Cell Membrane Structure and Function

• Cystic fibrosis is caused by the failure of transporter proteins to regulate the movement of substances in and out of cells.

Cell Membrane Structure

• The basic structure of all cell membranes is the lipid bilayer with many embedded proteins.
• Membranes act as continuous, selectively permeable barriers.
• Phospholipid molecules in the plasma membrane have hydrophilic heads that interact with water and hydrophobic tails that interact with each other to form a barrier to hydrophilic molecules.

Fluid Mosaic Model

• Describes the organization of cell membranes.
• Phospholipids drift and move like a fluid.
• The bilayer comprises a mosaic mixture of phospholipids, steroids, proteins, and other molecules.

Variations on the Model

• Membrane composition varies in terms of different kinds and numbers of carbohydrates attached to membrane proteins and different kinds of phospholipids.
• Membrane fluidity differs; some proteins are attached to the cytoskeleton while others drift around.
• Archaeans have more rigid membranes compared to bacteria or eukaryotes.

Membrane Proteins

• The cell membrane's function includes the roles played by many proteins associated with the lipid bilayer.
• Each type of protein in a membrane has a special function such as adhesion, recognition, acting as receptors, enzymes, or transport proteins (active and passive).

Common Types of Membrane Proteins

• Passive transporters allow ions or small molecules to cross a membrane to an area of lower concentration.
• Active transporters pump ions or molecules through membranes to an area of higher concentration and require energy input, such as ATP.
• Receptors initiate change in a cell's activity by responding to an outside signal.
• Cell adhesion molecules help cells stick to each other and to the extracellular matrix.
• Recognition proteins identify cells as self (belonging to one's own body or tissue).
• Enzymes speed up reactions without being altered by them.

Main Concepts

• Cell membranes have a lipid bilayer that acts as a barrier between the external environment and the internal cell environment.
• Diverse proteins, either embedded in or positioned on the surface of the bilayer, perform most membrane functions.

Movement Across the Membrane

• Ions and molecules move from one region to another in response to gradients.
• Selective permeability defines a cell membrane's ability to control which substances enter or exit and how much.
• Selective permeability enables the cell to maintain a difference between its internal and external environments.
• Selective permeability supplies the cell with nutrients, removes wastes, and maintains volume and pH.

Concentration Gradients

• Concentration refers to the number of molecules or ions of a substance per unit volume of fluid.
• A concentration gradient defines the concentration difference between two adjacent regions.
• Molecules move from a region of higher concentration to one of lower concentration.

Diffusion

• It's the net movement of molecules down a concentration gradient.
• Diffusion moves substances into, through, and out of cells.
• Substances diffuse depending on its own concentration gradient, irrespective of gradients of other solutes.

Rate of Diffusion

• The rate of diffusion depends on factors such as size, temperature, steepness of the concentration gradient, charge, and pressure.
• Gases and nonpolar molecules diffuse freely across a lipid bilayer.
• Ions and large polar molecules need help to cross the membrane.

Passive and Active Transport

• Molecules and ions diffuse across a lipid bilayer only when aided by a transport protein.
• Passive transport does not require energy input.
• Passive transport allows a specific solute, such as glucose, to follow its concentration gradient via a transport protein, and gated passive transporters change shape when specific molecules bind.
• Active transport requires energy input.
• Active transport moves a solute up its concentration gradient to the concentrated side of the membrane.
• Calcium pumps are an example of active transport.
• Active transporters move calcium ions across muscle cell membranes to the sarcoplasmic reticulum.

Cotransport

• In cotransport, an active transport protein moves two substances across a membrane.
• Sodium-potassium pumps move Na+ out of the cell and K+ into the cell

Diffusion and Membrane Transport

• Gradients facilitate the directional movement of substances across membranes.
• Transport proteins work with or against these gradients, ensuring the maintenance of water and solute concentrations.

Membrane Trafficking

• Endocytosis and exocytosis use vesicles.
• Vesicles help the cells take in or expel.
• Vesicles helps get rid of particles that cannot be transported by transport proteins and substances in bulk.
• Membrane trafficking is the formation and movement of vesicles formed from membranes, involving motor proteins and ATP.

Exocytosis

• Exocytosis includes the fusion of a vesicle with the cell membrane, releasing its contents to the surroundings.
• Endocytosis is the formation of a vesicle from the cell membrane, taking in materials near the cell surface.
• Bulk-phase endocytosis captures extracellular fluid in a vesicle and brings it in.
• Receptor-mediated endocytosis occurs when specific molecules bind to surface receptors enclosed by an endocytic vesicle.
• Phagocytosis involves pseudopods engulfing a target particle which then turns into a vesicle, which fuses with a lysosome in the cell.

Membrane Cycling

• Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane.
• The proteins, in new membranes, and lipids are made in the ER.
• The proteins, in new membranes, and lipids are then modified in Golgi bodies.
• Vesicles fuse with the plasma membrane.

Water Movement

• Water diffuses across cell membranes through osmosis.
• Osmosis is driven by tonicity and countered by turgor pressure.
• Osmosis is the movement of water down a concentration gradient through a selectively permeable membrane from a region of lower to a region of higher solute concentration.

Tonicity

• Tonicity is the relative measure in solute concentrations in two fluids that are seperated by a selectively permeable membrane.
• For two fluids, the one with the lower solute concentration is hypotonic, while the one with higher solute concentration is hypertonic, and water diffuses from hypotonic to hypertonic.
• Isotonic fluids have the same solute concentration.

Effects of Fluid Pressure

• Hydrostatic pressure happens because a volume of fluid exerts pressure against surrounding things like membrane, tube, or cell wall.
• Osmotic pressure is the volume of hydrostatic pressure that can prevent water from going into cytoplasmic fluid.

Tonicity in Plant Cells

• Turgor pressure within a plant cell relies on the tonicity of the solution that it is bathed in.