Membrane Function

Introduction to Membranes and Energy

• Composed mainly of a phospholipid bilayer, also containing other lipids (cholesterol), protein, carbs

General properties of a phospholipid bilayer

• Phospholipid Bilayer: Composed mainly of phospholipids.

  • Contains other lipids (e.g., cholesterol), proteins, and carbohydrates.

• Phospholipids:

  • Structurally, they consist of two fatty acid tails and a phosphate group head.

1. Self-assembling in Water: Phospholipids spontaneously arrange into bilayers in aqueous environments due to hydrophilic (water-attracting) and hydrophobic (water-repelling) interactions.

    Fatty Acids: long-chain organic acids, which can be classified into two types:

• Saturated Fatty Acids: Have no double bonds between carbon atoms, resulting in higher melting points.

• Unsaturated Fatty Acids: Contain one or more double bonds, leading to lower melting points.

2. Phospholipid bilayers made with unsaturated fatty acids are more flexible at colder temperatures than bilayers made with saturated

3. Individual molecules are not static (fluid mosaic)

• Molecules within the membrane are not static, illustrating the fluid mosaic model.

• Example: Staining membrane proteins from different cells with different colors and fusing them to observe distribution.

4. Semi-permeability: Membranes allow certain molecules to pass while restricting others:

• Phospholipids are Permeable to:

• Hydrophobic molecules

• Small molecules (e.g., water)

• They are Impermeable to:

• Large molecules

• ions

Membrane Proteins

• Held in membrane due to their:

  • hydrophobic and hydrophilic nature

  • attachment to cytoskeleton

  • there are outward facing amino acids with hydrophilic side chains, integral protein, amino acids with hydrophobic side chains, and peripheral protein

• Functions of Membrane Proteins:

  1. Enzymes

  2. Signal transduction

  3. Transport (may or may not require the use of the cell’s stored energy)

  4. Attachment to Cytoskeleton

  5. Cell Recognition

  6. Cell-to-cell connection

Movement Across Membranes

• Passive Transport: Diffusion across a membrane. Results from the random movement of objects. The net movement is always from an area of high to low concentration. Does not require energy from the cell. Eventually equals out.

  • Factors Affecting Rate of Diffusion:

    1. Temperature (velocity of particles)

    2. Concentration gradient

    3. Ability to pass through the membrane.

• Facilitated Diffusion: Materials not permeable to phospholipid bilayers may pass through integral proteins

• Most water passes through aquaporins (channels)

• Cholesterol prevents the passage of water across most biological membranes

• Osmosis: diffusion of water:

  • Water moves from areas with low solute (dissolved substance) concentration to areas with high solute concentration.

  • More free water molecules exist on side with smaller solute despite equal total numbers of water molecules. Net movement goes from the free water side to the other side.

Salt Concentration Comparison Terms

• Isotonic: same salt concentration.

• Hypotonic: lower salt concentration than another solution, causing cells to swell.

• Hypertonic: higher salt concentration than another solution, causing cells to shrink.

• Solutions with different salt concentrations can cause cells to lose or gain water. Hypotonic- bursts, Hypertonic- shrinks

• Organisms with cell walls can use water pressure to give them structure and movement. ex. seeds on a plant dispose by exploding

Active Transport

• Movement of substances against their concentration gradient, requiring potential energy stored by the cell

  • Energy: the ability to do work, cannot be created or destroyed, only converted from one form to another

• 4 Forms of Energy Important to Living Things:

  1. Heat: Movement randomization of particles. (living things cant convert this to other forms of energy)

  2. Kinetic Energy: The net movement of particles.

  3. Potential Energy: bonds of molecules, concentration gradients.

  4. Light:

• Energy released during chemical reactions (from bonds) is used to:

  1. change the shape of proteins

  1. move molecules

  2. create new bonds

ATP (Adenosine Triphosphate)

• Acts as short term energy storage and transport unit

• Consists of Adenine, a phosphate group, and ribose

• Cycle of ATP+H2O (hydrolysis) and ADP absorbing and creating phosphate and energy

• ATP as an Energy Source:

  1. Phosphorylation: Addition of phosphate to a molecule. Carried out by kinase enzymes

  2. Binding to a molecule and hydrolyzing

Active Transport

• Used for:

  • Transport against a concentration gradient

  • Transport of impermeable substances (substances that can’t diffuse through a membrane)

  • Both are accomplished by membrane bound proteins

• Examples:

  • Protein Ion Pumps:

  • Use ATP to transport ions (e.g., Na+, K+ ions).

    1. The protein’s shape causes Na+ binding.

    2. Na+ binding onto the protein induces phosphorylation.

    3. Phosphorylation prompts a shape change, releasing Na+ on the opposite side and..

    4. Binding K+, and releasing phosphate

    5. Releasing phosphate causesprotein to change shape and…

    6. Release K+.

  • Result: More Na+ ions are pumped to the outside, than K+ is pumped to the inside, leading to a charge difference across the membrane (membrane potential):

  • The outside has a positive charge compared to the inside.

• Co-transport Mechanism:

  • Active transport by one protein can drive transport of another (e.g., sucrose passage through the cotransporter requires H+ to also pass. the cotransporter does not use ATP).

  • A proton pump uses ATP to create a H+ concentration gradient

Bulk Transport Mechanisms

• Endocytosis and Exocytosis:

  • Transport by movement of the membrane

  • Used for bringing large molecules into and out of the cell

• Exocytosis: Cells involved have golgi bodies

• Example Highlight: Plant root tip cells and their interactions with surrounding environments during these processes.