Transport across cell membranes

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All membranes around and within all cells have the same basic structure and are known as what?
Plasma membranes
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What is the purpose of the cell-surface membrane?
Forms a boundary between the cytoplasm and the surrounding environment. It allows there to be different conditions inside and outside the cell. It controls the movement of substances in and out of the cell
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In every membrane there are 5 molecules that form its structure. What are they?
Phospholipids Proteins Cholesterol Glycolipids Glycoproteins
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Why is a phospholipid an important feature of the membrane?
The hydrophilic head point to the outside of the cell attracted by water on both sides. The hydrophobic tails point into the cell membrane repelled by water
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Lipid-soluble material moves through the phospholipid bilayer. What are the functions of the phospholipid bilayer?
1. Allow lipid soluble material through 2. Prevent water soluble substances entering and leaving 3. Make the membrane flexible and self-sealing
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Proteins are interspersed within the membrane. They are embedded in the phospholipid bilayer in what two main ways? (Part 1)
1.The proteins on the surface of the bilayer act to give mechanical strength or in conjunction with glycolipids as cell receptors for molecules such as hormones
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Proteins are interspersed within the membrane. They are embedded in the phospholipid bilayer in what two main ways? (Part 2)
2.Other proteins span the entire phospholipid bilayer. Some are protein channels which form water-filled tubes to allow water soluble ions to diffuse across. Others are carrier proteins that binds to ions/molecules then change shape to move them
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What are the functions of the proteins in the membrane?
1. Provide structural support 2. Act as channels to move water-soluble substances 3. Act as receptors 4. Helps cells adhere together 5. Allow active transport via carrier proteins
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What are the functions of cholesterol?
1. Add strength to membranes 2. reduce lateral movement of other molecules including phospholipids 3. Make the membrane less fluid at high temp 4. Prevent leakage of water and dissolved ions from the cell
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What are the functions of glycolipids?
1. The carbohydrate of the glycolipid extends to the watery environment outside the cell where it acts as a receptor. 2. Help maintain the stability of the cell 3. Helps cells to attach to one another and form tissues
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What are the functions of glycoproteins?
1. Act as recognition sites 2. Heps to attach to one another to form tissues 3. Allows cells to recognise each other
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Most molecules don't diffuse across a membrane because they are:
1. Not soluble in lipids and therefore cannot pass through the phospholipid layer 2. Too large to pass through the channels in the membrane 3. Of the same charge as the charge of thee protein 4. Electrically charged so have difficulty passing through
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The exchange of substances between cells and the environment occurs in ways that require metabolic energy (active transport) and ways that do not (passive transport) which type is diffusion?
Passive
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What does passive transport mean?
It just means that energy comes from the motion of particles and not an external factor such as ATP
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What is diffusion?
The net movement of molecules or ions from a region of high concentration to low concentration until evenly distributed.
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What is facilitated diffusion?
Some membranes are not readily permeable to molecules and only small non-polar molecules. The movement of larger and polar molecules is made easier by transmembrane channels and carriers.
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What type of transport is facilitated diffusion?
Passive as there is no external input of ATP
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What are the 2 types of proteins used in facilitated diffusion?
Carrier proteins and protein channels.
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What are carrier proteins?
When a molecule that is specific to the protein present, it binds with the protein. This cause it to change shape in a way that the molecule is released to the inside of the membrane
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What are protein channels?
Proteins form water-filled hydrophilic channels across the membrane. They allow specific water-soluble ions to pass through They are selective to specific ions if that ion isnt present then it will not open.
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What is osmosis?
The passage of water from a region where it has higher water potential to a region of lower water potential through a selectively permeable membrane
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What is solute?
Any substance that is dissolved in solvent. Solute and solvent form solution
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What unit is water measured in?
kPa (kilopascals)
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What greek letter is water represented by?
Psi Ψ
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What is water potential and what is the water potential of pure water?
The pressure created by water molecules under standard conditions of 25 degrees and 100 kPa. Pure water has a water potential of zero
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The addition of solute to pure water will do what to the water potential?
It will lower it
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The water potential of a solution (solute+water) must always be what?
Less than zero
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The more solute means what?
The lower the number (more negative)
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What would happen if you put red blood cells in pure water?
It will absorb water by osmosis as it has a low water potential in their water cytoplasm.
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What happens when red bloods cells absorb too much water?
The membrane cannot stretch to a great extent so it will break bursting the cell and releasing its contents. In this case this is called haemolysis.
