Transport across cell membranes

Question 1

What is the function of the cell surface membrane?

Answer 1

It is the plasma membrane that surrounds cells and forms the boundary between cell cytoplasm and the environment.
It allows different conditions to be established inside and outside the cell.
It controls the movement of substances in and out of the cell.

Question 2

Which molecules make up the cell-surface membrane?

Answer 2

Phospholipids
Proteins
Cholesterol
Glycolipids
Glycoproteins

Question 3

What is the function of phospholipids?

Answer 3

Allows lipid-soluble substances to enter and leave the cell.
Prevent water-soluble substances entering and leaving the cell.
Make the membrane flexible and self-sealing.

Question 4

What is the structure of phospholipids?

Answer 4

The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides.
The hydrophobic tails point into the centre of the cell membrane, repelled by the water on both sides.

Question 5

What is the function of the proteins in the membrane?

Answer 5

Provide structural support.
Act as channels transporting water-soluble substances across the 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 example for hormones.

Question 6

How are proteins embedded in the phospholipid bilayer?

Answer 6

There are peripheral proteins and transmembrane proteins.

Question 7

What are peripheral proteins?

Answer 7

Some proteins occur in the surface of the bilayer and never extend completely across it.
They act either to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones.

Question 8

What are transmembrane proteins?

Answer 8

Other proteins completely span the phospholipid bilayer from one side to the other.
There are protein channels and carrier proteins.

Question 9

What are protein channels?

Answer 9

A type of transmembrane protein that forms water-filled tubes to allow water-soluble ions to diffuse across the membrane.

Question 10

What are carrier proteins?

Answer 10

A type of transmembrane protein that bind to ions or molecules like glucose and amino acids, then change shape in order to move these molecules across the membrane.

Question 11

What is cholesterol?

Answer 11

They occur within the phospholipid bilayer of the cell-surface membrane.
They add strength to the membrane without making it rigid.
Cholesterol molecules are very hydrophobic so help to prevent loss of water and dissolved ions from the cell.
They also pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules without making the membrane rigid.

Question 12

What is the function of cholesterol in the membrane?

Answer 12

Reduce lateral movement of other molecules including phospholipids.
Make the membrane less fluid at high temperatures.
Prevent leakage of water and dissolved ions from the cell.

Question 13

What are glycolipids?

Answer 13

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 where it acts as a cell-surface receptor for specific chemicals, for example the human ABO blood system operates as a result of glycolipids.

Question 14

What is the function of glycolipids?

Answer 14

Act as recognition sites.
Help maintain the stability of the membrane.
Help cells to attach to one another and so form tissues.

Question 15

What are glycoproteins?

Answer 15

Carbohydrate chains are attached to extrinsic proteins on the outer surface of the cell membrane.
These glycoproteins also act as cell-surface receptors, more specifically for hormones and neurotransmitters.

Question 16

What is the function of glycoproteins?

Answer 16

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 organism’s own cells.

Question 17

Why do most molecules not freely diffuse across the membrane?

Answer 17

They are:
Not soluble in lipids and therefore cannot pass through the phospholipid bilayer.
Too large to pass through the channels in the membrane.
Of the same charge as the charge on the protein channels and so are repelled.
Electrically charged (polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the bilayer.

Question 18

What is the function of the membranes within cells?

Answer 18

Control the entry and exit of materials in discrete organelles such as mitochondria and chloroplasts.
Separate organelles from cytoplasm so that specific metabolic reactions can take place within them.
Provide an internal transport system (endoplasmic reticulum).
Isolate enzymes that might damage the cell e.g. lysosomes.
Provide surfaces on which reactions can occur, e.g. protein synthesis using ribosomes on RER.

Question 19

What is the fluid in the fluid-mosaic model?

Answer 19

The individual phospholipid molecules can move relative to one another.
This gives the membrane a flexible structure that is constantly changing in shape.

Question 20

What is the mosaic in the fluid-mosaic model?

Answer 20

The proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic.

Question 21

How are substances exchanged?

Answer 21

The exchange of substances between cells and the environment occurs either through active transport or by passive transport.
Active transport requires metabolic energy.
Passive transport doesn’t, for example, diffusion.

Question 22

What is diffusion?

Answer 22

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.

Question 23

What is important to understand to understand diffusion?

Answer 23

All particles are constantly in motion due to the kinetic energy that they possess.
This motion is random, with no set pattern to the way the particles move around.
Particles are constantly bouncing off one another as well as off other objects, like the sides of the vessel in which they’re contained.

Question 24

What is simple diffusion?

Answer 24

Only small, non-polar, lipid-soluble molecules can diffuse across the membrane.
For example, carbon dioxide and oxygen.
It moves down the concentration gradient.

Question 25

What is facilitated diffusion?

Answer 25

The movement of charged ions and polar molecules is made easier by transmembrane channels and carriers that span the membrane.
It is a passive process, it relies on the inbuilt motion of the diffusing molecules, there is no normal input of ATP from respiration.
It occurs at specific points on the plasma membrane where there are protein channels and carrier proteins.

Question 26

How do protein channels work?

