Gas exchange surfaces and transport across cell membranes

Question 1

What is the function of the glycoprotein?

Answer 1

enable cells to recognise another cell as foreign or familiar
(cell-cell recognition)

Question 2

What is the function of glycolipids?

Answer 2

maintain stability of the cell membrane and facilitate cellular recognition.

Question 3

What is the function of the phospholipid bilayer?

Answer 3

A barrier to the passage of molecules and ions in and out of the cell

Question 4

What is the function of channel proteins?

Answer 4

Allow for transport across the membrane without using energy and without binding to the substance.

Question 5

What is the function of carrier proteins?

Answer 5

Transport large molecules across the membrane
Three different types; uniporter, antiporter and symporter

Uniporters - Move a single substance down the gradient
Antiporters - Moves one molecule up the gradient and another molecule down the gradient
Symporters - Moves 2 molecules in the same direction across the membrane

Question 6

What is the role of cholesterol in the cell membrane ?

Answer 6

• Regulates membrane fluidity
• Increases mechanical strength and stability
• Increases fluidity at low temperatures, stops it from becoming too rigid
• Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures
• Cholesterol binds to hydrophobic tails of phospholipids stabilising them and causing phospholipids to compact closely together

Question 7

What is meant by facilitated diffusion?

Answer 7

The passive movement of larger or polar molecules (e.g. ions) across a membrane, from a region of high concentration to a region of low concentration through a carrier or channel protein - once equilibrium is reached diffusion stops

Question 8

How does the concentration gradient affect diffusion?

How does temperature affect diffusion?

How does surface area affect the rate of diffusion?

How does the size of the molecule affect the rate of diffusion?

polarity of molecule

Answer 8

Steeper gradient means faster diffusion. Greater difference in free moving molecules

higher temp - faster rate of diffusion as particles have more kinetic energy so they can move around faster

increased SA - rate of diffusion increases

small molecule - faster diffusion
large molecule - slower diffusion

uncharged, non polar molecules diffuse quicker

Question 9

Definition of osmosis

Answer 9

The net movement of water molecules from an area of high water potential to an area of lower water potential, down a concentration gradient, across a semi permeable membrane

Question 10

Explain endocytosis

Answer 10

-Particles are enclosed in vesicles made from cell surface membrane and transported into the cell.
-The cells way of receiving large packages of materials
-Fluid cell membrane folds around the substance
‘endo’ = enters the cell

Question 11

Explain exocytosis

Answer 11

-Membrane bound vesicles fuse with plasma membranes before releasing the substances within them into their surroundings

-The cells way of ‘emptying’ contents
-Commonly found in glands secreting hormones
-These molecules re-produced in cells and packaged in vesicles
‘exo’ = leaves the cell

Question 12

Definition of active transport

Answer 12

The active movement of desired molecules across a cell membrane, from an area of lower concentration to an area of higher concentration, involving the use of carrier proteins and also requiring energy

Question 13

Describe the structure of a cell membrane (5 marks)

Answer 13

The cell membrane has a phospholipid bilayer.

The heads of the phospholipid are hydrophilic meaning they are attracted to water.

Cell membrane also has glycoproteins which recognise other cells as familiar or foreign. (cell-cell recognition)

The cell membranes glycolipids facilitate cellular recognition.

Cholesterol maintains stability and fluidity of the cell membrane

extra points:
-Fluid mosaic model
-Carrier proteins- move molecules across the cell membrane
-Channel proteins- Allow for transport across the membrane without using energy and without binding to the substance

Question 14

Are phosphate heads hydrophilic or hydrophobic

Are phosphate tails hydrophilic or hydrophobic

Answer 14

Heads - Hydrophilic

Tails - Hydrophobic

Question 15

What is Fick’s law?

Answer 15

SA x concentration gradient / Length of diffusion path

Question 16

Explain how specialised exchange systems are maximised for diffusion

Answer 16

-Alveoli increase the SA of the mammalian lung, larger SA means faster rate of diffusion
-High concentration gradient is maintained through respiration and lots of blood flow. Lots of blood flow means more oxygen can be extracted from air and can pass into lungs making the bloody oxygen rich

Question 17

Why do multicellular organisms need specialist exchange surfaces?

Answer 17

-To facilitate the exchange of substances with their environment
-For larger multicellular organisms there is not enough SA to absorb enough nutrients, gas exchange and remove waste products for the total volume of all the cells

Question 18

How does SA:VOL affect transport of molecules?

Answer 18

Lower SA:Vol the further distance molecules need to travel to reach all parts of the organism. Diffusion alone is not sufficient, so specialised gas exchange surfaces are required

Question 19

Name 3 features of an efficient gas exchange surface

Answer 19

large SA
Thin/short distance for diffusion
Steep concentration gradient, maintained by blood supply or ventilation.

Question 20

How is the mammalian lung adapted for gas exchange?

Answer 20

Alveoli provide a large SA
Good blood supply maintains a steep concentration gradient
Only one cell thick, short diffusion distance
Large capillary network

Question 21

What is meant by the fluid mosaic model?

Answer 21

Cell membranes are fluid, and have a mosaic-like arrangement of proteins

Question 22

How are large molecules moved across a cell membrane?

