Membrane Structure and Function

Concept 7.1 Cellular membranes are fluid mosaics of lipids and proteins


*Phospholipids are amphipathic molecules- it has both a hydrophilic region and hydrophobic region.

  • A Fluid Mosaic Model is the membrane; is a fluid structure with a "mosaic" of various proteins embedded in or attached to a double layer or bilayer of phospholipids.

Membrane Models

  • A phospholipid bilayer was the main "fabric" of a membrane.
  • 1935- The " sandwich model"; phospholipids layer between two layers of proteins. There was two problems:the generalization htat alll membranes of the cell are identical was challenged. Also the placement of the proteins.
  • The membrane proteins are not bery soluble to water because the membrane proteins are amphipathic.
  • Singer and Nicolson proposed that membrane proteins were dispersed and individually inserted into the phospholipids layers.
  • Freeze Factor had demonstrated that proteins are embedde in the phospholipids bilayer or membrane. Basically, it splits the membrane right along the middle.

The Fluidity of Membranes

  • It is rare for molecules to "flip-flop" transversely across the membrane. In order to do so, the hydrophilic part of the molecule must cross the hdrophobic core of the membrane.
  • A membrane remains fluid as temperature decreases until the phospholipids settle into a closely packed arrangement and the membrane solidifies.
  • Cholesterol- " temperature buffer" for the membrane; the membrane must be fluid to work properly.
  • The lipid composition of cell membranes can change as an adjustment to changing temperature.

Membrane Proteins and their Functions
A membrane is a collection of deferent proteins embedded in a fluid matrix, the membranes function is to protect the cell and to diffuse differnt protens into the cell.
The Role of Membrane Carbohydrates in Cell-Cell Recognition

Synthesis and Sidedness of Membranes

Concept 7.2 Membrane structure results in selective permeability


  • The membrane is a great example of supramolecular structure

The Permeability of the Lipid Bilayer

  • Hydrophobic molecules (ex: hydrocarbons, carbon dioxide, oxygen) can dissolve in the lipid bilayer of membrane
  • Without the aid of membrane proteins
  • Hydrophobic core impedes hydrophillic substances from passing rapidly

Transport Proteins

  • Cell membranes are permeable to specific ions and variety of polar molecules
  • Hydrophillic avoid contact with lipid bilayer by passign thru transport proteins
  • channel proteins- have hydrophillic channels certain molecules or ions use
  • aquaporins
  • Carrier proteins-hold and shape

Concept 7.3 Passive transport is diffusion of a substance across a membrane with no energy investment


  • Molecules have a type of energy called thermal motion
  • Thermal motion causes diffusion

:Diffusion : the tendency for molecules of any substance to spread out evenly into the available space

  • The spreading of molecules is usually random
    • the spreading of a population of molecules may be directional
Concentration gradient
a simple rule of diffusion, without outside forces a substance will diffuse from from its highest area of concentration to an area of lower concentration

Note: diffusion is a spontanious process where each substance diffuses into each other untill an equillibrium is reached THERE ARE NO EXEPTIONS (that I know of).

Passive transport
the diffusion of a substance across a biological membrane, as the name implies the cell does not have to extend energy to diffuse, butt the membrane types can and to have selective permeability and so different membranes have different effects of different molecules.

Effects of Osmosis on Water Balance

the diffusion of water across a selectively permeable membrane.
  • Osmosis is very important process to cells and is ecential to all life
    • For Example= if you had a pipe-like glass container shaped like a U, put a special membrane that would only allow water to pass and filled one side with a 1 molar solution of sugar water and filled the other half with a .5 molar solution of sugar water then the water from the .5 molar solution would spread into the 1 molar solution so as to dillute it and reach equillibrium.

