Question

Which statements are consistent with the fluid-mosaic model of membranes? (a) All membrane proteins are bound to the interior of the membrane. (b) Both proteins and lipids undergo transverse (flip-flop) diffusion from the inside to the outside of the membrane. (c) Some proteins and lipids undergo lateral diffusion along the inner or outer surface of the membrane. (d) Carbohydrates are covalently bonded to the outside of the membrane. (e) The term mosaic refers to the arrangement of the lipids alone.

Short answer

Statements (c) and (d) are consistent with the fluid-mosaic model of membranes.

Step-by-step solution

Understanding the Fluid-Mosaic Model

The fluid-mosaic model describes the structure of cell membranes. It highlights that membranes are flexible (fluid) and consist of various molecules such as lipids, proteins, and carbohydrates (mosaic). Examine each statement with this in mind.

Analyzing Statement (a)

Statement (a) states that all membrane proteins are bound to the interior of the membrane. This is incorrect as membrane proteins can be peripheral or integral, and they can be found both inside and outside the membrane.

Analyzing Statement (b)

Statement (b) mentions that both proteins and lipids undergo transverse (flip-flop) diffusion. This is mostly true for lipids but not commonly for proteins because flip-flop of proteins is rare due to the energy barrier. Hence, this statement is not fully correct.

Analyzing Statement (c)

Statement (c) mentions lateral diffusion of proteins and lipids along the surface of the membrane. This is correct as both proteins and lipids move laterally within the layer, which is a fundamental aspect of the fluid-mosaic model.

Analyzing Statement (d)

Statement (d) states that carbohydrates are covalently bonded to the outside of the membrane. This is correct. Carbohydrates are often found on the extracellular surface of the membrane attached to proteins or lipids (forming glycoproteins and glycolipids).

Analyzing Statement (e)

Statement (e) claims that the term mosaic refers to the arrangement of the lipids alone. This is incorrect. The term mosaic describes the diverse assortment of proteins, lipids, and carbohydrates within the membrane.

Key concepts

cell membrane structure

The cell membrane is a vital component of all living cells. It defines the boundary of the cell, providing structural support and regulating the movement of substances in and out. The fluid-mosaic model is used to describe the structure of the cell membrane.

The 'fluid' aspect signifies that the membrane is not rigid. The lipid molecules in the membrane are in constant motion, allowing for flexibility and fluidity. The 'mosaic' aspect describes the mixed composition of the membrane.

• The membrane is primarily composed of phospholipids, proteins, and carbohydrates.
• Phospholipids form a bilayer, providing a semi-permeable barrier.
• Proteins can be found floating in or on the lipid bilayer.
• Carbohydrates are often attached to proteins and lipids on the extracellular surface.

lateral diffusion

Lateral diffusion is a key characteristic of the fluid-mosaic model. It refers to the movement of lipids and proteins within the plane of the membrane.

Unlike transverse diffusion ('flip-flop'), which is rare for proteins, lateral diffusion is common and essential for membrane function.

• Lipids can move within the same layer of the bilayer.
• This lateral movement allows the membrane to be flexible and self-healing.
• Proteins also move laterally, facilitating various functions, such as cell signaling and substance transport.

membrane proteins

Membrane proteins play a critical role in the cell membrane's functions. They can be categorized into two main types: integral and peripheral.

Integral proteins are embedded within the lipid bilayer and can span across it.

• These proteins are involved in transporting molecules and ions.
• They often form channels or pores through which substances can pass.

Peripheral proteins are found on the surface of the membrane and are not embedded within the lipid bilayer.

• These proteins typically interact with integral proteins or with the polar heads of the phospholipids.
• They are involved in signaling and structural support.

membrane lipids

Membrane lipids form the foundational structure of the cell membrane. The most common lipids in the membrane are phospholipids, cholesterol, and glycolipids.

Phospholipids form a bilayer with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• This arrangement creates a semi-permeable membrane.

Cholesterol is interspersed within the phospholipid bilayer.

• It helps to maintain membrane integrity and fluidity.

Glycolipids are lipids with carbohydrate groups attached.

• These are involved in cell recognition and interaction.

membrane carbohydrates

Membrane carbohydrates are crucial for cell-cell recognition and communication. They are typically found on the extracellular surface of the cell membrane.

Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids).

• Glycoproteins play a key role in immune responses and interactions with other cells.
• Glycolipids contribute to cell recognition and signal transduction.

The carbohydrate portion of these molecules extends from the membrane's surface,

• forming a 'glycocalyx' that can protect the cell and participate in cellular interactions.