4.0 Transport Across Cell Membranes

Question 1

What is a plasma membrane?

Answer 1

The name given to all membranes in and around all cells and membrane bound organelles. They all have the same basic structure.

Question 2

What is the structure and function of the plasma membrane?

Answer 2

Phospholipid bilayer, cholesterol, proteins, glycoproteins, glycolipids. Separates cell contents from external environment. Controls what enters and leaves the cell. Identifies cell as self. Acts as a receptor for various chemicals. site of chemical reactions.

Question 3

What is the cell-surface membrane?

Answer 3

The name given specifically to the plasma membrane that surounds cells and forms the boundary between the cytoplasm and the environment.

Question 4

How do phospholipids arrange themselves to form the bulk of the plasma membrane?

Answer 4

The hydrophillic heads point out and are attracted to water on both sides of the membrane. The hydrophobic tails point into the membrane, repelled by water on both sides.

Question 5

What is the role of the phospholipid bilayer in the plasma membrane?

Answer 5

Lipid soluble material is able to move into and out of the cell. Water soluble material is unable to move in or out of the cell. The membrane is flexible and self healing.

Question 6

How are proteins embedded in the plasma membrane?

Answer 6

Either they occur on the surface only, and don’t cross the membrane, or completely span the bilayer from one side to the other.

Question 7

What is the role of proteins which are embedded on the surface of a plasma membrane?

Answer 7

They either give mechanical support, or work as cell receptors for molecules such as hormones.

Question 8

What is the role of proteins which span the width of the plasma membrane?

Answer 8

Some are protein channels, water filled tubes allowing water soluble ions to diffuse across the membrane. Others are carrier proteins that bind to molecules and change chape to move them across the membrane.

Question 9

What are the overall roles of proteins in the membrane?

Answer 9

1- Structural support;
2- Channels for water soluble substances;
3- allow active transport;
4- cell-surface receptors for identifying cells;
5- cell adhesion;
6- act as receptors for e.g. for hormones.

Question 10

How does cholesterol interact with water?

Answer 10

Very hydrophobic

Question 11

What does cholesterol do to other molecules in the membrane?

Answer 11

They hold phospholipids tightly together. They prevent water loss.

Question 12

What is the role of cholesterol in the membrane?

Answer 12

Reduce lateral movement of other molecules (esp phospholipids). Make the membranes less fluid at high temperatures. Prevent leakage of water and dissolved ions.

Question 13

What is the difference between passive and active transport?

Answer 13

Passive uses only the energy of the motion of particles. Active transport uses external energy source, such as ATP.

Question 14

How can the movement of particles be described?

Answer 14

1 - Particles are constantly in motion due to kinetic energy.
2 - Particle motion is random.
3 - Particles are constantly bouncing of each other and their surroundings.

Question 15

What is the definition of diffusion?

Answer 15

The net movement of molecules or ions fomr a region where they are highly concentrated to one where their concentration is lower unil evenly distributed.

Question 16

What kind of molecules can diffuse through the cell-surface membrane? Give examples.

Answer 16

Small, non-polar molecules. E.g. oxygen and carbon dioxide.

Question 17

What does polar mean?

Answer 17

The molecule has a positive charge on one side and a negative charge on the other

Question 18

Which molecules do not diffuse easily across the plasma membranes. Why?

Answer 18

Charged ions and polar molecules. Due to the hydrophobic nature of the fatty-acid tails of the phospholipids in the membrane.

Question 19

Define the term facilitated diffusion

Answer 19

Movement of molecules from a high concentration to a low concentration, across a partially permeable membrane, via specific channel or carrier proteins. It is passive, does NOT require ATP

Question 20

Where does the energy for facilitated diffusion come from?

Answer 20

The inbuilt kinetic energy of the molecules.

Question 21

What is a channel protein?

Answer 21

A protein which creates a water filled hydrophillic channel through the cell membrane through which ions can pass. It is selective to a specific ion.

Question 22

What is a carrier protein?

Answer 22

A protein which changes shape to allow larger molecules to pass through the membrane. In facilitated diffusion this requires no energy, in active transport it requires ATP.

Question 23

State 3 factors that affect the rate of diffusion

Answer 23

Surface area, number of channel or carrier proteins, concentration gradient

Question 24

What is the definition of osmosis?

Answer 24

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

Question 25

What is meant by water potential?

Answer 25

The pressure applied by water molecules.

Question 26

What substance has the highest possible water potential of 0 kPa?

Answer 26

Pure water

Question 27

As more solute is added to a solution, what happens to the water potential?

Answer 27

The water potential decreases. It becomes more negative.

Question 28

Where does the water move from and to?

Answer 28

Water moves from an area of higher water potential to a lower water potential. From less negative to more negative.

Question 29

What symbol is used to represent water potential?

Answer 29

Ψ

Question 30

How can the water potential of a cell or tissue be determined?

Answer 30

Place the substance in various solutions of different water potentials. Where there is no net change of mass, (no gain or loss of water), the water potential must be the same inside the cell/tissue as in the external solution.

Question 31

Where does the energy for osmosis come from?

Answer 31

The molecules’ kinetic energy.

