1. Topic 2.4 – Plasma Membranes
IB Biology
S.Dosman
Adapted by Laura Sugden

2. Homework 2.4
1. Draw a basic phospholipid molecule and identify which part(s) are hydrophobic and which part(s) are hydrophilic. 2. Explain how phospholipids are arranged in the plasma membrane and how this arrangement allows a cell membrane to maintain its structure. 3. Explain how phospholipids are arranged in the plasma membrane and how this arrangement allows a cell membrane to maintain its structure. 4. What is a glycoprotein? 5. List and give a brief explanation of the 6 functions of membrane proteins. 6. Particles are in constant random motion. Explain the role of random motion in particles moving across a semi-permeable membrane. 7. What is a concentration gradient? 8. What role do membrane proteins play in diffusion? 9. Compare active and passive transport. 10. Use the sodium-potassium pump to explain how active transport can be used to move particles against a concentration gradient.

3. Hydrophobic
Having an aversion to water; tending to coalesce and form droplets in water.
Hydrophobic is a term used to describe how molecules react with water. Molecules that are hydrophobic have an aversion to water and are often referred to as “water fearing”.

4. Hydrophilic Having an affinity for water.
Hydrophilic is a term used to describe how molecules react with water. Molecules that are hydrophilic have an affinity for water. Hydrophilic is often referred to as “water loving”.

5. Integral protein
Typically a transmembrane protein with hydrophobic regions that completely spans the hydrophobic interior of the membrane.
The integral proteins in the plasma membrane penetrate the lipid bilayer from one side to the other.
These proteins control the entry and exit of specific molecules from the cell. They also have a hydrophobic and hydrophilic region which helps keep them in place in the membrane.

6. Integral Proteins

7. Peripheral protein
A protein appendage loosely bound to the surface of a membrane and not embedded in the lipid bilayer.
The protein molecules are not fixed in one spot of the membrane and they actually float in the fluid phospholipid bilayer or are attached to an integral protein.
Peripheral proteins are known as glycoproteins because they have a carbohydrate attached. The function in immune responses and are involved in cell to cell recognition.

8. Peripheral Proteins

9. Phospholipid
A molecule that is a constituent of the inner bilayer of biological membranes, having a polar, hydrophilic head and a non-polar, hydrophobic tail.
A phospholipid molecule is composed of glycerol, a 3-carbon compound with two fatty acids joined to two carbons and a phosphorylated alcohol molecule.

10. Osmosis
The diffusion of water across a selectively permeable membrane.
The movement occurs from an area of higher concentration to an area of lower concentration.
Water can move by osmosis across the plasma membrane in both directions very easily because the plasma membrane is very permeable to water.

11. OSMOSIS

12. Osmotic balance
Also known as water balance, osmotic balance is obtained when there are as many water molecules moving into an area as there are moving out.
Osmotic balance is a form of dynamic equilibrium.

13. Passive transport
Passive transport is the diffusion of substances across a biological membrane.
This occurs without the use of cellular energy.

14. Diffusion
The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area.
This movement of particles happens as a result of the random motion of particles and the existence of a concentration gradient.

15. Diffusion

16. Facilitated diffusion
The spontaneous passage of molecules and ions, bound to specific carrier proteins, across a biological membrane down their concentration gradient.

17. Active transport
The movement of a substance across a biological membrane against its concentration or electrochemical gradient with the help of energy input and specific transport proteins.
This movement of particles occurs against the concentration gradient or from an area of low concentration to an area of high concentration with the use of ATP.

18. Active Transport

19. Endocytosis
The cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surround the substance and pinch off to form an intracellular vesicle.
Cells can bring in solids and liquids using this process.

20. Pinocytosis
A type of endocytosis in which the cell ingests extracellular fluid and its dissolved substances.

21. Phagocytosis
A type of endocytosis involving large, particulate substances, accomplished mainly by macrophages, neutrophils and dendritic cells.

22. Exocytosis
The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane.

23. Adenosine Triphosphate (ATP)
An adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed. This energy is used to drive endergonic reactions in cells.

24. The head of the phospholipid is hydrophilic.
1. Draw a basic phospholipid molecule and identify which part(s) are hydrophobic and which part(s) are hydrophilic.
The head of the phospholipid is hydrophilic.
The fatty acid tails are hydrophobic.

25. 2. Explain how phospholipids are arranged in the plasma membrane and how this arrangement allows a cell membrane to maintain its structure.
The phospholipid molecules are arranged in the plasma membrane due to the way they react with water.
Because the polar heads of the molecules are hydrophilic, they are arranged so that they are always facing the internal and external fluid environment of the cell.

26. 2. Explain how phospholipids are arranged in the plasma membrane and how this arrangement allows a cell membrane to maintain its structure.
Due to the fact that the fatty acid tails are hydrophobic, they face inwards towards each other and away from the fluid environment.
The fatty acid tails in the center of the membrane are attracted to each other and the phosphate heads are attracted to the surrounding water and this makes the structure of the plasma membrane very stable.

27. They are used for cell to cell recognition.
3. What is a glycoprotein?
A glycoprotein is a membrane protein that is covalently attached to a carbohydrate.
They are used for cell to cell recognition.

