3.2.3 Transport across cell membranes

Question 1

Plasma membranes

Answer 1

All membranes around and within all cells

Question 2

Phospholipid bilayer

Answer 2

• Hydrophilic phosphate heads: point to outside of the cell membrane on both sides
• Hydrophobic hydrocarbon tails: point into the centre of the cell membrane

Question 3

Functions of phospholipids in the plasma membrane

Answer 3

• Allow lipid-soluble substances to enter + leave the cell
• Prevent water-soluble substances to enter + leave the cell
• Make the membrane flexible and self-sealing

Question 4

Proteins in the plasma membrane

Answer 4

• Some occur on the surface of the bilayer = give mechanical support, or along with glycolipids act as cell receptors
• Some span the bilayer: protein channels, carrier proteins

Question 5

Protein channels

Answer 5

Form water-filled tubes to allow water soluble ions to diffuse across membrane

Question 6

Carrier proteins

Answer 6

Bind to ions or molecules (e.g. glucose, amino acids) then change shape in order to move these molecules across the membrane
* active transport

Question 7

Functions of proteins in the plasma membrane

Answer 7

• structural support
• act as channels to transport water-soluble substances across the membrane
• carrier proteins allow active transport across the membrane
• form cell-surface receptors for identifying cells
• help cells adhere together
• act as receptors (e.g. for hormones)

Question 8

Cholesterol

Answer 8

• very hydrophobic so prevent loss of water + dissolved ions from the cell
• pull together the fatty acid tails of the phospholipid molecules to limit their movement

Question 9

Functions of cholesterol in the plasma membrane

Answer 9

• reduce lateral movement of other molecules (including phospholipids)
• make the membrane less fluid at high temperatures
• prevent leakage of water + dissolved ions from the cell

Question 10

Glycolipids

Answer 10

Carbohydrate covalently bonded with a lipid

Question 11

Glycolipids in the plasma membrane

Answer 11

Carbohydrate portion extends from the phospholipid bilayer into the water environment outside the cell where it acts as a cell-surface receptor for specific chemicals

Question 12

Functions of glycolipids in the plasma membrane

Answer 12

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

Question 13

Glycoproteins

Answer 13

Act as cell-surface receptors, specifically for hormones + neurotransmitters

Question 14

Role of the plasma membrane

Answer 14

To control the movement of substances into + out of the cell

Question 15

Why do most molecules not freely diffuse across the plasma membrane?

Answer 15

The molecules are:
* insoluble in lipids: cannot pass through the phospholipid bilayer
* too large to pass through the protein channels
* of the same charge as the charge on the protein channels - so even if they are small enough, they are repelled
* polar so cannot pass through the non-polar hydrophobic tails in the phospholipid bilayer

Question 16

Fluid-mosaic model of the cell surface membrane

Answer 16

The way in which all the various molecules are combines into the structure of the plasma membrane

Question 17

Why is the fluid mosaic model called the fluid mosaic model?

Answer 17

• Fluid: individual phospholipid molecules can move relative to one another -> makes the membrane flexible (it is constantly changing shape
• Mosaic: the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern

Question 18

Passive processes: explanation for simple diffusion

Answer 18

The energy comes from the natural motion of particles (not from an external source)

• All particles are constantly in random motion due to the K.E. they possess
• particles are constantly bouncing off each other and other objects (e.g. the side of the container)

Question 19

Diffusion definition

Answer 19

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower, until they are evenly distributed

Question 20

How is facilitated diffusion different to simple diffusion?

