Chapter 2: Basic Principles of Physiology-Membrane Transport

What type of membrane is the plasma membrane?

A semi-permeable membrane

What does semi-permeable mean?

It allows some substance to cross while restricting other

What are the 3 functions of the plasma membrane?

• Barrier

• Regulation of Transport

• Communication with other cells

What two environments does the plasma membrane separate as a barrier?

The intracellular and extracellular environments

What does intracellular mean?

Inside the cell

What does extracellular mean?

Outside of the cell

What does the plasma membrane regulate?

The exchange of substances across the membrane

What is the communication function of the plasma membrane?

Communication with other cells

How does the plasma membrane being semi-permeable relate to transport?

Being semi-permeable allows the membrane to control which substances cross, so it can regulate transport across the membrane

What are the 3 major types of membrane transport?

• Diffusion (Passive)

• Active Transport

• Vesicular Transport

What are the two types of diffusion?

• Simple diffusion

• Facilitated diffusion

In what direction do substances move during diffusion?

From high concentration to low concentration

Does passive diffusion require energy from the cell?

No

What is simple diffusion?

Passive movement of a substance directly across the membrane

What is facilitated diffusion?

Passive movement across the membrane with the help of a transport protein

Connection card: What do simple diffusion and facilitated diffusion have in common?

Both are passive and move substances from high → low concentration

What is the major difference between passive diffusion and active transport?

Passive diffusion does not require energy, while active transport requires energy

What does vesicular transport use to transport substances?

Vesicles, which are small membrane-bound sacs

How does diffusion help the plasma membrane perform its transport function?

Diffusion allows substances to move across the plasma membrane from higher to lower concentration

What process is shown in this figure?

Passive diffusion across the plasma membrane

What does the arrow at the bottom of the diffusion figure represent?

Time passing from left to right

Where is the initial concentration of molecules highest in this figure?

In the extracellular fluid (outside the cell)

Where is the initial concentration of molecules lowest in this figure?

In the cytoplasm (inside the cell)

In what direction is the net movement of molecules during diffusion?

High concentration → Low concentration

What is concentration gradient?

A difference in concentration between two areas

What happens to the distribution of molecules over time during diffusion?

The molecules become more evenly distributed

Does diffusion shown in this figure require cellular energy?

No. It is passive transport

Explain what is happening in the diffusion figure from left to right

Molecules initially have a higher concentration outside the cell than inside. Over time, they diffuse across the plasma membrane from high to low concentration, becoming more evenly distributed

What is diffusion?

The tendency of molecules to disperse (spread out) to achieve equilibrium of concentration

In what direction does net diffusion occur?

From high concentration to low concentration

What does moving “down the concentration gradient” mean?

Moving from high concentration → low concentration

What causes diffusion?

Random molecular motion

Is diffusion passive or active?

Passive

Does passive diffusuion require cellular energy?

No

How long does diffusion proceed?

Until equilibrium is reached

What does equilibrium mean in this diffusion figure?

The molecules have become evenly distributed in concentration

What is happening in figure (a)?

There is a high concentration of molecules on the left and a low concentration on the right.

What does figure (b) represent?

Equilibrium, where the molecules are evenly distributed in concentration

Connection card: Explain the entire diffusion figure from (a)→(b)

Molecules move from high → low concentration due to random molecular motion, moving down their concentration gradient until equilibrium is reached.

What is simple diffusion?

Movement of molecules directly through the plasma membrane from high → low concentration

Do molecules undergoing simple diffusion need a transport protein?

No. They move directly through the membrane

What types of molecules can undergo simple diffusion?

• Hydrophobic molecules

• Lipophilic molecules

What does lipophilic mean?

• Lipid-loving or soluble in lipids

What are some examples of molecules capable of simple diffusion?

• Ethanol

• Vitamins A, D, & E

• O₂

• CO₂

• H₂O

What is the diffusion of H₂O called?

Osmosis

What three factors affect the rate of simple diffusion?

• Concentration gradient

• Size of molecules

• Lipid solubility

What shows that the bottom figure represents simple diffusion?

Small molecules move directly through the lipid bilayer

What is happening in the osmosis figure?

Water crosses the semipermeable membrane toward the side with more solute, causing the water level on that side to rise

Connection card: How are diffusion, simple diffusion, and osmosis connected?

• Diffusion = movement of concentration from high → low

• Simple diffusion = movement of substance directly through membrane

• Osmosis= Diffusion of H₂O

What is facilitated diffusion?

Passive movement across the membrane with the help of a membrane transport protein

Does facilitated diffusion require cellular energy?

No it is passive

In what direction do molecules move during facilitated diffusion?