Answer 22

Requires vesicles.
Endocytosis = Cell membrane forms a vesicle and engulfs the material, which enters the cytoplasm
Exocytosis = Vesicle fuses with cell membrane to release contents from cell

Question 23

Describe why single-celled organisms dont need a specialised gas exchange surface

Answer 23

-They have a high SA:Vol ratio meaning concentration gradient is high.
-Due to their small size, the diffusion distance is short meaning gasses can diffuse quickly

Question 24

Explain why people with emphysema are given air with a higher concentration of oxygen than atmospheric air (2 marks)

Answer 24

-They need a large concentration gradient to maintain a fast rate of diffusion, they currently have a small SA:Vol ratio.
-They are given air with abundant oxygen to ensure there is enough in the blood so oxygen is delivered to muscles and organs.

Question 25

Explain the effect of temperature on cell membranes. (2 marks)

Answer 25

-Higher temperature means greater breakdown of the cell membrane
-High temperatures cause enzymes and proteins to become denatured

Question 26

Explain why the Phospholipids are arranged in two layers in a cell surface membrane (3 marks)

Answer 26

-The phospholipid has 2 fatty acids, a hydrophilic head and hydrophobic tail.
-Due to there being 2 layers, the bilayer can hold more things in the membrane e.g. cholesterol, glycolipids…
-There is a bilayer because there is water inside and outside the cell

Question 27

Name the bond that joins fatty acid molecule to a glycerol molecule in a phospholipid

Answer 27

Ester bond

Question 28

Explain why ATP is required for movement of sodium ions into the cell (2 marks)

Answer 28

Sodium ions are being moved against the concentration gradient using active transport

Question 29

Endocytosis and exocytosis are processes that move large molecules into a cell or out of a cell.
Compare and contrast the processes of endocytosis and exocytosis. (3 marks)

Answer 29

Similarities - Both processes involve vesicles and use of energy from ATP (2)

Differences - Exocytosis involves molecules leaving the cell whereas endocytosis involves molecules entering the cell (1)

Exocytosis involves vesicles fusing with cell membrane whereas endocytosis involves the formation of new vesicles

Question 30

Describe how glucose molecules move into the cell (2 marks)

Answer 30

Through facilitated diffusion using carrier proteins.

Question 31

Describe the function of carrier proteins in a cell surface membrane (4 marks)

Answer 31

-Involved in facilitated diffusion (1)
-Movement of { large molecules / polar molecules / ions } (1)
- (facilitated diffusion) from a high concentration to a low concentration (1)
-Involved in active transport (1)
- Needs ATP to move molecules against concentration gradient (1)

Question 32

Explain how the structure of the human lungs enables rapid gas exchange (4)

Answer 32

-Many alveoli provide a large surface area (1)
- {alveoli / capillaries} have walls that are one cell thick providing a short distance for diffusion (1)
-High concentration gradient maintained by {circulation / ventilation} (1)
-Extensive capillary network around alveoli provides large
surface area for gas exchange (1)

Question 33

Emphysema is a disease of the respiratory system that affects the structure of the lungs.

Explain why people with emphysema are given air with a higher concentration of oxygen than atmospheric air (2)

Answer 33

-Smaller surface area of alveoli (with emphysema) (1)
-Therefore need a larger concentration gradient (1)
-To maintain the rate of diffusion (1)

Question 34

Diffusion and active transport are mechanisms by which molecules can enter cells.
Compare and contrast these two mechanisms (3)

Answer 34

Similarities

-Both move molecules through the {phospholipid bilayer / cell
surface membrane} (1)
-(in both) molecules can move through proteins (1)

Differences

-Diffusion occurs down a concentration gradient whereas active transport occurs against a concentration gradient (1)
-Diffusion is {passive / does not require ATP} whereas active transport requires ATP (1)

Question 35

Give one function of the glycoproteins found in the cell surface membrane (1)

Answer 35

cell - cell recognition

Question 36

Describe how mineral ions are taken up by root hair cells (3)

Answer 36

• Mineral ions are taken up by active transport
• Through carrier proteins
• This requires ATP

Question 37

Compare and contrast diffusion and active transport

Answer 37

Similarities:
- Both move molecules through a phospholipid bilayer

• Molecules can both move through proteins

Differences:
- Diffusion occurs down a concentration gradient whereas active transport occurs against a concentration gradient

• Diffusion is passive whereas active transport requires ATP

Question 38

The vacuoles in beetroot cells contain molecules of betalain, a red pigment.

Betalains are large polar molecules.

These molecules can leave beetroot cells if the vacuole membrane and the cell surface membrane are damaged.

Explain why betalain molecules cannot move through intact cell membranes.
(3)

Answer 38

• Betalain molecules are too large to move through cell membrane
• There are no carrier/channel proteins for betalain molecules to move through
• Betalain molecules are polar and are repelled by hydrophobic fatty acid tails

Question 39

Describe the structure of the cell surface membrane (3)

Answer 39

• Cell membrane is mainly phospholipids and protein
• Phospholipids for a bilayer
• Proteins float in the phospholipids like fluid mosaic model

Question 40

Explain how the structure of a phospholipid molecule contributes to the partial permeability of a cell surface membrane (3)

Answer 40

Phospholipid molecule contains polar/hydrophilic head and non-polar/hydrophobic fatty acid chains

Allows non-polar molecules to pass through the membrane

Polar molecules cannot pass through phospolipid

Question 41

Explain how phospholipids form a cell surface membrane (3)

Answer 41

• Hydrophilic parts associate with water
• Hydrophobic parts repel water
• A bilayer forms with hydrophobic parts pointing towards each other

Question 42

Compare and contrast the structures of a phospholipid and a triglyceride (3)

Answer 42

Similarities:
- Both contain glycerol and fatty acid chains
- Both contain ester bonds

Differences:
- Triglycerides contain three fatty acid chains whereas phospholipids contain two fatty acid chains
- Phospholipids contain a phosphate group whereas triglyceride does not