Plasma Membrane Dynamics and Cell Transport Mechanisms

Plasma Membranes are mostly Lipids and Proteins arranged in a Fluid Mosaic Model

A typical cell membrane has a composition of:

Lipids:  40-60% – arranged in a double lipid bilayer.

Protein: 30-50% – proteins which are inserted either partly or completely through bilayer.

Carbohydrate: 5-10% – carbohydrates which attach to extracellular fluid (ECF) side.

These percentages can vary significantly depending on the specific type of cell in the body.

Cytoplasm with organelles


Figure 1. This is the typical diagrammatic representation of a eukaryotic cell. The extracellular fluid (ECF) is kept separate from the intracellular fluid (ICF) by the plasma membrane.

General Function of Plasma Membranes

  1. Physical Barrier: The plasma membrane (PM) acts as a barrier; it separates the inside of the cell, containing ICF, from the outside of the cell, containing ECF. It creates the boundary of the cell and isolates it from other cells and structures.
  2. Regulation of Exchange: Anything that goes into or out of a cell must do so by crossing the plasma membrane. Exchange with the environment occurs across this membrane, either by slipping through the membrane or by being transported across by protein channels or protein carriers.
  3. Structural Support: Structural proteins are tethered to the internal or intracellular aspect of the plasma membrane in order to create the internal structural support for the cell. This internal framework is referred to as the cytoskeleton of the cell. For example, this helps create the shape of cells, like the distinctive biconcave disc shape of the red blood cell.
  4. Communication and Cell ID: Signals from the external environment of the cell are transferred into the internal compartment across the plasma membrane. This often involves receptors that sit on the external aspect of the plasma membrane to receive the signal. Signal molecules are called ‘ligands’ and they bind to receptors, much like substrates bind to enzymes. There are also molecules (glycoproteins and glycolipids) which attach to the external surface of the plasma membrane to help identify the cell as self. For example, these flags or markers are what make up the blood typing of a red blood cell (A, B, AB or O).

Membrane Lipids

  1. Phospholipids – usually about 75% of lipid content.

The polar glycerol-phosphate head of a phospholipid is the hydrophilic end and a nonpolar fatty acid tail is the hydrophobic end. The entire molecule is amphiphilic, meaning it can mix with both water and lipid environments. The phospholipids are arranged in two rows, called the lipid bilayer and this functions as a barrier that only lipid-soluble molecules can penetrate. They also provide a framework for membrane proteins. Some lipids are involved in cellular communication. Some common phospholipids found in plasma membranes include phosphatidyl choline and sphingomyelin.

  1. Cholesterol – usually about 20 – 30% of lipid content.

This 4 ringed lipid structure inserts into the hydrophobic center with the nonpolar fatty acid tails. The more cholesterol in the plasma membrane the more insulative the membrane will be. For example, the myelin sheath membrane (which insulates axons of nerve cells) is about 30% cholesterol, while other mammalian cell membranes may be about 20% cholesterol.


Cholesterol helps to stabilize the plasma membrane. It functions to keep membranes impermeable and yet flexible. Membranes with higher cholesterol concentrations are less permeable to ions, water, and other small molecules. Presumably cholesterol blocks the openings between phospholipid tails through which these small molecules could otherwise pass. Mammals maintain a relatively constant Tb, so the “plasticizing” effect of cholesterol is not as important as it is in poikilothermic animals and plants that cannot maintain a constant body temperature.

  1. Glycolipids – usually about 5% of the lipid content.

The prefix glyco means ‘glucose’ or ‘sugar’, so a glycolipid is a small amount of a sugar attached to a large amount of lipid. Glycolipids are found on the external surface of the plasma membrane and act as a cell markers. This helps identify the cell as self to defense cells of the body.


Other Phospholipid Arrangements

  1. Micelles are small droplets with hydrophobic tails forming the interior; the hydrophilic heads form the exposed boundary. Important in digestion and absorption of fats in digestive tract.
  2. Liposomes are larger hollow spheres with phospholipid bilayer walls. Their hollow core can be loaded with water-soluble molecules. Can be used as a drug delivery system.

Membrane Carbohydrates

Plasma membrane carbohydrates attach to both lipids and proteins. The Glycocalyx is a protective layer on cell surface formed by Glycoproteins – when glucose attached to membrane proteins and Glycolipids – when glucose attached to membrane lipids. The carbohydrates of the glycocalyx play a critical role in identifying cells; for example, the carbohydrates of the glycocalyx in human blood cells differentiate the main ABO blood groups from one another.

Membrane Proteins

  1. Associated Proteins

Also termed peripheral or extrinsic proteins. They are attached loosely to membrane-spanning proteins or to polar regions of phospholipids. They do not span the plasma membrane!

  1. Integral Proteins and Membrane-Spanning Proteins

Also termed intrinsic proteins. These are tightly bound into the phospholipid bilayer. Some integral proteins only extend partway into the membrane, others are membrane-spanning. Membrane-spanning proteins have segments that cross the membrane multiple times. Loops extend into extracellular and intracellular regions. Carbohydrates attach to extracellular loops and phosphates attach to intracellular loops. When amino acids are linked to each other, they can form an a-helix that has an exterior layer of nonpolar side groups and a central core composed of the polar amino and carboxyl groups. This ties the protein so firmly to the membrane that it can only be freed by disrupting the phospholipid bilayer with detergents.

