7/29/2019 Edexcel Unit 1 Notes Transport Across Membranes

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Adam Clarke www.brain-freeze.co.uk

2.3 Transport across membranes

Cell membranes membranes include those that surround the organelles and the cell surface membrane

the cell surface membrane (outer cell membrane) forms the boundary of the cell where

anything that leaves or enters must pass through it

all membranes act as barriers to control what passes / allowing fluids on both sides to

have different compositions

this makes it possible to have right conditions for a particular reaction in one part of a cell

and different conditions elsewhere

membranes carry out chemical processes take place on membrane surface

the cell surface membrane must also be flexible to allow the cell to change shape

chemical secretions made by the cell are contained in membrane storage called ves icles

Membrane structure

made mainly of lipids and proteins arranged in a specific way

lipids polar lipids one end joined to a polar group many are phospholipids

phosphate group forming the polar part

fatty acid chains of a phospholipid are neutral and insoluble in water

the phosphate head carries a negative charge and is soluble in water

when the phospholipids contact the water two parts behave differently

polar phosphate is hydrophilic dissolves readily in water

lipids tails are hydrophobic insoluble in water

molecules tightly packed in water form monolayer hydrophilic heads in water, tails in

the air

micelles hydrophilic heads point outwards and all the hydrophobic tails are hidden inside water on both sides phospholipid form a bilayer with the hydrophilic with the

hydrophilic heads pointing into the water while the hydrophobic tails are protected in the

middle.

such structure is named the unit membrane and it is the basis of all membranes

Membrane proteins

bilayer of lipid as a fluid system many proteins / other molecules floating within it

proportion of phospholipids containing unsaturated fatty acids in the bilayer seems to

affect how freely the moving proteins float about in the membrane

many have a hydrophobic part in the lipid bilayer + hydrophilic part to carry out otherprocesses

membrane functions mainly help substances cross membrane

form pores / channels permanent/temporary allow specific molecules to move

through

some are gated channels open/shut depending on conditions in cell

some are active carrier system- use energy to move molecules

others are gaps in the lipid bilayer that allow ionic substances through the membrane in

either direction

they can act as specific receptor molecules / may be enzymes (internal cell membrane) to

control reactions linked to that membrane / glycoproteins proteins + carbohydrate allowcells to recognise each other floating proteins in lipid fluid mosaic model.

7/29/2019 Edexcel Unit 1 Notes Transport Across Membranes

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Adam Clarke www.brain-freeze.co.uk

Fluid mosaic model

Developed by S.J. Singer and Garth Nicolson in 1972 to describe the structure of cell

membranes.

The membrane is described as fluid because of its hydrophobic components such as lipids /

membrane proteins that move sideways through the membrane this means that the

membrane is not solid but more fluid.

It is described as mosaic where it consists of multiple macromolecules e.g. proteins,

glycoproteins, phospholipids, glycolipids, lipoproteins and sometimes cholesterol.

The model describes the membrane as a lipid bilayer (where protein is spread around it) and

it is as such because of the phospholipid component that can fold over to produce a double

layer when it is placed in water or a similar polar surrounding. Such feature is significant for

the membranes features of cellular transport and recognition of cells.