Stroll Through the Playlist (a Biology Review)


Captions are on! Click CC button at bottom right to turn off. Petunia, we have so many videos now. Every once in a while, I’ll come across
one of your pictures and be reminded all about the topic again —I love them so much. Oh cool, thanks. Remember the danger guppy? I LOVE the danger guppy. Ha! Which video was that for again? Classification. Remember, we were talking about how a scientific
name is much more reliable than a common name? Especially a made up common name? Oh yeeeah. Sometimes I forget what I’ve drawn. You…forget? Well, every video has like 200 pictures. If I don’t go back and watch the videos,
I forget. And I mean, we have like more than 50 videos
soooo that would take a long time. I guess I’d be more inclined to do that
if we had like a TL;DR version. A what? You know – too long, didn’t read? A summary of sorts? Like a refresher just kind of covering the
main points. A refresher… Now hang on, I don’t actually mean that
we need to create – But we DO. I mean, we’ve made quite a few videos now
in our biology playlist. And if someone was reviewing, we could have
this recap video, this stroll through the playlist! Yeah but– Now this one video would be way longer than
our short videos, obviously, but it could be a useful study tool to connect the main
pieces of the content together. Granted, it would only have main points. Not everything. Well…I guess that would be helpful but – Also, this stroll would be meant to be paused a lot. There’s so much vocabulary in biology. We’ll get Gus in on this; he can hold up
the “pause” sign so people know when to pause the video so we can even ask the viewer
questions. And if the questions are difficult to answer,
that may be a good indicator to check out the video it corresponds to. Are you ready to stroll, Petunia? Uhhhh… Pinky: Actually this is going to be kind of
a brisk stroll. And because it only covers a short part of
each concept, never forget biology is full of more exceptions and details than we can
cover. But that’s great for more exploring. We start with characteristics of life. What makes an organism alive or not alive
anyway? Life is difficult to define, and there are
exceptions when looking at characteristics of life. We went through some characteristics between
my bathtub grown pony (a long story) and a real pony, but we didn’t want to put numbers
on the characteristics of life because we didn’t want to suggest that these are the
only characteristics that one could argue. So here’s your first pause question- can
you think of some characteristics of life to include? [PAUSE] We also noted in the video they could
certainly be titled differently, but here they are! But this may get you thinking of what’s
living and what’s not. When studying biology, the study of life,
it’s important to understand the biological levels of organization. Meaning these levels start small. The smallest living unit being the cell—that’s
part of the cell theory after all. The cell theory includes that the cell is
the smallest living unit in all organisms, that all living things are made up of cells,
and what else? [PAUSE] Ah, yes, that all cells come from
pre-existing cells. So cells combined together make up tissues,
tissues make up organs, organs make up organ systems, organ systems are part of an organism! An individual organism. Individuals can be part of a population- they’re
all the same species. A community—now you’re including different
species. Can you keep leveling up? [PAUSE] So the next larger level after community
would be ecosystem…at that level you’re including abiotic factors which are nonliving
factors. Rocks. Or temperature. Next level is biome. And then with biomes combined, all parts of
the living world- the biosphere. Let’s focus on living organisms. Biomolecules are part of living organisms. We mentioned four of these major macromolecules—can
you name them here with their building blocks? [PAUSE] Carbohydrates, lipids, proteins, and
nucleic acids. And here are their building blocks: monosaccharides,
fatty acid & glycerol, amino acids, and nucleotides. These building blocks are considered true
monomers for carbs, proteins, and nucleic acids. Can you think of some important functions
for any of these biomolecules? [PAUSE] Ok, Petunia, bring out some functions. These are just SOME functions—we wouldn’t
exist without these large molecules of life! And their structures are—just beautiful—-we
included a popular mnemonic to remember some of the major elements they contain in their
structures as well. Most enzymes are made of proteins. Can you describe some of the vocabulary associated
with the enzyme? [PAUSE] Well, you can see this enzyme has
an active site where a substrate binds. Enzymes can speed up reactions. Enzymes have the ability to break down or
