Firstly, when it comes to plant cells, they may have both primary and secondary cell walls. The primary cell wall is a more flexible, more permeable one that will allow the cell to continue to grow in size. Upon maturation i.e. cell stops expanding, another layer i.e. secondary cell wall may be formed between the primary cell wall and plasma membrane. Lignin is added to the secondary cell wall, making it more rigid and strong for mechanical support (main constituent for wood), and less permeable to water when compared to the primary cell wall. The composition of the 2 cell walls is different but cellulose is present in both. The hydrophobic nature of lignin allows it to transport water as an integral part of xylem. Also, wood consists mainly of secondary cell wall that hold it up against gravity.
(http://www.helium.com/items/1633330-the-structure-and-function-of-lignin-in-plant-cells) “Though useful to plants as they increase in size and girth, lignin does have its drawbacks. Beans, for example become enedible as they age due in part to lignin deposits and as paper is made, used and recycled, lignin tends to remain as it is hard and difficult to dissolve so each time you recycle paper, the content of lignin increases and this impairs the quality of the paper. Finding bio-friendly ways of removing it without resorting to chemicals like dioxin is a challenge for the future.”
2. Thylakoids
Indeed, thylakoids are the flatted sacs of a granum. ☺ Someone mentioned that the thylakoid membrane system is continuous with the inner membrane of the chloroplast – that is wrong. As I had highlighted in class, the thylakoid membrane system is independent. However, it is derived from the pinching of the inner membrane of the chloroplast envelope.
3. ECM versus interstitial tissue fluid (loved it!)
ECM is different from the interstitial tissue fluid. Do they interact? Yes they do but they are also different in composition and derivation (interstitial fluid comes from leakages of blood capillaries while ECM are from the secreted products of cells, released via exocytosis. Collagen as a macromolecules is released in soluble form but assembled readily extracellularly via H-bonding. When we consider the role of ECM with its collagen and proteoglycan mixture, we can think of the cellulose cell wall with its criss-crossing fibrils that is very much porous but offers support to the cell. In this case ECM keeps the cells of a tissue together and supporting them, helping to coordinate activity etc. Just as water can pass through the porous cell wall, interstitial fluid can pass through ECM.
https://docs.google.com/viewer?a=v&q=cache:MeINS3Mhm5IJ:microfluidics.epfl.ch/twiki/pub/PBCCourse/ReadingMaterial2012/Annu_Rev_Biomed_Eng_2007_Swartz.pdf+&hl=en&gl=sg&pid=bl&srcid=ADGEESjPtBIxgTsSeIIIjyYfMp9iXUu6-qdakvXt9ffhsWg0FZhHkLK__UyMzUFW0STu91KoTSCgMgbwwSFt5rPJTNFPpOccmXqigDOEjDjcYTlNMgQLeeYT2nRqCp2CPCcwFjpsO6wr&sig=AHIEtbQADXQBPg8rMRK2fN9Kp3od6PEynA
THE INTERSTITIAL SPACE
Cells reside in highly specialized extracellular matrices (ECMs) that provide mechan- ical support, determine mechanical properties, and importantly impart extracellular signals to the cell, both through the ECM molecules and the cytokines that they bind. The specific components of this ECM also vary greatly according to tissue type, but typically include the family of collagens, proteoglycans, laminins, and, in pathological cases, fibrin. The specific composition of the ECM largely determines the resistance to fluid flow, most notably fibrillar collagen and proteoglycans.
INTERSTITIAL FLUID FLOW
Interstitial Fluid It is estimated that up to 20% of the body’s mass is made up of interstitial fluid (24), and much of this fluid is in constant motion, albeit slowly….interstitial fluid pressure (IFP) results from numerous factors, including exercise, blood pressure, tissue metabolism, hydration, ECM composition, and cell density (31). For example, the integrity of the ECM is important in maintaining healthy IFP, with damage to connective components of the tissue such as collagen leading to a loss of ECM tension and subsequent change in IFP. Besides such passive structural changes, cells also play important active roles, maintaining tension in the ECM in a β1 integrin-dependent manner and therefore helping to regulate IFP (32).”
4. Collagen (youth and old age)
http://www.smartskincare.com/skinbiology/skinbiology_collagen.html
http://www.science20.com/news_releases/shock_some_collagen_repair_skin_treatments_actually_work
http://www.prevention.com/beauty/beauty/collagen-drinks-do-they-actually-help-skin (on the not-so-effective collagen drink discussed in class)
With age, there is increased collagen degradation than with synthesis. The collagen fibers are also subjected to much damage with oxygen radicals and other agents that disturb the orderly structure of collagen, making it weaker than expected. As with any other cells, one may also expect fibroblasts which synthesise collagen to be less effective over time due to mutations. Nevertheless, one should note that the beauty of collagen is more than skin deep. Collagen is found in many other tissues as well and its degradation means more than just getting more wrinkles!
