Cell Size (Surface Area to Volume Ratio)

My AP Biology Thoughts

Unit 2 Cell Structure and Function

Welcome to My AP Biology Thoughts podcast, my name is Adrienne Li and I am your host for episode #56 called Unit 2 Cell Structure and Function: Cell‌ ‌Size‌. Today we will be discussing ‌the concept of surface‌ ‌area‌ ‌to‌ ‌volume‌ ‌ratio and how it contributes to cell size and efficiency.

Segment 1: Introduction to Cell‌ ‌Size‌

To start with the basics, cells are incredibly small and on average have a diameter of 100 micrometers. To understand why cells are so small, it’s important to address the concept of surface area to volume ratio. A cell’s surface area is the amount of cell membrane available for diffusion and how much diffusion that can happen at one time. On the other hand, its volume is amount of cytoplasm contained within the cell membrane and how long It takes to get from the membrane to the center of the cell by diffusion. Accordingly, as the ratio between surface area and volume increases, the cell efficiency increases because an increased surface area allows more nutrients and waste to enter and exit the cell. This is why cells are so small because it allows for more reactions to occur. In order to increase surface area, cells take on a complex folding shape and once it reaches a point where the surface area doesn’t allow enough nutrients to pass, the cell divides or dies. However, cells aren’t too small either because then, hereditary material and organelles would not be able to fit and the surface area would not be large enough to adequately exchange materials.

Segment 2: More About Cell‌ ‌Size‌

To illustrate cell size and the importance of surface area to volume ratio in real life, an example is seen in red blood cells which deliver oxygen to body tissues. They have a biconcave disc shape where the diameter at the thickest point is 2–2.5 micrometers and 0.8–1 micrometers at the thinnest point. Because they have that impression in the middle, it increases the surface area of the red blood cell. As a result, it increases the rate that oxygen is exchanged and allows the red blood cell to function efficiently. This is also seen in mitochondria where its inner membranes, called the cristae, have complex folds. This is where the electron transport chain is located which passes electrons from NADH and FADH2 to molecular oxygen and creates an electrochemical gradient that stores energy used to create ATP during chemiosmosis. Since these folds increase the surface area of the mitochondria, it increases the rate of ATP synthesis because there is more space for more embedded proteins to carry out reactions. Another example is seen in gut epithelial cells which play a role in digestion and water and nutrient absorption. These cells have microvilli which are integral to their function because the hair like protrusions increase the surface area so nutrients are absorbed quicker into the blood stream.

Segment 3: Connection to the Course

To dig deeper into cell size and its connection to Cell Structure and Function, we can take a look at the three previous examples. In each example, the cell or organelle’s structure allows it to have the greatest surface area. With an increased surface area, more reactions occur which increases the cell’s efficiency. Accordingly, this is why cells are the size they are and have the structures they have because it allows them to function optimally. If the red blood cell wasn’t shaped like a concave disc, it would have less surface area and diffuse oxygen at a slower rate. If the mitochondria’s inner...