Topic 2: Transport Across Cell Membranes

Question 1

What is a plasma membrane?

Answer 1

Any membrane around or within a cell. All have the same basis structure.

Question 2

What is the cell-surface membrane?

Answer 2

Cell-surface membranes form the boundary between cell cytoplasm and the environment

Question 3

What are the functions of the cell-surface membrane?

Answer 3

• Maintains conditions inside + outside the cell
• Controls movement of substances into and out of the cell
• Cell signalling and recognition

Question 4

Name the molecules found in the cell-surface membrane

Answer 4

• Phospholipids
• Proteins (carrier and channel)
• Cholesterol
• Glycoproteins
• Glycolipids

Question 5

How are phospholipids arranged in the cell-surface membrane?

Answer 5

Form a bilayer with the hydrophilic heads pointing out and hydrophobic tails pointing in due to the attraction and repulsion with water

Question 6

What types of molecules can pass through the phospholipid bilayer of the cell-surface membrane

Answer 6

Lipid soluble molecules
Smaller, less polar and uncharged molecules move more quickly through

Question 7

How are proteins arranged in the cell-surface membrane?
What different types are there?

Answer 7

Embedded in the phospholipid bilayer
Extrinsic proteins - on the surface give mechanical support / act as receptors
Intrinsic proteins - span the width of the membrane, can be protein channels (water-filled tubes allowing water-soluble ions to move) or carrier proteins (bind to ions or molecules and change shape to let them cross)

Question 8

What are the functions of proteins in the cell-surface membrane?

Answer 8

• 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 e.g for hormones

Question 9

Describe the functions of cholesterol within the cell-surface membrane

Answer 9

Embedded in the phospholipid bilayer
- Very hydrophobic so prevents loss of water and dissolved ions from the cell
- Pulls together fatty acid tails of phospholipids, reducing lateral movement without making the whole membrane rigid
- Makes membrane less fluid at higher temperatures

Question 10

How are glycolipids arranged within the cell-surface membrane?

Answer 10

A carbohydrate covalently bonded with a lipid. The carbohydrate portion extends into the external environment where it acts as a cell-surface receptor

Question 11

What are the functions of glycolipids in the cell-surface membrane?

Answer 11

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

Question 12

How are glycoproteins arranged within the cell-surface membrane?

Answer 12

Carbohydrate chains attached to extrinsic proteins on the outer surface. Act as cell-surface receptors for hormones and neurotransmitters

Question 13

What are the functions of glycoproteins in the cell-surface membrane?

Answer 13

• Act as recognition sites
• Help cells attach to one another and form tissues
• Allows cells to recognise one another

Question 14

What are the functions of membranes within cells?

Answer 14

• Control entry + exit of materials in discrete organelles
• Separate organelles from cytoplasm so specific metabolic reactions can occur within them
• Provide an internal transport system e.g endoplasmic reticulum
• Isolate enzymes that may damage the cell e.g lysosomes
• Provide surfaces on which reactions can occur

Question 15

Describe the permeability of the cell-surface membrane

Answer 15

• Molecules not lipid-soluble can’t pass through phospholipid bilayer
• Molecules too large to pass through channels in membrane can’t pass freely
• Molecules of the same charge as the protein channels are repelled
• Polar (electrically charged) molecules have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer

Question 16

Describe the fluid-mosaic model of the cell surface membrane

Answer 16

Fluid - there is constant lateral movement of individual phospholipid molecules because the forces holding them together are weak - membrane constantly changes shape

Mosaic -the molecules embedded in the phospholipid bilayer (proteins, glycolipids, glycoproteins, cholesterol) vary in shape, size and pattern like a mosaic

Question 17

What is simple diffusion?

Answer 17

The net movement of molecules or ions from a region of high concentration to a region of low concentration until an equilibrium is reached

Question 18

Describe simple diffusion

Answer 18

• All particles are moving constantly due to their kinetic energy
• They constantly bounce off each other and other objects
• Particles concentrated in one area of a closed vessel will distribute themselves evenly throughout the vessel

Uses particles’ natural kinetic energy = a passive process

Question 19

What is the rate of diffusion affected by?

