Cell Structure and Interactions

Cell Structure and Interactions

Cell Structure

The cell is the fundamental structural and functional unit of the body. Organelles, specialized subcellular structures, carry out many cellular functions. A cell consists of three main components:

1. Plasma (cell) membrane: Separates the cell's internal environment from the external environment.
2. Cytoplasm and organelles: The cytoplasm is the aqueous content within the plasma membrane but outside the nucleus. Organelles are subcellular structures with specific functions.
3. Nucleus: Contains the cell's DNA (genetic material) and is the site of ribosome production.

Structure of the Cell

• Plasma (cell) membrane: A selectively permeable barrier.
• Cytoplasm: The aqueous content of the cell.
• Organelles: Subcellular structures within the cytoplasm that perform specific functions (excluding the nucleus).
• Nucleus: The largest organelle, containing DNA and responsible for ribosome production.

Cellular Components: Structure and Function

Functions of the Plasma Membrane

1. Acts as a barrier separating intracellular and extracellular environments.
2. Surrounds the cell and gives it form.
3. Regulates the exchange of substances across the membrane.
4. Helps identify the cell to other cells (e.g., immune cells).
5. Participates in intercellular signaling.
6. Maintains differences in ion concentration between the interior and exterior of the cell.

Structure of the Plasma Membrane

• Double layer of phospholipids: Acts as a selective barrier, allowing lipid-soluble molecules to pass while restricting charged or polar substances.
• Proteins: Perform specialized functions and selective transport.
  • Peripheral proteins: Partially embedded in one face of the membrane.
  • Integral proteins: Span the membrane from one side to the other.

Fluid-Mosaic Model

The plasma membrane is described by the fluid-mosaic model, resembling a continually moving sea of fluid lipids containing a mosaic of various proteins that can move laterally.

Functions of Membrane Proteins

1. Ion channels: Allow specific ions, such as potassium (K^+$), to pass.
2. Carriers/Transporters: Selectively move polar substances or ions across the membrane.
3. Receptors: Recognize and bind specific molecules (e.g., insulin receptors).
4. Enzymes: Catalyze specific chemical reactions on the inner or outer surface of the cell.
5. Linkers: Anchor proteins in the plasma membranes of neighboring cells or to protein filaments inside and outside the cell.

Peripheral Proteins

Support the plasma membrane, anchor integral proteins, and participate in mechanical activities such as moving materials and organelles within cells and changing cell shape during cell division.

Glycocalyx

The outer leaflet of the plasma membrane is attached to a system of glycoproteins, glycolipids, and polysaccharides, forming the glycocalyx around the membrane. It acts as a molecular signature for cell recognition, which is crucial in immune responses.

Interactions Between Cells and the Extracellular Environment

The extracellular environment includes all constituents of the body located outside the cells. It consists of a fluid compartment and a matrix of polysaccharides and proteins. Interactions between the intracellular and extracellular environments occur across the plasma membrane.

• Importance of these interactions:
  1. Nutrient supply.
  2. Waste removal.
  3. Cell communication via chemical regulators.

Body Fluid

The body's water content is divided into:

1. Intracellular compartment: 67% of total body water.
2. Extracellular compartment: 33% of total body water.
   • Blood plasma (20%): Inside the vascular system.
   • Interstitial fluid (80%): Outside the vascular system.

Extracellular Matrix

Interstitial fluid resides within a gel-like extracellular matrix, which consists of protein fibers (collagen and elastin) and a ground substance (glycoproteins and proteoglycans). It provides structural support and maintains cell shape and integrity.

Integrins

Integrins are glycoproteins on the cell surface that connect the cytoskeleton inside the cell with the extracellular matrix. They facilitate adhesion and relay signals between the intracellular and extracellular compartments.

Categories of Transport Across the Plasma Membrane

The plasma membrane is selectively permeable, generally impermeable to proteins and nucleic acids. Transport processes are categorized by energy requirements or mechanisms involved.

Passive vs. Active Processes

1. Passive transport: No energy input required; uses kinetic energy of molecules or ions.
2. Active transport: Requires energy (primarily from ATP breakdown) to move substances against concentration gradients.

Passive processes include:

• Simple diffusion: Nonpolar molecules move through the phospholipid bilayer.
• Protein channels: Inorganic ions move through the plasma membrane.
• Facilitated diffusion: Small organic molecules bind to a carrier protein, which changes conformation to release the molecule on the other side.

