Topic 5. The Plasma Membrane

Structure & FunctionSeptember 26, 2005 Biology 1001

5.1 Obtaining Raw Materials for Metabolism

All organisms, whether autotrophs or heterotrophs, must obtain certain molecules from the extracellular environment for the chemical reactions that occur in the cell Eg. Heterotrophs need organic molecules like glucose, water and

minerals such as Na+ or Ca2+

Autotrophs need CO2, H2O & minerals

Such materials must pass through the cell wall and the plasma membrane The plasma membrane acts as a selectively permeable barrier

screening certain materials from entering the cell while allowing others to enter at different rates

**Note: Section 5.2 was covered in Topic 3**

Section 5.3 The Structure and Function of the Plasma Membrane Plasma membranes of cells consist mostly of lipids

and proteins, with some carbohydrates as well The most abundant lipids are the phospholipids The phospholipids and most membrane proteins are

amphipathic molecules, that is they have both hydrophilic (polar) and hydrophobic (non-polar) regions

The membrane is a fluid structure with a mosaic of various proteins embedded in or attached to a bilayer of phospholipids – this is the fluid mosaic model

The fluid mosaic model of membranes

Figure 7.3

More features of the plasma membrane Carbohydrates are located on the external (extracellular) side

of the membrane where they are covalently bonded to lipids (as glycolipids) or proteins (as glycoproteins) and function in cell-cell recognition

The two faces of the membrane also differ in lipid composition and protein constituents – membranes are sided

Features of the fluid mosaic model

The fluidity Both the lipids and proteins drift laterally, the lipids at a faster rate Fluidity is important to maintain membrane permeability and for

proteins to function properly Cholesterol acts a “temperature buffer” to maintain fluidity Also, the proportion of unsaturated fatty acids can change

The mosaicness Membranes are a collage of different kinds of proteins These proteins determine the specific functions of the membrane They can be transmembrane proteins or peripheral proteins

PASSIVE TRANSPORTDIFFUSION & OSMOSIS

What determines the direction of transport?

Diffusion is the tendency for the molecules of a substance to spread out evenly into the available space as a result of thermal motion (heat energy)

The substance therefore moves spontaneously down its concentration gradient from an area of greater concentration to an area of lesser concentration Only the concentration of the particular substance is important

Such passive transport requires no energy expenditure by the cell, and accounts for much of the traffic across cell membranes

Diffusion

Osmosis – The Diffusion of Water Osmosis is a special case of diffusion – the diffusion of water

molecules down their concentration gradient

Note: When attempting to determine the direction of movement of water molecules, one must consider the concentration of water relative to the total concentration of solutes

Water Balance of Cells What happens to a cell when it is placed in a solution?

Terminology Tonicity is the ability of a solution to cause a cell to lose or gain water A solution is isotonic to a cell if it has the same concentration of solutes A solution is hypertonic to the cell if it contains more total solutes A solution is hypotonic to the cell if it contains less total solutes

A cell without a wall does best in a isotonic environment; it loses water and shrivels in a hypertonic one, it gains water and lyses in a hypotonic one

A cell with a wall maintains turgidity in a hypotonic environment; plasmolyzes in a hypertonic one, and is flaccid in an isotonic environment

OsmoregulationThe Control of Water Balance

Figure 7.13

Membranes Are Selectively Permeable

Not all substances pass through the cell membrane and those that do do so at different rates

Two features contribute to this selective permeability – the polarity of the molecule and the presence or absence of transport proteins for particular molecules Small non-polar molecules such as O2 pass through relatively easily

Larger or polar molecules like H20 and glucose pass much more slowly In the latter case transport proteins may assist the passage

Transport proteins work by creating a hydrophilic channel through the membrane, or by carrying the molecule across the membrane

Passive Transport – Facilitated Diffusion

Diffusion is related to the direction of movement of molecules – down their concentration gradient

But we still have to consider the selective permeability of the membrane – not all substances pass at the same rate Substances will diffuse at different rates due to differing size and

polarity, and the presence or absence of transport proteins

When transport proteins are required but the direction of movement is still determined by diffusion down a concentration gradient, it is called facilitated diffusion

Both diffusion (including osmosis!) and facilitated diffusion are passive transport processes

Active Transport Certain transport proteins carry substances against their

concentration gradient

This is called active transport because the cell must expend energy

Enables the cell to maintain internal concentrations of molecules different from the environmental concentrations

The classic example is the sodium-potassium pump of animal cells which exchanges sodium for potassium across the membrane to keep sodium higher outside and potassium higher inside

Passive Vs. Active Transport

Figure 7.16 – The Na+-K+ Pump

Active Transport

Bulk Transport Across Plasma Membranes For larger molecules and particles transport involves vesicles

During exocytosis, the cell secretes particles by fusing vesicles to the plasma membrane

During endocytosis, the cell uptakes particles by surrounding them with plasma membrane that pinches off to form an intracellular vesicle