Unit 2: Cell Membrane and Permeability - Subjective Questions

Biological membranes are mainly composed of lipids, proteins, and carbohydrates.

• Lipids: Phospholipids form the basic bilayer. Each phospholipid has a hydrophilic phosphate head and hydrophobic fatty acid tails. Cholesterol is also present and helps regulate membrane fluidity and stability.
• Proteins: Integral and peripheral proteins perform functions such as transport, enzymatic activity, signal reception, cell adhesion, and communication.
• Carbohydrates: Short carbohydrate chains are attached to lipids and proteins, forming glycolipids and glycoproteins. They are involved in cell recognition, protection, and cell-to-cell interactions.

The relative proportions of these components vary depending on the type and function of the membrane.

A phospholipid consists of:

• A polar, hydrophilic head containing a phosphate group.
• Two nonpolar, hydrophobic fatty acid tails.

When phospholipids are placed in water, their hydrophilic heads interact with water, while their hydrophobic tails avoid water. Therefore, they spontaneously arrange into a phospholipid bilayer:

• The hydrophilic heads face the surrounding water on both sides.
• The hydrophobic tails face inward toward each other.

This arrangement forms a stable, selectively permeable barrier and provides the basic structural framework of the cell membrane.

The Fluid Mosaic Model describes the cell membrane as a dynamic structure made of different components.

• Fluid: Phospholipids and some proteins can move laterally within the membrane, giving it flexibility and fluidity.
• Mosaic: Different proteins, lipids, and carbohydrate-containing molecules are arranged in a pattern resembling a mosaic.
• Bilayer: The membrane consists of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.
• Proteins: Integral proteins are embedded in the bilayer, whereas peripheral proteins are attached to the membrane surface.
• Carbohydrates: Carbohydrate chains occur mainly on the external surface as components of glycoproteins and glycolipids.

This model explains how membranes remain flexible while performing transport, communication, recognition, and enzymatic functions.

Membrane fluidity refers to the ability of membrane lipids and some proteins to move within the plane of the bilayer. It is essential for membrane flexibility, transport, fusion, and cell signaling.

Important factors include:

• Temperature: Increasing temperature increases fluidity, while decreasing temperature reduces fluidity.
• Fatty acid saturation: Unsaturated fatty acids contain double bonds that create bends in their tails, preventing tight packing and increasing fluidity. Saturated fatty acids pack closely and reduce fluidity.
• Fatty acid chain length: Shorter hydrocarbon chains have weaker interactions and increase fluidity. Longer chains reduce fluidity.
• Cholesterol: At high temperatures, cholesterol restrains phospholipid movement and decreases excessive fluidity. At low temperatures, it prevents tight packing and helps maintain fluidity.

Cells adjust lipid composition to maintain suitable membrane fluidity.

Saturated fatty acid tails:

• Contain no carbon-carbon double bonds.
• Are relatively straight.
• Pack closely together.
• Increase membrane thickness and stability.
• Decrease membrane fluidity.

Unsaturated fatty acid tails:

• Contain one or more carbon-carbon double bonds.
• Have bends or kinks in their structure.
• Cannot pack tightly.
• Increase the spacing between phospholipids.
• Increase membrane fluidity and flexibility.

Thus, membranes containing more unsaturated fatty acids remain more fluid, especially at lower temperatures.

Cholesterol is located between phospholipid molecules in the bilayer and acts as a fluidity buffer.

• At high temperatures, cholesterol restricts the movement of phospholipid molecules. This reduces excessive fluidity and makes the membrane less permeable to some small molecules.
• At low temperatures, cholesterol prevents phospholipids from packing too closely together. This prevents the membrane from becoming rigid or solid.
• Cholesterol also increases mechanical stability and reduces membrane permeability to certain water-soluble substances.

Therefore, cholesterol helps maintain a relatively stable membrane state over a range of temperatures.

Membrane proteins are classified according to their association with the lipid bilayer.

