Physiology LG

Topic I: Foundations of Physiology

• Cellular Physiology

• Animal Form and Function

• Homeostasis and Integration

Target Learning Outcomes

After accomplishing the topic, the student can:

1. distinguish the relationship of animal form and function

2. discuss the factors affecting homeostasis based on feedback loop and thermoregulation
   mechanisms

3. explain cellular mechanisms in transport regulation and communication

Questions

1. Animal Cell Structures and Their Functions

• Plasma membrane – selectively permeable barrier that regulates what enters and leaves the cell.

• Cytoplasm – region where many cellular metabolic reactions occur.

• Nucleus – contains DNA and controls gene expression and cellular activities.

• Nucleolus – produces rRNA and assembles ribosomal subunits.

• Ribosomes – synthesize proteins.

• Rough ER – synthesizes and processes proteins destined for secretion, membranes, or organelles.

• Smooth ER – lipid synthesis, detoxification, and Ca²⁺ storage.

• Golgi apparatus – modifies, sorts, and packages proteins and lipids.

• Mitochondria – produce ATP through cellular respiration.

• Lysosomes – digest macromolecules and damaged cellular components.

• Peroxisomes – break down fatty acids and detoxify harmful substances.

• Cytoskeleton – provides structure, enables movement, and helps transport materials within the cell.

• Centrosome/centrioles – organize microtubules and help form the mitotic spindle.

How do cells communicate?

Cells communicate through cell signaling, in which a signaling cell releases or presents a signal that is detected by a receptor on another cell.

Major mechanisms:

• Direct contact – membrane-bound signals or cell junctions.

• Paracrine signaling – signals act on nearby cells.

• Autocrine signaling – a cell signals itself.

• Endocrine signaling – hormones travel through body fluids/blood to distant cells.

• Synaptic signaling – neurons release neurotransmitters across synapses.

2. Role of the Plasma Membrane in Homeostasis

The plasma membrane maintains homeostasis by selectively controlling the movement of substances into and out of the cell.

It:

• allows needed substances such as nutrients and ions to enter;

• removes wastes;

• maintains ion and concentration gradients;

• regulates water movement;

• allows cells to receive external signals through membrane receptors.

Key idea: The plasma membrane helps keep the cell's internal environment stable despite changes outside the cell.

3. Diffusion vs. Active Transport

Diffusion is the net movement of particles down their concentration or electrochemical gradient.

Active transport moves substances against their gradient using energy, usually through membrane proteins.

4. Na⁺/K⁺ Pump, Membrane Potential, and Action Potential

Na⁺/K⁺ Pump

The Na⁺/K⁺-ATPase uses ATP to transport:

3 Na⁺ out of the cell + 2 K⁺ into the cell

This maintains:

• high Na⁺ concentration outside;

• high K⁺ concentration inside;

• the ion gradients needed for electrical signaling.

Because more positive charge is moved out than in, the pump also contributes directly to making the inside of the cell relatively negative.

Membrane Potential

Membrane potential = electrical voltage difference across the plasma membrane.

The Na⁺/K⁺ pump establishes the ion gradients, while ion channels, especially K⁺ leak channels, are primarily responsible for the resting membrane potential.

Action Potential

An action potential is a rapid, temporary change in membrane potential, especially in neurons and muscle cells.

1. Resting potential – membrane is polarized.

2. Threshold – sufficient depolarization activates voltage-gated Na⁺ channels.

3. Depolarization – Na⁺ enters the cell.

4. Repolarization – Na⁺ channels inactivate and K⁺ exits.

5. Hyperpolarization – K⁺ channels remain open briefly.

6. Return to resting state – ion gradients and membrane channels restore the resting condition.

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5. Which Cells Can Transduce a Signal Without Molecules Crossing the Membrane?

Cells with membrane receptors can transduce signals without the signaling molecule entering the cell.

The signal molecule, or ligand, binds to a receptor on the cell surface. The receptor then activates an intracellular signaling pathway.

Examples:

• Neurons – neurotransmitters bind to membrane receptors.

