Cell Membrane Cell Membrane & Transport of & Transport of

materialsmaterialsSection 6Section 6

Cell/Plasma MembraneCell/Plasma Membrane• Cell membrane surrounds all cells • Cell membrane consists of

phospholipid & protein molecules• This forms the fluid mosaic model

Phospholipid BilayerPhospholipid Bilayer• Phospholipids arranged into a

double layer (bi-layer) • Hydrophilic heads form

hydrogen bonds with water on either side of the cell

• Hydrophobic tails face inwards

• Cholesterol is also a component of the bilayer

• Structure is fluid, yet is a stable boundary

• Tiny molecules can pass through easily

ProteinsProteins• Proteins are spread

throughout the phospholipid bilayer.

• FUNCTIONS:• Support• Channels :allowing small

molecules through the membrane

• Carriers: actively pump molecules across

• Enzymes: catalysing reactions at the membrane

• Receptors: for hormones to attach to

• Markers: Identify cell’s blood/tissue type

Other membranesOther membranes• Many organelles have

membranes• Nucleus – double membrane• Mitochondrion – folded inner

membrane• Endoplasmic Reticulum & Golgi –

membranes can pinch off = vesicles

• All membranes are selective barriers

• Entry/exit of materials is regulated

Modes of transport - Modes of transport - diffusiondiffusion

• Molecules/ions move along a concentration gradient

• Process is passive (no energy)

• Small molecules diffuse easily through tiny pores

• Larger molecules cannot diffuse

• Membrane is selectively permeable

OsmosisOsmosis• Movement of water from

HWC to LWC through a selectively permeable membrane

• In red blood cells:• In hypotonic solution –

burst• In isotonic solution – no

change• In hypertonic solution –

shrink (plasmolysed)

Active TransportActive Transport• Movement of molecules against

a concentration gradient• Requires ATP energy• Protein molecules are carriers –

recognise specific ions/molecules

• Sodium/Potassium pump exchanges ions back and forth

• Important in nerve cells• Affected by temperature,

oxygen, & glucose availability

Endocytosis/ExocytosisEndocytosis/Exocytosis• Endocytosis: a cell

engulfing large particles• A ‘pouch’ is formed – forms

an intracellular vesicle• 1) Phagocytosis • – engulfing of solid particles • (e.g. against bacteria)• 2) Pinocytosis – engulfing of

liquid • Exocytosis: Vesicles from

inside, fuse with membrane, and contents expelled

• e.g. enzymes, hormones secreted