Cellular activityHuman Biology Stage 3

Text: Chapter 2

KeywordsDiffusionOsmosisFluid mosaic modelPhospholipid bilayer

HydrophobicHydrophilic

Channel proteinsCarrier proteins

Carrier mediated transportFacilitated diffusionActive transport

Vesicular transportEndocytosis

PinocytosisPhagocytosis

ExocytosisReceptor proteins

Review: diffusion & osmosisDiffusion is the movement

of molecules from an area of high concentration to an area of low concentration

Osmosis is a special case of diffusion. It is the diffusion in water from a region of high concentration through a differentially permeable membrane

Both diffusion and osmosis are passive process, they require no energy input from the cell

The cell membrane

The cell membrane surrounds the whole cell and separates the contents of the cell from the extracellular fluid

Function of the cell membraneA physical barrier: separates

the cell from the extracellular fluid

The regulation of the passage of materials: the membrane controls the movement of materials into/out of cells

Sensitivity: the cell membrane is the first part of the cell affected by any changes in the extracellular fluid

Support: cell membrane is attached to microfilaments to give cell support

Structure of cell membranesThe main structure of

the membrane is composed phospholipid molecules, which are lipids that contain a phosphate group

The phospholipids are arranged in 2 layers called a bilayer

Structure of cell membranesEach phospholipid

molecule has a hydrophilic head (water loving) and a hydrophobic tail (water hating)

They are arranged so that their heads are on the outside and tails on the inside

Structure of cell membranesA model of the cell

membrane is called the fluid mosiac model

The membrane is said to be fluid because the molecules are constantly changing position

Its said to be mosiac as it is composed of many kinds of molecules

Structure of cell membranesEmbedded in the phospholipid bilayer are

cholesterol and protein molecules

Membrane proteinsMany of the proteins are

involved in the movement of substances through the membrane

There are 4 types of membrane proteins, each with a different function: Channel proteins Carrier proteins Receptor proteinsCell-identity markers

Membrane transport: Channel proteins & carrier proteins

Channel proteins are like open doors

They allow certain types of molecules (mostly ions) to flow freely in and out of the cell

The cell may be able to close these ‘doors’ when required

Channel proteins allow passive diffusion across the membrane (osmosis)

Carrier proteins are involved in carrier mediated transport

These proteins bind to molecules to be transported and help their passage across the membrane

They bind to the molecule, change shape and then release the molecule

Membrane transport: Channel proteins & carrier proteins

Membrane transport: Carrier mediated transport

There are 2 main types of carrier mediated transport :

Facilitated diffusion is a passive process which substances move along the concentration gradient. The protein changes shape and the molecule is released through the membrane

Membrane transport: Carrier mediated transport

Active transport requires an energy input as the substance is being transported across the membrane against the concentration gradient

Membrane transport: Carrier mediated transport

Carrier proteins are specific - they will only bind to a particular molecule

Carriers can become saturated. Once all the available carriers are occupied, any increase in the concentration of molecules cannot increase the rate of movement

Carrier activity is regulated by substances ie hormones

Membrane transport: Vesicular transport

Vesicular transport is the movement of substances across the cell membrane in membranous bags called vesicles

Membrane transport: Vesicular transportEndocytosis is taking

liquids or solids into the cell by vesicular transport Pinocytosis is taking in

liquids Phagocytosis is taking

in solids Exocytosis is when the

contents of a vessicle inside the cell are passed to the outside of the cell

Membrane receptorsMembrane receptor

proteins are sensitive to certain molecules outside the cell, and when those molecules are present they bind with the receptor protein

The receptor and the bound molecule then trigger changes in the cell.

Membrane receptorsReceptor proteins are

specific. Each type of receptor protein will bind to only one specific molecule.

There is a limited number of receptors proteins in the membrane of each cell, so when each receptor is bound to a molecule there can be no further increase in the rate of the cell’s activity

Membrane receptorsDifferent cells have

different types and numbers of receptor proteins

Receptor proteins also function for communication between cells

Receptor proteins are binding sites for hormones

You tube animations

https://www.youtube.com/watch?v=RPAZvs4hvGA

Bozeman Science14 minutes