TLDR;
This lecture explains the mechanisms of passive and active transport across the cell membrane. It details the structure and function of the cell membrane, differentiating between simple and facilitated diffusion in passive transport. The lecture further explains active transport, including co-transport mechanisms like symport and antiport, and vesicular transport processes such as endocytosis (pinocytosis, receptor-mediated endocytosis, and phagocytosis) and exocytosis.
- Passive transport doesn't require energy, moving substances from high to low concentration areas.
- Active transport requires energy to move substances against their concentration gradient.
- The cell membrane acts as a semi-permeable barrier, regulating the passage of substances.
Introduction to Cell Membrane and Transport [0:06]
The lecture begins by introducing the concept of transportation across the cell membrane, which is the outer covering of the cell, providing protection and serving as a channel for transporting materials. The cell membrane functions as a semi-permeable membrane, allowing only certain substances to pass through while acting as a barrier to others. Small hydrophobic molecules like carbon dioxide, oxygen, and lipids can easily pass through the membrane. Larger molecules and ions require transporter proteins to facilitate their movement, either with or without energy.
Passive vs. Active Transport [1:57]
The discussion moves to the two main types of transport: passive and active. Passive transport occurs without energy, while active transport requires energy. Gradients are important for moving materials through the membrane, either passively or actively. Passive transport is further divided into simple diffusion and facilitated diffusion. Simple diffusion allows molecules to pass through the membrane without assistance, while facilitated diffusion requires transporter molecules (proteins) to help substances cross the membrane.
Facilitated Diffusion and Aquaporins [4:23]
Facilitated diffusion involves transport proteins that temporarily bind substances and move them across the membrane. This process doesn't require energy and involves both carrier and channel proteins. Examples of substances transported via facilitated diffusion include glucose, amino acids, and small ions. The movement of water through the membrane also involves facilitated diffusion, utilising specific proteins called aquaporins. Aquaporins are membrane proteins that facilitate water transport.
Co-transport: Symport and Antiport [6:06]
The lecture introduces co-transport, where the transport of one molecule depends on the transfer of another. Co-transport can be either symport or antiport. A symport system moves two molecules in the same direction, while an antiport system moves two molecules in opposite directions, also known as counter-transport. Examples include the sodium-glucose transporter (symport) and the sodium-potassium transporter (antiport). A uniport system transports one molecule in one direction.
Endocytosis and Exocytosis: Vesicular Transport [9:56]
The discussion shifts to vesicle-mediated transport, including endocytosis and exocytosis, which involve the transport of large substances and changes in the shape of the membrane. Exocytosis involves the movement of materials out of the cell by fusing vesicles with the plasma membrane, while endocytosis involves the movement of materials into the cell. An example of exocytosis is the release of insulin from pancreatic cells into the bloodstream.
Types of Endocytosis: Pinocytosis, Receptor-Mediated, and Phagocytosis [12:06]
Endocytosis is further divided into pinocytosis, receptor-mediated endocytosis, and phagocytosis. Pinocytosis involves the internalisation of solutes by the cell membrane forming inward and creating a pinosome. Receptor-mediated endocytosis involves receptor molecules on the cell surface attaching to molecules for internalisation, followed by the plasma membrane folding inward to form a coated vesicle. Phagocytosis involves the engulfment of large particles, such as food particles, forming a food vacuole that fuses with lysosomes for digestion.
Summary of Active and Passive Transport [17:38]
In summary, active transport proceeds against the concentration gradient and requires energy input, while passive transport proceeds as diffusion from high to low concentration areas without requiring energy. Active transport involves transporter proteins that bind molecules and require ATP hydrolysis to facilitate the process.