Question

Как функции плазматической мембраны связаны с ее строением? Как вещества поступают в клетку? Чем отличаются поверхностные структуры мембран растительных и животных клеток?

Answer 1

Основные функции наружной мембраны
ограничительная.  Наружная мембрана обеспечивает целостность клетки, не давая её содержимому (растворимым веществам цитоплазмы) смешаться с окружающей средой или межклеточной жидкостью.
транспортная.  Важное свойство мембраны, связанное с выполнением этой функции - избирательная проницаемость. Некоторые вещества свободно прходят через липидный бислой за счет диффузии, для других он практически непроницаем. За транспорт таких веществ в клетки и из клеток отвечают особые транспортные белки.
рецепторная. Белки-рецепторы, имеющиеся на наружной мембране любой клетки, обеспечивают восприятие сигналов из внешней среды, их передачу в клетку и запуск ответной реакции. Самый распротсраненный вид сигналов - химические вещества, которые связываются с рецепторами.
образование межклеточных контактов.

Answer 2

Functions of the Plasma Membrane and its Relationship with Structure

The plasma membrane is a vital component of all cells, serving several important functions. It acts as a selectively permeable barrier, regulating the movement of substances into and out of the cell. Additionally, it plays a crucial role in cell signaling and communication. The functions of the plasma membrane are closely related to its structure.

The plasma membrane is composed of a phospholipid bilayer with embedded proteins. The phospholipids have a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails. This arrangement forms a barrier that separates the intracellular environment from the extracellular environment. The proteins embedded in the membrane have various functions, including transport, cell adhesion, and signal transduction.

Transport: One of the primary functions of the plasma membrane is to regulate the transport of substances into and out of the cell. The phospholipid bilayer acts as a barrier to most molecules, preventing their free passage. However, the membrane contains specialized transport proteins that facilitate the movement of specific substances across the membrane. These proteins can be categorized into two types: passive transport proteins (such as channels and carriers) and active transport proteins (such as pumps). Passive transport proteins allow the movement of substances down their concentration gradient without the need for energy, while active transport proteins require energy to move substances against their concentration gradient.

Cell Signaling: The plasma membrane also plays a crucial role in cell signaling and communication. It contains receptor proteins that bind to specific signaling molecules, such as hormones or neurotransmitters. When a signaling molecule binds to its receptor, it triggers a series of intracellular events that ultimately lead to a cellular response. This process is essential for coordinating various cellular activities and maintaining homeostasis.

Cell Adhesion: The plasma membrane is involved in cell adhesion, which is the process by which cells stick together. It contains proteins called adhesion molecules that help cells attach to each other and to the extracellular matrix. Cell adhesion is crucial for maintaining tissue integrity, organizing cells into tissues and organs, and facilitating cell migration during development and wound healing.

Cellular Uptake of Substances

Cells require various substances, such as nutrients and signaling molecules, to function properly. These substances enter the cell through different mechanisms:

Passive Diffusion: Small, non-polar molecules, such as oxygen and carbon dioxide, can passively diffuse across the plasma membrane. This process occurs down the concentration gradient, from an area of higher concentration to an area of lower concentration.

Facilitated Diffusion: Larger or charged molecules, such as glucose or ions, require the assistance of transport proteins to cross the plasma membrane. This process is called facilitated diffusion and does not require energy. Transport proteins, such as channels and carriers, facilitate the movement of specific substances across the membrane.

Active Transport: Some substances need to be transported against their concentration gradient, from an area of lower concentration to an area of higher concentration. This process is called active transport and requires the expenditure of energy. Active transport is mediated by specific proteins called pumps, which use energy (usually in the form of ATP) to move substances across the membrane.

Endocytosis: In endocytosis, the plasma membrane invaginates and forms a vesicle that engulfs extracellular material. There are two main types of endocytosis: phagocytosis and pinocytosis. Phagocytosis involves the engulfment of large particles, such as bacteria or cellular debris, while pinocytosis involves the uptake of fluid and small solutes.

Surface Structures of Plant and Animal Cell Membranes

While both plant and animal cells have plasma membranes, there are some differences in their surface structures:

Plant Cell Membrane: In addition to the plasma membrane, plant cells have an additional outer layer called the cell wall. The cell wall is composed of cellulose and provides structural support and protection to the cell. It is permeable to water and small solutes but restricts the movement of larger molecules. The plasma membrane of plant cells is located just inside the cell wall.

Animal Cell Membrane: Animal cells lack a cell wall and have only the plasma membrane surrounding the cell. The plasma membrane of animal cells is in direct contact with the extracellular environment.

In summary, the plasma membrane serves several important functions in cells, including regulating the transport of substances, facilitating cell signaling, and mediating cell adhesion. The structure of the plasma membrane, with its phospholipid bilayer and embedded proteins, is closely related to these functions. Substances enter cells through processes such as passive diffusion, facilitated diffusion, active transport, and endocytosis. Plant and animal cell membranes differ in the presence of a cell wall in plant cells, which provides additional support and protection.