3: Movement of Substances Across Cell Membrane

Question 1

Which model is used to illustrate the structure of the cell membrane?

Answer 1

The fluid mosaic model, which was proposed by Singer and Nicolson in 1972.

Question 2

What is the fluid mosaic model?

Answer 2

The fluid mosaic model is a model illustrating the structure of the cell membrane. It suggests that the cell membrane is mainly made up of phospholipids and proteins. The phospholipid molecules are arranged in a bilayer. The protein molecules are interspersed among the phospholipid molecules.

Question 3

Why is the cell membrane described as fluid and mosaic?

Answer 3

The cell membrane is described fluid because the phospholipid molecules can move laterally in the cell membrane. It is described as mosaic because the protein molecules are interspersed among the phospholipid molecules in a mosaic pattern.

Question 4

What is the difference between phospholipid and triglycerides?

Answer 4

Triglycerides are the most common type of lipids. A triglyceride molecule is formed from one glycerol molecule and three fatty acid molecules. Phospholipids are another type of lipid, whose structure is similar to that of triglycerides except that one of the fatty acid molecules is replaced by a phosphate group.

Question 5

What are the chemical features of the heads and tails of phospholipids?

Answer 5

The phosphate group is polar, carrying a charge. It forms the hydrophilic “head” of the phospholipid molecule. It is attracted to water molecules. On the other hand, the fatty acids are non-polar. They form the hydrophobic “tails” of the phospholipid molecule. They are repelled by water molecules.

Question 6

Why do phospholipids form a bilayer?

Answer 6

Due to the chemical nature that a phospholipid molecule has a hydrophilic head and hydrophobic tail, they arrange themselves in a bilayer. Their hydrophobic tails point inwards while their hydrophilic heads face the aqueous environment inside and outside the cell membrane.

Question 7

How are protein molecules interspersed in the phospholipid bilayer?

Answer 7

Some proteins are attached to the surface of the phospholipid bilayer, some are embedded half-way in the bilayer, and other span the entire bilayer. Carbohydrates are attached to some of these proteins to form glycoproteins.

Question 8

List five examples of membrane protein classifications according to their functions.

Answer 8

channel proteins, carrier proteins, antigens, receptors, and enzymes

Question 9

What are the functions of channel proteins?

Answer 9

Channel proteins provide channels across the membrane for transporting certain ions and small, polar molecules.

Question 10

What are the functions of carrier proteins?

Answer 10

Carrier proteins are involved in active transport to carry substances across the membrane.

Question 11

What are the functions of antigens?

Answer 11

Antigens are glycoproteins for cell recognition. For example, some white blood cells recognise antigens on our body as “self” and do not attack. Antigens on bacteria are recognised as “foreign” and they are attacked.

Question 12

What are the functions of receptors?

Answer 12

Receptors are glycoproteins for receiving chemical messengers (eg. hormones) outside the cell. These chemical messengers ay then turn on certain activities in the cell.

Question 13

What are the functions of enzymes?

Answer 13

They speed up (catalyse) reactions in the cell.

Question 14

What are the features of the structure of the cell membrane?

Answer 14

It is differentially permeable, flexible, and strong enough to support the cell.

Question 15

How is the cell membrane being differentially permeable related to its properties and functions?

Answer 15

The phospholipid layer is permeable to non-polar substances. Small, non-polar molecules (eg. oxygen and carbon dioxide) move across the cell membrane through the bilayer.
The phospholipid bilayer is impermeable to polar substances and ions. Small, polar molecules (eg. water and glucose) and certain ions move across the cell membrane through channel proteins or carrier proteins.
Molecules that are larger than the pores in the phospholipid bilayer and channel proteins, and do not fit the shape of the carrier proteins cannot move across the cell membrane.

Question 16

How is the cell membrane being flexible related to its properties and functions?

Answer 16

Since the phospholipid molecules in the cell membrane can move laterally, the cell membrane is fluid in nature and is flexible. This property allows the cell membrane to change shape and fuse during processes like phagocytosis and cell division.

Question 17

Why is the cell membrane strong enough to support the cell?

