Transcript Cell Membranes CXH File

Membranes in Cells
Objective:
• To explain the structure and function of the plasma membrane
Cells have many membranes:
plasma membrane
tonoplast
outer mitochondrial membrane
inner mitochondrial membrane
outer chloroplast membrane
nuclear envelope
Membranes have several functions:
1. They are partially permeable, controlling what passes through
them.
2. They are involved in cell signalling (communication between
cells).
3. They provide attachment sites for enzymes and other
molecules involved in metabolism.
4. They can allow electrical signals to pass along them.
5. They produce different compartments inside cells.
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
Membranes are flexible and able to break and
fuse easily
Neutrophil engulfing
anthrax bacteria.
Fill your plate with some
water. Add some drops
of oil (phospholipid).
The oil should form
micelles, just like a
membrane. Use a
cocktail stick to move a
drop of oil close to
another one. What
happens?
5 μm
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
Cytoplasm (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 inward and
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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 bilayer 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.
Activity:
Watch the demonstration of the fluid mosaic model.
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 their tails.
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
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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
channel 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) channel
protein 6) hydrophilic channel; 7)Click
phosphate
group;
8) fatty acid; 9) phospholipid;
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answers
10) glycocalyx; 11) phospholipid bilayer
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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 unevenly
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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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.