Homeostasis and Cell Transport Chapter 5 Table of Contents Section 1 Passive Transport Section 2 Active Transport

Homeostasis and Cell TransportChapter 5

Table of Contents

Section 1 Passive Transport

Section 2 Active Transport

Section 1 Passive TransportChapter 5

Objectives

• Explain how an equilibrium is established as a result of diffusion.

• Distinguish between diffusion and osmosis.

• Explain how substances cross the cell membrane through facilitated diffusion.

• Explain how ion channels assist the diffusion of ions across the cell membrane.

Passive Transport

• NO ENERGY REQUIRED to move substances across membrane -- water, lipids, and other lipid soluble substances.

Chapter 5 Section 1 Passive Transport

• Types:– Diffusion– Osmosis– Facilitated Diffusion– Filtration

•Involves the movement of molecules across the cell membrane •without an input of energy by the cell.


Diffusion

• Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, driven by the molecules’ kinetic energy until a state of dynamic equilibrium* is reached.

• Occurs because of Brownian Motion, i.e., the random movement of particles.

*Dynamic equilibrium – continued movement of molecules with no net change in concentration

– Concentration gradient, i.e., the difference in concentration across space.

Chapter 5

Click below to watch the Visual Concept.

Visual Concept

Concentration Gradient


Chapter 5

Diffusion



Osmosis

• Osmosis is the diffusion of water across a membrane.

Chapter 5


Visual Concept

Osmosis



Osmosis, continued

• Direction of Osmosis– The net direction of osmosis is determined by the

relative solute concentrations on the two sides of the membrane.


Osmosis, continued

• Direction of Osmosis– When the solute concentration outside the cell is

higher than that in the cytosol, the solution outside is hypertonic (more solute, less water) to the cytosol, and water will diffuse out of the cell.

– Plasmolysis = cell’s shrinking due to water loss


Osmosis, continued

• Direction of Osmosis– When the solute concentration outside the cell is

lower than that in the cytosol, the solution outside is hypotonic (less solute, more water) to the cytosol, and water will diffuse into the cell.

– Cytolysis = cell’s bursting due to water gain


Osmosis, continued

• Direction of Osmosis– When the solute concentrations outside and inside

the cell are equal, the solution outside is isotonic (same solute, same water), and there will be no net movement of water.

Chapter 5

Hypertonic, Hypotonic, Isotonic Solutions


Chapter 5


Visual Concept

Comparing Hypertonic, Isotonic, and Hypotonic Conditions



Osmosis, continued

• How Cells Deal With Osmosis– To remain alive, cells must compensate for the

water that enters the cell in hypotonic environments and leaves the cell in hypertonic environments.

– Cells in multicellular organisms respond to hypotonic environments by pumping solutes out of the cytosol (RBCs cannot compensate for changes in solute concentration)

– Contractile vacuoles are organelles that regulate water levels in paramecia.


Facilitated Diffusion

• Diffusion of molecules across a membrane when they are not soluble in lipids or are too large (e.g. glucose) to pass through pores in membrane

• In facilitated diffusion, a molecule binds to a carrier protein on one side of the cell membrane.

• The carrier protein (specific for one type of molecule) then changes its shape and transports the molecule down its concentration gradient to the other side of the membrane.

Chapter 5

Facilitated Diffusion



Diffusion Through Ion Channels

• Ion channels are proteins, or groups of proteins, that provide small passageways across the cell membrane through which specific ions can diffuse.

• Ions important in cell function include sodium, potassium, calcium, and chloride

Chapter 5

Ion Channels


Section 2 Active TransportChapter 5

Objectives

• Distinguish between passive transport and active transport.

• Explain how the sodium-potassium pump operates.

• Compare endocytosis and exocytosis.


• Active transport moves molecules across the cell membrane from an area of lower concentration to an area of higher concentration.

• Unlike passive transport, active transport requires cells to expend energy.

• Some types of active transport are performed by carrier proteins called cell membrane pumps.

Cell Membrane Pumps


Cell Membrane Pumps, continued

• Sodium-Potassium Pump– The sodium-potassium pump moves three Na+

ions into the cell’s external environment for every two K+ ions it moves into the cytosol.

• Animal cells must have a higher concentration of Na+ ions outside the cell and a higher concentration of K+ ions inside the cell

– ATP supplies the energy that drives the pump.

Chapter 5

Sodium-Potassium Pump


Sodium-Potassium Pump

• The exchange of three Na+ ions for two K+ ions creates an electrical gradient across the cell membrane– Outside becomes positively charged relative to the

inside, which becomes negative• Difference in electrical charge is important for

the conduction of electrical impulses along nerve cells

Chapter 5Section 2 Active Transport


Movement in Vesicles

• Endocytosis– In endocytosis, cells ingest external fluid,

macromolecules, and large particles, including cells by folding around them and forming a pouch.

– The pouch then pinches off and becomes a membrane-bound organelle called a vesicle.

*Some vesicles fuse with lysosomes, and their contents are digested by lysosomal enzymes


Movement in Vesicles, continued

• Endocytosis– Endocytosis includes pinocytosis, in which the

vesicle contains solutes or fluids, and phagocytosis, in which the vesicle contains large particles or whole cells.

• Bacteria and viruses are ingested in this way– Receptor-mediated endocytosis – molecules are

brought into the cell via coated pits (proteins)

Chapter 5


Visual Concept

Endocytosis



Movement in Vesicles, continued

• Exocytosis– In exocytosis, vesicles made by the cell fuse with

the cell membrane, releasing their contents into the external environment.

• Used to release large molecules, such as proteins, waste products, or toxins that would damage the cell if they were released within the cytosol

Chapter 5


Visual Concept

Exocytosis


Chapter 5

Endocytosis and Exocytosis


Documents

Homeostasis and Cell Transport Chapter 5 Table of Contents Section 1 Passive Transport Section 2 Active Transport