Cells - Transport across Cell Membranes

Question 1

Why are there proteins embedded in the plasma membrane?

Answer 1

The proteins allow some substances to pass through which can’t be easily transported through the phospholipid bilayer. Non-polar molecules can easily pass through.

Question 2

What is the protein channel?

Answer 2

A protein completely spanning the phospholipid bilayer which form water-filled tubes to allow water-soluble ions to diffuse across the membrane.

Question 3

What is a carrier protein?

Answer 3

A protein completely spanning the phospholipid bilayer which bind to ions or molecules then change shape in order to move these molecules across the membrane.

Question 4

Phospholipids form a bilayer. They are important components of cell-surface membranes. What is their structure?

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 of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides.

Question 5

What are the functions of phospholipids in the membranes?

Answer 5

• Allow lipid-soluble (non-polar) substances to enter and leave the cell.
• Prevent water-soluble (polar) substances entering and leaving the cell.
• Make the membrane flexible and self-sealing.

Question 6

Proteins are interspersed throughout the cell surface membrane. What are the two main ways they are embedded in the phospholipid bilayer?

Answer 6

Extrinsic proteins: 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 or as enzymes. May have carbohydrates bound to them.

Intrinsic proteins: Other proteins completely span the phospholipid bilayer from one side to the other. Some are protein channels, which form water-filled tubes to allow water-soluble ions to diffuse across the membrane. Others are carrier proteins that bind to ions or molecules like glucose and amino acids, then change shape in order to move these molecules across the membrane. They are made of protein only.

Question 7

What are the functions of the proteins in the membrane?

Answer 7

• 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
• helps cells adhere together
• acts as receptors, for example for hormones

Question 8

What is cholesterol?

Answer 8

• Cholesterol molecules occur within the phospholipid bilayer of the cell-surface membrane.
• They add strength and some rigidity to the membranes, particularly at high temperatures.
• Cholesterol molecules are very hydrophobic and therefore play an important role in preventing 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 but without making the membrane as a whole too rigid.

Question 9

What are the functions of cholesterol in the membrane?

Answer 9

• reduce lateral movement of the other molecules including phospholipids
• make the membrane less fluid at high temperatures
• prevent leakage of water and dissolved ions from the cell

Question 10

What is a glycolipid?

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

Question 11

What are the functions of glycolipids in the membrane?

Answer 11

• act as recognition sites
• help maintain the stability of the membrane
• help cells to attach to one another and so form tissues (acts as binding sites)

Question 12

What are glycoproteins?

Answer 12

Carbohydrate chains are attached to many 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 13

What are the functions of glycoproteins in the membrane?

Answer 13

• act as recognition sites
• helps cells to attach to one another and so form tissues
• allows cells to recognise one another, e.g. lymphocytes can recognise an organism’s own cells.

Question 14

Why do most molecules not freely diffuse across the cell-surface membrane?

Answer 14

Because many are:

• not soluble in lipids and therefore cannot pass through the phospholipid layer
• too large to pass through the channels in the membrane
• of the same charge as the charge on the protein channels and so, even if they are small enough to pass through, they are repelled
• electrically charged (polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer

Question 15

What are the functions of membranes within cells?

Answer 15

• 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, e.g. 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 rough endoplasmic reticulum.
• Has binding sites.
• Confers antigenic properties to the cell.

Question 16

What is the fluid-mosaic model?

Answer 16

Fluid because the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly 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 17

Cells have many membranes. What are they?

Answer 17

• nuclear envelope (double membrane)
• outer chloroplast membrane
• thylakoid membrane
• tonoplast (vacuole membrane)
• lysosome membrane
• outer mitochondrial membrane
• inner mitochondrial membrane (cristae)
• rough and smooth endoplasmic reticulum membrane
• golgi apparatus membrane

Question 18

What is diffusion?