Carrier-Mediated vs. Non-Carrier-Mediated Transport

1. Carrier-mediated transport: Requires carrier proteins that change conformation to translocate molecules or ions.
2. Non-carrier-mediated transport: Does not involve carrier proteins.

Carrier-Mediated Transport

Divided into:

1. Facilitated diffusion: Small organic molecules bind to a specific carrier protein, which changes conformation to release the molecule (e.g., glucose).
2. Active transport: Carrier-mediated transport from lower to higher concentration, requiring ATP.

Non-Carrier-Mediated Transport

Includes:

1. Simple diffusion of lipid-soluble molecules through the phospholipid bilayer.
2. Simple diffusion of ions through membrane channel proteins.
3. Simple diffusion of water (osmosis) through aquaporin channels.

Diffusion and Osmosis

Diffusion is the passive movement of molecules or ions from an area of higher concentration to lower concentration until equilibrium is reached. Osmosis is the net diffusion of water through a membrane.

Diffusion Through the Plasma Membrane

• Nonpolar molecules (O2$) and uncharged molecules ($CO2) can penetrate the phospholipid barrier.
• Small inorganic ions (Na^+$,$K^+$) can pass through protein channels.
• Water diffuses through aquaporins.
• Larger polar molecules (e.g., glucose) require carrier proteins.

Rate of Diffusion

The rate of diffusion depends on:

1. The magnitude of the concentration difference.
2. The membrane permeability.
3. The temperature.
4. The surface area of the membrane.

Osmosis

Osmosis is the net diffusion of water across a selectively permeable membrane. It requires:

1. A selectively permeable membrane.
2. A solute concentration difference.
3. Relative impermeability to the solute.

The rate of water diffusion occurs from the water is more concentrated on one side of the membrane than on the other side; that is, when one solution is more dilute than the other. Higher water concentration to lower water concentration

Osmotic Pressure

Osmotic pressure is the pressure needed to stop osmosis. It depends on solute concentration; higher solute concentration results in greater osmotic pressure. Pure water has an osmotic pressure of zero.

Molarity and Molality

• One mole of any substance contains $6.02 ims 10^{23}$ molecules (Avogadro’s number).
• A one-molar (1.0 M) solution contains one mole of solute dissolved in water to make 1 liter of solution.
• A one-molal (1.0 m) solution contains one mole of solute dissolved in 1 liter of water at 4°C.

Osmolality

The osmolality of a solution is the sum of the molalities of each solute. For example, a solution of 1.0 m glucose plus 1.0 m fructose has an osmolality of 2.0 Osm/L. Electrolytes like NaCl ionize, yielding two ions; thus, 1.0 m NaCl has a total concentration of 2.0 Osm.

Tonicity

Tonicity measures the effective osmotic pressure gradient between two solutions and affects cell volume. It depends on non-penetrating solutes.

Solutions can be:

1. Isotonic: Solute concentration is equal inside and outside the cell; no net water movement.
2. Hypotonic: Solute concentration is lower outside the cell; water moves into the cell, causing swelling and potential lysis.
3. Hypertonic: Solute concentration is higher outside the cell; water moves out, causing the cell to shrink (crenation).

Regulation of Blood Osmolality

Maintaining a constant osmolality of extracellular fluid is crucial to prevent neuronal damage and to maintain normal neural activity. Increases in plasma osmolality can be caused by dehydration or salt ingestion. Mechanisms defend plasma osmolality, usually preventing changes of more than 1% to 3%.

Increased plasma osmolality stimulates osmoreceptors in the hypothalamus, leading to:

1. Increased nerve impulse production.
2. Stimulation of the posterior pituitary.
3. Release of antidiuretic hormone (ADH) or vasopressin.
4. Promotion of water retention by the kidneys, resulting in less urine output.

Carrier-Mediated Transport

Cells uptake glucose, amino acids, and other organic molecules from the extracellular environment using carrier proteins. These molecules are too large and polar to pass through the lipid barrier directly.

Characteristics of carrier proteins:

1. Specificity: Interact only with specific molecules.
2. Competition: Different molecules compete for the same carrier.
3. Saturation: Concentrations above the transport maximum do not further increase transport.

Facilitated Diffusion

Involves an integral membrane protein (carrier or transporter) assisting a specific substance across the membrane from higher to lower concentration (no ATP required). Different tissues may have slightly different forms of carrier proteins (e.g., GLUT3 in neurons, GLUT4 in adipose tissue and skeletal muscles).