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What would happen if a red blood cell was placed in something with a lower water potential?
Water would leave the cell and the cell would shrink and shrivel. It would become darker as the haemoglobin is more concentrated as there is more of it than there is water.
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What is the difference between a plant and animal cell when a plant cell is placed in something with a lower water potential?
It wont shrivel unlike the animal cell. It will shrink but its cell wall will not change. Its the protoplast which contains the nucleus and cytoplasm that pulls away from the cell wall and shrinks
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What is incipient plasmolysis?
Where the protoplast completely pulls away from the cell wall when it a state of water loss and the cell is said to be plasmolysed.
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What type of transport is osmosis?
Passive
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What is active transport?
The movement of particles or ions into or out of a cell from a region of low concentration to a region of high concentration using ATP and carrier proteins.
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In active transport what is ATP used for?
1. directly move molecules 2. individually move molecules using a concentration gradient which has already been set up by active transport. (This is known as co-transport)
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How does active transport differ from passive transport?
1. Energy in the form of ATP is needed 2. Substances are moved against the concentration gradient 3. Carrier protein molecules act as pumps 4. The process is very selective
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What are the steps of direct active transport of a single molecule? (part 1)
1. The carrier proteins span the plasma membrane and bind to the molecule or ion to be transported 2. The molecule or ion binds to receptor sites on the carrier protein
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What are the steps of direct active transport of a single molecule? (part 2)
3. On the inside of the cell ATP binds to the protein causing it to split into ADP & a phosphate molecule. The protein molecule then changes shape & opens to the opposite side of the membrane.4.molecule/ion is released to the other side of the membra
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What are the steps of direct active transport of a single molecule? (part 3)
5. The phosphate molecule is released from the protein which causes the protein to revert to its original shape, ready for the process to be repeated. The phosphate molecule then recombines with the ADP to form ATP during respiration
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Sometimes more than on molecule or ion may be moved in the same direction at the same time by active transport, occasionally one molecules is moved into the cell the same time as another. What is one example of this?
Sodium-potassium pump
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What are microvilli?
The epithelial cells lining the ileum possess microvilli. They are finger-like projections of the cell-surface membrane. They provide more surface area for the insertion of carrier proteins through diffusion, facilitated diffusion & active transport
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How else can you increase transport across cell membranes?
Increase the number or carrier and protein channels
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What is the role of diffusion in absorption? (part1)
Carbohydrates and proteins are being digested continuously. There is more amino acids and proteins in the ileum then there is in the blood so the concentration gradient goes down which glucose moves by facilitated diffusion from the ileum into the
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What is the role of diffusion in absorption? (part2)
blood. Given that the blood is being constantly circulated by the heart, the glucose absorbed into the blood is being continuously being removed by cells as they use it up during respiration. This helps maintain the concentration gradient between
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What is the role of diffusion in absorption? (part3)
inside the ileum and the blood. This means the rate of movement by facilitated diffusion across the epithelial cell-surface membranes is increased.
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Why is active transport also needed in absorption?
At best, diffusion only results in equilibrium of concentrations. This means that not all glucose and amino acids are absorbed and some may pass out of the body. By using active transport also this wont then happen
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The actual mechanism of glucose being absorbed into the blood is an example of what?
Co-transport
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Why is the mechanism of glucose being absorbed into the blood co-transport?
Because either glucose or amino acids are drawn into the cells along with sodium ions that have been actively transported out by the sodium-potassium pump
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What are the steps of the co-transport of glucose into the blood? (part 1)
Sodium ions are actively transported out of epithelial cells by the sodium-potassium pump into the blood. This takes place in one type of carrier protein molecule found in the cell-surface membrane of the epithelial cells
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What are the steps of the co-transport of glucose into the blood? (part 2)
This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells
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What are the steps of the co-transport of glucose into the blood? (part 3)
Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier (co-transport protein) in the cell-surface membrane. As the sodium ions diffuse in through this second carrier protein they
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What are the steps of the co-transport of glucose into the blood? (part 4)
carry amino acids, or glucose molecules in with them. 4. The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.
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What are the causes of diarrhoea?
1. Damage to epithelial cells lining the intestine 2. loss of microvilli due to toxins 3. excessive secretion of water due to toxins
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To treat diarrhoeal diseases we have to rehydrate the patient however this is ineffective for what two reasons?
1. Water is not being absorbed from the intestine 2. Drinking water does not replace the electrolytes that are being lost from the epithelial cells
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