Transport Across Cell Membranes

Question 1

Two types of membranes?

Answer 1

All cells are surrounded by a membrane.

In eukaryotic cells, many of the organelles are surrounded by a membranes too.

1. 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.
2. 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.

Question 2

What Is A Plasma Membrane?

Answer 2

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.

Question 3

Fluid-Mosaic Model?

Answer 3

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.

Question 4

Phospholipid Properties?

Answer 4

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.

Question 6

Phospholipid Functions?

Answer 6

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.

Question 7

Cholesterol In Plasma Membranes?

Answer 7

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.

Question 8

What Is The Purpose Of Proteins In A Plasma Membrane?

Answer 8

• 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.

Question 9

What Structures Form The Plasma Membrane?

Answer 9

• Phospholipids (to form the phospholipid bilayer),
• Proteins (protein channels and carrier proteins),
• Glycolipids,
• Glycoproteins,
• Cholesterol.

Question 10

Glycolipids In Plasma Membrane?

Answer 10

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.

Question 11

Glycoproteins In A Plasma Membrane?

Answer 11

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.

Question 12

Simple Diffusion?

Answer 12

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.

Question 13

Facilitated Diffusion?

Answer 13

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.

Question 14

What Molecules Diffuse Through Plasma Membrane And What Molecules Don’t?

Answer 14

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.

Question 15

Carrier Proteins?

Answer 15

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.

Question 15

Protein Channels?

Answer 15

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.

Question 16

Diffusion Is Proportional To?

Answer 16

Diffusion is proportional to the concentration gradient.

E.g. if the concentration gradient increases, diffusion also increases.

Question 17

Functions Of Membranes Within Cells?

Answer 17

• 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.

Question 18

Practical: investigating the permeability of cell membranes

Answer 18

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.

1. Use a scalpel to carefully cut five equal sized pieces of beetroot. Rinse the pieces to remove any pigment released during cutting.
2. 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.
3. 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.
4. Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.
5. 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.

6. You can connect the colorimeter to a computer and your software to collect the data and draw a graph of the results.

Question 19

How does temperature effect membrane permeability?

Answer 19

On a graph, this looks like a ‘v’ shape, but with the bottom of the V on 0 degrees.

1. 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.

2. 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.
3. 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.

Question 20

How to investigate the effects of solvent on permeability of a cell membrane?

Answer 20

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.

Question 21

Rate of simple diffusion depends on?

Answer 21

1. 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.
2. The thickness of the exchange surface. The thinner the exchange service, the faster the rate of diffusion.
3. This is the third year. The larger the surface area, the faster the rate of diffusion.

Question 22

Rate of facilitated diffusion depends on?

Answer 22

1. 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.
2. 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.

Question 23

Osmosis?

Answer 23

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.

Question 24

Rate of diffusion depends on?

Answer 24

1. 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.
2. The thickness of the exchange surface - the thinner the exchange surface, the faster the rate of osmosis.
3. The surface area of the exchange surface - the larger the surface area, the faster the rate of osmosis.

Question 25

Practical: investigating water potential?

Answer 25

Use potatoes to find the water potential of plant tissue.

You need to make up several solution of different, known concentrations to test the cylinders in. You can do this using a serial dilution technique:

1. Start with a known concentration of sucrose solution (e.g. 2M). Set up five test tubes in a rack.
2. Add 10cm3 of the initial 2M solution to the first test tube and 5cm3 of distilled water to the other four test tubes.
3. Then use a pipette to draw 5cm3 of the solution from the first test tube, add it to the distilled water in the second test tube and mix the solution thoroughly. You now have a 10cm3 solution that’s half as concentrated as the first one (1M).
4. Repeat the process three more times to create solutions of 0.5M, 0.25M and 0.125M.

Question 26

Practical: making solutions of different concentration by finding the scale factor.

