Chapter 7- Membrane Structure and Function

Fluid mosaic model:

Model of arrangement of molecules in the plasma membrane. In F.M.M the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

How can substances not normally transported across the membrane gain entrance?

Molecules that can pass freely through the membrane follow concentration gradients, moving from the higher concentration area to the region of lower concentration. These processes take no energy and are called passive transport. The molecules that cannot pass freely across the phospholipid bilayer can be carried across the membrane in various processes that require energy and are therefore called active transport.

Why can certain molecules pass though the cell membrane?

Nonpolar molecules such as hydrocarbons are hydrophobic; they can therefore dissolve in the lipid bilayer of the membrane and cross it easily, without the aid of proteins.

How does cotransport and active transport work?

o Active Transport: the movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy. o Cotransport: The coupling of the downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

What are the differences in behavior of plant in animal cells in various tonic solutions?

o Animal cells. An animal cell fares best in an isotonic environment unless it has special adaptions that offset the osmotic uptake or loss of water. o Plant cells. Plant cells are turgid and generally healthiest in a hypotonic environment, where the uptake of water is eventually balanced by the wall pushing back on the cell.

How does passive transport work?

o There are three main types of passive transport: diffusion, facilitated diffusion, and osmosis. o Passive transport is a movement of biochemical and other atomic or molecular substances across cell membranes without need of energy input. Unlike active transport, it does not require an input of cellular energy because it is instead driven by the tendency of the system to grow in entropy.

Sodium potassium pump

A transport protein in the plasma membrane of animal cells that actively transports sodium out of the cell and potassium into the cell.

What role do carbohydrates play in cell membrane?

Carbohydrates covalently linked to proteins (glycoproteins) or lipids (glycolipids) are also a part of cell membranes, and function as adhesion and address loci for cells. The Fluid Mosaic Model describes membranes as a fluid lipid bilayer with floating proteins and carbohydrates. They help a cell to be recognized as a certain type of cell and are important for holding cells together.

Describe various bulk transport processes cells use.

Endocytosis and exocytosis

How is membrane fluidity maintained?

If unsaturated fatty acids are compressed, the "kinks" in their tails push adjacent phospholipid molecules away, which helps maintain fluidity in the membrane. The ratio of saturated and unsaturated fatty acids determines the fluidity in the membrane at cold temperatures.

Based on your knowledge of the cell membrane's chemistry, where would steroid hormones most likely interact with their target cell?

Inside the target cell's cytoplasm or nucleus

Intergral:

Integral: Penetrate the hydrophobic interior of the lipid bilayer. Majority are transmembrane which span the membrane; other integral extend into the hydrophobic interior.

What are the different roles for these proteins?

Intregral and Peripheral

Peripheral

Peripheral Proteins: are not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to expose parts of the integral proteins.

Composition of Membranes

Phospholipids and Proteins

How do phospholipids move?

Phospholipids can move laterally and allow water and other small molecules to pass through into or out of the cell.

What types of molecules can pass through the cell membrane, and which can not.

Small and nonpolar (hydrophobic) molecules can freely pass through the membrane, but charged ions and large molecules such as proteins and sugars are barred passage. Small uncharged lipid molecules can pass through the lipid innards of the membrane.

Selective permeability:

Something the plasma membrane does to allow some substances to cross it more easily than others. The ability of the cell to discriminate in its chemical exchanges with its environment is fundamental to life.

How does tonicity affect a cell?

The ability of a solution surrounding a cell to cause that cell to gain or lose water. The tonicity of a solution depends on its concentration of solutes that can not cross the membrane relative to that inside the cell.

Membrane potential

The difference in electrical chare (voltage) across a plasma membrane due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances.

Electrochemical gradient

The diffusion gradient of an ion, which is affected by both the concentration difference of an ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential. (an electric force)

How does osmosis work?

The diffusion of free water across a selectively permeable membrane. The balance of water across the cell membranes.

How does diffusion work

The movement of particles of any substance so that they spread out into the available space. Net movement of a substance from a region where it is more concentrated to an area of less concentration. Molecules move randomly, yet in a diffusion of a popular of molecules may be directional.

How are phospholipids arranged in the membrane?

The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. ach phospholipid molecule has a head and two tails. The head "loves" water (hydrophilic) and the tails "hate" water (hydrophobic). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell.

Molecular components of the plasma membrane.

The principal components of the plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrate groups that are attached to some of the lipids and proteins. A phospholipid is a lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group.

How are proteins arranged within the membrane bilayer?

The proteins are not randomly distributed, groups are associated in long lasting specialized patches, where they carry out common functions. As Integral or peripheral.

Fluid mosaic model:

The proteins are not randomly distributed, groups are associated in long lasting specialized patches, where they carry out common functions. The lipids themselves appear to form define regions as well. Also in some regions the membrane may be much more packed with proteins than shown.

What are the 6 functions of membrane proteins

Transport, enzymes, signaling, cell-cell recognition, intracellular joining, attachment to ECM/cytoskeleton

Freeze Fracturing:

Using electron microscopy and a technique called "freeze fracture," which splits frozen cell membranes apart, allows visualization of the membrane structure and the organization of proteins within the sea of phospholipids.

A cell might increase its membrane fluidity (flexibility) by increasing the amount of _________ in the cell membrane.

unsaturated fatty acids