Topic 2B: Cell membranes Flashcards
Why are plasma membranes described as a fluid-mosaic model?
Due to the mixture and movement of the phospholipids, proteins, glycoproteins and glycolipids it is made of.
Cell Membrane Components: Phospholipids:
• Create the phospholipid bi-layer; they contain a hydrophilic head which attracts with water, and a hydrophobic tail which is repelled by water.
Cell Membrane Components: Cholesterol:
• Restricts the lateral (side to side) movement of other molecules, which makes the membrane less fluid at high temperatures, preventing water and dissolved ions from leaking out of the cell which can cause dehydration or cell bursting.
Cell Membrane Components: Proteins:
• Embedded across the cell-surface membrane.
• Either peripheral (don’t expand completely across the width of the membrane), or integral (expand completely across the width of the membrane).
What is the role of peripheral proteins?
• Provide mechanical support, or are connected to proteins or lipids to form glycoproteins or glycolipids.
• Glycoproteins and glycolipids are involved in cell recognition (receptors).
What is the role of integral proteins?
• Channel proteins or carrier proteins involved in the transport of molecules across the membrane.
- Channel proteins form a tube filled with water to enable water-soluble ions to diffuse.
- Carrier proteins bind to other carrier proteins or larger molecules e.g. glucose and amino acids, or change shape to transport molecules to the other side of the membrane.
What is the partially permeable membrane?
A membrane where only certain molecules can diffuse across it.
What molecules can diffuse across a partially permeable membrane?
Lipid-soluble substances (e.g. some hormones like oestrogen), and very small molecules (e.g. CO2, H2O)
What molecules can’t diffuse across a partially permeable membrane?
Water-soluble substances (e.g. sodium ions) and larger molecules (e.g. glucose).
What is active transport?
The movement of molecules/ions from an area of low concentration to high concentration via energy from respiration (ATP), and a carrier protein.
Describe the process of the sodium-potassium pump:
• The molecule/ion binds to the receptor sites of the protein.
• ATP is hydrolysed into ADP + Pi, and energy is released. The inorganic phosphate binds to the protein, causing the protein to change shape and open on the other side of the membrane.
• The molecules/ions are released on the other side of the membrane, and as the inorganic phosphate is released from the protein, the protein regains its original shape.
Describe the process of co-transport of glucose from the ileum lumen into the blood:
- Sodium ions are actively transported from the epithelial cell into the blood via the sodium-potassium pump (using a carrier protein).
- This causes sodium ions in the ileum lumen to diffuse into the epithelial cells via the sodium-glucose co-transporter protein. Microvilli on the surface of epithelial cells increases the surface area for the co-transporter protein.
The co-transporter protein also carries glucose into the cell, increasing the concentration of glucose inside the cell. - Glucose is diffused from the epithelial cell into the blood via a protein channel by facilitated diffusion.
