Active transport occurs when cells move molecules across the plasma membrane against their concentration gradients, effectively moving them "uphill" from an area of lower concentration to an area of higher concentration. This process requires an input of energy, typically adenosine triphosphate (ATP) or other forms of energy. Due to this significant energy requirement, the membrane structures that facilitate active transport are often referred to as "pumps."

Active Transport

Passive transport is a mode of membrane transport where molecules move along their concentration gradient, from an area of higher concentration to an area of lower concentration. Unlike active transport, this process does not require the cell to expend energy.

Passive Transport

Group translocation is a mechanism used to transport substances into bacterial cells. As a molecule is moved against its concentration gradient into the cell, it is chemically modified, which prevents it from needing to be transported against an unfavorable gradient once inside. A common example is the bacterial phosphotransferase system, a series of carriers that phosphorylates sugars like glucose as they enter. Because this phosphorylation is already required during the early stages of sugar metabolism, the system is considered energy neutral.

Group Translocation

A primary function of the plasma membrane is controlling the transport of molecules into and out of the cell. This continuous regulation allows the cell to maintain internal conditions within a specific range, regardless of changes in the external environment. Cells utilize various modes of transport, broadly categorized into passive and active mechanisms, to move substances across the plasma membrane.

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The plasma membrane, also referred to as the cell membrane or cytoplasmic membrane, is a fundamental structure found in all prokaryotic and eukaryotic cells. It encloses the cytoplasm and acts as a semipermeable barrier, exhibiting selective permeability to control the flow of metabolites by allowing specific molecules to enter or leave the cell while restricting the passage of others.

Plasma Membrane

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Microbiology covers the scope and sequence requirements for a single-semester microbiology course for non-majors. The book presents the core concepts of microbiology with a focus on applications for careers in allied health. The pedagogical features of the text make the material interesting and accessible while maintaining the career-application focus and scientific rigor inherent in the subject matter. Microbiology’s art program enhances students’ understanding of concepts through clear and effective illustrations, diagrams, and photographs.

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The structure of the plasma membrane is frequently described using the fluid mosaic model. This model illustrates the membrane as having a mosaic-like composition made up of a diverse array of lipid and protein components. It also emphasizes the fluid nature of the membrane, as these components possess the ability to move dynamically within the plane of the membrane.

Fluid Mosaic Model

Membrane Transport

Certain prokaryotic cells, specifically cyanobacteria and photosynthetic bacteria, possess specialized membrane structures that allow them to perform photosynthesis. These structures are formed by infoldings of the plasma membrane that enclose photosynthetic pigments like green chlorophylls and bacteriochlorophylls. The specific names for these structures vary by organism: they are called thylakoids in cyanobacteria, and chromatophores, lamellae, or chlorosomes in photosynthetic bacteria.

Prokaryotic Photosynthetic Membranes

The plasma membrane of eukaryotic cells is a phospholipid bilayer with embedded peripheral and integral proteins. Similar to prokaryotic plasma membranes, its components move within the plane of the membrane according to the fluid mosaic model, but unlike prokaryotic membranes, it contains sterols and specialized lipids.

Eukaryotic Plasma Membrane

The plasma membrane is composed primarily of a phospholipid bilayer, various embedded or associated proteins, and carbohydrate chains attached to lipids or proteins, forming glycolipids and glycoproteins, respectively.

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