T1 Cell Biology

1. For each of the following chemical interactions or bonds, provide two distinct examples found within cells and explain their significance in cellular processes. [8 marks] i. Electrostatic attraction Enzyme-Substrate Interactions : Enzymes often have charged amino acid residues on their active sites that interact with oppositely charged substrates. These electrostatic attractions help enzymes bind to and catalyze reactions with their substrates.

Ion Channels : In cell membranes, ion channels contain charged residues that interact with ions (e.g., sodium, potassium) and facilitate their transport across the membrane via electrostatic attractions.

ii. Hydrogen bonds Complementary Base Pairing in DNA : Hydrogen bonds form between specific base pairs in DNA. Adenine (A) forms two hydrogen bonds with thymine (T), and guanine (G) forms three hydrogen bonds with cytosine (C).

Protein Secondary Structure : In proteins, hydrogen bonds play a crucial role in stabilizing secondary structures like alpha helices and beta sheets. These bonds form between the carbonyl group of one amino acid and the amide group of another along the polypeptide chain.

iii. Hydrophobic interaction Protein Folding : During protein folding, hydrophobic amino acid side chains tend to cluster together in the protein's interior to avoid contact with water. This hydrophobic effect contributes to the protein's overall stability.

Micelle Formation : In cell membranes, the hydrophobic tails of phospholipids aggregate together in the lipid bilayer, while the hydrophilic heads interact with water. wate This arrangement is essential for membrane structure

iv. Van der Waals Protein Tertiary Structure : In the three-dimensional structure of proteins, van der Waals interactions occur between nonpolar amino acid side chains. These interactions contribute to the compact folding of the protein.

Molecular Packing in Lipid Bilayers : In the lipid bilayer of cell membranes, van der Waals forces are responsible for the close packing of lipid molecules, helping to maintain membrane integrity

2. In cellular biology, cell membrane fluidity is a dynamic property crucial for cellular function. (A) Explain the concept of cell membrane fluidity and its importance in cellular biology. [3 marks] Cell membrane fluidity refers to the dynamic and flexible nature of the lipid bilayer in the cell membrane. In other words, phospholipids can move within the lipid bilayer via "flip-flop" and 'side-to-side" motions . It plays a crucial role in cellular biology as it impacts various cellular processes, including membrane protein function, cell signaling, cell division and the ability of the cell to adapt to changing environmental conditions (extracellular vesicles secretion, cell adhesion & migration, endocytosis & exocytosis).

(B) Discuss two factors that influence membrane fluidity. [4 marks] lipid composition: saturated vs unsaturated fatty acids and temperature unsaturated more fluid.

protein content (more protein, more rigid) fatty acid length (increase fatty acid chains, increase rigidity) lipid raft content (specialised membrane microdomains, high cholestrols and sphingolipids) more lipid raft more rigid

purpose of lipid raft: some receptor require to be present as a dimer and not a monomer to receive the ligand. if molecules are too far from each other so cannot form dimer. lipid raft can help to form dimer.

Two factors that influence membrane fluidity are:

Lipid Composition: The types of lipids present in the membrane can greatly affect fluidity. Unsaturated fatty acids introduce kinks in the lipid tails, preventing close packing and increasing fluidity. Saturated fatty acids, on the other hand, pack tightly and reduce fluidity. Temperature: Temperature affects membrane fluidity. Higher temperatures increase fluidity, making the membrane more permeable. Lower temperatures decrease fluidity and can lead to reduced membrane flexibility

(C) Provide two examples of how cells regulate membrane fluidity in response to changing environmental conditions. [4 marks]

1. incoperating cholesterols into membrane
2. Lipid desaturation eg. ocean fish maintain fluidity at low temperature by increasing mono or poly-unsaturated fatty acids

Cells can regulate membrane fluidity in response to changing environmental conditions. Cholesterol: Cells can adjust membrane fluidity by incorporating cholesterol molecules into the lipid bilayer. Cholesterol acts as a temperature buffer, increasing membrane stability at high temperatures and preventing excessive rigidity at low temperatures.

Lipid Desaturation: To increase fluidity, cells can introduce more unsaturated fatty acids into their membrane lipids. For instance, in response to cold temperatures, some organisms, like fish, modify the lipid composition of their cell membranes to maintain fluidity by increasing mono- or poly-unsaturated fatty acids.

3.(A) What is facilitated diffusion, and how does it differ from simple diffusion? [2 marks] simple diffusion without transport protein

facilitated diffusion uses a transport protein due to bigger size and they are charged

both must be from high concentration to low concentration due to concentration gradient

Cells can regulate membrane fluidity in response to changing environmental conditions. Cholesterol: Cells can adjust membrane fluidity by incorporating cholesterol molecules into the lipid bilayer. Cholesterol acts as a temperature buffer, increasing membrane stability at high temperatures and preventing excessive rigidity at low temperatures.

Lipid Desaturation: To increase fluidity, cells can introduce more unsaturated fatty acids into their membrane lipids. For instance, in response to cold temperatures, some organisms, like fish, modify the lipid composition of their cell membranes to maintain fluidity by increasing mono- or poly-unsaturated fatty acids.

(B) Explain the role of the transport protein involved in deliver glucose into cell. [2 marks] glucose binds to transport protein transport protein chamges shape glucose moves across cell membrane but only down the concentration gradient kinase enzyme reduces glucose concentration inside the cell by transforming glucose into glucose -6-phosphate transporter proteins always bring glucose into cell

two main types of glucose transporter:

• facilitated diffusion glucose transporters (GLUT)
• sodium-glucose linked transporters (SGLTs and Secondary active transport

Glucose transport into cells is facilitated by glucose transporters (GLUT proteins) which is a type of facilitated diffusions. These transporters bind to glucose molecules and allow their passage through the cell membrane, from high to low concentrations, ensuring efficient glucose uptake into the cell

(C) How does facilitated diffusion maintain homeostasis in cells, and why is it important for various physiological processes? [2 marks] A mutation in serine 226 of SLC2A1, which encode GLUT1 can cause GLUT1 dificiency syndrome (G1D) neurological disease (movement disorders, epilepsy, and developmental delay)

Serine 226 is the phosphorylation site of PKC (Protein Kinase C)

Facilitated diffusion plays a crucial role in maintaining homeostasis by allowing cells to regulate the transport of specific substances in and out of the cell. This controlled movement of molecules ensures that essential nutrients, ions, and other substances are available in the right concentrations for cellular processes. Without facilitated diffusion, many physiological processes, such as nutrient uptake and waste removal, would be disrupted, impacting cell function and overall organism health