Week 3 - Membrane

Question 1

What is the cell membrane

Answer 1

The cell membrane is a semipermeable phospholipid bilayer which surrounds a cell

Question 2

What substances is the membrane consist of

Answer 2

• 40-50% glycerophospholipids
• 50-60% proteins
• 5% glycolipids (sphingolipids)

Question 3

What are the functions of the cell membrane

Answer 3

• Acts as a cellular barrier
• Controls the flow of molecules in and out of the cell
• Has cell recognition
• Produces enzyme reactions
• Electrochemical gradient maintenance

Question 4

Explain the cell membrane as a cellular barrier

Answer 4

The membrane forms a physical barrier that separates the internal environment of the cell from the external environment

Question 5

Explain how the cell membrane controls the flow of molecule in and out of the cell

Answer 5

The membrane is a semipermeable barrier and its allows certain substances pass through while restricting the movement of others. This selective permeability helps maintain the internal balance necessary for cellular function. Molecules move through the membrane through either passive or active transport.

Question 6

Explain how the membrane has cell recognition

Answer 6

The cell membrane contains specific proteins and carbohydrates that bind specifically to biologically important substances that trigger biochemical responses in the cell such as immune responses, tissue development and cell signaling.

Question 7

How is the structure of the cell membrane formed

Answer 7

The main force driving the formation is the hydrophobic interactions of the lipid tails which force the hydrophilic heads to face the outside water environment and for the hydrophilic tails to attract each other using dispersion forces.

Question 8

What is the model which describes the dynamic movement of the cell membrane

Answer 8

Fluid Mosaic Model

Question 9

Explain the Fluid Mosaic Model

Answer 9

Membranes are dynamic structures and the proteins embedded in them are stable yet moving - moving like icebergs on the membrane (floating through), the lipids are also moving.
The model shows how there is a lateral motion of proteins which can ‘float’ along the plane of the membrane, lipids may also move laterally. Due to the oriented nature of the bi-layer diffusion is strong only laterally.

Question 10

What influences the fluidity of the cell membrane

Answer 10

The more unsaturated phospholipids there are in the cell membrane the more fluid the structure is due to the bent AA of the lipid which reduces the capacity for the lipids to compact close to each other

Question 11

What are the 2 major mechanisms for the movement of chemicals into and out of the cell

Answer 11

1. Passive Transport - going down a concentration gradient
2. Active Transport - going against a concentration gradient

Question 12

What are the different types of passive transport

Answer 12

• Simple Diffusion
• Facilitated diffusion
• Gap Junctions

Question 13

What type of molecules can undergo simple diffusion

Answer 13

For small neutral molecules e.g. O2, CO2 - which are moving down a concentration gradient

Question 14

What is simple diffusion

Answer 14

For small neutral molecules. including O2, CO2, they can diffuse directly through the lipid bilayer of the cell membrane. These molecules are moving down a concentration gradient.

Question 15

What type of molecules can undergo facilitated diffusion

Answer 15

larger or charged molecules moving down a concentration gradient

Question 16

What is facilitated diffusion

Answer 16

The diffusion of larger or charged molecules across the membrane which is facilitated by protein channels (integrated proteins) which act as channels for the molecules to pass through. This direction is also down a concentration gradient (high —> low). In facilitated diffusion a small amount of energy is expended as the transmembrane proteins change conformation to open and close to allow molecules to diffuse across the membrane.

Question 17

What are the different types of active transport

Answer 17

• Primary Active Transport
• Secondary Active Transport

Question 18

What is primary active transport

Answer 18

Primary active transport is where ATP is directly used to transport molecules against their concentration gradient.
E.g. sodium potassium pump

Question 19

Explain the steps of the sodium potassium pump

Answer 19

1. The sodium potassium pump is initially open on the intracellular side of the cell, which has binding room for 3 sodium specifically and ATP is used to change the protein’s configuration to open on the outside and push the sodium ions out (this is against their concentration gradient)
2. The pump then has 2 binding sites for potassium ions to attach to, coming from the excellular side. The pump then reverts back to its original position and opens in the intercellular side releasing the potassium ions into the cell (also against its concentration gradient)

Question 20

What is secondary active transport

Answer 20

Secondary active transport transports molecules against their concentration gradient however the ATP used is indirectly.

