Chapter 7 Membranes

Essential knowledge: Cell membranes are selectively permeable due to their structure.

• Function of the Membrane: – Cell membranes separate the internal

environment of the cell from the external environment.

• Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model.

The Fluid Mosaic Model

Fluid Mosaic Model: Cell membranes are made of the phospholipids, cholesterol, protein, glycoprotein and glycolipids. These molecules can be fixed in place or move laterally through the fluid like phospholipid bilayer:

Phospholipid molecules:

• Give the membrane both hydrophilic and hydrophobic properties.

Embedded proteins:

• Can be hydrophilic, with charged and polar side groups, or hydrophobic, with nonpolar side groups.

• Give cell membranes of different cell types their unique properties

Cholesterol:

• Help make the cell membrane stronger and less fluid.

Glycoproteins and Glycolipids: • Proteins or Lipids with short branched chains of

carbohydrate attached.• Serve as identification of cell types or make the

extracellular matrix.• In which organelle was the sugar attached to the

protein or lipids?

Membranes are Selectively Permeable• Molecules pass through the Membrane

through specific mechanisms depending on size, polarity or charge.

example Type of movement PictureSmall polar and nonpolar molecules

N2, CO2 Diffusion directly through membrane (in-between phospholipids)

Large polar hydrophilic molecules

Glucose Embedded protein channels or pumps

Water H2O Through embedded protein channels called aquaporins

Ions Na+, K+, Cl-, Mg2+, Ca2+

Embedded protein channels or pumps

Diffusion and Osmosis

• Substances will move from a region of high concentration to low concentration.

• Once dynamic equilibrium is established molecules still move, but there is no net movement.

Diffusion

Osmosis

• External environments can be hypotonic, hypertonic or isotonic to internal environments of cells. – This will determine the direction of net movement

of molecules into or out of the cell.

Essential knowledge: Growth and dynamic homeostasis are maintained by the constant movement of molecules across

membranes.

• Passive transport does not require the input of metabolic energy; the net movement of molecules is from high concentration to low concentration (diffusion/osmosis)

• Passive transport plays a primary role in the import of resources and the export of wastes.

• Membrane proteins play a role in facilitated diffusion of charged and polar molecules through a membrane.

Active transport

• Active transport requires energy to move molecules against the concentration gradient.

• Energy is used by proteins embedded in the membrane to move substances across the membrane and to establish and maintain concentration gradients.

• Membrane proteins are necessary for active transport.

Passive Transport: Molecules must go with the concentration gradient

• Facilitated Diffusion: Diffusion through a membrane protein.

Active Transport

• ATP-Dependent Transport: • Uses ATP to change the shape

of a protein to pump material against concentration gradient.

Active Transport is used by Neurons to maintain a potential difference (voltage) across the membrane

Na+ / K+ ATPase

•Pumps 3 Na+ out and 2 K+ in •Requires one ATP to perform this work.

Co-Transport (a form of Active Transport):

• Moves one molecule against its concentration gradient by moving another molecule down its concentration gradient.

• The images used in this handout are from either Campbell Reece 7th edition or BioVisions Inner life of the Cell.