Human Biology Stage 3 Text: Chapter 2

Keywords Diffusion Osmosis Fluid mosaic model Phospholipid bilayer Hydrophobic Hydrophilic Channel proteins Carrier proteins Carrier mediated transport Facilitated diffusion Active transport Vesicular transport Endocytosis Pinocytosis Phagocytosis Exocytosis Receptor proteins

Review: diffusion & osmosis Diffusion 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 membrane A 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 membranes The 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 membranes Each 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 membranes A 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 membranes Embedded in the phospholipid bilayer are cholesterol and protein molecules

Membrane proteins Many 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 proteins Cell-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 transport Endocytosis 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 receptors Membrane 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 receptors Receptor 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 receptors Different cells have different types and numbers of receptor proteins Receptor proteins also function for communication between cells Receptor proteins are binding sites for hormones

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