2.5* Flashcards
What is a fluid mosaic model?
Theory of cell membrane structure with proteins embedded in a sea of phospholipids.
What is a glycolipid?
Lipid/phospholipid with a chain of carbohydrate molecules attached.
What is a plasma membrane?
A cell surface membrane.
What is a glycoprotien?
protein with a chain of carbohydrate molecules attached.
Cell membranes form a barrier to separate the cells contents from the cells or organelles exterior environment. How do molecules pass into, out of or through the membrane?
Some very small molecules diffuse straight through the cells membrane, in-between its structural molecules.
Some substances dissolve in the lipid layer and pass through.
Other substances pass through special protein channels or are carried by other proteins.
How is the permeability of a membrane described and why?
Membranes are described as partially permeable, this is because they do not let all type of molecule pass though them.
The properties of the component molecules of the cell membrane determines its permeability, i.e. which molecules it allows pass through.
What is the purpose of a plasma membrane?
Separates the cells components from its external environment.
Regulates transport of materials in and out of the cell.
May contain enzymes involved in specific metabolic pathways.
Has antigens so that the organism’s immune system can recognise the cell as being ‘self’ and not attack it.
May release chemicals to signal other cells.
Contains chemical receptors for such chemical signal, and so is a site for cell communication or signalling
What a the inner membranes of mitochondria called and what is their purpose?
Mitochondria have inner folded membranes called cristae. These give a large surface area for some of the reactions of aerobic respiration and localise some of the enzymes needed for respiration to occur.
What a the inner membranes of chloroplasts called and what is their purpose?
The inner membranes of chloroplasts, called thylakoid membranes, house chlorophyll. On these membranes some of the reactions of photosynthesis occur.
Where and why is there some digestive enzymes on the plasma membranes.
There are some digestive enzymes on the plasma membrane of epithelial cells that line the small intestine, and these enzymes catalyse some of the final stages in the breakdown of certain types of sugars.
Who proposed a model that allowed the passage of molecules through the membrane, and when?
In 1972 Singer and Nicolson proposed a model.
What did the fluid mosaic model explain?
Their structure explained how cell membranes could be more dynamic and interact more with the cells environment. It was called the fluid mosaic model.
What did the fluid mosaic model propose?
That the fabric of the membrane consisted of a phospholipid bi-layer (double layer) with proteins floating in it, making up a mosaic patten. The lipid molecules can change places with each other, and some proteins may move giving fluidity.
In a membrane explain the two layers of phospholipid molecules.
The fabric of the membrane is the lipid bilayer made up of two layers of phospholipid molecules. Their hydrophilic heads are in contact with the watery exterior or watery interior (cytoplasm). The hydrophobic tail regions are in the centre of the membrane, away from the water.
Name all the proteins in the fluid mosaic model.
Channel protein, peripheral protein, carrier protein (intrinsic) and extrinsic proteins.
Give some extra detail on channel proteins.
Water-filled channel inside the channel protein is lined with hydrophobic heads and hydrophilic tails.
How far do channel and carrier proteins span, explain.
Channel and carrier proteins span the whole lipid bilayer - they are integral proteins and their membrane-spanning regions interact with the hydrophobic region of the phospholipid bi-layer.
Describe a phospholipid.
Their spherical phosphate head has a charge and is hydrophilic.
Their fatty acid tails (2) are hydrophobic.
What is the purpose of the carbohydrate chain?
Carbohydrate molecules (glycocalyx) on the outside of the membrane are very hydrophilic and attract water with dissolved solutes, helping the cell interact with its watery environment and obtain dissolved substances.
What is a carbohydrate chain called and what is its structure.
Glycoprotiens are carbohydrate chains attached to a protein molecule.
Glycolipids are carbohydrate chains attached to a lipid.
What may an extrinsic protein act as?
Protein not spanning the lipid bilayer may act as an enzyme.
How thick is a phospholipid bilayer.
about 7 nm in width.
What maintains mechanical stability and flexibility?
Eukaryotic cell membranes contain cholesterol, which is important for helping to regulate the fluidity of the membrane, maintain mechanical stability and resist the effects of the temperature changes on the structure of the membrane.
What is the top of a channel protein called?
A protein receptor site.
Spanning the membrane are various proteins, what are the different purpose’s they serve?
Some of them have pores and act as channels to allow ions which have an electrical charge and are surrounded by water molecules, to pass through.
Some proteins are carriers and, by changing their shape, carry specific molecules across the membrane.
