2.5 Biological Membranes Flashcards
What does a cell surface membrane create
An enclosed space separating the internal cell environment from the external environment
What do intracellular membranes form
Compartments within the cell, such as organelles including the nucleus, mitochondria and RER and vacuoles
How do membranes control the exchange of materials
They are partially permeable
What different ways can substances cross membranes
Diffusion, facilitated diffusion, osmosis and active transport
How do membranes play a role in cell signalling
Acting as an interface for communication between cells
What does fluid mosaic model of membranes explain
How biological membranes are arranged to form cell membranes
How does the fluid mosaic model describe cell membranes as ‘fluid’
The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
How does the fluid mosaic model describe cell membranes as ‘mosaics’
The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
What are the 4 main components included in the fluid mosaic model
Phospholipids
Cholesterol
Glycoproteins and glycolipids
Transport proteins
What do phospholipids form
The basic structure of the membrane (the phospholipid bilayer)
Phospholipid head
Hydrophilic (water loving)- attracts water
Phospholipid tail
Hydrophobic (water hating)- repels water
How is phospholipid bilayer formed
Head faces out towards water on either side of the molecule
The centre of the membrane is hydrophobic
What does phospholipid bilayer act as
barrier to most water-soluble substances
What does phospholipid bilayer ensure
water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in
How can phospholipids be chemically modified to act as signalling molecules
Moving within the bilayer to activate other molecules (eg. enzymes)
Being hydrolysed, which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm
What does cholesterol do
increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures)
How does cholesterol increase fluidity of the membrane
Stops there phospholipid tails packing too closely together
How does cholesterol work stabilise the cell membrane at higher temperatures
Stopping the membrane from becoming to fluid
Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
The impermeability of the membrane to ions is also affected by cholesterol
Role of cholesterol (strength)
increases the mechanical strength and stability of membranes (without it membranes would break down and cells burst)
How are glycolipids and glycoproteins able to act as receptor molecules
Contain carbohydrate chains that exits on the surface
Role of glycolipids and glycoproteins
Bind with certain substances at the cell’s surface
Three main receptor types
Signalling receptors for hormones and neurotransmitters
Receptors involved in endocytosis
Receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell
Cell adhesion
The process by which cells interact and attach to neighbouring cells through specialised molecules (glycoproteins and glycolipids) on the outer layer of the cell surface membrane
What do transport proteins do
create hydrophilic channels to allow ions and polar molecules to travel through the membrane
Two types of transport proteins
Channel (pore)
Carrier proteins
Transport protein specificity
Each transport protein is specific to a particular ion or molecule
What do transport proteins allow
Transport proteins allow the cell to control which substances enter or leave
What factors affect permeability of cell membranes
Temperature
Solvent conditions
What major components in cells membranes are affected by temperature
Proteins and lipids
What happens to lipids as temperature increases
Lipids become more fluid
What does increased fluidity lead to in the cell membrane
increased fluidity reduces the effectiveness of the cell membrane as a barrier to polar molecules, meaning polar molecules can pass through
How do higher temperatures affect any diffusion taking place through the cell membrane
any diffusion taking place through the cell membrane will also occur at a higher speed (due to increased kinetic energy)
Are changes in membrane fluidity reversible and why
Yes-If temperatures decrease, the lipids will return to their normal levels of fluidity)
How can temperature cause denaturation in the cell membrane
At a certain temperature (often around 40°C) many proteins (including those in cell membranes) begin to denature
This disrupts the membrane structure, meaning it no longer forms an effective barrier
What happens when the protein in the cell membranes begin to denature
This disrupts the membrane structure, meaning it no longer forms an effective barrier
As a result, substances can pass freely through the disrupted membrane
Is proteins denaturing in the cell membranes reversible
No it is irreversible
How does solvent concentration affect permeability of cell structure
Organic solvents can increase cell membrane permeability as they dissolve the lipids in the membrane, causing the membrane to lose its structure
Why do we use beetroot to investigate how different factors affect membrane structure and permeability
Beetroot cells contain a dark purple-red pigment
The higher the permeability of the beetroot cell membrane, the more of this pigment leaks out of the cell
Why do we rinse beetroot pieces
To remove any pigment released during cutting
Investigating the effect of temperature on membrane permeability method
Using a scalpel, cut five equal-sized cubes of beetroot
Add the beetroot pieces to five different test tubes, each containing the same volume of water (e.g. 5cm3)
Put each test tube in a water bath at a different temperature (e.g. 10℃, 20℃, 30℃, 40℃, 50℃) for the same length of time
The time should be long enough to allow the pigment to diffuse into the water (e.g. around 30 minutes)
Remove the beetroot pieces, leaving just the coloured liquid in the five test tubes
Use a colorimeter to measure how much light is absorbed as it passes through each of the five samples of coloured liquid
The higher the absorbance, the more pigment must have been released, due to a greater membrane permeability
