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CELL MEMBRANE, CYTOSKELETON & CELL-CELL INTERACTIONS. Chapter 5. A. Cell Membrane Structure Cell membrane is a phospholipid bilayer embedded with mobile proteins. Phosphate head of phospholipid is hydrophilic . Fatty acid tails are hydrophobic . Proteins.

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A. Cell Membrane Structure

Cell membrane is a phospholipid bilayer embedded with mobile proteins.

Phosphate head of phospholipid is hydrophilic.

Fatty acid tails are hydrophobic.

Proteins

slide3
Types of membrane proteins:
  • Transport proteins - move substances across membrane
  • Cell surface proteins - establish “self”
  • Cellular adhesion molecules (CAMs)- enable cells to stick to each other
  • Receptor proteins - receive & transmit messages into a cell
slide4
Additional molecules may be associated with proteins & phospholipids:
  • cholesterol
  • sugar molecules
    • glycoproteins
    • glycolipids
slide5
B. Movement Across Membranes

Cell membranes are selectively permeable.

1. Simple diffusion (passive)

Substance moves across phospholipids from an area of high to an area of low concentration, without using energy.

Substance moves down its concentration gradient

Ex. O2, CO2

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Simple Diffusion

Cell membrane

Transport protein

Diffusion continues until dynamic equilibrium is reached.

slide7
Movement of water across membranes by simple diffusion is called osmosis.

Water is driven to move because the membrane is impermeable to solute(s).

slide8
Tonicity

Refers to differences in solute concentration on either side of a semipermeable membrane.

  • Isotonic - both solutions have the same solute concentrations.
  • Hypotonic - solution with the lowersolute concentration.
  • Hypertonic - solution with the highersolute concentration.
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What is effect of immersing a plant cell in a hypertonic or hypotonic solution?

Cell immersed in hypertonic solution

Cell immersed in hypotonic solution

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2. Facilitated Diffusion (passive)

Substance moves through a transport protein from an area of high to an area of low concentration, without using energy.

Substance moves down its concentration gradient

Ex. glucose

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Facilitated Diffusion

Cell membrane

Transport protein

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Cell:Environment:

1% sucrose3% sucrose

1% glucose2% glucose

1% fructose1% fructose

97% water94% water

Assume cell membrane is permeable to water, glucose & fructose, but impermeable to sucrose.

In which direction will sucrose, glucose, fructose & water move?

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3. Active Transport (active)

Substance moves through a transport protein from an area of low to an area of highconcentration; requiresenergy.

Substance moves against its concentration gradient

Ex. ions (Na+, K+, Cl-)

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Active transport

Cell membrane

Transport protein

ATP

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4. Cotransport

The active transport of one substance is coupled to the passive transport of another.

Ex. sucrose (plant cells)

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5. Exocytosis, Endocytosis & Transcytosis

Movement of large particles across membranes with the help of vesicles.

Exocytosis - vesicles move particles out of the cell.

Ex. release of enzymes from head of sperm; neurotransmitter release

slide19
Endocytosis - vesicles move particles into the cell.

Three types of endocytosis:

Receptor-mediated endocytosis

Pinocytosis

Phagocytosis

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Transcytosis - combines endocytosis & exocytosis.

Vesicles rapidly transport particles through cells.

Ex. transport of nutrient monomers through cells lining digestive tract & into the bloodstream

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C. Cytoskeleton

The structural framework of a cell.

1. Microtubules - hollow, thick elements made of the protein tubulin.

Functions:

  • move chromosomes apart during cell division
  • form cilia & flagella
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2. Microfilaments - long, thin elements made of the protein actin.

Functions:

  • connect cells to each other
  • move vesicles & organelles within cytoplasm
  • help cells move
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3. Intermediate filaments - elements with diameters in between that of microtubules & microfilaments.

Made of various proteins (ie. keratin)

Functions:

  • maintain cell shape
  • connect cells to each other & to underlying tissue (skin cells)
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D. Intercellular Junctions

Structures that connect cells of multicellular organisms to form tissues.

1. Animal cell Connections

Tight Junctions - cell membranes of adjacent cells are fused, creating a tight seal.

Ex. cells lining small intestine; cells lining capillaries in brain

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Desmosomes - intermediate filaments weld cell membranes of adjacent cells together in isolated spots.

Ex. skin cells

Gap Junctions - channels that link the cytoplasm of adjacent cells, allowing exchange of materials.

Ex. heart muscle cells

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2. Plant Cell Connections

Plasmodesmata - channels that link the cytoplasm of adjacent plant cells, allowing the exchange of cytoplasm & organelles.

Ex. cells conducting water & nutrients

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E. Cell-Cell Interactions

1. Cell Adhesion

Process that uses membrane proteins called cellular adhesion molecules(CAMs) to direct the migration of cells.

Various CAMs function in sequence to:

  • guide WBCs to injury sites
  • guide embryonic cells to help form placenta
  • establish nerve connections involved in learning & memory
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2. Signal Transduction

Process by which cells receive, amplify, & respond to outside stimuli.