build up the substrates that they act upon. And ta-da: products! An example of why we care? Well, consider the specific, different digestive
enzymes that are specific for breaking down fats or sugars or proteins. But enzymes typically have a specific temperature
and pH range that they need to be in to work correctly. And what happens if enzymes can’t stay in
their ideal temperature or pH range? [PAUSE] That’s right, they can denature. Enzymes play a major role within cells. We have oh so many videos on cells that you
may wish to explore. We explain the differences between prokaryotic
cells and eukaryotic cells using the popular mnemonic that “pro” rhymes with no and
“eu” rhymes with do but what does that actually reference to? [PAUSE] Prokaryotic cells have no nucleus
nor the other fancy membrane bound organelles. But “eu” rhymes with do and eukaryotic
cells do have a nucleus and other membrane bound organelles. Prokaryotes include bacteria and archaea. Eukaryotes include plants, animals, protists,
and fungi. Can you think of some things that prokaryotic
cells would have in common with eukaryotic cells? [PAUSE] So just to name a few: DNA, cytoplasm,
ribosomes and a cell membrane would be included in both. In our “Intro to Cells” video, we explore
a lot of membrane-bound organelles that would be found exclusively in eukaryotes such as
the nucleus, endoplasmic reticulum, golgi apparatus, and mitochondria. Plant cells and animal cells can have some
differences between them as well. Let’s consider the cell membrane, also known
as a plasma membrane. It is a part of all living cells so why is
it so important? [PAUSE] Remember all cells have a membrane—regardless
of whether or not they may have a cell wall. The membrane is a big deal for homeostasis
because it controls what goes in and out of the cell. The membrane is made up of these phospholipids
which have polar heads and nonpolar tails. Some molecules move passively through the
membrane without a need for added energy- that’s called passive transport. Simple diffusion—and facilitated diffusion
(which is through a protein)—are examples of passive transport. In those cases, solutes travel with the gradient. Active transport though can involve using
ATP to force molecules to move in the opposite direction of the gradient. So is this example simple diffusion, facilitated
diffusion, or active transport and how do you know? [PAUSE] Well it’s not active transport—you
can tell the molecules are traveling with the gradient without a need for ATP. It’s not simple diffusion because it does
seem to require a protein. It’s facilitated diffusion! And that’s passive. Water molecules can travel directly across
a semi-permeable membrane as they are so small, or they can travel through proteins called
aquaporins – that is more efficient. Water traveling through the membrane is called
osmosis. Like diffusion, water molecules do travel
from an area where there is a high concentration of water molecules to an area of low concentration
of water molecules. But we mention there’s another way to look
at osmosis. You can also look at it as water traveling
to areas where there is a higher solute concentration—as the water concentration is less there. So to determine the net movement of water
in osmosis, look for the hypertonic area, the areas of high solute concentration. A cell that is placed in a salty solution
can lose water because the net movement of water is to the area of high solute concentration. One reason why you should not drink a lot
of salt water…it’s very dehydrating. Check to see if you can explain this graphic
using the vocabulary hypertonic, hypotonic, and isotonic. [PAUSE] Let’s move beyond the membrane here and
take a look at these organelles: the mitochondria and chloroplasts. In eukaryotes, cellular respiration involves
the mitochondria and photosynthesis involves the chloroplasts. Cellular respiration involves the breakdown
of glucose (sugar) to yield ATP. All organisms must make ATP in some way or
another. Yes, this includes plants. And amoebas. If oxygen isn’t available, some organisms—like
bacteria or yeast—can do anaerobic respiration or fermentation to make their ATP. So what do these chemical equations [cellular
respiration and photosynthesis] have in common? [PAUSE] Well one thing that is interesting
is that these reactants and products are switched here. Although that doesn’t mean they are simply
the reverse of each other. Keep in mind that they have many different
steps within them that make them very different. Photosynthesis produces glucose (sugar) using
sunlight energy. Not everything can do photosynthesis. In eukaryotic cells, it occurs in the chloroplasts. So moving beyond the mitochondria and chloroplasts,
