Chapter 4: Transport Across Cell Membranes

Question 1

Define plasma membrane:

Answer 1

The membrane which surrounds all cells and organelles (as they all have the same basic structure).

When surrounding the cell it is known as the cell surface membrane (boundary between cell cytoplasm and environment).

Question 2

What 5 molecules form the cell membrane structure?

Answer 2

Phospholipids

Proteins

Cholesterol

Glycolipids

Glycoproteins

Question 3

What is a phospholipid?

Answer 3

A water soluble lipid with a phosphate group which forms a bilayer due to the hyrophilic, polar ‘head’ and hydrophobic non polar tail.

Important component of the cell-surface membrane.

Question 4

Define amphipathic molecule:

Answer 4

The compination of hydrophobic and hydrophilic components in a molecule.

Question 5

What are the 3 main functions of the phospholipid bilayer?

Answer 5

Allow lipid-soluble substances in and out.

Prevent water-soluble substances entering or leaving.

Make the membrane flexible and self-sealing.

Question 6

What are the 2 ways which proteins can be found in the CSM?

Answer 6

• Transmembrane protein
• Embedded protein

Question 7

Define transmembrane protein:

Answer 7

A protein spanning the phospholipid bilayer wither found as a protein channel or carrier protein.

Question 8

Define embedded protein:

Answer 8

A protein found on the surface of the bilayer which acts as support or a receptor.

Question 9

What are the 6 main functions of membrane proteins?

Answer 9

• Stuctural support
• Channels for water soluble substances
• Active transport across membrane (carrier proteins)
• Cell-surface receptors to identify cells
• Adherance between cells
• Receptors

Question 10

Where is cholesterol found in the CSM?

Answer 10

Embedded in the hydrophobic area (as they are also hydrophobic).

Question 11

What are the 5 main functions of cholesterol in the CSM?

Answer 11

• Strength
• Prevent water loss
• Prevent soluble ion loss
• Limit movement of fatty acid tails by pulling them together
• Control fluidity

Question 12

What is the structure of a glycolipid?

Answer 12

A carbohydrate covalently bonded with a lipid. They are extensions of phospholipids.

Question 13

What are the main functions of glycolipids?

Answer 13

Cell-surface receptor (e.g. blood groups)

Stability of the membrane

Attatchment to other cells

Question 14

What is the structure of a glycoprotein?

Answer 14

Carbohydrate chains attached to extrinsic proteins on the outer surface of the cell membrane.

Question 15

What are the main functions of glycoproteins?

Answer 15

Cell surface receptors (mainly hormones and neurotransmitters)

Recognition sites for other cells

Attachment sites to form tissues

Question 16

Why don’t most molecules diffuse across the CSM freely?

Answer 16

• Not lipid soluble
• Too large to pass through channels
• Same charge as protein channels
• Polar so cant pass through the non-polar tails

Question 17

Why is the cell-surface membrane described as having a ‘fluid-mosaic model’?

Answer 17

Fluid: molecules can move around each other. Membrane has a flexible structure so is constantly changing shape.

Mosaic: variety of different molecules like the pattern of a mosaic.

Question 18

Define diffusion:

Answer 18

The net movement of ions or molecules from a regin of high concentration to one of lower concentration until evenly distributed.

Question 19

What type of transport can be used to describe diffusion? Why?

Answer 19

Passive

There is no external source of energy, only the inate energy of the system.

Question 20

Fill in the gaps:

All particles are in _____ _____ and this is completely ______ due to the kinetic energy they posess.

As a result, the particles are constantly _______ ___ each other and other objects.

Answer 20

constant motion

random

bouncing off

Question 21

Name 3 factors which affect the rate of diffusion:

Answer 21

Distance of concentration gradient

Surface area

Difference in concentration

Question 22

Give the equation for Ficks Law:

Answer 22

Rate of diffusion ∝ (Surface area x concentration gradient) / Length of diffusion pathway

Question 23

Define facilitated diffusion:

Answer 23

The movement of molecules through transmembrane channels and carriers that span the membrane. The two proteins involved are protein channels and carrier proteins.

Question 24

How does facilitated diffusion occur in protein channels?

Answer 24

Water filled hydrophilic channels are formed across the membrane, allowing specific water soluble ions to pass through. The channels are selective so only change shape for specific ions and molecules. Looser binds are formed than with carrier proteins.

