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The cell membrane
The cell membrane
The cell membrane is a very thin (7 - 10 nm thick) physical barrier between the interior (cytoplasm) and the exterior environment. It protects the cell organelles and other parts of the cell from the surrounding. It also controls the movement of substance across the cell membrane, which will be discussed in the next section. Besides, receptor proteins on the membrane are involved in cell signalling.
Structure and components of the cell membrane
Structure and components of the cell membrane
Fluid mosaic model
Fluid mosaic model
The cell membrane was first observed in the 17th century. Since then, the cell membrane theory developed from “plasma membrane” to “lipid membrane”. Today, the most widespread theory of the structure of the cell membrane is the “Fluid mosaic model” which was proposed by Singer and Nicolson in 1972.
The fluid mosaic model proposes that the cell membrane is consisted of two layers (a bilayer) of phospholipid with protein molecules embedded in it. The cell membrane is described as “fluid” because some proteins may move freely in the cell membrane, like a fluid.
(Fluid mosaic model, OpenStax / Wikimedia Commons / CC-BY-SA-3.0)
Phospholipid molecule
Phospholipid molecule
The fluid mosaic model proposes that phospholipid molecules of the two layers face in opposite directions, in a tail-to-tail manner. Each phospholipid molecule consists of a head, which is hydrophilic (water-liking), and a tail, which is hydrophobic (water-hating). While the cell membrane forms, the phospholipid molecules will form two layers with the hydrophilic heads passively pointing outwards due to the aqueous cytoplasm and extracellular environment.
(Phospholipid molecule, OpenStax / Wikimedia Commons / CC-BY-SA-4.0)
Transport proteins
Transport proteins
The two most basic transport proteins that can be found in the cell membrane is channel and carrier protein. They allow the movement of some substances across the cell membrane.
Cell adhesion molecules
Cell adhesion molecules
Cell adhesion molecules (CAM) are proteins on the surface of the cell membrane responsible for the cell binding with other cells or with extracellular matrix, which is molecules out of the cell that supports the cell.
Receptor
Receptor
Cell surface receptors are proteins involved in the cell-signalling process by receiving extracellular molecules and triggering changes within the cell. Receptors may bind to hormones, neurotransmitters, cytokines and growth factors etc.
Transport across the cell membrane
Transport across the cell membrane
Diffusion
Diffusion
Diffusion is the process of molecule movement along a concentration gradient, where particles move from a higher concentration region to a lower concentration region. There is no net movement of across the cell membrane once the molecules is evenly distributed. Since is a passive process, energy is not required. Diffusion occurs in gases and liquids when a concentration gradient exists.
For example, oxygen diffuses from the air sacs into the bloodstream by diffusion because the concentration of oxygen in the air sac is higher than that in the bloodstream
Osmosis
Osmosis
Osmosis is the process of water molecules moving across a differentially permeable membrane from high water potential region to low water potential region. Water potential is high when the concentration of solute is low. Thus, pure water has the highest water potential, which is defined as 0.
Active transport
Active transport
Diffusion and osmosis tends to allow substance or water molecules to move around to reach a state of equilibrium, making the concentration gradient less steep. However, sometimes some substances have to be transported against the concentration gradient, to obtain nutrient from the soil for example.
Active transport relies on carrier proteins. As mentioned before, carrier proteins require energy to work, so do active transport. Besides, because carrier proteins are specific, only substances that fit the shape of the carrier proteins can be transported by active transport.
Phagocytosis
Phagocytosis
Phagocytosis is an active process which engulfs particles to uptake nutrition or to toxic particles, defending our body against diseases etc. Phagocytosis involves a series of steps:
- A pseudopodium forms at the cell membrane to wrap around the target particle.
- The opening of the pseudopodium shuts to enclose the particle
- The cell releases enzymes into the vacuole to digest the particle
- The digested products diffuse to the cytoplasm
- Finally, the debris (leftover) will be release out of the cell by exocytosis (will be discussed later)
(process of phagocytosis)
Exocytosis
Exocytosis
Exocytosis is like the reverse of phagocytosis. It releases substances out of the cell through the following steps:
- A membrane is formed inside the cell, wrapping around the substance to be released
- The vesicle moves towards the cell membrane and fuses with the cell membrane
- The vesicle opens up and releases the substance being carried
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