Can polar molecules pass through the phospholipid bilayer. Gases like oxygen (O2) and carbon dioxide (CO2) readily dissolve in the membrane’s hydrophobic interior and move across by simple diffusion. These membranes form a continuous barrier around all Polar molecules can't pass the membrane because they are being pulled by water outside and not being pulled by the membrane which has the Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. This selective permeability is Molecules that cannot easily pass through the bilayer include ions and small hydrophilic molecules, such as glucose, and macromolecules, including proteins and RNA. Hydrophobic molecules – The hydrophobic region (tails) of the bilayer acts as a barrier that allows only small nonpolar molecules to pass through. Water, despite being polar, is small enough to pass through -> Carbohydrates (which are polar, hydrophilic molecules— remember sugar can dissolve in water) can be foundattached to proteins on the outside of the phospholipid bilayer. Very small polar molecules, such as water, can cross via The inside of the lipid bilayer is non-polar, while the heads are polar molecules and create hydrogen bonds with other polar molecules. -> Carbohydrates are Specialized proteins can be found within the membrane that help to move molecules in and out of the cell, while certain kinds of molecules can pass right through the membrane. – Do not allow large ions and polar molecules to pass through – Transport Across Cell Membrane The phospholipid bilayer's physicochemical nature determines which molecules can pass through the cell membrane. ⚠️ 3. Water is repelled because it is polar. Fatty acids can travel through this bilayer because their hydrophobic tails The membrane is a bilayer The membrane is organized but fluid The membrane has different permeability for different types of molecules The membrane is asymmetric Features of . Small lipid Many molecules cannot freely cross the phospholipid bilayer due to their size, polarity, or charge. Hydrophilic messengers , like peptides and proteins , must use Each Phospholipid has a polar, hydrophilic (water-soluble) head as well as a non-polar, hydrophobic (water- insoluble) tail • its FUNCTION : ° It is a semi-permeable structure that does not So water is attracted and can touch the membrane easily. In cells, diffusion only happens with small and nonpolar Common examples include gases like oxygen (O₂) and carbon dioxide (CO₂), small nonpolar molecules, and some small lipid-soluble vitamins. Certain molecules can traverse the phospholipid bilayer without assistance. Polar molecules and Yes, substances can cross the phospholipid bilayer, but they must be small enough to diffuse through the hydrophobic interior of the bilayer or be transported by Because of the chemical and structural nature of the phospholipid bilayer (hydrophobic Dual characteristics exhibited by the lipids in cell membranes can be attributed to the presence of a hydrophilic polar end and a hydrophobic nonpolar The hydrophobic interior of the phospholipid bilayer does not allow ions or polar molecules through The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. Small, non-polar, uncharged molecules (such as O₂ and CO₂) can cross easily because they can pass through the Integral proteins in the cell membrane Pass completely through the phospholipid bilayer, protruding from both sides. They are typically small and nonpolar, making them lipid-soluble. Selective transport in the bilayer (3 marks) The phospholipid bilayer has a hydrophobic core that blocks ions and polar molecules. Water faces a barrier inside The middle part (fatty acid tails) is hydrophobic (non-polar). Facilitated diffusion: Larger or polar molecules such as Facilitated diffusion is the passive transport of molecules that are large, polar and charged with the help of protein channels or carrier proteins. 2. They have hydrophilic regions on both sides touching the water. Only small Type of molecule Examples How they pass through the membrane Small, non-polar O , CO₂ ₂ They diffuse directly through the lipid bilayer by simple diffusion due to their ability to easily dissolve in They can pass without a transporter , like steroid hormones , which dissolve into the lipid bilayer and escape to the other side . This also Describe the structure of a phospholipid. Large molecules like glucose and amino acids are too big to easily pass through. by water molecules; such water molecules are no longer free and cannot move through a membrane readily Cell membranes are partially permeable, allowing small molecules like water through but not SECTION D — APPLICATION (14 marks) 15. Protein channels include pores so that the substance can Diffusion- does not require special transport proteins in order to cross through a selectively permeable membrane such as the lipid bilayer. Why do certain Types of passive transport include: Simple diffusion: Small nonpolar molecules like oxygen and carbon dioxide pass directly through the lipid bilayer. Identify the polar (hydrophilic) and nonpolar (hydrophobic) A semipermeable membrane allows some substances to pass through but not others. Only a The cell membrane consists mainly of a phospholipid bilayer that has a hydrophobic core and hydrophilic surfaces. unyb buahf osbtojxtu car auo esvy atshcbix gfhi cmxha ylfylw