BSC2085 Lesson 2

Cell Theory — Tenet 1

Cells are the building blocks of all plants and animals.

Cell Theory — Tenet 2

All cells come from the division of pre-existing cells.

Cell Theory — Tenet 3

Cells are the smallest units that perform all vital physiological functions.

Cell Theory — Tenet 4

Each cell maintains homeostasis at the cellular level.

Robert Hooke — Microscopy

Improved the compound microscope; first to see and name "cells."

Robert Hooke — Publication

Published the first comprehensive microscopy book, Micrographia, in 1665.

Human Cell Size — Typical

Most human cells are ~10-15 μm in diameter

Cell Size Limit — Rationale

Overly large cells cannot support themselves and may rupture.

Somatic cells

Include all non-meiotic cells (e.g., neurons, osteocytes, muscle cells, epithelial cells).

Sex (Germ) Cells

male sperm cells; female oocyte (egg).

Plasma membrane

The cell is surrounded by a ___________ that defines boundaries and separates inside from outside.

The plasma membrane is made of

Proteins and lipids; composition can vary between regions of the cell.

Cytoplasm — Definition

The region between the plasma membrane and nucleus containing organelles, cytoskeleton, inclusions, and cytosol/ICF.

Cytosol (ICF) — Definition

The clear gel component of cytoplasm (distinct from cytoplasm as a whole).

Extracellular Fluid (ECF) — Definition

Fluid outside cells, including tissue (interstitial) fluid, blood plasma, lymph, and cerebrospinal fluid (CSF).

Plasma Membrane — Functions

- Physical barrier: separates the outside of the cell from the inside of the cell.

- Regulates entry/exit of material (ions, nutrients, wastes).

- Responds to changes in the ECF: hormones and chemical signals bind membrane receptors to trigger intracellular changes.

- Provides support: gives shape and allows attachment to other cells or the extracellular matrix (ECM).

~98% of the plasma membrane is _____

Lipids (mostly phospholipids).

Phospholipids

make up 75% of membrane lipids

Phospholipid bilayer

Amphipathic: hydrophilic phosphate heads face water, hydrophobic tails face inward avoiding water.

Fluidity of phospholipids

Phospholipids drift laterally; flip-flop between sides of membrane is very infrequent; keeping the membrane fluid!!

Cholesterol

makes up ~20% of membrane lipids.

Cholesterol — Role

Holds phospholipids still and can stiffen or loosen the membrane.

Glycolipids

make up ~5% of membrane lipids

Glycolipids are

Phospholipids with short carbohydrate chains on the extracellular face

Glycolipids function

Contribute to the glycocalyx (carbohydrate coating) on the cell surface.

Glycocalyx — Definition

Carbohydrate component of glycoproteins and glycolipids located external to the plasma membrane; unique to each individual (except identical twins).

Glycocalyx functions:

- Protects the cell from physical and chemical injury.

- Contributes to immunity to infection.

- Cancer cells have altered glycocalyx; alterations can elicit immune responses against them.

- Important in transplant compatibility.

- Binds cells together in tissues (cell adhesion).

- Allows sperm to recognize and bind to eggs.

- Guides embryonic cells to their correct destinations.

Membrane Proteins — Abundance

Proteins are ~2% of membrane molecules but ~50% of membrane weight.

Transmembrane Proteins — Definition

Pass completely through the membrane; most are glycoproteins with hydrophilic regions (contacting cytoplasm/ECF) and hydrophobic regions (within lipid bilayer).

Transmembrane Proteins — Mobility

Some drift in the membrane; others are anchored to the cytoskeleton.

Peripheral Proteins — Definition

Adhere to one face of the membrane; inner-face peripheral proteins often tether to transmembrane proteins and the cytoskeleton.

Membrane Protein Function — Receptors

Receptors bind chemical signals and can trigger second-messenger production inside the cell.

Membrane Protein Function — Enzymes

Enzymes catalyze reactions, including digestion and second-messenger production.

Membrane Protein Function — Channels

Channel proteins allow hydrophilic solutes and water to pass; some are leak channels (always open), others are gated.

Gated Channels — Ligand-Gated

Open in response to chemical messengers.

Gated Channels — Voltage-Gated

Open/close in response to changes in membrane charge.

Gated Channels — Mechanically-Gated

Open in response to physical stress on the cell.

Membrane Protein Function — Carriers

Carriers bind solutes and transfer them across the membrane via conformational change.

Membrane Protein Function — Pumps

Carriers that consume ATP to move substances against their concentration gradient.

Membrane Protein Function — Identity Markers

Glycoproteins act as cell-identity markers (identification tags).

Cell adhesion molecules (CAMs)

Mechanically link a cell to another cell and to extracellular material.

Simple Diffusion — Definition

Net movement of particles from high to lower concentration without energy input.

Simple Diffusion cause

Results from constant, spontaneous molecular motion; molecules collide and bounce.

Simple Diffusion — Gradient flow

Substances diffuse down their concentration gradient.

Simple Diffusion — Medium

Occurs in air and water; does not require a membrane.

Simple Diffusion — Membrane Permeability

A substance diffuses through a membrane only if the membrane is permeable to it.

Diffusion Rate — Temperature

Higher temperature increases particle motion and raises diffusion rate.

Diffusion Rate — Molecular Weight

Smaller molecules move faster and diffuse faster.

