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So, we’re supposed to teach our students about evolution – but where to start? What topics to cover? And in what order should we cover them? And for each topic, what are the relevant learning standards? This sequence works for me:
Examples of evolution
So, we’re supposed to teach our students biology – but where to start? What topics to cover? And in what order should we cover them? And for each topic, what are the relevant learning standards? This sequence works for me:
Then we move on to types of cells
Now the bitty-gritty: Cell reproduction
For those teaching Honors Biology
has 2 layers of lipids (fats); hence > lipid bilayer.
Often we see simplified 2D drawings, showing a small section of the lipid bilayer.
Here we see the two layers, and some proteins.
The thick viscous liquid filling the cell. All the organelles float in it; and it’s also filled with millions of enzymes and other chemicals.
Here is a (false color) visualization of proteins in a cell’s cytoplasm – notice how densely packed this is.
The command-and-control center of the cell.
Here we see a more realistic image of the nucleus (lower left); we see mRNA copies of DNA coming out of the nucleus through nuclear pores.
If we magnify a cell, we can see “X” shaped chunks floating in the cell nucleus.
These chunks are called chromosomes. They are made of a chemical called DNA.
Here we see a cell nucleus being lysed (broken open) and all the chromosomes are spilling out on the right.
(The color was added by hand to make it easier to tell them apart.)
We can then cut-and-paste each of the chromosomes, number them, and line them up (lower left.)
In humans we find 23 pairs of chromosomes in every cell.
These X shaped chromosomes are not solid; they are like objects made of wound-up yarn.
A chromosome could be unwound into a long, thing string.
This string is made of DNA molecules.
Each section of the chromosome has difference sequences of DNA.
A complete sequence of DNA is called a gene; it is an instruction on how to build a protein.
The Endomembrane system
In this simplified diagram we note that ribosomes are very tiny compared to the size of a cell.
They are just seen as little black dots. They are way tinier than any of the other organelles.
Here we see a more realistic 3D model of a cell;
On the right we can just barely see the ribosomes as small dots stuck to the ER.
On the left we see the ER magnified, and the ribosomes are a bit clearer. (Although they are still small; we don’t see any details.)
Some ribosomes also float freely in the cytoplasm.
Here we see mRNA copies of DNA coming out of a cell nucleus, and moving to a type of ribosome floating nearby.
And now you’ll always remember this:
and a photo
large central vacuole
Previous MCAS exams from the Massachusetts Department of Elementary and Secondary Education
Below you will find each released short-response question, open-response question, and writing prompt that was included on High School Biology MCAS tests; the scoring guide for each question; and a sample of student work at each score point for that question. Taken together, these provide a picture of the expectations for student performance on the MCAS tests.
A car is a working machine – made from smaller systems working together.
A cell is an organic, living machine – made from organelles working together.
No part of a car, by itself, is functioning.
No part of a cell, by itself, is alive.
What we a “functioning car” is the way that parts work together.
What we call “life” is the way that organelles work together.
Let’s compare the parts of a car with the parts of a cell
Consider: The transmission, the axles and the engine.
Yet disconnect just one of these systems, and we effectively no longer have a car.
We’d just have an inert 2000 pound block of metal and plastic.
A car is not a car unless the parts are connected and working together.
The same is true for cells.
Inside the engine, what would happen if we removed or froze the pistons?
We’d effectively no longer have an engine.
The same is true for organelles inside cells.
Similarly, consider the ways that organelles and organic molecules are interacting within our cells:
What would happen if we removed some of these parts?
We’d effectively no longer have a living cell.
You’ve seen parts inside cars. Now let’s look at the organelles (parts inside cells)
First, a note of caution about artwork: Many books simplify what a cell looks like, with a 2D black & white drawing, like this. This picture is “true” – but simplified.
In reality cells are 3D.
Organelles are suspended in cytoplasm throughout the cell.
Now let’s learn the job of each organelle using analogies:
|lipid bilayer||Controls which molecules go in/out of the cell||Walls and doors|
|cytoplasm||Suspends and holds all the organelles||the factory floor, where all the work is done.|
|nucleus||Chromosome (DNA) storage||Control center|
(made of DNA)
|Instructions for building and running the cell||blueprints and instructions|
|mitochondrion||Converts energy from food molecules into a form usable by the cell||Powerhouse|
|ribosomes||biological protein synthesis (translation)||Workers on the assembly line, building our product.|
|cytoskeleton||gives the cell its shape and mechanical resistance to deformation||Walls & studs, support and structure|
|golgi body||Packages proteins into vesicles inside the cell, and send them to their destination.||Receives product from ER. Like UPS, its packages and distributes the products.|
|Manufactures lipids and proteins||Assembly line which makes our products|
|lysosome||Contain enzymes that can break down virtually all kinds of biomolecules.||garbage disposal|
|vacuoles||Multiple uses, often related to storing molecules.||Storage|
Organelles unique to plant cells (TDB)
section coming soon:
large central vacuole
Organelles: The Details!
