Transport Across Cell Membranes Flashcards
(33 cards)
Two types of membranes?
All cells are surrounded by a membrane.
In eukaryotic cells, many of the organelles are surrounded by a membranes too.
- Cell-surface membrane surrounds cells. They are the barrier between the cell and its environment, controlling which substances enter and leave the cell. They are partially permeable - they let some molecules through but not others. Substances can move across the cell surface membrane by diffusion, osmosis or active transport.
- Remembrance around organelles divide the cell into different compartment. They act as a barrier between the organelle and the cytoplasm. They are also partially permeable and control what substances enter and leave the cell.
What Is A Plasma Membrane?
All membranes around and within cells have the same basic structure and are known as plasma membranes.
Also known as ‘cell-surface membrane’.
It surrounds cells and forms the boundary between the cell cytoplasm and the environment.
It allows different conditions to be established inside and outside a self.
It controls the movement of substances in and out of the cell.
They are permeable to small, non-polar molecules, like oxygen.
The fluid-mosaic model models the cell-surface/plasma membrane.
Fluid-Mosaic Model?
The fluid-mosaic model represents the cell-surface membrane.
They’re composed of lipids (mainly phospholipids), proteins and carbohydrates.
In the model, phospholipid molecules form a continuous, double later. They’re about 7 nm thick.
Cholesterol molecules are present within the bilayer.
Proteins are scattered throughout the bilayer. This includes channel proteins and carrier proteins, which allow large molecules and ions pass through the membrane.
Receptor proteins on the cell surface membrane allows the cell to detect chemicals released from other cells. The cell can then respond to this (e.g. insulin binds to liver cell - cell absorbs glucose).
Some proteins are able to move sideways through the bilayer, while others are fixed in position. Some proteins have a polysaccharide (carbohydrate) chain attached- these are called glycoproteins.
Some lipids also have a polysaccharide chain attached these are called glycolipids.
Fluid: Because the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is continuously changing in shape.
Mosaic: Because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic.
Phospholipid Properties?
Similar to lipids, except one of the fatty acid molecules is replaced by a phosphate molecule.
Fatty acids repel water (are hydrophobic) and phosphates attract water (are hydrophilic).
A phospholipid is made of two parts:
- hydrophilic ‘head’ which interacts with water but not with fat,
- hydrophobic ‘tail’ which orients itself away from water but mixes readily with fat,
Molecules that have two ends (poles) that behave differently in this way age said to be polar,
This means when these polar phospholipid molecules are placed in water, they position themselves so that the hydrophobic are as far away from water and possible and the hydrophilic are as close to water as possible.
The phospholipids are part of the bilateral of a plasma membrane.
The heads face outwards, whilst the tails face inwards. There’s two rows of them, the tails are touching. This causes the centre of the bilayer to be hydrophobic so that the membrane doesn’t allow water soluble substances (like ions) through it. It acts as a barrier to these dissolved substances.
Phospholipid Functions?
They’re polar, so I’m aqueous environments, they form a bilayer with hydrophobic layer inside of the membrane and hydrophilic layer outside.
Hydrophilic phosphate heads help to hold the surface of the cell-surface membrane together,
Phospholipids structure allows formation with glycolipids by combining with carbohydrates within the cell-surface membrane. These glycolipids are important in cell recognition.
They allow for lipid-soluble substances to enter and leave the cell.
They prevent water-soluble substances entering and leaving the cell.
They also make the membrane flexible and self-sealing.
Cholesterol In Plasma Membranes?
A type of lipid.
Present in all cell membranes, except bacteria.
Cholesterol molecules occur within the phospholipid bilayer of the cell surface membrane, between the phospholipids. They bind to the hydrophilic tails, causing them to pack closer together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid.
They add strength to the membrane.
Cholesterol helps to maintain the shape of animal cells (which don’t have cell walls). This is particularly important for cells that aren’t supported by other cells, for example red blood cells.
They are very hydrophobic and so prevent loss of water and loss of dissolved ions from the cells.
