Chapter 3
I. Extracellular vs. Intracellular fluid
II. Cell Anatomy
>A. The membrane
>>1. Nature and orientation of phospholipids; prevents the passage of charged, polar & large molecules
>>2. Membrane proteins: integral & peripheral. Functions:
>>>a. Part of cytoskeleton; anchors
>>>b. Recognition proteins; "identification cards"
>>>c. Enzymes
>>>d. Receptors: recognize/bind to specific substances that are important to the cell (eg., hormones)
>>>e. Carriers and channels: allow the passage of charged, polar, & somewhat large molecules
>>3. Membrane carbohydrates: ID cards, also create a thick sticky layer around the outside of cell (adhesion & lubrication)
>B. The Cytoplasm- cytosol & organelles
>>1. Non-membrane bound organelles
>>>a. Cytoskeleton-protein filaments that give the cell structure and shape, and allow movement both of the cell itself and of structures within the cell. For example, immune cells can creep around because of cytoskeletal proteins, and all cells can move structures, like organelles, around inside of the cell because of cytoskeletal proteins. To do so, a cell attaches the organelles to protein filaments and the organelles ratchet along the protein filaments kind of like riding a conveyer belt! Amazing!
>>>>1)Microfilaments: actin protein filaments
>>>>2)Intermediate filaments: various proteins such as keratin
>>>>3)Microtubules: tubulin proteins associating to form hollow tubes. Allow cellular movement, including cilia & flagella, movement of intracellular substances, and separation of DNA copies during cell division.
>>>>4)Thick filaments: myosin protein. Only in muscle; interacts with actin to produce movement.
>>>b. Centrioles: origination of microtubules
>>>c. Cellular extensions: microvilli, cilia, flagella. Cilia & flagella move, driven by microtubules arranged similarly as in centrioles.
>>>d. Ribosomes- can be free-floating or attached to Endoplasmic Reticulum. The sites of protein synthesis.
>>2. Membrane-bound organelles
>>>a. Endoplasmic Reticulum- smooth & rough; Rough ER is so named because it is associated with ribosomes. The functions of Smooth and Rough ER are slightly different, but for our purposes we shall consider them together. In general, ER contains enzymes that serve a variety of functions, ex. processing proteins, detoxifying substances, building non-protein substances (such as lipids). The ER also stores certain substances such as Ca2+, as we will see when we get to muscles.
>>>b. Golgi Complex
>>>c. Lysosomes
>>>d. Mitochondria
>C. Nucleus & Genetic material- nuclear pores, nucleoplasm, nucleoli *since nucleoli are where ribosomes are produced, and ribosomes are the sites of protein synthesis, cells with many nucleoli probably make lots of proteins.
III. Membrane Permeability
>A. Background physics: diffusion, osmosis, hydrostatic pressure
>>1. Diffusion
>>>a. Explanation- because of random motion of particles, particles tend to spread out and move from areas of high concentration to areas of lower concentration, until they are, on average, approximately equally spaced (equilibrium). They move down their concentration gradient. When particles move down their concentration gradient, energy is released (the particles slow down). A concentration gradient can be considered a form of potential (stored) energy.
>>>b. Factors that affect the rate of diffusion, or how quickly particles spread out and reach equilibrium (see book); one extra factor not mentioned explicitly by the book, but that we will see and use: an electrical gradient can affect the rate of diffusion of charged particles. So, for example, let's say Na+ is diffusing from outside of the cell to inside of the cell. It turns out the interior of the cell has slightly more negative charge than the exterior (we'll see why soon). So, Na+ will be diffusing into a negative environment. Since it is a cation, it is attracted to the relative abundance of negative charge, and will diffuse faster than it would if the charge difference did not exist. How will the rate of K+ diffusion OUT of the cell be affected by this electrical gradient?
>>>c. Similar particles diffuse somewhat independently of other particles; that is, (for example) Na+ will move down its OWN concentration gradient regardless of the total concentration of all solutes. Other factors, like electrical interactions, can affect this, but we will see that when we start studying the movement of substances along gradients, we will consider the gradient of each individual substance.