let’s take a look at this nucleus of a eukaryotic cell. Guess what’s in here? DNA! DNA is a nucleic acid, and nucleic acids are
one of the types of biomolecules. It contains your genetic information, and
your entire DNA code is found in almost all of your body cells, although genes can be
turned on or off in different cells. Let’s zoom into the monomer of DNA, a nucleotide. Nucleotides have a phosphate, deoxyribose,
and a nitrogenous base. Which part of these do you think is critical
for determining genetic information? [PAUSE] Yep, the base. Well, that is, the sequence of them. And this mnemonic, “Apples in the Tree;
Car in the Garage” can help you remember that the bases adenine and thymine pair together. Cytosine and guanine pair together. DNA can be tightly coiled and condensed into
these units called chromosomes. The number of chromosomes in humans is 46. How many do you receive from each parent? [PAUSE] Well, you would receive 23 from the
female parent and 23 from the male parent. That’s really important later on when we
talk about cell division, because chromosomes are more portable when it comes to cells dividing. Zooming back out, DNA is made up of two anti-parallel
strands. One strand runs 5’ to 3’—and the other
strand runs 3’ to 5’. Now, your body cells have to make copies of
their DNA. Why? [PAUSE] When you make a new body cell- which
you make body cells for growth and repair- you need DNA to go into that new body cell
as that is its genetic material. Hence the need for DNA replication. Making more DNA. We have some major key player enzymes here-
can you remember what these key players do? [PAUSE] DNA must be unwound by an enzyme called
helicase. Primase is an enzyme that sets down primers. Primers are needed because another enzyme
called DNA Polymerase requires them in order to start building. DNA Polymerase builds the new strand in the
5’ to 3’ direction only. And because of that directional building,
one of these new strands will be a lagging strand as DNA polymerase has to keep racing
up here next to where the unwinding is going on. This causes fragments on the lagging strand
known as Okazaki fragments. Ligase can eventually be involved in sealing
those fragments together. So we mentioned that you have to replicate
DNA before you make new cells. That’s a controlled event that happens in
something known as the cell cycle. Do you remember the cell cycle phases, often
shown in a pie chart like this? [PAUSE] The cell cycle includes G1 (the cell
is growing), S phase (synthesis of DNA- that’s when the DNA replicates), G2 (cell grows some
more to prepare for dividing), and then M phase which includes mitosis and cytokinesis. G1, S, and G2 are all part of interphase so
the cell is not dividing during that time. But once it enters M phase, it divides. There are checkpoints that control whether
a cell can continue through the cycle. If a cell doesn’t meet the checkpoint requirements,
it is either fixed or it must undergo apoptosis which means the cell destroys itself. This highly regulated cell cycle is controlled
by many different proteins- some that we mentioned included Cdk, cyclin, and p53. Cancer cells are body cells that do not respond
correctly to these checkpoints and tend to divide out of control. They can also have other problems such as
making too much of their own growth factors, not anchoring properly, and not functioning
correctly. Now, we mentioned this cell cycle has M phase
which includes mitosis. So what is mitosis? Mitosis is part of cell division. What kind of cells does it make? [PAUSE] In humans and many other organisms,
it makes identical body cells. Like skin cells making skin cells or stomach
cells making stomach cells. Great for growth of an organism or replacement
of worn out cells. During mitosis, chromosomes- which are condensed
forms of DNA and protein- can be moved more easily into the newly formed daughter cell. We went over the PMAT mnemonic to remember
the stages- prophase, metaphase, anaphase, and telophase. Cytokinesis splits the cytoplasm and completely
divides the actual cell. What’s really easy to confuse with mitosis? Meiosis. Kind of wish they didn’t sound so close. Anyway what kind of cells does meiosis make? [PAUSE] In humans and many other organisms,
meiosis makes gametes which are critical for sexual reproduction. Otherwise known as sperm and egg cells, these
gametes have half the number of chromosomes as a body cell. Gametes are haploid—meaning they have one
set of chromosomes. Body cells are diploid—meaning they have
two sets of chromosomes. PMAT happens twice here in meiosis. You have your starting cell here which is
diploid. It goes through prophase I, metaphase I, anaphase
I, and telophase I. Then cytokinesis happens and it makes 2 cells. Then those cells go through prophase 2, metaphase
