We are searching data for your request:
Upon completion, a link will appear to access the found materials.
I have read in books that Proteins and Peptides are fundamental components of cells which carryout important biological functions.Can any one explain me the structural difference between Proteins and Peptides?
In terms of structure: both are composed of amino acids.
A peptide is when at least two amino acid are linked together. A protein is composed of multiple amino acids and have a secondary, tertiary and even quaternary structure.
In terms of function: larger molecules have more complex functions.
Peptides can act as a intracellular or extracellular ligands which will activate a signal.
Proteins have a large array of functions (enzymatic, structural, receptors etc)
A Description of the Difference Between Carbohydrates, Proteins, Lipids and Nucleic Acids
Macromolecules are large molecules within your body that serve essential physiological functions. Encompassing carbohydrates, proteins, lipids and nucleic acids, macromolecules exhibit a number of similarities. For example, all except lipids are long chains made up of smaller building blocks, and digestion reduces the size of macromolecules so your body can absorb their component parts. However, they also demonstrate distinct differences.
Mouse Kallikreins mK13 and mK26
The protein sequence for mK13 (deduced from the nucleotide sequence of the mKlk-13 gene, or mGK-13, formerly) is 100% identical to the deduced amino acid sequence of PRECE or PRECE-1. The protein sequence for mK26 (deduced from PRECE-2 cDNA sequence) is 100% identical to the deduced amino acid sequence of pSGP-2 cDNA or EGF-BP type B. There is a nine amino acid difference between mK13 and mK26, at positions 8, 32, 111, 151, 168, 210, 211, 229, and 232, and an 11-nucleotide difference between the two cDNAs (coding region). PRECE-1 has an isoelectric point of 9.5-9.8, and comprises two subunits with molecular weights of 17 and 10 kDa. The crystal structure of mK13 has been reported  .
Structure of the S protein
With a size of 180–200 kDa, the S protein consists of an extracellular N-terminus, a transmembrane (TM) domain anchored in the viral membrane, and a short intracellular C-terminal segment . S normally exists in a metastable, prefusion conformation once the virus interacts with the host cell, extensive structural rearrangement of the S protein occurs, allowing the virus to fuse with the host cell membrane. The spikes are coated with polysaccharide molecules to camouflage them, evading surveillance of the host immune system during entry .
The total length of SARS-CoV-2 S is 1273 aa and consists of a signal peptide (amino acids 1–13) located at the N-terminus, the S1 subunit (14–685 residues), and the S2 subunit (686–1273 residues) the last two regions are responsible for receptor binding and membrane fusion, respectively. In the S1 subunit, there is an N-terminal domain (14–305 residues) and a receptor-binding domain (RBD, 319–541 residues) the fusion peptide (FP) (788–806 residues), heptapeptide repeat sequence 1 (HR1) (912–984 residues), HR2 (1163–1213 residues), TM domain (1213–1237 residues), and cytoplasm domain (1237–1273 residues) comprise the S2 subunit (Fig. 2a) . S protein trimers visually form a characteristic bulbous, crown-like halo surrounding the viral particle (Fig. 1a). Based on the structure of coronavirus S protein monomers, the S1 and S2 subunits form the bulbous head and stalk region . The structure of the SARS-CoV-2 trimeric S protein has been determined by cryo-electron microscopy at the atomic level, revealing different conformations of the S RBD domain in opened and closed states and its corresponding functions (Fig. 2b, c) [15, 16].
a Schematic representation of the SARS-CoV-2 spike. b–c The S protein RBD closed and opened status. d The S protein binds to ACE2 with opened RBD in the S1 subunit. e The six-helix structure formed by HR1 and HR2 of the S2 subunit.
In the native state, the CoV S protein exists as an inactive precursor. During viral infection, target cell proteases activate the S protein by cleaving it into S1 and S2 subunits , which is necessary for activating the membrane fusion domain after viral entry into target cells . Similar to other coronaviruses, the S protein of SARS-CoV-2 is cleaved into S1 and S2 subunits by cellular proteases, and the serine protease TMPRSS2 is used as a protein primer. Although the cleavage site of SARS-CoV is known, that of SARS-CoV-2 S has not yet been reported [18, 19].
Structure of the S1 subunit
The binding of virus particles to cell receptors on the surface of the host cell is the initiation of virus infection therefore, receptor recognition is an important determinant of viral entry and a drug design target.