Answer 17

The protein molecules interspersed among the phospholipid molecules give strength to the cell membrane. This makes the cell membrane strong enough to support the cell contents.

Question 18

How do substances move across membranes?

Answer 18

Substances move across membranes in four main ways: diffusion, osmosis, active transport and phagocytosis.

Question 19

What is diffusion?

Answer 19

Diffusion is the process of particles in liquids or gases distributing evenly.
Particles in liquids and gases can move about freely. They collide with each other and move randomly in all directions.
Particles in liquids or gases tend to distribute evenly. If there is a difference in the concentration of particles between two regions (a concentration gradient exists), there will be a net movement of particles from a region of higher concentration gradient to a region of lower concentration (down the concentration gradient) until the particles become evenly distributed.

Question 20

Is diffusion a passive or active process?

Answer 20

Diffusion is a passive process, which does not require energy.

Question 21

What are the factors affecting the rate of diffusion?

Answer 21

1. Concentration gradient: the greater the difference in concentration that exists between two regions/the steeper the concentration gradient, the higher the rate of diffusion.
2. Temperature: At higher temperatures, particles have more kinetic energy and collide with each other more frequently. This results in a higher rate of diffusion.
3. Size of particles: Small particles diffuse faster than large particles.
4. Distance: the shorter the distance between regions of difference concentrations, the higher the rate of diffusion.
5. Surface area: for the diffusion of substances through a membrane, the larger the surface area of the membrane, the higher the rate of diffusion.

Question 22

What is the importance of diffusion?

Answer 22

Diffusion enables cells to exchange materials with the environment. Cells obstinate useful materials like oxygen and nutrients, and remove waste like carbon dioxide by diffusion. Substances in cells are also distributed throughout the cytoplasm by diffusion.

Question 23

What is osmosis?

Answer 23

Osmosis is a special kind of diffusion which involves the movement of water molecules across a differentially permeable membrane. When two solutions of different concentrations are separated by a differentially permeable membrane, there will be a net movement of water molecules from the less concentrated solution to the more concentrated solution.

Question 24

Is osmosis an active or passive process?

Answer 24

Osmosis is a passive process, which does not require energy.

Question 25

What is water potential?

Answer 25

Water potential describes the tendency of water molecules to move from one place to another. It is affected by the presence of solute.
The presence of solute particles lowers the water potential of a solution. Pure water has the highest water potential defined as 0. All solutions have a water potential lower than that of pure water (a negative value). The higher the concentration of solute particles, the lower the water potential of the solution.

Question 26

How can osmosis be redefined in terms of water potential?

Answer 26

Osmosis is the net movement of water molecules across a differentially permeable membrane from a region of higher water potential to a region of lower water potential.

Question 27

What are hypotonic, isotonic, and hypertonic solutions?

Answer 27

A solution with a water potential higher than that of the cytoplasm of the cell is described as hypotonic. (eg. distilled water)
A solution with the same water potential as that of the cytoplasm of the cell is described as isotonic. (eg. 0.9% sodium chloride solution) Meanwhile, a solution with a water potential lower than that of the cytoplasm is described as hypertonic. (eg. 5% sodium chloride solution)

Question 28

How do animal cells change when bathed in a hypotonic solution?

Answer 28

The water potential of the bathing solution is higher than that of the animal cell. Since water molecules move from regions with higher water potential to those with lower water potential, there is a net movement of water from the bathing solution into the cytoplasm through the differentially permeable cell membrane by osmosis. The cell will have a net gain of water; it will swell and may finally burst.

Question 29

How do plant cells change when bathed in a hypotonic solution?

Answer 29

The water potential of the bathing solution is higher than that of the plant cell. Since water molecules move from regions with higher water potential to those with lower water potential, there is a net movement of water from the bathing solution into the cytoplasm through the differentially permeable cell membrane by osmosis. The cell will have a net gain of water; it will swell. There is turgor presure that presses the cell membrane against the cell wall. However, the swelling is resisted by the rigid cell wall. Water will eventually stop entering the cell, and the cell is said to be turgid.

Question 30

How do animal cells change when bathed in an isotonic solution?

