Today s Agenda: I. Review of organelles II. More important organelles III. Plasma membrane structure IV. Diffusion and transport Delve AP Biology Lecture 4: 10/9/11 Melissa Ko and Anne Huang I. Review of Organelles Structures in both prokaryotes and eukaryotes: plasma membrane, DNA, cytosol, cytoplasm, ribosomes Prokaryotic cells only: nucleoid, cell wall, capsule, pili, flagella Eukaryotic cells only: nucleus, membrane bound organelles Organelles in both animal and plant cells: Nucleus, ribosomes, ER, Golgi, mitochondria, peroxisome, cytoskeleton Animal cells only: Lysosomes, centrioles Plant cells only: Chloroplasts, central vacuole and tonoplast, cell wall, plasmodesmata II. More Important Organelles Nucleus Function: contains DNA [genetic information] o Chromatin: a complex of histone proteins and DNA, which later condense into chromosomes during cell division o Nucleolus: where rrna is synthesized. It leaves nucleus and combines with other proteins to make ribosomes o o Nuclear envelope: double membrane Nuclear pore: a hole in both membranes that allows larger macromolecules to pass through [ex: mrna]. The hole is lined by a protein structure called a pore complex Ribosome Function: synthesize proteins. Ribosomes can be free in the cytoplasm or attached to the ER. The proteins synthesized by ribosomes on the ER will be secreted or embedded on the plasma membrane o Made of rrna [from nucleolus] and protein, not membrane-bound o 2 subunits (40S and 60S in eukaryotes) Endoplasmic Reticulum Functions: o Rough ER: help synthesize proteins that will be secreted or attached to the cell membrane o Smooth ER: synthesize lipids [ex: steroids], metabolize carbohydrates, detoxification 1
o Network of membranous sacs called cisternae o Membrane separates internal environment of ER [lumen] from cytosol o Rough ER has ribosomes attached to it Golgi Apparatus Functions: help modify and ship proteins to where they need to go. It can also synthesize some polysaccharides o Flattened membranous sacs called cisternae, which separate the lumen from the cytosol. The cisternae have a cis and a trans side (cis faces ER) o Cisternal maturation model: Golgi is dynamic and the cisternae move from cis to trans Lysosome Function: helps the cell digest macromolecules o A vesicle containing enzymes that digest macromolecules o Inside of lysosome is acidic, which is the ideal ph for the digestive enzymes. That way if the lysosome accidentally breaks open, the digestive enzymes will not digest the entire cell [the ph of the cell is neutral, so the digestive enzymes will denature] o Enzymes are made in the ER and transferred to Golgi for further processing. Most lysosomes arise from budding off of the Golgi ***The ER, Golgi, and lysosomes part of the endomembrane system Mitochondria: cellular respiration, produce ATP o Double membrane, intermembrane space, cristae [foldings of inner membrane], matrix [inside] o Has its own DNA and ribosomes Chloroplasts: photosynthesis, converts sunlight into chemical energy o Double membrane o Another membranous system: thylakoids [flattened, interconnected sacs], one stack of thylakoids = granum [pl. grana], stroma is the fluid inside o Has its own DNA and ribosomes contained in stroma Endosymbiotic theory: Where did mitochondria and chloroplasts come from? Early prokaryotes may have engulfed other prokaryotes. Instead of being digested, the prokaryotes inside continued living and developed into organelles Evidence for the endosymbiotic theory: Mitochondria and chloroplasts have a double membrane They have their own DNA, and their DNA is circular [not linear like eukaryotes] They have their own ribosomes They divide through binary fission They are about the same size as bacteria Peroxisome: break down toxins 2
Function: oxidizing enzymes break down toxins into hydrogen peroxide plasma membrane, crystalline core made of enzymes Cytoskeleton: organizes the structure of the cell 3 main fibers: microtubules, microfilaments, and intermediate filaments [p. 113] III. Plasma Membrane Structure The plasma membrane is a thin barrier separating the extracellular space and cytoplasm Terms to know: Phospholipid (an amphipathic molecule) How does cholesterol affect the fluidity of the membrane? Fluid Mosaic Model Integral proteins Peripheral proteins Glycolipids and glycoproteins IV. Diffusion and Transport Diffusion: a natural phenomenon where all molecules will evenly spread out in available space. The molecules move around randomly and will make net movement from high to low concentration. It involves entropy and a tendency against organization of molecules. Osmosis: diffusion of water across a selectively permeable membrane Tonicity: how the surrounding solution affects the cell, if it causes