Diffusion Rate — Gradient Steepness

Steeper concentration gradients increase diffusion rate.

Diffusion Rate — Membrane Area

Larger membrane surface area increases diffusion rate.

Diffusion Rate — Permeability

Higher membrane permeability increases diffusion rate.

Osmosis — Definition

Net flow of water through a selectively permeable membrane from higher water (lower solute) to lower water (higher solute) concentration.

Aquaporins effect

Water can diffuse through the bilayer, but ____ (water channels) greatly enhance osmosis.

Cells can increase osmosis by installing more aquaporins.

Clinical applications of osmosis

Crucial for IV fluids; osmotic imbalances underlie diarrhea, constipation, and edema.

Osmotic Pressure — Definition

Hydrostatic pressure required to stop osmosis; increases as nonpermeating solute concentration rises.

Nonpermeating Solutes — Example

Large solutes like proteins cannot cross membranes and can create osmotic pressure.

Hydrostatic Pressure — Definition

Fluid pressure exerted on the membrane.

Reverse Osmosis — Definition

Applying mechanical pressure to override osmotic pressure; used to purify water (e.g., cruise ships desalinate seawater).

Osmolarity — Definition

Total osmotic concentration (quantity of non-permeating solutes) per liter of solution. measured in milliosmoles per liter (mOsm/L)

Typical Osmolarity

Blood plasma, tissue fluid, and intracellular fluid are ~300 mOsm/L.

Tonicity — Definition

Ability of a surrounding solution to affect cell volume and pressure; depends on concentration of nonpermeating solutes.

Hypotonic Solution — Effect

Causes cells to absorb water, swell, and possibly burst (lyse).

Hypotonic Solution

Has a lower concentration of nonpermeating solutes than ICF.

Hypertonic Solution — Effect

Causes cells to lose water and shrivel (crenate).

Hypertonic Solution

Has a higher concentration of nonpermeating solutes than ICF.

Isotonic Solution — Effect

No net change in cell volume.

Isotonic Solution

Nonpermeating solute concentrations in ECF and ICF are the same.

Isotonic Solution — Example

Normal saline (0.9% NaCl) is isotonic.

Carrier-Mediated Transport — Definition

Membrane proteins (carriers) move solutes into/out of cells or organelles.

Carrier Specificity

search for particular solutes:

a solute (ligand) binds a receptor site, carrier changes shape, and translocates the solute across the membrane.

Carrier Saturation — Concept

As solute concentration rises, transport rate increases until carriers are saturated (transport maximum, Tm).

Carrier Types — Uniport

Moves one solute type (e.g., calcium pump moves only Ca²⁺).

Carrier Types — Symport

Moves two or more solutes simultaneously in the same direction (cotransport). Ex. sodium-glucose transporters move both at same time and same direction

Carrier Types — Antiport

Moves two or more solutes in opposite directions (countertransport). Ex. Sodium-potassium pump moves Na⁺ out and K⁺ into the cell.

Mechanisms — Facilitated Diffusion

Carrier moves a solute down its concentration gradient without ATP; bind → conformational change → release to other side of membrane.

Mechanisms — Primary Active Transport

Carrier moves a solute up its gradient using ATP directly.

Primary Active Transport — Examples

Calcium pump (uniport) uses ATP to expel Ca²⁺ to where it is already more concentrated.

Sodium-potassium pump (antiport) uses ATP to expel Na⁺ and import K⁺.

Mechanisms — Secondary Active Transport

Carrier uses ATP indirectly; exploits gradients established by primary active transport.

SGLT — Function

Moves glucose into cells up its gradient while carrying Na⁺ down its gradient; used by kidney cells to reabsorb Na⁺ and glucose.

Secondary active transport — ATP Usage

Does not use ATP directly; depends on the Na⁺ gradient created by the Na⁺/K⁺ pump.

Na⁺/K⁺ Pump

- Each cycle consumes 1 ATP and exchanges 3 Na⁺ out for 2 K⁺ in.

- Keeps intracellular K⁺ higher and Na⁺ lower than in ECF.

- Maintains the Na⁺ gradient used for secondary active transport.

- Regulates solute concentration, and thus osmosis and cell volume.

- Maintains a negatively charged resting membrane potential.

- Produces heat.

Vesicular Transport — Definition

Moves large particles, fluid droplets, or many molecules at once via membrane-bounded vesicles (ATP-dependent).

Vesicles — Definition

Bubble-like membrane enclosures that transport cargo.

Endocytosis — Definition

Brings material into the cell.

Exocytosis — Definition

Releases material from the cell; essentially the reverse of endocytosis.

Endocytosis — Phagocytosis

"Cell eating": pseudopods engulf large particles; form a phagosome that fuses with a lysosome to become a phagolysosome for digestion.

Endocytosis — Pinocytosis

"Cell drinking": intake of ECF droplets containing useful molecules; forms pinocytic vesicles.

Endocytosis — Receptor-Mediated

Particles bind specific receptors; a pit forms and cytosolic side is coated with clathrin; forms a clathrin-coated vesicle directed internally.

Exocytosis

discharge material from cell, essentially the reverse of endocytosis

Exocytosis — Example

Release of insulin by endocrine cells of the pancreas.

Membrane Balance

Replaces plasma membrane lost during endocytosis.