Cell Membrane and Transport Notes

Mr Walker Biology Class

The Plasma Membrane

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-  S.J. Singer proposed the Fluid Mosaic Model to describe the cell membrane

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The membrane is semipermeable (imagine a fence or screen door)

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The phospholipid bilayer

Fluid Mosaic Model�

• Phospholipids are made up of a phosphate head that is polar and hydrophilic (water loving), as well as two fatty acid tails that are non-polar and hydrophobic (water "fearing"). ��When put into an aqueous environment, phospholipids automatically form the structure of a bilayer with the hydrophilic phosphate heads facing the aqueous environment, and the hydrophobic fatty acid chains facing each other.
• Aqueous environments contain water, so the water loving phosphate heads stay on the outside, whilst the water fearing fatty acid tails stay on the inside AWAY from the water. ��They form a bilayer in aqueous environments because one component of phospholipids does not like interacting with water whilst the other component does, and forming this bilayer allows both components to be "happy". 

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Jobs of the Cell Membrane

• Isolate the cytoplasm from the external environment
• Regulate the exchange of substances
• Communicate with other cells
• Identification

1.  The plasma membrane is a phospholipid bilayer with embedded proteins. 

What does the cell membrane do?

1. acts as a boundary layer to contain the cytoplasm (fluid in cell) �2) Regulates interactions between the cell and the environment.
1. Selective Permeability:
• The ability to allow certain substances through a membrane while keeping others out.
• Our cells need certain macromolecules/ions to enter and needs to keep wastes out.

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Channel Proteins - form small openings for molecules to diffuse through�Carrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cell�Gated Channels - similar to carrier proteins, not always "open"

Receptor Proteins - molecular triggers that set off cell responses (such as release of hormones or opening of channel proteins), binding site

Recognition Proteins - ID tags, to identify cells to the body's immune system�Enzymatic Proteins – carry out specific reactions

Figure 5.4a

Figure 5.4b

Figure 5.4c

Figure 5.4d

Figure 5.4e

Membrane Permeability 🡪   Transport Across Membrane

*Selectively or Differentially permeable – some things can cross, not others

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What things can pass?

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What cannot pass?

Passive Transport

• DOES NOT USE ENERGY!!!!
• Diffusion: The movement of a particle from an area of a high concentration to that of a low concentration. Remember….Salt Sucks….Sugar sucks.
• Equilibrium: the same amount of particles per space on both sides.

Osmosis

• Osmosis is the diffusion of water through a membrane.
• ISOTONIC: water will move through the membrane at the same rate.
• HYPOTONIC: More water outside the cell than inside. So water moves into the cell causing the cell to SWELL!
• HYPERTONIC: More water inside the cell than outside. So water moves out of the cell causing the cell to SHRINK!

Proteins in the cell membrane

• Channel proteins allow particles to diffuse into the cell.
• Facilitated Diffusion: A channel protein which allows large particles to enter the cell.
• PASSIVE TRANSPORT

Active Transport

• Active transport is the movement of particles from an area of a LOW concentration to an area of a HIGH concentration.
• USES ENERGY!!!!!!!

Na+ K+ Pump

• A carrier protein which uses energy to pump in 2 potassium ions and pump out 3 sodium ions

Endocytosis

• The movement of the cell membrane around a large particle to help bring the particle INTO the cell

Exocytosis

• The movement of the cell membrane around a large particle to help bring it out of the cell.