BIO 111 - Membrane Structure and Function

BIO 111: Membrane Structure and Function (Class 10)

Introduction to Membrane Function

Membranes are crucial structures in cells, and understanding their function is fundamental for comprehending subsequent chapters. The primary focus of this section is to explore how membranes work and what they accomplish.

The Fluid Mosaic Model

Cell membranes are not rigid structures. They are best described by the fluid mosaic model. This model emphasizes that membranes are highly dynamic and flexible, not static. The integrity of the membrane is primarily maintained by hydrophobicity, where the non-polar fatty acid tails of phospholipids cluster together.

To visualize the fluid mosaic model, imagine a bathtub filled with water, its surface packed with a single layer of ping-pong balls representing phospholipid heads. If you create a wave in the bathtub, the ping-pong balls move fluidly with the wave, demonstrating the non-rigid nature of the membrane. Now, add tennis balls to represent membrane-bound proteins. These tennis balls would also move up and down and around within the fluid layer of ping-pong balls, illustrating how proteins are embedded and can move within the fluid lipid bilayer.

Phospholipid Structure and Membrane Fluidity

Phospholipids consist of a phosphate head and two fatty acid tails:

• The phosphate head is charged and hydrophilic, meaning it interacts favorably with water.

• The hydrocarbon tails are non-polar and hydrophobic, meaning they cluster away from water.

The arrangement of fatty acid tails significantly impacts membrane fluidity:

• If both fatty acid tails were straight and saturated, they would stack more tightly, resulting in a less fluid membrane.

• If both fatty acid tails were bent and unsaturated, they would stack less tightly, leading to a more fluid membrane.
  The balance of saturated and unsaturated fatty acids is crucial for maintaining optimal membrane fluidity. In water, phospholipids naturally cluster, forming bilayers where the hydrophobic tails face each other, creating a boundary that partitions water to either side of the membrane, attracted to the hydrophilic phosphate heads.

Modified Phospholipids and Specialization

Phospholipids can be modified to carry out specialized functions:

• Glycolipids are formed when sugars are linked to a phospholipid.

• Other modifications include linking amino acids, such as serine (e.g., phosphatidylserine), to phospholipids. These specialized phospholipids are often involved in specific cellular responses, such as inflammation.

Cholesterol: Rigidity, Animal-Specificity, and Health Implications

Cholesterol is another lipid found in membranes with critical roles:

• Membrane Rigidity: The amount of cholesterol directly influences membrane rigidity. Higher cholesterol content leads to a more rigid membrane, while less cholesterol makes it less rigid. This rigidity can be localized, forming lipid rafts—platforms within the membrane that often contain higher concentrations of cholesterol and serve as organizational centers for signaling proteins.

• Animal Cell Specificity: Cholesterol is found only in animal cells. It is absent in plant cells (explaining why plant products like spinach do not contain cholesterol) and prokaryotes.

• Essential Precursor Molecule: Cholesterol is vital for animals because it serves as a precursor for synthesizing various hormones, including:

  • Sex hormones like testosterone and estrogen.

  • The stress hormone cortisol.

• Health Concerns: Due to its non-polar and hydrophobic nature, cholesterol does not readily break down in water and can accumulate. High dietary intake or genetic predispositions can lead to cholesterol buildup in blood vessels, forming plaque. Plaque decreases blood flow and can lead to serious cardiovascular issues:

  • Complete blockage in the heart causes a heart attack.

  • Complete blockage in the brain causes a stroke.

  • Coronary bypass surgery is a procedure to reroute blood flow around severely blocked arteries, often using a vein from the leg.

• Cholesterol Transport: Because cholesterol is so hydrophobic that it "sticks" in the membrane, it requires special transport to get into cells for hormone synthesis. It is transported via protein complexes called lipoproteins:

  • High-density lipoproteins (HDLs)

  • Low-density lipoproteins (LDLs)
    Monitoring HDL and LDL levels is a standard part of blood tests.

Functions of Membrane Proteins

Proteins embedded within or associated with the membrane perform a variety of crucial functions:

1. Transport

Membranes act as a barrier, especially to polar, charged, and large molecules. Transport proteins facilitate the movement of necessary substances (like glucose, a polar monosaccharide) across the membrane, allowing them to enter or exit the cell despite the hydrophobic barrier.

2. Enzymatic Activity

Membrane proteins can function as enzymes, which are biological catalysts that speed up specific reactions without being consumed in the process. Enzymes are highly specific to their substrates.

• Example: Lactose intolerance is due to the lack of the enzyme lactase, which rapidly breaks the glycosidic bond in lactose.

• Nomenclature: A general rule of thumb (though not absolute) is that enzyme names often end in "-ase" (e.g., sucrase acts on sucrose).

3. Cell Surface Receptors

Receptor proteins on the cell surface bind to specific signaling molecules (ligands) from the extracellular environment, triggering a response inside the cell. They act like a "baseball glove" catching a specific "baseball" or a "satellite dish" receiving a signal.

• This function is critical for cell communication, a topic covered in an entire chapter.