In summary:
- They reduce lateral movement of other molecules including phospholipids.
- They make the membrane less fluid at high temperatures.
- They prevent leakage of water and dissolved ions from the cell.
What Is The Purpose Of Proteins In A Plasma Membrane?
- Providing structural support,
- Act as channels for water-soluble substances for transport across membrane,
- Allow active transport across the membrane through carrier proteins,
- Form cell-surface receptors for identifying cells,
- Help cells adhere together,
- Act as receptors, for e.g. hormones.
What Structures Form The Plasma Membrane?
- Phospholipids (to form the phospholipid bilayer),
- Proteins (protein channels and carrier proteins),
- Glycolipids,
- Glycoproteins,
- Cholesterol.
Glycolipids In Plasma Membrane?
Glycolipids are made up of a carbohydrate covalently bonded with a lipid.
The carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell. Here, the carbohydrate acts as a cell surface receptors for specific chemicals (for example the human ABO blood system operates as a result of the glycolipids on the cell surface membrane).
Functions:
- Acts as a recognition site.
- It helps maintain the stability of the membrane.
- Glycolipids also help cells to attach to one another and so form tissues.
Glycoproteins In A Plasma Membrane?
These glycoproteins also act as cell-surface receptors, more specifically for hormones and neurotransmitters.
Functions of glycoproteins in membrane:
- Act as recognition sites,
- Help cells to attach to one another and so form tissues,
- Allows cells to recognise one another, for example lymphocytes can recognise an organisms own cells.
Simple Diffusion?
Diffusion is the net movement of molecules or ions from a region of high concentration to a low concentration until evenly distributed.
Molecules will diffuse both ways, but the net movement will be to the area of a lower concentration.
The concentration gradient is the path from an area of high concentration to an area of lower concentration. Particles diffuse down a concentration gradient.
Diffusion is a passive process - no energy needed.
In this process:
- All particles are constantly in motion due to the kinetic energy that they possess.
- Motion is random.
- Particles are constantly bouncing off one another as well as other objects, for example, the sides of the vessel in which they are contained.
Facilitated Diffusion?
Some larger molecules (e.g. amino acid, glucose) would diffuse extremely slowly through the phospholipid bilayer because they’re so big.
Charged ions and polar molecules cannot move through the plasma membrane also. This is because they’re water soluble, and the centre of the bilayer is hydrophobic.
The movement of these molecules is made easier (facilitated) by transmembrane channels and carriers that spanned the membrane.
Also a passive process.
It relies only on the inbuilt motion (kinetic energy) of diffusing molecules.
There is no ATP involved.
Protein channels and carrier proteins allow for facilitated diffusion.
Facilitated diffusion involves carrier proteins and protein channels.
What Molecules Diffuse Through Plasma Membrane And What Molecules Don’t?
Molecules must be one of the following to diffuse across the plasma membrane.
Must be:
- Soluble in lipids so they can pass through phospholipid bilayer.
- Small enough to pass through channels.
- Of a different charge as the charge on protein channels and so, even if they are small enough to pass through, they are not repelled.
- Not electrically charged (non-polar) because if they are electrically charger (polar), they will have difficulty passing through non-polar hydrophobic tails in the phospholipid bilayer.
Carrier Proteins?
Facilitated diffusion type.
Found in plasma membrane (phospholipid bilayer).
When a molecule (such as glucose) that is specific to the protein is present, it binds with the protein.
This causes it to change shape in such a way that the molecule is released to the inside of the membrane.
No external energy is needed for this.
The molecules move from a high concentrated to a low concentration, using only the kinetic energy of the molecules themselves.
Protein channels and carrier proteins have binding sites, not active sites.
Protein Channels?
Part of the cell-surface (plasma) membrane.
They form pores in the membrane for charged particles to diffuse through (down conc gradient).
These proteins form water-filled hydrophilic channels across the membrane.
They allow specific water soluble ions to pass through.
