Chapter 2 Cell Membranes

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  • Created by: Chloesn
  • Created on: 30-12-13 20:34
Membrane sizes?
7nm and 10nm thick
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Function of the plasma membrane?
The plasma (cell) membrane isolates the reactions inside the cell from the outside environment. It controls what enters the cell and what leaves. It’s a thin layer of lipid (fat) molecules
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What else about the plasma membrane?
-Partially Permeable -Communication -Controls what enters/leaves
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Components of the phospholipid bilayer?
Heads Tails Cholesterol Proteins Glycolipids Glycoproteins Channels
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Define Fluid Mosaic Model
Fluid Mosaic Model → In constant motion The way that it would look from above
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Components of a phospholipid and their properties?
Heads - Hydrophilic (water loving) - Have a tiny charge Tails - Hydrophobic (water hating) - Do not have a charge
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Role of the phospholipid?
Forms the bilayer which is the fundamental basis of the membrane in which all other components are embedded. Provides a barrier to water soluble (hydrophilic) substances - ions and charged molecules.
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Role of the cholesterol
Helps maintain fluidity preventing it from becoming too stiff in low temperatures and too fluid in high temperatures. Giving some eukaryotic cells mechanical stability.
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Role of the proteins and glycoproteins?
Form channels - hydrophilic opened/closed. Transporters move substances across up their concentration gradient. Receptor sites allowing specific molecules outside like - hormones - to bind and set up reaction. Recognition sites. Enzyme
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Why is cell signalling important?
A cell must stay in contact with its environment and with other cells in order to survive, they must be able to react to any changes. Multicellular organisms - Therefor it needs to pick up ‘signals’ at its surface to which it may need to respond.
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What is cell signalling?
Signals arrive at the plasma membrane from outside the cell as particular substances - i.e hormones - or changes in electrical potential - as it happens as nerve impulses. A receptor picks these signals up and brings about change in the cell.
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Three different ways in which cell signalling can occur?
Receptor acts as an ion channel - Receptor activates a G-protein - Receptor acts as an enzyme
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Receptor acts as an ion channel?
The signal is a chemical that attaches to a protein or glycoprotein acting as an ion channel. When the chemical attaches to the receptor, it makes the channel open and let the ions into the cell, bringing about a response.
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Receptor activates a G-protein?
A slightly more complex mechanism - the receptor in the plasma membrane interacts with another molecule, a g-protein. When the signal attaches to the receptor, the g-protein then activates an enzyme, which brings about a reaction inside the cell.
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Receptor acts as an enzyme?
This involves a receptor that is also an enzyme. It is made of two parts - when the signal molecule arrives, it slot into both of these parts, connecting them to another and forming them into an active enzyme - the enzyme then brings about a change
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The passive and active movements of substances into and out of cells?
Some passively (no energy required to make them move) - diffusion, facilitated diffusion, osmosis. Others are actively moved by the cell -(using energy to move them up their concentration gradient) - active transport.
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Diffusion?
Is the constant, random movement, spread evenly throughout a space. I
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Oxygen diffusion?
Inside aerobic respiration constantly uses up oxygen so concentration is low in the cell. Net movement is the overall direction of the movement of a substance is into the cell.
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Concentration gradient?
Initially more in one place than another → concentration gradient.
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Diffusion and concentration gradient and net movement?
Diffusion is the net movement of molecules or ions down their concentration gradient for them. (to a place of lower concentration).
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Facilitated diffusion?
Cells therefore provide special pathways through the plasma membrane - channel proteins. These lie in the membrane, one side to the other, forming a hydrophilic channel so ions can pass. they can pass through diffusion down their concentration grad..
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Osmosis?
Water molecules although carry a charge are very small can therefore pass through the lipid bilayer by diffusion. This movement of water down their diffusion gradient, through a partially permeable membrane is osmosis.
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High water potential?
A solution containing a lot of water, and under pressure.
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Low water potential?
A solution containing a lot of dissolved substances (solutes) and little water, not under pressure.
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Correct - Water concentration and water moves down a concentration gradient.
Water potential and Water tends to move down a water potential gradient from where there is a lot of water (high water potential) to where there is less of it (low water potential).
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The term used instead of isotonic and the state of a cell?
Normal, balanced. When a cell is in this solution the water potential gradient is maintained and so it's normal biconcave shape with a higher water potential outside the cell.
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What happens to an animal cell when the water potential gradient is very high outside?
Too much water enters by osmosis the cell swells and can eventually lysis (burst).
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What happens to an animal cell when the water potential outside the cell is lower than inside and what implications does this have?
the water leaves the cell causing it to shrink and become star shaped - crenated. The concentration of the solutes in the cytoplasm increases which can adversely affect metabolic reactions.
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What happens to a plant cell when the water potential gradient is very high outside?
The plant cell doesn't burst because as it swells it has to push against. The cell wall resists expansion of the cell, exerting a force called pressure potential. - Cell becomes stiff and turgid.
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What happens to a plant cell when the water potential outside the cell is lower than inside and what implications does this have?
If a lot of water is lost, it shrinks and doesn’t press outwards losing its turgor - said to become flaccid. The cell wall cannot cave much but the volume gets smaller and the plasma membrane can pull away often damaged - plasmolysis
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Active transport?
There are many instances where the cell has to take in or get rid of substances whose concentration gradient is the wrong way.
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Sodium and potassium in relation to active transport and cell needs?
Most cells need to contain a higher concentration of potassium ions and lower concentration of sodium ions than the concentration outside of the cell. To achieve this they constantly pump Na ions out and K ions in up their concentration gradient -ATP
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The sodium-potassium pump?
It is carried out by transporter proteins working closely with ATP supplying the energy. Sodium-potassium pump - ATP is used to change the shape of the of the transporter proteins, the shape change moves 3 sodium ions out of and 2 potassium ions in.
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Exocytosis?
Moving substances out of the cell - exocytosis - it is contained in a membrane bound sac - a vesicle - to be secreted outside the cell. The vesicle is moved along microtubules to the plasma membrane it then fuses with it to release the contents.
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Endocytosis?
Moving substances into the cell is - endocytosis - i.e a phagocyte engulfs a bacterium and fingers of cytoplasm fuse with one another to complete a ring around it where enzymes can then be secreted into the vacuole to digest it.
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What happens to a membranes permeability in high temperatures?
As the phospholipids get hotter they vibrate more moving more than previously, leaving temporary gaps for molecules to pass through. Protein molecules move too they may vibrate so much that they begin to come apart and lose shape leaving gaps.
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What happens to a membranes permeability in low temperatures?
If the temperature is very low it will decrease the membrane permeability, vibrating less and packing together tightly rarely providing pathways.
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What happens to a transporter proteins and channel proteins in low temperatures?
Protein channels remain in place, but transporter proteins may not work very well because low temperatures make it difficult to provide ATP - active transport. At low temperatures molecules and ions will be moving around less - few hitting the membra
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Function of the plasma membrane?

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The plasma (cell) membrane isolates the reactions inside the cell from the outside environment. It controls what enters the cell and what leaves. It’s a thin layer of lipid (fat) molecules

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What else about the plasma membrane?

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Card 4

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Components of the phospholipid bilayer?

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