Cell Membrane Transport (Passive and Active) and Hydrostatic and Oncotic Pressure

This review will discuss cell membrane transport via passive and active transport processes and the movement of fluid and solutes through the capillary wall (hydrostatic and oncotic pressure).

The passive transport processes (require no energy) that move fluid and solutes through the cell membrane include simple diffusion, facilitated diffusion, and osmosis.

In contrast, the transport process that requires energy to move solutes through the cell membrane is known as active transport.

What’s the purpose of transport processes in the cell? In a nut shell, the purpose is to maintain homeostasis in the body. The body wants equal amounts of fluid and solutes inside and outside the cell. In order to achieve this, it will use different transport processes to accomplish it.

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Therefore, let’s review these transport processes:

First, the cell membrane:

Cell Membrane

The cell membrane is a very cool structure. It is made up of a phospholipid bilayer that is composed of hydrophilic heads and hydrophobic tails that act as barrier to separate the extracellular space (outside of the cell) from the intracellular space (inside of the cell). This bilayer is where fluids and solutes pass to enter and exit the cell. Also, in this layer are special channel/carrier proteins that help move certain solutes in and out of the cell.

Now, one thing I want you to remember about this phospholipid bilayer is that it is really particular about what transport processes a solute uses to get inside and outside the cell. Some solutes have to use facilitated diffusion while others can use simple diffusion (more on that below).

Simple Diffusion

What is it? It is just as the name says, it is a very simple process. It’s the movement of molecules (ex: solutes) from a high concentration of solutes to a lower concentration of solutes. In other words, the solutes go down the concentration gradient (NOT against it).

Facts about Simple Diffusion:

• It requires no energy (ex: ATP) and is a form of passive transport.
• Molecules (solutes) move down the concentration gradient (high to low).
• It uses no special channel proteins to help with the movement of solutes. In regards to the cell membrane, the solutes simply go straight through the phospholipid bilayer.
• The solutes (molecules) that can move via simple diffusion must be tiny and non-charged (ex: oxygen, carbon dioxide etc.)
• The mass movement of solutes decrease once equilibrium has been achieved (hence equal amount of solutes on the inside vs. the outside of the cell).

Facilitated Diffusion

What is it? It’s just like simple diffusion (solutes move from high to low concentration). However, it’s a less simple process than simple diffusion because this transport process needs the assistance of special helper channel proteins (hence the solutes need to be facilitated).

Why does it need these proteins? Because this process transports big, polar, charged molecules like glucose and ions. They are unable to just simply pass through the phospholipid bilayer and need help.

Facts about Facilitated Diffusion:

• It requires no energy (ex: ATP) and is a form of passive transport.
• Molecules (solutes) move down the concentration gradient (high to low) with the assistance of special helper channel proteins.
• The solutes (molecules) that can move via facilitated diffusion are big, polar, charged molecules (ex: glucose, ions etc.)
• The mass movement of solutes decrease once equilibrium has been achieved (hence equal amount of solutes on the inside vs. the outside of the cell).

Osmosis

What is it? It is the movement of WATER (not solutes) through a semipermeable membrane. When trying to understand osmosis you can look at it two ways.

One way: Osmosis is where water moves from a high concentration of water to a lower concentration of water.

Second way: Osmosis is where water moves from a solution with a low solute concentration to a solution with a high solute concentration.

Think of it this way: Sodium attracts water. Therefore, water will move where there is a lot of sodium or high concentration of solutes.

Facts about Osmosis:

• It requires no energy (ex: ATP) and is a form of passive transport.
• Only water moves (not solutes) because the semipermeable membrane only allows water through.
• In regards to the cell:
  • If the cell is surrounded by a hypertonic fluid (high osmolarity=high solutes, low water), water will move from the inside of the cell to the outside of the cell. This can cause cell shrinkage.
  • If the cell is surrounded by a hypotonic fluid (low osmolarity=low solutes, high water), water will move from the outside of the cell to the inside of the cell. This can cause cell swelling and rupture.
  • If the cell is surrounded by an isotonic fluid (equal osmolarity=same solute and water concentration inside and outside the cell), there is no mass movement of water from inside the cell to the outside of the cell.
• In healthcare, we can use the osmotic process to treat certain fluid overload or deficit disorders by administering hypotonic, hypertonic, and isotonic IV fluids.