RBD situated in the S1 subunit binds to the cell receptor ACE2 in the region of aminopeptidase N. The S1 region contains the NTD and CTD, and atomic details at the binding interface demonstrate key residue substitutions in SARS-CoV-2-CTD. In addition, the SARS-CoV-2 S CTD binding interface has more residues that directly interact with the receptor ACE2 than does SARS-RBD (21 versus 17), and a larger surface area is buried with SARS-CoV-2 S CTD in complex with ACE2 than with SARS S RBD. Mutations of key residues play an important role in enhancing the interaction with ACE2. F486 in SARS-CoV-2, instead of I472 in SARS RBD, forms strong aromatic–aromatic interactions with ACE2 Y83, and E484 in SARS-CoV-2-CTD, instead of P470 in SARS RBD, forms ionic interactions with K31, which leads to higher affinity for receptor binding than RBD of SARS-CoV (Fig. 2d) [15, 16, 20, 21].
The RBD region is a critical target for neutralizing antibodies (nAbs), and SARS-CoV-2 and SARS-CoV RBD are
73%–76% similar in sequence. Nine ACE2-contacting residues in CoV RBD are fully conserved, and four are partially conserved. Analysis of the RBM (receptor-binding motif, a portion of RBD making direct contacts with ACE2) of SARS-CoV and SARS-CoV-2 revealed that most residues essential for ACE2 binding in the SARS-CoV S protein are conserved in the SARS-CoV-2 S protein. However, some studies showed that murine monoclonal antibodies (mAbs) and polyclonal antibodies against SARS-RBD are unable to interact with the SARS-CoV-2 S protein, revealing differences in antigenicity between SARS-CoV and SARS-CoV-2 . Similarly, a SARS-CoV RBD-specific antibody failed to block infection mediated by the S protein of SL-CoV–SHC014 , which suggests that the S1 RBD may not be an ideal drug target due to the highly mutable characteristic of broad-spectrum anti-CoV drugs.
Structure of the S2 subunit
The S2 subunit, composed successively of a FP, HR1, HR2, TM domain, and cytoplasmic domain fusion (CT), is responsible for viral fusion and entry.
FP is a short segment of 15–20 conserved amino acids of the viral family, composed mainly of hydrophobic residues, such as glycine (G) or alanine (A), which anchor to the target membrane when the S protein adopts the prehairpin conformation. Previous research has shown that FP plays an essential role in mediating membrane fusion by disrupting and connecting lipid bilayers of the host cell membrane .
HR1 and HR2 are composed of a repetitive heptapeptide: HPPHCPC, where H is a hydrophobic or traditionally bulky residue, P is a polar or hydrophilic residue, and C is another charged residue . HR1 and HR2 form the six-helical bundle (6-HB) (Fig. 2e), which is essential for the viral fusion and entry function of the S2 subunit . HR1 is located at the C-terminus of a hydrophobic FP, and HR2 is located at the N-terminus of the TM domain . The downstream TM domain anchors the S protein to the viral membrane, and the S2 subunit ends in a CT tail .
RBD binds to ACE2, and S2 changes conformation by inserting FP into the target cell membrane, exposing the prehairpin coiled-coil of the HR1 domain and triggering interaction between the HR2 domain and HR1 trimer to form 6-HB, thus bringing the viral envelope and cell membrane into proximity for viral fusion and entry . HR1 forms a homotrimeric assembly in which three highly conserved hydrophobic grooves on the surface that bind to HR2 are exposed. The HR2 domain forms both a rigid helix and a flexible loop to interact with the HR1 domain. In the postfusion hairpin conformation of CoVs, there are many strong interactions between the HR1 and HR2 domains inside the helical region, which is designated the “fusion core region” (HR1core and HR2core regions, respectively).
Targeting the heptad repeat (HR) has attracted the greatest interest in therapeutic drug discovery. The S protein is an important target protein for the development of specific drugs, while the S1 RBD domain is part of a highly mutable region and is not an ideal target site for broad-spectrum antiviral inhibitor development . In contrast, the HR region of the S2 subunit plays an essential role in HCoV infections and is conserved among HCoVs, as is the mode of interaction between HR1 and HR2 . A synthetic peptide derived from the stem region of the ZIKV envelope protein was demonstrated in 2017 to potently inhibit infection by ZIKV and other flaviviruses in vitro , implying antiviral efficiency of peptides derived from conserved regions of viral proteins. Peptides derived from the HR2 region of class I viral fusion proteins of enveloped viruses competitively bind to viral HR1 and effectively inhibit viral infection . Therefore, HR1 is a promising target for the development of fusion inhibitors against SARS-CoV-2 infection.
