Membrane and Transport Across the Membrane

Describe the molecular structure of the plasma membrane

1. The plasma membrane is a phospholipid bilayer with membrane proteins, glycoproteins, glycolipids and cholesterol embedded or attached.

2. Hydrophilic phosphate heads of phospholipids face outwards, in contact with the aqueous exterior and interior of the cell or organelle.

3. Hydrophobic fatty acid tails of phospholipids are sandwiched between hydrophilic phosphate heads of phospholipids, shielded away from the aqueous environment.

4. There are intrinsic proteins embedded in the membrane.

5. There are extrinsic proteins attached loosely to the membrane.

6. There are glycoproteins and glycolipids in the outer layer of the phospholipid bilayer.

7. There are cholesterol between phospholipids.

8. Hydrophilic -OH group of cholesterol interacts with hydrophilic phosphate heads of phospholipids via hydrophilic interactions.

9. Hydrophobic hydrocarbon skeleton of cholesterol interacts with hydrophobic fatty acid tails of phospholipids via hydrophobic interactions.

10. Fatty acid tails of phospholipids can be saturated or unsaturated.

11. The outer and inner layers of the membrane differ in composition and function.

12. The phospholipids and membrane proteins move freely and membrane proteins are scattered in the sea of phospholipids, hence it is a fluid mosaic model.

Explain why membranes with more unsaturated fatty acids are more fluid

1. Phospholipids with unsaturated fatty acids are less closely packed.

2. They contain one or more C=C double bonds, which causes kinks in the tail.

3. This prevents the phospholipids from packing closely together.

4. The membrane becomes more fluid and is prevented from freezing at low temperatures.

Explain why the fluid mosaic model is used to describe the membrane

1. Phospholipids and membrane proteins are always in constant motion unless they are anchored to the cytoskeleton or extracellular matrix.

2. Phospholipids and membrane proteins move laterally in the plane of the membrane.

3. Phospholipids are held together primarily by hydrophobic interactions between fatty acid tails.

4. Membrane proteins are scattered in the sea of phospholipids, forming a mosaic.

Explain why membranes are asymmetric in structure

1. The outer and inner layers of the membrane differ in composition and function.

2. There are different types or amounts of phospholipids, membrane proteins and cholesterol between the two layers.

3. There are different membrane proteins, glycoproteins and glycolipids in the membrane.

Describe the role of cholesterol in membranes

1. Cholesterol regulates membrane fluidity to ensure that the fluidity of the membrane does not fluctuate too much at extreme temperatures.

2. At low temperatures, kinetic energy of phospholipids is less and they move less vigorously, hence fluidity of the membrane is less.

3. Cholesterol increases membrane fluidity by preventing phospholipids from packing closely together.

4. This prevents the membrane from freezing at low temperatures.

5. At high temperatures, kinetic energy of phospholipids is greater and they move more vigorously, hence fluidity of the membrane is more.

6. Cholesterol decreases membrane fluidity by interacting with hydrophobic fatty acid tails of phospholipids and glycolipids via hydrophobic interactions.

Explain why intrinsic proteins are not easily removed

1. Intrinsic proteins are embedded in the membrane and not easily removed.

2. They span the entire phospholipid bilayer and are known as transmembrane proteins.

3. Hydrophilic (polar or charged) amino acid residues of intrinsic proteins interact with hydrophilic phosphate heads of phospholipids via hydrophilic interactions.

4. Hydrophobic (non-polar) amino acid residues of intrinsic proteins interact with hydrophobic fatty acid tails of phospholipids via hydrophobic interactions.

Explain why extrinsic proteins are easily removed

1. Extrinsic proteins are attached loosely to the surface of the membrane and are easily removed.

2. They are largely hydrophilic.

3. Hydrophilic amino acid residues of intrinsic proteins interact with hydrophilic phosphate heads of phospholipids via hydrophilic interactions.

Describe the various functions of membrane proteins

1. Channel or carrier proteins are transmembrane membrane proteins with a hydrophilic channel to shield polar molecules and charged ions from the hydrophobic core of the phospholipid bilayer. This transports them across the membrane.

2. Enzymes have their active site exposed to substrates in the cytosol for enzymatic reactions.

3. Receptors for cell signalling have their binding site exposed to the exterior of the cell for ligands to bind. This allows the cell to detect and respond to the external stimulus to trigger a cellular response within the cell. Such receptors usually have a carbohydrate side-chain.

4. Glycoproteins bind to proteins or glycoproteins or glycolipids of other cells during cell-cell recognition. They act as receptors involved in cell-cell recognition. This allows the cell to determine if other cells are the same or different from itself.

5. Glycoproteins binds to proteins or glycoproteins of adjacent cells in the correct orientation during cell-cell adhesion. This regulates cell growth and division and the formation of tissues.

