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Cell Biology

The Plasma Membrane

recall…Fluid Mosiac Model •  S.J. Singer •  Semipermeable membrane •  “fluid” portion is double layer of

phospholipids (=phospholipid bilayer) •  “mosaic” portion is the proteins and

carbohydrates embedded in the membrane

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recall…Role of the Cell Membrane •  Isolate the cytoplasm and organelles

from external environment •  Regulate the exchange of substances •  Cell communication •  Cell identification

1. Phospholipids

•  Primary structural building block of the plasma membrane

Membrane Composition:

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•  Phospholipids contain a hydrophyllic head and non-polar hydrophobic tail

•  Hydrogen bonds form between the heads and the watery environment inside and outside the cell

•  Hydrophobic interactions force tails to face inward •  Phospholipids are not bonded together which make the

double layer fluid

2. Cholesterol (eg. Lipoprotein) •  An important structural element

•  Provides strength and stability to the membrane •  Makes membrane less fluid

Image: Cholesterol within the animal cell membrane

3. Glycolipid •  Phospholipid with a CHO attached •  Provide energy •  Cellular recognition markers for specific elements

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4. Glycoprotein •  Protein with a CHO attached

•  Important for cell-cell interactions •  Important for white blood cell recognition •  Mucin (secreted in respiratory and digestive tracts)

•  protective coating, water retention capacity

5. Proteins

•  Serve many functions depending on structure and location

•  May be integral (embedded) or peripheral (on the inside surface of membrane)

Cell Cytoplasm

Extracellular Environment

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Embedded Proteins Channel Proteins

•  Form small openings for molecules to diffuse through Carrier Proteins

•  Binding site on protein surface selects certain molecules and allows them to pass through

Receptor Proteins •  Molecular triggers that set off cell responses (release of

hormones)

Cell Recognition Proteins •  ID tags to identify certain cells to the body’s immune system

Enzymatic Proteins •  Carry out metabolic reactions

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Transport Across a Cell Membrane What controls how substances Move thru the membrane?

•  Size of the molecule (selective permeability) •  Charge of the molecule

What moves easily thru the membrane?

•  Water •  Carbon dioxide

•  Oxygen

What does NOT move easily thru the membrane? •  Large molecules such as glucose and proteins •  molecules or atoms with charges, such as ions

(sodium, potassium, calcium)

PASSIVE TRANSPORT •  Does NOT require chemical energy from the cell •  Depends on permeability of the cell

•  Consider characteristics of the membrane (lipids, proteins, etc)

Three main types: 1.  Diffusion 2.  Facilitated transport 3.  Osmosis

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1.  Simple Diffusion

•  Movement of molecules from an area of HIGH concentration (more solute) to LOW concentration (less solute)

•  These molecules follow their concentration gradient •  Cells are able to dispose of O2, CO2, and H2O without

energy

n  http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

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2.  Facilitated Diffusion/Transport

•  Movement of a molecule from an area of [HIGH] to [lower] with with aid of proteins (carrier or channel proteins)

•  works as a facilitating pathway for larger molecules or charged ions (amino acids, glucose, Na+ and K+)

•  Cell membranes have specific proteins for each molecule

n  http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

•  Several factors determine rate of diffusion: •  Number of molecules in cell (mol) •  Permeability of a particular molecule •  Width of cell membrane •  Concentration of diffusing molecule (mole/cm3) •  Temperature (rate ↑’s as temp ↑’s)

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3.  Osmosis

•  Diffusion of WATER through a semi-permeable membrane

•  Water moves from high H2O potential and low [solute] to low H2O potential and hi [solute]

Given: A - 100% water B - 90% water

Question:

What direction does the water flow?

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Given: A - 100% water B - 90% water

Question:

What direction does the water flow? Water flows from A --> B

Given: A - 1% glucose B - 20% glucose

Question:

What direction does the water flow?

What direction does the glucose flow?

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Given: A - 1% glucose B - 20% glucose

Question:

What direction does the water flow? Water flows from A --> B

What direction does the glucose flow? Glucose flows from B --> A

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Types of Solutions: 1. Hypertonic •  Hyper = more ; tonic = solute •  Thus, a cell in a solute rich environment

•  Water flows outward from region of high [water] to low [water] (or low to high [solute])

•  Cell undergoes plasmolysis (fluid loss) and crenate (shrivel)

2. Hypotonic •  Hypo = less ; tonic = solute •  Thus, a cell in a solute poor environment

•  Water flows inward from region of high [water] to low [water] •  Cell undergoes cytolysis (cell breaks) and bursts

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3. Isotonic •  Iso = equal ; tonic = solute •  Thus, a [solute] both equal within and outside the cell

•  Water flows in and out of cell with net zero movement •  Equilibrium

n  http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

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ACTIVE TRANSPORT •  Requires chemical energy (ATP) •  Involves the movement of molecules “uphill” against a

concentration gradient •  Proteins involved are called pumps because they are used to

‘pump’ substances across the concentration gradients Eg. Sodium (Na) Pump

Types:

1.  Endocytosis (entering the cell) •  Phagocytosis

•  Pinocytosis

2.  Exocytosis (exiting the cell)

BULK TRANSPORT: Endocytosis and Exocytosis

•  A form of active transport (uses ATP) •  When particles are too big or you are moving in bulk

ENDOCYTOSIS (endo=inside; cyte=cell)

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Endocytosis: Phagocytosis •  Engulf large particles

Endocytosis: Pinocytosis •  engulf very small particles or liquids •  “sipping”

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EXOCYTOSIS (exo=outside; cyte=cell) •  Export of a secretory vesicle or vacuole that will

migrate to membrane’s inner surface and fuse with membrane

•  Contents are forced out of the cell •  Opposite of endocytosis

•  Examples: peptide hormones, steroid hormones, wastes

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Test Yourself 1.  Describe in your own words, the fluid mosaic model of a

plasma membrane. 2.  How is active transport different from facilitated

transport? 3.  Distinguish between phagocytosis and pinocytosis