Gas Exchange, Cell Membranes and Transport

Question 1

How are gas exchanges surfaces adapted for efficient diffusion?

Answer 1

• Large surface area to volume ratio
• They’re thin which provides a short diffusion pathway
• Concentration gradient of gases across the exchange surfaces

Question 2

How are the lungs adapted for efficient gas exchange?

Answer 2

• Lots of alveoli means there is a large SA:V ratio
• The alveolar epithelium and capillary endothelium are only 1 cell thick, giving a short diffusion pathway
  -They constantly bring CO2 from the capillary into the alveolus and take away oxygen from the alveolus into the capillary, this maintains a concentration gradient.

Question 3

How do you calculate the SA and V of a small cube with length, width and depth being 2cm?

Answer 3

SA = 226=24
V = 222=8

Question 4

What is Fick’s Law?

Answer 4

The rate of diffusion is proportional to the surface area multiplied by the difference in concentration over thickness of diffusion surface.

Question 5

What happens during inspiration and expiration?

Answer 5

• During inspiration the external intercostal muscles contract to raise the ribs, flattening the diaphragm, pushing down on the contents in the abdomen. The volume of chest cavity increases and pressure drops. - During exhalation the internal intercostal muscles contract to forcefully push out air. The diaphragm is relaxed so it’s pushed up, volume of chest cavity is decreased and pressure increases.

Question 6

What’s a vesicle?

Answer 6

Small membrane bound sacs containing a substance.

Question 7

What’s exocytosis?

Answer 7

• The release of substances, usually proteins from the cell
• Vesicles fuse with the cell membrane and the contents are released
• Also requires ATP

Question 8

What’s endocytosis?

Answer 8

Substances are taken into the cell by the creation of vesicles from the cell surface membrane. Part of the cell membranes engulfs the solid or liquid material to be transported.

Question 9

What is diffusion?

Answer 9

Is the passive net overall movement of particles from a region of high conc to low conc. Always occur when there’s a conc gradient. Small uncharged particles diffuse directly across the membrane as well as CO2.

Question 10

What is facilitated diffusion?

Answer 10

Polar molecules or ions cannot simply diffuse as the hydrophobic tails provide impenetrable barriers. They cross with the aid of proteins (channel and carrier). Each type of channel protein has a specific shape that permits the passage of a specific substance or the molecule binds to the carrier protein.

Question 11

What is osmosis?

Answer 11

Net movement of water molecules from a high conc of free water molecules to a low conc of free water molecules across a partially permeable membrane.

Question 12

What is active transport?

Answer 12

• Moves across a membrane against a concentration gradient, energy is supplied by ATP
• The substance to be transported binds to the carrier protein, one phosphate group is removed from ATP by hydrolysis forming ADP + Pi
• Lots of energy is released as bonds form between water and phosphate. This energy causes the carrier protein to change shape.

Question 13

What structures make up the plasma membrane?

Answer 13

Phospholipids, hydrophilic head and hydrophobic tails. Proteins, cholesterol, glycolipids and glycoproteins.

Question 14

What is the overall function of membranes?

Answer 14

• Control entry and exit of materials
• Separate organelles from cytoplasm
• Isolate damaging enzymes

Question 15

What is the function of cholesterol in the plasma membrane?

Answer 15

• They add strength
• Hydrophobic so plays an important role in preventing water loss
• They pull together fatty acid tails of the phospholipid, limiting their movement
• Makes the membrane less fluid at high temperatures

Question 16

What is the function of proteins in a plasma membrane?

Answer 16

• Provide structural support
• Acts as channels transporting water and soluble substances across the membrane
• Helps cells adhere together as well as acting as receptors

Question 17

What is the function of glycolipids in a plasma membrane?

Answer 17

• Help cells attach to one another whilst maintaining stability
• They can also act as recognition sites.

Question 18

What is the function of glycoproteins in a plasma membrane?

Answer 18

Carbohydrate chain attached to many extrinsic proteins, they act as cell surface receptors as well as recognition sites.

Question 19

Use Fick’s Law of Diffusion to explain the adaptations of mammalian gas exchange surfaces.

Answer 19

• Rate of diffusion is proportional to surface area, alveoli have a large surface
• Rate of diffusion is proportional to concentration gradient, breathing/blood flow maintains a difference in gas concentrations
• Rate of diffusion is inversely proportional to diffusion distance, walls of alveoli and capillary are one cell thick (flattened cells)

Question 20

Using the diagram and your knowledge of gas exchange surfaces, explain how the structure
of a flatworm is adapted to obtain oxygen from the water

Answer 20

• Short diffusion pathway
• Large SA:V ratio

Question 21

Flatworms do not have a heart or a circulatory system.
i) Explain why many animals need a heart and a circulatory system

Answer 21

• Heart is needed to pump blood around the body
• Circulatory system is an example of mass flow
• Many animals have a small SA:V ratio so a circulatory system is needed to overcomes limitations of diffusion

Question 22

In mammals, blood passes through the heart twice for each circulation of the body.
i) Suggest how this type of circulation enables mammals to carry out effective gas exchange.

Answer 22

• One side of the heart transports blood to the lungs and the other to the body
• Separation of oxygenated and deoxygenated blood
• Maintains a concentration gradient

Question 23

Describe what is meant by the term fluid mosaic with reference to cell membranes.

Answer 23

• Fluid refers to the movements of phospholipids in the membrane
• Mosaic refers to the random association of proteins

Question 24

The fluid mosaic model has been developed from the knowledge of the structure and properties of cell membranes. It can explain how molecules can enter and leave a cell.
(a) Describe the structure of a cell membrane.

Answer 24

• Phospholipid bilayer
• Head is on the outside because its hydrophilic and the fatty acid tails are on the inside as they are hydrophobic
• There are intrinsic and extrinsic proteins such as glycoproteins, glycolipids and cholesterol in the membrane

Question 25

Suggest two properties of molecules that enable them to enter a cell by diffusion.

Answer 25

• Small
• Uncharged/non polar