Cell Membrane Transport: Passive and Active Processes

• Biological membranes, also known as cell or plasma membranes, are selectively permeable, allowing some substances to pass while blocking others.
• These membranes have a fluid-like consistency, similar to oil, which facilitates the passage of ions and molecules.
• Rough Endoplasmic reticulum (rough ER) are located close to the nucleus and is composed of lipids, proteins, and carbohydrates.

Fluid Mosaic Model

• Describes the dynamic and semi-permeable structure of biological membranes, in which protein molecules "float" in a fluid phospholipid bilayer.
• It is called a mosaic due to the variety of molecules embedded within it.
• The model describes the amphipathic nature of the membrane, with both hydrophilic and hydrophobic regions.
• Functions to protect the inside of the cell from the outside environment.
• The extracellular side is composed of glycolipids, carbohydrates, and glycoproteins.
• The cytoplasmic side consists of sterol, microfilaments of the cytoskeleton, peripheral proteins, and integral proteins.

Phospholipid Bilayer

• Composed of phospholipids arranged in a bilayer.
• Hydrophilic heads face outward, interacting with water, and are polar.
• Hydrophobic tails face inward, shielded from water, and are nonpolar fatty acid chains.
• One tail is straight, while the other has a kink due to a cis double bond, enabling movement.
• The bilayer provides fluidity, facilitating movement of proteins and lipids.

Membrane Proteins

• Proteins are mobile within the phospholipids.
• Integral proteins are embedded within the hydrophobic core, often spanning the bilayer.
• Transmembrane proteins span across both layers and are composed of hydrophobic amino acid stretches, forming alpha helices that interact with lipid tails.
• Peripheral proteins are loosely attached to the surface, interacting with other proteins and lipids.
• Proteins function in transport, signal transduction, enzymatic activity, cell-cell recognition, and intercellular joining.
• Transport proteins transport energy, often creating energy as a byproduct or producing ATP.
• Signal transduction involves detecting something from the outside and triggering a response inside the cell, initiated by a signaling molecule attaching to a receptor.
• Enzymatic activity is sped up by enzymes that are part of the membranes
• Proteins help with enzymatic activities.
• Cell-cell recognition allows cells to recognize nearby cells.
• Attachment to the cytoskeleton and extracellular matrix (ECM) is important, and ECM is maintained by attachment to microfibrils to maintain the plasma membrane.

Carbohydrates and Cholesterol

• Carbohydrates, in the form of glycoproteins and glycolipids, are involved in cell recognition and signaling.
• Cholesterol maintains membrane stability and fluidity.

Movement Within the Membrane

• Lateral diffusion involves phospholipids moving side-to-side within a single layer.
• Transverse diffusion ("flip-flop") is rare.
• Phospholipids also rotate within the layer.
• All membrane lipids are produced in the smooth ER (SER).
• SER contains all phospholipid components to create the head and tail sections, contributing to the bilayer.
• Enzymes in the SER join fatty acids, glycerol, phosphate, and head groups to make phospholipids.
• After synthesis, phospholipids are inserted into one of the monolayers and distributed by flippases to the other bilayer.
• Flippases are enzymes that transfer specific lipids to the other bilayer, creating asymmetry and allowing the membrane to grow.
• Proteins can contribute to membrane asymmetry.

Membrane Fluidity

• Allows for rapid movement of membrane proteins within the bilayer.
• Facilitates diffusion of lipids and proteins to their correct lateral locations after insertion into the bilayer.
• Fusion of membranes mixes their molecules (e.g., during cell reproduction).
• Ensures equal distribution of membrane molecules in daughter cells after division.
• Hindered substance transport occurs If membranes are too rigid.

Membrane Protein Mobility and Function

Proteins can move within the bilayer and are often grouped together, embedded in the fluid matrix of the lipid bilayer.

• Proteins influence membrane-specific functions.
• An experiment by Larry Fry & Michael Edidin fused mouse and human cells, protein mixing was observed.
• Experiment demonstrated that membrane proteins move freely.

Selective Permeability of the Membrane

• Plasma membranes regulate what enters and exits the cell.
• The cell must alter materials with its surroundings
• Selective permeability regulates cell traffic.
• Hydrophobic (nonpolar) molecules (CO2, O2, hydrocarbons) diffuse very quickly.
• Rapid cell functioning requires specific membrane proteins, such as for CO2 and O2.
• O2 doesn't need transport proteins out of the cell for photosynthesis.
• Hydrocarbons can pass through the lipid bilayer, but are slow since their molecules are bigger than oxygen and carbon dioxide
• Hydrophilic (polar) molecules (ions, sugars) do not cross easily and require transport proteins.
• Transmembrane proteins accelerate hydrophilic molecule movement.

Transport Proteins

• Transport proteins enable the passage of hydrophilic substances across the membrane.
• Channel proteins provide a hydrophilic tunnel for molecules or ions, Aquaporins are an example
• Passive transport does not require energy and if it fits, then the transport happens.
• Carrier proteins bind to molecules and change shape to transport them across the membrane.
• These are specific to the substances
• Active transport requires energy for the change to occur.

