Membranes in cells
Chapter 2.3
Objectives of unit:
• Understand the structure and properties of the plasma membrane
• Investigate the properties of plasma membranes practically
• 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
Neutrophil engulfing
anthrax bacteria.
Cover credit:
Micrograph by Volker
Brinkmann, PLoS Pathogens
Vol. 1(3) Nov. 2005.
5 μm
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
Question: Explain why phospholipids form a
bilayer in plasma membranes (4).
• Phospholipids have a polar phosphate group which are
hydrophilic and will face the aqueous solutions
• The fatty acid tails are non-polar and will move away from an
aqueous environment
• As both tissue fluid and cytoplasm is aqueous
• phospholipids form two
layers
with
the hydrophobic tails facing
Click
to reveal
answers
inward
• and phosphate groups outwards interacting with the aqueous
environment
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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.
Freeze fracturing and etching
Using liquid nitrogen, biological tissue samples or cells are rapidly frozen to
immobilize cell constituents.
Cell membranes are composed of two layers of phospholipids, called a bilayer,
where the hydrophobic, or water-hating, lipid tails point to the inside of the
membrane and the hydrophilic, or water-loving, ends of the lipid molecule point
outward and toward the inside of the cell.
The frozen sample is cracked or fractured with a microtome, which is a knife-like
instrument for cutting thin tissue slices.
This causes the cell membrane to split apart precisely between the two layers
because the attraction between the hydrophobic lipid tails represents the weakest
point.
Following fracturing, the sample undergoes a vacuum procedure, called "freeze
etching".
The surface of the fractured sample is shadowed with carbon and platinum vapor to
make a stable replica, which follows the contours of the fracture plane.
Acid is used to digest organic material adhering to the replica, leaving a thin
platinum shell of the fractured membrane surface.
This shell is then analyzed by electron microscopy.
Activity:
Read pages 100 – 103 of your textbook
Answer questions 1 – 3 on page 103
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
Question 4: How can polar and non-polar
molecules pass through the membrane (2).
•Polar molecules require proteins to enable them to pass through
the membrane
•Non-polar molecules canClick
diffuse
directly through the phospholipid
to reveal answer
bilayer
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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
Question: Explain why the model for membrane
structure is known as the fluid mosaic model (3).
• The phospholipid molecules can move freely laterally and
makes the membrane fluid.
• The proteins are distributed throughout the membrane un
evenly and in a mosaic pattern.
to revealupon
the answers
• The agreed structureClick
is based
experimental and chemical
evidence and so is classed as a model.
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Question: Describe the structure and function of
the glycocalyx (3)
• Consists of glycoproteins
• Which are proteins with added carbohydrate chains
• Used for cell recognition/receptors
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There are different types of carrier proteins in the
membrane:
ATP
Carrier protein
(passive)
Gated-channel protein
Channel protein
Carrier protein
(active)
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
Question: Other than as carrier proteins state two
functions of membrane bound proteins (2).
• Receptors
• Enzymes
• Structural (attached to microtubules)
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Practical Activity:
Factors affecting membrane permeability
See practical sheets
Question 3: Describe an experiment by which you could
test to see whether alcohol concentration affected
membrane permeability (5).
• Same volume discs of beetroot
• Same volume of alcohol
• Same temperature
• Same time in alcohol
• Range of alcohol concentrations
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• Use colourimeter to read amount of pigment in solution
• Graph of colour intensity (% absorbance etc.) over alcohol
concentration
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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.