Cell membrane (lipid bilayer)
large central vacuole
6.MS-LS1-2. Develop and use a model to describe how parts of cells contribute to the cellular functions of obtaining food, water, and other nutrients from its environment,
disposing of wastes, and providing energy for cellular processes.
Biology High School Standards: Cells have specific structures and functions that make them distinctive. Processes in a cell can be classified broadly as growth, maintenance, and reproduction.
2.1 Relate cell parts/organelles (plasma membrane, nuclear envelope, nucleus, nucleolus, cytoplasm, mitochondrion, endoplasmic reticulum, Golgi apparatus, lysosome, ribosome, vacuole, cell wall, chloroplast, cytoskeleton, centriole, cilium, flagellum, pseudopod) to their functions. Explain the role of cell membranes as a highly selective barrier (diffusion, osmosis, facilitated diffusion, active transport).
By the end of the 12th grade, students should know that
- Every cell is covered by a membrane that controls what can enter and leave the cell. 5C/H1a
- In all but quite primitive cells, a complex network of proteins provides organization and shape and, for animal cells, movement. 5C/H1b
- Within the cells are specialized parts for the transport of materials, energy capture and release, protein building, waste disposal, passing information, and even movement. 5C/H2a
Cellular and molecular biology: Cell structure and organization, mitosis, photosynthesis, cellular respiration, enzymes, biosynthesis, biological chemistry
Animals probably evolved from marine protists, although no group of protists has been identified from an at-best sketchy fossil record for early animals.
Cells in primitive animals (sponges in particular) show similarities to collared choanoflagellates as well as pseudopod-producing amoeboid cells.
Multicellular animal fossils and burrows (presumably made by multicellular animals) first appear nearly 700 million years ago, during the late precambrian time….
All known Vendian animal fossils had soft body parts: no shells or hard (and hence preservable as fossils) parts.
Animals in numerous phyla appear at (or in many cases before) the beginning of the Cambrian Period ( 540 million years ago)
Nicole King explains “All animals, from sponges to jellyfish to vertebrates [animals with a backbone], can be traced to a common ancestor. So far, molecular and fossil evidence indicate that animals evolved at least 600 million years ago. The fossil record does not reveal what the first animals looked like or how they lived. Therefore, my lab and other research groups around the world are investigating the nature of the first animals by studying diverse living organisms….. Choanoflagellates are a window on early animal evolution. Both cell biological and molecular evidence indicate that choanoflagellates are the closest living relatives of multicellular animals.
Between 620 and 550 million years ago (during the Vendian Period) relatively large, complex, soft-bodied multicellular animals appear in the fossil record for the first time. While found in several localities around the world, this particular group of animals is generally known as the Ediacaran fauna, after the site in Australia where they were first discovered.
The Ediacaran animals are puzzling in that there is little or no evidence of any skeletal hard parts i.e. they were soft-bodied organisms, and while some of them may have belonged to groups that survive today others don’t seem to bear any relationship to animals we know. Although many of the Ediacaran organisms have been compared to modern-day jellyfish or worms, they have also been described as resembling a mattress, with tough outer walls around fluid-filled internal cavities – rather like a sponge.
A new study mapping the evolutionary history of animals indicates that Earth’s first animal–a mysterious creature whose characteristics can only be inferred from fossils and studies of living animals–was probably significantly more complex than previously believed… the comb jelly split off from other animals and diverged onto its own evolutionary path before the sponge. This finding challenges the traditional view of the base of the tree of life, which honored the lowly sponge as the earliest diverging animal. “This was a complete shocker,” says Dunn. “So shocking that we initially thought something had gone very wrong.”
But even after Dunn’s team checked and rechecked their results and added more data to their study, their results still suggested that the comb jelly, which has tissues and a nervous system, split off from other animals before the tissue-less, nerve-less sponge.
The presence of the relatively complex comb jelly at the base of the tree of life suggests that the first animal was probably more complex than previously believed, says Dunn.
Is this possible? for this to be true, it would seem that complex structures – neurons – have evolved twice! Independently? See here for more amazing details:
An infographic on the intersection of physics and health:
For most people the biggest cancer risk from radiation hovers in the sky above us giving us all warmth and light. There is no cancer risk from Wi-Fi or microwaves.
Wear sunscreen, but use WiFi without fear. (Image: Spazturtle/SMS (CC))
What is radiation, and where does it come from? nuclear chemistry
What is cancer? How is caused? Cancer