Types of Transport

• Passive transport does not require energy.
• Diffusion occurs down the concentration gradient, moving from high to low density
• A directional diffusion of a population occurs, with plasma membrane in the middle regulating the molecule's movement
• Dynamic equilibrium occurs when as many molecules cross the membrane in one direction as in the other
• No work must be done to move substances down the concentration gradient because solutes move down it naturally.
• Diffusion across a biological membrane is passive transport.
• The cell is able to perform passive transport because water is not needed
• Molecules spread evenly into space until until equilibrium is reached.
• Continues until equilibrium is reached.
• Osmosis describes the diffusion of water across a selectively permeable membrane which moves from a low to high concentration.
• Facilitated diffusion is when large/ charged molecules use transport proteins to cross
• Transport proteins speed up passive movement of molecules across the membrane.
• It is still considered passive transport because no energy is needed
• Channel proteins provide corridors for specific molecules or ions.
• Aquaporins facilitate the diffusion of water.
• Ion channels facilitate the diffusion of ions, and are called gated when they open/close in response to a stimulus.
• Carrier proteins undergo subtle shape changes to move solutes across the membrane.

Effects of Osmosis on Water Balance

• Tonicity refers to the effect of the surrounding solution on a cell.
• Cell walls play a role.
• Isotonic solution: solute concentration is equal.
• No net water movement.
• Plant cell is flaccid.
• Animal cell normal.
• Hypertonic solution: higher solute concentration outside the cell.
• Water leaves the cell, which means cell is shriveling (plasmolysis).
• Plant cell: plasmolyzed.
• Animal cell: shriveled.
• Hypotonic solution: lower solute concentration outside.
• Water enters the cell, leading to swelling.
• Plant cell: turgid.(normal for plants).
• Animal cell: lysed (bursts).

Tonicity and Food Preservation

• Moisture drawing out of food prevents spoiling.
• Replacing of water with a solute can draw out the water.
• Air interacts with food; without air, the contamination is lessened
• Salt and sugar are used in food preservation by creating a hypertonic environment.
• Bacteria growth is prevented via reduction, thus preventing spoilage.
• Plasmolysis occurs in a hypertonic solution, water leaves the vacuole, which causes the cytoplasm to shrink, however the cell wall remains intact.

Water Balance of Cells with Cell Walls

• Cell walls contribute to water balance.
• Plant cells in a hypotonic solutions cause water to enter creating turgidity or firmness in the cell.
• Plant cell in an isotonic solutions will cause the it to become flaccid ( limp).
• Plant cells in a hypertonic environment case cells to lose lose water, causing it to plt because the loss of water has led to plasmolysis

Active Transport

• Molecules are transported across the membrane using energy (ATP)
• Molecule movement occurs against their concentrations gradient. uses energy.
• Transport is performed by specific membrane proteins and Special membrane proteins.

Membrane Potential

• Voltage is created by differences in the distribution of positive and negative ions across the membrane from the voltage difference.

Electrochemical Gradient

• Two forces drive the diffusion of ions across the membrane: chemical and electrical.
• Chemical force is the ion's concentration gradient.
• The effect of membrane potential on ion movement is electrical force.

Electrogenic Pumps

• Transport proteins generate voltage across a membrane.
• Sodium-potassium pump occurs through the electrochemical gradient in animal cells.
• Proton pump occurs in plants, fungi, and bacteria.
• Electrogenic pumps are stored for cellular work.

Cotransport

• Movement of substances occurs by a membrane protein, using the active transport of one solute to drive another.
• Proton pumps are involved in cotransport.
• When a proton pump releases H+ ions, sucrose enters the cell via sucrose cotransport
• Less energy is needed for cotransport of sucrose, because every H⁺ ion, sucrose is also transported.
• The resulting gradient causes the active transport of nutrients into the cell.

Bulk Transport Across the Plasma Membrane

• Cells transport large quantities of substances via exocytosis and endocytosis.
• Small molecules and water pass through the lipid bilayer or transport proteins where large molecules use vesicles to cross through the membrane.
• Requires energy.
• During exocytosis. transport vesicles migrate, fuse with the membrane, and release their contents.
• Many secretory cells use exocytosis to remove waste.
• Endocytosis: macromolecule intake occurs by use of vesicles from the plasma membrane.
• A reversal of exocytosis involves proteins.
• Phagocytosis is "cell eating," example is Amoeba engulfing food
• Pinocytosis is "cell drinking" that that causes cells to need water in bulk.
• Receptor-mediated endocytosis is a specific, highly selective process, using specialized receptors in the plasma membrane that help in cell-to-cell recognition.
• Receptors bind with specific ions or molecules needed by the cell.
• The membrane forms transport the substance into the cell.

Explore the principles of cell membrane transport. This includes passive mechanisms like diffusion and osmosis, as well as the role of transport proteins in facilitated diffusion. Understand the selective permeability of biological membranes and the fluid mosaic model.