Other proteins may be attached to the carrier protein and function as enzymes, antigens or receptor sites for complimentary-shaped signalling chemicals such as hormones.
What is the total thickness of the cell membrane?
Between 5 and 10 nm
What is outside the cell membrane?
Outside the membrane is the glycocalyx - formed from the carbohydrate chains attached to either lipids (glycolipids) or proteins (glycoprotiens) in the membranes.
Do all membranes have the same composition?
No their membranes may have particular distributions of proteins in order to enable them to carry out their specific functions.
Describe the plasma membrane on a neurone.
In neurones (nerve cells) the protein channels and carriers in the plasma membrane covering the long wxon allow entry and exit of ions to bring about the conduction of electrical impulses along their length.
Describe the neurone sheath.
Neurones have a myelin sheath formed by flattened cells wrapped around them several times, giving several layers of cell membrane. The membrane forming the myelin sheath is about 20% protein and 76% lipid.
Describe the plasma membrane on a white blood cell.
The plasma membranes of white blood cells contain special protein receptors that enable them to recognise the antigens on foreign cells, usually from invading pathogens but also from tissue or organ transplants.
Describe the plasma membrane on a root hair cell.
Root hair cells in plants have many carrier proteins to actively transport nitrate iron
Describe the inner membranes of mitochondria.
The inner membranes of mitochondria are 76% protein and 24% lipid. This is because their inner membranes contain many electron carriers that are made of protein, and hydrogen ion channels associated with ATP synthase enzymes.
Why does eating glucose produce a quicker release of insulin than injecting it straight into the blood stream?
As well as cells in your pancreas having receptors to detect sugars, cells lining your intestines also have taste receptors for sugar.
Describe the process of discovering taste cells in the intestines.
For 50 years, scientists were mystified as to why eating glucose produces a quicker release of insulin than if glucose was injected straight into the blood stream, In 2007, they discovered that cells lining the intestines contain taste receptors for sugars, and when stimulated these cause a cascade of hormones that ultimately ends in release of extra insulin from the pancreas.
Are taste cells in the pancreas just a receptor for glucose?
In fact, these receptors also respond to artificial sweeteners, just as the receptors on your tongue so, so these sweeteners also cause a insulin surge.
Describe the permeability of cell membranes.
Cell membranes are partially permeable. This means that they allow some, but not all, substances to pass through. Substances may pass from inside the cell to the outside, or from the cell into its interior.
What is diffusion?
Movement of molecules from an area of high concentration or that molecule to an area of low concentration; it may or may not be across a membrane; it does not involve metabolic energy (ATP).
What is facilitated diffusion?
Movement of molecules from an area of high concentration of that molecule to an area of low concentration, across a partially permeable membrane via protein channels or carriers; it does not involve metabolic energy (ATP).
What do cells need to remove from their interior?
They need to remove the toxic metabolic waste products, such as carbon dioxide.
What raw materials do cells need to receive?
Biochemical processes that sustain life go on in cells. Cells therefore need to receive raw material or reactants for these reactions. They respire to make ATP, which provides cellular energy, to drive these biochemical reactions. Therefore they need oxygen and glucose.
What molecules do cells need to export?
They need to export some of the molecules that they make, such as enzymes, hormones or other signalling molecules.
Describe passive processes.
Some substances can pass across cell membranes without using any of the cells metabolic energy. These are described as passive processes because they use only the kinetic energy of the molecules and do not use ATP.
What happens when you have been active a long time concerning adenosine.
When you have been active a long time, a lot of ATP molecules are eventually broken down to adenosine. The build-up in concentration of adenosine eventually acts as a signal to part of your brain triggers you to feel sleepy.
What causes diffusion to work?
All molecules have kinetic energy and can move freely and randomly within a gas or liquid media. If there is a high concentration of a certain type of molecule in an area, then the molecules will bump into each other as they randomly move, and eventually they will spread further from each other. More will move to an area where they are in lower concentration, until eventually they become evenly dispersed.
All molecules have kinetic energy and can move freely and randomly within a gas or liquid media. Does the water need to be moving or disturbed for this to happen?
This will happen even if the medium is not mixed by stirring or shaking.
In diffusion what happens when molecules have moved down their concentration gradient?
When molecules have moved down their concentration gradient, they are still moving randomly but remain evenly dispersed so there is no net diffusion. They have reached equilibrium.
When there is a solute for example glucose in a higher concentration outside a cell than inside the cell what would the concentration gradient look like on a graph with concentration on the Y axis and outside to inside the cell on the X axis.