2, anaphase 2, and telophase 2. After cytokinesis, this results in 4 haploid
cells as these sperm cells shown here. These cells are all different from each other
due to independent assortment and a process known as crossing over. So what is crossing over again and when does
it happen? [PAUSE] Crossing over happens during prophase
I and it’s when pairs of homologous chromosome can transfer information between each other. So since meiosis is an important process for
making sperm and egg cells for sexual reproduction in humans and many other organisms, how is
this involved with the alleles and genes that a baby organism may inherit? Remember that in humans, a sperm cell has
23 chromosomes and an egg cell has 23 chromosomes. When they come together in a fertilized egg,
that is 46 chromosomes. Portions of the chromosomes are genes that
can code for specific traits. Many traits actually involve multiple genes. Genes can come in varieties known as alleles. Alleles are forms of a gene. For example, we talk about the trait of tasting
or not tasting the chemical PTC. If treating this as a single gene trait, we
would say the gene is a PTC tasting gene. But the allele that could be on a chromosome,
which is a form of the gene, could be tasting (in this case we used a capital letter T to
indicate it’s a dominant allele) or non-tasting (in this case, we used a lowercase letter
t to indicate it’s a recessive allele). In Mendelian inheritance, recessive alleles
are expressed if the dominant allele is not present. So someone who inherits a homozygous dominant
genotype of TT would have a phenotype that is PTC tasting. What would the phenotypes be of these other
two? [PAUSE] Someone who inherits a heterozygous
Tt genotype would have a phenotype that is also PTC tasting. Only someone who inherits a homozygous recessive
tt genotype would have a phenotype that is non PTC tasting. Again, assuming it is a single gene trait,
and as we mentioned in the video- it may be more complex than that. So speaking of alleles and genes, it’s time
for the super brisk stroll through different types of genetics we have covered. We started with basic Mendelian monohybrid
and dihybrid crosses. Could you explain, in your own words, how
to complete these Punnett squares and how to write out the genotype and phenotype ratios
of the offspring? [PAUSE] To get help with the answer to these
questions, check out the videos on these two topics specifically because there are multiple
steps to solving these. Then we talked about some non-Mendelian inheritance
including sex-linked traits and multiple alleles – if these look unfamiliar, you might want
to review those videos as well. We also mentioned incomplete dominance and
codominance. What is the difference between incomplete
dominance and codominance? [PAUSE] This graphic may help- notice in codominance
both alleles are expressed! In incomplete dominance, you can see how the
phenotype can have an almost “in-between” appearance of the two traits—there is not
complete dominance when both of these alleles are present. Finally, we have a video on pedigrees. Pedigrees can be used to track a trait of
interest whether it be a sex-linked trait or an autosomal trait. In a pedigree, individuals that are female
are represented by circles, males are represented by squares, and individuals that have the
trait being tracked are represented by circles or squares that are shaded. Now, when we’re talking about these fascinating
traits, you might wonder—how does DNA actually code for your traits? Well DNA can code for proteins and proteins
are involved with many traits. Proteins are involved in transport, in structure,
in acting as enzymes that make all kinds of materials, in protecting the body…and so
much more. Your eye color is due to proteins involved
in pigment production. So protein synthesis- that is making proteins-
is a big deal. Do you remember the two major steps in protein
synthesis? [PAUSE] First step is transcription—which
makes mRNA within the nucleus. The second step is translation—which takes
place in the ribosome and makes a chain of amino acids known as a polypeptide. Proteins can be made up of 1 or more of these
polypeptide chains. We also mention other forms of RNA such as
rRNA and tRNA as well as how to read a codon chart to determine which amino acids are produced. Proteins often need folding to be fully functional-
we have a video clip on protein folding and structure too. Now on the subject of this codon chart, you
will notice that the bases are read in threes to determine a specific amino acid. These three bases on the mRNA are known as
a codon. tRNA has an anticodon on it that complements
the mRNA codon. tRNAs also carry the corresponding amino acid. But what if there is a mutation in the DNA
