1.3 Membrane structure Flashcards
What are amphipathic molecules?
Amphipathic molecules are those that have both hydrophilic (water-loving) and hydrophobic (water-fearing) parts, allowing them to interact with both water and lipid environments.
Why are phospholipids considered amphipathic?
Phospholipids are considered amphipathic because they have hydrophilic phosphate heads that attract water and hydrophobic fatty acid tails that repel water, enabling them to form the bilayer structure of cell membranes.
Describe the structure and function of cholesterol in cell membranes.
Cholesterol is a steroid with a hydrophilic hydroxyl group and a hydrophobic ring structure and tail. It stabilizes membrane fluidity and permeability by restricting the movement of phospholipids and other molecules, thus playing a crucial role in the proper functioning of animal cell membranes.
What was the fundamental flaw of the Davson-Danielli membrane model as proposed in 1935?
The Davson-Danielli model suggested membranes had a uniform structure with protein layers outside a lipid bilayer, failing to account for the diverse functions and structures of different membranes. It was eventually disproven by evidence showing proteins embedded within the lipid bilayer.
Explain the fluid mosaic model proposed by Singer and Nicolson in 1972.
The fluid mosaic model describes biological membranes as phospholipid bilayers with proteins embedded throughout, allowing for a flexible and dynamic membrane structure. This model accounts for the diverse functions of membrane proteins, including transport, enzyme activity, and cell recognition.
Differentiate between integral and peripheral proteins in the cell membrane.
Integral proteins are embedded within the plasma membrane and often span its entire thickness, having both hydrophobic and hydrophilic regions. Peripheral proteins are attached to the exterior of the plasma membrane and are primarily hydrophilic.
How do glycolipids contribute to cell membrane function?
Glycolipids, composed of a phospholipid and a carbohydrate, play roles in maintaining cell membrane structure and facilitating cell recognition processes, helping to distinguish between self and non-self cells.
What evidence led to the proposal of the Davson-Danielli model, and how was it ultimately disproven?
The Davson-Danielli model was based on electron microscopy evidence and the assumption of a 2:1 lipid-to-cell membrane surface area ratio from earlier studies. It was disproven by freeze-fracture techniques and advancements in microscopy that showed proteins embedded within the lipid bilayer, supporting the fluid mosaic model.
List and describe the diverse functions of membrane proteins.
Membrane proteins perform a variety of functions including: Ion channels (allow specific ions to pass through the membrane), Receptor proteins (bind to external chemicals to regulate processes within the cell), Enzymes (speed up chemical reactions at the membrane), Carrier proteins (transport substances across the membrane), and Cell adhesion molecules (help cells to adhere to each other and to the extracellular matrix).
How does cholesterol affect the permeability of mammalian cell membranes to specific solutes?
Cholesterol modulates cell membrane permeability by making the membrane less permeable to small water-soluble molecules when temperatures are high, thus stabilizing the membrane and reducing fluidity. At low temperatures, it prevents tight packing of phospholipids, thereby increasing fluidity and preventing solidification, which can enhance or reduce the permeability to different solutes.
What evidence from electron microscopy led to the proposal of the Davson–Danielli model?
The Davson-Danielli model was proposed based on electron microscopy showing cell membranes as three-layered structures with two dark lines flanking a lighter space, suggesting a lipid bilayer sandwiched between two layers of proteins. This interpretation came from the appearance of proteins as dark and lipids as lighter in electron micrographs.
How was the Davson-Danielli model falsified, leading to the adoption of the Singer-Nicolson fluid mosaic model?
The Davson-Danielli model was falsified by evidence from freeze-fracture electron microscopy, which showed irregular, rough surfaces inside the phospholipid bilayer, indicating that proteins were embedded within the bilayer itself. This observation supported the fluid mosaic model, where proteins float within the lipid bilayer, not just outside it.
What function do channel proteins in cell membranes serve?
Channel proteins allow the passive transport of substances between the inside and outside of the cell without requiring energy.
What role do carrier proteins play in cell membrane function?
Carrier proteins bind to substances on one side of the membrane, change shape, and transport them to the other side. Those that use energy are called protein pumps.
How do certain proteins contribute to a cell’s ability to recognize self from non-self cells?
Certain proteins aid the cell in distinguishing between self and non-self cells, crucial for triggering an immune response.
