Plasma Membranes, Transport and Enzymes - Chapter 5

Characteristics of Membranes:
- Act as a great barrier and are semi-permeable.
- They restrict passage of many polar molecules.
- Comprised predominantly of phospholipids, which form a barrier allowing non-polar molecules to pass through, such as:
- Gases (e.g., Oxygen (O₂), Carbon Dioxide (CO₂))
- Lipids
- Water (H₂O) can also permeate, facilitated by aquaporin proteins.

Components of Phospholipids:
- Each phospholipid consists of a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.
- The hydrophilic head includes:
  - A phosphate group attached to a glycerol molecule.
- The hydrophobic tails consist of:
  - Saturated or unsaturated fatty acids, characterized by long hydrocarbon chains.
- Visual Representation:
- Structural formula, space-filling model, and symbolic representation illustrate the phospholipid's architecture.

Roles of Embedded Proteins:
- Embedded proteins in the membrane serve crucial communication roles, including:
- Acting as cell receptors that receive signals and relay information to the cell.
- Adding structural integrity to the membrane, which is pliable.

Integral Proteins:
- These proteins fully integrate into the membrane structure.
- Their hydrophobic areas dissolve in fatty acid tails, while hydrophilic regions are exposed to the cytosol or extracellular fluid.
Peripheral Proteins:
- These proteins do not penetrate completely and are located on the exterior or interior surfaces of the membrane.
- Principle of "like dissolves like" is applicable to understanding how these proteins associate with the membrane.

Transport Proteins:
- Essential for allowing specific ions or molecules to enter or exit the cell, particularly because many necessary nutrients are polar.

Diffusion:
- Molecules naturally move from areas of high concentration to areas of low concentration, a process which can occur across membranes and in other media, such as gases.
- This movement is known as passive transport and requires no energy (ATP).
Factors Influencing Diffusion:
- Concentration gradient
- Mass of solute
- Temperature
- Solubility
- Surface area and thickness of the membrane
- Distance to be traveled
Facilitated Diffusion:
- Involves the movement of solute across a membrane utilizing protein facilitators, transitioning from higher solute concentration to lower concentration without energy expenditure.

Definition:
- Osmosis refers to the diffusion of water across a membrane, crucial for maintaining cellular homeostasis.
- Water migrates toward areas of higher solute concentration, which is a mechanism through which the body manages dissolved molecules.
Tonicity:
- Refers to the concentration of impermeable solutes in a solution, classified as:
- Hypertonic: Higher solute concentration, resulting in water exiting cells, potentially causing them to shrink.
- Hypotonic: Lower solute concentration, leading to water entering cells, potentially causing them to swell or lyse.
- Isotonic: Equal solute concentration, maintaining cell size and function.

Definition & Mechanism:
- Active transport involves moving molecules or ions across membranes using energy (ATP) against their concentration gradient (from low to high).
- ATP causes conformational changes in proteins, allowing the transportation of materials.

Exocytosis:
- A process where vesicles containing materials fuse with the plasma membrane and release their contents into the extracellular fluid. The vesicle integrates into the membrane during this process.
Endocytosis:
- The process of internalizing substances where the cellular membrane engulfs materials to form a vesicle.
- Three Types to Note:
- Phagocytosis: Cell engulfs solid particulates (food).
- Receptor-Mediated Endocytosis: Specific molecules bind to receptors, triggering vesicle formation (important for many pathogens).
- Pinocytosis: The cell ingests extracellular fluid (water specifically).

Energy Requirements:
- All cells depend on energy, which cannot be created or destroyed but can change forms (Law of Conservation of Energy).

Potential Energy:
- Stored energy; specifically, the energy in chemical bonds.
- Energy in food is converted to ATP energy via chemical reactions involving bonds.
Kinetic Energy:
- Energy in motion, applicable to physical movement of objects and molecular activity.

Exergonic Reactions:
- Reactions that release energy, resulting in products with less energy than reactants.
Endergonic Reactions:
- Reactions that require energy input, yielding products with more energy than reactants.

Definition:
- ATP (Adenosine Triphosphate) serves as the primary energy currency of the cell.
Characteristics:
- Composed of a nucleotide structure; phosphodiester bonds present in ATP are high-energy.
- Forming these bonds is endergonic, whereas breaking them is exergonic.
Cycle of ATP:
- ATP synthesis is endergonic while ATP hydrolysis is exergonic, driving cellular work. ATP can be regenerated from ADP.

Enzymes:
- Biological catalysts that lower activation energy requirements for chemical reactions without being consumed or permanently changed in the process.
- Enzymes display high specificity and can be conceptualized as energy coupons.

Activation energy is the minimum energy necessary to initiate a chemical reaction, preventing most reactions from proceeding spontaneously without sufficient energy input.

Active Site: The specific region of an enzyme where substrate molecules bind and undergo a chemical reaction.

Types of Inhibitors:
- Competitive Inhibitors: Bind to the active site, competing with the substrate for binding.
- Non-competitive Inhibitors: Bind elsewhere, causing conformational changes to the enzyme that affect the active site and enzyme function.

A regulatory mechanism where the final product of a multi-step biochemical pathway inhibits an early enzyme, preventing excessive product accumulation and conserving energy.