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Measurement of Heterophoria

  • Within a small distance, either side of the horopter, objects can still be fused & seen as single. Strictly speaking, they fall on non-corresponding retinal points & there will be a small disparity
  • The zone on either side of the horopter within which it is still possible to see objects singly is known as Panum’s area
  • At periphery, Panum’s area is large, at fovea it is small
  • Panum’s Fusional Space indicates that:
    • Retinal correspondence is not just between pairs of points but between retinal areas centred on corresponding points
  • Panum’s Areas:
    • The receptive fields of cortical binocular neurons
    • Objects far enough from the horopter to be outside Panum’s area produce very large retinal disparities, so cannot be fused
    • They are seen as double


  • Fusion & Panum’s Areas
    • (You can only fuse images for so long before getting tired)
    • (As you move towards the periphery, the number of neurons sending info to the brain decrease, there are more neurons in the centre of the eye sending info to the brain – less space is dedicated in visual cortex to collect info from periphery)
    • Fusional stress – effect of horizontal shear (eye moving horizontally) – central diplopia before peripheral diplopia (fuses much worse horizontally than vertically, & centrally than peripherally. Receptive fields are larger for peripheral areas than central)
    • Fusional stress – effect of vertical shear: central diplopia before peripheral diplopia (fuses much worse vertically than horizontally, & centrally than peripherally)
    • Fusional stress – effect of cyclo shear: peripheral diplopia before central diplopia
  • Fusional eye movements (extremely quick)
    • Reaction time ~ 200 msec
    • Slow movement ~ 30°/sec
    • (cf. Saccades ~ 600°/sec) – saccades = how quickly eyes can move from one side to the other
    • Affected by alcohol, anoxia (lack of O₂)
  • Strabismus
    • The retinal images in each eye do not fall onto corresponding points
    • Adults who develop strabismus often have double vision because the brain is already trained to receive images from both eyes & cannot ignore the image from the turned eye
    • In a young child, (before the corresponding points are formed) the brain learns to ignore the image from one eye & sees only the image from the straight or better-seeing eye. The child then develops amblyopia


  • Heterophoria
    • Subjective tests are used to quantify the magnitude of heterophoria with refractive correction
    • Cover test (objective test) is insensitive to small eye movements below 2∆
    • Subjective tests will usually be performed in an eye exam if the cover test indicates a large phoria or poor fusional recovery or if there is a significant change in refractive correction (& therefore a possible change in phoria)
    • (If there is a tropia, there’s very little you can do to get info subjectively. Only for phoria can you rely on subjective tests)
  • Measurement of Heterophoria
    • Occluding one eye – The Cover Test
    • Distortion tests – e.g. the Maddox rod or red & green goggles
    • Independent object tests – e.g. the Maddox Wing or the synoptophore
    • Displacement tests – e.g. vertical prism dissociation to measure the horizontal phoria, horizontal prism dissociation to measure the vertical phoria
    • (do this measurement with distance VA after they’ve been refracted with their prescription in)
  • Distance Phoria – Maddox Road/Groove
    • With the distance Rx in place & both eyes un-occluded place the Maddox Rod over one eye (usually the RE)
    • Turn the Maddox Rod until the grooves are horizontal (produces a vertical line) – use horizontal base prims
    • Turn down the room lights, switch on the spotlight
    • Ask the Px to look at the spot
    • The Maddox Rod is a red or clear lens composed of a series of parallel plano-convex cylinders
    • The Px views a spotlight with one eye & the eye that views through the Maddox Rod forms a distorted (red or white line) image of the spotlight
    • The distorted image appears as a streak of light perpendicular to the axis of the cylinders
    • Can be used to measure cyclophoria
  • Patient Instructions:
    • “Can you see a red streak running from top to bottom to the side of the spot?”
    • “Which side of the spot is it?”
    • “Tell me when the streak passes through the spot”
    • (in 90% of these tests the streak & spot do not already overlap (naturally) so we have to fix this
  • Position of the streak (see diagram)
    • Spotlight is on the left fovea & seen as straight ahead
    • Streak falls on temporal retina of RE. This does not correspond with L fovea. Streak is ф° to the left of the L foveal visual direction
    • The two foveas correspond, they have the same visual direction
    • Therefore streak seen ф° to the left of the spot
  • If the Maddox Rod is on RE for e.g., if streak is on right side of spot then they have ESOphoria, if streak is on left side of spot they have EXOphoria (vice versa) – so same side = eso, opposite side = exo
  • Measuring the size of the phoria (Prism Bar)
    • Base in prism will bring the streak to the spot
    • The size of the phoria is the prism strength that puts the streak on the spot
  • When the correct prism is in place the streak falls onto the right fovea. The spot falls on the left fovea. The foveae are corresponding points so the streak is seen in the same direction as the spot.

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