Cell Membrane Composition

Plasma Membrane

• It is of great importance to the function of the cell.
• It is composed of a phospholipid bilayer with a phospholipid nature
  • The phospholipid bilayer is both hydrophilic and hydrophobic.
• The structure is a fluid mosaic.

Plasma Membrane Composition

• Proteins are a key component
  • These can be transmembrane, or surface proteins
• Lipids are another key component
  • These can be phospholipids or cholesterol
• Carbohydrates are important for membrane composition
  • Glycolipids and glycoproteins are lipids and proteins connected to a carbohydrate chain.
  • They are used for Identification

Cell Membrane Composition

• Several membrane proteins are present:
  • Carrier proteins move substances across the plasma membrane, altering shape to facilitate transport.
  • Cell recognition proteins, including glycoproteins, identify cells.
  • Channel proteins move substances across the membrane, without changing shape.
  • Enzymatic proteins assist in metabolic interactions on the membrane.
  • Junction proteins connect cells to extracellular materials, forming cellular junctions.
  • Receptor proteins regulate a cell's internal environment by responding to stimuli and altering shape.

Permeability of Plasma Membrane

• It demonstrates selective permeability.
  • It allows nonpolar and polar molecules to pass through.
    • It contains hydrophobic and hydrophilic heads.
  • Substances follow a concentration gradient from an area of high to low concentration which does not require energy input.
• Water flow is important
  • Aquaporins are channel proteins specifically for water transport.
• Ions and amino acids utilize carrier proteins
  • Carrier proteins must bind to an ion and change shape to facilitate transport and are highly specific.
• Bulk transport transports large quantities of molecules into or out of the cell with vesicles and the plasma membrane.

Diffusion and Membranes

• Diffusion is the spreading of solutes in a solution, moving along a concentration gradient between solutes and solvents.
• Several factors influence the speed of diffusion
  • These include the size of molecules, temperature, concentration, and pressure.

Diffusion and Osmosis

• Osmosis is the movement of water across a membrane.
  • Osmosis can be contrasted with diffusion.
  • Osmosis is important for water regulation.
  • It is directly related to osmotic pressure.
  • It also uses aquaporins.

Diffusion and Osmosis

• Water concentration in a system can greatly impact an organism:
  • Isotonic conditions mean water concentration is equal inside and outside cells, resulting in no net water movement.
  • Hypotonic conditions mean water concentration is less within the cell, leading to water influx and turgor pressure in plants
  • Hypertonic conditions mean water concentration is greater within the cell, leading to water efflux, plasmolysis, and crenation

Membrane Transport Mechanisms

• Facilitated transport:
  • It moves solutes across a membrane using a transporter protein, either carriers or channels.
  • It is a form of passive transport
    • It requires no energy
    • It follows the concentration gradient.
• Active transport involves crossing/diffusion that requires energy input.
  • It goes against the concentration gradient.
  • It utilizes a transporter protein and ATP hydrolysis, as seen in Na and K pumps.

Membrane Trafficking

• Vesicles transport substances into and out of cells:
  • Exocytosis involves a vesicle binding to the cell membrane and releasing its contents, such as enzymes, hormones, and neurotransmitters.
  • Endocytosis involves a vesicle forming and taking extracellular substances into the cytoplasm.
• The plasma membrane and endomembrane system are involved in both endocytosis and exocytosis.

Endocytosis

• There are three types:
  • Phagocytosis is a process where the cell membrane forms a pocket around large material.
    • It is used by white blood cells.
  • Pinocytosis forms a pocket around small material/multiple solutes.
  • Receptor mediated endocytosis is highly specific, where certain molecules bind to receptors.
• The usage of lysosomes is key to breaking down the contained material.

Cell Surface Specialization

• The Extracellular Matrix is involved in excretion from the cell, and is composed of proteins, polysaccharides, cell wall, capsule and works with the cytoskeleton
• Cellular Junctions connects adjacent cells
  • These junctions enable cells to communicate and share materials.

Cellular Junctions

• There are several types of cell junctions.
  • Tight junctions connect adjacent cells to form a seal, with plasma membranes touching and commonly found in intestines and the bladder.
  • Adhesion junctions connect adjacent cell cytoskeletons, with cell membranes that do not touch, and are subjected to large amounts of strain, and found in animals
  • Gap junctions: closable channels that connect the cytoplasm of cells allowing sharing of ions and nutrients, in response to stimuli, and found in Animals
  • Plasmodesmata are channels that permeate the cell wall, connect cytoplasm, share ions and nutrients, and found in plants.