What are membranes?
Membranes are the structures that separate the contents of cells from their environment. They also separate the different areas within cells from each other and the cytosol.
True or false? Some organelles are divided further by internal membranes
What is compartmentalisation?
The formation of separate membrane-bound areas in a cell
Why is compartmentalisation vital to a cell?
Because metabolism includes many different and often incompatible reactions. Containing the reactions in separate parts of the cell allows the specific conditions required for cellular reactions to be maintained and protects vital cell components.
True or false? All the membranes in a cell have the same basic structure
What is the name given to the cell surface membrane which separates the cell from its external environment?
Membranes are formed from a phospholipid bilayer. Explain what a phospholipid bilayer is.
The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane.
Fill in the gaps. Cells usually exist in ________ environments. The _______ of cells and organelles are also usually aqueous _______. _________ ________ are perfectly suited as membranes because the ______ surfaces of the ________ phosphate heads can interact with ______.
Aqueous, inside, environments, phospholipid bilayers, outer, hydrophilic, water.
How were membranes first seen? What did they look like?
Membranes were seen for the first time following the invention of electron microscopy, which allowed images to be taken with higher magnification and resolution. They looked like two parallel black lines.
What is the 'fluid-mosaic' model? Why was it given this name?
A model in which proteins occupy various positions in the membrane. It has this name because the phospholipids are free to move within the layer relative to each other and because the proteins vary in size, shape and position.
What is a glycoprotein?
A branching carbohydrate portion of a protein which acts as a recognition site for chemicals e.g. hormones
What is the role of a glycolipid?
A recognition site e.g. for cholera toxins
What is the function of cholesterol in the phospholipid bilayer?
What is the name of a protein molecule partly embedded?
What is the name of a protein molecule that spans the phospholipid layer?
What is the name of a protein molecule lying on the surface?
Do the hydrophilic heads of the phospholipid molecules point inwards or outwards?
Do the hydrophobic tails of the phospholipid molecules point inwards or outwards?
True or false? The components of plasma membranes play an important role in the functions of the membrane and the cell or organelle they are part of
Membrane proteins have important roles in the various functions of membranes. What is the name of the two types of proteins in the cell surface membrane?
Intrinsic and extrinsic proteins.
Describe the general structure of intrinsic proteins.
They have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane to keep them in place.
Give two examples of intrinsic proteins.
Channel proteins and carrier proteins
Describe the function of channel proteins.
Channel proteins provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes
What holds channel proteins in position?
Interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins
What roles do carrier proteins have and what feature of the carrier protein do these depend on?
Carrier proteins have an important role in both passive transport and active transport into cells. This involves the shape of the protein changing.
Fill in the gaps. Glycoproteins are _______ proteins. They are embedded in the cell-surface membrane with attached _______ chains of varying _______ and ______.
Intrinsic, carbohydrate (sugar), lengths, shapes.
What is the role of glycoproteins?
They have a role in cell adhesion (when cells join together to form tight junctions in certain tissues) and act as receptors for chemical signals.
Describe the cell communication or cell signalling process.
A chemical binds to the receptor which triggers a response from the cell. This causes a direct response inside the cell.
Give two examples of cell signalling.
- Receptors for neurotransmitters at nerve cell synapses. The binding of the neurotransmitters triggers or prevents an impulse in the next neurone.
- Receptors for peptide hormones, including insulin and glucagon, which affect the uptake and storage of glucose by cells.
Some drugs can act by binding to cell receptors. Give an example.
Beta blockers. These are used to reduce the response of the heart to stress.
What are glycolipids?
Lipids with attached carbohydrate chains.
Why are glycolipids sometimes refered to as cell markers or antigens?
Because they can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism)
Where are extrinsic proteins found in the bilayer?
On the surface of the bilayer.
Describe the general structure of extrinsic proteins.
They normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins.
True or false? Extrinsic proteins can be present in either layer and some move between the layers.
What is cholesterol? What is its role?
