Cellular Organization and Structure

Cellular Organization

• A cell represents the fundamental living, structural, and functional unit of the body
• Vital processes within cells are separated into specialized compartments.
• The rate of material flow in and out of the cell is regulated.
• Genetic instructions, encoded by DNA or RNA, facilitate the management of cellular activities.
• Cytology is the study of cell structures.
• Cell physiology concerns the study of their function.

General Cell

• Plasma membrane is synonymous with the cell membrane
• Nucleus refers to the cell's genetic material.
• Cytoplasm encompasses everything found between the nucleus and the cell membrane.
• The Cytosol is intracellular fluid.
• Organelles are specific structures within a cell that perform dedicated functions.

Representative Cell

• The organelles contained inside cells are not uniform, their composition varying, depending on cellular function and specialization.

Plasma membrane

• A flexible but sturdy barrier that surrounds the cytoplasm
• The fluid mosaic model describes the plasma membrane's structure.
• It is composed of a "sea of lipids" wherein proteins drift, akin to icebergs.
• The membrane comprises 50% lipids and 50% proteins, which are held together by weak hydrogen bonds.
• Lipids prevent passage of polar substances.
• Proteins act as "gatekeepers" that manage the trafficking of substances.
• For every protein molecule, about 50 lipid molecules can be found.

Lipid Bilayer

• Lipid molecules arrange themselves in two back-to-back layers
• The layers include phospholipids, cholesterol, and glycolipids,
• Cholesterol and glycolipids are dispersed among the bilayer of phospholipid molecules.

Phospholipids

• Make up approximately 75% of the membrane's lipids.
• Arrange in a bilayer consisting of two parallel molecule rows.
• The molecules are amphipathic, possessing both polar and nonpolar regions.
• Polar regions (heads) are hydrophilic and positioned to encounter the watery environment on either surface.
• Nonpolar regions (tails) are hydrophobic, aligning with each other in the membrane's interior.

Glycolipids

• Constitute approximately 5% of the lipids in the cell membrane.
• Carbohydrate groups make up a polar head that is present solely on the membrane's surface where the extracellular fluid is facing.

Cholesterol

• Accounts for 20% of lipids in membrane.
• Cholesterol molecules are interspersed among all other lipids found in both layers of the membrane.
• Cholesterol rings and hydrocarbon tail are nonpolar and positioned in the membrane's middle.

Membrane Proteins

• Integral proteins extend into or across the cell membrane and are also known as transmembrane proteins.
• Hydrophobic segments anchor the proteins in the phospholipid layer, making them amphipathic entities.
• Glycoproteins feature a sugar portion facing extracellular fluid, forming a glycocalyx "coat."
  • The Glycocalyx protects the cell from being digested.
  • The Glycocalyx gives cells unique characteristics, and creates stickiness that enables cells to bind together.
  • The Glycocalyx enables containment of a fluid layer, creating a slippery surface.
• Peripheral proteins are loosely linked to the membrane's inner or outer surface and may be detached without compromising its integrity.

Membrane Protein Functions

• Can serve as a channel, permitting particular substances to move through a water-filled pore.
• Act as transporters, binding to specific molecules and conveying them across the membrane by altering their own shape.
• Can function as receptors, recognizing certain ligands and so, altering the cell's function.
• Can act as an cell identity marker, allowing cells to recognize other, similar cells.
• Function the of membranes as linkers, allowing movement or modifying shape.
• Can acts as enzymes speeding up chemical reactions.

Membrane Fluidity

• The membrane is known to be a fluid-like structure.
• Membranes are self-sealing if punctured.
• Membrane fluidity is achieved due to membrane molecules' capacity to rotate and move freely.
• The existence of cholesterol helps reduce membrane fluidity.
  • Cholesterol generates membrane stiffness due to creation of hydrogen bonds with neighboring phospholipid heads

Selective Permeability

• Lipid bilayers allow passage of nonpolar, uncharged molecules like oxygen, carbon dioxide, and steroids.
• Lipid bilayers allow passage of water through transient gaps within the hydrophobic core, formed as phospholipids move around.
• Transmembrane proteins permit passage of small-to-medium sized polar and charged particles through their channels.
• The size constraints from the membrane prevent the passage of macromolecules, which are transported via vesicular transport.

Gradients Across Membranes

• Cell membranes maintain a concentration gradient through a difference in concentration, which is maintained on either side of the membrane.
  • There exists a relatively elevated concentration of O2 and Na+ outside the cell membrane.
  • The K+ and CO2 are at relatively higher concentrations inside the cell membrane.
• Cell membranes maintain an electrical gradient through a difference in charged ions.
• The electrical gradient creates membrane potential.
• Substances often travel in the direction of areas with lesser concentration
• Substances travel toward areas that are oppositely charged.
• Ions have electrochemical gradients.

Transmembrane Transport

• Substances cross the membranes by varied processes
• Mediated transport involves movement of materials aided by a transporter protein
• Nonmediated transport does not depend on transporter proteins.
• Active transport uses ATP to transport substances against their concentration gradient.
• Passive transport uses kinetic energy without expending ATP.
• Vesicular transport allows substances to move across membranes via exocytosis or endocytosis with the help of small vesicles that contain the transported substances.

Principles of Diffusion

• Random mixing of particles in a solution due to kinetic energy.
• Molecules move from areas of high concentration to low concentration
• Larger differences in concentration between two areas result to faster diffusion rate
• Higher temperatures lead to quicker diffusion.
• Larger molecules have slower rate of diffusion rate.
• Greater surface area allows quick diffusion rate
• Reduced diffusion occurs if there is an increased distance to travel.
• Diffusional equilibrium exists at uniform molecule distribution, and movement occurs no longer.