• Muscle cells – hormones or neurotransmitters activate membrane receptors.

• Many other cells respond to peptide/protein hormones, which cannot freely cross the lipid bilayer.

Key idea: The signal does not need to enter the cell; receptor activation converts the extracellular signal into an intracellular response.

6. How Are Hormones Transported?

Hormones are transported primarily through the bloodstream to their target cells.

• Water-soluble hormones such as peptide hormones and catecholamines dissolve in blood plasma and circulate relatively freely.

• Lipid-soluble hormones such as steroid hormones are poorly soluble in water, so many travel bound to carrier proteins in the blood.

Only cells with the appropriate receptors respond to a particular hormone.

Endocrine signaling: hormone → bloodstream → distant target cell.

7. Different Body Plans of Animals

Symmetry

• Asymmetry – no defined body symmetry; characteristic of many sponges.

• Radial symmetry – body parts arranged around a central axis; common in cnidarians.

• Bilateral symmetry – body can be divided into approximately equal left and right halves; characteristic of most animals.

Germ Layers

• Diploblastic – two embryonic tissue layers:

  • ectoderm

  • endoderm

• Triploblastic – three embryonic tissue layers:

  • ectoderm

  • mesoderm

  • endoderm

Body Cavities

• Acoelomate – no body cavity between the digestive tract and body wall.

• Pseudocoelomate – body cavity is present but not completely lined by mesoderm.

• Coelomate – true coelom completely lined by mesoderm.

Other Body-Plan Features

• Segmentation – body divided into repeated sections.

• Cephalization – concentration of sensory organs and nervous tissue toward the anterior/head region.

8. What Limits the Size, Shape, and Development of Animals?

Animal size, shape, and development are limited by physical, physiological, and biological constraints.

Important factors include:

• Surface area-to-volume ratio – as animals become larger, volume increases faster than surface area, making exchange of gases, nutrients, and wastes more difficult.

• Diffusion distance – diffusion becomes inefficient over long distances.

• Transport systems – larger animals require specialized circulatory and respiratory systems.

• Gravity and structural support – body size and shape are affected by the ability of tissues/skeletons to support the body.

• Energy requirements – larger and more active animals require greater energy and nutrient delivery.

• Temperature regulation – body size and shape affect heat gain and heat loss.

• Developmental/genetic constraints – genes and developmental processes determine how tissues and organs form.

Key idea: Animal body design is constrained by the need to exchange materials, transport substances, obtain energy, support the body, and maintain homeostasis.

9. Positive vs. Negative Feedback

Negative Feedback

Negative feedback counteracts a change and brings a system back toward its normal range.

Stimulus → response → reduces original stimulus

Examples:

• Body temperature regulation

• Blood glucose regulation

• Blood pressure regulation

Role: Maintains homeostasis and stability.

Positive Feedback

Positive feedback reinforces or amplifies a change.

Stimulus → response → increases original stimulus

Examples:

• Childbirth – oxytocin increases uterine contractions, which stimulates more oxytocin release.

• Blood clotting – activated platelets promote activation of additional platelets.

Role: Drives a process toward a particular endpoint rather than maintaining a stable condition.

10. Thermoregulation and Osmoregulation

Thermoregulation

Thermoregulation is the regulation of body temperature within an appropriate range.

Animals can be classified based on how body temperature is regulated:

• Endotherms – generate most body heat internally through metabolism.

• Ectotherms – rely heavily on environmental heat sources.

Mechanisms include:

• Vasodilation – increases heat loss.

• Vasoconstriction – reduces heat loss.

• Sweating – promotes evaporative cooling.

• Shivering – generates heat through muscle activity.

• Behavioral changes – moving into shade, sunlight, water, etc.

Osmoregulation

Osmoregulation is the regulation of water and solute concentrations in the body.

It maintains:

• appropriate water balance;

• ion concentrations;

• osmotic pressure of body fluids.

Major organs involved include the kidneys in vertebrates.

Key idea:

• Thermoregulation = control of temperature

• Osmoregulation = control of water and solute balance