Structure of eukaryotic cells

Cell-Surface Membrane (Plasma Membrane):

• Structure: A phospholipid bilayer with embedded proteins, cholesterol, and glycoproteins.

• Function: Controls the movement of substances in and out of the cell, cell signaling, and cell recognition.

Nucleus:

• Structure: Contains the cell's genetic material (DNA) organised into chromosomes. The DNA is bound to proteins (histones). Also contains one or more nucleoli.

• Function: Controls cell activities through gene expression, DNA replication, and RNA synthesis. The nucleolus is the site of ribosome assembly.

Mitochondria:

• Structure: Double-membraned organelle with a highly folded inner membrane (cristae) and a fluid-filled matrix.

• Function: The site of aerobic respiration, generating ATP (the cell's energy currency) through oxidative phosphorylation.

The structure of a mitochondrion.

Chloroplasts (Plants and Algae):

• Structure: Double-membraned organelle with stacks of thylakoids (grana) containing chlorophyll, and a fluid-filled stroma.

• Function: The site of photosynthesis, converting light energy into chemical energy (glucose).

The structure of a chloroplast.

Golgi Apparatus and Golgi Vesicles:

• Structure: A stack of flattened membrane-bound sacs (cisternae).

• Function: Modifies, sorts, and packages proteins and lipids into vesicles for transport to their final destinations.

Lysosomes:

• Structure: Membrane-bound organelles containing hydrolytic enzymes.

• Function: Break down worn-out organelles, cellular debris, and ingested material.

Ribosomes:

• Structure: Composed of rRNA and proteins, found free in the cytoplasm or attached to the rough endoplasmic reticulum (RER).

• Function: The site of protein synthesis.

Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER):

• Structure: A network of interconnected membrane-bound sacs and tubules. RER is studded with ribosomes, while SER lacks ribosomes.

• Function: RER synthesises and modifies proteins. SER synthesises lipids, detoxifies drugs and poisons, and stores calcium ions.

Cell Wall (Plants, Algae, and Fungi):

• Structure: A rigid layer outside the cell membrane, composed of cellulose (plants), chitin (fungi), or other polysaccharides.

• Function: Provides structural support, protection, and helps maintain cell shape.

Cell Vacuole (Plants):

• Structure: A large, membrane-bound sac filled with cell sap.

• Function: Maintains turgor pressure, stores water, ions, pigments, and other substances.

The structure of a eukaryotic animal cell.

The structure of a eukaryotic plant cell.

Cell specialisation

Understanding the structure and function of cellular components allows us to explain how cells are adapted to their specific roles. Cells become specialised as a result of different genes being expressed. This results in different proteins being produced and so different processes taking place within the cell. It is often possible to determine the function of a cell by studying what organelles are present and in what proportions. For example, cells that have many mitochondria often require a lot of energy produced by aerobic respiration, while a cell that produces and secretes many proteins would contain a lot of rough endoplasmic reticulum, Golgi apparatus, and vesicles. The shape of the cell is also often indicative of the function. 

Specialisation results in division of labour, allowing for greater efficiency and complexity.

Organisation

Similar cells are often aggregated (grouped/attached) together into tissues since they all perform the same function. Different tissues also often group together into organs to coordinate their functions. Organs group together into organ systems to perform one overall function. 

Definitions