Membrana Plasmática - Funções e Estrutura | Biologia com Samuel Cunha

Membrane Structure and Function

Overview of the Plasma Membrane

• The plasma membrane is a cellular envelope present in all cells, including both eukaryotic and prokaryotic cells.

• It serves to protect the cell by selectively allowing substances to enter or exit, a process known as selective permeability.

Composition of the Plasma Membrane

• The membrane primarily consists of proteins and lipids, with phospholipids being a major component. These have a polar head (hydrophilic) and non-polar tail (hydrophobic).

• The arrangement of phospholipids forms a bilayer where hydrophilic heads face outward towards water, while hydrophobic tails are shielded from it.

Fluid Mosaic Model

• Proposed by Singer and Nicolson in 1972, the fluid mosaic model describes the membrane's structure as dynamic, with molecules able to move laterally within the layer.

• This model illustrates that proteins can shift positions freely, contributing to membrane flexibility.

Molecular Movement Across the Membrane

• Molecules like CO2 can pass through the lipid bilayer easily due to their size and solubility.

• Oxygen (O2) also diffuses across; however, water requires specific channels called aquaporins for efficient transport.

Protein Functions in Membranes

• Proteins embedded in membranes can be integral (spanning across the membrane) or peripheral (attached to one side), facilitating various functions including transport.

• Integral proteins allow larger molecules and ions to cross while peripheral proteins often serve enzymatic roles.

Role of Cholesterol in Membranes

• Cholesterol contributes to membrane stability by preventing excessive fluidity; it is essential for maintaining structural integrity under varying temperatures.

Glycoproteins and Their Importance

Cell Membrane Structures and Functions

Glycocalyx: Structure and Function

• The glycocalyx is a structure formed by phospholipids that acts as an additional cellular envelope, present in some cells but not all.

• Its primary functions include protecting the cell from mechanical damage and retaining essential nutrients, such as glucose, which can be trapped within its structure for cellular use.

• The glycocalyx also plays a crucial role in cellular recognition; it helps the body identify transplanted organs to prevent rejection by recognizing them through these structures.

Cytoskeleton: Support and Shape

• The cytoskeleton consists of filamentous structures that provide shape and support to the cell, functioning similarly to a skeleton.

• A detailed examination of the membrane reveals its composition, including proteins embedded in a lipid bilayer with hydrophobic regions.

Cell Wall vs. Plasma Membrane

• The cell wall is an external layer providing rigidity to certain cells (e.g., plant cells), while animal cells lack this feature.