Chapter 4 Flashcards
What are plasma membranes?
All membranes around and within cells
What is the cell-surface membrane and what is its role?
The cell-surface membrane is the plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the environment. It allows different conditions to be established inside and outside a cell. It controls the movement of substances in and out of the cell.
Why are phospholipids important components of cell-surface membranes?
- One layer of phospholipids has its hydrophilic heads pointing inwards.
- The other layer of phospholipids has its hydrophilic heads pointing outwards.
- The hydrophobic tails of both phospholipid layers point into the centre of the membrane - protected from water on both sides.
What are the functions of phospholipids in the membrane?
- Allow lipid-soluble substances to enter and leave the cell.
- Prevent water-soluble substances entering and leaving the cell.
- Make the membrane flexible.
How are the proteins of the cell-surface membrane arranged?
Less symmetrically than the regular pattern of phospholipids.
How are the proteins of the cell-surface membrane embedded in the bilayer?
- Extrinsic proteins occur either on the surface of the bilayer or only partly embedded in it, but they never extend completely across it. They act either to give mechanical support to the membrane or as cell receptors for molecules such as hormones.
- Intrinsic proteins completely span the phospholipid bilayer from one side to the other. Some act as carriers to transport water-soluble material across the membrane whereas others are enzymes.
What are the functions of the proteins in the membrane?
- Provide structural support.
- Allow the transport of water-soluble substances by forming channels across the membrane.
- Allow active transport across the membrane by acting as ion carriers for sodium, potassium, etc.
- Form recognition sites by identifying cells.
- Help cells adhere together.
- Act as receptors.
What gives plasma membranes their different properties?
The different substances they contain - especially proteins.
Why is it called the fluid-mosaic model?
Fluid beause the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing shape.
Mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size, and pattern in the same way as the stones or tiles of a mosaic.
Is diffusion active or passive?
Passive.
All movement requires energy. So why is diffusion classed as passive?
The energy comes from the natural, inbuilt motion of particles, rather than from from ATP.
Define diffusion.
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower.
What are some factors that affect diffusion?
- Concentration gradient.
- Area over which diffusion takes place.
- Thickness of exchange surface.
What are features of facilitated diffusion?
It is a passive process. It occurs down a concentration gradient, but it differs in that it occurs at specific points on the plasma membrane where there are special protein molecules.
What is osmosis?
The net movement of water from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane.