Cholesterol is a lipid with a hydrophilic end and a hydrophobic end, like a phospholipid. It reguates the fluidity of membranes.
What is the importance of cholesterol molecules in the phospholipid bilayer?
The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.
How does cholesterol add stability to membranes without making them too rigid?
Cholesterol molecules are positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end intracting with the tails, pulling them together.
True or false? Proteins in the membrane forming organelles or present within organelles have to be in particular positions for chemical reactions to take place.
Give an example of where the position of proteins within the membrane is important.
The electron carriers and the enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of mitochondrion) for the production of ATP in respiration.
What happens if membranes lose their structure?
They lose control of the passage of substances into and out of the cells and so cell processs will be disrupted.
How does temperature affect the permeability of the membrane?
Phospholipids in a cell membrane are constantly moving.
When temperature is increased the phospholipids will have more kinetic energy and so will move more.
This makes the membrane more fluid and it begins to lose its structure.
If temperature continues to increase the cell will eventually break down completely
This increases the permeability of the membrane.
Which two proteins are affected by high temperatures? What happens? What will this result in?
Carrier and channel proteins in the membrane willl be denatured at higher temperatures. These proteins are involved in transport across the membrane so as they denature, membrane permeability will be affected.
Water, a polar solvent, is essential in the formation of the phospholipid bilayer. Explain why.
The non-polar tails of the phospholipids are orientated away from the water, forming a bilayer with a hydrophobic core. The charged phosphate heads interact with water, helping to keep the bilayer intact.
Many organic solvents are less polar than water for example alcohols, or are non-polar like benzene. What effect do organic solvents have on membranes?
Organic solvents will dissolve membranes, disrupting cells.
Why are alcohols used in antiseptic wipes?
They dissolve the membranes of bacteria in a wound, killing them and reducing the risk of infection.
Explain why alcoholic drinks don't destroy cells in the body.
Alcoholic drinks are less concentrated solutions of alcohol than pure or very strong alcohol solutions. It is these pure or very strong alcohol solutions that are toxic and so can destroy cells in the body.
Alcoholic drinks can't destroy cells but they do cause damage. Explain how.
The non-polar alcohol molecules can enter the cell membrane and the presence of these molecules between the phospholipids disrupts the membrane.
What happens when neuronal membranes are disrupted? Why?
When neuronal membranes are disrupted, nerve impulses are no longer transmitted as normal. When the membrane is disrupted it becomes more fluid and more permeable meaning that the membrane is no longer in tact.
What is diffusion?
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
What is the meaning of the word, equilibrium in terms of diffusion?
Equilibrium means a balance or no difference in concentrations.
True or false? Diffusion is an active process and will continue until there is a concentration equilibrium between the two areas.
False. Diffusion is a passive process.
The exchange of substances between cells and their environment or between membrane-bound compartmets within the cells and the cytosol is defined as either _______ or _______.
Active or passive
All movemnet requires energy. Where do passive processes get their energy from?
Passive processes utilise energy from the natural motion of partices.
Why does diffusion happen?
Diffusion happens because the particles in a gas or liquid have kinetic energy.
How, in diffusion, does the unequal distribution of particles become equal?
The particles move randomly and so, an unequal distribution of particles will eventually become an equal distribution.
True or false? Equilibrium doesn't mean that the particles stop moving, just that the movements are equal in both directions.
Explain why diffusion is slower over longer distances.
Particles move at high speeds and are constantly colliding, which slows down their overall movement. This means that over short distances diffusion is fast, but as diffusion distance increases the rate of diffusion slows down because more collisions have taken place.
Why are cells generally microscopic?
The movement of particles within cells depends on diffusion and a large cell would lead to slow rates of diffusion. Reactions would not get the substrates they need quickly eough or ATP would be supplied too slowly to energy-requiring processes.
How does temperature affect the rate of diffusion?
The higher the temperature the higher the rate of diffusion. This is because the particles have more kinetic energy and move at faster speeds.
How does concentration difference affect the rate of diffusion?