Rate of carrier-mediated facilitated diffusion is determined by:

1. Concentration gradient across the membrane.
2. The transport maximum (Tm), limited by the number of available carriers.

Active Transport

Movement of molecules and ions against their concentration gradients (from lower to higher concentrations). Requires energy. Processes exhibit a transport maximum and saturation.

Examples include transport of $Na^+$, $K^+$, $H^+$, $Ca^{2+}$, and $I^-$.

Two sources of cellular energy can drive active transport:

1. Primary active transport: ATP hydrolysis directly powers the carriers.
2. Secondary active transport (coupled transport): Energy stored in an ionic concentration gradient drives transport.

Primary Active Transport

ATP hydrolysis changes the shape of a carrier protein, which pumps a substance against its concentration gradient. Carrier proteins are often called pumps.

Examples:

1. $Ca^{2+}$ pump.
2. $Na^+/K^+$ pump.

$Ca^{2+}$ pump

Located in the plasma membrane of all cells and in the endoplasmic reticulum of striated muscle cells. Binding of cytoplasmic $Ca^{2+}$ activates ATPase, hydrolyzing ATP into ADP and Pi, causing a shape change that opens a passageway for $Ca^{2+}$ to the extracellular fluid or the endoplasmic reticulum.

$Na^+/K^+$ pump

Transports three $Na^+$ out of the cell for every two $K^+$ transported into the cell, both against their concentration gradients.

Secondary Active Transport (Coupled Transport)

Energy for the uphill movement of a molecule or ion comes from the downhill transport of $Na^+$ into the cell. ATP hydrolysis by $Na^+/K^+$ pumps maintains low intracellular Na+ concentrations. Coupled transport can be classified as:

1. Cotransport or symport: The other molecule or ion moves in the same direction as $Na^+$ (into the cell). Example: sodium-coupled glucose transporters (SGLT).
2. Countertransport or antiport: The other molecule or ion moves in the opposite direction (out of the cell). Example: $Na^+-Ca^{2+}$ antiport.

Bulk Transport

Molecules such as polypeptides and proteins are too large for carrier transport. Exocytosis and endocytosis provide bulk transport:

1. Endocytosis: Materials move into the cell via vesicles formed from the plasma membrane.
2. Exocytosis: Materials move out of the cell via vesicles fusing with the plasma membrane.

Both processes require energy from ATP.

Endocytosis

Three types:

1. Receptor-mediated endocytosis: Highly specific (e.g., cholesterol-containing LDLs, transferrin).
2. Phagocytosis: Cell eating (macrophages, neutrophils).
3. Pinocytosis: Cell drinking (absorptive cells in intestines and kidneys).

Transport of Materials into and out of Cells

The Membrane Potential

The membrane potential is the difference in charge or electrical gradient across the cell membrane, caused by:

1. Permeability properties of the plasma membrane (more permeable to $K^+$).
2. Action of $Na^+/K^+$ pumps.
3. Presence of nondiffusible negatively charged molecules (e.g., phosphate groups of ATP) inside the cell.

Equilibrium Potentials

The extent to which each ion contributes to membrane potential depends on its concentration gradient and membrane permeability. The K+ equilibrium potential ($E_K$) is -90 mV.

Resting Membrane Potential

The membrane potential of a cell not producing impulses, typically ranging from -65 mV to -85 mV (averaging -70 mV in neurons). It depends on the concentration ratio of ions and the specific permeability of the membrane to each ion. Changes in extracellular potassium concentration have the greatest effect on resting membrane potential.

Cell Signaling

Cell signaling refers to how cells communicate by releasing chemicals into the extracellular environment. It can be divided into:

1. Paracrine signaling
2. Synaptic signaling
3. Endocrine signaling

Target cells must have specific receptor proteins for regulatory molecules to respond. A cell can have millions of receptor proteins, allowing fine control over tissues and organs.

Receptors may be:

1. Intracellular (cytoplasm or nucleus) for nonpolar molecules.
2. On the outer surface of the plasma membrane for large or polar molecules.

G-Protein and Second Messengers

The binding of a polar regulatory molecule to its receptor activates an enzyme protein (effector) in the plasma membrane. G-proteins act as shuttle proteins between receptors and membrane effector proteins. The effector protein activates an enzyme, leading to the production of second messengers, which may be ions (Ca2+) or molecules (cAMP). For example, epinephrine uses cAMP to stimulate the heart.