the cell to gain or lose water Isotonic: no net movement of water Hypertonic: water will flow from cell to environment Hypotonic: water will flow from environment to cell For animal cells, hypertonic solutions can cause cell to shrivel up and die, whereas hypotonic solutions can cause cell to swell and burst or lyse. For plant cells with cell walls, it is preferable to have water flowing into cell. The plant cell is turgid when it has net water flowing into the cell and lots of pressure exerted against cell wall, which prevents cell from popping. The plant cell is flaccid when it has no net water flow and there is not much pressure on the cell wall. The plant cell is plasmolyzed when it is losing water; it will wilt and die. Osmoregulation: the process of regulating and controlling the amount of water flowing in and out of the cell Selective permeability Concentration gradient: difference in concentration across a membrane, a force that diffusion depends on Electrochemical gradient: a gradient caused by a difference of charge and concentration, important for ions These gradients provide two separate motivations for a molecule to move from one position to another 3
Membrane potential: the voltage across a membrane, which usually ranges from about -50 to -200 mv with a negative inside of the cell relative to the outside Given the character of the membrane, small and uncharged/nonpolar molecules are best for crossing the plasma membrane Transport proteins and channels/pores are categorized by energy usage as well as how they move molecules Passive transport: the diffusion of a molecule across the membrane, can happen with the help of protein, does not require energy to occur Active transport: uses energy to move a molecule against concentration gradient, transport requires energy (ex: sodium-potassium pump, proton pumps) Cotransport: simultaneous movement of two different molecules by the same carrier protein Pumps: rely on hydrolysis of ATP to provide energy for moving a molecule against its concentration gradient. It uses the energy currency of cell to make a nonspontaneous reaction happen through reaction coupling o Electrogenic pump: a pump that moves charged ions across the cell to produce a voltage. Voltage is defined as the difference in electric potential energy between two different positions, and it represents the potential to do work. An example is the proton pump. Facilitated diffusion: where a protein just helps a molecule across the membrane when it is chemically unfavorable for the molecule to cross phospholipid bilayer Ex: aquaporin is a channel protein that moves water across the membrane. Water is a small polar molecule, so it does not move fast enough across the membrane by diffusion alone Ion channel: opens for free movement of ions along electrochemical gradient, movement to opposite charge or low concentration, very rapid movement Gated channel: a regulated channel that needs to bind a ligand in order to open. It may also depend on conditions to open such as voltage Carrier Proteins Proteins bind a molecule to be moved and undergo conformational change to transfer molecule across membrane. It may be a case of facilitated diffusion as well, just moving molecule from high concentration to low concentration. The transport of some molecules needs to be from low concentration to high. For example, cells require moving much more glucose into cell than outside to meet their energy needs. How can a membrane protein roll a ball up the hill? It depends on the change in Gibbs free energy and coupling reactions Bulk transport involves the movement of vesicles, instead of transporting small molecules or proteins through a transport protein Exocytosis: the release of molecules/proteins through vesicle fusion. Vesicles carry proteins and molecules synthesized in organelles. They bud off of the Golgi apparatus, move on cytoskeleton fibers towards plasma membrane, and fuse with the membrane to release cargo. This is how hormones or neurotransmitters are secreted. It is also important for the creation of the ECM and cell wall 4
Endocytosis: the uptake of molecules/proteins from extracellular space. Swallowed molecules are caught in vesicles that bud from the plasma membrane. The vesicle moves towards the interior of the cell, may fuse and release to other organelles. Phagocytosis describes the membrane wrapping around to ingest large particles like a bacterium Pinocytosis is small, unregulated sampling from extracellular space Receptor-mediated endocytosis occurs in response to the binding of a desired ligand to a receptor. The caught cargo is specifically taken up into the cell The balance between exocytosis and endocytosis keeps the amount of membrane on inside and outside fairly constant 5