This Tiny Difference Between Human Cells and Bacteria Could Lead To New Antibiotics
You are not entirely the same person you were when you woke up this morning. That’s because complex living organisms like us are in a constant process of shedding dead cells and replenishing them. How many exactly? Well, in humans, we’re talking hundreds of billions of new cells every day. For example, around 2 million new red blood cells enter your blood stream every second to replace a similar amount that just died.
Don’t let anyone tell you you weren’t productive today.
Of course, mature red blood cells don’t have a nucleus and therefore don’t contain any nuclear DNA, but this ditching of genetic baggage is something that happens quite late in the maturation process.
When red blood cells are being formed they do need genetic instructions just like every other cell in your body does, from skin cells to bone cells to neurons. That means that the daily making of the new(-ish) you requires the production of an astronomical amount of DNA. This, in turn, relies on a ready supply of the four basic building blocks of DNA: the deoxyribonucleotides adenine (A), guanine (G), cytosine (C), and thymine (T).
If this production line isn’t working properly, you can see how it could be a very big problem.
DNA synthesis is such an ancient and fundamental part of life, that the processes involved are common across organisms from ancient bacteria to humans to blue whales. And while there may be some species specific variations here and there, the fundamental structures and behaviors of the proteins that carry out these processes tend to be highly similar. In other words, from an evolutionary perspective, if it works, and you need it, you keep it.
But every now and then, something does change and as long as it still does the job its needed for, that’s fine, too. On goes evolution.
Now, researchers at MIT have just discovered a slight difference in how humans produce the building blocks of DNA compared to how bacteria does it.
A major part of the DNA building block assembly line in both humans and bacteria is an enzyme called ribonucleotide reductase (RNR). RNR is essential for maintaining an adequate supply of the DNA building blocks (A,G,C, and T).
, via Wikimedia Commons" align=""] Nucleotides, the building blocks of DNA
We’ve known the structure of the bacterial version of RNR for a while now, but the human version has been elusive. This all came down to a method researchers often use to figure out the structure of proteins: X-ray protein crystallography. In a nutshell, it goes something like this: you make a super concentrated solution containing your protein of choice, and hope against hope that crystals of pure protein will form. If protein crystals do form and they’re good quality, you can shoot high energy X-rays at them, and these X-rays will bounce off the lattice-like arrangement of proteins in the crystal and create a diffraction pattern on a detector. Long story short, these diffraction patterns can then be used to reconstruct what the protein actually looks like. If the data are good enough, you can even achieve atomic level resolution, meaning you can actually distinguish one atom from another, and see which ones are interacting. It tells you a lot about how the protein functions. In the case of medically relevant proteins, it also means you can design a drug that will fit perfectly into the critical parts of the protein and stop it working, if that’s the effect you’re after.
One of the key bottlenecks in this process, is that some proteins, no matter how hard you try (and how much you hope), simply won’t form crystals. Consequently, their structures have remained a mystery. But over the past few years, something very interesting has been happening in another area of structural biology: cryo-Electron Microscopy. Cryo-EM uses electron beams, which have wavelengths much shorter than that of visible light (up to 100,000 times shorter, in fact), so it has been wonderful for revealing the structure of very, very small objects. Traditionally, EM could give you high enough resolution to get a get a good look at parts of a cell, and maybe large protein complexes, but teasing out the individual structures of proteins wasn’t really possible. They were just too small. Now, modified approaches in cryo-EM are enabling a detailed look at proteins down to the level of 3 Ångströms (0.0000000003 meters). This can not only reveal how several proteins interact with one another, it can also reveal the detailed bends, twists, and turns that dictate how a particular protein works. So, for all those proteins that would never crystallize, we now have an opportunity to finally see what’s going on.
Researchers at MIT used cryo-EM (housed at Scripps) to solve the structure of the human version of RNR and found some significant differences between human RNR and bacterial RNR. First of all, ours looks a bit different, we have extra protein subunits that aren’t present in the bacterial version. When the researchers took a close look at how all these protein sections were arranged, it appeared that our RNR carries out its function in a slightly different way to bacterial RNR.
“People have been trying to figure out whether there is something different enough that you could inhibit bacterial enzymes and not the human version,” says Catherine Drennan, a professor at MIT, who was a senior author on the paper along with Professor Joanne Stubbe (MIT) and Associate Professor Francisco Asturias (University of Colorado).
“By considering these key enzymes and figuring out what are the differences and similarities,” she says, “We can see if there’s anything in the bacterial enzyme that could be targeted with small-molecule drugs.”
This bodes well for the possible development of new antibiotics.