Question 25

How does facilitated diffusion occur in carrier proteins?

Answer 25

Carrier proteins span the plasma membrane, allowing molecules to bind to it, changing its’ shape and relearing the molecule to the other side of the membrane. These bonds are tighter than those in protein channels

Question 26

Define osmosis:

Answer 26

The movement of water from a region of high water potential to one of lower water potential through a selectively permeable membrane.

Question 27

Why is it only water that moves via osmosis?

Answer 27

The selectively permeable membrane only allows water across it, not the solute.

Question 28

Define hypertonic in terms of concentration and water potential:

Answer 28

High concentration outside cell.

Water potential is higher inside the cell.

Question 29

Define isotonic in terms of concentration and water potential:

Answer 29

Concentration and water potential are equal both inside and outside the cell.

Question 30

Define hypotonic in terms of concentration and water potential.

Answer 30

Low concentration outside cell

Water potential is higher outside cell

Question 31

Define water potential:

Answer 31

The pressure exerted by water

Question 32

What is te highest possible water potential value?

Answer 32

0 kPa

Question 33

How are red blood cells impacted by osmosis?

HINT: haemolysis

Answer 33

Red blood cells contain a variety of solutes dissolved in their cytoplasm so they absorb water by osmosis if placed in pure water causing them to burst. This is why red blood cells live in blood plasma.

Also they become shrivelled if placed in a solution with a lower water potential than its own.

Question 34

Define 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 are the 2 main uses of active transport?

Answer 35

To directly move molecules

To move molecules individually using a pre-established concentration gradient (co-transport).

Question 36

How is active transport different from passive transport?

Answer 36

Metabolic energy is needed in the form of ATP

Substances move against (up) a concentration gradient

Carrier protein molecules are used, acting as ‘pumps’. E.g. the sodium-potassium pump

The process is very selective

Question 37

Describe direct active transport:

Answer 37

1) Carrier mpolecules span the plasma membrane and contain binding sites for the molecules or ions being transported.
2) The molecule/ion binds to the receptor site on the protein, chaning the protein shape.
3) ATP binds to the carrier protein from the inside of the cell due to the shape change. This initiates the hydrolysis of ATP which causes the protein molecule to change shape again and open.
4) The molecule/ion is released into the cell due to the shape change.
5) The phosphate molecule is finally released from the carrier protein causing it to resume its initial shape.

Question 38

Name an example of more than one molecule/ion being moved via active transport:

Answer 38

Sodium-potassium pump

Question 39

What are microvilli?

Answer 39

Finger-like projections of the CSM on epithelial cells lining the ileum (part of the small intestine).

Question 40

What is the function of microvilli?

Answer 40

Increase SA for the insertion of protein channels and carrier proteins to increase transport across membranes.

Question 41

How does diffusion aid the absorption of carbohydrates and amino acids?

Answer 41

Generally more of them in the ileum than the bloodstream (due to constant flow of blood to maintain conc. gradient) so the can move through protein channels and carrier proteins via facilitated diffusion.

Question 42

Why is active transport necessary for the absorption of glucose and amino acids?

Answer 42

Diffusion can only equal the concentrations of gluscose and amino acids in the ileum and bloodstream so active transport is neccessary so that everything is absorbed.

Question 43

Describe the absorption of glucose and amino acids via co-transport:

Answer 43

This is an example of the sodium-potassium pump in use:

1) Sodium ions are actively transported out of epithelial cells by a carrier protein molecule, maintaining the concentration gradient of sodium in the epithelial cell. This is where potassium ions enter the cell.
2) There is a higher concentration of sodium ions in the lumen of the intestine than inside the cell so they diffuse down their concentration gradient into the cell through a different type of carrier protein (co-transport protein).
3) As htey diffuse in through the carrier protein, they carry the glucose/AA molecule with them.
4) The glucose/AA molecules pass into the bloodstream via facilitated diffusion.

Question 44

How does each molecule in the co-transport move in realtion to its concentration gradient?

Answer 44

Sodium: down concentration gradient

Glucose/ AA: against concentration gradient (with sodium conc. grad.)

Question 45

What form of transport does glucose move by in c-transport?

Answer 45

Indirect