4.1- Structure of the cell surface membrane

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  • Created by: Megan2413
  • Created on: 11-12-16 15:59
What is a cell-surface membrane?
It is a barrier between the cell and its environment
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What is the function of a cell-surface membrane?
To control what substances leave and enter the cell
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They are partially permeable- what does this mean?
They let some molecules and ions through but not others
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What three modes of transport can substances move across the cell-surface membrane by?
- Diffusion - Osmosis - Active transport
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How is the cell-surface membrane structure described as and why?
A fluid-mosaic- The phospholipid molecules form a (bilayer) which is 'fluid' as the phospholipids are constantly moving and the proteins are scattered around like tiles in a mosaic
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What components are found in a cell-surface membrane?
-Proteins - Phopholipids - Cholesterol - Glycoproteins - Glycolipids
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What three types of proteins are found in the cell-surface membrane?
- Channel proteins - Carrier proteins - Receptor proteins
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What are the two functions of surface proteins?
- They provide mechanical support for the membrane - act as receptors for hormones
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Where are channel and carrier proteins found in the structure of a cell surface membrane?
They span from one side of the bilayer to the other
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What do carrier and channel proteins do?
They allow water-soluble substances to diffuse through the membrane
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What is another function of proteins in the membrane?
They help cells adhere together
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Are all proteins able to move around in bilayer?
No
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Why do phospholipids form a bilayer?
Their heads are hydrophilic- they attract water and their tails are hydrophobic - they repel water. So they from a bilayer where the heads are facing outwards on both sides of the membrane
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Why does the centre of the phospholipid bilayer not let through water-soluble substances through (like ions)?
It contains the hydrophobic tails
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What is the function of the phospholipid bilayer?
They form a barrier against water soluble substances but let through lipid-soluble substances
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Cholesterol is a type of l....?
lipid
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Cholesterol is found in all cell membranes- except..?
Bacterial cell membranes
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What are the two functions of cholesterol in the cell-surface membrane?
- Helps maintain the shape of membrane (important in cells such as animal cells which don't have a cell wall) - Prevent leakage of water and dissolved ions from the cell
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How does cholesterol carry out its function?
It binds to the hydrophobic tails of phospholipids, which causes them to pack together more closely- restricting movement of the phospholipids making the membrane less fluid and more rigid
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What is the difference in location of a glycolipid and glycoprotein?
Glycolipids branch out from the phospholipid bilayer surface, whereas glycoproteins branch out from surface proteins in the phospholipid bilayer
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What are the three functions of a glycolipid?
- Acts as a recognition site for chemicals such as hormones - Helps maintain the structure of the membrane - Helps cells attach to one another to form tissues
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What are the three functions of a glycoprotein?
- Acts as a recognition site - Helps cells attach to one another to form tissues - Allows cells to recognise one another (e.g. lymphocytes can recognise an organisms own cells)
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What is a lymphocyte?
A type of white blood cell (leukocyte) that determine the specifity of the immune response to microorganisms
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How can cell-surface membrane permeability in different conditions be tested in a lab?
You can use beetroot to test the effect of different conditions- beetroot cells contain a red pigment that leaks out, the higher th epermeability of the membrane the more pigment that will leak out into a solution such as water
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How would membrane permeability be measured in this experiment?
By using a colorimeter, the absorbance of light can be measured using a blue filter and the higher the reading, the more permeable the membrane
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Why do temperatures between 0 and 45 increase membrane permeability?
The phospholipids become less packed together and the membrane is partially permeable, the increased temperature increases the energy in the phospholipid molecules causing them to move more, increasing the permeability of the membrane
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How would temperatures lower than 0 affect a membranes permeability?
The phospholipids don't have much energy hence don't move around causing them to be closely packed and the membrance is then rigid. But channel and carrier proteins deform, increasing the permeability. Ice crystals may form and pierce the membrane
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--> continuation
making it highly permeable when it thaws
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How do temperatures above 45 affect membrane permeability?
The phospholipid bilayer starts to melt causing the membrane to be more permeable. Water inside the cell expands, putting pressure on the membrane. Channel and carrier proteins deform so they can't control what goes in and out-increasing permeability
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Why may polar molecules not be able to pass through the phospholipid bilayer?
They can't pass through the non-polar hydrophobic tail centre of the bilayer
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Describe the structure of a channel protein
They form water-filles tubes which only allow water-soluble substances through
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What types of substances can't pass through protein channels?
- Large molecules - Molecules of the same charge as the channel as they're repelled - Lipid soluble substances
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What five functions do membranes within cells have?
- Control exchnage of substances in and out of organelles - Separate organelles from cytoplasm so specific metabolic reactions can take place within them - Provide internal transport medium - Isolate enzymes that may damage the cell - Provide -->
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Give an example of a membrane used as an internal transport medium inside a cell
Endoplasmic reticulum
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Give an example of an enzyme that may need to be isolated from the rest of a cell by the use of a membrane
Lysozymes inside lysosome organelle
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Give an example of a reaction taking place on the surface of a membrane inside a cell
Protein synthesis using ribosomes on the surface of RER
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Card 4

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Card 5

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