Drennan is now keen to see what other elusive structures the new cryo-EM approach will reveal.
The study was published in the Feb. 20 issue of the journal eLife.
Summary – mRNA vs tRNA
Among the three types of RNA, mRNA and tRNA are two types. Both are essential for the protein synthesis in a cell. However, the key difference between mRNA and tRNA is their function. mRNA carries the genetic information of a gene to produce a protein in three letter code while the tRNA brings amino acids to ribosome according to the codons specified in the mRNA sequence. mRNA is synthesized in the nucleus and transported to the cytoplasm. On the other hand, tRNA is present in the cytoplasm. mRNA a tRNA work cooperatively during the synthesizing polypeptide chain in the ribosome. Thus, this summarizes the difference between mRNA and tRNA.
1. Nature News, Nature Publishing Group. Available here
2. “Messenger RNA.” Wikipedia, Wikimedia Foundation, 2 Jan. 2019. Available here
1.”MRNA-interaction”By Sverdrup at English Wikipedia. (Public Domain) via Commons Wikimedia
2.”Peptide syn”By Boumphreyfr – Own work, (CC BY-SA 3.0) via Commons Wikimedia
Other Natural Amino Acids
The twenty alpha-amino acids listed above are the primary components of proteins, their incorporation being governed by the genetic code. Many other naturally occurring amino acids exist, and the structures of a few of these are displayed below. Some, such as hydroxylysine and hydroxyproline, are simply functionalized derivatives of a previously described compound. These two amino acids are found only in collagen, a common structural protein. Homoserine and homocysteine are higher homologs of their namesakes. The amino group in beta-alanine has moved to the end of the three-carbon chain. It is a component of pantothenic acid, HOCH2C(CH3)2CH(OH)CONHCH2CH2CO2H, a member of the vitamin B complex and an essential nutrient. Acetyl coenzyme A is a pyrophosphorylated derivative of a pantothenic acid amide. The gamma-amino homolog GABA is a neurotransmitter inhibitor and antihypertensive agent.
Many unusual amino acids, including D-enantiomers of some common acids, are produced by microorganisms. These include ornithine, which is a component of the antibiotic bacitracin A, and statin, found as part of a pentapeptide that inhibits the action of the digestive enzyme pepsin.
Protein aggregates caught stalling
Laura Pontano Vaites is in the Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, USA.
You can also search for this author in PubMed Google Scholar
J. Wade Harper is in the Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115, USA.
You can also search for this author in PubMed Google Scholar
Neurodegenerative diseases are often associated with genetic mutations that cause repetition of short sequences of nucleotides. In the disorders amyotrophic lateral sclerosis (ALS, also known as motor neuron disease) and frontotemporal dementia, such an expansion in a non-protein-coding region of the C9orf72 gene 1 , 2 , leads to aberrant translation products that contain repetitive stretches of glycine and alanine amino-acid residues. These ‘poly(GA)’ products form aggregates in neurons, and have been implicated in the disruption of a key cellular process in which complexes called proteasomes degrade proteins 3 , 4 . However, the biochemical basis for this disruption, and how it might promote disease, is poorly understood. Writing in Cell, Guo et al. 5 precisely map the organizational and structural features of poly(GA) aggregates and associated macromolecular complexes in neurons using a technique called 3D cryo-electron tomography (cryo-ET), to provide direct visualization of how proteasomes are disrupted by poly(GA) proteins.
Cryo-ET in 3D uses electron microscopy to view very thin, frozen but hydrated sections of a cell from various angles. The resulting images are combined to produce a 3D image called a tomogram. Guo et al. used 3D cryo-ET to visualize neurons that had been genetically engineered to express a poly(GA) tract that contained either 175 or 73 repeats. The tracts were fused with a green fluorescent protein that enabled their precise position to be determined using correlative light microscopy. The engineered protein mimics poly(GA) tracts that are produced from C9orf72 expansion, which take a long time to form in vivo. The authors found that poly(GA) proteins form highly clustered and often bifurcated twisted ribbon structures that are of relatively uniform thickness, but of variable length and width, similar to poly(GA) structures previously observed by conventional electron microscopy in vitro 6 .
The value of the authors’ work lies not only in their observation of the structure of poly(GA) aggregates in detail in cells, but in their comparison of poly(GA) with aggregates formed through a different genetic expansion — a glutamine repeat (poly(Q)) tract, which causes the neurodegenerative disorder Huntington’s disease, and which the same group analysed by 3D cryo-ET last year 7 . This comparison revealed structural differences that could explain dissimilarities in pathogenic mechanisms between the conditions.