The channels are selective, each opening in the presence of a specific ion.
If the particular ion is not present, the channel remains closed.
The ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side.
Diffusion Is Proportional To?
Diffusion is proportional to the concentration gradient.
E.g. if the concentration gradient increases, diffusion also increases.
Functions Of Membranes Within Cells?
- Controls the entry and exit of materials in organelles such as mitochondria and chloroplasts,
- Separates organelles from cytoplasm so that metabolic reactions can take place within the membrane,
- Provides an internal transport system, e.g. endoplasmic reticulum,
- Isolate enzymes that might damage the cell, e.g. lysosomes,
- Provides surface for which reactions can occur, e.g. protein synthesis using ribosomes on the rough endoplasmic reticulum.
Practical: investigating the permeability of cell membranes
Cell permeability is affected by different conditions, for example temperature and solvent concentration.
Beetroot cells contain a coloured pigment that leaks out. The higher the permeability of the membrane, the more pigment leaks out of the cell.
- Use a scalpel to carefully cut five equal sized pieces of beetroot. Rinse the pieces to remove any pigment released during cutting.
- Add the five pieces to 5 different test tubes. Each containing 5 cm³ of water. Use a measuring cylinder or pipette to measure the water.
- Place each test tube in a water bath at different temperatures, e.g. 10°, 20° for the same length of time. Use a stopwatch to measure time.
- Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.
- Use a colorimeter - a machine that passes light through the liquid and measures how much of the light is absorbed. Let the colorimeter stand for 5 minutes to stabilise. Take a measurement through pure water to calibrate it to zero before taking the measurements.
The higher the absorbance, the more pigment released, so the higher the permeability of the membrane.
- You can connect the colorimeter to a computer and your software to collect the data and draw a graph of the results.
How does temperature effect membrane permeability?
On a graph, this looks like a ‘v’ shape, but with the bottom of the V on 0 degrees.
- Temperatures below 0° - The phospholipids don’t have much energy, so they can’t move very much. They are packed closely together and the membrane is a rigid.
Channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane. Ice crystals may form and pierce the membrane, making it highly permeable when it thaws. Permeability is highest here.
- Temperature between zero and 45° - The phospholipids can move around and are not packed as tightly together. The membrane is partially permeable. As the temperature increases, the phospholipids move more because they have more energy. This increases the permeability of the membrane.
- Temperatures above 45° - The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable. Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins deform so they cannot control what enters and leaves the cell. This increases the permeability of the membrane.
How to investigate the effects of solvent on permeability of a cell membrane?
Surround cells in an increasing concentration of a solvent (such as alcohol or acetone).
Increasing the concentration of a solvent will increase the membrane permeability because the solvent dissolves the lipids in the cell membrane, causing it to lose its structure.
Rate of simple diffusion depends on?
- The concentration gradient. The higher the concentration gradient, the faster the rate of diffusion. This means that diffusion slows down over time because the two sides of the membrane decrease in concentration until it reaches an equilibrium.
- The thickness of the exchange surface. The thinner the exchange service, the faster the rate of diffusion.
- This is the third year. The larger the surface area, the faster the rate of diffusion.
Rate of facilitated diffusion depends on?
- The concentration gradient. The higher the concentration gradient, the faster the rate of diffusion (to a point). As equilibrium is reached, the rate of facilitated diffusion will level off.
- The number of channel and carrier proteins. Once all the proteins in a membrane are in use, facilitated diffusion cannot happen any faster. The greater the number of channel or carrier proteins in the cell membrane, the faster the rate of facilitated diffusion.
Osmosis?
Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential.
Water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution.
Pure water has the highest water potential. All solutions have a lower rate potential than pure water.
If two solutions have the same water potential, they’re said to be isotonic.
Rate of diffusion depends on?
- The water potential gradient - the higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time.
- The thickness of the exchange surface - the thinner the exchange surface, the faster the rate of osmosis.
- The surface area of the exchange surface - the larger the surface area, the faster the rate of osmosis.
