Biological Membranes, Membrane Transport,And Cell Signaling Ch. 4 & 5

The term "selectively permeable" refers to the membrane's ability to control the movement of certain molecules while allowing others to pass through. This means that the membrane restricts the movement of specific molecules across it, while still allowing other molecules to freely move. This term does not imply that only sodium or potassium movement is restricted, but rather that the membrane can selectively control the passage of different types of molecules.

A small, non polar molecule is more likely to diffuse through the cell membrane because non polar molecules can easily pass through the hydrophobic lipid bilayer of the membrane. The small size of the molecule allows it to fit between the lipid molecules, while the non polar nature ensures compatibility with the hydrophobic interior of the membrane. In contrast, large molecules are hindered by their size, and polar molecules are hindered by their inability to interact with the hydrophobic interior of the membrane. Amphipathic molecules have both polar and non polar regions, but their large size still limits their ability to diffuse efficiently through the membrane.

Neurotransmitters are chemical messengers released from the axon terminal of a neuron that directly communicate with other cells. They are responsible for transmitting signals across the synapse, the junction between two neurons or between a neuron and a target cell. Neurotransmitters play a crucial role in the functioning of the nervous system by facilitating the transmission of electrical impulses and regulating various physiological processes such as cognition, movement, and emotions.

Hormones are chemical messengers that are released into the bloodstream by endocrine glands. Once in the bloodstream, hormones can travel throughout the body and affect target cells in various organs and tissues. They play a crucial role in regulating numerous physiological processes, such as growth, metabolism, reproduction, and mood. Therefore, the correct answer is hormone.

Phagocytosis is a process in which cells engulf large particles, such as bacteria. This process involves the cell membrane forming a coated pit in response to a substance in the extracellular fluid binding to specific receptors on the cell membrane. The coated pit then invaginates and enters the cell, allowing the engulfment of the large particles. This explanation accurately describes the process of phagocytosis.

Gap junctions are specialized channels that directly connect the cytoplasm of adjacent cells, allowing for direct communication and exchange of small molecules and ions. These junctions play a crucial role in coordinating the activities of cells in tissues and organs. Unlike ligands, receptors, paracrine chemical messengers, and endocrine hormones, which involve signaling molecules that need to be released and received by target cells, gap junctions provide a direct pathway for electrical and chemical signals to pass between cells.

A receptor protein-tyrosine kinase catalyzes the phosphorylation of a protein inside the cell. This means that it adds a phosphate group to a protein molecule, which can have various effects on the protein's structure and function. Phosphorylation is an important mechanism in cell signaling, as it can activate or deactivate proteins, regulate enzyme activity, and control cellular processes such as growth, division, and differentiation. Therefore, a receptor protein-tyrosine kinase plays a crucial role in intracellular signaling pathways.

Molecules with a relatively high lipophilicity are capable of crossing the lipid bilayer directly. Lipophilicity refers to the ability of a molecule to dissolve in lipids or fats. Since the lipid bilayer is composed of phospholipids, which have a hydrophobic (water-repelling) tail, lipophilic molecules can easily pass through this hydrophobic region without the need for any additional energy or assistance from proteins or carrier molecules. Therefore, they can cross the membrane directly by moving through the lipid bilayer.

When the alpha subunit of the G protein is activated, it can stimulate ion channels to either open or close. This is because the alpha subunit can interact with the ion channels and trigger a conformational change that either allows the channel to open and allow ions to pass through or closes the channel and prevents ions from passing through. This activation of ion channels by the alpha subunit of the G protein is an important mechanism for regulating ion flow and cellular signaling.

The intracellular fluid is described as being rich in proteins and potassium. This means that within the cells of the body, the fluid contains a high concentration of proteins and potassium ions. This is important for maintaining cell function and regulating various cellular processes.

All three transport mechanisms - facilitated diffusion, primary active transport, and secondary active transport - require a protein for the transportation process. They are all specific for particular solutes and have a limit to the total number of molecules that can be transported per unit of time. Additionally, transport can be increased by increasing the number of transport molecules in the plasma membrane. However, the one statement that is NOT true for all three transport mechanisms is that they require energy. Facilitated diffusion does not require energy, while primary and secondary active transport both require energy in the form of ATP.

