Chapter 4

Question 1

Why are phospholipids important components of the cell surface membrane?

Answer 1

The hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane and thus are attracted to water on both sides
The hydrophobic tails of both phospholipids layers point into the centre of the cell membrane repelled by water on both sides

Question 2

What are the functions of phospholipids in the cell surface membrane?

Answer 2

1. Alllows lipid-soluble substances to enter and leave the cell
2. Prevents water soluble substances entering and leaving the cell.
3. Makes the membrane flexible and self sealing

Question 3

What three roles do proteins have in the cell surface membrane?

Answer 3

1. Give mechanical support to the membrane
2. Act as cell receptors for molecules in conjunction with glycolipids
3. Helps cells adhere together

Question 4

What functions do proteins that completely span the surface of the cell surface membrane hold?

Answer 4

1. Protein channels form water filled tubes allowing water soluble ions to diffuse across the membrane
2. Carrier proteins bind to ions or molecules then change shape in order to transport them across the membrane

Question 5

What is the role of glycolipids in the cell surface membrane?

Answer 5

• Acts as a cell surface receptor/recognition site for chemicals.
• Helps maintain the stability of the membrane
• Helps cells attatch to each other allowing them to form tissues

Question 6

What is the function of glycoproteins in the celll surface membrane?

Answer 6

• Act as recognition sites
• Helps cells attatch to each other to form tissues
• Allows cells to recognise each other e.g. lymphocytes

Question 7

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

Answer 7

• They are not lipid soluble so cannot pass through the phopholipid layer
• They are too large to pass through the protein channel
• They are of the same charge of the proteins channels so are repelled
• They are electrically charged/polar so do not readily pass through the non polar hydrophobic tails in the phospholipid bilayer

Question 8

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

Answer 8

• Fluid as the individual phospholipid moelcules can move relative to one another giving the membrane flexibitly
• Mosaic as the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern

Question 9

Why do particles distribute themselves evenly through a closed vessel?

Answer 9

Constantly in motion due to the kinetic energy they have
The motion is random
Particles are constantly being bounced of each other as well as other objects

Question 10

Define diffusion?

Answer 10

The net movement of particles or ions from a region of high concentration to a region of lower concentration until they are evenly distributed.

Question 11

What is facilitated diffusion?

Answer 11

The movement of charged ions and polar molecules which do not easily diffuse by transmembrane carriers and channels

Question 12

Why is facilitated diffusion a passive process?

Answer 12

It relies on the kinetic energy of the diffusing molecules. There is no external input of ATP from respiration. Occurs down a concentration gradient.

Question 13

Describe a protein channel?

Answer 13

Water filled hydrophobic channels that span the cell membrane, selectively allowing specific water soluble ions to pass through. The ions bind with the protein causing it to change shape so that it closes on one side of membrane and opens it the other side.

Question 14

Describe a carrier protein?

Answer 14

When a molecule that is specific to the protein it binds causing the molecule to be released to the inside of the membrane

Question 15

Define osmosis?

Answer 15

The passage of water from an a region where it has higher water potential to a region where it has lower water potential through a partially permeable membrane

Question 16

What is water potential?

Answer 16

Represented by psi and is measured in units of pressure (kPa).
The pressure created by water molecules.

Question 17

Describe how you would find the water potential of cells or tissues?

Answer 17

Place them in a series of solutions with different water potentials. Where there is no net loss or gain of water from the cells or tissues, the water potential must be the same as that of the external solution.

Question 18

Why would a cell become turgid?

Answer 18

The water potential of the external solution is higher and so water eneters the cell. The protoplast therefore swells

Question 19

Why would a cells condition be incipient plasmolysis?

Answer 19

When the external water potential of the cell is equal meaning water neither enters nor leaves

Question 20

Why would a cell become plasmolysed?

Answer 20

The external water potenial of the cell is lower meaning water leaves the cell, shrinking the protoplast.

Question 21

Define active transport?

Answer 21

The movement of molecules or ions inot or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.

Question 22

How is ATP used in active transport?

Answer 22

To directly move molecules
Individually move molecules using a concentration gradient which as already been set up by direct active transport (co-transport)

Question 23

How does active transport differ from passive transport?

Answer 23

Metabolic energy in the form of ATP is needed.
Substances are moved against a concentration gradient
Carrier protein molecules acting as pumps are involved.
The process is selective with only specific molecules being transported.

Question 24

How does direct active transport take place?

Answer 24

The carrier proteins spans the plasma membrane and binds to the molecule or ion needing to be transported to the other side, the molecule binds to a receptor site on the proteins carrier.
On the inside of the cell, ATP binds to the protein casuing it to split into ADP + Pi, this causes the protein molecule to change shape and open the opposite side of the membrane, the molecule is then released the other side of the membrane.
The phosphate molecule is released from the protein which causes the protein to revert to it’s original shape, the Pi molecule the recombines with ADP to form ATP during respiration.

Question 25

Dscribe the sodium-potassium pump?

Answer 25

Sodium ions are activey removed from the cell while potassium ions are actively taken in from the surroundings.