Human Phys - Exam 1

Chapter 3

Review of cell and plasma membrane

1) Regarding cellular physiology, describe the cellular components involved in protein synthesis

and describe how a protein (let's use a neurotransmitter as an example) is transported to the

nerve terminal.

The cellular components involved in protein synthesis include the nucleus, ribosomes, endoplasmic reticulum, and the Golgi apparatus. Each organelle has their own function that is specialized in protein creation. In the nucleus, DNA is transcribed to mRNA, which then travels to through the cytoplasm to reach the ribosomes on the rough ER. In the ribosome, the mRNA is translated into a polypeptide chain, forming the protein. The synthesized neurotransmitter is then transferred to the Golgi apparatus where it is packaged, enclosed in a vesicle, and moved along the cytoskeleton (motor proteins) to the nerve terminal. Once there, the vesicle fuses with the membrane via exocytosis. 

a) In protein synthesis, describe transcription and translation. Where and how are proteins made?

Transcription takes place in the nucleus. A gene’s DNA sequence is copied into mRNA and RNA polymerase unwinds the DNA and uses one strand as a template to synthesis the complementary mRNA strand. This mRNA carried the genetic code from the DNA out of the nucleus into the cytoplasm

Translation occurs in the cytoplasm at the ribosomes. Here, the ribosome reads the mRNA in sets of three codons. tRNA molecules bring the appropriate amino acids to the ribosome, where the ribosome links them together to form a polypeptide chain, which folds into a functional protein. 

In short, proteins are made first in the nucleus via transcription, and then assembled into their amino acid sequences in the cytoplasm during translation

b) After proteins are made, how are they made functional? What is primary, secondary,

tertiary, and quaternary structure, and how are these made?

After proteins are made, they must be folded and be modified to become functional. The four structures are primary, secondary, tertiary, and quaternary.

Primary - sequence of amino acids. This is made during translation, where amino acids are linked together in a specific sequence

Secondary - local folding into alpha helices or beta sheets that are stabilized by hydrogen bonds. Formed through hydrogen bonds between nearby amino acids

Tertiary - Further folding into a 3d shape. Develop as proteins fold further and are stabilized by bonds.

Quaternary - multiple protein subunits joining together. Multiple polypeptide chains join.

c) After the protein is created, how and where is it packaged, and how is it programmed to be

delivered to the appropriate cellular space?

They take place in the endoplasmic reticulum and the Golgi apparatus. In the ER,  the protein is folded, modified, and sent in transport vesicles to the Golgi apparatus where it undergoes further modification where it is then sorted and packaged into vesicles.

Proteins are “programmed” through signal sequences which are short amino acid tags that direct the protein to its correct location. The vesicles transport the protein to its final destination based on those signals. Motor proteins on the cytoskeleton help guide the vesicle.

d) Once packaged, how does the protein get transported? What cellular machinery is

important in this process?

The protein, packaged in a vesicle, is transported along the cell’s cytoskeleton with microtubules making up the main pathways. Motor proteins like kinesin and dynein attach to vesicles and “walk” along the cytoskeleton using ATP. Some important cellular machinery include microtubules (tracks), motor proteins (moving the vesicles), the vesicles themselves (carry the protein), and ATP (power the cellular machinery).

Homeostasis and feedback systems

Describe the components of negative feedback and how they function in a physiological

system. Compare this to positive feedback.

Negative feedback keeps physiological systems stable by reversing changes. It involves a sensor that detects a change , a control center that processes this information, and an effector that acts to bring the system back to its set point. For example, if your temperature rises, sweating helps cool you down or blood glucose regulation. In contrast, positive feedback amplifies changes. It involves a sensor and control center, but the effector increases the change, driving the system further from its set point. An example is blood clotting. A blood vessel is injured and platelets adhere to the site and release chemicals that attract more platelets. This continues until a clot is formed.

a) What are the components of a feedback system?

Sensor - detects change in a physiological system and sends information to the control center.

Control center - receives and processes the information from the sensor. Then, it determines the appropriate response for the variable.

Effector - carries out the response determined by the control center. It adjusts the variable until it is at the set point.

b) How does the stimulus cause a change?

It can alter a physiological variable from its normal set point. A stimulus is triggered when there is a deviation from the set point detected by the sensor. Then, the sensor sends information to the control center. The control center processes the information and decides on a corrective action. Lastly, the effector executes the response.

c) How does this differ for positive and negative feedback?

In negative feedback, the system counteracts the deviations from a set point to restore homeostasis.