4. Cell Surface Identity Markers

These proteins (often glycoproteins or glycolipids) enable cells to recognize each other and distinguish "self" from "non-self."

• Example: After a cat bite, the body's immune system recognizes foreign cells as non-self due to differing cell surface markers.

• Organ Transplants: Kidney transplant recipients require a "match" for cell surface markers. Even with a close match, immunosuppressive drugs are often necessary to prevent the recipient's body from attacking the transplanted organ.

5. Cell to Cell Adhesion

Adhesion proteins help cells of a specific tissue type stick together, enabling the formation and integrity of organs and tissues. These proteins are located on the outside of the cell and bind to similar proteins on adjacent cells.

6. Attachment to the Cytoskeleton

Membrane proteins can link the membrane to the intracellular cytoskeleton, providing structural support and helping to anchor membrane components. This involves various cytoskeletal proteins based on size:

• Smallest: Actin filaments

• Intermediate: Intermediate filaments

• Largest: Microtubules

• Proteins like integrins can span the membrane and link to actin or intermediate filaments, such as those involved in tight junctions and desmosomes.

• Example: The tight junctions lining the stomach prevent highly acidic contents (pH = 1) from dissolving the stomach cells by forming a seal that prevents even liquid from passing between cells.

Transmembrane Protein Structure and Function

Integral or transmembrane proteins span the entire membrane. Their structure allows them to create pathways through the hydrophobic lipid bilayer:

• Hydrophilic Pores: Transmembrane proteins often form a central core that is hydrophilic, providing a通道 for polar substances. This can be achieved through:

  • Beta-sheets: When a beta-sheet rolls up, its polar side chains can face inward, creating a hydrophilic channel.

  • Alpha-helices: A group of alpha-helices can also cluster to form a hydrophilic pore.

• Gated Channels: Some protein channels can act like gates, opening and closing to regulate the passage of specific molecules (analogous to opening a gate for cows to move between fields).

Examples of Transmembrane Proteins:

1. Single-pass Transmembrane Protein: A polypeptide chain passes through the membrane once, typically as a single alpha-helix. The N-terminal and C-terminal ends can be on opposite sides of the membrane (e.g., N-terminal extracellular, C-terminal intracellular).

2. Multi-pass Transmembrane Protein: A polypeptide chain passes through the membrane multiple times (e.g., a three-pass transmembrane protein shown with three alpha-helices). Each pass is connected by loops of polypeptide chain that lack a specific secondary structure.

3. Beta-Barrel Proteins: Formed by multiple beta-sheets rolled into a barrel shape, creating a pore. The N- and C-terminals can both be on the same side of the membrane.

Peripheral proteins associate with only one side of the membrane without spanning it.

Viral Entry: A Receptor-Mediated Process

Viruses, typically composed of nucleic acids (DNA or RNA) enclosed in a protein coat, are too large and charged to passively cross the cell membrane. They enter cells through receptor-mediated mechanisms:

• A virus's specific protein (e.g., a spike protein) binds to a complementary cell surface receptor (e.g., CD4 receptor).

• This binding can trigger a conformational change in another transmembrane protein, often a seven-pass transmembrane protein (also known as a serpentine protein due to its snake-like path through the membrane).

• Activation of this protein can initiate phagocytosis, where the cell engulfs the virus, allowing it to enter.

• Example: The roundworm C. elegans has a body composed of exactly 959 cells, and approximately 5% of its entire genome is dedicated to making these seven-pass transmembrane proteins, highlighting their biological importance.

Passive Transport Across Membranes

Passive transport is the movement of substances across a membrane without the direct expenditure of cellular energy.

Key Characteristics:

• No Energy Required: Driven by inherent physicochemical forces.

• Concentration Gradient Dependent: Substances move from an area of high concentration to low concentration (down their concentration gradient). This is a fundamental chemical principle; molecules tend to spread out until evenly distributed (e.g., perfume diffusing throughout a room).

Factors Affecting Permeability (Passive Transport):

• Non-polar molecules: Gases like oxygen (O_2) are non-polar and small, so the membrane is not a barrier at all. They easily diffuse across.

• Small polar molecules: Water (H_2O) is polar but exceptionally small, allowing it to pass through the hydrophobic membrane, though at a slower rate than non-polar molecules.

• Ions: Even the smallest ions, such as a hydrogen ion (H^+—a single proton), cannot cross the membrane due to their charge. The hydrophobic interior of the membrane strongly repels charged particles.

• Large polar molecules: Generally cannot cross the membrane without assistance, as they are both polar and too big.

• Steroid hormones: Hormones like estrogen and testosterone are large, but they are derived from cholesterol and are relatively non-polar, allowing them to pass across the membrane through passive diffusion, although they are not as hydrophobic as cholesterol itself. Cholesterol, being extremely hydrophobic, tends to stick within the membrane and requires special transport (e.g., LDL, HDL) to enter or exit cells.

Examination Information

The upcoming exam on Wednesday will cover Chapters 1 to 4. Students are encouraged to review these chapters thoroughly. The instructor emphasizes that personal self-worth is not defined by exam scores and expects the average score to be around a C. A word of prayer will be offered before the exam for those who wish to attend.