Cell Membrane & Transport

Question 1

What is main structure of the cell membrane composed of?

Answer 1

Phospholipid molecules, which are lipid molecules containing a phosphate group

Question 2

How are the phospholipids arranged?

Answer 2

In two layers, known as a phospholipid bilayer, with heads on outside and tails on inside.

Question 3

Describe the structure of a phospholipid molecule

Answer 3

Each phospholipid has a hydrophilic (water-loving) head and a hydrophobic (water-hating) tail

Question 4

Describe the movement of a phospholipid bilayer?

Answer 4

They drift from place to place with their heads and tails moving, keeping the membrane fluid.

Question 5

Why is the membrane known as a fluid?

Answer 5

Because the molecules of which it is made are constantly changing position.

Question 6

Why is the membrane known as a mosaic?

Answer 6

Because it is composed of many different kinds of molecules.

Question 7

What proteins are found in the bilayer?

Answer 7

Cholesterol and protein molecules.

Question 8

Why are cholesterol proteins important?

Answer 8

Are important for the function, integrity and stability of the membrane

Question 9

What protein molecules are found in the membrane?

Answer 9

Receptor proteins, channel proteins, carrier proteins and cell-identity markers

Question 10

How are the protein molecules positioned in the FM model?

Answer 10

Some of these molecules extend from one side of the membrane to the other, while others are bound to the membrane surface.

Question 11

What percentage of the membrane is proteins?

Answer 11

Only about 2%, yet they make up about 55% of the mass of the membrane - because proteins are very large molecules.

Question 12

What are the functions of the cell membrane?

Answer 12

1. Acts as a physical barrier
2. Regulates the passage of materials
3. Is sensitive to changes
4. Helps support the cell

Question 13

How does the cell membrane act as a physical barrier?

Answer 13

Separates cytoplasm from extracellular fluid around cell which is important because their compositions are very different.

Question 14

How does the cell membrane regulate the passage of materials?

Answer 14

Controls movement of materials into and out of cell

E.g the entry of ions and nutrients, the removal of wastes and the release of secretions.

Question 15

Why is the cell membrane sensitive to changes?

Answer 15

It’s first part of cell affected by changes in extracellular fluid. Also has receptors that are sensitive to particular molecules in its immediate environment.

Question 16

How does the cell membrane provide support?

Answer 16

Internal part is attached to microfilaments of cell’s cytoskeleton giving support to whole cell.

Also connections between membranes of adjacent cells, providing support to whole tissue.

Question 17

What are receptor proteins?

Answer 17

CM has limited RPs. Certain molecules outside the cell bind to RP, influencing changes in the cell.

Only one type of receptor protein will bind to one molecule as they are site specific.

The limited number of RPs influences the rate of cellular activity.

Question 18

What are channel proteins?

Answer 18

1. Found in cell membrane + allow substances to pass in/out of cell.
2. Water soluble substances can enter/leave cell through channels if small enough to fit.
3. Large molecules (glucose) can’t pass - facilitated diffusion

Question 19

What are carrier proteins?

Answer 19

Can change shape to bind to molecules (glucose) so they can transported across cell membrane.

Question 20

What are the characteristics of carrier proteins?

Answer 20

• Are specific; will only bind to a particular molecule at a time.
• Carriers can become saturated. Once all available carriers are occupied, any increase in concentration of molecules transported cannot increase rate of movement.
• Carrier activity is regulated by substances such as hormones which are important in coordinating activities of carrier proteins

Question 21

What are cell-identity markers?

Answer 21

Serve as recognition markers for body’s immune system - If immune system recognises these cells they won’t be destroyed.

Question 22

What does the rate of diffusion depend on?

Answer 22

1. Concentration Gradient
2. Size/Nature of Diffusing Particles
3. Distance Diffusion Takes Place
4. Area Diffusion Takes Place

Question 23

How does the concentration gradient affect the rate of diffusion?

Answer 23

The greater difference in concentration between the 2 regions, the greater the rate of diffusion

Question 24

How does the size/nature of diffusing particles affect the rate of diffusion?

Answer 24

Smaller molecules diffuse faster than larger ones.

Question 25

How does distance affect the rate of diffusion?

Answer 25

Short distances between the two regions of different concentrations, the greater the rate of diffusion

Question 26

How does area affect the rate of diffusion?

Answer 26

A larger surface area, the greater the rate of diffusion

Question 27

What is a hypotonic solution?

Answer 27

Water enters, cell swells and may burst

Question 28

What is an isotonic solution?

Answer 28

No net movement of water, cell remains normal size

Question 29

What is a hypertonic solution?

Answer 29

Water leaves, cell shrinks and cremates

Question 30

What types of proteins are found in the CM?

Answer 30

Peripheral and Integral

Question 31

What is a peripheral proteins?

Answer 31

Are attached to one side of the membrane only

Question 32

What is an integral protein?

Answer 32

Located through the membrane.

Question 33

What is the function of integral proteins?

Answer 33

- Responsible for transporting ions/small molecules across the membrane - May be polar in nature or moving against their concentration gradient

Question 34

What is osmosis?

Answer 34

Diffusion of a solvent (water) through a differentially permeable membrane to balance concentration of another substance.