Passive transport refers to the movement of molecules or ions across a membrane without the need for energy. The movement of sodium through ion channels is a passive transport mechanism because it occurs down its concentration gradient, without the need for ATP or any other energy source. In contrast, cotransport of glucose with sodium, transport of sodium and potassium across the membrane by the Na+/K+ pump, and movement of calcium out of the cytosol all require energy in the form of ATP. Movement of sodium out of the cytosol can also be an active process, depending on the specific mechanism involved.

The affinity of a receptor for its ligand refers to the strength of the binding between the ligand and receptor. It indicates how likely the ligand is to bind to the receptor and form a stable complex. A higher affinity means a stronger binding interaction, while a lower affinity indicates a weaker binding. Therefore, the correct answer is "the strength of binding between ligand and receptor."

A disaccharide is least likely to cross the plasma membrane because it is a larger molecule composed of two sugar units joined together. The plasma membrane is selectively permeable and allows only small, non-polar molecules to pass through. Disaccharides are larger and more polar than other options such as fatty acids, oxygen, and steroids, making it difficult for them to pass through the membrane. Water, although polar, is a small molecule and can easily cross the plasma membrane through specialized channels called aquaporins.

CO2 is the correct answer because it is a small, nonpolar molecule that can easily dissolve in the lipid bilayer of the cell membrane. Being nonpolar, CO2 does not interact strongly with the polar head groups of the lipid molecules, allowing it to pass through the membrane without the need for a transport protein. In contrast, water, glucose, Ca+2, and ethanol are all polar or charged molecules that cannot easily pass through the nonpolar lipid bilayer without the assistance of specific transport proteins.

Primary active transporters are unique in their ability to couple the movement of ions directly to ATP. This means that these transporters use the energy from ATP hydrolysis to actively transport ions across a membrane against their electrochemical gradient. This process is essential for maintaining ion concentration gradients and is crucial for various cellular processes such as nerve conduction, muscle contraction, and nutrient uptake.

The magnitude of a cell's response to a specific hydrophilic ligand is not affected by the lipid solubility of the ligand. Hydrophilic ligands are not able to cross the cell membrane easily due to their hydrophilic nature. Therefore, the lipid solubility of the ligand does not play a role in determining the magnitude of the cell's response. The other factors listed, such as ligand concentration, number of receptors, receptor upregulation, and affinity of the receptor for the ligand, can all influence the cell's response to the ligand.

As cells become more active and oxidative phosphorylation increases, the demand for oxygen also increases. This leads to a decrease in the concentration of oxygen in the blood cells, creating a concentration gradient between the blood cells and the cell. According to Fick's law of diffusion, substances move from an area of higher concentration to an area of lower concentration. Therefore, the concentration gradient for oxygen increases, and oxygen movement into the cell also increases.

Estrogen, being a steroid hormone, acts by altering transcription of mRNA. Steroid hormones, including estrogen, bind to intracellular receptors located in the cytoplasm or nucleus. Once bound, the hormone-receptor complex enters the nucleus and binds to specific DNA sequences called hormone response elements (HREs) located in the promoter region of target genes. This binding leads to the recruitment of transcription factors and other co-regulators, ultimately resulting in the transcription of specific mRNA molecules. These mRNA molecules are then translated into proteins, which mediate the physiological effects of estrogen in the cell.

Lipophilic hormones are hydrophobic and cannot dissolve in the aqueous plasma. Therefore, they bind to carrier proteins in the blood to be transported to their target cells. Once inside the target cell, the lipophilic hormones dissociate from the carrier proteins and bind to receptors located inside the cell. This allows the hormones to exert their effects on the target cell.

The fluid compartment with high sodium concentration contains only trace amounts of protein is called interstitial fluid. This fluid is found between the cells and tissues of the body and plays a crucial role in maintaining the balance of electrolytes and nutrients. It allows for the exchange of substances between the cells and blood vessels, providing them with necessary nutrients and removing waste products. Unlike plasma, which is the liquid component of blood and contains proteins, interstitial fluid has very low protein content.