Cell Membranes and Transport Flashcards
Describe the structure of a phospholipid molecule.
There is a hydrophilic head containing phosphate group and two hydrophobic fatty acid tails.
Why is the membrane model described as a fluid mosaic model?
It is described as “fluid” because both the phospholipids and the proteins can move about by diffusion. The word “mosaic” describes the pattern produced by the scattered protein molecules when the surface of the membrane is viewed from above.
Describe the features of the fluid mosaic model.
- Phospholipid Bilayer: Forms the basic structure; hydrophobic tails face inward, and hydrophilic heads face outward.
- Fluidity:
• More unsaturated fatty acid tails increase fluidity.
• Longer fatty acid tails and lower temperatures reduce fluidity. - Proteins:
• Found in the inner layer, outer layer, or spanning the membrane (transmembrane proteins).
• Hydrophobic regions face inward; hydrophilic regions face the aqueous environment.
• Some proteins float freely, while others are fixed. - Glycoproteins and Glycolipids: Short carbohydrate chains attached to proteins and lipids face outside the membrane.
- Cholesterol: Stabilizes the membrane and affects fluidity.
Why is it difficult for polar molecules or ions to pass through membranes?
The non-polar (hydrophobic) tails of phospholipids form a barrier that prevents the passage of polar molecules and ions.
What is the function of membranes as a barrier?
Membranes prevent water-soluble substances like sugars, amino acids, and proteins from leaking out of the cell and block unwanted water-soluble molecules from entering.
What is cholesterol’s structure and position in the membrane?
Cholesterol is a small molecule with hydrophilic heads and hydrophobic tails, fitting between phospholipid molecules with its head at the membrane surface.
What role does cholesterol play in membrane stability?
Cholesterol strengthens membranes by reducing their fluidity and preventing them from breaking, which helps avoid cell rupture.
How does cholesterol prevent the passage of ions and polar molecules?
The hydrophobic regions of cholesterol molecules act as a barrier, particularly in the myelin sheath, preventing ion leakage that could slow nerve impulses.
What happens to phospholipid tails at low temperatures, and how does cholesterol counteract this?
At low temperatures, phospholipid tails pack closely together. Cholesterol prevents excessive packing, maintaining membrane fluidity.
Why is maintaining membrane fluidity important?
Proper membrane fluidity ensures that cells can survive in colder temperatures and function effectively.
Summary of roles of phospholipids and cholesterol.
Phospholipids:
• Create a hydrophobic barrier, blocking polar and water-soluble molecules.
• Prevent leakage of water-soluble substances.
• Can act as signaling molecules when modified.
Cholesterol:
• Provides mechanical stability by reducing membrane fluidity.
• Prevents membrane rupture.
• Blocks ion and polar molecule passage.
• Maintains membrane fluidity at low temperatures by preventing phospholipid tails from packing too closely.
What role do carbohydrate chains on glycoproteins and glycolipids play?
They help act as receptor molecules by binding with specific substances at the cell surface.
How do glycolipids and glycoproteins function as cell markers or antigens?
They allow cells to recognize each other, which is important in growth, development, and immune responses.
What is the function of transport proteins in membranes?
Transport proteins provide hydrophilic channels or passageways for ions and polar molecules to pass through the membrane.
How are transport proteins specific in their function?
Each transport protein is specific to a particular ion or molecule.
How do proteins contribute to the function of organelle membranes?
In mitochondria and chloroplasts, membrane proteins play a role in respiration and photosynthesis processes.
Summary of Molecule Functions
• Carbohydrate Chains on Glycoproteins/Glycolipids:
• Act as receptors and cell markers (antigens), allowing cells to recognise each other.
• Enable cell-cell recognition for growth, development, and immune responses.
• Transport Proteins:
• Provide hydrophilic pathways for ions and polar molecules.
• Specific to particular ions or molecules (channel and carrier proteins).
• Membrane Proteins in Organelles:
• Support respiration in mitochondria.
• Aid photosynthesis in chloroplasts.
• Some membrane proteins are enzymes.
Define diffusion.
The net movement of molecules or ions from a region of higher concentration to a region of lower concentration down a concentration gradient, as a result of the random movements of particles.
What are the factors that affect the rate of diffusion?
- The steepness of the concentration gradient - steeper the concentration gradient of a substance, faster the rate of diffusion of that substance.
- Temperature - higher the temperature, faster the rate of diffusion.
- The nature of the molecules or ions - large molecules diffuse more slowly than small molecules. Non-polar molecules such as glycerol, alcohol and steroid hormones diffuse much more slowly than polar molecules.
- Surface area - greater the surface area faster the rate of diffusion. The surface area of cell membranes can be increased by folding. Larger the cell, the smaller its surface area in relation to its volume.
Define facilitated diffusion.
The diffusion of a substance through a transport protein (channel protein or carrier protein) in a cell membrane; the protein provides hydrophilic areas that allow the molecule or ion to pass through the membrane, which would otherwise be less permeable to it.
What are channel proteins, and what is their primary function?
Channel proteins are proteins with water-filled pores that allow charged substances, usually ions, to diffuse through the membrane.
What does it mean for a channel protein to be ‘gated’?
Gated channel proteins have a part of their structure that can open or close the pore to control ion exchange.
How do gated channel proteins operate in nerve cells?
• One type allows sodium ions (Na⁺) to enter during the production of an action potential.
• Another type allows potassium ions (K⁺) to exit during repolarisation.
Do gated channel proteins always require energy?
Some gated channel proteins require energy in the form of ATP to operate the gate.