First, the aggregates formed in each case are themselves structurally distinct. Poly(Q )proteins form fibril structures that show little branching and are less densely packed than poly(GA) ribbons 7 .
Second, when the authors used powerful computational approaches to search for known macromolecular complexes in each aggregate, they found many proteasomes incorporated in poly(GA) aggregates (Fig. 1). Indeed, biochemical data suggested that as many as 50% of the proteasome complexes in the neuron become highly entangled within poly(GA) ribbons. Removal of proteasomes from their normal location in cells through this sequestration mechanism might explain the reduced proteasomal activity in cells harbouring these aggregates 4 , 8 . Complexes called ribosomes, which mediate protein production and are comparable in size to proteasomes, were largely excluded from poly(GA) ribbons, suggesting that poly(GA) aggregates are actively recruited or retained by proteasomes. By contrast, poly(Q) fibrils did not contain proteasomes, but formed close contacts with membranes from multiple types of organelle. This interaction leads to deformation of the membranes around organelles, such as the endoplasmic reticulum. Such deformation might alter pathways involved in protein translation, trafficking and degradation 7 .
Figure 1 | Contrasting mechanisms of aggregate toxicity. a, In some cases of the neurodegenerative disorder amyotrophic lateral sclerosis, long chains of glycine and alanine amino-acid residues (dubbed poly(GA) tracts) aggregate in neurons. Guo et al. 5 show, through high-resolution structures in cells, that poly(GA) tracts form ribbon-like aggregates that capture protein complexes called proteasomes, which normally process other proteins for degradation. Such capture causes proteasome stalling, providing an explanation for the toxicity of this aggregate. Poly(GA) aggregates do not bind membrane-bound organelles such as vesicles and the endoplasmic reticulum (ER). b, By contrast, repetitive tracts of the amino acid glutamine (poly(Q) tracts), which are associated with Huntington’s disease, form fibril-like aggregates 7 . These aggregates deform the membranes of vesicles and the ER, suggesting that different aggregates cause neurodegeneration through different mechanisms.
The proteasome consists of a barrel-shaped core particle in which substrate cleavage takes place, and one or two regulatory particles that cap the ends of the barrel, restricting access to the core so that only proteins tagged with the pro-degradation molecule ubiquitin can enter. Regulatory particles have been observed in multiple conformations 9 , 10 , indicating that proteasomes progress through a reaction cycle that involves ground, committed and substrate-engaged states. Guo et al. used computational particle averaging to quantify the proteasomal states (technique reviewed in ref. 11) 11 , and found both ground and substrate-engaged states within poly(GA) aggregates. They also found a large increase in the proportion of doubly capped proteasomes (indicating engagement with substrate) compared with control neurons that did not contain poly(GA) products. Almost one-quarter of the proteasomes within aggregates adopted a conformation recently described 9 as substrate-engaged yet stalled, in which the substrate becomes trapped in the barrel. And that proportion rose to 36% for those proteasomes closest to poly(GA) ribbons.
Why might this stalling occur? The authors’ tomographic reconstructions revealed numerous regions of electron density located between a poly(GA) ribbon and the site where the protein RAD23 binds to the proteasome. RAD23 is involved in recruiting ubiquitin-tagged proteins to the proteasome and is known to be enriched in poly(GA) aggregates 8 . Thus, this electron density could indicate RAD23-associated ubiquitin that is attached to proteins within the aggregate. Which protein or proteins the proteasome is choking on is currently unclear, although possibilities include the poly(GA) peptides themselves, which probably inhibit proteasome activity directly. Regardless of the mechanism, it seems likely that depletion of proteasomal activity in the cell proper would be detrimental to protein-degradation pathways, thereby contributing to cellular toxicity.
These results raise several important questions. First, pathogenesis in cases of ALS driven by C9orf72 expansion has been linked both to changes mediated by poly(GA) formation and to changes caused by reduced production of C9orf72 protein — but what are the relative contributions of each mechanism? C9orf72 is part of a complex involved in autophagy 12 , a process by which cellular material, including proteins, is degraded and recycled. It is therefore possible that reduced C9orf72 levels conspire with poly(GA)-dependent proteasome inhibition to increase neuronal toxicity. Second, is toxicity promoted by the capture of other proteins within poly(GA) aggregates? One candidate is the autophagy cargo receptor p62, which is known to accumulate in poly(GA) aggregates 8 . Third, several molecular machines involved in disassembling aggregates do not accumulate in poly(GA) structures, but the reasons for this are unclear.
