It requires an energy input. Glucose can be taken into cells by active transport if the concentration of glucose inside the cell is already quite high.

This requires carrier proteins. Types of transport Partially permeable cell membranes All cells have a cell membrane. While this process still consumes ATP to generate that gradient, the energy is not directly used to move the molecule across the membrane, hence it is known as secondary active transport. Both antiporters and symporters are used in secondary active transport.

Co-transporters can be classified as symporters and antiporters depending on whether the substances move in the same or opposite directions across the cell membrane. Secondary active transport brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside the plasma membrane because of the action of the primary active transport process, an electrochemical gradient is created.

If a channel protein exists and is open, the sodium ions will be pulled through the membrane. This movement is used to transport other substances that can attach themselves to the transport protein through the membrane. Many amino acids, as well as glucose, enter a cell this way. This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP.

The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP. Secondary Active Transport : An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport. Endocytosis takes up particles into the cell by invaginating the cell membrane, resulting in the release of the material inside of the cell.

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: the plasma membrane of the cell invaginates, forming a pocket around the target particle.

The pocket pinches off, resulting in the particle being contained in a newly-created intracellular vesicle formed from the plasma membrane. Phagocytosis : In phagocytosis, the cell membrane surrounds the particle and engulfs it. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil.

In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane.

The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly-formed compartment endosome.

When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly-formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane. Pinocytosis : In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off.

A variation of endocytosis is called pinocytosis. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome. Potocytosis, a variant of pinocytosis, is a process that uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin.

The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae singular caveola are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Receptor-Mediated Endocytosis : In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane. A targeted variation of endocytosis, known as receptor-mediated endocytosis, employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances. In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane.

If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by the failure of receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood.

Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells. Privacy Policy. Skip to main content.

Organization at the Cellular Level. Search for:. Transport Across Membranes. Diffusion Diffusion is a process of passive transport in which molecules move from an area of higher concentration to one of lower concentration. Learning Objectives Describe diffusion and the factors that affect how materials move across the cell membrane.

Key Takeaways Key Points Substances diffuse according to their concentration gradient; within a system, different substances in the medium will each diffuse at different rates according to their individual gradients. After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another, a state known as dynamic equilibrium.

Several factors affect the rate of diffusion of a solute including the mass of the solute, the temperature of the environment, the solvent density, and the distance traveled. Key Terms diffusion : The passive movement of a solute across a permeable membrane concentration gradient : A concentration gradient is present when a membrane separates two different concentrations of molecules. Osmosis Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration.

Learning Objectives Describe the process of osmosis and explain how concentration gradient affects osmosis. Key Takeaways Key Points Osmosis occurs according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes.

Osmosis occurs until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. Osmosis occurs when there is a concentration gradient of a solute within a solution, but the membrane does not allow diffusion of the solute.

Key Terms solute : Any substance that is dissolved in a liquid solvent to create a solution osmosis : The net movement of solvent molecules from a region of high solvent potential to a region of lower solvent potential through a partially permeable membrane semipermeable membrane : A type of biological membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally by specialized facilitated diffusion.

Tonicity Tonicity, which is directly related to the osmolarity of a solution, affects osmosis by determining the direction of water flow. Learning Objectives Define tonicity and describe its relevance to osmosis. Key Takeaways Key Points Osmolarity describes the total solute concentration of a solution; solutions with a low solute concentration have a low osmolarity, while those with a high osmolarity have a high solute concentration.

Water moves from the side of the membrane with lower osmolarity and more water to the side with higher osmolarity and less water. In a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the cell; water enters the cell. In a hypertonic solution, the extracellular fluid has a higher osmolarity than the fluid inside the cell; water leaves the cell.

In an isotonic solution, the extracellular fluid has the same osmolarity as the cell; there will be no net movement of water into or out of the cell. Key Terms osmolarity : The osmotic concentration of a solution, normally expressed as osmoles of solute per litre of solution.

Examples Tonicity is the reason why salt water fish cannot live in fresh water and vice versa. Facilitated transport Facilitated diffusion is a process by which molecules are transported across the plasma membrane with the help of membrane proteins.

Learning Objectives Explain why and how passive transport occurs. Key Takeaways Key Points A concentration gradient exists that would allow ions and polar molecules to diffuse into the cell, but these materials are repelled by the hydrophobic parts of the cell membrane. Facilitated diffusion uses integral membrane proteins to move polar or charged substances across the hydrophobic regions of the membrane.

