MCAT Study Guide Biology Ch. 5 – Cell 2017-08-15T06:45:06+00:00

IV.          5.2:  THE ORGANELLES


Surrounded by nuclear envelope; in eukaryotes, this is where replication, transcription, and splicing occurs (translation in cytoplasm)


In eukaryotes, linear ds-DNA

1.     Chromosome ― a piece of the genome

2.     Telomere ― ends of chromosome that protects it during replication

3.     Centromere ― center of chromosome

4.     Locus ― location of a gene on a chromosome

5.     Heterochromin ―densely packed  chromatin that is inaccessible, genes are usually turned off

6.     Euchromatin ―more  loosely packed chromatin that allows genes to be activated

7.     Nuclear matrix ― protein scaffolding that gives support and structure to the nucleus; analogous to the cytoskeleton in cytoplasm


Little nucleus, where ribosomes are partially assembled(the rRNA part only; the protein part is assembled outside)


Composed of 2 lipid bilayer membrane, the area between congruous with the rough ER

1.     Punctuated with large nuclear pores that allow small proteins and molecules through to the nucleus

2.     Nuclear localization sequence ― a sequence of AAs on proteins that allow them to pass through the envelope by specific transport mechanisms


Site of oxidative phosphorylization, have their own circular DNA

1.     Matrix ― inside the inner membrane, contains pyruvate dehydrogenase and enzymes of the Krebs cycle

2.     Inner membrane ― impermeable to free diffusion of polar substances

3.     Outer membrane ― has large pores that allows for free diffusion of small molecules


1.     Smooth ER ― contains enzymes involved in steroid hormone biosynthesis (gonads) or in degredation of environmental toxins (liver)

2.     Rough ER ― site of protein synthesis for proteins that enter the secretory pathway (free ribosomes in cytoplasm make other proteins

a)     Rough ER proteins ― get secreted, get put in membranes (integral membranes, ER membrane, Golgi apparatus membranes), or put inside the above organelles

b)     Signal sequence ― sequence that the signal recognition particle (on ribosomal ER) recognizes; translation occurs and pushes protein into lumen of ER

(1)   After translation, the signal sequence is removed

(2)   These proteins are then sent through the Golgi apparatus to the plasma membrane

c)     Transmembrane domains ― similar to signal sequence, but they are located in middle of protein, not removed after translation, and occur on integral membrane proteins

3.     Other functions of rough ER:

a)     Modification of proteins (adding of saccharides); though this also occurs in the GA

b)     Disulfide bond formation

4.     Other notes:

a)     Default pathway is to plasma membrane

b)     Proteins going elsewhere need targeting signals

c)     Proteins made by free ribosomes that are headed to a different organelle need localization signals


1.     3 functions:

a)     1 ― Modification of proteins made in the RER

b)     2 ― Sorting and sending proteins to their correct destinations

c)     3 ― Synthesizing other macromolecules (like polysaccharides) to be secreted

2.     Vesicle traffic is unidirectional (mostly)

3.     GA sometimes makes proteins that are stored in secretory vesicles and are released only at certain times

4.     Constitutive secretory pathway ― proteins finished in the GA that are sent immediately to the cell surface


Membrane-bound organelle that degrade macromolecules by hydrolysis

1.     Acid hydrolases ― enzymes in lysosomes, only active at pH of 5


Essential for lipid breakdown, also assist in detox of drugs and chemicals

1.     Byproduct of enzymes produce H2O2, but peroxisomes contain catalase, which converts it to H2O and O2


V.          5.3:  THE PLASMA MEMBRANE

A.     MEMBRANE STRUCTURE ― lipid bilayer (phospholipids, glycolipids, and cholesterol)

1.     Hydrophobic center of membrane makes it difficult for polar molecules to pass through

2.     Cell surface receptors ― bind extracellular molecules (like hormones) and signal the cell appropriately

3.     Integral membrane proteins ― embedded in cell membrane

4.     Transmembrane domains ― regions of proteins that cross membranes

5.     Peripheral membrane proteins ― stuck to the integral membrane proteins by H+ bonding


1.     Review of Diffusion and Osmosis

2.     Passive Transport

a)     Simple Diffusion

b)     Facilitated Diffusion: Channels

c)     Facilitated Diffusion:  Carriers

d)     Pores and Porins

e)     Kinetic Concerns

3.     Active Transport

a)     Primary Active Transport

b)     Secondary Active Transport

4.     NA/K ATPase and the Resting Membrane Potential

5.     Endocytosis and Exocytosis




1.     Microtubules

2.     Eukaryotic Cilia and Flagella

3.     Microfilaments

4.     Intermediate Filaments


1.     Tight Junctions

2.     Desmosomes

3.     Gap Junctions



Chapter 5 Summary

  • Know the structures and functions of the nucleus, mitochondria, ribosomes, RER, smooth ER, GA, lysosomes, peroxisomes
  • RER is site of translation of proteins to be either secreted from cell, inserted into membrane, or targeted to lysosome, ER, or GA
  • Signal sequences are specific AA sequences that direct proteins in translation to the RER and secretory pathway (RER → GA → final location)
  • Post-translational modification can occur in the RER or GA
  • All cellular membranes are composed of lipid bilayers; these act as selective barriers that regulate which molecules can cross
  • Hydrophobic molecules (O2, CO2, steroids) cross membrane by simple diffusion
  • Hydrophilic, polar molecules must cross membrane with the help of special membrane protein in process called facilitated diffusion
  • Active transport uses energy to move molecules against their concentration gradients
    • Primary AT – uses ATP directly
    • Secondary AT – uses previously established gradient by primary active transporter
  • Na/K ATPase – primary active transporter that moves 3 Na+ out for every 2 K+ it moves in
    • Helps establish the resting potential and maintain osmotic balance, and sets up Na+ gradient that can be used for secondary active transport
  • G-proteins help transduce signals from the extracellular ligands across the membrane
    • They change the levels of cAMP or calcium (2nd messengers) which changes the metabolic enzyme pathways active in the cell
  • Microtubules form centrioles, cilia, and flagella
  • Microfilaments participate in contractile activity
  • Tight junctions form seal between cells so the flow of molecules across entire cell layer is regulated
  • Desmosomes form general adhesions between cells
  • Gap junctions for connections between cells that allow the flow of cytoplasm from cell to cell
  • DNA replication occurs during S-phase of interphase and cell division occurs during M-phase (mitosis)
  • Mitosis is comprised of 4 major phases:  prophase, metaphase, anaphase, telophase

MCAT Study Guide Biology - Kim Matsumoto

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