USC Bridge 2.2 plasma membrane

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Plasma membrane

Selectively permeable- has a hydrophobic bilayer- allows hydrophobic molecules and small molecules like O2 can sneak between the membrane. Large hydrophilic molecules cannot pass through the membrane ie: glucose, sodium ions, hydrogen ions.

Separates interior and exterior of the cell

called cell membrane because it is the outermost coating of the cell

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Forms the basic fabric of the membrane. Constructed of phospholipids which have a polar head that is charged and hydrophilic, they lie on the inner and outer surface of the membrane. It also has a uncharged nonpolar tail that is hydrophobic which lines up the center of the membrane.

maintains its spherical shape

allows them to reseal when torn

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Membrane proteins

Allow cell communication with the environment

Proteins make up about half of the mass of plasma membrane

have specialized membrane functions

two types: integral proteins and peripheral proteins


Integral proteins

Firmly inserted into the lipid bilayer, some protrude from one membrane face only, but most are transmembrane proteins that span the entire width of the membrane and protrude on both sides


Peripheral proteins

Not imbedded in the lipid layer, loosely attached inside or outside on integral proteins and are easily removed without disturbing the membrane.

some peripheral proteins are called enzymes others are motor proteins.

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6 membrane protein tasks:

  1. Transportation
  2. receptors for signal transduction
  3. attachment to the cytoskeleton and extracellular matrix
  4. enzyme activity
  5. intercellular joining
  6. cell cell recognition

Transport proteins

Protein on the membrane that provide a channel for certain molecules to enter the cell


Receptor molecules

Bond to signal molecules and can then emit second messengers which trigger changes inside a cell

Receptors are thus important links in the system of communication among cells

some signal molecules such as hormones are also proteins


Structural proteins

Shape and anchor cells, serve as tracks along who cell parts can move.

Bind cells together making organized units such as muscles, ligaments, and the tendons that bind muscles to bones

silk of spiders and hair of mammals are structural proteins


Cell enzymes

A protein that changes the rate of a chemical reaction without changing into a different molecule in the process

promote and regulate all chemical reactions in cells

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Tendency of molecules or ions to move from an area where they are in higher concentration to an area where they are in lower concentration. Can also be described as molecules moving down their concentration gradient.

Diffusion is based on concentration, collision, molecular size, and if a molecule is a lipid soluble molecule


Passive process

No energy is required for substances to move across the plasma membrane

go across via diffusion or a channel.

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Facilitated Diffusion

Certain hydrophilic molecules are transported passively down their concentration gradient ie: glucose, amino acids, and ions

the transported substance either binds to protein carriers in the membrane, or moves through water filled protein channels.


Carrier-mediated facilitated diffusion

Substances bind to protein and carries it through


Channel-mediated facilitated diffusion

Smaller molecules move through water filled channels, selected based on size.

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Active process

Energy (ATP) is required to move substance across the plasma membrane


Active transport (Solute pumps)

Move solutes like ions (Na, K, Ca) against the concentration gradient. Requires cells to expend energy (ATP).

two types: primary and secondary active transport


Vesicular transport

Large particles are transported through the cellular membrane in sacs or vesicles

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Cellular ATP energy

Chemical energy released when glucose is broken down is captured in ATP

ATP directly powers chemical reactions in cells and offers immediate usable energy needed by body cells

structure of ATP- adenine- containing RNA nucleotide with two additional phosphate groups


Structure of ATP

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Uses of ATP in cells for transport

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Primary active transport

Required energy come directly from ATP hydrolysis

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Secondary active transport

Required energy is obtained indirectly from ionic gradients created by primary active transport


Phospholipids orient themselves in aqueous solutions such that:

The polar heads face the interior and exterior of the cell with the tails forming the center of the membrane


When movement of Na ions down their concentration gradient drives the transport of other substances across the membrane it's called:

Secondary active transport

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