Week 3 - Membrane Flashcards
What is the cell membrane
The cell membrane is a semipermeable phospholipid bilayer which surrounds a cell
What substances is the membrane consist of
- 40-50% glycerophospholipids
- 50-60% proteins
- 5% glycolipids (sphingolipids)
What are the functions of the cell membrane
- 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
Explain the cell membrane as a cellular barrier
The membrane forms a physical barrier that separates the internal environment of the cell from the external environment
Explain how the cell membrane controls the flow of molecule in and out of the cell
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.
Explain how the membrane has cell recognition
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.
How is the structure of the cell membrane formed
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.
What is the model which describes the dynamic movement of the cell membrane
Fluid Mosaic Model
Explain the Fluid Mosaic Model
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.
What influences the fluidity of the cell membrane
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
What are the 2 major mechanisms for the movement of chemicals into and out of the cell
- Passive Transport - going down a concentration gradient
- Active Transport - going against a concentration gradient
What are the different types of passive transport
- Simple Diffusion
- Facilitated diffusion
- Gap Junctions
What type of molecules can undergo simple diffusion
For small neutral molecules e.g. O2, CO2 - which are moving down a concentration gradient
What is simple diffusion
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.
What type of molecules can undergo facilitated diffusion
larger or charged molecules moving down a concentration gradient
What is facilitated diffusion
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.
What are the different types of active transport
- Primary Active Transport
- Secondary Active Transport
What is primary active transport
Primary active transport is where ATP is directly used to transport molecules against their concentration gradient.
E.g. sodium potassium pump
Explain the steps of the sodium potassium pump
- 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)
- 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)
What is secondary active transport
Secondary active transport transports molecules against their concentration gradient however the ATP used is indirectly.
Example of secondary active transport and the steps
The transport of lactose Permease
The pumping of lactose is driven by the H+ ion gradient and flow
- 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)
- 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
What are ports
Ports are the proteins which enable different types of transport through the membrane
What are the different types of ports
- Uniport
- Symport
- Antiport
What is a uniport
A port which transports 1 type of particle/molecule in one direction. This type of port is utilized in passive transport.
What is a symport
Transport 2 types of particle/molecules in one direction. This type of port is utilized in active transport (secondary)
What is an antiport
Transports 2 types of particle/molecule in different directions. This type of port is utilized in active transport - primary
What type of transport and what port is used in the movement of AA, ions and carbohydrates
- Passive transport
- Facilitated diffusion
- Uniport
What type of transport and what port is used in the sodium potassium pump
- Active transport
- Primary active transport
- Antiport
What type of transport and what port is used in the transport of lactose permease
- Active transport
- Secondary active transport
- Symport
What transports cholesterol and triaclyglycerols
lipoproteins
What is the difference between HDL,LDL,VLDL, and chylomicrons
The ratio between cholesterol and triglycerides which make up the different types of lipoproteins
How and what controls the synthesis of cholesterol
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.
Explain the steps of the movement of cholesterol from the liver
- 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.
- 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)
Explain the steps of the transport of cholesterol back to the liver
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
- High density lipoproteins synthesized in the liver collects excess cholesterol from peripheral tissues
- While in the plasma, free cholesterol in HDL is cholesteryl esters
- In the liver HDL binds to the liver cell surface and transfers the cholesteryl esters to the liver cells
- These cholesteryl esters are recycled and used for the synthesis of steroid hormones and bile acids
What is high LDL and low HDL a symptom of
A faulty cholesterol transport and warning of possible atherosclerosis.
Causes the diameter of the artery to decrease due to cholesterol easter building up
What are the functional types of membrane proteins
- Receptor Proteins
- Transport Proteins
- Enzymes
What are receptor proteins
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.
What are transport Proteins
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