or mRNA? When we talk about mutations, we first mentioned
gene mutations. This can include substitution, deletion, or
insertion. Do you remember which of these are more likely
to result in a frameshift mutation? [PAUSE] A frameshift is a shift in the reading
frame. Bases are read in threes so a frameshift mutation
is more commonly caused by an insertion or deletion. If you add or delete a base, it’s possible
to change the entire reading frame depending on where it occurs. With substitution, you typically would only
affect one codon. Now not every change in the base means the
amino acid will be different. See how all of these codons still code for
the amino acid leucine? We also discussed chromosomal mutations. Can you name and describe some chromosomal
mutations? [PAUSE] We mentioned duplication, deletion,
inversion, and translocation. As mentioned, mutations can be neutral. They can also be harmful or, potentially,
even beneficial. But the mutations are random- the organism
doesn’t will itself to mutate or have some certain trait. This is a good time to talk about natural
selection. Take these frogs, sitting on logs. They are all the same species. There can be variety though within the species-
due to processes like independent assortment and crossing over during meiosis or from mutations. The frogs in this population with a darker
color blend into this particular environment more easily. A predator may have a higher chance of consuming
the lighter, easier to see green frogs. The darker green frogs may have more fitness
than the lighter frogs. Fitness, in the biological sense, is determined
by not how strong they are or how long they live—but by how many offspring they have. These darker green frogs pass down their DNA
to their offspring. The new baby frogs will have DNA from their
parents. The lighter green frogs are being selected
against since they are easier to see in this particular habitat. Over a long period of time, you could expect
to see a higher frequency of darker frogs in the population. This mechanism of evolution is known as natural
selection, which acts on populations. So how does natural selection compare to genetic
drift?Well both genetic drift and natural
selection are mechanisms of evolution. In natural selection, organisms with traits
that result in high reproductive fitness tend to be more frequent in a population over time. But with genetic drift, the organisms that
survive and have offspring were randomly selected—they are not necessarily more biologically fit-
instead it’s more that the organisms won the game of chance from an event. Check out the bottleneck effect and founder
effect which are forms of this. We mention in our natural selection video
an example involving bacteria and antibiotic resistance that continues to be a great concern
in our world. But let’s talk more about bacteria in general. Bacteria are unicellular prokaryotes; some
can make their own food (they’re autotrophs) and some consume organic material (they’re
heterotrophs). Being prokaryotes, they don’t have a nucleus
or other membrane-bound organelles, but they still have genetic material, cytoplasm, and
ribosomes. Bacteria can come in a range of shapes. Bacteria often get a reputation for being
bad pathogens, and there are many that can be, although not all bacteria are harmful. Bacteria can also be very helpful for organisms
and ecosystems. Can you think of some examples of bacteria
being helpful?Some examples of helpful bacteria
roles include breaking down food in our digestive system, acting as decomposers, making some
foods that we eat, and fixing nitrogen for plants. But as for harmful bacteria, they can be treated
with antibiotics. Examples of bacterial infections include strep
throat, tooth decay, or tetanus. When we start thinking about bacteria, our
minds may wander to viruses. How are bacteria and viruses similar and how
are they different?If you watch our viruses video, you
will hear some reasons why viruses are not considered to be living organisms although
debate still exists on calling them non-living. Unlike bacteria, viruses are not prokaryotes;
viruses don’t even consist of cells. But viruses do have genetic material (DNA
or RNA). Viruses typically have a protein coat known
as a capsid. Some viruses have envelopes, and some diseases
that viruses cause include the common cold, HIV, or influenza (the flu). Unlike bacteria though, viruses don’t respond
to antibiotics. While bacteria can reproduce by splitting
in something called binary fission, viruses actually require a host to reproduce. Viruses reproduce using the lytic or lysogenic
cycle- definitely something to revisit if you have forgotten. While viruses are not considered to be living