>>2. Osmosis: the diffusion of water molecules. When two solutions are separated by a membrane that allows the passage of water but not solutes, water diffuses into the area with the lower concentration of WATER molecules (the area with the higher concentration of solutes)
>>3. Hydrostatic Pressure/Filtration- the movement of water directed by a mechanical force (pushing, ex. heart beat pushes blood)
>B. Permeability of Cell Membranes: protein carriers/channels, and invaginations/evaginations of the membrane allow the passage of charged, polar, and large molecules into and out of cells.
>>1. Passive mechanisms: do not require ATP (energy). Rely on diffusion, which happens without the input of energy as a result of concentration gradients of specific particles.
>>>a. Leak channels & gated channels- do not change structure as a result of binding with the specific particle they allow through. Most cells are freely permeable to water as they maintain many leak channels for water to pass.
>>>b. Facilitated diffusion- protein carriers that change structure when they bind to the particle they transport.
>>2. Active mechanisms: require ATP. Can move particles against their concentration gradient.
>>>a. Active transport- uses carrier proteins. Sodium-Potassium exchange pump is an active transport mechanism that maintains more sodium out of cell than in, and more potassium inside cell than out.
>>>b. Vesicular transport- invagination or evagination of membrane: allows passage of large, or many, particles.
>>>>1) Endocytosis-bringing stuff into the cell. Includes pinocytosis, phagocytosis, receptor-mediated endocytosis.
>>>>2)Exocytosis- dumping stuff out of the cell. Can be waste material, or material the cell makes specifically for export, like the proteins in saliva.
>>3. Another category of transport that can be passive or active: coupled transport. This is a situation in which protein channels are used to transport more than one substance into or out of the cell at the same time. These channels will only open when both substances bind to the channel. For example, a channel called the sodium-potassium exchange pump moves 3 sodiums out of the cell and 2 potassiums into the cell at the same time. In this case, both ions are pumped against their concentration gradient so the pump must use energy.
Another example is a carrier that moves sodium and glucose into the cell at the same time. However, in this instance sodium is allowed to move down its concentration gradient, so no energy is required. The cell uses the concentration gradient of sodium to move glucose against its gradient. So, even though glucose is moved against its gradient the cell does not need to expend energy.
When two substances are moved in the same direction (such as sodium and glucose), it is called symport. When two substances are moved in opposite directions (such as sodium and potassium), it is called antiport.
>C. The transmembrane potential- Cells maintain a slightly negative charge interior to the membrane relative to the charge outside of the membrane (simply by putting more positive ions outside the membrane). By keeping charged particles separated, the cell maintains an electrical potential, which is a form of potential (stored) energy. One of the primary mechanisms by which cells do this is the sodium-potassium exchange pump, which is a carrier protein that pumps 3 sodiums out and 2 potassiums in at a time. The concentration & electrical (electrochemical) gradient for sodium is particularly high, and if it could, it would rush into cells. The volt is a unit of measure of electrical potential. Each cell type has a characteristic resting electrical potential measured in mV.
IV. Protein Synthesis- Genes "code" for specific proteins. When the cell needs a protein, like collagen or an enzyme, the gene that codes for that protein is activated & transcribed.
>A. Transcription- Occurs in the nucleus. The two complementary strands of DNA separate at the gene. A strand of RNA is made by matching complementary bases with one of the DNA strands. RNA does not contain thymine (T), but instead contains uracil (U) that also hydrogen bonds to adenine (A). Transcription occurs in the nucleus, and the type of RNA made is messenger (mRNA). Once the mRNA strand is complete, it will leave the nucleus through nuclear pores.
>B. Translation- Occurs in the cytoplasm, at ribosomes (either free or on ER). mRNA nestles between the two ribosomal subunits (large & small). Transfer RNA (tRNA) delivers amino acids. Every 3 bases (a codon) on the mRNA codes for an amino acid, and each tRNA has 3 bases that are complementary to the codons of mRNA (anticodon). Each tRNA carries a specific amino acid. Amino acids are delivered by tRNAs based on the specific sequence of codons of the mRNA. As amino acids are delivered in sequence, they are joined together by peptide bonds.
V. Cell Life Cycle
>A. Overview of phases of the cell life cycle
>>1. Interphase
>>2. Mitosis- division of the nucleus
>>>a. Prophase
>>>b. Metaphase
>>>c. Anaphase
>>>d. Telophase
>>3. Cytokinesis- new cells separate/divide
>B. Back to interphase: DNA replication
>C. Rates of cell divsion in different types of cells- see text