Transport across the plasma membrane: Passive transport, Active Transport, For Class 10th, 11th,  12th, and NEET

Transport across the plasma membrane: Passive transport, Active Transport, For Class 10th, 11th,  12th, and NEET

Transport across the plasma membrane

A selectively permeable barrier that separates the cell from the extracellular environment is the plasma membrane. Because of its permeability, it allows for the easy entry of vital molecules like glucose, amino acids, and lipids, the retention of metabolic intermediates, and the exit of waste products. In other words, plasma membranes selectively permit some compounds to pass through while blocking others. When a membrane is selectively permeable or semi-permeable, only specific chemicals can pass through it. The cell would be destroyed if the membrane were to lose its selectivity since it would be unable to preserve homeostasis or function.

There are two different forms of transport procedures:

A portion of the cell’s energy may be used to transport molecules across the plasma membrane. The transport is referred to as active if energy is used. Molecules are using passive transport if they can cross the plasma membrane without requiring any energy.

Passive transport

 The substances use their kinetic energy, which is an inherent property of the moving particles, to flow down their concentration or voltage gradient and across the membrane. The cell does not supply any energy.

Three different processes—Diffusion, Osmosis, and Filtration—can allow molecules to flow across a membrane passively.

Diffusion: Transport that is done passively is called diffusion. Until the concentration is the same throughout a space, a single substance tends to travel from an area of high concentration to an area of low concentration. Diffusion is the process by which certain substances pass through the plasma membrane and others diffuse through the cytoplasm of the cell. There is no energy used in diffusion. Contrarily, as the gradient is removed, potential energy associated with concentration gradients dissipates. Each distinct material in a medium, like the extracellular fluid, has its concentration gradient that is independent of the gradients of other substances. Each substance will also diffuse under that gradient. The different compounds in the medium will diffuse at varying speeds throughout a system.

Osmosis: A specific instance of diffusion is osmosis. Osmosis is the process of moving water across a semi-permeable membrane from a zone of high concentration to one of low concentration to balance the concentrations of the water.Most cell membranes are permeable to water, and since water diffusion is crucial to every living thing’s biological operation, a specific term called osmosis has been created to describe it. It is the net passage of a solvent through a membrane with a narrow range of permeation. Water acts as the solvent in living systems and travels across plasma membranes by osmosis from one region of higher water concentration to another region of lower concentration. In other words, water flows from a region with a lower concentration of solutes to a region with a higher concentration of solutes across a selectively permeable membrane. There are two ways that water molecules can cross a plasma membrane. Aquaporins and the lipid bilayer are both traversed for this to occur (integral membrane proteins that function as water channels).

Filtration: The capillaries most often use the final type of passive transfer. Diffusion occurs quickly through capillaries because of their thin membranes, which are only one cell thick. However, keep in mind that animals have blood pressure. Water and other tiny solutes that have dissolved in the water can be forced right through the capillary membrane by the pressure at which blood flows through them. In essence, the capillary membrane serves as filter paper, letting fluid around the body’s cells while preventing big molecules from entering the tissue fluid.

Active Transport

Active transport is the energy-intensive movement of a material along a concentration gradient, or from a lower concentration to a higher concentration, across a cell membrane. The cell membrane contains special proteins that serve as specialized protein “carriers.” Active transportation is powered by ATP produced during respiration (in mitochondria).

Types of active transport

i) Primary transport

ii) Secondary transport

 Primary transport

All types of solutes are transported across a membrane against their concentration gradient through primary active transport, also known as direct active transport, which directly employs chemical energy (such as from adenosine triphosphate or ATP in the case of the cell membrane). Primary active transport is demonstrated by the human intestines’ ability to absorb glucose. Other energy sources for primary active transport include photon energy and redox energy (chemical reactions like oxidation and reduction) (light). The mitochondrial electron transport chain, which uses the reduction energy of NADH (nicotinamide adenine dinucleotide, reduced form) to carry protons across the inner mitochondrial membrane against their concentration gradient, is an example of primary active transport employing redox energy. The proteins involved in photosynthesis are an illustration of primary active transport using light energy.

