0:07 Let's start by going over exactly what
0:10 we'll cover in this video. First, we'll
0:12 discuss what diffusion is and the
0:15 factors that affect it. Next, we'll
0:18 explore why multisellular organisms need
0:20 specialized exchange surfaces.
0:22 Then, we'll examine various exchange
0:25 surfaces in different organisms. And
0:27 finally, we'll take a look at how these
0:30 exchange surfaces are adapted to
0:32 function efficiently.
0:34 Let's begin with understanding what
0:37 diffusion is. Diffusion is the spreading
0:40 out of particles of any substance in
0:43 solution or particles of a gas resulting
0:46 in a net movement from an area of higher
0:48 concentration to an area of lower
0:50 concentration. This process happens
0:53 naturally without requiring energy.
0:56 It's occurring all around us and within
0:58 our bodies constantly. For example,
1:01 oxygen and carbon dioxide are exchanged
1:04 in our lungs by diffusion. Waste
1:06 products like ura also diffuse from our
1:09 cells into the blood for excretion by
1:12 the kidneys. Several factors affect how
1:14 quickly diffusion happens. The first is
1:17 the concentration gradient. The bigger
1:19 the difference in concentration, the
1:21 faster the rate of diffusion.
1:24 Temperature also plays a role. Higher
1:26 temperatures give particles more kinetic
1:28 energy, causing them to move more
1:31 rapidly. And finally, the surface area
1:35 available for diffusion is crucial. The
1:37 larger the surface area, the faster
1:39 substances can diffuse. For
1:42 single-sellled organisms like ammoa,
1:44 diffusion works efficiently for all of
1:46 their needs. This is because they have a
1:48 relatively large surface area compared
1:50 to their volume. We call this the
1:54 surface area to volume ratio. However,
1:56 as organisms become larger and more
2:00 complex, this ratio decreases. If you
2:02 imagine a cube getting bigger, its
2:05 volume increases more rapidly than its
2:08 surface area. This means that for large
2:10 multisellular organisms, simple
2:13 diffusion across the body surface would
2:16 be far too slow to supply all cells with
2:18 what they need. This is why larger
2:21 organisms evolve specialized exchange
2:24 surfaces and transport systems. These
2:26 surfaces allow efficient transfer of
2:29 materials between the organism and its
2:31 environment. Let's look at how exchange
2:33 surfaces in different organisms are
2:36 adapted for efficient diffusion. All
2:38 effective exchange surfaces share
2:40 certain features that help maximize the
2:42 rate of diffusion. First, they have a
2:45 large surface area. This increases the
2:47 amount of material that can be exchanged
2:50 at any one time. For example, the human
2:53 lungs contain millions of tiny air sacks
2:55 called alvoli. These alvioli
2:58 collectively provide an enormous surface
3:02 area for gas exchange. Second, exchange
3:04 surfaces are very thin providing a short
3:07 diffusion path. This minimizes the
3:09 distance that substances need to travel
3:12 during diffusion. The alvoli walls in
3:15 our lungs are just one cell thick
3:18 allowing gases to pass through quickly.
3:21 Third, in animals, exchange surfaces
3:23 have a good blood supply. This maintains
3:26 a steep concentration gradient by
3:28 rapidly carrying away substances that
3:31 have diffused in. It also brings new
3:34 substances to diffuse out. Our lungs
3:36 have a dense network of capillaries
3:39 surrounding each alvolas for this purpose.
3:40 purpose.
3:43 Fourth, for gas exchange in animals,
3:46 there's often a mechanism to ventilate
3:49 the exchange surface. This ensures that
3:51 fresh supplies of substances are
3:53 constantly brought to the exchange
3:56 surface. In humans, this happens through
3:59 breathing, which brings fresh air into
4:02 the lungs. Now, let's look at specific
4:04 examples of exchange surfaces in other
4:08 organisms. In fish, gills serve as the
4:11 respiratory exchange surface. Water
4:13 flows over gill filaments in one
4:16 direction while blood flows through the
4:19 capillaries in the opposite direction.
4:21 This countercurren flow maximizes the
4:23 concentration gradient and thus the
4:27 efficiency of gas exchange. And in
4:29 plants, the leaves are the main gas
4:32 exchange surfaces. Leaves have tiny
4:35 pores called domata which can open and
4:38 close to control gas exchange. Inside
4:40 the leaf, there are air spaces between
4:43 the cells that increase the surface area
4:45 available for gas exchange. Going back
4:47 to the example of animals, they possess
4:49 a small intestine across which
4:52 absorption of digested food occurs
4:54 across the intestinal lining. This
4:56 lining is folded into finger-like
4:59 projections called villi which are
5:01 themselves covered in even smaller
5:03 microvilli. This arrangement
5:05 dramatically increases the surface area
5:08 available for absorption. Understanding
5:11 diffusion and exchange surfaces helps us
5:14 appreciate how organisms have evolved to
5:16 overcome the challenges of size and
5:18 complexity. These adaptions enable
5:20 efficient exchange of materials which is
5:23 essential for survival.
5:25 In our next episode, we'll explore
5:28 another important transport mechanism,
5:30 osmosis, and how it differs from
5:32 diffusion along with the energy
5:36 requiring process of active transport. [Music]
