Section 2 - Cells: 4. Transport across cell membranes

Question 1

What is a plasma membrane

Answer 1

All membranes around and within all cells, with the same basic structure, such as the cell surface membrane

Question 2

What is the function of membranes within cells

Answer 2

• Control the entry and exit of materials in discrete organelles (eg. mitochondria)
• Separate organelles from the cytoplasm, so reaction can occur within them
• Provide internal transport system (eg. endoplasmic reticulum)
• Isolate enzymes that might damage cells (eg. lysosomes)
• Provide surfaces on which reactions can occur (eg. Protein synthesis in ribosomes on RER)

Question 3

What are the basic structure and components of the cell surface membrane

Answer 3

Phospholipid bilayer containing:
- Proteins
- Cholesterol
- Glycolipids
- Glycoproteins

Question 4

Why is the phospholipid bilayer key for the function of the cell surface membrane

Answer 4

• Hydrophobic tails on the inside of the layer allows lipid soluble substances to enter and leave the cell
• Prevents water soluble substances from entering or leaving the cell
• Lack of bonds between the lipids (only attraction) makes the membrane flexible and self-sealing

Question 5

What is the function of the proteins in the cell surface membrane

Answer 5

• Provide structural support
• Acts as channels transporting water soluble substances through the membrane
• Allows active transport through carrier proteins
• Form cell-surface receptors of identifying cells
• Help cells stick together
• Act as receptors (eg. hormones)

Question 6

What is the difference between intrinsic and extrinsic proteins in the cell membrane

Answer 6

• Intrinsic (integral) = embedded in the membrane, passing all the way through
• Extrinsic (peripheral) = on the outer or inner surface of the membrane, not passing all the way through

Question 7

What are the two types of transport proteins in the cell membrane

Answer 7

• Channel proteins: Facilitated diffusion and active transport
• Carrier proteins: Active transport

Question 8

What is the function of cholesterol in cell surface membranes

Answer 8

• Add strength to the membrane
• Hydrophobic, so help to prevent water leakage and the loss of dissolved ions
• Reduce lateral movement of other molecules such as phospholipids
• Make the membrane less fluid at high temperatures

Question 9

What is the structure of a glycolipid

Answer 9

Carbohydrate covalently bonded to a lipid

Question 10

What is the function of glycolipids in the cell surface membrane

Answer 10

• Act as recognition site (carbohydrate portion extends from the phospholipid bilayer)
• Help to maintain membrane stability
• Can attach to one another so form tissues

Question 11

What is the structure of a glycoprotein

Answer 11

A carbohydrate attached to the extrinsic proteins on the outer surface of the cell membrane

Question 12

What is the function of glycoproteins in the cell surface membrane

Answer 12

• Act as recognition site
• Allows cells to recognise one another (eg. lymphocytes can recognise self cells)
• Can attach to one another so allow the formation of tissues

Question 13

What type of molecules can’t pass through the cell surface membrane by diffusion alone

Answer 13

• Not lipid-soluble
• Too large
• Of the same charge as the protein channels (repelled)
• Polar: can’t pass through the non polar tails of the phospholipid bilayer

Question 14

Why is the arrangement of the cell surface membrane known as the fluid-mosaic model

Answer 14

• Fluid: Individual phospholipid molecules can move relative to each other, so the membrane is flexible
• Mosaic: proteins within the membrane vary in shape, size and pattern, so look like a mosaic

Question 15

What is the difference between passive and active transport

Answer 15

Passive transport occurs without the need for metabolic energy, whereas active transport does require this external energy input.

Question 16

What is diffusion

Answer 16

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower, down the concentration gradient, until an equilibrium is reached

Question 17

Why does diffusion naturally occur

Answer 17

• All particles are constantly in motion due to their kinetic energy
• The motion is random with no set patterns
• The particles are constantly bouncing off each other and the walls of the container
• Diffusion will occur both ways as the particles naturally move, but the net movement is larger down the conc. gradient, as there are more particles coming from one side, until an equilibrium is reached

Question 18

What is the difference between simple and facilitated diffusion

Answer 18

• Simple diffusion = small, non-polar molecules through the phospholipid bilayer without the need for transport proteins
• Facilitated diffusion = larger, charged ions or polar molecules with the use of channel proteins or carrier proteins

Question 19

How are channel proteins used for facilitated diffusion

Answer 19

Water-filled hydrophilic channels that allows certain ions to pass through the membrane.