Secondary transport

On the other hand, secondary active transport enables one solute to travel downward (along its electrochemical potential gradient) to produce enough entropic energy to propel the movement of the other solute upward (from a low concentration region to a high one). Contrary to noncoupled or uniport transport, which facilitates the transport of just one component, this is sometimes referred to as coupled transport. Antiport and symport are the two basic types of linked transport. In antiport, two ion or other solute species are pushed across a membrane in opposition to one another, whereas in symport transport, two species move in the same direction.

Major Active Transport examples

Reabsorption of salts, glucose, and amino acids by the kidney’s proximal convoluted tubule of the nephron.

Cell membranes contain a sodium/potassium pump (especially nerve cells)


Large molecules, particles, bacteria, and other creatures migrate over the cell membrane in this manner. It entails the joining of vesicles (which contain the target or victim) with cell membranes, such as when bacteria enter macrophages. The Golgi body first prepares the materials that will be secreted.

Pinocytosis “Cell drinking” (pinocytosis) This is an example of endocytosis, where huge molecules (DNA, protein) are taken up from a solution by tiny, transient vesicles.

Phagocytosis (‘cell eating’). This occurs when a cell takes in solid substances, such as when an amoeba feeds or when phagocytes engulf germs.

Describe osmosis.

Osmosis is the process of moving water across a semi-permeable membrane from a zone of high concentration to one of low concentration to balance the concentrations of the water. Because osmosis is a passive transport system, it runs without any energy.

Osmosis is a particular kind of diffusion that describes how water molecules travel. Osmosis happens when a membrane has different concentrations of water molecules on each side of it. Until equilibrium is attained, water molecules will diffuse over the membrane.

A specific illustration of diffusion is osmosis. It is the movement of water along the water potential gradient across a partially permeable membrane from a more diluted solution to a more concentrated solution.

Note that neither osmosis nor diffusion requires ATP energy to function.

A partly permeable membrane allows certain substances to flow through but not others; for example, it allows the passage of solvent molecules but not all of the bigger solute molecules.

Because they permit the passage of some solutes in addition to water, cell membranes are referred to as selectively permeable. When certain solutes are present, the membrane is stimulated to open particular channels or activate active transport mechanisms, allowing the flow of those molecules across the membrane.

Examples of osmosis in action include:

1. Plant roots absorbing water

2. The proximal and distal convoluted tubules of the nephron reabsorb water.

3. Water absorption by the alimentary canal, which includes the stomach, small intestine, and colon.

Is osmosis limited to water?

From a high energy or concentration zone to a low energy or concentration region, only water or another solvent can travel. In every medium, whether it be liquid, solid, or gas, diffusion can take place. Osmosis only takes place in liquid media. Osmosis needs a semipermeable membrane.

What occurs throughout osmosis?

Water diffuses into or out of cells through osmosis. A cell that has water entering it may swell or possibly explode. When cells are inserted into a hypotonic solution, this occurs. Cells submerged in hypertonic fluids experience this.

Osmosis: a passive transport or not?

Osmosis: passive transport. Osmosis is the process by which water diffuses through a semipermeable membrane following the gradient of water concentration across the membrane. A specific instance of diffusion is osmosis. Water flows from a region of higher concentration to one of lower concentration, just like other substances.

What does tonicity mean to you?

Tonicity, or the water potential of two fluids separated by a semipermeable cell membrane, is a measurement of the effective osmotic pressure gradient. One solution’s tonicity can be categorized into three different ranges: hypertonic, hypotonic, and isotonic.

What impact does tonicity have on cells?

Tonicity. A solution’s tonicity is determined by how it affects a cell’s volume. Isotonic solutions are those that do not alter a cell’s volume. A cell will enlarge in a hypotonic solution while contracting in a hypertonic one.

