Cell structure and function (cell membrane) - SDL Wksheet A

Endocytosis vs Exocytosis (opposite processes)
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis

http://www.youtube.com/watch?v=K7yku3sa4Y8&list=FL-cFTJ8NxBjonnjuwSQgSsw&index=52&feature=plpp_video

Endocytosis
- Definition: Cellular uptake of biological molecules (eg. proteins) and particulate matter via formation of new vesicles (called endosomes in general) from the plasma membrane.

- Importance: This is required as most substances required by the cells cannot pass through the plasma membrane.

- Types:

(i) Phagocytosis ("cellular eating") - process of engulfing solid particles (eg. bacteria, food) by the cell membrane to form a phagocytic vacuole/ phagosome by phagocytic cells (eg. white blood cells). The resulting phagosome subsequently fuses with primary lysosomes to form secondary lysosomes. Hydrolases present in lysosomes hydrolyse the solid particles and the nutrients released will then diffuse out into the cytosol for use in other metabolic processes.

(ii) Pinocytosis ("cellular drinking") - non-specific process where a cell creates a pinocytic vacuole around a droplet of extracellular fluid and brings it into the cell. The resulting pinocytic vesicle/ pinocytic vacuole subsequently fuses with primary lysosomes to form secondary lysosomes. Hydrolases present in lysosomes hydrolyse the particles and the nutrients released will then diffuse out into the cytosol for use in other metabolic processes.

(iii) Receptor-mediated endocytosis - Specific binding of extracellular substances (eg. LDL) to protein receptors (eg. LDL receptor) on the cell surface membrane, triggering the formation of a vesicle, and the subsequent internalising of the substances.

Cell structure and function tutorial - identifying rER vs sER

rER: granular (due to presence of ribosomes on its surface), more regular, flattened cisternae, continuous with outer membrane of nuclear envelope

sER: non-granular, less regular, tubular (more "rounded") cisternae

Cell structure and function tutorial - Ribosomes

Location of ribosomes
- Eukaryotic ribosomes in the cell are found in the cytosol as either free ribosomes, or ER-bound ribosomes. As the bound ribosomes are bound to the ER, the proteins made by the bound ribosomes are fed into the lumen of the ER where it can be modified (eg. glycosylation) (picture below).
- Modification of proteins helps it to fold properly or targets it to the correct location in the cell. Further modification of proteins occur in the GA.


- Both free and bound ribosomes function to carry out protein synthesis. The difference is that the proteins made by the free ribosomes are for more "localised" use whereas the proteins made by the bound ribosomes are usually transported to other parts of the cell (eg. inserted into the membrane), or secreted out of the cell for use.
- Ribosomes may be found as polyribosomes/polysomes (complex of mRNA with many ribosomes - to increase rate of translation/proteins synthesis) (picture below).

Types of ribosomes
- Ribosomes are made up of a small subunit and a large subunit.
- There are 2 types of ribosomes, 70S and 80S. 80S ribosomes are bigger than 70S ribosomes. "S" refers to Svedberg, a unit of sedimentation coefficient used for centrifugation. Prokaryotic cells (bacteria), mitochondria and chloroplasts have only 70S ribosomes, while eukaryotic cells (include plant and animal cells) have 80S ribosomes (in their cytosol) PLUS 70S ribosomes (in their mitochondria/chloroplasts).

Synthesis of ribosomal subunits (a little complex; you may draw out the process to improve understanding)
- Ribosomal subunits (both small and large) are composed of rRNA and proteins.
- rRNA is synthesised by the nucleolus. Newly-synthesized rRNA remains in the nucleolus to wait to be assembled with the ribosomal proteins (aka the protein component of ribosome).
- The ribosomal proteins (like other proteins) are synthesised by the ribosomes in the cytoplasm. Once made, they will be imported into the nucleus (through the nuclear pore; import regulated by the nuclear pore complex of proteins) to the nucleolus where it is assembled with the rRNA. Hence, assembly of ribosomal subunits (picture below) is carried out by the nucleolus. Once assembled, the ribosomal subunits are then exported out of the nucleus and into the cytoplasm.

Practical: Videos for use of a microscope

Using:
http://www.youtube.com/watch?v=X-w98KA8UqU&list=FL-cFTJ8NxBjonnjuwSQgSsw&index=10&feature=plpp_video

Focusing:
http://www.youtube.com/watch?v=scEhgAiazzU&feature=autoplay&list=FL-cFTJ8NxBjonnjuwSQgSsw&lf=plpp_video&playnext=1

Wet mount:
http://www.youtube.com/watch?v=jjevU-XMVzU&list=FL-cFTJ8NxBjonnjuwSQgSsw&index=13&feature=plpp_video

Clarification

1) Chloroplast is bound by a double membrane but it has another set of membrane inside of it called the thylakoid membrane.

Biology Flashcards for your smartphone

Revision flash cards, courtesy of Mr Chee!

1) Go to app store, search for "iflashcards".
2) After downloading, go to "card management" --> "browse cards by account" --> search for "frozespot171".
3) Proceed to view and save cards.

Terminologies

Dear all,

I understand that some of you may be confused about the terminologies polar/nonpolar/hydrophilic/hydrophobic. These terminologies are probably more Chemistry-based than Biology-based so I may not be an expert in this, but let me try my best to clarify these terms. I think you will learn more about these terms in Chemistry so maybe you will have a better idea then. The examples I have used below are from Wikipedia.

Polarity is dependent on (1) the difference in electronegativity between atoms in a compound and (2) the asymmetry of the compound's structure.

(1) The main reason why a molecule is POLAR is due to the presence of a large electronegativity difference between its atoms. This leads to unequal sharing of electrons in a covalent bond.

For example, water (H2O) is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen, as oxygen is more electronegative than hydrogen and will hence pull shared electrons towards itself more. Due to the polar nature of the water molecule itself, polar molecules are generally able to dissolve in water. Hence, polar and nonpolar molecules are also called hydrophilic and hydrophobic molecules, respectively.

On the contrary, oxygen (O2) is a nonpolar molecule as the oxygen atoms are of equal electronegativity. Methane (CH4) is also a nonpolar molecule because carbon shares the electrons with hydrogen almost equally.

(2) The shape (you can google "VSEPR" to see all the possible shapes of molecules) of the molecule also determines whether a molecule is polar or nonpolar. A molecule is POLAR if there is asymmetrical arrangement of polar covalent bonds.

A phosphate group (PO4) has a tetrahedral (asymmetrical) shape, so there is an imbalance between the 4 oxygens and creates and stronger pull to one side of the molecule and hence making it polar.

On the contrary, boron trifluoride molecule (BF3) has a trigonal planar (symmetrical) arrangement of three polar bonds at 120°. This results in a more equal distribution of electrons in the molecule so it is nonpolar.

Hope this helps.