Finally, although poly(GA) is the most abundant repetitive protein produced by C9orf72 expansion, it is not the only one — mutation can also produce tracts of glycine–arginine (poly(GR)) and proline–arginine (poly(PR)). How do the structures of these other aggregates compare to that of poly(GA) proteins? Most data on poly(GR) and poly(PR) aggregates indicate that they do not accumulate proteasomes, suggesting alternative toxicity mechanisms 13 , 14 . Further analysis by 3D cryo-ET, and analysis of natural products of C9orf72 expansion rather than the engineered product used in the current study, might clarify the similarities and differences between the aggregates that occur in patients and the model aggregate structures studied by Guo and colleagues.
In sum, the current work highlights the unprecedented resolution of 3D cryo-ET for visualizing fundamental processes within cells 11 . Moreover, it sets the stage for a more comprehensive mechanistic understanding of aggregate-associated neurodegenerative diseases.
Skin Basics 1.2.1 - Skin Cells - Function, Structure & Protein Babies
Hello everyone, and welcome back! Today’s lesson may not sound very exciting because it probably won’t explain much to do with skincare, and maybe you’re just holding out until the upcoming lesson where we get started on that spooky-sounding film on your face known as the acid mantle.
But if you stick around, my goals for this lecture (as well as part 2, coming soon) are to give you an even better understanding of what your skin does all day, a more thorough base knowledge for when we move onto future lessons, and to give you a liiittle bit more confidence when trying to browse PubMed on your own! After all, a good teacher should give you the tools to keep learning even without their guidance.
your skin, hair, and nails all form your body’s integumentary system
your skin’s layers are the hypodermis, dermis, and epidermis
your hypodermis is not often considered to be part of your integument
the hypodermis has adipocytes, as well as fibroblasts and macrophages
the hypodermis is mainly composed of fat tissue
the dermis has fibroblasts, as well as macrophages and mast cells
the dermis is mainly composed of collagen fiber, elastic fiber, and reticular fiber
the epidermis has keratinocytes and melanocytes, as well as Langerhans cells and Merkel cells
your hair follicles contain a cluster of cells known as the hair matrix, which has keratinocytes and melanocytes
your hair matrix is responsible for building a hair shaft
In today’s lesson, we are going to learn what each of these cells do, and (hopefully) why we should care. Let’s get started!
A lot of people I come across have this idea that humans and other living organisms have cells inside them.
While that’s technically true, a more accurate description would be that people and other organisms are cells -- they are made of either one, single cell (like bacteria) or many, many cells, working with and alongside each other to form your cute little face.
Think of a cell as a lego. They have different shapes and serve different purposes, and putting them together can build all kinds of creations. Those creations don’t just have legos inside of them -- they are legos!
So, what do these things do?
Cells metabolize and grow. A cell can break large molecules into smaller ones to produce energy, and a cell can use this energy to string small molecules together to create larger molecules, helping it to grow.
Cells divide. A cell can make a copy of its DNA once it has grown, and can then divide into two. These two cells will generally continue the cycle of growing, copying, and dividing until there is no longer a need for any new cells (e.g., once a cut has healed).
Cells make proteins. This process is pretty cool (well, I think so, at least), but it’s a little complicated, so we’ll come back to this after we take a look at the cell’s structure.
Okay, so they can do stuff. But what’s in a cell that allows it to actually do this stuff?
Remember those textbook renderings of cell structures from high school? Those images were probably of a eukaryotic cell, because that’s the type of cell that plants, animals, and you are built with. There is another type, prokaryotic cells, which are generally what bacteria are made of, but we’ll get to those on another day (probably not). For now, let’s focus on those eukaryotic cells more specifically, we’ll focus on the type found in animals.
On that note, here’s yet another textbook rendering to help jog your memory!
Though it may look like it, a cell isn’t just an oddly shaped blob -- kind of like how you aren’t simply an oddly shaped blob that somehow manages to eat, poop, and go to work. Your body is filled with organs that help you accomplish all of that eating and pooping, and a cell has its own tiny organs, known as organelles, that help it to eat and poop as well!
So let’s sharpen up that hazy recollection of your high school biology class, and revisit your cell’s structure and the organelles inside of it.
Plasma or Cell Membrane - The plasma boundary of the cell. It decides what can come in and what can go out.
Cytosol - The jelly filling of your cell, 80% of which is water. Your cell’s organelles are all floating around inside of this stuff.
Cytoplasm - A catchall term for everything inside of the cell, including cytosol and organelles, apart from the nucleus.
