Membranes & Membrane Transport (DP IB Biology)

The two main categories of membrane proteins are integral proteins and peripheral proteins.

Integral proteins are partially hydrophobic (amphipathic) proteins that are embedded in the phospholipid bilayer, either across both layers or just one layer.

Channel proteins form holes or pores through which molecules can travel, while carrier proteins change shape to transport a substance across the membrane.

False.

The protein content of membranes can vary depending on the function of the cell.

Receptor proteins bind to specific molecules like peptide hormones, neurotransmitters, or antibodies, generating signals that trigger reactions inside the cell.

Peripheral proteins are hydrophilic proteins attached to either the surface of integral proteins or to the plasma membrane via a hydrocarbon chain, and can be inside or outside the cell.

Immobilised enzymes are integral proteins with the active site exposed on the surface of the membrane, which can be inside or outside the cell.

Glycoproteins act as cell markers or antigens for cell-to-cell recognition, such as the ABO blood group antigens that differ slightly in their carbohydrate chains.

False.

Each transport protein is specific to a particular ion or molecule.

The main function of the phospholipid bilayer in the plasma membrane is to provide a barrier to the movement of some substances into and out of the cell.

Osmosis is the diffusion of water molecules, from a dilute solution to a solution with a higher solute concentration, across a partially permeable membrane.

Aquaporins are channel proteins that allow water to pass through membranes more freely during osmosis.

Both types of diffusion involve the passive movement of substances from high to low concentration, however simple diffusion does not require a transport protein and facilitated diffusion does require a transport protein.

The two types of proteins that enable facilitated diffusion are channel proteins and carrier proteins.

Large molecules, polar molecules, and ions typically require facilitated diffusion to cross cell membranes.

Gated channel proteins can control the exchange of ions by opening or closing the pore through movement of part of the protein on the inside surface of the membrane.

False.

Carrier proteins change shape to transport substances, while channel proteins have a fixed shape.

Channel proteins are pores with a fixed shape that allow specific substances to pass through, while carrier proteins can switch between two shapes to transport substances across the membrane.

Active transport is the movement of molecules and ions across a cell membrane, from a region of lower concentration to a region of higher concentration, using energy from respiration.

False.

Active transport requires carrier proteins, not channel proteins.

A pump protein is a carrier protein that uses energy from adenosine triphosphate (ATP) to transfer specific particles across membranes. This is used in active transport.

Selective permeability is the ability of the membrane to differentiate between different types of molecules, only allowing some molecules through while blocking others.

Glycoproteins are cell membrane proteins that have a carbohydrate chain attached on the extracellular side.

Glycolipids are lipids with carbohydrate chains attached, located on the outer surface of cell membranes.

The carbohydrate chains enable glycoproteins and glycolipids to act as receptor molecules, allowing them to bind with substances at the cell surface.

False.

Glycoproteins and glycolipids are located on the extracellular (outside) side of cell membranes.

Some glycoproteins and glycolipids act as cell markers or antigens, allowing the immune system to determine whether a cell belongs in the body or is a pathogen.

Membranes are described as 'fluid' because the phospholipids and proteins can move around within their own layers.

The four main components of the fluid mosaic model are phospholipids, cholesterol, glycoproteins and glycolipids, and integral and peripheral proteins.

False.

Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles, and also within organelles.

The scattered pattern produced by the proteins within the phospholipid bilayer gives membranes their mosaic appearance when viewed from above.

Glycoproteins are represented with a carbohydrate chain attached on the extracellular side of the membrane.

False.

Peripheral proteins do not extend into the hydrophobic region of the membrane.

In the membrane, cholesterol's OH group is positioned next to the phosphate heads, with the rest positioned next to the tails of the phospholipids.

The two hydrophobic tails of a phospholipid are made up of fatty acids and the hydrophilic head of a phospholipid is a phosphate group.

False.

Saturated fatty acids have no double bonds and are fully "saturated" with hydrogen atoms.

Unsaturated fatty acids have lower melting points than saturated fatty acids because unsaturated fatty acids have kinks in their structure from double bonds, preventing them from packing tightly together.

Saturated fatty acids have higher melting points, which makes membranes stronger at higher temperatures.

Cholesterol holds fatty acid tails together, increasing membrane stability by preventing excessive fluidity at high temperatures.

At lower temperatures, cholesterol prevents stiffening of the membrane.

Cholesterol helps prevent water-soluble substances from diffusing across the membrane by fitting into the‎ spaces between phospholipids.

Endocytosis is a process in which cells take in substances from outside of the cell by engulfing them in a vesicle.

Exocytosis is the process of transporting materials from within a cell to the exterior of the cell.

False.

Both endocytosis and exocytosis are forms of active transport and require energy.

Phagocytosis is an example of endocytosis that transports solid materials, such as the engulfing of bacteria by phagocytic white blood cells.

A vesicle is a small, membrane-bound sac within a cell that is filled with liquid. They perform a variety of roles such as transport, storage and communication within the cell.

The formation of vesicles is an active process and involves a small region of the plasma membrane being pinched off.

Nicotinic acetylcholine receptors open in response to the neurotransmitter acetylcholine, allowing ions (such as Ca²⁺ or Na⁺) to enter, changing the membrane potential and potentially generating an action potential.

False.

The sodium-potassium pump transports ions against their concentration gradients using ATP.

Indirect active transport uses the energy from sodium ions moving down their gradient to move glucose against its gradient, without directly using ATP for the glucose transport itself.

Sodium (Na⁺) and potassium (K⁺) channels are examples of voltage-gated ion channels in neurons.

Sodium-dependent glucose cotransporters use the energy from sodium ions moving down their concentration gradient to transport glucose against its concentration gradient.

In the small intestine, this mechanism allows efficient absorption of glucose into the bloodstream.

In the nephron, sodium-dependent glucose cotransporters enable the reabsorption of glucose from the filtrate back into the blood, preventing glucose loss in urine.

The term describes the surface proteins that allow cells to adhere to each other and form tissue is cell-adhesion molecules (CAMs).

CAMs are proteins on the cell surface that help cells stick to each other, allowing them to form tissues by creating stable cell–cell connections.

False.

Different types of CAMs are used for different types of cell–cell junctions.

CAMs enable cells to stick together, allowing them to form organised tissues and structures essential for the complex functions of multicellular organisms.

Cell-cell junctions provide structural cohesion and communication pathways between cells, ensuring tissue coordination.