Membranes in cells
Chapter 2.3
Objectives of unit:
• Understand the structure and properties of the plasma membrane
• Explain passive transport mechanisms of diffusion and facilitative diffusion,
including the role of transporter and carrier proteins
• Define the process of osmosis
• Explain the process of active transport and the role of proteins and ATP
• Explain the processes of endocytosis and exocytosis
• Describe the properties of gas exchange surfaces in living organisms
• Explain how the structure of the mammalian lung is adapted for rapid
gaseous exchange
Cells have many membranes:
plasma membrane
tonoplast
outer mitochondrial membrane
inner mitochondrial membrane
outer chloroplast membrane
nuclear envelope
Membranes are flexible and able to break and
fuse easily
Membranes allow cellular compartments to have
different conditions
pH 4.8
Contains digestive
enzymes, optimum
pH 4.5 - 4.8
lysosome
Membrane acts as
a barrier
pH 7.2
cytosol
Membranes are mainly made of phospholipids
phosphate group
hydrophilic
head
phosphoester bond
glycerol
ester bond
fatty acid
hydrophobic
tail
The polar hydrophilic heads are water soluble
and the hydrophobic heads are water insoluble
Hydrophobic (water-hating) tail
air
aqueous solution
Hydrophilic (water-loving) head
Phospholipids form
micelles when
submerged in water
In 1925 Gorter and Grendel proposed that the unit
membrane is formed from a phospholipid bilayer
Extracellular space (aqueous)
Phosphate heads
face aqueous
solution
phospholipid
bilayer
Cytosoplasm (aqueous)
Hydrophobic tails
face inwards
Initial studies showed that the plasma membrane
had layers:
Scientists also found that protein were present in membranes so
Davson-Danielli proposed in 1935 the following model for
membrane structure:
Protein
Phospholipid
bilayer
The development and use of electron microscopes
showed that the Davson-Danielli model was incorrect
In the early 1970s Singer and Nicholson used techniques such as
freeze-etching to confirm the lipid bilayer.
They also showed that the proteins were distributed throughout
the protein in a mosaic pattern.
In addition they found that the membrane was fluid and had
considerable sideways movement of molecules within it.
Hence they proposed the Fluid-Mosaic Model for Plasma
Membrane Structure.
The fluid mosaic model of the plasma
membrane:
The proteins can move freely through the lipid bilayer.
The ease with which they do this is dependent on the number of
phospholipids with unsaturated fatty acids in the phospholipids.
Fat-soluble organic molecules can diffuse through
the bilayer but polar molecules require proteins
Fat-soluble molecules
Polar molecules
Extracellular
space
Cytosoplasm
(aqueous)
hydrophilic pore
Osmosis
•Water is a polar molecule
•Water must pass through a
protein to enter or leave the
cell
•The movement of water is
OSMOSIS
•Osmosis is the movement of
water from an area of less
negative water potential to an
area of more negative water
potential through a partially
permeable membrane.
OSMOSIS IN PLANT CELLS
The membrane contains many types of protein:
carbohydrate chain
Glycocalyx: For cell
recognition so cells group
together to form tissues
Receptor: for
recognition by
hormones
glycoprotein
peripheral protein
Enzyme
or
signalling
protein
integral protein
carrier protein
hydrophilic channel
Question: Label the diagram (11marks)
4
1
5
6
Note: label the proteins based on location
or structure, e.g. you do not need to
identify receptors and enzymes.
3
2
7
10
9
11
8
1) carbohydrate; 2) glycoprotein; 3)integral protein; 4) peripheral protein; 5) carrier
protein 6) hydrophilic channel; 7)Click
phosphate
group;
8) fatty acid; 9) phospholipid;
to reveal
answers
10) glycocalyx; 11) phospholipid bilayer
click to cover answers
There are different types of carrier proteins in the
membrane:
ATP
Carrier protein
(passive)
Gated-channel protein
Channel protein
Carrier protein
(active)
Facilitated Diffusion
• Facilitated Diffusion
occurs through a carrier
protein
• Polar molecules move
from an area of high
conc. to an area of low
conc.
• No ATP is required
Active Transport
• Active transport occurs
through a carrier
protein
• Polar molecules move
from a low to a high
concentration
• ATP energy is required
Membrane bound proteins allow chemical processes
to occur on membranes in a sequential manner:
proteins
membrane
Cyt c
Q
I
III
II
Enzyme and transporter proteins
involved in aerobic respiration in the
inner mitochondrial membrane
IV
ATP synthase
Summary
• The unit membrane consists of a phospholipid bilayer
• Phospholipids consist of a polar, hydrophilic phosphate head and a nonpolar, hydrophobic tail consisting of fatty acid chains.
• Proteins also occur in the membrane and float freely throughout it.
• The model for membrane structure is known as the fluid mosaic model.
• Peripheral proteins occur on the inner or outer face of the membrane and
integral proteins extend through both lipid layers.
• Membrane bound enzymes occur allowing structured metabolic pathways.
• Glycoproteins form the glycocalyx and allow cell to cell recognition.
• Receptor proteins can act as binding sites for hormones and other
substances and can transmit the information to the interior of the cell.
• A variety of carrier proteins allow for the controlled movement of substance
through the membrane using both passive diffusion or active transport.
• Non-polar, lipid soluble molecules diffuse through the phospholipid bilayer.
• Ionic, polar molecules require carrier proteins to enable them to pass
through the membrane.
• Membrane structure loses integrity with high temperature or presence of
organic solvents such as alcohol, thereby increasing permeability.