6. Membrane proteins bind to components of the cytoskeleton to maintain cell shape and stabilise the location of certain membrane proteins.

Describe the various functions of glycoproteins and glycolipids

1. Glycoproteins and glycolipids are primarily involved in cell-cell recognition, cell-cell communication and cell-cell adhesion.

Describe the various functions of all membranes

1. The membrane forms a hydrophobic boundary between the external environment and cytoplasm of the cell and cytoplasm of the cell and organelle. The membrane is partially permeable. Only non-polar molecules can move across the membrane directly while polar molecules and charged ions cannot. This regulates the movement of substances in and out of the cell and organelle.

2. The membrane provides compartmentalisation within the cell and organelle. This ensures a constant internal environment within the cell and organelle is maintained, optimal conditions required for enzymatic processes are maintained and prevents intermediates of one pathway from interfering with another. This allows several metabolic processes to occur simultaneously and independently. For example, Krebs cycle occurs in the mitochondria while glycolysis occurs in the cytosol. Both occur simultaneously and independently without interfering with each other.

3. The membrane provides site of attachment of enzymes, receptor proteins and other proteins involved in maintaining cell shape. This allows for a more efficient reaction sequence, a more efficient cell signalling pathway and maintains the shape of the cell and organelle as membrane proteins are attached to the cytoskeleton.

4. The membrane is extensively folded to increase its surface area for more enzymes and proteins to be attached. This increases the rate of reaction.

Describe the various functions of cell surface membranes only

1. The cell surface membrane receives the external stimulus. Receptors attached in the cell surface membrane detect and respond to changes in the external environment.

2. Receptors attached in the cell surface membrane bind to proteins or glycoproteins or glycolipids of neighbouring cells for cell-cell recognition, cell-cell communication and cell-cell adhesion. This regulates cell growth and division and the formation of tissues.

3. The cell surface membrane forms finger-like extensions to increase its surface area for absorption of nutrients. This increases the rate of absorption of nutrients.

4. The cell surface membrane of phagocytes extend outwards, forming pseudopia to surround the particles. The particles are taken into the cell via phagocytosis.

Define simple diffusion

1. Simple diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration, down the concentration gradient.

2. It does not require energy via the hydrolysis of ATP.

3. It does not require specific transport proteins.

Define facilitated diffusion

1. Facilitated diffusion is the net movement of polar molecules and charged ions from a region of lower concentration to a region of higher concentration, down a concentration gradient via specific transport proteins.

2. It does not require energy via the hydrolysis of ATP.

3. It requires specific transport proteins.

Describe the structure of channel proteins

1. Channel proteins are transmembrane proteins.

2. Channel proteins are specific as only molecules or ions of a specific shape and a specific charge can pass through.

3. They have a hydrophilic channel to shield polar molecules or charged ions from the hydrophobic core of the phospholipid bilayer. This transports them across the membrane.

4. They can become saturated. When all the channel proteins have been used up, rate of simple diffusion is maximum.

5. They can be inhibited. When an inhibitor binds at the pore of the channel, the diffusing molecule or ion is prevented from passing through.

Describe the structure of carrier proteins

1. Carrier proteins are transmembrane proteins.

2. Carrier proteins are specific as only molecules and ions of a specific shape and a specific charge can pass through.

3. Shape of the binding site of the carrier protein is complementary to the shape of the molecule and ion transported.

4. The binding site of the carrier protein is exposed to the extracellular matrix. When the molecule or ion binds at the binding site, it induces a conformational change in the carrier protein, resulting in the molecule or ion being released to the other side of the membrane.

5. They can become saturated. When all the carrier proteins have been used up, rate of facilitated diffusion is maximum.

6. They can be inhibited. An inhibitor with shape similar to the molecule or ion competes and binds to the binding site of the carrier protein.

Define osmosis

1. Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, down a water potential gradient.

2. It does not require energy via the hydrolysis of ATP.

Describe what happens when cells are placed in a solution of higher water potential

1. In animal cells, water potential of solution is greater than water potential of the cell.

2. There is a net movement of water molecules from the solution into the cell via osmosis.

3. Volume of the cell increases and the cell expands.

4. The cell bursts due to the lack of the cellulose cell wall to prevent further uptake of water into the cell.

5. In plant cells, water potential of solution is greater than water potential of the cell.

6. There is a net movement of water molecules from the solution into the cell via osmosis.

7. Volume of the cell increases and the celll expands

8. The cell does not burst due to the cellulose cell wall which prevents further uptake of water into the cell.

Describe what happens when cells are placed in a solution of equal water potential

