Anatomy & Physiology: Homeostasis, Biochemistry, and Cell Membrane Transport Study Notes

Homeostasis and Feedback Mechanisms

Elements of a Homeostatic Control System

Homeostasis refers to the body's ability to maintain stable internal conditions despite changes in the external environment. The control system involves several key components:

• Receptor: Detects changes (stimuli) in the environment.

• Input: Information sent along the afferent pathway to the control center.

• Control Center: Processes the information and determines the appropriate response.

• Output: Information sent along the efferent pathway to the effector.

• Effector: Carries out the response to restore balance.

Example: Body temperature regulation involves receptors in the skin and brain, a control center in the hypothalamus, and effectors such as sweat glands and blood vessels.

Negative vs. Positive Feedback Mechanisms

Feedback mechanisms help maintain homeostasis by responding to changes in the body:

• Negative Feedback: Reduces or shuts off the original stimulus. Examples include temperature control, blood sugar regulation, and blood pressure.

• Positive Feedback: Enhances or exaggerates the original stimulus, often leading to a cascade effect. Examples include blood clotting and labor contractions.

What happens if homeostasis fails? Imbalances can lead to disease or dysfunction, such as shock due to blood loss or overshooting immune responses.

Biochemistry Fundamentals

Types of Chemical Mixtures

• Solution: Homogeneous mixture, solute particles are evenly distributed (e.g., salt water).

• Colloid: Heterogeneous mixture, particles are not evenly distributed but do not settle out (e.g., cytoplasm).

• Suspension: Heterogeneous mixture, large particles settle out (e.g., blood cells in plasma).

Factors Affecting Chemical Reactions

• Temperature: Higher temperature increases reaction rate.

• Concentration: Higher concentration of particles or substrate increases reaction rate.

• Particle Size: Smaller particles increase reaction rate.

• Catalysts: Substances that increase the rate of reaction without being consumed (e.g., enzymes).

ATP and Cellular Energy

ATP (Adenosine Triphosphate) is the primary energy carrier in cells. It stores energy in small packets that can be released easily and gradually. ATP is used in muscle contraction, active transport, and many other cellular processes.

• ATP Cycle: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$ (energy released)

• Examples of ATP use: Muscle contraction, active transport, biosynthesis.

Electrolytes and pH

• Electrolytes: Substances that conduct electricity in water (e.g., Na+, K+, Ca2+, Cl-).

• pH: Measures acidity or alkalinity. Lower pH = more acidic, higher pH = more alkaline.

Formula: $\text{pH} = -\log_{10}[\text{H}^+]$

Enzymes

Enzymes are biological catalysts, usually proteins, that speed up biochemical reactions by lowering activation energy. They are specific to substrates and do not change during the reaction.

• Substrate: The molecule upon which an enzyme acts.

• Product: The result of the enzymatic reaction.

Macromolecules: Carbohydrates, Lipids, Proteins, Nucleic Acids

Carbohydrates

• Monosaccharides: Simple sugars (e.g., glucose, galactose).

• Disaccharides: Two monosaccharides joined (e.g., lactose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen).

Other Building Blocks

• Lipids: Phospholipids (cell membranes), steroids (hormones), triglycerides (energy storage).

• Proteins: Enzymes, structural components, transporters.

• Nucleic Acids: DNA and RNA, store genetic information.

Cell Membrane Transport

Types of Transport Mechanisms

• Passive Transport: No energy required; includes simple diffusion, facilitated diffusion (carrier or channel).

• Active Transport: Requires energy (ATP); includes primary and secondary active transport, vesicular transport.

Osmosis and Tonicity

Osmosis is the movement of water across a semipermeable membrane. Tonicity describes the effect of a solution on cell volume:

Solution Type	Effect on Cell	Example (Egg Experiment)
Hypotonic	Cell swells	Egg appears swollen (water enters)
Hypertonic	Cell shrinks (crenation)	Egg shrivels (water leaves)
Isotonic	No net change	Egg appears normal

Membrane Potential and Na+/K+ Pump

The Na+/K+ pump maintains the membrane potential by moving 3 Na+ ions out of the cell and 2 K+ ions into the cell against their concentration gradients, using ATP.

• Creates a negative charge inside the cell (resting membrane potential ≈ -70 mV).

• Vital for muscle and nerve cell function.

Equation: $3\ \text{Na}^+_{\text{out}} + 2\ \text{K}^+_{\text{in}}$ (per ATP hydrolyzed)

Summary Table: Key Terms and Functions

Term	Definition/Function
Receptor	Detects changes in the environment
Control Center	Processes information and determines response
Effector	Carries out the response
Enzyme	Biological catalyst, speeds up reactions
ATP	Main energy carrier in cells
Electrolyte	Conducts electricity in solution
pH	Measures acidity/alkalinity

Additional info: Academic context and examples have been expanded for clarity and completeness.