The greater the difference in concentration between two regions the faster the rate of diffusion because the overall movement from the higher concenration to lower concentration will be larger.
Does a concentration gradient go from low to high concentration or from high to low concentration?
High to low concentration
Which way along a concentration gradient does diffusion go? Why?
Diffusion takes place down a concentration gradient because it takes a lot more energy to move substances up a concentration gradient.
What is simple diffusion?
Simple diffusion is diffusion in the absence of a barrier or membrane.
How do you calculate rate of diffusion?
True or false? Distance travelled/time is not affected by changes in surface area, whilst volume/time varies depending on the surface area.
Diffusion across membranes involves particles passing through the phospholipid bilayer. Can all particles and molecules diffuse across the membrane?
It can only happen if the membrane is permeable to the particles or molecules. Non-polar molecules can diffuse easily.
Fill in the gaps. The ________ interior of the membrane _______ substances with a positive or negative charge (ions), so they _______ easily pass through. Polar molecules, such as _____ (H2O) with partial positive and _______ charges can diffuse through ________, but only at a very _____ rate.
The hydrophobic interior of the membrane repels substances with a positive or negative charge (ions), so they cannot easily pass through. Polar molecules, such as water (H2O) with partial positive and negative charges can diffuse through membranes, but only at a very slow rate.
Do all polar molecules pass through membranes with the same degree of difficulty?
No, small polar molecules pass through more easily than larger ones.
Why are membranes described as partially permeable?
Only certain molecules are able to pass through.
How does surface area affect the rate at which molecules or ions diffuse across membranes?
The larger the area of an exchange surface, the higher the rate of diffusion.
How does the thickness of membrane affect the rate at which molecules or ions diffuse across membranes?
The thinner the exchange surface, the higher the rate of diffusion
True or false? Facilitated diffusion is still a passive process, which does not require energy from respiration.
What is facilitated diffusion?
Diffusion across a membrane through protein channels.
Membranes with protein channels are partially permeable. Do you agree? Explain your answer.
Membranes with protein channels are selectively permeable not partially permeable. This is because most protein channels are specific to one molecule or ion.
'Facilitated diffusion can also involve carrier proteins.' Do you agree?
Yes. They change shape when a specific molecule binds.
In facilitated diffusion, the movement of the molecules is ______ a concentration gradient.
What is the rate of facilitated diffusion dependent on?
- concentration gradient
- membrane surface area
- membrane surface thickness
- number of channel proteins
How does the number of channel proteins affect the rate of facilitated diffusion?
The more protein channels, the higher the rates of diffusion overall.
Name a biological process that depends on the presence of a concentration gradient.
The transmission of nerve impulses.
What is active transport?
Active transport is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration up a concentration gradient.
Active transport requires energy and carrier proteins. Why does it require energy? Where does the energy come from?
Energy is needed as the particles are being moved up a concentration gradient, in the opposite direction to diffusion.
Metabolic energy is supplied by ATP.
What do the carrier proteins in active transport act as and where are they found?
Carrier proteins span the membranes and act as 'pumps'
Describe the general process of active transport form outside to inside a cell.
- The molecule or ion binds to receptors in the channel of the carrier protein on the outside of the cell.
- On the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate
- Binding of the phosphate molecule to the carrier protein causes the protein to change shape- opening up on the inside of the cell.
- The molecule or ion is released to the inside of the cell
- The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP
- The carrier protein returns to its original shape
What is meant by active transport being described as selective?
Specific substances are transported by specific carrier proteins.
Why is bulk transport used in some situations rather than active transport?
Large molecules such as enzymes, hormones and whole cells like bacteria are too large to move through channel or carrier proteins so bulk transport is used.
Define the terms endocytosis, phagocytosis and pinocytosis.
Endocytosis- the bulk transport of material into cells
Phagocytosis- a type of endocytosis for solids
Pinocytosis- a type of endocytosis for liquids
Describe the process of endocytosis.