What impact does hypotonicity have on red blood cells?

A solution with a low solute concentration and a high water concentration is called a hypotonic solution. As a result, since the concentration of water in the RBC is reduced, water would naturally enter the cell through osmosis if it were placed in a hypotonic solution.

What is the Blood’s tonicity?

The osmolarity of normal saline, which is 9 grams of sodium chloride dissolved in 1 litre of water, is quite similar to the 290 mOsm/L osmolarity of sodium chloride in the blood. As a result, blood plasma and normal saline are practically isotonic.


The net movement of molecules from locations with high concentrations to areas with low concentrations of that molecule is known as diffusion. Collisions with other molecules in the medium cause the Brownian motion, which is this erratic movement.

The phrase “going down the concentration gradient” refers to the migration of particles from a high concentration to a low concentration.

Diffusion is the net passive transfer of particles (atoms, ions, or molecules) from regions of higher concentration to those of lower concentration. It keeps on until all areas have the same amount of the chemical.

Major biological examples of diffusion include:

• Gas exchange at the alveoli, where oxygen is transferred from the air to the blood and carbon dioxide is transferred from the blood to the air.

• Gas exchange for photosynthesis: oxygen from the leaf to the air and carbon dioxide from the air to the leaf.

• Gas exchange for respiration: carbon dioxide travels oppositely from tissue cells to the circulation than oxygen does.

• Acetylcholine is a transmitter that is transferred at synapses from the presynaptic to the postsynaptic membrane. Water diffuses over a semipermeable membrane by osmosis.

Short distance, vast surface area, and high concentration differential all contribute to a high diffusion rate (Fick’s Law). Large molecules slow diffusion while high temperatures speed it up.

Facilitated Diffusion

This is the passage of particular molecules through the membrane using a particular carrier protein and a concentration gradient. Thus, each carrier has its structure and only permits one molecule (or one set of closely related compounds) to pass through, similar to how enzymes do.

Selection is based on size, shape, and charge. Glucose and amino acids are typical substances that enter or exit cells in this manner.

It uses no energy from the cell and is passive. The concentration gradient of glucose will be maintained high if the molecule is altered upon entry into the cell ((glucose + ATP → glucose phosphate + ADP), resulting in constant one-way traffic.


 When a molecule reaches equilibrium, or when there are equal amounts of that molecule throughout the entire medium, it will continue to diffuse in this manner. For this reason, the food colouring granules distribute until the water is uniformly coloured.


You took the shell off the eggs before making your bare ones. Currently, a semipermeable membrane encloses them. The size, shape, or charge of the molecule as well as the properties of the membrane’s pores dictate what is allowed to pass through the membrane. In your studies, the egg membrane is solely permeable to water, therefore all the changes you observe are the result of the passage of water through the membrane.


Osmoregulation is the process of maintaining an appropriate level of blood or cell cytoplasm concentration.

(a) Freshwater amoebae use a contractile vacuole to remove extra water from their cytoplasm as a result, they require more oxygen and ATP for respiration than isotonic (marine) amoebae.

(b) The kidneys keep the blood—and by extension, the entire body—at the proper concentration.


When the exterior solution is more diluted than the cell sap in the vacuole, turgor occurs, which is the pressure of the enlarged cell contents against the cell wall.

Turgor’s function in plants

• Mechanical assistance for non-woody soft tissue, such as leaves.

• The guard cells’ altered shape, which creates the stomatal opening between them.

• Increasing the growth of young, immature plant cells.

Water potential.

The propensity of water to flow from one location to another is known as water potential.

Values are inherently negative.

• Water usually moves downward, or in the direction of the larger negative number.

• Pressure is a unit (kPa)

• Although calculations are not predetermined, this formula might be:

Pressure Potential (p) plus Solute Potential (s) equals Water Potential (W).

The force exerted by the cell wall on its contents is known as pressure potential.

• Since this is zero for animal cells, in animals:

Water Potential (ψ) = Solute Potential (ψs)

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