High concentration → Low concentration

What two types of proteins are show for facilitated diffusion?

• Channel proteins

• Carrier proteins

What does it mean that a channel protein is selective?

It allows specific substances to pass

What does it mean that a channel protein is gated?

The channel can be opened or closed to regulate movement

What substances does the slide associate with channel proteins?

Small ions

How does a carrier protein move a molecule across the membrane?

The molecule binds to the carrier, the protein changes shape, and the molecule is moved across

What is conformational change?

A change in the shape of a protein

What substances does the slide associate with carrier proteins?

Larger molecules, such as glucose and amino acids

How can you recognize a channel protein in a figure?

It forms a passageway or channel through the membrane that can be opened or closed

How can you recognize a carrier protein in a figure?

A molecule binds to the protein, and the protein changes shape to move it across

Connection card: What is the major difference between simple diffusion and facilitated diffusion?

• Simple Diffusion → moves substances directly through the lipid bilayer

• Facilitated Diffusion → moves substances with the help of a membrane protein

What does the rate of simple diffusion depend on?

The concentration gradient

What happens to the rate of simple diffusion as the concentration gradient increases?

The rate of simple diffusion increases

What two things determine the rate of facilitated diffusion?

The concentration gradient and the number of available transport proteins

What is transport maximum?

The maximum transport rate reached when all available transporters are occupied

What does “occupied” mean when discussing transport proteins?

The transport proteins are already being used by molecules

What happens to facilitated diffusion once all transport proteins are occupied?

The transport rate levels off at its maximum

What does flux into the cell mean on the graph?

The rate of movement into the cell

Which line on the graph represents simple diffusion?

The straight line that continues increasing

Why does the facilitated/mediated transport curve level off?

Because all available transport proteins become occupied

What does the plateau of the mediated transport curve represent?

Maximal flux/ transport maximum

Connection Card: Why does facilitated diffusion reach a transport maximum while simple diffusion does not in the graph?

Facilitated diffusion depends on a limited number of transport proteins, while simple diffusion does not require transport proteins

What does it mean when transport protein is regulated?

Its activity is being controlled

What are four ways transport proteins can be regulated according to the slide?

1. Binding of regulatory molecule

2. Changes in membrane potential

3. Covalent modification

4. Insertion or removal of transport proteins

How can binding of a regulatory molecule regulate a transport protein?

A regulatory molecule binds to the protein and changes its activity

What is membrane potential?

The electrical difference across the cell membrane

How can membrane potential regulate transport proteins?

Changes in membrane potential can change the transport protein activityWh

What type of covalent modification does the professor give as an example?

Phosphorylation

What is phosphorylation?

The addition of a phosphate group to a molecule/protein

ow can phosphorylation regulate a transport protein?

Adding a phosphate group can change the protein’s activity

How can the number of transport proteins in the membrane be regulated?

Transport proteins can be inserted into or removed from the membrane

Connection card: Why can inserting more transport proteins into the membrane increase transport?

It creates more available transport proteins for substances to use.

In other words: More transport proteins = more opportunities for substances to cross.

What is active transport?

Transport of molecules against their concentration gradient

In what direction does the active transport move molecules relative to their concentration gradient?

Low concentration → high concetration

Does active transport require energy?

Yes

Connection Card: Why does active transport require energy?

Because molecules are being moved against their concentration gradient

In other words: Moving low→ high is like moving uphill and requires energy

What are primary active transporters also called?

Pumps

What energy source is used directly by primary active transporters?

ATP

What are the three primary active transporters listed on the slide?

• H⁺-ATPase

• Ca²⁺-ATPase

• Na⁺/K⁺-ATPase

What type of active transport is Na⁺-dependent cotransporter?

Secondary active transport

What provides the energy for Na⁺-dependent secondary active transport?

The Na⁺ concentration gradient

What is the major difference between primary and secondary active transport?

• Primary → directly uses ATP

• Secondary → uses energy stored in an ion gradient, such as Na⁺

Connection Card: How do diffusion and active transport differ in direction and energy use?

Diffusion: high → low, passive

Active transport: low → high, require energy

What type of transport is Na+/K+-ATPase?

Primary active transport

Why is the Na⁺/K⁺-ATPase considered primary active transport?

Because it directly uses ATP to transport ions

How many Na+ and K+ ions are moved by the Na+/K+-ATPase per cycle?

3Na+ OUT and 2 K+IN

Where do the 3 Na+ions initially bind to the Na+/K+pump?

On the inside of the cell

What happens to ATP after Na⁺ binds to the pump?

ATP is broken down and a phosphate is added to the pump.

What causes the Na⁺/K⁺ pump to change shape and release Na⁺ outside?

Phosphorylation of the pump