Anatomy & Physiology | University of Florida
Difficulty: Introductory | Prerequisites: None, though a basic grasp of biology terminology helps.
This material sits right at the start of any anatomy and physiology course. Every system you study later (nervous, cardiovascular, digestive, and so on) is built from cells, so understanding what a cell contains, how its membrane works, and how substances move in and out is foundational. If you can explain organelle function, membrane composition, and transport mechanisms, the rest of the course will make considerably more sense. If you missed early lectures, start here.
Cells are the smallest living units, each packed with organelles that handle energy production, protein synthesis, waste disposal, and genetic storage. The plasma membrane controls what enters and leaves through a mix of passive and active transport. Grasping these basics is the price of admission to everything that follows in A&P.
Nucleus
The membrane-bound organelle that houses DNA and directs cell activities such as growth, metabolism, and reproduction. Think of it as the cell's command centre.
Mitochondria
Organelles that generate ATP through cellular respiration. In simple terms, these are the cell's power stations (the "powerhouse of the cell").
Golgi apparatus (Golgi body / Golgi complex)
A stack of membrane-bound sacs that modifies, sorts, and packages proteins and lipids for delivery elsewhere. Think of it as the cell's post office.
Lysosomes
Vesicles filled with digestive enzymes that break down waste, worn-out organelles, and foreign material. In simple terms, these are the cell's recycling centres.
Ribosomes
Small structures (free in the cytoplasm or bound to the ER) where amino acids are assembled into proteins following mRNA instructions. Think of them as the cell's protein factories.
Cell membrane (plasma membrane)
The selectively permeable phospholipid bilayer that surrounds every cell, controlling what enters and exits. In simple terms, it is the cell's gatekeeper.
Phospholipid bilayer
Two layers of phospholipid molecules arranged with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward. This arrangement is what makes the membrane selectively permeable.
Cholesterol
A lipid embedded in the membrane that modulates fluidity, keeping the membrane stable across a range of temperatures.
Carrier proteins
Membrane proteins that change shape to shuttle specific molecules across the membrane, used in both passive and active transport.
Receptor proteins
Membrane proteins that detect chemical signals (e.g. hormones) outside the cell and trigger a response inside.
Marker proteins (cell-identity markers)
Proteins on the cell surface that identify the cell type, important for immune recognition.
Membrane potential
The difference in electrical charge across the plasma membrane, maintained largely by the Na+/K+ pump. Essential for nerve impulses and muscle contractions.
Na+/K+ pump (sodium-potassium pump)
An active-transport protein that moves 3 Na+ out and 2 K+ into the cell per ATP consumed, maintaining the electrochemical gradient.
Diffusion
The passive movement of molecules from an area of higher concentration to lower concentration, down the concentration gradient. No energy required.
Facilitated diffusion
Passive transport that uses carrier or channel proteins to move molecules (e.g. glucose, ions) across the membrane. Still no ATP needed.
Osmosis
The diffusion of water across a selectively permeable membrane, from a region of lower solute concentration (hypotonic) to higher (hypertonic).
Active transport
Movement of molecules against their concentration gradient, requiring ATP. The Na+/K+ pump is the classic example.
Endocytosis
The process by which a cell engulfs substances from outside by wrapping its membrane around them. Think of the cell eating or drinking.
Exocytosis
The reverse: vesicles fuse with the plasma membrane to release their contents outside the cell.
Houses the cell's DNA, organised into chromosomes
Surrounded by a double membrane (nuclear envelope) with nuclear pores that regulate traffic in and out
Contains the nucleolus, where ribosomal RNA (rRNA) is produced
Coordinates gene expression, controlling which proteins the cell makes and when
Double-membrane organelles with an inner membrane folded into cristae (increased surface area for ATP production)
Site of cellular respiration: glucose + oxygen are converted to ATP, carbon dioxide, and water
Contain their own DNA, evidence supporting the endosymbiotic theory
Cells with high energy demands (muscle cells, liver cells) contain more mitochondria
Receives proteins and lipids from the endoplasmic reticulum
Modifies them (e.g. adding carbohydrate chains), sorts them by destination, and packages them into vesicles
Sends finished products to the plasma membrane (for secretion), to lysosomes, or back to the ER
Membrane-bound sacs containing hydrolytic enzymes that work best at acidic pH (~5)
Digest ingested bacteria, worn-out organelles, and cellular debris (autophagy)
Defective lysosomes lead to lysosomal storage diseases (e.g. Tay-Sachs)
Made of rRNA and protein; assembled in the nucleolus
Free ribosomes float in the cytoplasm and produce proteins used inside the cell
Bound ribosomes attach to the rough endoplasmic reticulum and produce proteins destined for export or membrane insertion
Rough ER: studded with ribosomes; synthesises and folds proteins for secretion or membrane use
Smooth ER: lacks ribosomes; synthesises lipids, detoxifies drugs, and stores calcium ions
Each phospholipid has a hydrophilic (polar) head and two hydrophobic (nonpolar) fatty-acid tails
In water, phospholipids spontaneously arrange into a bilayer: heads face the aqueous environment on both sides, tails face inward
This arrangement makes the membrane selectively permeable: small nonpolar molecules (O2, CO2) pass freely; large or charged molecules cannot without help
Wedged between phospholipid tails
At high temperatures, it restrains phospholipid movement and reduces fluidity
At low temperatures, it prevents tight packing and maintains fluidity
Net effect: membrane stays functional across a range of temperatures
Integral (transmembrane) proteins span the full bilayer; many serve as channels or carriers
Peripheral proteins sit on one surface and are involved in signalling and structural support
Four functional categories to know:
Carrier proteins: ferry specific molecules across (active or passive)
Receptor proteins: bind signalling molecules (hormones, neurotransmitters) and trigger intracellular responses
Marker proteins: display carbohydrate tags that identify the cell to the immune system
Enzymes: catalyse reactions at the membrane surface
The inside of a resting cell is roughly -70 mV relative to the outside
Maintained primarily by the Na+/K+ pump (3 Na+ out, 2 K+ in per ATP) and by leak channels
Critical for nerve impulse conduction and muscle contraction
General anaesthetics work partly by altering membrane fluidity, which changes how ion channels and receptors behave. Understanding membrane composition helps explain why different anaesthetic agents have different onset times and side-effect profiles.
