plasmalemma

8/14/2019 plasmalemma

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BIOLOGY:

Membrane Structure and Function


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History of the Membrane Idea 1925 Gorter & Grendelhydrophobic tails

inward. 1940s Daniel and Davsonsandwich model:

(protein, phospholipid, and protein.)

1972 Singer and Nicholsonfluid mosaicmodel.


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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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Functions of the Plasma Membrane1. Regulates the passage of materials into and

out of the cell.

2. Receives chemical messages from other

cells, e.g. hormones, growth factors,

neurotransmitters. Maintains structural and chemical

relationships with other cells.

Protects the cell, helps in cell movement,secretion, and in transmitting impulses.


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Plasma Membrane as a Fluid At body temperature, consistency of olive oil.

Each phospholipid molecule can movesideways at ~ 2 mm/s

Most proteins are free to drift along it.

Cholesterol stiffens and strengthens the

membrane, helping to regulate fluidity.


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Figure 8.12 The water balance of living cells


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Membrane-Assisted Transport Large marcomolecules are

transported into or out of thecell by vesicle formation.

Endocytosis- Cells take in

substances with vesicles Phagocytosis solid material.

Pinocytosis - Liquid particles.

Exocytosis- Vesiclessecrete substances out of cell


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Plasma Membrane Structure

Plasma membrane is a phospholipid bilayer.

Hydrophilic polar heads face outside, andhydrophobic nonpolar tails face each other.

Proteins may be peripheral orintegral.

Peripheral proteins arefound on the inner

membrane surface.

Integral proteins areembedded in the

membrane.


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Carbohydrate Chains In animal cells, the carbohydrate chains give

the cell a sugar coat, called the glycocalyxwhich helps protect the cell

adhesion between cells

in the reception of

signal molecules

cell-to-cell recognition.

give a fingerprint(tissue rejection)

give rise to A, B, and O

blood groups


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Protein Functions Channel Proteins - pass

molecules through

Carrier Proteins - bondwith substance to help itthrough

Cell Recognition

Proteins - Help bodyrecognize foreignsubstances and itself.

Receptor Proteins -Protein changes shape tobring about cellularchange.

Enzymatic Proteins -Carry out metabolic

reactions directly.


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Transport by Carrier Proteins Carrier proteins combine with molecules

which are then transported through themembrane.

Facilitated Transport

(facilitated diffusion)

molecules move with

the gradient by combining

with carrier proteins.

Active Transport molecules move against

the gradient by combining

with carrier proteins.

(requires ATP)


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Figure 8.15 The sodium-potassium pump: a specific case of active transport


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Cell Surface Modifications Cell Surfaces in Animalsjunctions between cells

Adhesion Junctions Intercellular filamentsbetween cells.

bladder & stomach

Tight Junctions

Impermeable barriers. intestines and kidneys.

Gap Junctions

Membrane channels join

Allows small ions &

molecules to pass

between cells.


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Cell Surface Modifications Extracellular Matrix

Meshwork of polysaccharides

and proteins Can be flexible or hard,

as in bone.

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