Cirvello #2: Structure and Function of the Cell

Question 1

Cytoplasm

Answer 1

A semisolid matrix composed of proteins, fats, and other molecules suspended in water.

Question 2

Physiological Roles of Cell Membrane

Answer 2

Maintain an internal cellular environment independent of the extracellular environment.

Allow cells to communicate with other cells.

Allow for the absorption of extracellular products into the cell and the converse—release of cell-produced products into the extracellular space.

Question 3

Phospholipid

Answer 3

Composed of a glycerol backbone attached to up to three side chains. The side chains are usually fatty acids–long chains of saturated carbons (no double bonds) with a terminal COOH group.

In many cases, the first and second carbon of glycerol have attached fatty acid side chains with the third carbon attached to a phosphate group alone (phosphatidic acid), combined with choline (lecithin), combined with ethanolamine (cephalin), or inositol (phosphatidylinositol) as examples.

Phospholipids are amphipathic in that one end of the molecule is hydrophilic while other end is hydrophobic.

Question 4

Fluid Mosaic Model

Answer 4

Discovered by Singer and Nicholson. Explains how the cell can move, change its shape, repair small tears in the membrane, and change its composition over time. The fluid nature of the membrane also allows molecules suspended within the membrane to move and to redistribute themselves.

Recent work has discovered that there are lipid rafts within the cell membrane. Lipid rafts are membrane regions with less fluid lipids because of the presence of cholesterol, other lipids, and proteins. These lipid rafts are thought to have different functions than the rest of the cell membrane and act as transport shuttles moving proteins over the cell surface.

Question 5

Channel Proteins

Answer 5

Class of integral proteins. Form a tiny pore or channel through membrane that specific molecules pass through.

Usually constructed of several subunits that have TMS regions facing the hydrophobic membrane and hydrophilic amino acids that make up the pore walls.

The hydrophilic amino acids that line the pore wall determine the nature and type of ions that can move through the channel because of charge and size.

For ions to move through the channels, they must first lose their water shells and dehydrate. They are then drawn through by interactions with amino acid side chains.

Although channels are generally considered ion specific, it’s more accurate to say that the channels preferentially transport one ion, but other ions may be able to pass through at a slower rate or under specific conditions.

Opening and closing tightly regulated. Some channels are left open, or leak, and allow free movement of specific ions down their concentration gradient. Channels that are usually closed can be opened by chemicals (known as ligands) binding to outside of the channel (ligand-gated channels) or by electrical charges in the cell membrane (voltage-gated channels).

Question 6

Transporter/Carrier Proteins

Answer 6

Integral membrane proteins that move macromolecules from one side of the cell membrane to the other. The transporter/carrier protein has specific binding sites for ligands. The binding of these ligands to the protein induces a molecular change in the transporter/carrier protein that causes the ligand to be moved to the other side of the membrane.

Question 7

Structural Proteins

Answer 7

Linking one cell to another.

Question 8

Recognition Proteins

Answer 8

Many cells have molecules on their outer surfaces that can be recognized by other cells or components of the immune system. The majority of these molecules are glycoproteins (proteins with attached sugars); the rest are glycolipids (lipids with attached sugars).

This recognition process is an important one, as not only does it allow the correct cells to join together to form tissue, it alters the body when it’s infected with foreign agents.

Glycoproteins are either integral or peripheral with the sugar exposed to the extracellular space.

Question 9

Attachment Proteins

Answer 9

Recent research has greatly increased our knowledge of how membrane-bounded proteins act as attachment sites to other cells or to the extracellular matrix. Proteins like the cadherins, integrins, catenins, and vinculin provide a link between the cytoskeleton of a cell and other cells of attachment sites.

Question 10

Receptor Proteins

Answer 10

Most are found on extracellular face of cell membrane, though some are found on intracellular face, or in the cytoplasm. A receptor molecule has both a ligand-binding domain (a region that binds a ligand) and an effector domain, which is part of the signaling pathway. Ligands are molecules that bind to the receptor protein as a key fits into a lock. This binding event elicits a change in the shape of the receptor that, in turn, activates another protein or causes a change within the cell. This second change is the signal or message the ligand carries to the cell. The ability of a ligand to bind to a specific receptor molecule depends on the structure of the ligand and the receptor because its structure dictates its function. Receptor proteins are critical components in cell-to-cell communication and for the maintenance of homeostasis.