1. In animal cells, water potential of solution is equal to water potential of the cell.

2. There is no net movement of water molecules between the solution and the cell via osmosis.

3. The cell neither shrinks nor swells.

4. In plant cells, water potential of solution is equal to water potential of the cell.

5. There is no net movement of water molecules between the solution and the cell via osmosis.

6. The cell becomes flaccid due to the lack of turgor pressure.

Describe what happens when cells are placed in a solution of lower potential

1. In animal cells, water potential of solution is lower than water potential of the cell.

2. There is a net movement of water molecules from the cell into the solution via osmosis.

3. Volume of the cell decreases and the cell shrinks.

4. In plant cells, water potential of solution is lower than water potential of the cell.

5. There is a net movement of water molecules from the cell into the solution via osmosis.

6. Volume of the cell decreases and the cell shrinks and is flaccid due to the lack of turgor pressure.

Define active transport

1. Active transport is the movement of molecules or ions from a region of lower concentration to a region of higher concentration, against the concentration gradient.

2. It requires energy via the hydrolysis of ATP.

3. It requires specific transport proteins.

4. The phosphate group from ATP is transferred to the carrier protein to induces a conformational change in the carrier protein, resulting in the molecule or ion being released to the other side of the membrane.

Describe the role of active transport

1. Active transport ensures the continual uptake of nutrients even when their concentrations outside the cell is lower than inside the cell.

2. It ensures the continual removal of unwanted substances, even when their concentrations outside the cell is higher than inside the cell.

3. It maintains the optimal internal concentration of molecules or ions inside the cell or organelle.

Define endocytosis

1. Endocytosis is the uptake of particles into the cell via formation of vesicles from the cell surface membrane.

2. It requires energy via the hydrolysis of ATP.

Describe what happens during phagocytosis

1. The cell surface membrane of phagocytes extend outwards, forming pseudopodia that surrounds the particles./ The cell surface membrane of phagocytes invaginates, forming a depression that surrounds the particles.

2. The particles are taken into the cell via phagocytosis, forming a phagosome.

3. The phagosome fuses with a lysosome to form a phagolysosome.

4. The hydrolytic enzymes in the lysosome digest the particles.

5. Useful substances are absorbed into the cytoplasm for use by the cell.

6. For example, the uptake of antigens by phagocytes such as macrophages and neutrophils.

Describe what happens during receptor-mediated endocytosis

1. Shape of the particle is complementary to shape of binding site of the protein receptor.

2. The particle binds to the binding site of the specific receptor on the cell surface membrane.

3. The cell surface membrane invaginates, forming a depression to surround the particles.

4. The particles enter the cell via receptor-mediated endocytosis, forming a phagosome.

5. The phagosome fuses with a lysosome to form a phagolysosome.

6. The hydrolytic contents in the lysosome digest the particles,

7. Useful substances are absorbed into the cytoplasm for use by the cell.

8. Most receptors are recycled back to the cell surface membrane for reuse.

9. For example, the uptake of influenza virus by respiratory epithelial cells.

Define exocytosis

1. Exocytosis is the release of particles out of the cell via fusion of vesicles with the cell surface membrane.

2. It requires energy via the hydrolysis of ATP.

Explain what happens during endocytosis

1. The secretory vesicle containing the protein or secretory substance buds off from trans-face of the GA, travels along the microtubules of the cytoskeleton and the membrane of the secretory vesicle fuses with the cell surface membrane.

2. The contents of the secretory vesicle is released out of the cell via exocytosis.

State the similarities between simple and facilitated diffusion

1. Both do not require energy via the hydrolysis of ATP.

2. Both involve the net movement of molecules down a concentration gradient.

State the differences between simple and facilitated diffusion

1. During simple diffusion, non-polar molecules are transported while during facilitated diffusion, polar molecules and charged ions are transported.

2. During simple diffusion, molecules diffuse directly across the phospholipid bilayer while during facilitated diffusion, molecules or ions are transported across the phospholipid bilayer via specific transport proteins.

State the similarities between osmosis and facilitated diffusion

1. Both do not require energy via the hydrolysis of ATP.

2. Both involve the net movement of molecules down a concentration gradient.

3. Both occur across a membrane.

State the differences between osmosis and facilitated diffusion

1. During osmosis, only water molecules are transported while during facilitated diffusion, polar molecules and charged ions are transported.

2. During osmosis, water molecules can move directly across the phospholipid bilayer or via aquaporins while during facilitated diffusion, molecules or ions are transported across the phospholipid bilayer via specific transport proteins.

State the similarities between active transport and facilitated diffusion

1. Both require specific transport proteins.

2. Both occur across a membrane.

State the differences between active transport and facilitated diffusion

1. Active transport occurs against a concentration gradient while facilitated diffusion occurs down a concentration gradient.

2. Active transport requires energy via the hydrolysis of ATP while facilitated diffusion does not require energy via the hydrolysis of ATP.

3. Active transport only involves carrier proteins while facilitated diffusion involves both carrier and channel proteins.