Answer 19

• Concentration gradient (larger difference in concentration = faster rate)
• Surface area (increased = faster rate because a greater area for diffusion to occur)
• Length of diffusion pathway (thinner exchange surface = faster rate of diffusion)

Question 20

Give the equation that explains the factors affecting rate of diffusion

Answer 20

Fick’s Law:

Rate of diffusion is proportional to:

(surface area * difference in concentration) / length of diffusion pathway

Question 21

What type of molecules can undergo simple diffusion across membranes?

Answer 21

• Small + non-polar molecules can slip through gaps set up by the movement of phospholipid molecules

*Water is polar but very small so can diffuse directly over the membrane, but very slowly

Question 22

Describe the two ways to classify transport across membranes

Answer 22

Passive - uses the natural (kinetic) energy of particle motion, not an external source

Active - uses energy released from an external source to move molecules/ions e.g the hydrolysis of ATP

Question 23

What is facilitated diffusion?

Answer 23

Transmembrane channels help charged ions and polar molecules move from an area of high concentration to an area of low concentration

Passive process but only occurs at specific points on the plasma membrane where there are carrier or channel proteins

Question 24

Describe the role of protein channels in facilitated diffusion

Answer 24

Form water-filled hydrophilic channels across the membrane. Allow only specific water-soluble ions to pass (only opens in the presence of that ion to control the entry and exit)
Ions bind to the protein, making it change shape in such a way that the ion is released to the other side of the membrane

Question 25

Describe the role of carrier proteins in facilitated diffusion

Answer 25

Span the plasma membrane.
When a large molecule specific to the protein is present, they bind, making the protein change shape and release it to the inside of the membrane
Only uses the kinetic energy of the molecules, not an external source

Question 26

What is osmosis?

Answer 26

The movement of water from a high (less negative) water potential to a region of lower (more negative) water potential through a selectively permeable membrane

Question 27

What is water potential?
What is the water potential of distilled water?

Answer 27

The pressure created by water molecules. Usually measured in kPa and represented by the Greek letter ψ
Under standard temp/pressure (25C, 100kPa), pure water has a ψ of 0.
So, the more solute in a solution, the lower its water potential
ψ of a solution will always be negative

Question 28

How would you find the water potential of a cell or tissue?

Answer 28

• Place them in a series of solutions with different water potentials
• When there is no net gain/loss of mass from the cells, the water potential of the cells must be the same as the external solution

Question 29

Explain osmosis

Answer 29

• All molecules are in random motion due to their kinetic energy
• One solution will have a lower concentration of solute molecules than the other (higher water potential)
• This means there are less barriers between the water molecules and the membrane so they are more likely to cross the membrane
• The selectively permeable plasma membrane only lets water across
• There is a net movement of water molecules down the water potential gradient until a dynamic equilibrium is reached

Question 30

Describe the osmosis in animal cells in a :
- Solution with a higher ψ than the cell
- Solution with an equal ψ to the cell
- Solution with a lower ψ than the cell

Answer 30

• Water enters cell by osmosis, cell swells + bursts
• No net movement of water and no change to the cell
• Water leaves cell by osmosis, cell shrinks and shrivels

Question 31

Describe and explain the osmosis in plant cells in a:
- Solution with a higher ψ than the cell
- Solution with an equal ψ to the cell
- Solution with a lower ψ than the cell

Answer 31

Plant cells can be divided into 3 parts - central vacuole, cell wall, protoplast ( cytoplasm and its contents).
Unlike animal cells, can’t control external composition of fluids

• Higher - water enters cell, protoplast swells + pushes against cell wall, cell is turgid ( cell wall only capable of limited expansion = pressure builds on it that resists further entry of water - cell becomes turgid)
• Equal - no net movement of water, no change to protoplast, cell in incipient plasmolysis
• Lower - water leaves cell, protoplast shrinks + completely pulls away from cell wall, cell is plasmolysed

Question 32

What is active transport?