5. Endosymbiotic theory
As the story goes on the origin of eukaryotic cells:
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter4/animation_-_endosymbiosis.html
(animation; good)
http://www.biology.iupui.edu/biocourses/N100/2k2endosymb.html
(review of evidence as discussed in class)
6. Less cholesterol in plants than in animals
This is one reason the doctor may ask you to eat more veggies than meat – to reduce cholesterol intake. Anyway, while it is true that there is less cholesterol in plants, unlike animal cells where cholesterol is the sterol that dominates, plant cells produces an array of sterols like sitosterol, stigmasterol and 24-methylcholesterol which often predominate. Both sitosterol and 24-methylcholesterol helps to regulate membrane fluidity and permeability in a similar way cholesterol is to mammalian cell membranes.
7. If it is less energetically favourable, why do phospholipids bother to flip-flop?
Ahh. I had answered this question before some years back :P. A familiar feeling settled in when I located my source and saw my own note. While we talked about PL, we did not go further to discuss about the different types of PL present like phosphatidylcholine or phosphatidylserine etc. At the plasma membrane, enzymes known as flippases make use of ATP to flip the different types of PLs for an asymmetrical (PL) bilayer. The different PLs are known for the different roles they play in sending different signals within the cell or to other cells. For example, when apoptosis (programmed cell death) is engaged, phosphatidylserine, which tend to be located on the cytosolic side of the membrane will be flipped to the extracellular side where they serve as recognition markers for phagocytes to be engulfed.
8. Phagocytosis of encapsulated bacteria
mmm..I cannot remember the question very well but if I am correct, here is my response….. phagocytes (a type of white blood cells) have receptors on their plasma membranes that recognise and bind to specific protein markers (i.e. toll-like receptors) on the surface of the pathogens for phagocytosis to occur (cell-to-cell recognition). The associated question is on the capsule of bacteria which helps to protect the bacteria from phagocytosis by hiding the bacterial surface. The capsule is composed of polysaccharides and the type of sugar actually vary from one species of bacteria to another, making them good markers to differentiate amongst the bacteria as well. The capsule is important because it limits the ability of phagocytes to engulf it such that the bacterium can remain virulent/pathogenic. If a bacterium loses its capsule, it is no longer effective; nonpathogenic. For encapsulated bacteria, the antibodies produced in our body can assist in their destruction. Opsonization whereby (specific) antibodies bind to the specific sugar markers and help phagocytes to engulf the bacterium. As such, the polysaccharides are used as antigen/markers in vaccine to stimulate the production of antibodies in our body that can help the phagocytes. http://www.youtube.com/watch?v=TYXn2FhVVGM
9. Why flagellum of sperm is membrane bound
Just a note, the flagellum of sperm moves in a wave-like motion while that of bacteria is rotational. Also the flagellum of sperm is made of microtubules while bacteria’s is of protein flagellin. Consistent with the purpose of membrane in compartmentalization and regulating the interior environment, the beating of the flagellum is regulated by Ca2+ entry. Mammalian sperm are stored in a quiescent state in the epididymis and become capacitate/active/mature only after release into the female reproductive tract. In other words, chemical signals within the female reproductive tract helps to capacitate/activate the mammalian sperm including the facilitation of Ca2+ entry for the beating of flagellum. However, non-mammalian sperms do not require this capacitation stage; they are ever-ready although their movement is also dependent on ion exchange http://en.wikipedia.org/wiki/Capacitation The possible reason as to why bacteria do not have membrane bound flagellum is to be mobile while making sure a conducive and tightly controlled environment for the cell. For the sperm cell whose sole purpose is for fertilization, the entry of Ca2+ is more than to get the flagellum moving but for the whole maturation/capacitation process.
10. How does one get embryonic stem cell?
Typically from excess embryos from fertility treatment. Embryonic stem cells are taken from blastocyst, a ball of cells developed from the zygote after 4th -5th of fertilization. It consists of 3 parts: the inner cell mass, trophoblast and blastocyst cavity. The inner cell mass which is the source of ESC, will develop into fetus while the trophoblast will develop into placenta and other supporting tissues. As such, the inner cell mass cannot form an entire organism by itself – that is the argument for its use but highly debatable. Totipotent – able to differentiate into any cell types including the extraembryonic tissues such as placenta which nourishes the embryo – can form an entire organism. ESC of the inner cell mass is only pluripotent as it cannot differentiate into the extraembryonic tissues.
http://www.pbs.org/newshour/science/stem-cells/cellbasics.html .
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