The Body in Motion

Biology, Seventh Edition

Chapter 5

Biological

Membranes

Copyright © 2005 Brooks/Cole — Thomson Learning

Solomon • Berg • Martin

Biology, Seventh Edition CHAPTER 5 Biological Membranes

• Biological membranes

• Complex, dynamic structures made of lipid and protein molecules

• Perform many functions

– Define cell as a compartment

– Regulate passage of materials

– Participate in chemical reactions

– Transmit signals between cell interior and the environment

– Act as part of energy transfer and storage



• Biological membranes

• Physically separate cell interior from extracellular environment

• Form compartments within eukaryotic cells

• Plasma membrane

– Regulates passage of materials

– Participates in biochemical reactions

– Receives information about environment

– Communicates with other cells



Phospholipids form bilayers in water

Phospholipids in water Detergent in water



• Fluid mosaic model

• Membranes consist of fluid phospholipid bilayer with a mosaic pattern of associated proteins

• Phospholipid molecules are amphipathic and contain

– Hydrophobic regions

– Hydrophilic regions



Plasma membrane of mammalian red blood cell



• Membrane properties

• Orderly arrangement of phospholipid molecules make the cell membrane a liquid crystal

– Allow molecules to move rapidly

– Proteins move within membrane

• Lipid bilayers are

– Flexible

– Self-sealing

– Can fuse with other membranes



Detailed structure of the plasma membrane



• Integral membrane proteins

• Embedded in the bilayer

• Transmembrane proteins

• Integral proteins that extend completely through the membrane

• Peripheral member proteins

• Associated with the surface of the bilayer



• Membrane proteins, lipids, and carbohydrates

• Asymmetrically positioned to bilayer

• Sides have different composition and structure

• Function of member proteins

– Transport of materials

– Acting as enzymes or receptors

– Cell recognition

– Structurally linking cells



Asymmetry of the plasma membrane



Functions of membrane proteins



• Membranes are selectively permeable

• Physical processes

– Osmosis

– Diffusion

• Carrier-mediated processes

– Channel proteins

– Carrier proteins



Diffusion: net movement of a substance from a region of greater to lower concentration



Osmosis: water passes through selectively permeable membrane from region of higher concentration to lower


CHAPTER 5 Biological Membranes


• Osmotic pressure

• Concentration of dissolved substances in a solution

• Isotonic: equal solute concentration

• Hypertonic: loses water in plasmolysis

• Hypotonic: gains water and swells

• Turgor pressure

• Internal hydrostatic pressure in walled cells



Response of animal cells to osmotic pressure




Turgor pressure and plasmolysis




• Facilitated diffusion

• Occurs down a concentration gradient

• Active transport

• Moves ions or molecules against a concentration gradient

• Cotransport

• ATP-powered pump maintains a concentration gradient



Sodium-potassium pump




• Cells expend metabolic energy to carry on physiological processes

• Exocytosis

• Endocytosis

– Phagocytosis

– Pinocytosis

– Receptor-mediated endocytosis



Exocytosis



Phagocytosis



Pinocytosis




Receptor-mediated endocytosis



• Cells communicate by cell signaling

• Signaling molecules include

• Neurotransmitters

• Hormones

• Regulatory molecules



• Cell signaling involves

• Synthesis and release of signaling molecule

• Transport to target cells

• Reception by target cells

• Signal transduction

• Response by the cell

• Termination of signal



Signal transduction




• Cells in close contact often develop intercellular junctions

• Anchoring junctions

– Desmosomes

– Adhering junctions

• Tight junctions

• Gap junctions

• Plasmodesmata



Desmosomes




Tight junctions




Gap junctions




Plasmodesmata



The Body in Motion

Chapter 5

Biological

Membranes

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Chapter 5

Biological

Membranes

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