• Integral membrane proteins: These are firmly embedded in the phospholipid bilayer. Their hydrophobic regions interact with the membrane interior.
• Transmembrane proteins: These are a type of integral protein that spans the entire bilayer. Their hydrophobic regions face the fatty acid tails, while their hydrophilic regions project into the aqueous environments on either side.
• Peripheral membrane proteins: These are loosely attached to the inner or outer surface of the membrane. They may interact with integral proteins, phospholipid heads, or cytoskeletal elements.

The structure of each type is related to its specific role in transport, signaling, attachment, or enzymatic activity.

Membrane proteins perform several important functions:

• Transport: Channel proteins and carrier proteins move ions and molecules across the membrane.
• Enzymatic activity: Some proteins catalyze reactions at the membrane surface.
• Receptors: Receptor proteins bind specific signaling molecules such as hormones or neurotransmitters.
• Cell recognition: Glycoproteins help cells identify one another.
• Cell adhesion: Adhesion proteins attach cells to neighboring cells or to the extracellular matrix.
• Intercellular communication: Junctional proteins form connections that allow cells to exchange information or materials.
• Attachment: Membrane proteins anchor the membrane to the cytoskeleton or extracellular structures.

Thus, membrane proteins are essential for both the structural and physiological functions of cells.

Channel proteins:

• Form hydrophilic pores through the membrane.
• Allow specific ions or molecules to pass through.
• Usually transport substances down their concentration or electrochemical gradient.
• May be gated by voltage, chemicals, or mechanical stimuli.
• Do not usually bind and completely enclose the transported substance.

Carrier proteins:

• Bind specifically to the substance being transported.
• Change their shape to move the substance across the membrane.
• Can function in facilitated diffusion or active transport.
• Transport is generally slower than movement through an open channel.

Both types provide selective pathways through the hydrophobic interior of the membrane.

Passive transport is the movement of substances across a membrane without the direct use of cellular energy in the form of ATP. Substances move down their concentration or electrochemical gradient.

Its main types are:

• Simple diffusion: Small nonpolar molecules, such as oxygen and carbon dioxide, move directly through the lipid bilayer.
• Facilitated diffusion: Polar molecules or ions move through specific channel or carrier proteins.
• Osmosis: Water moves across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.

Passive transport continues until equilibrium is reached, although individual molecules continue moving randomly.

Simple diffusion is the direct movement of small, nonpolar molecules through the phospholipid bilayer. It does not require membrane proteins or ATP.

Facilitated diffusion is the movement of ions or polar molecules through specific channel or carrier proteins. It also does not require ATP and occurs down the concentration gradient.

Similarities:

• Both are passive processes.
• Both move substances down a concentration or electrochemical gradient.
• Neither requires direct ATP consumption.

Differences:

• Simple diffusion occurs through the lipid bilayer, whereas facilitated diffusion requires membrane proteins.
• Facilitated diffusion is more selective and can become saturated when all transport proteins are occupied.
• Simple diffusion is mainly used by small nonpolar molecules, while facilitated diffusion transports ions and larger polar molecules.

Osmosis is the passive movement of water across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

• Hypotonic solution: The external solution has a lower solute concentration than the cell. Water enters the cell. Animal cells may swell and burst, while plant cells become turgid because of the cell wall.
• Isotonic solution: The solute concentration is approximately equal inside and outside the cell. There is no net movement of water, and animal cells maintain their normal shape. Plant cells may become flaccid.
• Hypertonic solution: The external solution has a higher solute concentration than the cell. Water leaves the cell. Animal cells shrink, while plant cells undergo plasmolysis as the membrane pulls away from the cell wall.

Active transport is the movement of substances across a membrane against their concentration or electrochemical gradient. It requires energy, usually supplied by ATP, and uses specific transport proteins.

Differences from passive transport:

• Active transport requires cellular energy, whereas passive transport does not directly require ATP.
• Active transport moves substances from lower to higher concentration, whereas passive transport moves substances from higher to lower concentration.
• Active transport depends on pumps or carrier proteins, while passive transport may occur through the lipid bilayer, channels, or carriers.
• Active transport can maintain concentration gradients, whereas passive transport tends to reduce them.

Examples of active transport include the sodium-potassium pump and proton pumps.

The sodium-potassium pump is an ATP-powered transport protein found in the plasma membrane of animal cells.