organisms, bacteria are. So are archaea, protists, fungi, plants, and
animals. We mention that archaea are unicellular prokaryotes
and many can live in extreme environments; they can be either autotrophs or heterotrophs. Protists are mostly unicellular but can be
multicellular- this diverse group can be made up of autotrophs or heterotrophs. Fungi are typically multicellular but they
can be unicellular. Fungi are heterotrophs; many can act as decomposers. We’ll get to plants and animal systems a
bit later. So how do we classify living organisms? Well, first of all, all of life can be organized
into three domains. Can you recall what those domains are? Those domains are Bacteria, Archaea, and Eukarya. Consider looking at the classification video
to refresh your memory of characteristics of these domains. But we can get more specific than domains,
right? Can you remember those taxonomy levels that
come after domain?They are Kingdom, Phylum, Class, Order,
Family, Genus, and Species. And this was our mnemonic to help you remember,
but you may have one that is more memorable. The thing about classification is that it
is changing as we learn more about relatedness from DNA evidence. Scientific names tend to be able to be used
everywhere, often having Latin or Greek roots, and they are definitely more reliable than
common names which can vary by language or location. Or…in this case…be completely made up. Let’s take some time to focus on a kingdom
that provides a significant amount of the oxygen that we breathe. A talented kingdom of autotrophs, which means,
they make their own food. Plants. And if they are going to make their own food
using photosynthesis, they are definitely going to need to have structure that helps
them do so. To do photosynthesis, plants need water. How do they get water? Nonvascular plants get their water by osmosis. Kind of like soaking up water like a sponge. How is that different from a vascular plant?Vascular plants have two major types
of vessels. The xylem, which carries water, and the phloem,
which can carry photosynthesis products such as sugar, throughout the plant. How about light? We mention that plant cells have chloroplasts
to capture light energy. To do photosynthesis, plants need carbon dioxide. Many plants have these fascinating little
openings—pores really—called “stomata.” Stomata have a major role in gas exchange. Gases like CO2 can flow in through these openings. Guard cells can control the opening and closing
of the stomata. When might stomata need to be closed?One example is on a very hot day when
the plant has low water. So staying on the topic of plants, how do
they reproduce? Well, many plants can reproduce asexually
as mentioned with my spider plants. But many plants, spider plants included, can
reproduce sexually. We only covered sexual reproduction in flowering
plants at the time of this stroll, otherwise known as reproduction in angiosperms. Angiosperms typically have petals to attract
pollinators and many offer nectar to attract them as well. Many angiosperms have sepals which protect
the developing flower bud. Ok, so do you remember the male and female
parts that can be within a flower structure?Male parts of the flower include the
anther and filament—this whole thing here is the stamen. Female parts of the flower include the stigma,
style, and ovary—this whole thing here is the pistil. Can you describe the pollination and fertilization
process in angiosperms using those terms?Simplified a bit, pollen is brought
from an anther to the sticky stigma. Possibly by a pollinator. That’s pollination. Next comes fertilization. For this to happen, a pollen tube is formed. A generative cell from within the pollen can
divide into two sperm cells which can travel down the style to the ovary, into an ovule,
where one sperm cell will fertilize an egg—giving rise to a zygote. Inside the ovule, another sperm cell will
fertilize two polar nuclei which gives rise to the endosperm. The endosperm provides food for the baby plant. Because this fertilization process involved
sperm cells joining two different things (the egg and the polar nuclei)—we call this double
fertilization. These fertilized ovules can develop into seeds. The ovary can give rise to a fruit- and that
fruit can be very useful for helping the seeds get dispersed. But, while angiosperms bear fruit- keep in
mind it may not be how you might imagine a fruit. So we talked about plant structure and how
some plants reproduce. We already mentioned how plants provide a
lot of the oxygen that we breathe. But it’s not just about oxygen. Plants are also critical as part of food chains
and food webs. As autotrophs, plants are producers. If you remember, in a food chain, we start
with producers. Then we move into the consumers, which are