Active Transport

What is it? This type of transport moves solutes in the opposite direction as diffusion and uses energy to do so in the form of ATP. Therefore, active transport moves solutes (molecules) from a low concentration of solutes to a high concentration of solutes with the assistance of ATP (an energy source). This transport also uses special helper protein channels to move the solutes.

Facts about Active Transport:

• Active form of transport
• Uses energy in the form of ATP
• Moves solutes AGAINST the concentration gradient (not down)
  • Moving against the concentration gradient requires a lot of effort and this is why ATP is enlisted to help move the solutes
• The sodium-potassium pump is an example of active transport: this is where 3 sodium ions are moved out of the cell and 2 potassium ions move in the cell.

Transport within Capillary Wall: Oncotic & Hydrostatic Pressure

Oncotic and hydrostatic pressure work in opposite ways to move fluid across the capillary wall. Oncotic pressure pulls, while hydrostatic pressure pushes.

Oncotic Pressure

What is it? It’s the “pulling” effect on water across the capillary wall. The goal of oncotic pressure is to keep water inside the intravascular space (the plasma).

Facts about Oncotic Pressure:

• It’s also called colloidal (colloid) osmotic pressure. Therefore, if you hear or see that term it means the same thing.
• Oncotic pressure within the capillary wall is influenced by a colloid called albumin.
• Albumin likes to hang out in the intravascular space and is too big to pass through the capillary wall. Therefore, high concentrations of it reside in the plasma (intravascular space). This creates high colloidal osmotic pressure (hence oncotic pressure). The result is that through the process of osmosis, water is pulled from the interstitial space (tissue space) into the intravascular space.
  • What happens if albumin levels are low in the plasma…hypoalbuminemia?
    • Oncotic pressure decreases along with its “pulling” effect on water. Water will not be drawn to stay in the intravascular space and will leave and go into the interstitial space. This will lead to tissue swelling.
    • This is why patients with low albumin levels will have edema.

Hydrostatic Pressure

What is it? It’s the “pushing” effect on water across the capillary wall. The goal of hydrostatic pressure is to move fluid and solutes out of the capillary wall into the interstitial space so nutrients and fluid can replenish this space and the cell. This process is known as filtration.

Facts about Hydrostatic Pressure:

• Hydrostatic pressure is defined as the force of a fluid within a restricted space.
  • When thinking about our body, the restricted space is our blood vessels (capillaries), and the fluid is our blood.
• Hydrostatic pressure is created by our heart’s contractions, and is this pressure intensity varies through circulatory system. Therefore, where do you think hydrostatic pressure is going to be the highest….in the arteries or veins? Arteries!
  • The goal of the arteries is to take nutrient rich blood away from the heart so it can replenish cells. Consequently, there has to be a lot of pressure to do this.
    • As a result, hydrostatic pressure is the highest at the arterial end of the capillary.
  • The goal of the veins is to take blood that is exhausted of nutrients and contains wastes back to the heart to be replenished. Therefore, not as much pressure is needed for the blood to go back to the heart.
    • As a result, hydrostatic pressure is the lowest at the venous end of the capillary.

Now, test your knowledge on this content with cell membrane transport quiz and oncotic/hydrostatic pressure quiz.

_References:

_{Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Transport of Small Molecules. Available from: https://www.ncbi.nlm.nih.gov/books/NBK9847/}

_{Hydrostatic pressure: Meaning and examples of use. Hydrostatic Pressure | meaning and examples of use. (n.d.). https://dictionary.cambridge.org/us/example/english/hydrostatic-pressure}

_{Lopez MJ, Hall CA. Physiology, Osmosis. [Updated 2023 Mar 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK557609/}

_{Merriam-Webster. (n.d.). Osmolarity. In Merriam-Webster.com dictionary. Retrieved March 08, 2023, from https://www.merriam-webster.com/dictionary/osmolarity}

_{The capillary wall (2018, July) Centers for Disease Control and Prevention. Available at: https://www.cdc.gov/dengue/training/cme/ccm/page71620.html.}