Theme B - Cells B2.1 Membrane and Membrane Transport

check with syllabus and notes and slides

What are cell membranes composed of and what is its basis?

• lipids like phospholipids, glycolipids and sterols

• proteins

• small amounts of carbohydrates in the form of glycolipids and glycoproteins

Lipid bilayers are the basis of the cell membrane

Why do phospholipids spontaneously form bilayers in water?

Phospholipids are amphipathic. They have hydrophilic phosphate heads that are attracted to water and face the aqueous environment, and hydrophobic fatty acid tails that repel water and face inward, shielding themselves from the aqueous solution.

Explain why the lipid bilayer acts as an effective barrier between aqueous solutions.

The hydrophobic (non-polar) core formed by the fatty acid tails prevents hydrophilic particles (ions and polar molecules) from passing through freely.

• Small or large hydrophilic (polar) particles are unable to freely diffuse across the membrane

• Small or large hydrophobic (non-polar) particles freely diffuse across the membrane

What is the advantage of having bilayers for phospholipid?

Bilayers minimize disruption to the hydrogen-bonding network of water, ensuring energetically stability

By hiding the tails inside the bilayer, water molecules are "freed" from making rigid cages. They can move naturally and maintain their optimal hydrogen-bonding network. Ultimately, it means phospholipid bilayers form spontaneously not because the tails attract each other, but because water actively pushes them together to save its own network.

Compare the structure and function of integral and peripheral membrane proteins

• Integral proteins: Structurally hydrophobic on part of their surface and are embedded in the hydrocarbon chains of the membrane (many extend across the membrane). Function: Transport (channels/pumps) and receptors.

• Peripheral proteins: Structurally hydrophilic on their surface and are not embedded, but remain attached to the surface or to integral proteins. Function: Cell signaling and maintaining cell shape.

Outline the roles of glycoproteins and glycolipids in the cell membrane.

Both consist of carbohydrate chains attached to proteins or lipids facing the extracellular environment. They are primarily involved in cell adhesion (forming tissues) and cell-to-cell recognition (acting as receptors/antigens).

Draw a cell membrane (6)

• Phospholipids (drawn as a circle head with two distinct wavy lines for tails).

• Integral proteins (embedded in the bilayer).

• Peripheral proteins (on the surface).

• Glycoproteins (protein with a carbohydrate chain attached).

• Glycolipids (phospholipid head with a carbohydrate chain attached).

• Cholesterol (positioned between phospholipids in the hydrophobic core).

What is passive transport?

Movement of molecules across a cell membrane that does not require cellular energy (ATP)

• Simple diffusion

• osmosis

• facilitated diffusion

Explain what is meant by the selective permeability of cell membranes.

It is the property of the membrane that allows it to control which substances pass through. Small, non-polar molecules pass through easily, while large, polar, or charged particles are restricted and require specific transport proteins to cross.

Describe the simple diffusion of oxygen (O2) and carbon dioxide (CO2) across membranes

Both are small, non-polar molecules that pass directly through the phospholipid bilayer. It is a passive process where molecules move down their concentration gradient (from high to low concentration) due to the net movement, without using ATP, resulting from their random kinetic energy.

Functions of membrane proteins (5)

• Catalysis through enzymatic activity

• Attachment to other cells or to the extracellular matrix

• Transport of ions & polar particles

• Reception of signalling molecules in chemical and neural signalling

• Recognition of markers for cell identification (e.g. immunity)

What is osmosis and explain (5)

• the movement of water molecules from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration)

  • Water particles move randomly in all directions because they have kinetic energy (random movement of particles)

  • Water movement is affected by unequal distribution of solutes across the membrane

    • Substances become dissolved in water (i.e. solutes) because they form hydrogen bonds with water

  • Impermeability of membranes to solutes & difference in solute concentration causes osmosis

The role of aquaporins? (3)

• Transport channels (integral) that enable water molecules to move across the membrane by osmosis by forming hydrophilic channels that span across the membrane.

• Bidirectional, i.e., water can flow in either direction

• The amino acids lining the inner pore are selective for water molecules

What is facilitated transport? (3)

the movement of particles down their gradient across membranes by transport proteins without using any energy (ATP)

Explain how the structure of channel proteins makes membranes selectively permeable and carries out facilitated diffusion (3)

• Channels can only perform passive transport

  • Contain a hydrophilic pore (opening) that allows specific polar ions to diffuse through it

  • The outer surface of the channel is hydrophobic because it is embedded in the hydrophobic core of the lipid bilayer

  • Diffusion when channels are open, not when they are closed

What are the carrier proteins? (4)

• Carriers can perform both passive and active transport

  • Contain a solute-binding site that attaches to specific solutes

  • Binding of solute causes a conformational change in the protein structure of the carrier, which translocates the solute across the lipid bilayer & releases it on the other side

  • When involved in passive transport they are commonly referred to as carriers, but when performing active transport they are called pum

Flashcard 8

• Front: Explain how channel proteins assist in facilitated diffusion.

• Back: Channel proteins create hydrophilic pores through the membrane. They allow specific ions or polar molecules to pass passively down their concentration gradient without the expenditure of energy (ATP).

Flashcard 9

• Front: Explain how water moves across membranes by osmosis, including the role of aquaporins.

• Back: Osmosis is the net movement of water molecules from a region of lower solute concentration to a region of higher solute concentration across a partially permeable membrane. Aquaporins are specialized channel proteins that significantly increase membrane permeability to water, allowing it to flow much faster.

Flashcard 10

• Front: Explain how pump proteins transfer particles across membranes against a concentration gradient.

• Back: This is active transport. A specific solute binds to the pump protein. ATP binds to the protein and is hydrolyzed, providing the energy to cause a conformational (shape) change in the protein. This change translocates the solute across the membrane and releases it against its concentration gradient.