- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Structure-function of cell membranes : Where are membranes found in cells ?
A Typical Prokaryotic Cell
• Cell wall
• Plasma membrane
• Cytoplasm
• Free DNA
• Ribosomes
0.1 – 10μm Depending on the species
B2.1.1 (a) What are phospholipids? What are their properties? What do they form?
(b) What structures do phospholipids spontaneously form in water?
(a) Cell membranes are primarily composed of phospholipids.
Properties:
- They are Amphipathic (meaning they have both hydrophilic and hydrophobic)
- Phospholipids are made from a polar head, which is hydrophilic
- It contains phosphate and glycerol
- Also contains two non-polar fatty acid tails which are hydrophobic
(b) Phospholipids spontaneously arrange into a bilayer.
Hydrophobic tail regions face inwards and are shielded from the surrounding polar water/fluid, the hydrophilic phosphate and glycerol in the head region attracts to the water outside and inside the cell. Phospholipids are held together in a bilayer by hydrophobic interactions (weak associations) of the fatty acid tails.
B2.1.2- How do lipid bilayers serve as barrier around cells? What is the basis of this?
The lipid bilayers in cell membranes act as selective barriers , which are impermeable to and block the entry of large molecules, ions, and polar substances, due to the hydrophobic region in the middle made of fatty acid tails.
This selective permeability is essential for maintaining the internal environment of the cell;
B2.1.3 – What molecules can use simple diffusion to cross membranes?
An example of simple diffusion is the exchange of oxygen and carbon dioxide across cell membranes , these molecules are small and non-polar and can cross the cell plasma membrane easily, without using energy following their concentration gradients;
This process is vital for cellular respiration, where oxygen is required for energy production, and carbon dioxide is a waste product to be expelled
B2.1.4- (a)What are Integral and peripheral proteins in membranes?
(b) What are some of the functions of proteins in the membrane?
(a) Integral proteins are embedded in one or both of the lipid layers of a membrane
Peripheral proteins are attached to one or other surface of the bilayer , they have diverse structures, locations and functions, including transport channels and receptors.
(b) Transport Proteins: Facilitate molecule movement in and out of cells, including channel and carrier proteins.
Channel Proteins: Form pores for molecule passage.
Carrier Proteins: Change shape to transfer molecules across the membrane.
Recognition: Act as cellular ‘name tags’ for cell-cell recognition, crucial in immune system functioning.
Receptors: Bind to chemical signals like hormones, triggering intracellular reactions.
Enzymes: Catalyse reactions, e.g., glucose-6-phosphatase in the endoplasmic reticulum.
Cell Adhesion & Motility: Aid in cell adherence and movement.
B2.1.5- How do water molecules move across membranes?
Water moves across membranes always via osmosis:
This due the random movement of water molecules, water moves from areas of lower solute concentration to areas of higher solute concentration.
Solutes cannot pass the through the membrane easily as it is not very permeable to them, water can move through aquaporins, which are specialized channel proteins facilitating water movement.
B2.1.6- What are channel proteins what process of membrane transport uses them? Give examples.
Channel proteins are proteins which cross the plasma membrane, they allow specific ions to diffuse through when open due to the amino acids which make up the inside of the channels, so they only attract certain ions/molecules
e.g. glucose; through the GLUT channels contributing to selective permeability;
B2.1.7 – What are pump proteins? What process uses them? Give examples.
Active transport is the passage of materials against a concentration gradient (from low to high).
This process requires the use of protein pumps which use the energy from ATP to translocate the molecules against the concentration gradient.
The hydrolysis of ATP causes a conformational change in the protein pump resulting in the forced movement of the substance.
Protein pumps are specific for a given molecule, allowing for movement to be regulated (e.g. to maintain chemical or electrical gradients).
e.g. Na+/K+ pump which is involved in the generation of nerve impulses
3 sodium pumped out for every 2 potassium pumped in to the axon
B2.1.8 – What is permeability? How are membranes selectively permeable?
Permeability is ability of a membrane to allow molecules to pass through.
Selective permeability is when a membrane does not allow the free movement of all molecules and is permeable only to certain molecules due to specific channel proteins; pump proteins, which only allow specific molecules to pass.
e.g. Calcium or sodium ions
B.2.1.9 – What is the structure, function and location of glycoproteins and glycolipids?
