• When a carrier protein transports two or more substances in the same direction, it constitutes a symport e.g. Na- glucose symport.
• When a carrier protein transports two substances in opposite direction, it is known as antiport. 
• Eg. Na+ – H+ antiport in proximal convoluted tubules in the kidney or Na+-K+ ATPase pump.
• Exocytosis is a type of vesicular transport. Vesicles with contents are transported to the cell membrane. 
  • Then they fuse with the cell membrane with the discharge of their contents outside the cell.
• Pinocytosis is basically endocytosis and the ingested material is liquid.
• In endocytosis, the sequence of events proceeds in almost the opposite direction of exocytosis.

Facilitated diffusion is a form of transporter-mediated membrane transport that does not require energy input. Just as in passive diffusion, the transport of ionized and non-ionized compounds across the plasma membrane occurs down their electrochemical potential gradient. Therefore, steady state will be achieved when the electrochemical potentials of the compound on both sides of the membrane become equal.

A i.e. Na⁺

A i.e. Sodium symport

Glucose and Na⁺ share the same Co-transporter (or symport) – the sodium dependent glucose transporter: (SGLT – sodium glucose co- transporter)Q.

Sodium dependent glucose transporter (symport)

– Since the intracellular Na^t concentration is low in intestinal & renal cells, Na^t moves into the cells along its concentration gradient.

– Glucose moves with the Na⁺ & released into the cell.

–  The Na+ is then transported to lateral intercellular spaces & the glucose is transported by GLUT-2 into the interstitium & then into capillaries.

– This is an example of secondary active transport i.e. the energy for glucose transport is provided by the active transport of Na⁺ out of cells.

Also know: Glucose mechanism also transport glactoseQ. Fructose utilizes different mechanism, it is transported by facilitated diffusionQ.

Glucose is incorporated in ORS to facilitate Na* absorptionQ.

C i.e. Na+ with glucose symport

• In this along with Na⁺, another substance is carried by the carrier protein called symport. 
• The energy is obtained from the diffusion process of Na⁺ due to the concentration gradient across the cell membrane.
• Na+ depended on glucose co-transporters (SGLT, members of a large family transporter. Gene name SLC5A).
• Facilitative Na+ independent sugar transporter (GLUT family, gene name SLC2A).
• The transport of most hexoses is dependent on Na+ in the intestinal lumen.
• The glucose and galactose both uses the same transport SGLT-1. 
• SGLT-1 transports either glucose or galactose along with Na⁺ from intestinal lumen to cell (Secondary active transport).
• Fructose is absorbed by facilitated diffusion from the intestinal lumen into the enterocytes by GLUT-5. 
• This process is Na⁺ independent.

C i.e. Carrier protein

• To move substances against a concentration or electrochemical gradient, the cell must utilize energy in the form of ATP during active transport.
• Primary active transport, which is directly dependent on ATP, moves ions across a membrane and creates a difference in charge across that membrane.
• Secondary active transport, created by primary active transport, is the transport of a solute in the direction of its electrochemical gradient and does not directly require ATP.
• Carrier proteins such as uniporters, symporters, and antiporters perform primary active transport and facilitate the movement of solutes across the cell’s membrane.

Ans.B i.e. It requires a carrier protein

• Facilitated diffusion is a form of passive (no energy) transport that occurs along an electrochemical gradient and requires no energy. The rate of transport is not always proportionate to the concentration gradient because it is mediated through carrier proteins.
• At lower concentrations before the carrier protein becomes saturated (or before V_max is reached), the rate of facilitated diffusion is proportionate to concentration gradient whereas, at higher concentrations when carrier protein becomes saturated (V_max has reached) the rate of facilitated diffusion becomes constant.