Answer 26

You can make sucrose solutions of any concentration by finding the scale factor. For example, if you want to make 15cm3 of 0.4M sucrose solution.

1. Start with a solution of a known concentration, e.g. 1M.
2. Find the scale factor by dividing the solution by the concentration of the solution would want to make. In this case, the scale factors is 1 / 0.4 = 2.5.
3. This means that the solution you want to make it 2.5 times weaker than the one you have. To make the solution 2.5 times weaker, use 2.5 times less of it, e.g. 15 / 2.5 = 6. Transfer this amount to a clean test tube.
4. Top up the test tube with distilled water to get the volume you want to make. In this case you want to make 15cm3 of solution, so you need to add: 15 - 6 = 9cm3 of distilled water.

Question 27

Practical: finding water potential water potential of potato cells.

Answer 27

1. Use a cork borer to cut potato’s into identically sized chips, about 1cm in diameter.
2. Divide the chips into groups of three and measure the mass of each group using a mass balance.
3. Place one group into each of your sucrose solutions.
4. Leave the chips in the solutions for at least 20 minutes (making sure that they all get the same amount of time).
5. Remove the chips and pat dry gently with a paper towel.
6. Weigh each group again and record results.
7. Calculate the percentage change in mass for each group.
8. Use the results to make a calibration curve, showing the percentage change in mass against sucrose concentration.

The potato chips will gain water, and therefore mass, in solutions with a higher water potential than the chips, and lose water in solution with a lower water potential.

In a graph, the water potential of the sucrose and water is the same at where the line crosses the x axis. Find the concentration at this point, then look up the water potential for that concentration of sucrose (e.g. in a text book).

Question 28

Active transport?

Answer 28

Active transport requires energy to move molecules and ions across the membrane, usually against a concentration gradient.

There are two proteins involved in active transport:

• carrier proteins,
• co-transporters.

Question 29

Carrier proteins in active transport?

Answer 29

Carrier proteins are involved in active transport.

The process is very similar to facilitated diffusion - a molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane.

Two differences:

• Active transport usually moves salute from low to high concentration. In facilitated diffusion, this is opposite.
• Active transport requires energy whilst facilitated diffusion does not. ATP is a common source of energy - produced by respiration.

Question 30

Co-transporters in active transport?

Answer 30

These are a type of carrier protein.

1. Co-transporters bind two molecules at a time.
2. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient.

For example, sodium ions move into the cell down a concentration gradient. This means glucose into the cell to, against its concentration gradient.

Question 31

Factors effecting rate of active transport?

Answer 31

When active transport moves molecules and ions against the concentration gradient, a decreasing concentration gradient doesn’t affect the rate of active transport.

It is affected by:
1. The speed of individual carrier proteins. The faster they work, the faster the rate of active transport.

2. The number of carrier protein is present. The more proteins that are, the faster the rate of active transport.
3. The rate of respiration in the cell and the availability of ATP. If respiration is inhibited, active transport can’t take place.

Question 32

How is glucose absorbed?

Answer 32

Can you close is absorbed into the bloodstream in the small intestine.

In the ileum, the concentration of glucose is too low for glucose to diffuse out into the blood. So glucose is absorbed from the lumen of the ileum by co-transport.

1. Sodium ions are actively transported out of the ileum epithelial cells, into the blood, by the sodium-potassium pump. This creates a concentration gradient - high concentration of sodium ions in the linen of the ileum than inside the cell.
2. This causes sodium ions to diffuse from the lumen of the ileum into the epithelium cells, down their concentration gradient. They do this via the sodium-glucose co-transporter proteins.
3. The co-transporter carries the glucose into the cell with the sodium. As a result, the concentration of glucose inside the cell increases.
4. Glucose diffuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion.

Question 33

Channel proteins and carrier proteins differences?

Answer 33

Chanel - don’t change shape. They diffuse charged ions and polar molecules down them.

Carrier - change shape. They diffuse large molecules down them.