Diffusion example

• A crystal dye when placed in water, spreads from high concentration area to areas of low concentration.
• Far right cylinder shows equilibrium state after uniform dye distribution.

Osmosis

• Net movement of a solvent (usually water) via a selectively permeable membrane from high to low concentration.
• This can occur directly through the lipid bilayer.
• It can also happen through transmembrane proteins that function as water channels, namely aquaporins.
• Occurs when the membrane is water permeable but not permeable to certain solutes.

Water Osmosis

• Pure water on the left side of a container.
• The membrane separating both sides is impenetrable to solution.
• Water moves to the right side from left to right until the generated hydrostatic pressure begins forcing solvent towards the opposite (left) direction.

Tonicity Effects

• Volume of cell remains constance in normal isotonic solutions.

Tonicity and RBCs

• Normally the osmotic pressure inside of a cell balances with that outside of it, and thus cell volume remains constant.
• When red cells undergo fluid changes in lab the following occurs:
• Water rapidly enters the cell.
• Water gets transferred inside and outside at equal rates.
• Water rapidly goes out of the cell.

Tonicity and Cells

• An isotonic solution maintains equal water concentration inside and outside of the cell resulting in no net water movement.
• A hypotonic solution facilitates hemolysis because there is a higher water concentration outside instead of inside the cell.
• A hypertonic solution leads to cell crenation because the lower water concentration is outside the cell instead of inside.

Lipid Diffusion

• Aids in wastes excretion and nutrients absorption.
• Nonpolar, hydrophobic compounds can easily diffuse.
• Diffusing compounds include: oxygen, fatty acids, carbon dioxide, steroids, small alcohols, nitrogen, ammonia, and fat-soluble vitamins (A, E, D, & K).

Membrane Channels

• The presence of specific compounds determines which type of ions (K+, Cl–, Na+, or Ca+2) will utilize a particular membrane channel.
• While still fast in transit overall, diffusion by membrane channels takes place slower than that which is done through membranes directly.
  • 1 million K+ per second have been found to transverse individual channels.
• In the gate of ion channels, there exists potential to be either in an opened state, or in a controlled, closed state.

Facilitated Diffusion

• Substance binds to a specific transporter
• The transporter protein changes shape to move the substance.
• Facilitated diffusion proceeds down a concentration gradient only.
• Without concentration differences, movement does not occur.
• The rate depends on: Steepness of concentration gradient AND the amount of transporter proteins.

Glucose and Proteins

• A transport protein binds glucose so it can change shape.
• As glucose moves across, Kinase reduces its concentration by transforming it.
• Transporter proteins are known to attract only glucose into the cell.

Active Transport

• The process involves movement of charged substances or polar against its gradient.
• Requires constant energy input.
  • Hydrolysis of ATP is primary active transport.
  • Stored energy from ionic concentration gradients are secondary active transport.
• Involves saturation and maximums
• Examples include amino acids, monosaccharides, Na+, K+, H+, Ca+2, I- Cl-.

Transporter Proteins

• Transporter proteins are also called pumps.
  • 40% of ATP is required to work against gradients in these pumps.
  • Thousands of pumps exist inside each cell.
    • ATPase moves potassium into the cytosol and sodium out to maintain osmotic pressure.
    • Sodium is continually pumped out to prevent it from getting inside the cell.
    • Potassium contributes the osmotic pressure to the cytosol.

Sodium Potassium Sources

• Three NA+ ions can be removed from cells by two K+ ions.
• ATP fuels the split.
• K+ and NA+ can bind.

Active Transport type 2

• This uses stored energy in an ion gradient for transport to other objects.
  • Route for sodium leaks creates energy for other substance transport.
    • Glucose and amino acids rush inwards with Na+ using Symporter proteins.
    • Calcium and hydrogen can be pushed outward as Na+ rushes in via Antiporters.

Digitalis and Muscle

• Digitalis slows sodium to allow Na+ buildup in heart cells.
• Lower concetration gradient allows antiporters to slow, so that calcium remains inside cardiac cells.
• Heart contractions are therefore strengthened.
• NA/Ca balance in the cytosol is important

Particulate Transport

• Endocytosis: Engulfing by cells. Includes eating (phagocytosis) and drinking (pinocytosis).
  • In phagocytosis, receptor proteins bind to a particle, which is then consumed by WBCs or macrophages. Viruses are engulfed.
  • In pinocytosis, cells uptake fluids.
• Receptor mediated: Selective input by cells like HIV.
• Exocytosis: Expelling cell contents like neurotransmitters, hormones etc. It also replaces the membrane.

Endocytosis/Exocytosis

• A cell membrane invaginates and engulfs substances to be taken to the interior.
• Ligands binding to surface receptors causes the membrane to fold over.
• Digestive enzymes and receptor separate.
• Vesicles can join for transmembrane passage.

Pinocytosis and Phagocytosis

• Pinocytosis uptakes dissolved solutes into a pinocytic vesicle. This is then digested via lysomal enzymes.
• Residues become exposed for the exterior via exocytosis.
• The digestion of a phagosome similarly causes residues to be exposed.
• Unlike Pinocytosis, Pseudopods form during phagocytosis.

Cytosol

• Cytosol is found in cells, accounting for 55% of its volume.
  • The composition of cytosol is 75-90% water in addition with several other components such as: -Suspended electrical charges are found in organic molecules, lipids and proteins. -Dissolved ions, in addition to several simple sugars, can also be found inside organic molecules.
    • Inclusions found in lipids and/or molecules of glycogen
• Chemical reactions constantly are taken place in the cells’s cytosol through ATP, synthesis, and through the building of more blocks