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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.
Which video was that for again?
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.
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
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?
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.
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
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
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
[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.
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
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
Guess what’s in here?
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.
[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
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?
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
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
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
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
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.
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
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
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
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
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
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.