Question 20

How are carrier proteins used for facilitated diffusion

Answer 20

Change shape when molecules bind to them, opening on one side and closing on the other, allowing certain specific molecules to pass through

Question 21

What is the difference between channel proteins and carrier proteins

Answer 21

Channel:
- Fixed position
- high transport rate
- Movement of water soluble ions

Carrier:
- Each side opens/closes when molecule binds to it
- Low transport rate
- Movement of water soluble and insoluble molecules

Question 22

What is osmosis

Answer 22

The passage of water from a region with a higher water potential to a region with lower water potential, through a selectively permeable membrane

Question 23

What is water potential

Answer 23

Pressure created by water molecules (KPa)
Unit = Greek letter psi: Ψ

Question 24

What is the water potential of pure water in standard conditions (25°C and 100KPa)

Answer 24

0Ψ

Question 25

How does the addition of a solute effect the water potential of a solution

Answer 25

Lowers it (more negative)

Question 26

How would you work out the water potential inside a cell or tissue

Answer 26

Place them in a series of solutions with different water potentials.
Plot a graph of change in mass against the concentration, and where there is no net water gain/loss, the water potential of the solution is the same as inside the cells

Question 27

What is a hypotonic solution

Answer 27

Less concentrated solution with less solutes (higher Ψ) in comparison to other solution

Question 28

What is an isotonic solution

Answer 28

A solution with the same solute concentration (and therefore Ψ) than another solution

Question 29

What is a hypertonic solution

Answer 29

More concentrated solution with more solutes (less Ψ) in comparison to other solution

Question 30

What happens to animal cells when placed in solutions with different water potentials

Answer 30

• Lower Ψ inside the cell - in hypotonic solution: cell swells and bursts (as no cell wall)
• Equal Ψ - isotonic: no change
• Higher Ψ inside the cell - in hypertonic solution: cell shrinks and shrivels

Question 31

What happens to plant cells when placed in solutions with different water potentials

Answer 31

• Lower Ψ inside the cell - in hypotonic solution: cell swells and becomes turgid (supported by cell wall)
• Equal Ψ - isotonic: no change (incipient plasmolysis occurs - about to be plasmolysed as protoplast begins to pull away from the cell wall)
• Higher Ψ inside the cell - in hypertonic solution: plasmolysis occurs, as the protoplast (inside cell wall) pulls away from the cell wall

Question 32

What is Active transport

Answer 32

The movement of molecules or ions into or out of a cell from a region of lower concentration, against the concentration gradient, using ATP and carrier proteins

Question 33

Describe the process of direct active transport

Answer 33

• Molecule or ion to be transported binds to receptor sites on the carrier protein
• On the inside of the Cell, ATP binds to the carrier protein causing it to spilt into ADP, with a phosphate molecule remaining attached to the protein causing it to change shape.
• The protein opens to the opposite side of the membrane, so the molecule or ion is released to the area of higher concentration (against the conc. gradient)
• The phosphate molecule is release from the protein, so it returns to it’s original shape

Question 34

What is a sodium-potassium pump

Answer 34

Sodium is actively removed from the cell while potassium is actively and simultaneously taken in from the surroundings

Question 35

Why is a sodium-potassium pump required for co-transport

Answer 35

Sodium ion can leave without affecting Ψ, as they are placed with potassium, giving a constantly low sodium conc. in the epithelial cells, so they can enter from the ileum, carrying the glucose

Question 36

How is the rate of movement across membranes increased

Answer 36

• Increase the number of channel and carrier proteins in a given area of membrane
• Increase the Surface area, so more transport proteins can be present (eg. microvilli lining the ileum)

Question 37

What is the ileum

Answer 37

The last and largest section of the small intestine, where molecules from digestion such as glucose and amino acids are absorbed from, into the bloodstream

Question 38

What is co-transport

Answer 38

The coupled movement of substances across a membrane via carried proteins, through active transport and facilitated diffusion

Question 39

Describe the process of co-transport

Answer 39

• Sodium ions are actively transported out of the epithelial cells, replaced by potassium as to not effect Ψ (sodium-potassium pump)
• This means there is a lower conc. of sodium ions in the epithelial cells than in the ileum
• Sodium ions then enter the epithelial cells from the ileum by facilitated diffusion, carrying either a glucose molecule or an amino acid out if the ileum with them
• The Glucose/amino acids in the epithelial cells can enter the blood stream by facilitated diffusion

Question 40

Why is oral rehydration therapy needed to treat diarrhoeal diseases (eg. cholera)

Answer 40

Diarrhoeal diseases result in insufficient fluid intake and/or excessive fluid loss, causing dehydration.

Just drinking more water won’t rehydrate the cells, as the disease is cause by damage to epithelial cells/loss of microvilli due to toxins, so the added water won’t be absorbed.
Water will also not replace the lost electrolytes

Question 41

How does oral rehydration therapy work

Answer 41

A rehydration solution is taken and is absorbed by alternate pathways and different types of carrier proteins
(eg. As sodium is absorbed, it lowers the Ψ of the cell, drawing in more water by osmosis)

Question 42

What should a rehydration solution contain

Answer 42

• Water: to rehydrate tissue
• Sodium ions: to replace lost ions and to lower the Ψ of cells, so water enters by osmosis
• Glucose: to stimulate the intake of sodium ions and provide energy
• Potassium ions: to replace lost ions and stimulate appetite
• Other electrolytes (eg. Chloride and citrate ions): to replace lost ions and prevent and electrolyte imbalance leading to metabolic acidosis where the blood pH increases