Mader: Biology 8th Ed.

Membrane Structure and Function

Chapter 5

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Mader: Biology 8th Ed.

Outline

• Membrane Models

– Fluid-Mosaic

• Plasma Membrane Structure and Function

– Protein Functions

– Plasma Membrane Permeability

!Diffusion

!Osmosis

!Transport Via Carrier Proteins

• Cell Surface Modifications

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Mader: Biology 8th Ed.

Membrane Models

• Fluid-Mosaic Model

– Membrane is a fluid phospholipid bilayer in which protein molecules are either partially or wholly embedded.

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Mader: Biology 8th Ed.

Fig. 5.1

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Mader: Biology 8th Ed.

Fluid-Mosaic Model

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Mader: Biology 8th Ed.

Plasma Membrane Structure and Function

• Plasma membrane separates internal environment from the external environment.

• Hydrophilic polar heads face outside, and hydrophobic nonpolar tails face each other.

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Mader: Biology 8th Ed.

Plasma Membrane Structure and Function

• Proteins may be peripheral or integral.

– Peripheral proteins are found on the inner membrane surface.

– Integral proteins are embedded in the membrane.

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Fig. 5.3

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Mader: Biology 8th Ed.

Protein Functions

• Channel Proteins - Involved in passage of molecules through membrane.

• Carrier Proteins - Combine with substance to aid in passage through membrane.

• Cell Recognition Proteins - Help body recognize foreign substances.

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Mader: Biology 8th Ed.

Protein Functions

• Receptor Proteins - Allow molecule binding, causing protein to change shape and bring about cellular change.

• Enzymatic Proteins - Carry out metabolic reactions directly.

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Mader: Biology 8th Ed.

Plasma Membrane Permeability

• Plasma membrane is differentially permeable.

– Passive Transport - No ATP requirement.

!Molecules follow concentration gradient.

– Active Transport - Requires carrier protein and ATP.

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Mader: Biology 8th Ed.

Crossing Plasma Membrane

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Mader: Biology 8th Ed.

Diffusion

• Diffusion - Movement of molecules from a higher to a lower concentration until equilibrium is reached.

– Down concentration gradient

• A solution contains a solute (solid) and a solvent (liquid).

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Diffusion

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Mader: Biology 8th Ed.

Osmosis

• Osmosis - Diffusion of water across a differentially (selectively) permeable membrane due to concentration differences.

– Osmotic pressure is the pressure that develops due to osmosis.

!The greater the osmotic pressure, the more likely water will diffuse in that direction.

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Osmosis

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Mader: Biology 8th Ed.

Osmosis

• Isotonic Solution - Solute and water concentrations both inside and outside the membrane are equal.

• Hypotonic Solution - Solution with a lower concentration of solute than the solution on the other side of the membrane.

– Cells placed in a hypotonic solution will swell.

!Lysis

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Mader: Biology 8th Ed.

Osmosis

• Hypertonic Solution - Solution with a higher concentration of solute than the solution on the other side of the membrane.

– Cells placed in a hypertonic solution will shrink.

!Plasmolysis

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Mader: Biology 8th Ed.

Transport by Carrier Proteins

• Carrier proteins combine with a certain molecules which are then transported through the membrane.

– Facilitated Transport

!Small molecules follow concentration gradient by combining with carrier proteins.

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Transport by Carrier Proteins

– Active Transport

!Small molecules move against their concentration gradient by combining with carrier proteins.

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Membrane-Assisted Transport

• Large marcomolecules are transported into or out of the cell by vesicle formation.

– Exocytosis - Vesicles fuse with plasma membrane as secretion occurs.

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Membrane-Assisted Transport

– Endocytosis - Cells take in substances by vesicle formation.

!Phagocytosis - Large, solid material.

!Pinocytosis - Liquid or small, solid particles.

!Receptor-Mediated - Specific form of pinocytosis using a coated pit.

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Mader: Biology 8th Ed.

Cell Surface Modifications

• Cell Surfaces in Animals

– Junctions Between Cells

!Adhesion Junctions

" Intercellular filaments between cells.

!Tight Junctions

"Form impermeable barriers.

!Gap Junctions

"Plasma membrane channels are joined (allows communication).

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Cell Surface Modifications

– Extracellular Matrix!Meshwork of polysaccharides and

proteins in close association with the cell that produced them.

• Plant Cell Walls– Plants have freely permeable cell wall, with

cellulose as the main component.!Joined by plasmodesmata that spans cell

wall and contains strands of cytoplasm that allow passage of material between cells.

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Mader: Biology 8th Ed.

Review

• Membrane Models

– Fluid-Mosaic

• Plasma Membrane Structure and Function

– Protein Functions

– Plasma Membrane Permeability

!Diffusion

!Osmosis

!Transport Via Carrier Proteins

• Cell Surface Modifications

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