Question 11

Receptors Linked to Channel Proteins

Answer 11

Many integral membrane receptor proteins are associated with channels. When a ligand binds to this type of receptor molecule, the associated channel changes its conformation, either opening or closing. This change in channel opening affects permeability of a specific ion through the membrane.

Question 12

Solute & Macromolecule Movement

Answer 12

Four means by which solutes and macromolecules can pass through the cell membrane.

1) Directly through the membrane. Hydrophobic molecules can pass directly through the hydrophobic interior of the cell membrane.
2) Through membrane proteins. There are many different types of channels in the cell membrane. Usually, but not always, ion selective, and may be nongated, ligand gated, or voltage gated. The specificity of the channel depends on the size of the channel and the amino acids that line hte pore.
3) Via carrier proteins. Large polar molecules cannot pass through membrane channels because a channel large enough to pass through would be lethal to the cell. Specific carrier proteins in cell membrane transport large polar solutes. Some carries don’t need energy, but others do.
4) In vesicles. Larger macromolecules can be transported across cell membrane in vesicles. The small sac pinches off from the membrane to bring contents into the cell (endocytosis), or an intracellular sac fuses with the cell membrane to release its contents into the extracellular space (exocytosis). In some cases, material is picked up on one side of the cell and is released to the other side of the cell (transcytosis).

Question 13

Diffusion

Answer 13

The physical process by which solutes move through a liquid or gas phase from an area of high to low concentration. This motion doesn’t stop when concentration is distributed evenly, but there is no net movement because movement in all directions is random and balanced in all directions.

Question 14

Diffusion Dependent On

Answer 14

1) Concentration gradient (Greater concentration gradient, greater the net movement of particles.)
2) Temperature (Brownian motion is faster at higher temperatures, so diffusion is faster at higher temperatures.)
3) Surface Area (Diffusion occurs more quickly over a larger surface area.)
4) Particle Size (Larger particles diffuse more slowly than small particles.)
5) Solvent Viscosity (Viscosity is a physical measurement of how easily a liquid flows; thick viscous solutions impede the diffusion of particles.)
6) Membrane Thickness (Thicker the membrane, more time needed to diffuse through it.)

Question 15

Osmotic Pressure

Answer 15

Net movement of water. The movement of water reaches equlibrium when the weight of the column of water is equal to the pressure of water diffusing down its concentration gradient.

Question 16

Hypertonic & Hypotonic

Answer 16

A solution is hypotonic to another solution if it has a lower osmolarity; conversely, a solution is hypertonic to another solution if it has a higher osmolarity.

Hypotonic solution, greater solute concentration is inside so water will travel into cell. Hypertonic higher concentration of solute is outside so water will exit the cell.

Osmolarity is the concentration of solutes.

Question 17

DNA

Answer 17

Within the nucleus, genetic material is composed of DNA and associated proteins that are dispersed throughout the nucleus as narrow strands. The proteins are both structural and regulatory. Histones are the mature structural proteins. Various transcription factors are among the regulatory proteins. The DNA-protein is called chromatin. Chromatin is typically spread throughout the nucleus but is more condensed and highly stained in certain areas. Condensed chromatin is more tightly coiled and, therefore, less accessible for transcription. During cell division, the chromatin condenses to form densely coiled chromosomes.

DNA contains the instructions to construct all the proteins found in the body. These proteins carry out all of the aspects essential to life. The function of a protein is determined solely by its structure, making it possible to identify the parts of proteins that make them suitable for structural components or as enzymes. Not all cells produce the same proteins even though they all have the same DNA. Differentiation of cells into different forms is dependent of the production of either different proteins or the same proteins at higher or lower levels.

Question 18

Transcription Factors

Answer 18

Responsible for the regulation of gene transcription. Gene transcription is the process by which the information within genes is converted to mRNA and transported to the ribosomes for the production of proteins.

Question 19

Protein Synthesis

Answer 19

Responsible for cytoskeleton development and hence cell shape, as well as all of the biochemical properties of the cell: transport, channels, and metabolism, among others.