Answer 18

The movement of a substance from an area of higher concentration to an area of lower concentration, along the usual concentration gradient until evenly distributed. It is caused by the random movement of particles due to kinetic energy. It does not require any energy other than heat and so is a passive process.

Question 19

What are the factors affecting the rate of movement across cell membranes?

Answer 19

• surface area
• number of channel or carrier proteins
• differences in gradients of concentration or water potential

Question 20

What is facilitated diffusion?

Answer 20

Facilitated diffusion is a passive process. It relies only on the inbuilt motion (kinetic energy) of the diffusing molecules. There is no external input of ATP from respiration. Like diffusion, it occurs down a concentration gradient, but it differs in that it occurs at specific points on the plasma membrane where there are special protein molecules. The proteins pass substances across the membrane faster than otherwise possible, however it can be limited by the number of proteins. The two types of proteins that are involved are protein channels and carrier proteins. Each has a different mechanism.

Question 21

Which proteins are responsible for facilitated diffusion?

Answer 21

• carrier proteins

- channel proteins

Question 22

How are carrier proteins responsible for facilitated diffusion?

Answer 22

Carrier proteins span the plasma membrane. 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 region where they are highly concentrated to one of lower concentration, using only the kinetic energy of the molecules themselves. This is often done by co-transport.

Question 23

How are channel proteins responsible for facilitated diffusion?

Answer 23

Protein channels are proteins that 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. In this way, 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 24

What is active transport?

Answer 24

Active transport is the process by which a cell takes up a substance against a concentration gradient (e.g. from a lower concentration to a higher concentration).

It uses protein carrier molecules that are highly specific but also requires energy. This comes in the form of a molecule called ATP which is produced during respiration. Cells in which lots of active transport takes place, such as the epithelial cells of the small intestine, have many mitochondria which produce ATP by hydrolysis for this process.

Question 25

Give an example of where active transport is used.

Answer 25

The sodium/potassium pump found in all animal cell membranes. Continually uses ATP to actively pump sodium out of the cell and potassium into the cell. This creates concentration gradients across the membrane.

Question 26

What is co-transport?

Answer 26

Carrier proteins can sometimes transport two particles at once. Both must be present for transport to take place. Often both an active and passive process as one particle is moved with the concentration gradient and the other is moved against it. Substances can be moved in the same (symport) or opposite (antiport) direction.

Question 27

What uses diffusion directly across phospholipid bilayers?

Answer 27

small, lipid-soluble, uncharged molecules

Question 28

What uses facilitated diffusion using channel proteins?

Answer 28

small, polar molecules and ions

Question 29

What uses facilitated diffusion using carrier proteins?

Answer 29

larger, lipid-insoluble molecules

Question 30

Describe the solute concentration outside of the cell, the water potential outside of the cell, and the net movement of water of a hypotonic solution.

Answer 30

solute concentration outside of cell - lower
water potential outside of cell - higher
net movement of water - in to cell

Question 31

Describe the solute concentration outside of the cell, the water potential outside of the cell, and the net movement of water of a isotonic solution.

Answer 31

solute concentration outside of cell - same
water potential outside of cell - same
net movement of water - no net movement

Question 32

Describe the solute concentration outside of the cell, the water potential outside of the cell, and the net movement of water of a hypertonic solution.

Answer 32

solute concentration outside of cell - higher
water potential outside of cell - lower
net movement of water - out of cell

Question 33

What is osmosis?

Answer 33

Osmosis is defined as 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. It is a special case of diffusion in that the concentrations of solutes in the water can affect how it occurs.

Question 34

What is a solute? What is a solvent?

Answer 34

Since water is a polar molecule, many substances dissolve in it. These dissolved substances are termed solutes, and water is a solvent.

Question 35

What is water potential?

Answer 35

Water potential measures the concentration of free water molecules. It is a measure of the tendency of these molecules to diffuse to another area. The more free water molecules, the higher the water potential. Water potential is the pressure created by water molecules.

Question 36

What is water potential measured in?

