What does the fluid mosaic model state about the membrane?
The membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.
How are proteins bound to the surface of the membrane?
Proteins are loosely bound to the surface of the membrane.
What role do carbohydrates play on the plasma membrane?
Carbohydrates on the external side of the plasma membrane vary among species, individuals, and cell types, allowing cells to recognize each other.
What is the characteristic of plasma membranes regarding permeability?
Plasma membranes are selectively permeable, regulating the cell’s molecular traffic.
What is passive transport?
Diffusion of a substance across a membrane with no energy investment.
Which type of molecules can easily cross the lipid bilayer of the membrane?
Hydrophobic (nonpolar) molecules, such as hydrocarbons.
Which type of molecules do not cross the membrane easily?
Polar molecules, such as sugars.
Define diffusion.
The tendency for molecules to spread out evenly into the available space.
What is the direction of diffusion regarding concentration?
From higher to lower concentration.
What is meant by ‘diffusion down a concentration gradient’?
Substances diffuse from where they are more concentrated to where they are less concentrated.
Why is the diffusion of a substance across biological membranes considered passive transport?
Because no energy is expended by the cell to make it happen.
What is facilitated transport?
Transport proteins speed the passive movement of molecules across the plasma membrane.
Integral protein
Penetrate the hydrophobic interior of the lipid bilayer
What change do carrier proteins undergo during transport?
A subtle change in shape that translocates the solute-binding site across the membrane
What is osmosis?
The diffusion of water across a selectively permeable membrane
In which direction does water diffuse during osmosis?
From the region of lower solute concentration to the region of higher solute concentration
What is tonicity?
The ability of a surrounding solution to cause a cell to gain or lose water
What characterizes an isotonic solution?
Solute concentration is the same as inside the cell; no net water movement across the plasma membrane
What happens to a cell in a hypertonic solution?
The cell loses water
What occurs to a cell in a hypotonic solution?
The cell gains water
What role do cell walls play in plant cells?
They help maintain water balance
What happens to a plant cell in a hypotonic solution?
It swells until the wall opposes uptake; the cell is now turgid (very firm)
What occurs when a plant cell becomes flaccid?
The cell loses water and may wilt
What is plasmolysis?
The process where the membrane pulls away from the wall in a hypertonic environment