In positive feedback, the system amplifies change, driving the variable further from the set point.

d) In what ways does feedforward differ from positive and negative feedback?

Feedforward involves detecting potential changes early and initiating a response to prevent those changes from disrupting the system. With negative feedback, the response is to changes that have already occurred and is working to return the system to its set point. With positive feedback, it amplifies changed that have occurred, pushing the system further away from the set point to achieve a specific goal.

e) Describe a physiological system for each (do not use the ones in the video or PowerPoint)

Negative: Blood pressure regulation. When blood pressure rises, sensors in the blood vessels detect the increase. The info is sent to the brain’s control center, which then signals the heart to slow down and blood vessels to dilate, reducing the blood pressure back to normal levels

Positive: Lactation. When a baby suckles at the breast, it stimulates sensory receptors in the nipple. This sends signals to the brain to release more prolactin and oxytocin. These hormones increase milk production and release, which encourages the baby to suckle more, further stimulating milk production

Membrane diffusion and transport

Describe Gibbs's free energy and Van’t Hoff’s equation to explain diffusion through a

semipermeable membrane. How does this differ for simple diffusion, facilitated diffusion, and

active transport?

Gibbs's free energy measures the spontaneity of a process, including diffusion through a semipermeable membrane. It shows whether a process will occur naturally. Van’t Hoff’s equation relates this concept to diffusion by linking Gibbs's free energy change to concentration differences across a membrane. For simple diffusion, molecules move from areas of higher to lower concentration due to the natural tendency to increase entropy, driven by a negative Gibbs free energy change. Facilitated diffusion involves carrier or channel proteins aiding this movement, but it still relies on concentration gradients and a negative Gibbs free energy change, just with additional help to cross the membrane. Active transport, however, moves molecules against their concentration gradient, which requires energy input and results in a positive Gibbs free energy change, unlike the spontaneous nature of simple and facilitated diffusion.

a) Describe the plasma membrane and how it can be a semipermeable membrane.

The plasma (cell) membrane is a flexible barrier that surrounds and protects the cell. It is made up of a phospholipid bilayer with hydrophilic heads and hydrophobic tails with the tails pointed inward and the heads pointed outwards. This creates a semi-permeable membrane that allows for certain molecules (small, nonpolar molecules) to pass through while blocking others (larger, polar molecules). Larger and polar molecules need specific protein channels to enter and exit the cell. The membrane is crucial for maintaining homeostasis and regulation of the internal environment

b) Compare and contrast simple diffusion, facilitated diffusion, and active transport. How are

membrane channels and pumps involved in diffusion and active transport?

Simple diffusion allows small, nonpolar molecules to pass through the phospholipid bilayer from high concentration to low concentration without energy. Facilitated diffusion uses membrane protein channels to help larger or polar molecules to cross the membrane from high concentration to low concentration. Active transport moves substance against their concentration gradient (low to high) using ATP and pumps. EX: the sodium potassium pump.

c) Describe the component of Gibbs Free energy and how each variable is involved in

diffusion

ΔG=ΔH−TΔS

ΔH - The change in enthalpy. In diffusion, this is usually negligible because diffusion primarily concerns entropy rather than heat changes

T - the temperature in Kelvin. This affects the energy available for molecular movement. Higher temperatures increase molecular movement and this facilitates diffusion

ΔS - change in entropy. During diffusion, molecules move from high to low concentration, which increases disorder and contributes to negative ΔG, which drives the diffusion process.

• Change in ΔH (Enthalpy): If ΔH increases (more energy is required), ΔG becomes less negative or more positive, making the process less spontaneous. Conversely, if ΔH decreases (less energy required), ΔG becomes more negative, making the process more spontaneous.

• Change in T (Temperature): Increasing the temperature increases the TΔ S term. If ΔS is positive (increased disorder), a higher temperature makes ΔG more negative, enhancing the spontaneity of the process. If ΔS is negative (decreased disorder), a higher temperature makes ΔG less negative or more positive, potentially making the process non-spontaneous.

• Change in ΔS\Delta SΔS (Entropy): If ΔS increases (more disorder), the TΔS term becomes larger and more negative, which makes ΔG more negative and increases the spontaneity of the process. If ΔS decreases (less disorder), TΔS is smaller, making ΔG less negative or more positive, reducing spontaneity.

In summary, increasing temperature and entropy generally promotes spontaneity by making ΔG more negative, while an increase in enthalpy can make a process less spontaneous if it outweighs the effects of temperature and entropy