Answer 26

They 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.
There is control over the entry and exit of ions.
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 27

How do carrier proteins work?

Answer 27

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, only uses the kinetic energy of the molecules themselves.

Question 28

What is osmosis?

Answer 28

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.

Question 29

What does selectively permeable mean?

Answer 29

Cell-surface membranes and other plasma membranes such as those around organelles are selectively permeable.
They are permeable to water molecules and a few other small molecules, but not to larger molecules.

Question 30

What is a solute?

Answer 30

Any substance that is dissolved in a solvent, for example, water.
The solute and solvent together form a solution.

Question 31

What is water potential?

Answer 31

Water potential is the pressure created by water molecules.
Measured in kiloPascals (kPa).
Under standard conditions of temperature and pressure (25°c and 100kPa), pure water has a water potential of 0.

Question 32

How can you find the water potential of cells or tissues?

Answer 32

Place them in a series of solutions of different water potentials.
Where there is no net gain or loss of water from the cells or tissues, the water potential inside the cells or tissues must be the same as that of the external solution.

Question 33

How is water potential affected?

Answer 33

The addition of a solute to pure water will lower its water potential.
The water potential of a solution must always be less than 0 (a negative value).
The more solute that is added (the more concentrated a solution), the lower (more negative) its water potential.

Question 34

What is hypotonic?

Answer 34

One of two solutions with more water and less solute.
(Higher water potential)

Question 35

What is hypertonic?

Answer 35

One of two solutions with less water and more solute.
(Lower water potential)

Question 36

What is isotonic?

Answer 36

Solutions that have the same amount of water and solute.

Question 37

What do animal cells do in hypertonic solutions?

Answer 37

The cell contents shrink and the membrane wrinkles.

Question 38

What do animal cells do in a hypotonic solution?

Answer 38

The animal cell will eventually burst - haemolysed.

Question 39

What do plant cells do in hypertonic solutions?

Answer 39

The water leaves the cell.
The protoplast shrink and the plant membrane pulls away from the cell wall - plasmolysis.

Question 40

What do plant cells do in hypotonic solutions?

Answer 40

Water enters the cell.
The swelling protoplast push against the cell wall which will stop the cell getting any larger - turgid.

Question 41

What is the protoplast?

Answer 41

The part of the cell consisting of the outer cell-surface membrane, nucleus, cytoplasm and the inner vacuole mebrane.

Question 42

What is the central vacuole?

Answer 42

It contains a solution of salts, sugars and inorganic acids in water.

Question 43

What is the cellulose cell wall?

Answer 43

A tough, inelastic covering that is permeable to even large molecules.

Question 44

What do plant cells do in isotonic solutions?

Answer 44

Water neither enters or leaves.
The condition of the cell is incipient plasmolysis.
The protoplast is beginning to pull away from the cell wall.

Question 45

How does osmosis affect animal cells?

Answer 45

Cell surface membranes are very thin (7nm) and, although they’re flexible, they cannot stretch to a great extent.
The cell-surface membrane will therefore break, bursting the cell and releasing its contents.

Question 46

How are red blood cells affected by osmosis?

Answer 46

If a red blood cell is place in pure water it absorbs water by osmosis because it has a lower water potential.
The cell-surface membrane will break - haemolysis.
To prevent this happening red blood cells live in a liquid of the same water potential - blood plasma.

Question 47

What is active transport?

Answer 47

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.

Question 48

In active transport, what is ATP used for?

Answer 48

To directly move molecules.
To individually move molecules using a concentration gradient which has already been set up by (direct) active transport.
This is co-transport.

Question 49

How is active transport different to passive transport?

Answer 49

Metabolic energy in the form of ATP is needed.
Substances are moved against a concentration gradient.
Carrier protein molecules which act as ‘pumps’ are used.
The process is very selective, with specific substances being transported.

Question 50

What is the process of direct active transport of a single molecule or ion?

Answer 50

The carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it.
The molecule or ion binds to receptor sites on the carrier protein.
On the inside of the cell/organelle, ATP binds to the protein, causing it to split into ADP and a phosphate molecule.
The protein molecule then changes shape and opens to the opposite side of the membrane, releasing the ion.
The phosphate molecule is released from the protein which causes the protein to revert back to its original shape, ready for the process to be repeated.
The phosphate molecule then recombines with the ADP to form ATP during respiration.

Question 51

How are active transport and facilitated diffusion different and similar?

Answer 51

They both use carrier proteins.
Facilitated diffusion occurs down a concentration gradient while active transport occurs against.
Facilitated diffusion does not require metabolic energy, while active transport does.
The metabolic energy is provided by ATP.

Question 52

What is the sodium-potassium pump?

Answer 52

Sodium ions are actively removed from the cell/organelle while potassium ions are actively taken from the surroundings.
This process is essential to a number of important processes, including the creation of a nerve impulse.

Question 53

How many molecules or ions are typically transported in active transport?

Answer 53

Usually only a single molecule or ion at one time.
Sometimes more than one may be moved in the same direction at the same time by active transport.
Occasionally, the molecule or ion is moved into the cell at the same time as a different one is being removed from it.
An example is the sodium-potassium pump.