Why do beetroot pieces have to equally sized
The pieces must have the same dimensions so that they all have equal surface areas and volumes, as these factors could affect the rate at which the pigment leaks out
General pattern of effect of temperature on membrane permeability practical
as temperature increases, membrane permeability also increases
Why does higher temperature increase permeability of membrane
As temperature increases, the phospholipids within the cell membrane move more because they have more energy
Increased movement means the phospholipids are not as tightly packed together, increasing the permeability of the membrane
How does volume of water inside the cells expanding increase the permeability of the membrane
the volume of water inside the cells expands, putting pressure on the membrane, causing channel and carrier proteins to deform so they can no longer control what enters and leaves the cell
How to improve beetroot practical
conduct several repeats, using different parts of the beetroot and find a mean
Limitations of beetroot practical
Some parts of beetroot tissue have more pigment in their cells than others
The beetroot pieces may not be identical in size and shape
Diffusion
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration (down a concentration gradient)
What is the random movement of molecules or ions in diffusion caused by
the natural kinetic energy of the molecules or ions
What happens as a result of diffusion
molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space
Factors affecting affect diffusion
Steepness of concentration gradient, temperature, surface area and properties of molecules or ions
How does steepness of concentration gradient affect diffusion
This is the difference in the concentration of the substance on the two sides of the surface
A greater difference in concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion
How does temperature affect rate of diffusion
Molecules and ions have more kinetic energy at higher temperatures
They move faster, resulting in a higher rate of diffusion
How does surface area affect rate of diffusion
The greater the surface area, the greater the number of molecules or ions that can cross at one moment
How do properties of molecules or ions affect rate of diffusion
Large molecules diffuse more slowly than smaller ones as they require more energy to move
Uncharged and non-polar molecules diffuse directly across the phospholipid bilayer
Non-polar molecules diffuse more quickly than polar ones as they are soluble in the non-polar phospholipids bilayer
Facilitated diffusion
The passive movement of molecules down a concentration gradient (high to low) across a membrane, and it involves special carrier and channel proteins
Why is facilitated diffusion important
Certain substances cannot diffuse through the phospholipid bilayer of cell membranes. These include:
Large polar molecules such as glucose and amino acids
Ions such as sodium ions (Na+) and chloride ions (Cl-)
Channel proteins
Water-filled pores which allow charged substances (ions) to diffuse through the cell membrane
How do channel proteins control the exchange of ions
The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
How do carrier proteins differ to channel proteins
Carrier proteins can switch between two shapes
What does carrier proteins being able to switch between two shapes mean
the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
What is the net diffusion (movement) of molecules or ions into or out of a cell
down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
How can rate of diffusion be investigate
timing the diffusion of ions through different sized cubes of agar
Agar cube rate of diffusion investigation practical method
The cubes are then placed into boiling tubes containing a diffusion solution (such as dilute hydrochloric acid)
Agar cube rate of diffusion investigation measurements
The time taken for the acid to completely change the colour of the indicator in the agar blocks
The distance travelled into the block by the acid (shown by the change in colour of the indicator) in a given time period (eg. 5 minutes)
Active transport
the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
What does active transport require
carrier proteins (each carrier protein being specific for a particular type of molecule or ion)
How does active transport use carrier proteins differently to facilitated diffusion
The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane
How is the energy required for active transport provided
The energy required is provided by ATP (adenosine triphosphate) produced during respiration. The ATP is hydrolysed to release energy
Why is active transport important
The reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
The absorption of some products of digestion from the digestive tract
The loading of sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant
The loading of inorganic ions from the soil into root hairs
Examples of bulk transport of larger quantities of materials
Large molecules such as proteins or polysaccharides
Parts of cells
Whole cells eg. bacteria
Endocytosis
Bulk transport into cells
Exocytosis
Bulk transport out of cells
What type of process in endocytosis and exocytosis
These two processes require energy and are therefore forms of active transport
What is the process of endocytosis
the cell surface membrane engulfs material, forming a small sac (or ‘endocytic vacuole’) around it
Two forms of endocytosis
Phagocytosis and pinocytosis
Phagocytosis in terms of endocytosis
This is the bulk intake of solid material by a cell
Cells that specialise in this process are called phagocytes
The vacuoles formed are called phagocytic vacuoles
Example of phagocytosis
the engulfing of bacteria by phagocytic white blood cells
Pinocytosis
This is the bulk intake of liquids
If the vacuole (or vesicle) that is formed is extremely small then the process is called micropinocytosis
Process of exocytosis
materials are removed from, or transported out of, cells (the reverse of endocytosis)