1. Three sodium ions bind to the pump on the cytoplasmic side.
2. ATP phosphorylates the pump, causing a conformational change.
3. The pump releases the three sodium ions outside the cell.
4. Two potassium ions bind to the pump from the extracellular side.
5. The phosphate group is released, causing the pump to return to its original shape.
6. The two potassium ions are released into the cytoplasm.

Overall, one ATP molecule is used to move three sodium ions out and two potassium ions into the cell. This maintains ion gradients, contributes to the resting membrane potential, regulates cell volume, and supports nerve impulse transmission.

Primary active transport uses energy directly from ATP hydrolysis to move substances against their electrochemical gradients. An example is the sodium-potassium pump, which transports sodium ions out of the cell and potassium ions into the cell.

Secondary active transport does not use ATP directly at the transport protein. Instead, it uses the energy stored in an electrochemical gradient created by primary active transport.

• Symport: Two substances move in the same direction. For example, sodium and glucose may enter an intestinal cell together.
• Antiport: Two substances move in opposite directions. For example, a sodium-calcium exchanger uses sodium movement to drive calcium export.

Both mechanisms ultimately depend on energy supplied by cellular metabolism.

The electrochemical potential of an ion is determined by two forces:

• The chemical gradient, which results from a difference in ion concentration across the membrane.
• The electrical gradient, which results from a difference in charge across the membrane.

An ion moves in the direction favored by the combined effect of these forces. For example, a positively charged ion may be attracted to a negatively charged side of the membrane even if its concentration gradient opposes that movement.

Electrochemical gradients are important for:

• Generating membrane potentials.
• Transmitting nerve impulses.
• Driving secondary active transport.
• Producing ATP in mitochondria and chloroplasts.

Transport proteins may allow ions to move down this gradient or use energy to move them against it.

Endocytosis is the process by which a cell takes in materials from the extracellular environment by forming an inward vesicle from the plasma membrane. It requires energy and involves membrane deformation.

Major types include:

• Phagocytosis: The cell engulfs large particles, microorganisms, or cellular debris. It is common in specialized immune cells.
• Pinocytosis: The cell takes in extracellular fluid and dissolved substances in small vesicles.
• Receptor-mediated endocytosis: Specific molecules bind to receptors on the cell surface, and the membrane forms a coated vesicle to internalize them selectively.

Endocytosis is important for nutrient uptake, removal of materials, regulation of surface receptors, and defense against pathogens.

Exocytosis is the process by which materials inside a vesicle are released outside the cell. The vesicle moves toward the plasma membrane, fuses with it, and discharges its contents into the extracellular space.

The process involves:

1. Formation of a secretory or transport vesicle.
2. Movement of the vesicle toward the plasma membrane.
3. Recognition and docking of the vesicle at the membrane.
4. Fusion of the vesicle membrane with the plasma membrane.
5. Release of the vesicle contents outside the cell.

Exocytosis is used to secrete hormones, neurotransmitters, digestive enzymes, and extracellular matrix components. It also adds lipids and proteins to the plasma membrane.

Endocytosis:

• Moves substances from outside to inside the cell.
• The plasma membrane folds inward and forms a vesicle.
• Requires cellular energy.
• Includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.
• Examples include uptake of bacteria, fluids, cholesterol, and nutrients.

Exocytosis:

• Moves substances from inside to outside the cell.
• An intracellular vesicle fuses with the plasma membrane.
• Requires cellular energy.
• Used for secretion and membrane recycling.
• Examples include release of neurotransmitters, hormones, and digestive enzymes.

Both processes are forms of bulk transport and are important for maintaining membrane composition and cell communication.

Receptor-mediated endocytosis is a highly selective process for taking specific substances into the cell.

• Receptor proteins located on the extracellular surface bind particular ligands, such as cholesterol-containing lipoproteins or hormones.
• Binding causes the membrane to gather around the receptor-ligand complexes.
• A vesicle forms and carries the selected materials into the cell.
• The vesicle may later fuse with an endosome, where the ligand and receptor can be separated and processed.

This process demonstrates membrane specificity because only molecules that fit particular receptors are efficiently internalized. It allows cells to obtain essential materials even when their extracellular concentration is relatively low.