Protein synthesis requires the conversion of info encoded in DNA into a polypeptide chain. Genes are not present as a continuous sequence in DNA but are broken up into exons that code for proteins and introns that don’t. The presence of these allow for alternative splicing of mRNA, allowing the cell to produce far more different proteins than the total number of gens. mRNA is typically transcribed with both of these sequences, and is termed a pre-mRNA. Enzymes in the nucleus remove the intron regions and splice the exon regions together in a spliceosome to produce mRNA. Also in the nucleus, 200-300 adenine nucleotide bases are added to the 3’ end of the mRNA. This poly A tail directs the mRNA out of the nucleus and to a ribosome in the cytoplasm. These changes to the mRNA strand are collectively called posttranscriptional processing.

The mRNA must be read in the ribosomal complex to produce protein. tRNAs transport individual amino acids to the ribosomal complex. Each tRNA has a figure-eight structure with an anticodon sequence that’s complementary to the codon on the mRNA. The start codon is always AUG, which codes for the amino acid methionine. The complementary anticodon is UAC. A tRNA with the UAC anticodon will deliver the amino acid methionine to the new or growing peptide chain. All proteins start with a methionine that can be removed during posttranslational processing. The anticodon at the base of the tRNA binds a complementary codon of an mRNA in a ribosome. Adjacent amino acids attached to different tRNA are covalently bonded to each other through a peptide bond. The ribosome moves along the mRNA, matching three bases at a time and adding amino acids to the growing polypeptide chain. When the ribosome reaches one of the “stop” codons, the ribosome releases both the polypeptide and mRNA. The process of polypeptide synthesis is termed translation.

As mentioned previously, the instructions carried by mRNA are written in three-letter codes, or codons. With four bases, it’s possible to write 64 three-letter codons. There are 21 amino acids with the human body, so more than 1 codon can encode for a specific amino acid.

Question 20

Polyribosome

Answer 20

After a ribosome reads the initial part of an mRNA, another ribosome can attach to the mRNA to begin to make an additional polypeptide. This results in a cluster of ribosomes around mRNA known as the polyribosome. Each ribosome in this complex produces the same polypeptide. This is an efficient mechanism to make many polypeptide molecules from the same mRNA.

Question 21

mRNA Additional Info

Answer 21

mRNA contains additional info at the 5’ and 3’ ends. The additional info affects the stability of the mRNA and contains either untranslated material or an additional polypeptide sequence. The additional peptide sequence has specific functions:

1) It can act as a leader sequence that directs the growing peptide to its correct state within the cell (a cellular zip code)
2) or it can be a presequence or prosequence, an additional peptide sequence that is important for storage of the protein or acts to keep a protein inactive until it’s secreted.

In the case of insulin, the propeptide sequence is cleaved by proteases after the proinsulin is secreted into the blood, converting the protein to its final, active form. These sequences can direct either the addition of modifications, such as in the Golgi apparatus, where sugars or lipids are added, or the final location within the cell, such as for mitochondrial proteins made in the cytoplasm. These modifications of proteins are termed posttranslational processing.

Question 22

Translation

Answer 22

Because DNA is physically separated from ribosomes, the cell first produces mRNA to serve as the blueprint telling the ribosomes to produce specific proteins. The mRNA leaves the nucleus through the nuclear pore complex and binds to ribosomes in the cytoplasm. The process of reading the mRNA and converting the info to a protein is called translation. Messenger RNA has a half-life (time required to lose half of the mRNA) that is tightly regulated. Either synthesizing new mRNA or increasing the half-life of existing mRNA can increase the levels of a protein.

Question 23

The Transmembrane Potential

Answer 23

The electrical charge inside the cell membrane is slightly negative, and the outside slightly positive (more positive ions outside, more negative ions inside).

- this unequal charge produces a difference in electrical potential (potential difference) across the cell membrane (transmembrane potential).
- transmembrane potential is expressed in millivolts (mV)
- the resting potential of an undisturbed cell ranges from -10 mV to -100 mV, depending on the type of cell. 
- the ability of the cell membrane to keep electrical charges separated results in potential energy across the membrane.  
- transmembrane potential provides electrical energy to muscles, the nervous system and some glands.