Glycoproteins are carbohydrate structures linked to proteins in membranes
Glycolipids are carbohydrate structures linked to lipids in membranes
They are both exclusively on the extracellular side.
They are used in crucial for cell adhesion and cell recognition.
e.g. receptors.
B2.1.10 – (a) What is the fluid mosaic model of membrane structure?
(b) Draw a diagram to represent the fluid mosaic structure of the membrane
(a) The Fluid mosaic model was proposed by Singer and Nicolson.
It says that both integral and peripheral proteins are embedded in the fluid bilayer, forming a mosaic pattern.
Lipids and proteins can move laterally within the membrane, meaning it is fluid.
Fluidity depends on fatty acid types in phospholipids and the cholesterol content.
(b)
AHL Only – B2.1.11—What is the relationships between fatty acid composition of lipid bilayers and their fluidity?
The fatty acid composition of the membrane can affect its fluidity, if there are more unsaturated fatty acids, which contain double bonds, leading to lower melting points. This makes the lipid bilayer more fluid and flexible. If there are more saturated fatty acids which do not have double bonds, this results in higher melting points.
AHL Only – B2.1.12—What is cholesterol? What is the impact of cholesterol on membrane fluidity in animal cells?
Cholesterol is hydrophobic found embedded within the lipid bilayer ,between hydrophobic fatty acid tails ,as it has a hydrophilic region as well, making it amphipathic.
Function:
- It acts as a fluidity regulator;
- Cholesterol reduces fluidity ,making membranes more stable at higher temperatures;
- It also prevents crystallisation at lower temperatures.
AHL Only – B2.1.13— (a) What is membrane fluidity?
(b) What is endocytosis? What is an example?
(c) What is exocytosis? What is an example?
(a) Fluidity is the ability of the membrane to move in a flexible way. It also describes the way that membranes can fuse and the way membranes can form smaller regions of membrane without breaking.
(b) Endocytosis is a process where large amounts of substances can enter the cell. During endocytosis, the membrane can wrap around and pinch off, forming a vesicle due to fluidity of membrane.
It can remain unbroken
e.g. phagocytosis of bacteria by phagocytes.
(c) Exocytosis is a process where large amounts of proteins are synthesised by rough endoplasmic reticulum , which are packaged into vesicles which pinch-off or bud-off from the rough endoplasmic reticulum and are carried to the golgi apparatus ,vesicles fuse with the flattened-sac membranes of the golgi, modification and processing of proteins to put them in their final form takes place ,vesicles bud-off again and travel to the plasma membrane or other locations in cell, fuse with the membrane to secrete contents outside the cell.
AHL Only – B2.1.14—How are gated ion channels used in neurons?
Voltage-gated channels open and close in response to electrical charge. They are carrier proteins.
If there is a change in voltage around the channel ,causes it to open, potassium channel open, when there are more positive charges inside the cell than outside.
K+ can flow through down the concentration gradient and aids in repolarisation of axon as positive potassium flow down concentration gradient out of cell.
AHL Only – B2.1.15—What is the sodium-potassium pump? How is it an example of as an example of an exchange transporter?
Active transport of sodium and potassium uses energy from ATP to pump. The Sodium potassium pump transports 3 sodium ions OUT of cell for every 2 potassium ions IN. Sodium ions bind to interior of pump on inside of axon. ATP hydrolysis allow phosphate to bind to pump, which causes a conformational change (change in shape) of pump, releasing sodium outside the cell.
2 potassium bind to pump outside of the cell causing the release of phosphate, causing a conformational change in the pump, releasing potassium inside the cell.
AHL Only – B2.1.16—What are sodium-dependent glucose co-transporters? How are they an an example of indirect active transport?
Sodium-dependent glucose co-transporters facilitate glucose transport into cells, alongside sodium ions. It is a form of indirect active transport. Sodium ions are pumped out of cells, leading to a concentration gradient, as they flow back down their gradient the energy can be used to transport glucose into cells.
AHL Only – B2.1.17—How do cells adhere to form tissues? What are CAMs?
Cell-Adhesion Molecules (CAMs) are proteins that allow cells to adhere to each other, forming stable tissues
Different forms of CAMs are used in different types of cell-cell junctions.