The cell-surface membrane first invaginates (bends inwards) when it comes in to contact with the material to be transported. The membrane enfolds the material until eventually the membrane fuses, forming a vesicle. The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing within the cell.
Describe the process of exocytosis.
Vesicles, usually formed by the Golgi apparatus, move towards and fuse with the cell surface membrane. The contents of the vesicle are then released outside of the cell.
What is the difference between facilitated diffusion and active transport?
Both use carrier proteins but facilitated diffusion occurs down a concentration gradient and therefore does not require energy.
Fill in the blanks. Energy in the form of _____ is required for movement of _______ along the cytoskeleton, changing the _______ of cells to engulf materials, and the _______ of cell membranes as ________ form or as they meet the _____________ membrane.
Energy in the form of ATP is required for movement of vesicles along the cytoskeleton, changing the shape of cells to engulf materials, and the fusion of cell membranes as vesicles form or as they meet the cell-surface membrane.
What is a solute?
A solute is a substance dissolved in a solvent forming a solution
What is concentration?
The amount of solute in a certain volume of aqueous solution.
What is water potential?
Water potential is the pressure exerted by water molecules as they collide with a membrane or container.
What is the unit of measurement for water potential?
Pressure pascals (Pa) or kilopascals (kPa)
What is the symbol for water potential?
At standard temperature and atmospheric pressure (25ºC and 100kPa) what is the water potential of pure water?
What is the highest possible value for water potential? Give a reason for your answer.
0kPa. The presence of a solute in water lowers the water potential below zero.
True or false? All solutions have negative water potentials.
What is the trend linking concentration and water potential.
The more concentrated the solution, the more negative the water potential.
Higher water potential = _____ concentrated
Lower water potential = _____ concentrated
Higher water potential = Less concentrated
Lower water potential = More concentrated
When solutions of different concentrations are separated by a partially permeable membrane, the water molecules can move between the solutions but the solutes usually cannot. Where will the net movement of water be?
The net movement of water will be from the solution with the higher water potential to the solution with the lower water potential. This will continue until the water potential is equal on both sides of the membrane.
What is hydrostatic pressure?
Hydrostatic pressure is the pressure that builds up as a result of the diffusion of water in to a solution in a closed system.
What is the unit of measurement for hydrostatic pressure?
True or false? At cellular level, hydrostatic pressure is relatively large and potentially damaging.
If an animal cell is placed in a solution with a higher water potential than that of the cytoplasm, where will water move to and by what process? What will this result in?
Water will move into the cell by osmosis which will increase the hydrostatic pressure inside the cell.
How thin is the cell-surface membrane?
What will happen to an animal cell if hydrostatic pressure inside increases?
The cell-surface membrane is unable to stretch and so will not be able to withstand the inceased pressure. It will break and the cell will burst, an event called cytolysis.
What is cytolysis?
When a cell bursts as a result of increased hydrostatic pressure.
If an animal cell is placed in a solution that has a lower water potential then the cytoplasm, what will happen to the cell?
It will lose water to the solution by osmosis down the water potential gradient. This will cause a reduction in the volume of the cell and the cell-surface membrane to 'pucker' (crenation)
What is crenation?
When a cell surface membrane shrinks.
How can multicellular animals prevent either cytolysis or crenation?
They usually have control mechnisms to make sure their cells are continuously surrounded by aqueous solutions with an equal water potential (isotonic).
What is the meaning of the word isotonic?
Equal water potential inside and outside the cell.
Are plants able to control the water potential of the fluid around them?
What is turgor?
Turgor is the increased hydrostatic pressure in plant cells pushing the membrane against the rigid cell walls.
When is a cell said to be turgid?
When the turgor pressure increases and it resists the entry of further water.
When plant cells are placed in a solution with a lower water potential than their own, water is lost from the cells by osmosis. What does this lead to?
This leads to a reduction in the volume of the cytoplasm which eventually pulls the cell-surface membrane away from the cell wall- the cell is said to be plasmolysed.
What is the meaning of the term plasmolysed?
When the cell-surface membrane pulls away from the cell wall.