Cytoskeleton - As the name implies, this is a network of microtubules (little tubes) that maintain a cell’s shape, as well as directing the flow of traffic for those little organelles inside of your cell’s cytoplasm.
Cilia - Present all over the exterior of most cells, these are hundreds of little hair-like extensions from the cell membrane made up of cytoskeleton tubes. They can either work like an antenna for molecules or they sweep stuff over the cell’s surface (dusting off mucus from your esophagus, for example).
Flagella - Almost the same as cilia, they are just longer, they move a little differently, and there are significantly fewer of them per cell (only 1 to 8, depending on the cell). They work to help a cell swim (sperm, anyone?).
Centrioles - These are another set of microtubules that are bundled together, and they help the cell split when it’s time to divide.
Nucleus - The command center of a cell, containing most of your cell’s genetic material. It’s surrounded by a nuclear membrane (also called a nuclear envelope), protecting the DNA inside from the surrounding cytoplasm. Whether or not a cell has a nucleus is one of the main differences between a eukaryotic cell and a prokaryotic cell, by the way.
Nucleolus - It’s not a typo! It’s a structure inside of the nucleus that creates each of the two subunits of ribosomes. Cells that are particularly active in making proteins may have more than one.
Ribosomes - These things are made of rRNA (r as in ribosomal) and proteins, and are composed of two subunits, as mentioned above. They help your cells make proteins.
Lysosomes - Sort of like a garbage disposal, these organelles are little membrane pockets full of enzymes that digest any waste produced by the cell.
Mitochondria - Known as the cell’s powerhouse, these organelles generate the energy a cell needs to function. This is done through a complicated process called cellular respiration, in which they take in molecules and convert them into energy. (Fun Fact: Evidence suggests that mitochondria were once an ancient, free-living bacteria -- prokaryotic cells on their own -- that decided to just take up residence inside of eukaryotic cells! Evolution is pretty neat.)
Endoplasmic Reticulum (ER) - This membrane-bound organelle has two regions known as the Rough ER (RER, rough because it’s studded with ribosomes) and the Smooth ER (SER, smooth because it’s ribosome free). It makes a few changes to any proteins being made that will either end up being secreted or used in the cell’s membrane. It also makes lipids (fat).
Golgi Apparatus/Body/Compex - Located close to the ER, it’s a stack of membrane-bound pouches that sort of works as your cell’s post office, sorting through proteins and sending them where they’re addressed. (Fun Fact: It gets its weird name from the physician who discovered it in the late 1800’s, Camillo Golgi.)
Vesicles - If the golgi apparatus is the post office, then these are the envelopes. The protein gets packaged inside of a vesicle before being shipped.
There’s a ton of stuff floating around in one, tiny cell! But that’s because your cells have a lot of work to do. So now that we’ve gone over the organelles and their various chosen career paths, let’s digress so we can look at this mysterious process of making a protein (protein synthesis), a job that requires the employment of quite a few of these itty bitty workers.
Picture a DNA double helix: two strands that wind around each other, with ladder rungs going from top to bottom.
Your DNA is very important, so it is bunkered down inside of a cell’s nucleus for protection. Some portions of a twisty DNA ladder have recipes for building proteins stored within the rungs. When a protein needs to be made, the two strands in the relevant section unwind and separate, allowing a recipe to be copied since the DNA can’t leave its safehouse. The recipe is copied onto something that can leave, a single strand of mRNA (m stands for messenger), a process known as transcription.
Once free to go, the mRNA exits the nucleus and enters the cytoplasm, where it goes to greet a nice ribosome to share this recipe with. A ribosome has no sense of what is appropriate behavior for a first date though, so it immediately locks its subunits around the strand and begins analyzing all of the data that was copied onto the mRNA’s rungs.
As you may know, if you want to cook something, you’ll need to get the ingredients. To cook up a protein, those ingredients will be various amino acids.
So while the ribosome reads the recipe, it will call over a tRNA (t stands for transfer). A charged tRNA will have an amino acid attached to it, so the ribosome will bring over whichever tRNA is charged with the correct amino acid it needs for each step of the recipe. This process is called translation.
The ribosome will dock the tRNA to the rungs of the mRNA. Then the ribosome moves to the next step of the recipe, and calls over another tRNA carrying the next ingredient. The two amino acids from each tRNA then link together with a peptide bond. The first tRNA leaves the party once his amino acid has been discharged, and the process continues until the ribosome has finished reading the recipe, and a whole polypeptide chain of amino acids has been produced! Whew, that was long.