Answer 30

The water potential of the bathing solution is the same as that of the animal cell. Therefore, there is no net movement of water through the differentially permeable cell membrane. Water will enter and leave the cell at the same rate. As a result, there will be no net gain or loss of water in the cell. The volume of the cell will remain the same.

Question 31

How do animal cells change when bathed in a hypertonic solution?

Answer 31

The water potential of the hypertonic solution is lower than that of the cytoplasm. Since water molecules move from regions with higher water potential to those with lower water potential, there is a net movement of water molecules from the inside of the cell to the bathing solution through the differentially permeable cell membrane by osmosis. The cell will have a net loss of water; it will shrink and become wrinkled.

Question 32

What is the name of the condition in which red blood cells burst with the release of haemoglobin?

Answer 32

Haemolysis

Question 33

How do plant cells change when bathed in an isotonic solution?

Answer 33

The water potential of the bathing solution is the same as that of the plant cell. Therefore, there is no net movement of water through the differentially permeable cell membrane. Water will enter and leave the cell at the same rate. As a result, there will be no net gain or loss of water in the cell. The volume of the cell will remain the same.

Question 34

What is the importance of osmosis?

Answer 34

Osmosis is the main process by which water enters or leaves cells in all organisms. The absorption of water in human small intestines and in plant roots are also carried out by osmosis.

Question 35

How do plant cells change when bathed in a hypertonic solution?

Answer 35

The water potential of the hypertonic solution is lower than that of the cytoplasm. Since water molecules move from regions with higher water potential to those with lower water potential, there is a net movement of water molecules from the inside of the cell to the bathing solution through the differentially permeable cell membrane by osmosis. The cell will have a net loss of water. The vacuole will shrink and the cell membrane will pull away from the cell wall. This condition is called plasmolysis and the cell is said to be flaccid.

Question 36

What is active transport?

Answer 36

Active transport involves carrier proteins on the cell membrane. Only substances that fit the shape of the carrier proteins can be transported across the cell membrane by active transport.
In active transport, the substance to be transported first binds to a specific carrier protein. This carrier protein then changes its shape using energy. Finally, the substance is released on the other side of the cell membrane.

Question 37

What are the features of active transport?

Answer 37

Active transport is an active process which requires energy. It can be used to transport substances against the concentration gradient.
As energy is required to change the shape of carrier proteins, active transport occurs only in living cells, which can release energy by respiration. Any condition that prevents respiration like the presence of cyanide or a lack of oxygen will slow down or stop active transport.

Question 38

What is the importance of active transport?

Answer 38

Active transport is important for organisms to obtain useful substances from the environment to maintain life. For example, the absorption of glucose in the small intestine of humans and the absorption of minerals from the soil into the roots of plants involve active transport.

Question 39

What is phagocytosis?

Answer 39

Phagocytosis is the process of
“cell-eating” for particles too large to move across the cell membrane by diffusion or active transport.
It starts with the infolding of the cell membrane to form a pit, or the extension of the cell membrane to form foot-like pseudopodia to surround a particle. A small vacuole is then formed to enclose the particle. The cell finally releases enzymes to digest the particle. The digested products then diffuse to the cytoplasm.

Question 40

What is the importance of phagocytosis?

Answer 40

Phagocytosis is important for nutrition of some unicellular organisms. For example, amoeba engulfs food particles by phagocytosis.
Phagocytosis is also importance for body defence against diseases. For example, in humans and other mammals, certain types of white blood cells engulf harmful microorganisms by phagocytosis.

Question 41

Is phagocytosis an active or passive process?

Answer 41

It is an active process that requires energy and a living membrane.

Question 42

What is endocytosis and two main forms of endocytosis?

Answer 42

Endocytosis is the process by which large molecules are transported into a cell. Phagocytosis and pinocytosis are two main forms of endocytosis. Pinocytosis is similar to phagocytosis except the cell takes in a liquid instead of solid particles.

Question 43

What is exocytosis?

Answer 43

Exocytosis is the reverse of endocytosis. In exocytosis, substances are released out of the cell. It is important for cells to remove wastes and secrete chemicals like hormones and enzymes.