Question 32

Why must there be a selectively permeable membrane involved in osmosis?

Answer 32

This only allows water molecules to move across, not the solute molecules.

Question 33

Describe what would happen to a red blood cell placed in a solution with a more negative water potential than that of its cytoplasm

Answer 33

Water would move OUT, down a WP gradient, shrinking and shrivelling (also known as crenation)

Question 34

Describe what would happen to a liver cell placed in a solution with a more positive WP than its own cytoplasm

Answer 34

Water would move IN, down a WP gradient, swelling the cell and causing it to burst (also known as crenation) due to the presence of no cell wall

Question 35

Describe what would happen to a root hair cell placed in a solution with lower WP than its own cytoplasm/vacuole

Answer 35

Water would move OUT, down a WP gradient, causing the cell to become flaccid and then plasmolysed

Question 36

Describe what would happen to a guard cell placed in a solution of less negative WP than its own cytoplasm

Answer 36

Water would move IN, down a WP gradient, causing the cell to swell and become turgid.

Question 37

If the water potential is equal in and out of the cell, what would happen?

Answer 37

Water would move IN, down a WP gradient, causing the cell to swell and become turgid.

Question 38

What is the protoplast?

Answer 38

The cell surface membrane, nucleus, cytoplasm and vacuole membrane.

Question 39

What does turgid mean?

Answer 39

This is when the protoplast pushes against the cell wall.

Question 40

What does plasmolysed mean?

Answer 40

This is when water has left the cell, and it’s volume therefore decreases. This means the protoplast no longer pushes against the celluluose cell wall.

Question 41

What does incipient plasmolysis mean?

Answer 41

Any further loss of water would lead to plasmolysis.

Question 42

What is the definition of active transport?

Answer 42

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 43

How is active transport different to passive transport?

Answer 43

Metabolic energy is required (ATP). Substances move against the concentration gradient. Carrier proteins are always involved. The process is very selective.

Question 44

What are the two methods of active transport?

Answer 44

It can directly move molecules. It can also be involved in co-transport.

Question 45

What are the stages in direct active transport?

Answer 45

1 - Carrier proteins span the membrane, molecule/ion to be transported binds.
2 - substance moves from low to high concentration. 3 - ATP binds to protein inside the cell, causing it to change shape and open on opposite side (ATP -> ADP)
4 - substance is released on other side of membrane.
5 - Phosphate molecule released from protein and carrier returns to original shape.

Question 46

What is the similarity between facilitated diffusion and active transport?

Answer 46

They both use carrier proteins.

Question 47

What are the differences between facilitated diffusion and active transport.

Answer 47

FD occurs down a concentration gradient, AT occurs against a concentration gradient.
FD is passive, AT uses metabolic energy in the form of ATP.

Question 48

Sometimes 2 molecules are moved at once by active transport, one in and one out of the cell, give an example of this

Answer 48

The sodium-pottasium pump. Sodium ions are actively removed from the cell, whilst pottasium ions are actively taken in from the surroundings.

Question 49

What is the ileum?

Answer 49

The small intestine.

Question 50

The epithelial cells of the ileum possess microvilli, how are these colectively known?

Answer 50

As a “brush border”.

Question 51

What is the purpose of the microvilli in the ileum?

Answer 51

To increase surface area to aid absorption.

Question 52

Why is there typically a higher concentration of glucose and amino acids in the ileum compared to the blood?

Answer 52

1 - carbohydrates and proteins are constantly being broken down.
2 - the glucose and amino acids being absorbed are constantly being removed as the blood flows away from the ileum.
3 - glucose in the blood is constantly being used in respiration.

Question 53

Why is active transport also required alongside diffusion, to absorb nutrients from the ileum?

Answer 53

Diffusion only results in concentrations on both sides becoming equal, this will lead to some loss of glucose and amino acids from the body.

Question 54

What is the process by which amino acids and glucose are absorbed from the ileum?

Answer 54

Co-transport

Question 55

What is the basic principle of co- transport of glucose or amino acids from the ileum?

Answer 55

The molecules are taken into the cells along with sodium ions which have been actively transportedd out by the sodium potassium pump.

Question 56

What are the stages involvlved in the cotransport of glucose from the ileum?

Answer 56

1 - Sodium ions are actively pumped out of the epithelial cells via a sodium-potassium pump, into the blood.
2 - This creates a low concentration of sodium inside the cell compare to the lumen of the ileum.
3 - Sodium ions diffuse into the epithelial cells through a carrier protein. as they dothey carry glucose (or amino acid) molecules with them.
4 - This increases the concentration of glucose in the epithelial cell, so it moves via facilitated diffusion into the blood plasma.

Question 57

Sodium ions and glucose/amino acid molecules move from the ileum to the epithelial cells together, in which direction compared to their concentrations?

Answer 57

Sodium moves down the concentration gradient. Glucose/amino acids move up the concentration gradient.

Question 58

What is powering the movement of glucose/amino acid molecules into the epithelial cells from the ileum?

Answer 58

The sodium ion concentration gradient. (Sodium ions diffuse across the membrane, taking with them the other molecule). this is indirect active transport.