Question 21

Example of secondary active transport and the steps

Answer 21

The transport of lactose Permease
The pumping of lactose is driven by the H+ ion gradient and flow

1. The pump takes up both lactose and hydrogen from outside of the cell and then changes shape, depositing both substances inside the cell (pump = symport protein)
2. The hydrogen ions that entered the cell are later returned to the outside by the act of the proton pump which is a form of primary active transport which creates sodium and hydrogen ion gradients at the expense of ATP

Question 22

What are ports

Answer 22

Ports are the proteins which enable different types of transport through the membrane

Question 23

What are the different types of ports

Answer 23

• Uniport
• Symport
• Antiport

Question 24

What is a uniport

Answer 24

A port which transports 1 type of particle/molecule in one direction. This type of port is utilized in passive transport.

Question 25

What is a symport

Answer 25

Transport 2 types of particle/molecules in one direction. This type of port is utilized in active transport (secondary)

Question 26

What is an antiport

Answer 26

Transports 2 types of particle/molecule in different directions. This type of port is utilized in active transport - primary

Question 27

What type of transport and what port is used in the movement of AA, ions and carbohydrates

Answer 27

• Passive transport
• Facilitated diffusion
• Uniport

Question 28

What type of transport and what port is used in the sodium potassium pump

Answer 28

• Active transport
• Primary active transport
• Antiport

Question 29

What type of transport and what port is used in the transport of lactose permease

Answer 29

• Active transport
• Secondary active transport
• Symport

Question 30

What transports cholesterol and triaclyglycerols

Answer 30

lipoproteins

Question 31

What is the difference between HDL,LDL,VLDL, and chylomicrons

Answer 31

The ratio between cholesterol and triglycerides which make up the different types of lipoproteins

Question 32

How and what controls the synthesis of cholesterol

Answer 32

Serum cholesterol levels control cholesterol synthesis in the liver.
There is a negative feedback loop depending on the concentration of serum cholesterol levels. If there is high serum cholesterol due to diet, the synthesis of cholesterol in the lever is low and vice versa.

Question 33

Explain the steps of the movement of cholesterol from the liver

Answer 33

1. Cholesterol is synthesized in the liver and it needs to be transported through blood while encapsulated in a lipoprotein due to its hydrophobic nature. Cholesterol as well as triglycerides ad other lipids first starts in a Very Low Density Lipoprotein (VLDL) and is carried in plasma to capillaries of muscle or fat tissues.
2. In the capillaries triglycerides are removed and the density of the lipoprotein increases as the size of the molecule decreases and it becomes a Low Density Lipoprotein (LDL)

Question 34

Explain the steps of the transport of cholesterol back to the liver

Answer 34

High Density Lipoproteins (HDL) are synthesized in the liver and is the return pathway for cholesterol. HDL picks up excess cholesterol from tissues and transports it back to the liver for excretion or recycling

1. High density lipoproteins synthesized in the liver collects excess cholesterol from peripheral tissues
2. While in the plasma, free cholesterol in HDL is cholesteryl esters
3. In the liver HDL binds to the liver cell surface and transfers the cholesteryl esters to the liver cells
4. These cholesteryl esters are recycled and used for the synthesis of steroid hormones and bile acids

Question 35

What is high LDL and low HDL a symptom of

Answer 35

A faulty cholesterol transport and warning of possible atherosclerosis.
Causes the diameter of the artery to decrease due to cholesterol easter building up

Question 36

What are the functional types of membrane proteins

Answer 36

• Receptor Proteins
• Transport Proteins
• Enzymes

Question 37

What are receptor proteins

Answer 37

These proteins can bind to signaling molecules such as hormones, neurotransmitters or growth factors. The binding of these signaling molecules to the receptor initiates cellular responses allowing the cell to respond to its environment.

Question 38

What are transport Proteins

Answer 38

They facilitate the movement of ions, molecules and other substances across the cell membrane. These proteins play a crucial role in maintaining cellular homeostasis by regulating the passage of specific substances in and out of the cell. There are different types of transport proteins, each with distinct mechanisms for moving substances across the cell membrane e.g. uniport, symports, antiport