Answer 36

Osmosis can be quantified using water potential. Water always moves from a high water potential to a low water potential. Water potential is measured in units of pressure kiloPascals (kPa), where the highest water potential (that of pure water) is 0 kPa and lower
water potentials go into negative numbers. The more concentrated the solution, the lower the number.

Question 37

Describe osmosis in plant cells.

Answer 37

Under isotonic conditions, there is no net movement of water. In a hypotonic environment, vacuoles fill with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. In a hypertonic environment, vacuoles lose water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the centre of the cell.

Question 38

Describe osmosis in animal cells.

Answer 38

Under isotonic conditions, there is no net movement of water. In a hypotonic environment, water enters the cell, which may burst (lysis). In a hypertonic environment, water leaves the cell, which shrivels (crenation).

Question 39

Why are all particles constantly in motion?

Answer 39

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.

Question 40

Can diffusion occur between different concentrations of different compounds?

Answer 40

Diffusion ONLY occurs between different concentrations of the same substance.

Question 41

Why don’t charged ions and polar molecules diffuse easily?

Answer 41

Because of the hydrophobic nature of the fatty-acid tails of the phospholipids in the membrane. The movement of these molecules is made easier by transmembrane channels and carriers that span the membrane. This process is therefore called facilitated diffusion.

Question 42

What is diffusion proportional to?

Answer 42

Diffusion is proportional to the difference in concentration between two regions (the concentration gradient).

Question 43

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

Answer 43

One way of finding the water potential of cells or tissues is to 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 44

When is a dynamic equilibrium established in osmosis?

Answer 44

At the point where the water potentials on either side of the plasma membrane are equal, a dynamic equilibrium is established and there is no net movement of water.

Question 45

Explain osmosis in animal red blood cells.

Answer 45

Animal cells, such as red blood cells, contain a variety of solutes dissolved in their watery cytoplasm. If a red blood cell is placed in pure water, it will absorb water by osmosis because it has a lower water potential. Cell surface membranes are very thin and, although they are flexible, they cannot stretch to any great extent. The cell-surface membrane will therefore break, bursting the cell and releasing its contents (in red blood cells this is called haemolysis). To prevent this happening, animal cells normally live in a liquid which has the same water potential as the cells. In this example, the liquid is the blood plasma. This an red blood cells have the same water potential. If a red blood cell is placed in a solution with a water potential lower than its own, water leaves by osmosis and the cell shrinks and becomes shrivelled.

Question 46

What is the central vacuole?

Answer 46

Contains a solution of salts, sugars and organic acids in water.

Question 47

What is the cellulose cell wall?

Answer 47

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

Question 48

What is ATP used for in active transport?

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 known as co-transport.

Question 49

How does active transport differ from passive forms of transport?

Answer 49

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

Question 50

Describe the 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. As a result, the protein molecule changes shape and opens to the opposite side of the membrane.
• The molecule or ion is then released to the other side of the membrane.
• The phosphate molecule is released from the protein which causes the protein to revert 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

What is the difference between active transport and facilitated diffusion?

Answer 51

Both use carrier proteins but facilitated diffusion occurs down a concentration gradient, while active transport occurs against a concentration gradient. This means that facilitated diffusion does not require metabolic energy, while active transport does. The metabolic energy is provided in the form of ATP.

Question 52

What are different carrier proteins involved in?

Answer 52

Different carrier proteins are involved in facilitated diffusion and active transport but any given protein carrier is very specific about what it carries and by which method.

Question 53

What is the sodium-potassium pump?

Answer 53

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

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

Question 54

What is exocytosis and endocytosis?

Answer 54

Exocytosis and endocytosis transport large particles.

In endocytosis, the cell membrane binds to a large molecule resulting in the formation of a vesicle. The vesicle containing the large molecules then enters into the cell.

In exocytosis, vesicles containing molecules of substances are fused with the inside of the cell surface membrane and the molecules are secreted from the cell.

Question 55

Where does co-transport occur?