28. Functions of membrane proteins
4. List and give a brief explanation of the 6 functions of membrane proteins.
Functions of membrane proteins
Description
Hormone binding sites
A site exposed on the outside of the membrane allows one specific hormone to bind based on shape. A signal is then transmitted to the inside of the cell.
Immobilized Enzymes
Enzymes located in membranes either catalyze reactions inside or outside the cell, depending on whether the active site is on the inner or outer surface. Grouped so that a metabolic pathway may occur
Cell adhesion
Proteins from adjacent cells may hook together to provide either permanent or temporary connections. This forms junctions referred to as gap junctions and tight junctions.
Cell to cell communication
The proteins involved in cell to cell communication have an attached molecule of carbohydrate which provides an identification label for the cell (glycoproteins).
Channels for Passive Transport
Channels are passages through the center of membrane proteins. Each channel allows one specific substance to pass through from high concentration to low concentration.
Pumps for active transport
Pumps release energy from ATP and use it to move specific substances across the plasma membrane. The energy is used to change the shape of the protein.

29. 5. Particles are in constant random motion
5. Particles are in constant random motion. Explain the role of random motion in particles moving across a semi-permeable membrane.
Particles are in constant random motion in liquids and gasses. If the particles are permeable to the membrane they can diffuse across.
Particles will move from an area where they are in high concentration to an area of lower concentration. Once the concentration of the particle on both sides of the membrane is equal the particles will continue moving and this state is called equilibrium.

30. 6. What is a concentration gradient?
A concentration gradient can be defined as an increase or decrease in the density of a chemical substance in an area. Cells often maintain concentration gradients of ions across their membranes.
In other words, it is the difference of concentration (high or low) of a substance on either side of the plasma membrane.

31. 7. What role do membrane proteins play in diffusion?
Membrane proteins can be used as channels to allow substances to pass across the plasma membrane from high concentration to low concentration in facilitated diffusion.
They can also be used as pumps during active transport to allow substances to move against the concentration gradient (low to high). This requires the use of ATP to change the shape of the membrane protein.

32. 8. Compare active and passive transport.
Passive transport is the diffusion of particles across a biological membrane. This occurs without the use of cellular energy.

33. 8. Compare active and passive transport.
Active Transport is the movement of particles across biological membranes using energy from ATP. This movement of particles occurs against the concentration gradient or from an area of low concentration to an area of high concentration.

34. 8. Compare active and passive transport.
A cell can expend energy to move substances across the membrane against the concentration gradient. Active transport allows a cell to maintain concentration levels inside the membrane different from those in its external environment. The cell will use membrane proteins as pumps to move these substances.

35. Each pump only transports particular substances.
9. Use the sodium-potassium pump to explain how active transport can be used to move particles against a concentration gradient.
The cell can hydrolyze ATP and use the energy released to move substances across the plasma membrane and against the concentration gradient. The energy is used to change the shape of membrane protein “pumps”.
Each pump only transports particular substances.

36. 9. Use the sodium-potassium pump to explain how active transport can be used to move particles against a concentration gradient.
The particle will enter the pump on the side with a lower concentration and bind to a specific site for that type of particle.
Energy from ATP is used to change the shape of the pump, the particle is released on the side of higher concentration, and the pump returns to its original shape.

37. 9. Use the sodium-potassium pump to explain how active transport can be used to move particles against a concentration gradient.
An example of this is the sodium-potassium pump in animal cells. There is a high concentration of potassium ions inside cells and a high concentration of sodium ions outside of the cells. The cell maintains these concentrations using the sodium-potassium pump; a membrane protein. (see diagram page 149 Campbell-Reece)

39. 10. Explain how vesicles are used to transport materials within a cell between the RER, Golgi apparatus and plasma membrane.
The ribosomes attached to the RER synthesize proteins and release them into the RER.
The proteins travel through the RER and are then released from the RER in vesicles formed from the RER membrane.

40. Explain how vesicles are used to transport materials within a cell during endocytosis and exocytosis.
The vesicle travels through the cytoplasm and delivers the proteins to the Golgi apparatus which then modifies the proteins. The proteins are then sent off inside a vesicle formed from the membrane of the Golgi apparatus and are carried to the plasma membrane.
The vesicle fuses with the plasma membrane and the contents are released outside the cell (exocytosis).

41. 10. Explain how vesicles are used to transport materials within a cell during endocytosis and exocytosis.
Endocytosis occurs when the plasma membrane is pulled inwards and will form a “pocket” around a particular substance. The substance will become enclosed in the vesicle which is then pinched off and begins moving through the cytoplasm. Cells can bring in solids and liquids using this process.

42. 10. Explain how vesicles are used to transport materials within a cell during endocytosis and exocytosis.
Exocytosis is internal vesicles will fuse with the plasma membrane and the contents of the vesicle are released into the external environment of the cell. The cell can secrete substances they produced this way or excrete waste products.

43. 10. Explain how vesicles are used to transport materials within a cell during endocytosis and exocytosis.
The phospholipid molecules which make up the plasma membrane are in a fluid state which allows them to change shape.
This fluidity of the plasma membrane allows it to form vesicles that can be pinched off or ones that can fuse with it.
During endocytosis and exocytosis these vesicles are formed without any damage to the plasma membrane.