PARTS OF THE CELL
Plasma membrane : surrounds and contains the cytoplasm of the cell.
Cytoplasm : composed of a semi-liquid part (cytosol) in which we find the organs and cells.
Core : houses the> part of the cell’s DNA. The shape, size, structure and function of our body cells are highly variable.
2- PLASMA MEMBRANE: Cell membrane or plasmalema. Fine membrane that separates the interior from the outside of the cell and regulates the exchange of substances between the inside and outside. Visible to the electron microscope.
Molecular organization : it is a mixture of lipids and proteins (50 lipid molecules per protein) and some carbohydrates as part of glycolipids and glycoproteins.
– Lipids :
o Phospholipids: they form a bilayer, with the polar heads out of the membrane and the apolar portion enclosed in the membrane. Ex: cholesterol.
o Glycolipids: only found on the outer surface. It has adhesion and recognition and communication functions between cells (nerve cells, muscle cells).
– Proteins :
o Integrals or intrinsic: they extend through the lipid bilayer. The majority are glycoproteins and act as identification markers.
o Peripheral or extrinsic: loosely attached to the outer or inner surfaces of the membrane.
Functions of membrane proteins:
– Conveyor channels : allow the passage of specific solutes through the membrane.
– Receptors : they bind ligands, hormones or neurotransmitters, which function as cell recognition sites.
– Binding proteins : fix the proteins of the plasma membrane to the protein filaments inside and outside the cell.
– Enzymatic activity : catalyze chemical reactions.
– Anchoring : for the cytoskeleton.
– Cellular identity markers
– * Provision “fluid mosaic” → proteins float like icebergs in a sea of lipids.
Fluidity of the membrane : the fluidity of the membrane is > in the places where the double bonds abound, in the fatty acid channels of the lipids and form a bilayer. Cholesterol gives it resistance to bilayer, but it reduces fluidity when the body temperature is normal.
Its fluidity allows reactions to occur in the thickness of the membrane and makes possible the movement of its components.
Membrane permeability: Selective membrane permeability allows some substances to pass > easily than others.
The lipid bilayer is permeable to most non-polar molecules without electrical charge. It is impermeable to ions and charged or polar molecules, with the exception of water and urea. The “channels” and transported increase the permeability of the plasma membrane in some medium to small polar charged substances, such as ions, which cannot cross the lipid bilaye
The selective membrane permeability determines the existence of concentration gradient (difference in concentrations between 2 substances).
3- TRANSPORTATION THROUGH THE PLASMA MEMBRANE.
Passive processes : do not require energy expenditure.
– Diffusion : movements of molecules across the membrane from an area of > [] to another of < [] (in favor of a gradient of []).
The [] depends on the volume (small better than large), the quality of the molecule (better liposoluble and those with specific channels).
The membrane has pores that allow the passage of glucose and water, the balance is not a static state, but compensatory.
The factors that influence the degree of diffusion are the magnitude of the [] gradient, temperature, mass of the substance being diffused, surface area, diffusion distance.
– Diffusion through the ionic membrane channels :
The molecules that diffuse through the lipid bilayer are non-polar or hydrophobic (O², CO², N², steroids, fat-soluble vitamins, in addition to water and urea that are polar and uncharged).
Selective ion channels for K, Cl, Na, Ca allow these small inorganic ions (which are too hydrophilic to pass into the non-polar interior of the membrane) to cross the plasma membrane.
* Which ion is the most abundant?
Inside the cell the K and outside it the Na
– Osmosis : diffusion of water through the plasma membrane in the presence of a solute for which the membrane is not permeable. As a result of osmosis, there is a ↑ of volume and pressure on one side of the membrane and a ↓ of volume and pressure on the other. Under normal conditions, the osmotic pressure of the intracellular fluid = extracellular fluid.
Effects of osmosis on cells:
- Isotonic: the 2 liquids have the same osmotic pressure. Red blood cells maintain their normal shape.
- Hypertonic: > pressure. Red blood cells undergo creation.
- Hypotonic: < pressure. Red blood cells suffer hemolysis.
– Dialysis : diffusion in which the selectively permeable nature of a membrane causes the separation between the smallest particles of a solute and the largest.
– Facilitated diffusion : it is the movement of substances through the membrane, in favor of a gradient of []. Ex → Glucose.
– Filtration : passage of water and solutes of small to medium size through the membrane thanks to the hydrostatic pressure of the blood (PA). It takes place at the capillary level. Ex → Urine in the kidney.
Active processes : require energy consumption. The energy comes from the metabolism to force the particles to cross the membrane.
– Active transport: it involves movements of substances through the membrane by means of a transport molecule against the electrochemical gradient, with energy consumption. Can be:
o Primary active processes: energy derives directly from the degradation of ATP.
- Ca pump: 2 Ca ions penetrate the pump and the ATP associates its activating center.
The energy released from the ATP changes from pump to pump and releases the Ca to the other side.
- Na-K pump: operates on the plasma membrane of all human cells.
3 Na ions from the cytosol bind to the pump protein.
The binding of Naendechains the hydrolysis of ATP in ADP, a reaction through which a phosphate group binds to the pump protein and generates a change in the conformation that releases the 3Na⁺ and the protein form favors the union of 2K⁺.
The binding of 2K⁺ leads to the release of the phosphate group and again modifies the shape of the protein. It releases the K⁺ within the cytosol.
o Secondary active processes: the energy comes from the stored (in the form of potential energy) in the ion gradients (Na⁺e H⁺). 2 types:
o Simporte or cotrasporte: the 2 molecules move in the same direction.
o Antiport or counter transport or exchangers: the 2 molecules move in the opposite direction.
Volume transport: allows substances to enter or leave the cell without actually crossing the plasma membrane. They require a lot of energy.
– Endocytosis: passage of substances into the cell by entrapment in a part of the membrane, which emerges from the rest forming an intracellular vesicle.
o Phagocytosis: the cell emits projections or pseudopods that encompass large solid particles. A large vesicle or phagosome is formed that fuses with the lysosomes.
o Pinocytosis: the membrane is invaded, so that a small drop of extracellular liquid material penetrates the cell, forming a pinocytic vesicle.
– Exocytosis: leaving the cell of substances such as proteins or other cellular products by fusion of a secretory vesicle with the membrane.