Answer 55

Co-transport uses ions to move substances into and out of cells. This occurs particularly in epithelial cells of the ileum. Here sodium and potassium ions are pumped out of the epithelial cell by active transport into the blood leaving a lower concentration in the cell. This causes these ions to move in from the lumen by facilitated diffusion, which at the same time brings glucose and amino acids into the cell. These then diffuse from a high concentration in the epithelial cell to a low concentration in the blood.

Question 56

What is the effect of dissolved solutes on water potential?

Answer 56

The presence of solutes (e.g. sucrose) lowers water potential because the solutes restrict the movement of water molecules.

Question 57

What has the highest water potential?

Answer 57

pure water

Question 58

Which direction does water diffuse in regarding water potentials?

Answer 58

Water always diffuses from regions of less negative to more negative water potential.

Question 59

What is water potential determined by?

Answer 59

• the solute potential
• the pressure potential

water potential = solute potential + pressure potential

Question 60

What is the pressure potential?

Answer 60

The pressure potential is the hydrostatic pressure to which water is subjected (e.g. by a plant cell wall). In other words, it is a measure of the pressure that develops inside a cell as a consequence of the inflow of water. The pressure potential is usually positive and is zero when cells are in equilibrium. It is sometimes called turgor or wall pressure.

Question 61

What is the solute potential?

Answer 61

The solute potential is a measure of the reduction in water potential due to the presence of solute molecules. In other words, a measure of the extent to which solute concentrations lower the water potential. It is the negative component of water potential, sometimes referred to as the osmotic potential or osmotic pressure.

Question 62

What happens with a hypotonic solution?

Answer 62

• less negative solute potential
• less negative water potential
• loses water by osmosis

Question 63

What happens with a hypertonic solution?

Answer 63

• more negative solute potential
• more negative water potential
• gains water by osmosis

Question 64

What is turgor?

Answer 64

When the contents of a plant cell push against the cell wall, they create turgor (tightness) which provides support for the plant body. When cells lose water, there is a loss of turgor and the plant wilts. Complete loss of turgor from a cell is called plasmolysis and is irreversible.

Question 65

What happens when the external water potential is the same as that of the cell?

Answer 65

When the external water potential is the same as that of the cell, there is no net movement of water.

Question 66

What happens when the external water potential is more negative than the water potential of the cell?

Answer 66

When the external water potential is more negative than the water potential of the cell, water leaves the cell and, because the cell wall is rigid, the plasma membrane shrinks away from the cell wall. This process is termed plasmolysis and the cell becomes flaccid (pressure potential = 0). Full plasmolysis is irreversible; the cell cannot recover by taking up water.

Question 67

What happens when the external water potential is less negative than the water potential of the cell?

Answer 67

When the external water potential is less negative than the water potential of the cell, water enters the cell. A pressure potential is generated when sufficient water has been taken up to cause the cell contents to press against the cell wall. The pressure potential rises progressively until it offsets the solute potential. Water uptake stops when the water potential of the cell = 0. The rigid cell wall prevents cell rupture. Cells in this state are turgid.

Question 68

How can you increase the rate of movement across membranes, regarding epithelial cells?

Answer 68

The epithelial cells lining the ileum possess microvilli. These are finger-like projections of the cell-surface membrane. The microvilli provide more surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can take place. Another mechanism to increase transport across membranes is to increase the number of protein channels and carrier proteins in any given area of membrane (i.e. increase their density).

Question 69

What is the role of diffusion in absorption?

Answer 69

As carbohydrates and proteins are being digested continuously, there is normally a greater concentration of glucose and amino acids within the ileum than in the blood. There is therefore a concentration gradient down which glucose moves by facilitated diffusion from inside the ileum into the blood. Given that the blood is constantly being circulated by the heart, the glucose absorbed into it is continuously being removed by the cells as they use it up during respiration. This helps us to maintain the concentration gradient between the inside of the ileum and the blood. This means the rate of movement by facilitated diffusion across epithelial cell-surface membranes is increased.