Channel proteins can aid in the facilitated diffusion of substances by forming a hydrophilic passage through the plasma membrane through which polar and charged substances can pass. Channel proteins can be open at all times, constantly allowing a particular substance into or out of the cell, depending on the concentration gradient; or they can be gated and can only be opened by a particular biological signal. Carrier proteins aid in facilitated diffusion by binding a particular substance, then altering their shape to bring that substance into or out of the cell.

In other words, the greater the surface area of the cell compared to its volume, the more efficient the cell will be in performing its functions. It is interesting to note that as a cell gets bigger, its volume will increase more than its surface area. Let's look at what happens if you double the size of a cell:. So you can see that there is a negative relationship between size and efficiency in cells. The bigger they get the more difficult it is for them to take up materials fast enough.

Diffusion across the cell membrane happens because of the concentration gradient between the intracellular and extracellular environments. We have already seen that diffusion means the movement of substances from areas of high concentration to areas of low concentration. However, the rate of diffusion is dependent upon the concentration gradient. The concentration gradient is calculated as the difference in concentration per centimeter.

Imagine a boy rolling a ball down a hill. If the hill is very steep, the ball will roll faster. If a concentration gradient is steep, that is to say it represents a rapid change from high concentration to low concentration, then substances will move down it faster - just like the ball! A typical cell membrane is very thin. The reason for this is to keep the distance between internal and external concentrations short. This helps create a steeper concentration gradient, enabling the movement of substances in and out of the cell.

When you take a deep breath, the concentration of oxygen in the lungs is increased. The lungs are full of air with a high oxygen concentration compared to a lower oxygen concentration in the blood. Therefore, oxygen diffuses into the bloodstream. The movement of substances in and out of the cell by diffusion is known as passive transport. However, sometimes substances will not diffuse across the membrane and need to be chemically assisted. This is known as active transport.

A typical situation in which active transport is required is when a substance must travel against the concentration gradient. Clearly in this case diffusion will not help at all! Active transport always occurs across the cell membrane and it requires an input of extra energy to push the particles up the concentration gradient. The energy for active transport is provided by the process of respiration.

The cell membrane has specialised molecules incorporated into it. These carrier molecules absorb the energy of respiration in order to assist other substances in crossing the cell membrane.

Osmosis is exactly the same mechanism as diffusion but it is a term used to apply specifically to the movement of water molecules. So when water molecules H 2 O are transferred across a partially permeable membrane from an area of higher to an area of lower concentration, which is called osmosis. Let's just pause here a moment to give some definitions of a few important terms we've used:. Biologists will often refer to a solution which contains a large amount of solute as having a 'concentrated solution' but you can also think of that as a solution with a low concentration of water molecules.

So the concept of high and low concentration is always relative to the molecules you are referring to! An animal cell is surrounded by a partially permeable membrane. Because osmosis enables water to flow so freely through the cell system, it can do a lot of harm as well as good. The greatest danger is that of lysis. In facilitated diffusion , substances move into or out of cells down their concentration gradient through protein channels in the cell membrane.

Simple diffusion and facilitated diffusion are similar in that both involve movement down the concentration gradient. The difference is how the substance gets through the cell membrane. In simple diffusion, the substance passes between the phospholipids; in facilitated diffusion there are a specialized membrane channels. Charged or polar molecules that cannot fit between the phospholipids generally enter and leave cells through facilitated diffusion.

Note that the substance is moving down its concentration gradient through a membrane protein not between the phospholipids. The types of membrane transport discussed so far always involve substances moving down their concentration gradient.

It is also possible to move substances across membranes against their concentration gradient from areas of low concentration to areas of high concentration. Since this is an energetically unfavorable reaction, energy is needed for this movement.

The source of energy is the breakdown of ATP. If the energy of ATP is directly used to pump molecules against their concentration gradient, the transport is called primary active transport. Note that the substance indicated by the triangles is being transported from the side of the membrane with little of the substance to the side of the membrane with a lot of the substance through a membrane protein, and that ATP is being broken down to ADP.

In some cases, the use of ATP may be indirect. On the left side of the picture below, a substance represented by an X is being transported from the inside of the cell to the outside even though there is more of that substance on the outside indicated by the letter X being larger on the outside of the cell.