Water Balance of Cells Without Walls

the ability of a solution to cause a cell to gain to lose water
  • Tonicity is very important to cells
    • the main factor in tonicity is the concentration of solutes in a solution that cannot be obsorbed by through the cell membrane and vice versa
a solution that has the same equillibrium as the cell
  • When an animal or plant cell is placed in such a solution it is already at a stable equillibrium and does not have to give up or take in excess water (flaccid state).
a solution that has a higher concentration of non absorbable solutes than inside the cell
  • When an animal or plant cell is placed in this solution both loose their water to the sorrounding solution rendering them in a plasmolyzed or shriveled state.
a solution that has little or no excess solutes compared to the cell
  • When an animal cell is placed in this type of solution it will explode do to the water it must obsorb to try to reach an equillibrium (destroys cell in the process) unlike a plant cell that does not burst due to its cell wall (both cells are in the turgid state).
the control of water balance in a cell, is an adaptation for cells that live in hypertonic or hypotonic solutions.

Water Balance of Cells With Walls
-cells of plants, prokaryotes, Fungi, and some protist have walls
- when a cell with a wall is put in a Hypotonic solution the wall helps the cell maintain water balance
- when the cell is so full that the wall can’t expand it exerts back pressure which stops further water uptake. At this point the cell is turgid or firm. If the cell did not have a wall it would blow up. Unless it had a vacuole which gets rid of excessive water.
- In a isotonic solution a plant cell becomes flaccid or limp. Because they are not losing or gaining water to their surroundings.
- In a hypertonic solution both plant an animal cell lose H2O to their surroundings and shrivel and die.

Facilitated Diffusion: Passive Transport Aided by Proteins
-Polar ions and molecules that cannot pass through the lipid
Bilayer are helped by transport proteins which is called facilitated diffusion. Two types of transport protein are channel and carrier proteins.
- channel proteins are simple corridors that let certain molecules or ions pass through the cell membrane.
- Ion channels are like gated channels in which a stimulus causes to open or close.
- Carrier proteins which change the shape of the solute which lets it either move the solute in or out if the cell.

Concept 7.4 Active transport uses energy to move solutes against their gradients

The Need for Energy in Active Transport
Maintenance of Membrane Potential by Ion Pumps
**Cotransport: Coupled Transport by a Membrane Protein*
Single ATO-powered pumps can transport a specific solute and can indirectly drive the active transport or several other solutes, into a mechanism called the cotransport. A substance that has been pumped across a membrane can do its work as moves back, across the membrane by diffusion. For example plant cells use gradient of hydrogen ions generated by it proton pumps to dive the active transport, of amino acids, sugars, and others into the cell. Then one specific transport protein returns the hydrogen ions to transport the sucrose in the cell, s they hydrogen ions use the common transport protein to diffuse down, to the electrochemical gradient controlled by the proton pumps. The plant uses a mechanism of sucrose-H+ contransport to load sucrose produced by photosynthesis, into the specialized cells in the veins of leaves. Which means sugar can be distributed by the vascular tissue to the non-photosynthetic organs such as the roots. *

Concept 7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

-Water and small solutes enter and leave the cell by passing through the lipid bilayer of the plasma membrane.
-Proteins and polysaccharides, as well as larger particles, generally cross the membrane by different means.
-Cells secreet macromoleculese by the fusion of vesicles with the plasma membrane.
-A transport vesicle that has budded from the Golgi Apparatus moves along microtubes of the cytoskeleton to the plasma membrane.
-Many secretory cells use exocytosis to export their products.
-Example!: Certain cells in the pancreas manufacture the hormone insulin and secreet it into the blood by exocytosis.
-When plant cells are making walls, exocytosis delivers proteins and certain carbohydrates from the Golgi Apparatus to the outside of the cell.
-The Cell takes in macromolecules and particulate matter by forming new vesicles from the plasma membrane.

  • Phagocytosis - Engulfs food within a membrane/vesicle.
  • Pinocytosis - Gulps extracellular fluid
  • Receptormedicated Edocytosis - Cholesterol.
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