Chapter 5 Notes

5.1 Passive Transport

How do cell membranes help maintain homeostasis?

By controlling what substances may enter or leave the cell

What is passive transport?

Process where some substances can cross the cell membrane without the input of energy

Diffusion

What is diffusion?

The movement of molecules from an area of higher concentration to an area of lower concentration

What is a concentration gradient?

The difference in the concentration of molecules across a distance

What causes differences to occur?

Driven entirely by the molecules kinetic energy.  Molecules are in constant motion and they travel in straight line until they hit an object and bounce off in another straight line.

How do molecules move from areas of concentration?

Molecules tend to move from areas where they are more concentrated to areas where they are less concentrated, or “up” their concentration gradient.

What is equilibrium?

When the concentration of molecules is the same throughout the space molecules occupy.

When equilibrium is reached, molecules do not stop moving.  Explain this statement:

Molecules till move randomly but balance one another because of the many directions they travel

Diffusion Across Membranes

What happens to molecules on one side of cell membrane if they can pass through?

Diffuse from area of higher to lower concentration on the other side

What is simple diffusion?

Diffusion across a membrane, only allows certain molecules to pass through the membrane.

What does simple diffusion depend on?

The size and type of molecules and on the chemical nature of the membrane

What molecules may pass through membranes?

Molecules that can dissolve in lipids such as non-polar carbon dioxide and oxygen.  Molecules that are very small but not soluble in lipids may also diffuse across by moving through the pores in the membrane.

Osmosis

What is osmosis?

The process by which water molecules diffuse across a cell membrane from an area of higher concentration to an area of lower concentration.

What does osmosis not require and what type of transport is this?

Cells do not expend energy – passive transport

Direction of Osmosis

What does net direction of osmosis depend on?

Relative concentration of solutes on the two sides of the membrane.

What is hypotonic and what occurs?

When the concentration of solute molecules outside the cell is lower than the concentration in the cytosol.  Water diffuses into the cell until equilibrium is established.

What is hypertonic and what occurs?

When the concentration of solute molecules outside the cell is higher than the concentration in the cytosol.  Water diffuses out of the cell until equilibrium is established.

What is isotonic and what occurs?

When the concentration of solutes outside and inside the cell are equal – water diffuses into and out of the cell at equal rates, no net movement of water occurs.

How are hypo and hyper related?

If solution outside cell is hypo, inside cell must be hyper.  If solution outside cell is hyper, inside cell must be hypo.

How does water tend to flow?

Diffuse from hypotonic solutions to hypertonic solutions

How Cells Deal with Osmosis

What cells usually live in isotonic solutions?

Cells of vertebrate animals on land and of most other organisms living in the sea

What live in hypotonic environment and what must they do?

Unicellular freshwater organisms, must rid themselves of excess water that enters by osmosis

What are contractile vacuoles and what do they do?

Organelles that remove water – collect the excess water and then contract, pumping the water out of the cell – requires cell to expend energy.

How do cells of multi-cellular organisms respond to hypotonic environments?

Pumping solutes out of the cytosol so water is less likely to diffuse into the cell

What is turgor pressure?

The pressure that water molecules exert against the cell wall

What happens in plant cells in hypertonic environments?

Water leaves cells through osmosis and cells shrink away from the cell walls and turgor pressure is lost.

What is plasmolysis?

Condition where cells shrink – reason that plants wilt if they don’t receive enough water

Why can’t red blood cells compensate for changes in the solute concentration?

Lack contractile vacuoles, solute pumps, and cell walls

What is cytolysis?

The bursting of cells

Facilitated Diffusion

What is facilitated diffusion?

Type of passive transport used for molecules that cannot readily diffuse through cell membrane even when there is a concentration gradient across the cell membrane.

What molecules may use facilitated diffusion?

May not be soluble in lipids, or too large to pass through the pores in the membrane.

What are carrier proteins?

Specific proteins that assist in the movement of molecules across the cell membrane

What are the two important properties of facilitated diffusion?

1. Can help substances move either into or out of a cell, depending on the concentration gradient

2. Carrier proteins involved in facilitated diffusion are each specific for one type of molecule

Diffusion Through Ion Channels

What do ion channels do?

Transport ions from higher to lower concentrations

What type of transport are ion channels?

Passive

What ions are transported?

Sodium (Na⁺), Potassium (K⁺), Calcium (Ca²⁺), and chloride (Cl^-)

Why do ions not diffuse?

Not soluble in lipids so cannot diffuse across phospholipid bilayer without assistance

What kinds of stimuli may cause ion channels with “gates” to open or close?

Stretching of the cell membrane, electrical signals, or chemicals in the cytosol or external environment

5. 2 Active Transport

What is active transport and what does it require?

When cells must move materials from an area of lower concentration to an area of higher concentration (up their concentration gradient).  Cells must expend energy.

Cell Membrane Pumps

What are cell membrane pumps?

Carrier proteins that serve in active transport

What supplies energy for cell membrane pumps?

ATP

What is the sodium-potassium pump?

Active transport of Na⁺ ions and K⁺ ions up their concentration gradient

How is ion concentration in cells distributed?

Na⁺ ions outside the cell and K⁺ ions inside the cell

How many ions are exchanged in a complete cycle?

3 Na⁺ ions and 2 K⁺ ions

At top speed how many ions can be pumped every second?

450 Na⁺ ions and 300 K⁺ ions

What does the pump create and why is it important?

Creates an electrical gradient – used for conduction of electrical impulses along nerve cells

Movement in Vesicles

What substances may be too large to pass through the cell membrane by transport processes?

Macromolecules and nutrients

What process do cells use and what type of transport are they?

Endocytosis and exocytosis – active transport

Endocytosis

What is endocytosis?

The process by which cells ingest external fluid, macromolecules, and large particles, including other cells

How does endocytosis occur?

External materials are enclosed by a portion of the cell’s membrane, which folds into itself and forms a pouch, the pouch pinches off from the cell membrane and becomes a membrane-bound organelle called a vesicle

What are the two main types of endocytosis and what are they based on:

1.  Pinocytosis – involves the transport of solutes or fluids

2.  Phagocytosis – the movement of large particles o whole cells – many unicellular organisms feed by phagocytosis, certain cells in animals use phagocytosis to ingest bacteria and viruses that invade the body

What are phagocytes?

Cells that allow lysosomes to fuse with vesicles that contain ingested bacteria and viruses – lysosomal enzymes then destroy the bacteria and viruses before they can harm the animal

Exocytosis

What is exocytosis?

The process by which a substance is released from the cell through a vesicle that transports the substance to the cell surface and then fuses with the membrane to let the substance out of the cell.

What do cells use exocytosis for?

To release large molecules such as proteins, waste products, or toxins.  They would damage the cell if they were released within the cytosol

What may cells in the nervous and endocrine system use exocytosis for?

To release small molecules that control the activities of other cells