There is a high concentration of the molecules on one side of the membrane and a low concentration on the other side. The difference between them forms a gradient (slope). The steeper the slope, the faster the molecules will diffuse down that gradient.
What molecules can pass through the membrane by simple diffusion?
Some molecules such as oxygen and carbon dioxide, which are small, pass through cell membranes by simple diffusion.
How do fat-soluble molecules pass through the cell membrane?
Fat-soluble molecules such as steroid hormones, even if they are larger, can diffuse through the cell membranes as they dissolve in the lipid bilayer. They still move down their concentration gradient.
How does water pass through the lipid bi-layer?
Water molecules are a special case. Since they are polar and insoluble in lipid the phospholipid bi-layer would seem an impenetrable barrier. However, water is present in such great concentrations that significant direct diffusion does happen.
In membranes where a very high rate of water movement what might there be?
In membranes where a very high rate of water movement there may indeed be specific water channel proteins known aquaporins to allow water molecules to cross the membrane without the challenge of moving through a lipid environment.
How is the concentration gradient maintained within cells?
Many molecules entering cells then pass into organelles and are used for metabolic reactions; this maintains the concentration gradient and keeps more of the molecules entering the cell.
How is the concentration gradient of oxygen diffusing into the cytoplasm maintained?
Oxygen diffusing into the cytoplasm of respiring cells the diffuses into mitochondria and are used for aerobic respiration.
How is the concentration gradient of carbon dioxide diffusing into the palisade mesophyll maintained?
Carbon dioxide diffusing into the palisade mesophyll cells of a plant leaf will then diffuse into chloroplasts and be used for photosynthesis.
Factors that affect the rate of simple diffusion;
What does diffusion rely on?
Simple diffusion relies on the molecules own kinetic energy, and so factors that alter this kinetic energy will affect the rate of diffusion.
Why is temperature a factor that affects simple diffusion?
As temperature increases, molecules have more kinetic energy, so their rate of diffusion will increase. Conversely, as they lose heat their rate of diffusion will slow down.
Why is diffusion distance a factors that affects the rate of simple diffusion?
The thicker the membrane across which molecules have to diffuse, the slower the rate of diffusion.
Why is surface are a factors that affects the rate of simple diffusion?
More diffusion can take place across a larger surface area. Cells specialised for absorption have extensions to their cell surface membranes, called microvilli. These increase the surface area.
Why is the size the of the diffusing molecule a factors that affects the rate of simple diffusion?
Smaller ions or molecules diffuse more rapidly than larger molecules.
Why is concentration gradient a factors that affects the rate of simple diffusion?
The steeper this gradient (the more molecules there are on one side of the membrane compared with the other side), the faster the diffusion to the side where there are fewer molecules, down the gradient.
How do you investigate factors that affect the rate of diffusion using universal indicator.
Using model cells to which you have added universal indicator solution.
What type of molecules pass through the cell membrane using facilitated diffusion? and why?
Small molecules that have polarity (opposite charges at either end of the molecule), such as ions that have electrical charge, are insoluble in lipid because they cannot interact with the hydrophobic tails of the lipid bi-layer This means that they diffuse through water filled protein channels (pores) embedded in the membrane.
How big are the water filled channel proteins (pores)?
0.8 nm in diameter.
Why facilitated diffusion is needed;
What reduces the permeability of the membranes to small water soluble molecules?
Cholesterol molecules within the membrane.
How does glucose diffuse across the membrane?
Glucose molecules are too large to diffuse through the water filled protein channel in the membrane, but they can bind to a transmembrane carrier protein, which then opens to allow the glucose to pass out on the other side of the membrane. There are specific carrier proteins for different types of molecules.
How do cells us transmembrane proteins control the types of molecule that pass in or out?
Different cell types have membranes with differing proportions of transmembrane protein channels and transmembrane protien carriers. This allows cells to control he types of molecule that pass in or out.
What membranes have many channel proteins specific to either sodium or potassium ions an why?
Neurone plasma membranes have many channels specific to either sodium or potassium ions. The diffusion of these ions into and out of the neurone axon is crucial for the conduction of nerve impulses.
At synapses in the neurone what other ion channels may there exist in the plasma membrane?
At synapses (gaps between the neurones), there are also calcium ion channels and there may be chloride ion channels.
What ions channels would the plasma membrane on epithelial cells have and what purpose would this serve?
The plasma membrane of epithelial cells that line your airways have chloride ion channels, and these play a crucial role in regulating the composition of mucus to trap particles and pathogens.
What is osmosis?
The passage of water molecules down their water potential gradient, across a partially permeable membrane.