Answer 32

The movement of molecules or ions into or out of a cell from a region of low concentration to a region of high concentration using carrier proteins and energy released from the hydrolysis of ATP

Question 33

Explain active transport

Answer 33

• The molecule/ion to be transported binds to the specific receptor sites on the carrier protein that spans the plasma membrane
• On the inside of membrane, ATP binds to carrier protein. Carrier protein is linked to ATP hydrolase which hydrolyses ATP into ADP and an inorganic phosphate
• The inorganic phosphate molecule stays bound to the protein, energy is released causing the protein to change shape and release the molecule to the opposite side of membrane
• Phosphate molecule is released from protein - it reverts back to original shape.
• Phosphate molecule recombines with ADP to form ATP in respiration

Question 34

What is ATP used for in active transport?

Answer 34

• Directly moving molecules
• Co-transport - individually moving molecules using a concentration gradient that has been set up by (direct) active transport

Question 35

How does active transport differ from passive transport?

Answer 35

• Requires metabolic energy in the form of ATP
• Substances move against a concentration gradient
• Carrier protein molecules are involved (*not channel proteins)
• Process is very selective - transports specific substances

Question 36

What is the sodium-potassium pump?

Answer 36

The process of moving Na+ and K+ ions across the plasma membrane by active transport
Maintains the large excess of Na+ outside the cell and K+ inside

Question 37

Explain the sodium-potassium pump

Answer 37

• Carrier protein has an affinity for (is attracted to) Na+ ions that bind to it on the inside of the cell
• ATP is hydrolysed, the inorganic phosphate bound to the protein causes it to change shape
• Protein loses its affinity to Na+ and releases them outside
• Protein now has an affinity with K+ which enter
• The inorganic phosphate group unbinds, reverting the protein back to its original shape
• It loses its affinity to K+ which are released to the inside

Question 38

What is bulk transport?
Describe the two types

Answer 38

Occurs when substances larger than single molecules need to enter/leave the cell. Also requires energy released from the hydrolysis of ATP

• Exocytosis - substances leave the cell, fusion of a micelle/golgi vesicle with the cell-surface membrane
• Endocytosis - substances enter the cell, engulfing by the cell-surface membrane to form a vesicle

Question 39

What is digestion?

How is the ileum adapted to increase the rate of absorption of digested products

Answer 39

The hydrolysis of large insoluble molecules into smaller soluble ones

• Intestine walls have villi (1mm projections with blood vessels) to increase surface area
• Each epithelial cell has microvilli (0.6 um, collectively a brush border) to increase surface area
• Epithelial cells have lots of mitochondria to produce ATP for active transport
• Increased density of carrier and channel proteins (more active transport)
• Muscle contraction moves villi in and out of gut lumen, keeping them in contact with a high concentration of digested material

Question 40

How are villi adapted for absorbing substances?

Answer 40

• Large surface area - more carrier proteins for a greater rate of active transport/facilitated diffusion
• Thin wall - molecules travel a shorter distance = greater rate of absorption
• Good supply of blood vessels - takes absorbed molecules away maintaining a steep concentration gradient = greater rate of absorption

Question 41

Describe when diffusion occurs in digestion in the ileum

Answer 41

Diffusion is used at first because there is often a higher concentration of glucose and amino acids in the gut than blood (because carbs + proteins are digested continuously)

Diffusion stops when equilibrium is reached, which would waste 50% of what we eat, so co-transport is used

Question 42

Explain co-transport in the ileum

Answer 42

• Na+ ions actively transported out epithelial cells into blood by Na-K pump. Occurs in one type of carrier protein
• Decreases concentration of Na+ ions inside cell so sets up a conc gradient with the gut lumen
• Na+ ions diffuse into epithelial cells from lumen through a co-transport carrier protein in the cell-surface membrane
• As they diffuse in, they carry an amino acid or glucose molecule with them
• Increases conc of glucose/amino acid inside epithelial cell, which diffuse down their concentration gradient into blood by facilitated diffusion through a carrier protein

Question 43

Describe co-transport’s function in the ileum.
Describe its use of energy

Answer 43

Moves glucose against its concentration gradient from the ileum lumen into the cell, and along its concentration gradient from the cell into the blood

Is an indirect form of active transport because the energy doesn’t come from ATP. The electrochemical gradient of Na+ ions (lots of charge outside cell, very little inside) is an energy source instead.