heterotrophs. Heterotrophs have to consume other things. So we have primary consumers, secondary consumers,
tertiary consumers—we could keep going. The arrows point to the direction of the energy
flow. We could arrange this into an energy pyramid. The producers at the base here—in trophic
level 1—- actually contain the most energy. The primary consumers here—in trophic level
2—actually only receive approximately 10% of the energy from the level below. Meaning, let’s say you have plants here
that had 10,000 kilocalories of energy. Can you complete the rest of the pyramid with
approximately how much of the energy would be within each trophic level?Well the next level here—the primary
consumers in trophic level 2, would only receive 1,000 kilocalories of energy. The secondary consumers in trophic level 3,
would receive 100 kilocalories of energy! Tertiary consumers in trophic level 4 would
receive approximately 10 kilocalories of energy. Energy can be lost as heat or undigested. Ecosystems typically do not have a single
food chain. Instead, they tend to have a food web. A food web is made up of multiple food chains
that interact together. This can show the importance of biodiversity:
the variety of organisms living in a given area. Biodiversity can contribute to the sustainability
of a community. But how do they develop? This takes us to our ecological succession
video. Ecological succession is a process—over
time— of organisms in an ecological community. In primary succession, the area this is happening
in generally is brand new without soil. An example could be a volcano lava flow that
has cooled and left behind this new area with no soil present. Usually you have a pioneer species, which
is a name for the species that colonizes first. Lichen or moss for example. After pioneer species colonize the area, they
slowly break down rock into smaller, more plant friendly substrate—and over time,
contributing more organic matter in newly formed soil which will support plants. Small vascular plants like grasses can come
in. Shrubs can follow. Then trees. Animals continue to move into the area. How long this takes can vary…but it’s
often hundreds of years before you get a climax community going. So how is this different from secondary succession?With secondary succession, you’re
talking about an area that once had plants and animals and an ecological community going
on. But then there is an ecological disturbance
such as a forest fire or human activity. The soil is still there and that’s kind
of the big key point here, because your initial species starting out could be small plants
as there is already soil there. Secondary succession can then follow a similar
sequence to primary succession after that point. See our video for more details and an understanding
of why this succession sequence tends to happen. Communities make up ecosystems, and in order
for these ecosystems to function—we’ve got to have cycling. You probably learned about the water cycle
in elementary school- learning about the carbon cycle and the nitrogen cycle tends to be explored
later on in junior high or high school. So let’s recap that from our Nitrogen and
Carbon cycle video. Carbon is often known as a building block
in life: you will find it in the four big biomolecules. Can you think of examples where you might
find carbon?Some examples: Carbon is dissolved
in the ocean. It is in rocks and fossil fuels. It is in living organisms. It can be in the atmosphere. Consider carbon dioxide in the atmosphere. It is taken in by organisms that perform photosynthesis. If the photosynthetic organism is eaten by
an animal, it becomes part of that animal too. And the animal that eat that animal. Both the plants and animals do cellular respiration
which releases carbon dioxide. When the plants and animals die, the carbon
can be released and stored in sediment. Over a very long time, they can even be converted
into fossil fuels. The burning of fossil fuels produces carbon
dioxide, and this has also led to the concern of excessive carbon dioxide in the atmosphere. Now for nitrogen. Nitrogen is important in building proteins
and nucleic acids. Let’s look at how it can cycle. Nitrogen can be found in the atmosphere, but
it needs to be “fixed” before it can be used well. Some plants have nitrogen fixing bacteria
living in their roots—the nitrogen is fixed by these bacteria into a form of nitrogen
known as ammonia and ammonium. Nitrifying bacteria in the soil can convert
the ammonium to nitrates and nitrites, forms of nitrogen that plants can also easily use
and assimilate. Animals can eat those plants and obtain nitrogen. When both plants and animals decompose, decomposers
help return ammonia and ammonium to the soil in a process known as ammonification where
