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Why are cells so small?

• 40 trillion cells in human body (4x10^13)
• Because most chemical reactions in cells require diffusion:
• Diffusing molecules don’t travel long distances because of their “random walk”
• Distance travelled is proportional to the square root of time


SA to V ratio

- the volume increases by power of 3
- Surface increases by power of 2
- Everything must diffuse from outside into the cell (ex. Oxygen, glucose)
- Chicken egg is largest single cell
- Why can this be so big?- there is no diffusion happening in the beginning, once the egg is fertilized, the cells that develop are very tiny. The cells then that are metabolically active are very small.


Cells show two organizational patterns

• Prokaryotes are very small. They have no nucleus and few membrane-enclosed compartments. (split into bacteria and archaea)
• Eukaryotes have a membrane-enclosed nucleus and many other membrane-enclosed compartments called organelles, and can therefore be larger than prokaryotes.


Prokaryotic Structure

• Nucleoid without membrane: have DNA floating around
• Flagella: allows them to float around
• Cell wall provides stability to organism
• Can eat all kinds of stuff (eukaryotes are very limited to what they can eat)


Eukaryotic Strucure

• LOTS of compartments which allow for more chemical reactions
• Nucleus: biggest compartment (consists of nuclear envelope, nucleolus and chromosomes)
• Have centrioles: structures that occur in animal cells, but not plant cells


Plant Cell

- similar but a couple of exceptions: cell wall (cellulose, rigid- doesn’t need sodium potassium pump)
- If not enough water, plant can wilt (cell wall isn’t rigid)
- Vacuole- specialized lysosome (water storage area)
- Chloroplasts (where photosynthesis takes place)
- Compartments similar in size than eukaryotes


The endomembrane system

- nucleus is surrounded by two membranes: ER and nucleus membrane
• Golgi apparatus: stack of membranes that communicate with ER membranes
• Vesicles: belong to endomembrane system
• The lumen of the vesicle is identical with the outside environment


Formation of vesicles

- start off with a membrane- enclosed organelle
- it forms a vesicle (released into cytosol) which then fuses with the plasma membrane
* The orientation of the membrane is always preserved (cytosolic surface of membrane remains the cytosolic surface)


The Nucleus

• Has loosely packed sections of chromosomes on outside of nucleolus
• Densely packed sections are further out of the nucleolus
• The nucleolus: makes ribosomes and has DNA and chromosomes
• Nuclear envelope surrounds it (consists of two membranes)


What happens in the nucleus?

• DNA is replicated
• DNA is transcribed into mRNA or rRNA by RNA polymerases
• rRNA is not translated; it directly folds into a 3D structure
• Ribosomes are made of 4 rRNAs and about 80 proteins in the nucleolus (RNAs largely determine structure and function)


Nuclear envelope

• mRNA gets shipped out of cell into the cytosol
• Building blocks of DNA and RNA enzymes are shipped back into the nucleus


Rough endoplasmic Reticulum

• Membrane proteins are made here
• continuous
• studded with ribosomes


The Signal Hypothesis; Why do ribosomes sit on the ER?

- mRNA is floating in cytosol
1. Signal sequence (amino acids at end of ribosome) is synthesized
2. Signal binds to SRP
3. SRP binds to receptor
4. Protein is synthesized into ER
5. Protein synthesis is complete
- from here, they have to travel


Smooth Endoplasmic Reticulum

• Lipids are made here
-Other reactions take place here (ex. in liver, ER is very big because lots of detoxifying reactions take place there (add OH group to make it more water soluble- to get rid of unwanted chemicals)


Animal Pancreatic Cells

- Have a lot more rough ER in order to make digestive enzymes (proteins)


Animal Testis cell

- has more smooth ER in order to export lipid- soluble signals


Golgi apparatus

• The sorting station (decides where the protein goes)
- a stack of membranes
- has cis side facing the ER
- have trans side facing the plasma membrane


Process of sorting proteins

*Exocytosis- something exits the cell*
1. Ribosome deposits protein in the ER
2. Protein exits ER
3. Protein enters Golgi for processing
4. Protein exits Golgi
5. Protein exits the cell


Pulse- chase experiment

- wanted to see if proteins travel through different organelles
- Used radioactive amino acids
- Built into proteins and you could see them on electro-micrographs
- Added for one minute; “pulse”
- Can track where they go afterwards
- After labelling, there was a lot of signals at the rough ER
- Therefore, this is where the proteins must be made, then into vesicles an finally into the cell membrane


Receptor mediated endocytosis

*when something enters the cell*
- important for lipids
1. Macromolecules bind to receptors
2. Early endosome forms
3. Processing (fuses with other vesicles(membrane proteins) to lower the pH)
4. Digestive enzymes received (vesicle from Golgi apparatus fuses)
5. Mature lysosome is formed (function is to enter the cell, lower the pH then special enzymes digest what ever is in the lysosome)


Process: Phagocytosis

• refers to entire organism being eaten at once
• eventually organism ends up in lysosome then destroyed
1. Detection
2. Phagosome formation
3. Delivery to lysosome and digestion
4. Release of particles into cytosol


Process: Autophagy

AKA self eating (start by eating muscle that is nonessential)
1. Organelle surrounded by membrane
2. Delivery to lysosome
3. Recycling


Protein sorting and vesicle transport

1. Proteins are tagged
2. Proteins are sorted
3. Vesicles bud
4. Proteins interact with receptors
5. Delivery