want many peripheral opportunities to take in nutrients and get rid of wastes. Imagine you are going to a concert at a big venue downtown. When you arrive, you park your car a million miles away (there are soooo many people there), enter the venue, and find your seat. The concert is so amazing that everyone stays until after the encore performance. But then, the mad rush occurs. All 50,000 people want to leave the venue at the same time. There are only 5 parking lot exits open - and so you wait, for hours…..
That is a lot of volume to leave through such a limited amount of surface. Very inefficient. Imagine how much faster people could leave if they could drive over curbs and barriers? This is like the cell. The people are the proteins, carbs, gases, wastes that need to cross the phospholipid membrane. And while the membrane does have a say in what can cross when (for a future episode on transport), the greatest surface area per volume of space will always accomplish the task best.
A few more examples. You are cold, and shivering, and the hair on your arms stands up on end. Why? The hair raised on your arm creates a greater surface area which traps heat. Another – your intestines – their primary role is to take in nutrients from your digestive tract (across a surface) into your bloodstream. Your intestines are folded into columns called villi, which have further folds in them called microvilli – and why is this necessary? The greater the folds the more surface area, meaning more efficient nutrient absorption in the gut. Some estimates have the amount of surface area in the small intestine alone is equal to half of a tennis court.
To recap….
Why are there no elephant sized amoebas? As cells get larger, their volume increases at a faster rate than their surface area. A supersized amoeba would either starve or be poisoned by its one waste products. Not a pretty picture.
Today’s Question of the day is about calculations.
Question: Which has a greater surface area to volume ratio – a spherical cell with a radius of 40 um or 20 um?