Simple diffusion: small, nonpolar molecules (O2, CO2, steroid hormones) move down their concentration gradient directly through the bilayer
Facilitated diffusion: larger or polar molecules (glucose, ions) move down their gradient through channel proteins or carrier proteins
Osmosis: the diffusion of water across a selectively permeable membrane, from low solute concentration (hypotonic side) to high solute concentration (hypertonic side)
Isotonic solution: water in = water out, cell volume stable
Hypotonic solution: water moves into the cell, cell swells (animal cells may lyse)
Hypertonic solution: water moves out, cell shrinks (crenation in red blood cells)
Moves molecules against their concentration gradient
Primary active transport: the Na+/K+ pump uses ATP directly to move 3 Na+ out and 2 K+ in; maintains membrane potential and cell volume
Secondary active transport: uses the gradient created by primary active transport to move another substance (e.g. glucose co-transport with Na+ in the intestine)
Endocytosis: the cell membrane wraps around material and pulls it inward as a vesicle
Phagocytosis ("cell eating"): engulfs large particles or whole cells
Pinocytosis ("cell drinking"): takes in small droplets of extracellular fluid
Receptor-mediated endocytosis: specific molecules bind to receptors, triggering vesicle formation
Exocytosis: vesicles fuse with the membrane and release contents outside (e.g. neurotransmitter release, hormone secretion)
Secretory vesicles: transport packaged proteins from the Golgi to the membrane for exocytosis
Students often think osmosis moves solute. It does not. Osmosis is the movement of water toward the higher solute concentration.
Students confuse facilitated diffusion with active transport because both use proteins. The distinction is energy: facilitated diffusion requires no ATP; active transport does.
"Selectively permeable" does not mean nothing gets through. It means the membrane allows some substances to pass and blocks others, depending on size, polarity, and charge.
Cholesterol is sometimes assumed to be harmful in membranes. In reality, it is essential for maintaining proper membrane fluidity and stability.
⚠️ Be able to compare and contrast passive vs active transport (direction relative to gradient, energy requirement, protein involvement)
⚠️ Know the components of the plasma membrane and what each does (phospholipids, cholesterol, the four protein types)
⚠️ Predict what happens to a cell placed in hypotonic, hypertonic, or isotonic solutions
⚠️ Explain how the Na+/K+ pump maintains membrane potential and why that matters for nerve and muscle function
⚠️ Distinguish between endocytosis subtypes (phagocytosis, pinocytosis, receptor-mediated)
True or False: Osmosis is the movement of solute across a membrane. (False, it is the movement of water.)
Fill in the blank: The Na+/K+ pump moves ___ Na+ out and ___ K+ in per ATP. (3, 2)
True or False: Facilitated diffusion requires ATP. (False)
Fill in the blank: Cholesterol in the membrane modulates ___ and ___. (fluidity, stability)
True or False: A cell placed in a hypertonic solution will swell. (False, it will shrink/crenate.)
Q: Name the four functional categories of membrane proteins and give one role for each.
A: Carrier proteins (transport molecules across the membrane), receptor proteins (detect external signals such as hormones), marker proteins (identify the cell type for immune recognition), enzymes (catalyse chemical reactions at the membrane surface).
Q: A red blood cell is placed in a 0.2% NaCl solution. What happens to the cell and why?
A: The 0.2% NaCl solution is hypotonic relative to the cell's interior. Water moves into the cell by osmosis, causing it to swell and potentially lyse (haemolysis).
Q: Explain why the Na+/K+ pump is classified as active transport.
A: It moves Na+ and K+ against their respective concentration gradients, which requires energy in the form of ATP. Passive processes move substances down their gradient without energy input.
Q: How does cholesterol affect membrane fluidity at high and low temperatures?
A: At high temperatures, cholesterol reduces fluidity by restraining phospholipid movement. At low temperatures, it prevents phospholipids from packing too tightly, maintaining fluidity. The result is a membrane that stays functional across a temperature range.
Q: Compare phagocytosis and pinocytosis.
A: Phagocytosis ("cell eating") engulfs large particles or whole cells by extending pseudopods around them. Pinocytosis ("cell drinking") takes in small droplets of extracellular fluid along with dissolved solutes. Both are forms of endocytosis.
Membrane transport connects directly to homeostasis and feedback mechanisms: the Na+/K+ pump is what creates the electrochemical gradient that nerve cells rely on to fire action potentials. When you study the nervous system, everything starts with this pump.
Organelle function links to metabolism and energy: mitochondria produce the ATP that powers active transport, and the Golgi packages proteins that become membrane receptors and carriers.
Membrane composition also matters in pharmacology. Drug delivery often depends on whether a molecule is lipid-soluble (crosses membranes easily) or water-soluble (needs a transporter).
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