Cell Membranes and Transport

Membranes and Membrane Transport

• Lipid and protein molecules assemble into biological membranes.
• A substance's ability to pass through a biological membrane is determined by characteristics like size, charge, and solubility.

Lipid Bilayers as the Basis of Cell Membranes

• Phospholipids are amphipathic, containing hydrophilic heads and hydrophobic tails.
• In aqueous environments, phospholipids form bilayers, creating the basic structure of cell membranes.

Lipid Bilayers as Barriers

• Lipid bilayers act as selective permeability barriers.
• Hydrophobic cores prevent large molecules, hydrophilic particles, ions, and polar molecules from crossing.
• Membranes effectively separate aqueous environments inside and outside the cell.

Simple Diffusion Across Membranes

• Small, non-polar molecules like oxygen (O2) and carbon dioxide (CO2) diffuse directly across phospholipid bilayers.
• This diffusion occurs without the need for transport proteins.

Integral and Peripheral Proteins in Membranes

• Integral proteins are embedded within the lipid bilayer, spanning the membrane.
• Integral proteins function as channels or transporters.
• Peripheral proteins attach to the membrane surface, playing a role in signaling and maintaining cell shape.

Movement of Water Molecules Across Membranes by Osmosis and the Role of Aquaporins

• Osmosis involves water moving across membranes from lower to higher solute concentrations.
• Aquaporins are specialized channel proteins that facilitate rapid water transport.
• Aquaporins are crucial in cells requiring high water permeability.

Channel Proteins for Facilitated Diffusion

• Channel proteins allow specific ions or molecules to pass through membranes.
• This process occurs by facilitated diffusion, which allows selective permeability.

Pump Proteins for Active Transport

• Pump proteins use energy from ATP to move specific particles across membranes.
• This movement occurs against the concentration gradient.
• This process is known as active transport.

Selectivity in Membrane Permeability

• Facilitated diffusion and active transport enable selective permeability.
• These mechanisms allow cells to regulate their internal environment.

Structure and Function of Glycoproteins and Glycolipids

• Glycoproteins and glycolipids have carbohydrate chains on the extracellular side of membranes.
• These structures play roles in cell recognition and adhesion.

Fluid Mosaic Model of Membrane Structure

• The fluid mosaic model describes the membrane as a dynamic structure.
• Proteins and lipids move laterally within the bilayer.

Organelles and Compartmentalization

• Organelles in cells are adapted to their specific functions through their structure.
• Compartmentalization in cells offers advantages, such as increased efficiency by concentrating resources.

Organelles as Discrete Subunits of Cells Adapted to Specific Functions

• Organelles are specialized structures within cells that perform distinct functions.
• The nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus are examples of organelles.

Advantage of the Separation of the Nucleus and Cytoplasm into Separate Compartments

• In eukaryotes, the separation between the nucleus and cytoplasm allows for post-transcriptional modifications before translation.
• This separation enhances gene expression regulation.

Advantages of Compartmentalization in the Cytoplasm of Cells

• Compartmentalization allows for the concentration of metabolites and enzymes.
• The concentration of material enhances metabolic efficiency.

Cell Specialization and Stem Cells

• Stem cells play key roles in multicellular organisms, including development and repair.
• Differentiated cells are adapted to their specialized functions through specific structures and activities.

Production of Unspecialized Cells Following Fertilization and Their Development into Specialized Cells by Differentiation

• Following fertilization, cells undergo differentiation, leading to specialized cell types with distinct functions.
• Cell differentiation is vital for forming complex tissues and organs.

Properties of Stem Cells

• Stem cells can divide indefinitely and differentiate into various cell types.
• A stem cell's differentiation depends on its potency.

Location and Function of Stem Cell Niches in Adult Humans

• Specific microenvironments, such as bone marrow and hair follicles, maintain stem cells.
• These microenvironment niches regulate stem cell proliferation and differentiation.

Differences Between Totipotent, Pluripotent, and Multipotent Stem Cells

• Totipotent cells can form all cell types.
• Pluripotent cells can form most but not extra-embryonic tissues.
• Multipotent cells are limited to specific lineages.

Cell Size as an Aspect of Specialization

• Cell size varies depending on function.
• Neurons, red blood cells, and muscle fibers showcase cell size variation.

Surface Area-to-Volume Ratios and Constraints on Cell Size

• A larger surface area-to-volume ratio facilitates efficient exchange of materials.
• The surface area-to-volume ratio influences cell size limitations.

Water Potential (Ψ)

• Water potential measures the potential energy of water in a system.
• Water potential helps determine the direction of water movement.
• Water potential is influenced by solute concentration and pressure.
• Water potential is represented by Ψ (psi) and measured in megapascals (MPa).
• Pure water at standard conditions has a water potential of zero.
• The overall water potential equation is: Ψ = Ψ_s + Ψ_p
• Water moves from regions of higher (less negative) to lower (more negative) water potential, which is influenced by the surrounding solution's tonicity.

Components of Water Potential

• Solute potential (Ψ_s) or osmotic potential accounts for the effect of dissolved solutes on water potential.
• The presence of solutes lowers the water potential, making it more negative.
• Pressure potential (Ψ_p) refers to the physical pressure exerted on or by the water within a system.
• Turgor pressure in plant cells is an example of pressure potential.
• In plant cells, turgor pressure contributes positively to pressure potential.
• By the plasma membrane of the cell wall, turgor pressure maintains cell rigidity and structural integrity.

Water Movement in Cells

• Hypotonic solutions have higher water potential/lower solute concentration than the cell.
• This may cause animal cells swell/burst or increase in plant cells.
• Hypertonic solutions have lower water potential/higher solute concentration than the cell.
• This may cause animal cells shrivel or plant cell membranes pull away from cell wall.
• Isotonic solutions have the same water potential as the cell's environment.
• Water movement is not impacted and the cell maintains its shape.

Importance in Plants

• Water potential is crucial for understanding water movement in plants.
• Water potential is important in processes like nutrient uptake and turgor pressure maintenance.
• Turgor pressure is vital for structural support and growth in plants.