Question 24

Transmembrane Potential Requirements

Answer 24

The 3 main requirements for a transmembrane potential are:

1. A concentration gradient of ions (Na+, K+) across the cell membrane
2. The membrane be selectively permeable through membrane channels
3. Passive and active transport mechanisms maintain a difference in charge across the membrane (resting potential = -70 mV)

Question 25

Chemical and Electrical Gradients

Answer 25

1. Chemical gradients:
   - concentration gradients of ions (Na+, K+) across the membrane
2. Electrical gradients:
   - the charges of positive and negative ions are separated across the membrane, resulting in a potential difference.
   - positive and negative charges attract one another
   - if charges are not separated, they will move to eliminate potential difference, resulting in an electrical current
   - how much current a membrane can restrict is called its resistance

Question 26

Electrochemical Gradient

Answer 26

Electrochemical gradient:

1. the sum of chemical and electrical forces acting on an ion (Na+, K+) across a cell membrane is the electrochemical gradient for that ion.
2. chemical gradient of potassium tends to move potassium out of the cell, but the electrical gradient of the cell membrane opposes this movement
3. the transmembrane potential at which there is no net movement of a particular ion across the cell membrane is the equilibrium potential for that ion (K+ = -90 mV, Na+ = +66 mV).
4. the electrochemical gradient is a form of potential energy

Question 27

Resting Potential Maintain

Answer 27

Active forces maintain the cell membrane’s resting potential (e.g.; in neuron = -70 mV). The cell actively pumps out sodium ions (Na+), and pumps in potassium ions (K+). The sodium-potassium exchange pump (the carrier protein sodium-potassium ATPase), powered by ATP, exchanges 3 Na+ for each 2 K+, balancing the passive forces of diffusion.

Question 28

The Sodium-Potassium Exchange Pump

Answer 28

A stimulated neuron can produce 1000 action potentials per second (changes in transmembrane potential). Maintaining the concentration gradients of Na+ and K+ over time in order for the neuron to continue firing additional action potentials requires the sodium potassium pump, which in turn requires energy in the form of ATP
(1 ATP for each exchange of 2K+ pumped in for 3 Na+ pumped out).
If a cell ran out of ATP, neurons would stop functioning!

Question 29

Changes in the Transmembrane Potential

Answer 29

The transmembrane potential rises or falls in response to temporary changes in membrane permeability resulting from opening or closing specific membrane channels.

It is primarily the membrane’s permeability to sodium and potassium ions that determines transmembrane potential. Sodium and potassium channels are either passive or active.

Passive channels (leak channels) are always open, but their permeability changes according to conditions.

Active channels (gated channels) open and close in response to stimuli. At the resting potential, most gated channels are closed.

Question 30

Gated Channels

Answer 30

Gated channels can be in one of 3 conditions:

1. closed, but capable of opening
2. open (activated)
3. closed, and not capable of opening (inactivated)

There are 3 classes of gated channels:
1. chemically regulated channels:
open in response to the presence of specific chemicals (e.g. ACh) at a binding site
found on the dendrites and cell body of a neuron
2. voltage-regulated channels:
respond to changes in the transmembrane potential
characteristic of excitable membrane
found in axons of neurons, sarcolemma of skeletal muscle fibers, and cardiac muscle cells
have an activation gate (opens) and an inactivation gate (closes)
3. mechanically regulated channels:
open in response to distortion of the membrane
found in sensory receptors for touch, pressure and vibration

Question 31

Carrier-Mediated Transport

Answer 31

In carrier-mediated transport, integral proteins bind ions and organic substrates and carry them across the cell membrane.

The 3 characteristics of carrier-mediated transport are:

1. Specificity: one transport protein carries only specific substances
2. Saturation limits: The rate of transport depends on the number of available transport proteins, not concentration.
3. Regulation: cofactors such as hormones can regulate transport.
   
   • Cotransport: one carrier transports 2 substances in the same direction at the same time.
   • Countertransport: one substance is moved in while another is moved out.

Question 32

Secondary Active Transport

Answer 32

In secondary active transport, the concentration gradient of one substance drives the active transport of another substance in the same direction, without the immediate use of ATP.
- In the case of glucose transport, the resulting concentration of Na+ within the cell later requires ATP energy to pump the Na+ back out.