it can be reused again. There are also denitrifying bacteria! In denitrification, they can convert nitrates
and nitrites back into nitrogen gas. This is just one example of cycling, but keep
in mind that this happens in both aquatic and terrestrial environments. So you can see there’s a balance with these
elements and living organisms in an ecosystem. Let’s talk about some of the ecological
relationships among living organisms. In the ecological relationships video, I mention
my fascination with antlions. Antlions are predators of ants. Ants are their prey. This is known as predation. Antlions have to compete with other predators-
like this jumping spider for example. Competing for a food resource is an example
of competition. We also mentioned three symbiotic relationships:
symbiotic relationships are specific types of relationships where different species live
together. Can you recall what occurs in the three symbiotic
relationships that we mention: commensalism, parasitism, and mutualism?In commensalism, one organism benefits
and the other is neither helped nor harmed: it’s neutral. Many barnacle species can attach themselves
to moving things. On a free whale ride, this barnacle can get
access to food since it’s a filter feeder and the whale may travel to nutrient rich
waters. However, in this example with this particular
whale and these barnacles, the whale was neither helped nor harmed. In parasitism, one organism benefits and the
other one is harmed by a parasite. Parasites can live inside or on their host. Mutualism is an example of a symbiotic relationship
where both organisms involved benefit. Our example had been an acacia tree being
protected by acacia ants. The acacia tree provides a home- and possibly
nutrients. But you know, one of my favorite examples
of mutualism is the good bacteria. They can live in our digestive system and
help us digest our food. So speaking of systems in the human body-
our short video on that topic only goes into basic functions of eleven body systems. Here they are up here for you in alphabetical
order—can you give a general function for each of these?The circulatory system helps transport
gases and nutrients. The digestive system is involved with both
the mechanical and chemical breakdown of food. The endocrine system is involved with producing
important signals known as hormones. The excretory system is involved with excreting
waste material as done by the kidneys or skin. The immune/lymphatic system helps defend our
body against pathogens such as viruses and harmful bacteria. The integumentary system—long, fancy word
for a large organ- your skin—can protect against water loss and serve as a barrier. The muscular system is involved with allowing
for movement. The nervous system coordinates both voluntary
and involuntary responses. The reproductive system allows for the ability
to reproduce. The respiratory system is involved with gas
exchange. And the skeletal system is critical for structure
and support. Those are very basic functions mentioned and,
of course, this doesn’t include structures. But the big takeaway we hope you have from
our body systems video is that these systems don’t work in isolation! They work together. If you’re nervous about a test—which we
hope you’re not because we have confidence that you’re going to do great—but if you
were nervous, you can get an adrenaline rush. Your endocrine system secretes adrenaline,
a hormone, that can cause your heart—involved in the circulatory system—to speed up its
beating. Your breathing rate, which is involved with
your respiratory system, can increase. These are all systems working together. And that’s relevant for the end. Because in this stroll through our playlist,
you’ve seen how we’ve been connecting these concepts together. Because that’s the thing that is so cool
about biology: it’s all connected. We hope this video helps you to identify your
strengths and areas that you might want to go back and explore. We also hope that you recognize that beyond
any test you’re studying: it is so important to be able to answer, “Why does this content
matter?” If there is a topic in this video that still
doesn’t seem to matter beyond just studying for a test—please check out our full video
on that topic—because that’s something that we really try to address in each and
every video. Don’t forget we also have a video with study
strategies that you may want to check out, and we have helpful GIF animations and comics
on our website that you might find useful. And…if you are studying for something big…
it is our sincere amoebic wish that you will feel confident about your learning. Well, that’s it for the Amoeba Sisters,
and we remind you to stay curious.

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