How does exocytosis happen
The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body
These vesicles then travel to the cell surface membrane
Here they fuse with the cell membrane and release their contents outside of the cell
Example of exocytosis
the secretion of digestive enzymes from pancreatic cells
Osmosis
the diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane down a concentration gradient
Partially permeable
allows small molecules (like water) through but not larger molecules (like solute molecules)
Water potential
the tendency of water to move out of a solution
Water potential of a dilute solution
High
Water potential of a concentrated solution
Low
What potential of pure water
The water potential of pure water (without any solutes) at atmospheric pressure is 0kPa, therefore any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)
Osmosis in animal cells
Animal cells can lose and gain water as a result of osmosis
As animal cells do not have a supporting cell wall (unlike plant cells), the results of this loss or gain of water on the cell are severe
Animal cells losing water
If an animal cell is placed in a solution with a lower water potential than the cell (such as a concentrated sucrose solution)
Water will leave the cell through its partially permeable cell surface membrane by osmosis and the cell will shrink and shrivel up
Cremation
The loss of water in an animal cell which is usually fatal for the cell
Animal cells gaining water
If an animal cell is placed in pure water or a dilute solution, water will enter the cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential
The cell will continue to gain water by osmosis until the cell membrane is stretched too far and the cell bursts (cytolysis), as it has no cell wall to withstand the increased pressure created
Lysis
The disintegration of a cell by rupture of the ell wall or membrane
occurs when the cell is in a hypotonic environment
Hypotonic environment
the solution outside of the cell has a lower solute concentration than the inside of the cell
Isotopic environment
the solution outside of the cell has the same solute concentration as the inside of the cell
Movement of water in animal cells in isotonic environment
The movement of water molecules into and out of the cell occurs at the same rate (no net movement of water) and there is no change to the cells
Osmosis in plant cells
Like animal cells, plants cells can also lose and gain water as a result of osmosis
As plant cells have a supporting cell wall, the results of this loss or gain of water on the cell are less severe than in animal cells
Plant cells losing water
If a plant cell is placed in a solution with a lower water potential than the plant cell (such as a concentrated sucrose solution), water will leave the plant cell through its partially permeable cell surface membrane by osmosis
Process of plant cells losing water
As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
The protoplast gradually shrinks and no longer exerts pressure on the cell wall
As the protoplast continues to shrink, it begins to pull away from the cell wall
Protoplasm
All o the contents of a bacterial or plant cell except for the cell. The living parts of the cell
Plasmolysis
The process of contraction or shrinkage of the protoplasm of a plant cell and is caused due to the loss of water in the cell
Plant cells gaining water
If a plant cell is placed in pure water or a dilute solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis, as the pure water or dilute solution has a higher water potential than the plant cell
Process of plant cells gaining water
As water enters the vacuole of the plant cell, the volume of the plant cell increases
The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell – the inelastic cell wall prevents the cell from bursting
The pressure created by the cell wall also stops too much water from entering and this also helps to prevent the cell from bursting
When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid
Why is turgidity important for plants
as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight
What happens if plants do not receive enough water
the cells cannot remain rigid and firm (turgid) and the plant wilts
Investigating water potential using potato cylinders method
The required number of potato cylinders are cut (one for each of the solutions you are testing – or more than one per solution if you require repeats)
They are all cut to the same length and, once blotted dry to remove any excess moisture, their initial mass is measured and recorded before placing into the solutions
They are left in the solutions for a set amount of time (eg. 30 minutes), usually in a water bath (set at around 30o)
They are then removed and dried to remove excess liquid
The final length and mass of each potato cylinder is then measured and recorded
What does a positive percentage change in potato mass indicate
the potato has gained water by osmosis (net movement of water from the solution into the potato) meaning the solution had a higher water potential than the potato
The gain of water makes the potato cells turgid
What does a negative percentage change in mass of potato indicate
the solution had a lower water potential than the potato
The potato cylinder in the strongest sucrose concentration will have decreased in mass the most as there is the greatest concentration gradient in this tube between the potato cells (higher water potential) and the sucrose solution (lower water potential)
What happens when potato loses mass
More water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the potato cylinder – the potato cylinders will feel floppy
What happens in mass of potato has neither decreased or increased
it means there was no overall net movement of water into or out of the potato cells
The solution that this particular potato cylinder was in had the same water potential as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient and therefore no net movement of water into or out of the potato cells
