Cell surface (Plasma) MembranesTransport across a partially-permeable surface

LearningObjectives

Describe a

phospholipid bilayer Describe what is meant by ‘fluid mosaic’. Understand the role of intrinsic proteins Explain what selective permeability means

The Phospholipid Bilayer

Cell membranes are made of phospholipid, like a layer of oil. The phospholipid membrane has two (bi) layers of

phospholipid molecules. The phospholipid molecule has two ends (a hydrophobic

(water hating) and a hydrophilic (water loving end). This is the principal of how all cell membranes are formed.

Fluid Mosaic model (Singer and Nicholson 1972)

Intrinsic and Extrinsic Proteins

Intrinsic proteins go all the way through the phospholipid bilayer, some have holes (channels) to let molecules through.

Extrinsic proteins are only in the upper or lower layer only. They often have sugar chains on their surface which often act as ‘markers’ to identify the cell as ‘friendly’ (cell recognition), for cell adhesion, or hormone binding sites.

Membrane Transport – getting substances across the membrane

The intrinsic proteins are specialized, each one carries specific molecules across the bilayer

They help with selective permeability, only allowing certain molecules through

Sometimes this happens by active transport and sometimes by facilitated diffusion

Some substances can pass directly through the phospholipid bilayer (small and fat soluble – non polar) water, although polar is small enough to pass

Facilitated diffusion

Some proteins have ‘holes’ through them. These simple protein channels let through molecules of the right shape only

This happens by diffusion: molecules move from an area of high concentration to an area of low concentration

Carrier Proteins

The binding of the molecule to be transerred causes deformation of the protein

This mechanism helps deliver the molecule but does not require energy

It will not occur against a concentration gradient

Active Transport Is made possible by

carrier proteins using energy to actively shift molecules across a membrane.

This can happen against a concentration gradient

It requires ATP

Summary of transport across the membrane

Effect of heat

Heat will affect permeability of the phospholipid bilayer, the phospholipids become more fluid.

Small ‘holes’ may appear Extreme heat may cause big holes or

destroy the bilayer completely