Question 70

What is the role of active transport in absorption?

Answer 70

At best, diffusion only results in the concentrations either side of the intestinal epithelium becoming equal. This means that not all the available glucose and amino acids can be absorbed in this way and some may pass out of the body. The reason why this does not happen is because glucose and amino acids are also being absorbed by active transport. This means that all the glucose and amino acids should be absorbed into the blood.

Question 71

Give an example of co-transport.

Answer 71

The mechanism by which glucose and amino acids are absorbed into the blood from the small intestine.

Question 72

Describe the sodium-potassium pump system.

Answer 72

1. Sodium ions are actively transported out of the epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of of protein-carrier molecule found in the cell-surface membrane of the epithelial cells.
2. This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells.
3. Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier (co-transport protein) in the cell-surface membrane. As the sodium ions diffuse in through this second carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them.
4. The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.

Question 73

Why is the movement of glucose/amino acids into the epithelial cells not considered as active transport?

Answer 73

Both sodium ions and glucose/amino acid molecules move into the cell, but while the sodium ions move down their concentration gradient, the glucose molecules move against their concentration gradient. It is the sodium ion concentration gradient, rather than ATP directly, that powers the movement of glucose and amino acids into the cells. This makes it an indirect rather than a direct form of active transport.

Question 74

What is flaccid?

Answer 74

Plant cells or tissue that is drooping as a result of a lack of water.

Question 75

What is lysis?

Answer 75

The disintegration of a cell by rupture of the cell wall or membrane.

Question 76

What is plasmolysis?

Answer 76

Contraction of the cell membrane of a plant cell, resulting in it pulling away from the cell wall, as a result of a loss of water.

Question 77

What is turgid?

Answer 77

A plant cell that contains the maximum volume of water it can. Additional entry of water is prevented by the cell wall stopping further expansion of the cell.

Question 78

What is turgor pressure?

Answer 78

Pressure exerted by a plant cell membrane against the plant cell wall as a result of the cell taking up water.

Question 79

What is water potential?

Answer 79

The pressure created by water molecules. It is the measure of the extent to which a solution gives out water. The greater the number of water molecules present, the higher (less negative) the water potential. Pure water has a water potential of zero.

Question 80

What phosphorylates sodium ions?

Answer 80

ATP (leads to a conformational change)

Question 81

What’s a conformational change?

Answer 81

A change in the shape of a macromolecule, often induced by environmental factors.

Question 82

What are aquaporins?

Answer 82

Channel proteins that allow water to pass through the membrane.

Question 83

How is ATP used in active transport?

Answer 83

Active transport requires the hydrolysis of ATP into ADP plus an inorganic phosphate.

Question 84

What are examples of endocytosis?

Answer 84

Phagocytosis is endocytosis which allows for the uptake of solids.

Pinocytosis is endocytosis which allows for the uptake of fluids.

Question 85

Where does absorption of products of digestion occur?

Answer 85

• Absorption of products of digestion begins in the duodenum.
• Majority of the absorption occurs in the ileum.
• Specifically, the absorption occurs at the microvilli surface of the epithelial cells which make up the villi.

Question 86

How are cells adapted for rapid transport? (factors affecting rate of transport)

Answer 86

1. Surface Area
   The villi increase surface area in the ileum and hence increase rate of glucose and sodium transport.
2. Number of Channel Proteins
   The more channel proteins present, the faster facilitated diffusion is.
3. Number of Carrier Proteins
   The more carrier proteins present, the faster the facilitated diffusion or active transport.
4. Number of Aquaporins
   The more aquaporins present, the faster the osmosis.

Question 87

What are the limitations of simple diffusion imposed by the nature of the phospholipid bilayer?

Answer 87

If the particles can move through the lipid bilayer by simple diffusion, then there is no limit to the number that can fit through the membrane. The rate of diffusion increases linearly as we add more particles to one side of the membrane.