4.2: CELL TRANSPORT

Solutions

Homogeneous mixtures of two or more components.

Solvent and solute

components of a solution

Solvent

Present in largest amount; dissolves other substances.

water

In the human body → __ is the primary solvent.

Solute

Present in smaller amounts; dissolved in the solvent.

Solute

So tiny that molecules cannot be seen with the naked eye and do not settle out.

Intracellular fluid

Extracellular fluid (Interstitial fluid)

Types of Body Fluids

Intracellular fluid

Nucleoplasm + cytosol inside the cell.

Intracellular fluid

Contains small amounts of gases (O₂, CO₂), nutrients, salts, dissolved in water.

Extracellular fluid (Interstitial fluid)

Fluid that continuously bath

Extracellular fluid (Interstitial fluid)

A rich, nutritious, unusual "soup" with thousands of ingredients

Nutrients: amino acids, sugars, fatty acids, vitamins

Regulatory substances: hormones, neurotransmitters

Salts and waste products

nutrients and other substances of extracellular

extracting needed substances at specific times and rejecting the rest.

Healthy cells survive by __

Selective Permeability

Allows some substances to pass while excluding others.

Selective Permeability

Nutrients enter; undesirable/unnecessary substances are blocked.

Selective Permeability

Retains valuable proteins and other molecules inside.

Selective Permeability

Permits waste products to exit.

Selectivity

__ is maintained only in healthy, unharmed cells.

passive, active

two membrane transport

Passive processes

Do not require ATP.

Active processes

Require ATP (metabolic energy)

diffusion and filtration

Passive Process

diffusion

Passive membrane transport where molecules/ions move from higher concentration to lower concentration.

diffusion

Driving Force: Kinetic energy of molecules.

diffusion

Concentration Gradient: Molecules move down their gradient.

Smaller

Higher

__ molecules → faster diffusion

__ temperature → faster diffusion

Small enough to pass through pores (channels)

Lipid-soluble

Assisted by a membrane carrier

Molecules can diffuse through the plasma membrane if they are:

Simple diffusion

Osmosis

Tonicity

Facilitated diffusion

Types of Diffusion

Simple diffusion

Unassisted diffusion of solutes (lipid-soluble substances such as fats, fat-soluble vitamins, O₂, CO₂).

Osmosis

Diffusion of water across a selectively permeable membrane.

aquaporins

In osmosis, water passes through __.

aquaporins

protein water channels

Osmosis

Continuous process; water moves down its concentration gradient.

Tonicity

Ability of a solution to alter cell size/shape by changing water content.

isotonic

hypertonic

hypotonic

tonicity

Isotonic

Same solute + water concentration as cell.

Isotonic

Cells gain and lose water equally.

Interstitial fluid

__ is isotonic.

Hypertonic

Higher solute concentration than inside cell.

Hypertonic

Water moves out of cell → cell shrinks.

Hypertonic

Used medically to draw water out of tissues into bloodstream.

Hypotonic

Lower solute concentration than inside cell.

Hypotonic

Water moves into cell → cell swells, risk of lysis (rupture).

Hypotonic

Used in rehydration therapy (must be slow).

Facilitated diffusion

For lipid-insoluble or large/charged solutes.

Facilitated diffusion

Uses membrane proteins like channel and carrier

Facilitated diffusion

Example: glucose, certain ions.

Filtration

Passive transport where water and solutes are forced through a membrane by hydrostatic (fluid) pressure.

Filtration

Pressure gradient (from high → low pressure

Filtration

Selectivity: Limited; only large molecules (e.g., blood cells, proteins) are held back.

Filtration

Example: Kidney filtration (water + solutes pass into tubules).

Active Transport (Solute Pumping)

Vesicular Transport

Active Processes

Active Transport (Solute Pumping)

Uses ATP + protein carriers (solute pumps) to move molecules.

Active Transport (Solute Pumping)

Against concentration/electrical gradient (uphill).

Active Transport (Solute Pumping)

Highly selective; each pump moves specific substances.

Active Transport (Solute Pumping)

Example: Sodium-Potassium Pump

Sodium-Potassium Pump

Moves 3 Na⁺ out, 2 K⁺ in (against gradients).

Sodium-Potassium Pump

Powered by ATP phosphorylation.

Sodium-Potassium Pump

Essential for nerve impulse transmission.

Vesicular Transport

Bulk transport via membrane vesicles (ATP-dependent).

Exocytosis ("out of the cell")

Endocytosis ("into the cell")

Vesicular Transports

Exocytosis ("out of the cell")

Secretes hormones, mucus, wastes.

Exocytosis ("out of the cell")

Secretory vesicle fuses with plasma membrane and ruptures, releasing contents.

Exocytosis ("out of the cell")

Uses docking proteins for vesicle-membrane recognition.

Endocytosis ("into the cell")

Engulfs extracellular material into vesicles.

Endocytosis ("into the cell")

Vesicles often fuse with lysosomes for digestion.

phagocytosis

pinocytosis

receptor-mediated

types of endocytosis

Phagocytosis ("cell eating")

Engulfs large particles (bacteria, dead cells).

Phagocytosis ("cell eating")

Protective, not nutrient-related.

Phagocytosis ("cell eating")

Example: macrophages, WBCs.

Pinocytosis ("cell drinking")

Engulfs droplets of extracellular fluid.

Pinocytosis ("cell drinking")

Common in absorption-specialized cells.

Receptor-mediated endocytosis

a highly selective type of endocytosis by which cells take up specific ligands

Receptor-mediated endocytosis

Receptors bind specific molecules → vesicle internalization.

Substances: enzymes, hormones, cholesterol, iron.

Receptor-mediated endocytosis substances

Receptor-mediated endocytosis

Exploited by viruses (e.g., flu virus entry).