Question 35

How does water move during osmosis?

Answer 35

Water will move from an area of low concentration to an area where the solute is high in concentration. As more water moves into the high concentration, solution will become diluted, lowering concentration. At same time, as water moves out of the area of low concentration the concentration of solution will increase

Question 36

Why does water diffuse this way?

Answer 36

Occurs because, if there are equal volumes in both areas, where there is more solute there will be less solvent - concentration of water is therefore lower.

Question 37

Why can't large molecules directly cross the membrane?

Answer 37

Large polar molecules, such as glucose, and ions, such as sodium ions, are unable to cross the cell membrane directly as they're repelled by hydrophobic tails in phospholipid bilayer.

Question 38

What type of molecule can enter the cell membrane?

Answer 38

Water molecules are small enough to be able to pass through the cell membrane, since they can fit between the lipid tails

Question 39

How else can water cross the membrane?

Answer 39

Water also crosses membrane by passing through protein channels

Question 40

What is simple diffusion?

Answer 40

A passive process resulting from random movement of ions and molecules.

Question 41

What is a concentration gradient?

Answer 41

1. The difference in concentration that brings about diffusion 2. The greater the difference in concentrations, the ‘steeper’ the diffusion gradient and the faster the rate of diffusion

Question 42

What is net diffusion?

Answer 42

Movement of liquid or gas molecules from places of higher concentration to places of lower concentration, along a concentration gradient.

Question 43

Why does net diffusion happen?

Answer 43

Because there will also be some molecules moving against concentration gradient in opposite direction. Difference in concentrations = more particles moving from the area of high concentration to the area of low concentration. The same concentrations = same number of particles moving in each direction.

Question 44

What substances can enter through simple diffusion?

Answer 44

- Alcohol, steroids and other fat-soluble substances. - Oxygen and carbon dioxide

Question 45

Why does oxygen diffuse into the cell?

Answer 45

Because it's continually used up inside the cell for respiration. The concentration of oxygen inside the cell is therefore lower than the oxygen concentration outside the cell - Concentration difference = net diffusion of oxygen into cell

Question 46

Why does carbon dioxide diffuse into the cell?

Answer 46

It is continually produced inside the cell by respiration. Higher concentration of carbon dioxide inside the cell means there will be net diffusion of carbon dioxide out of the cell.

Question 47

What type of substances are unable to pass through the lipid portion of the membrane?

Answer 47

Water-soluble substances are unable to pass directly through the lipid portion of the membrane and hence require other modes of transport

Question 48

What is osmotic pressure?

Answer 48

The pressure needed to keep it in equilibrium with pure water. High solutes in solution = higher osmotic pressure

Question 49

What is facilitated diffusion?

Answer 49

A passive process where substances are transported through a protein along the concentration gradient

Question 50

What happens to glucose during carrier-mediated facilitated diffusion?

Answer 50

The glucose attaches to a binding site on the specific carrier protein. The protein changes shape and the molecule is released on the other side of the membrane.

Question 51

What proteins in facilitated diffusion allow transportation?

Answer 51

1. Channel proteins - form protein channels 2. Carrier proteins - allow carrier-mediated transport.

Question 52

What is the function of channel proteins in the cell membrane during facilitated diffusion?

Answer 52

These channels provide a pathway for the hydrophilic particles to cross the cell membrane without coming in contact with the hydrophobic inner portion.

Question 53

How are protein channels structured for their function?

Answer 53

Protein channels are very small in diameter, but water and ions can easily get through - larger molecules are too big to fit through the channels.

Question 54

What is active transport?

Answer 54

The use of energy to move substances, usually ions, across a cell membrane against the concentration gradient - Doesn't depend on concentration gradient

Question 55

Why does active transport require energy?

Answer 55

Requires energy in form of ATP because substances are transported across the membrane against the concentration gradient, from lower to higher concentration

Question 56

How can cells use active transport?

Answer 56

Using active transport, a cell can take in or pass out substances regardless of their concentrations inside or outside the cell.

Question 57

Why type of molecules can pass through the bilayer?

Answer 57

The lipid bilayer is permeable to a few small uncharged molecules such as oxygen and carbon dioxide

Question 58

What is vesicular transport?

Answer 58

The movement of substances across cell membrane in membranous sacs called vesicles.

Question 59

What type of process is vesicular transport?

Answer 59

An active process, because energy from the cell is needed to form the vesicles.

Question 60

What is endocytosis?

Answer 60

Taking liquid or solids into cell by vesicular transport

Question 61

Describe the process of endocytosis

Answer 61

1. Cell membrane folds around a droplet of liquid or a solid particle until droplet/particle is completely enclosed. 2. Vesicle formed then pinches off and is suspended in the cell’s cytoplasm.

Question 62

What is the difference between pinocytosis and phagocytosis?

Answer 62

1. Cells enfold/take in drops of liquid = pinocytosis; 2. Cells surround/take in solid particles = phagocytosis.

Question 63

What is exocytosis?

Answer 63

When contents of a vesicle inside cell are passed to outside.

Question 64

Describe the process of exocytosis

Answer 64

1. A vesicle formed inside cell migrates to cell membrane and fuses with membrane. 2. The contents of vesicle are then pushed out into extracellular fluid.