Cell Membrane, Cytoplasm, and Cell Wall: Structure and Function

Cell Membrane

1. Membrane: The cell membrane is a thin lamina enveloping the whole cell. It separates the internal content of the cell from the external environment and controls the passage of substances. It also surrounds organelles such as chloroplasts and vacuoles.

Composition and Structure

• Lipids: 50%
• Proteins: 40%
• Carbohydrates: 10%

The structure is modeled as a "fluid mosaic model".

Lipids

• Phospholipids: form a lipid bilayer. They have mobility. The head is polar (hydrophilic) and the two tails are non-polar (hydrophobic) fatty acid chains.
• Cholesterol: a steroid found in the membrane. It is important in the hydrophobic area. It increases fluidity, resistance and stability.

Only in animal cells.

Proteins

• Transmembrane proteins: cross the membrane from one side to the other. They are proteins with a carbohydrate part on the exterior (glycoproteins).
• Peripheral proteins: are proteins bound to lipids (lipoproteins). They are found on the surface of the membrane, where they can be found in the hydrophilic parts.

Glycolipids

They are found in the membrane and act on the exterior, with functions such as: recognition of substances, receptors, immunity and cell-to-cell communication. They give the membrane an "asymmetric" structure on each side.

Functions

1. Receiving signals from the exterior: chemical molecules arrive and trigger a response.
2. Cell identity: for example, blood groups.
3. Transport of molecules between the interior and the exterior:

Small molecules:

• Passive transport (down a gradient, in favor, with no energy expense): simple diffusion, where substances pass directly through the phospholipid bilayer or through pores formed by proteins; and facilitated diffusion, which uses transport proteins.
• Active transport (against a gradient, from low to high concentration, with energy expense in the form of ATP): sodium-potassium pump, which moves 3 Na⁺ out of the cell and brings 2 K⁺ into the cell against their gradient.

Large molecules:

• Endocytosis (entry via vacuoles): phagocytosis and pinocytosis.
• Exocytosis (exit via vacuoles).

Cell Wall

The cell wall is a thick and rigid layer surrounding the plant cells. Its main component is what gives the plant protection. Composition: mainly cellulose. Its skeleton endures even after the cell dies.

Structure: a network of cellulose fibers embedded in water, mineral salts and hemicellulose, which hardens and thickens the wall.

Functions:

1. Transport and exchange of molecules.
2. Gives the cell rigidity and prevents its rupture (when turgor pressure builds up, without it the cell would burst).

Cytoplasm

2. Cytoplasm: the space between the plasma membrane and the nuclear envelope.

Cytosol

The internal medium of the cell is the cytosol. In it are the cytoskeleton and the ribosomes. It is formed by 85% water, with lipids, carbohydrates and salts.

Functions: carries out metabolic reactions such as glycolysis.

Organelles without membrane

Cytoskeleton

Found in eukaryotic cells. It is a set of fibrous protein structures found in the cytoplasm (microfilaments, intermediate filaments and microtubules).

Functions:

1. Gives shape and consistency to the cell.
2. Enables internal movement and controls the cell.
3. Serves as a means of communication between the different parts of the cell.

Centrioles

A hollow, cylindrical organelle formed by microtubules associated in groups of 3, forming 9 groups. Found in animal and plant cells. It has a basal body structure (9 triplets) placed perpendicularly.

Functions:

1. Plays an important role in cell division.
2. Forms cilia and flagella.

Cilia and Flagella

Cilia and flagella are fine hair-like structures that protrude from the surface of eukaryotic cells. They are numerous. Cilia and flagella are typical of protists (protozoa and unicellular algae) and of some animal cells. Cilia are short and numerous; flagella are long.

Composition and Structure: their structure is very similar to that of the basal bodies: a group of 9 doublets of microtubules. They move by sliding the microtubules past each other through the action of dynein (a protein bridge that joins the doublets).

Functions:

1. Propulsion: in algae and protozoa, for swimming.
2. Capture of food: in protozoa.
3. Displacement over the cell surface of fluids.