THE CELL MEMBRANE

The Key to Cellular Transport

Some Membrane Terms

Many substances can diffuse across biological membranes, but some are too large or too strongly charged to cross the lipid bilayer.

If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it.

A membrane is impermeable to substances that cannot pass across it.

Most biological membranes are selectively permeable, meaning that some substances can pass across them and others cannot.

Characteristics of the Cell Membrane Made of phospholipids – arranged in two layers called a bilayer Selectively permeable

-it controls “selects” what can enter or leave the cell

-some materials are allowed through others aren’t Fluid Mosaic Model - made of different molecules – phospholipids,

proteins,

cholesterol

- the molecules aren’t static but can move “fluid”

Lipid bi-layer – the heart of the membrane

Phospholipids are arranged into 2 layers

- called a bilayer

Phospholipids: Phosphate Heads Polar/hydrophillic – attracted to water

and charged particles

Phospholipids: Lipid Tails

Nonpolar/hydrophobic – repel water and charged particles

Cholesterol

Found in bilayer – helps to hold the membrane together

while adding to fluidity of the membrane

Glycoproteins

Proteins with carbohydrates attached – identifies the cell type

-allows it to bind with other cells of the same type

Receptor Proteins

recognize and bind with substances outside of the cell,

causing changes inside the cell

Enzymes

Allow reactions to occur- light reactions and ETS occur with enzymes in membrane

Transport Proteins

Proteins that extend across the membrane – provide a way for large or charged molecules to cross the membrane

Types of Transport Proteins

Ion Channels- allow certain charged particles to cross the membrane – passive transport

Carrier Proteins – bind to a specific molecule and carry it

across the membrane

- facilitated diffusion – proteins carry molecules with the concentration gradient /no cellular energy is required

- active transport – proteins carry molecules against the concentration gradient/ cellular energy is required