Answer 20

The movement of polar molecules across the plasma membrane

Only occurs at specific points on the plasma membrane where there are special protein molecules (protein channels and carrier proteins)

Question 21

Mechanism of protein channels for facilitated diffusion

Answer 21

• Hydrophilic channels which allow specific water-soluble ions to pass through
• Selective (only open in the presence of a specific ion)
• The ions bind with the protein causing it to change shape in a way that closes it on one side and opens it on the other

Question 22

Mechanism of carrier proteins

Answer 22

When a molecule that is specific to the protein is present, it binds with the protein which causes it to change shape and release it to the inside of the membrane

• The molecules move from high -> low conv

Question 23

Osmosis definition

Answer 23

The passage of water from a region where it has high water potential to a region where it has lower water potential through a selectively permeable membrane

Question 24

Solute

Answer 24

Any substance that is dissolved in a solvent

Question 25

Water potential of water under standard conditions

Answer 25

0

Question 26

Value of water potential

Answer 26

• Adding pure water increases its water potential
• The value is always less than 0

Question 27

How to find the water potential of cells or tissues

Answer 27

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

Question 28

Why is the environment around a cell in an animal the same water potential as that of the cytoplasm inside the cell?

Answer 28

To avoid bursting of the cell which would release its contents

• in humans, the blood is the same water potential as that of the cell

Question 29

Conditions of cell

Answer 29

• Turgid
• Incipient plasmolysis
• Plasmolysed

Question 30

Turgid

Answer 30

Cell is in higher water potential solution

Question 31

Incipient plasmolysis

Answer 31

equal

Question 32

Plasmolysed

Answer 32

Cell is in lower water potential solution - protoplast is completely pulled away from the cell wall

Question 33

Protoplast

Answer 33

Outer cell-surface membrane

Question 34

Active Transport

Answer 34

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

Question 35

What is ATP used for in active transport?

Answer 35

• Directly move molecules
• Co-transport

Question 36

How does active transport (active process) differ from passive forms of transport?

Answer 36

• Metabolic energy in the form of ATP is needed
• Substances are moved against a concentration gradient
• Carrier proteins are involved, which act as ‘pumps’
• The process is selective

Question 37

Process of direct active transport

Answer 37

• Carrier proteins span the plasma membrane and bind to the molecule or ion to be transported on one side of it
• The molecule or ion binds to receptor sits on the carrier protein
• On the inside of the cell/organelle, ATP binds to the protein, causing it to split into ADP and P1. This causes the protein molecule to change shape and open to the opposite side of the membrane
• The molecule or ion is released to the other side of the membrane
• The P1 is released from the protein -> the protein reverts to its original shape (ready for the process to be repeated)
• The P1 recombines with ADP to form ATP during respiration

Question 38

Difference between facilitated diffusion and active transport

Answer 38

• Facilitated occurs down a concentration gradient, so doesn’t require metabolic energy
• Active transport occurs against a concentration gradient, so requires metabolic energy

Question 39

Sodium-potassium pump

Answer 39

Sodium ions are actively removed from the cell/organelle while potassium ions are actively taken in from the surroundings

Question 40

Role of diffusion in absorption

Answer 40

During digestion, there is a higher concentration of glucose + amino acids in the ileum compared to the blood

So glucose moves into the blood by facilitated diffusion

Question 41

Role of active transport in absorption

Answer 41

With diffusion, glucose + amino acids can only be absorbed until the concentrations are the same on either side of the ileum = not all of the glucose + amino acids

So instead glucose + amino acidsa re drawn into the cells along with sodium ions that have been actively transported by the sodium-potassium pump

Question 42

Process of co-transport

Answer 42

1. Sodium ions are actively transported out of the epithelial cells by the sodium-potassium pump, into the blood. This takes place in 1 type of protein-carrier molecule found in the plasma membrane of the epithelial cells
2. This maintains a higher conc. of sodium ions in the lumen compared to the epithelial cells
3. Sodium ions diffuse into the epithelial cells down a concentration gradient through a different type of protein carrier in the plasma membrane, and as they do this they can either carry amino acids or glucose into the cell within them
4. The glucose/amino acids pass into the lumen by facilitated diffusion using another type of carrier

Question 43

Purpose of co-transport

Answer 43

Indirect form of active transport