We’re not quite done yet, though. o(╥﹏╥)o
Many of the proteins that are produced need to leave the cell eventually, either because the protein is needed elsewhere in the body, or because it is to be used by the cell’s membrane. When this is the case, the polypeptide chain needs to be looked over by the ER before getting shipped off, in a process known as translocation.
Our little ribosome in this scenario will be bound to the surface of the rough endoplasmic reticulum, rather than floating free.
As this ribosome begins reading the mRNA, one of the first sections of the chain he builds will function as a signal, letting everyone know that this recipe is meant to make a protein that needs to be secreted. When this announcement is made, the ribosome pauses his translation and gives the RER a moment to start taking in the polypeptide chain. Once the ribosome finishes translating and the polypeptide chain is completely fed into the RER (the interior of the RER is called the lumina), it gets trimmed up, modified, and folded into the proper shape of an actual protein.
Our brand new protein baby might get used by the RER. You know, because he’s selfish. If it’s not used, it will then make its way through the RER and get packaged into a vesicle for safe passage to the Golgi apparatus.
Upon arrival, the Golgi modifies the protein a teensy bit more, packages it into a new vesicle, and labels it to ensure it will be shipped to the correct address. It sends out the vesicle-bound protein, and it exits (or gets absorbed by) the cell membrane, free to go on its merry way.
And that’s it for today! I hope you guys learned something new, or at least enjoyed this refresher course on basic cell structure. Next time, we will be taking a closer look at those specialized cells found in each layer. Woohoo!
ѧѦ ѧ ︵͡︵ ̢ ̱ ̧̱ι̵̱̊ι̶̨̱ ̶̱ ︵ Ѧѧ ︵͡ ︵ ѧ Ѧ ̵̗̊o̵̖ ︵ ѦѦ ѧ ︵͡︵ ̢ ̱ ̧̱ι̵̱̊ι̶̨̱ ̶̱ ︵ Ѧѧ ︵͡ ︵ ѧ Ѧ ̵̗̊o̵̖ ︵ ѧѦ ѧ
I am SO sorry for the crazy long delay with this one, guys.
Kindergarten started on the 17th, and I thought that meant I’d have more time but it’s not so. They’ve already started asking me to volunteer for upcoming school events, scheduling Flex testing, and assigning homework.
I also had to start studying tax law, as my husband’s boss (an independent contractor) has begged me to start working for him as a bookkeeper and administrative assistant, two jobs I have zero experience in. I’m not sure why he thinks my Type A personality is enough to get the job done. Luckily, I have QuickBooks on my side.
Additionally, I had actually planned to cover the specialized cells in this post. It wasn’t until I was just about to start researching epidermal cells that I had the bright idea to do a character count. I was WAY over Reddit’s text submission limit! That's why this is 1.2.1, and not just 1.2, haha.
Gosh, I could’ve had this post out centuries ago if I had thought to check that earlier.
I left out a lot of info in this lesson, such as the process of cellular respiration, the process of lipid synthesis, other optional places a protein could be sent, etc. But I felt those details weren’t necessary for the purpose of this series. I figured protein synthesis might prove more relevant, as the specialized cells I’ll be covering next week will be responsible for synthesizing a bunch of proteins.
Anyway, notes will be in the comments as usual! And of course, feel free to ask questions. :)
Oh, many thanks to /u/brownskinned for reminding me -- I had a question for the readers: Many redditors in the Intro post had requested that a section of the Skin Basics curriculum be dedicated to ingredients/products. I want to avoid making product recommendations, but I'm thinking I might go ahead with a section on individual ingredients. Each ingredient included will receive:
why the ingredients might work the way they're meant to
whether or not studies support the intended results
(For example: collagen supplements, they're meant to increase plumpness and reduce wrinkles, the company/consumers think it does this because collagen already in the skin functions in a certain way, and studies show that taking collagen supplements is/isn't effective.)
Your gastrointestinal tract digests proteins to release the individual amino acids it contains. These building blocks, once absorbed and transported throughout your body, can then join together in a variety of ways to create a new protein as the need arises. Digestion of this macronutrient begins in your stomach, where acid relaxes the coiled structure of the protein, allowing a protein-digesting enzyme to act. As the partially digested molecule travels to your small intestine, additional enzymes continue to eat away at the protein until only individual amino acids remain, ready for absorption by your small intestine.
A writer since 1985, Jan Annigan is published in "Plant Physiology," "Proceedings of the National Academy of Sciences," "Journal of Biological Chemistry" and on various websites. She holds a sports medicine and human performance certificate from the University of Washington, as well as a Bachelor of Science in animal sciences from Purdue University.