Structure of eukaryotic cells (3.2.1.1)
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Animals, fungi and plants are all eukaryotic organisms. Eukaryotic cells are complex structures characterised by a defined nucleus and membrane–bound organelles, each with specific functions that contribute to the overall functioning of the cell.
The nucleus is a large organelle that houses chromosomes which are linear DNA molecules bound to proteins. Within it, dense regions called nucleoli serve as the sites of ribosome production, while the remaining space is filled with nucleoplasm.
Enclosing the nucleus is a double membrane, continuous with the endoplasmic reticulum, that regulates the movement of substances into and out of the organelle.
The rough endoplasmic reticulum (RER) is a network of interconnected, flattened membrane sacs called cisternae, which are continuous with the nuclear envelope. Ribosomes attached to its surface give it a rough appearance.
The RER is the site of synthesis and transport of proteins destined for secretion or insertion into cell membranes. Proteins enter the cisternae, where they are folded and modified before being transported to the Golgi apparatus for further processing.
The smooth endoplasmic reticulum (SER) is a network of membranous tubules and sacs which extends throughout the cytoplasm. It is similar in structure to the rough endoplasmic reticulum, but lacks attached ribosomes and therefore appears smooth.
The SER is involved in the synthesis and transport of lipids, including phospholipids, cholesterol and steroid hormones.
The Golgi apparatus is a stack of flattened, single membrane–bound sacs called cisternae. It modifies proteins and lipids received from the endoplasmic reticulum, for example, by adding carbohydrate chains.
These molecules are then sorted and packaged into vesicles for transport to the cell surface for exocytosis or to other organelles, such as lysosomes.
Ribosomes are small, non–membrane–bound organelles made of ribosomal RNA (rRNA) and protein. They are found free in the cytoplasm or attached to the rough endoplasmic reticulum and are the site of protein synthesis.
Eukaryotic cells contain larger 80S ribosomes, whereas prokaryotes, mitochondria and chloroplasts contain smaller 70S ribosomes.
Mitochondria are double membrane–bound organelles that are the site of the later stages of aerobic respiration and ATP production. Cells with high metabolic activity, such as muscles and neurones, contain many mitochondria.
The inner membrane is folded into cristae, increasing the surface area for respiratory enzymes and electron carriers. Mitochondria with more densely folded cristae have a greater capacity for aerobic respiration and ATP production.
The fluid-filled interior is called the matrix and contains enzymes, mitochondrial DNA and ribosomes.
Lysosomes are single membrane–bound vesicles formed from the Golgi apparatus that contain hydrolytic (digestive) enzymes, including lysozymes. They digest cellular debris, worn-out organelles and pathogens such as bacteria, and also play a role in apoptosis (programmed cell death).
Lysosomes are particularly abundant in phagocytic cells such as macrophages and neutrophils.
The cell–surface membrane, also called the plasma membrane, separates the cell’s contents from the environment surrounding the cell. The partially or selectively permeable membrane controls the movement of substances and is involved in cell signalling and recognition.
The membrane consists of a phospholipid bilayer containing proteins, cholesterol and carbohydrate-containing molecules.
Chloroplasts are double membrane–bound organelles found in plant and algal cells that are the site of photosynthesis.
Chloroplasts contain flattened membrane sacs called thylakoids, which contain chlorophyll and are stacked into grana connected by lamellae. These membranes are the site of the light-dependent reactions. The fluid-filled stroma contains enzymes involved in the light-independent reactions.
Chloroplasts also contain circular DNA, ribosomes and starch grains for temporary carbohydrate storage.
A cell wall is a freely permeable, rigid outer layer surrounding the cell–surface membrane in plants, algae and fungi. Plant and algal cell walls are mainly made of cellulose, whereas fungal cell walls are made of chitin.
The cell wall provides mechanical strength and support, helping maintain cell shape and preventing the cell from bursting when water enters by osmosis. The resistance of the cell wall to expansion allows turgor pressure to develop, making plant cells rigid.
In plant cells, the vacuole is a large, membrane-bound sac containing cell sap, a solution of water, sugars, amino acids, ions, pigments, and wastes.
The vacuole is surrounded by a selectively permeable membrane called the tonoplast, which regulates the movement of substances into and out of the vacuole. This helps maintain osmotic balance and ) within the cytoplasm and can isolate harmful substances.
The uptake of water into the vacuole maintains turgor pressure, keeping plant cells rigid and helping support the plant.
In eukaryotic cells, protein production and secretion require the coordinated action of several organelles:
- mRNA is transcribed from DNA in the nucleus.
- It travels to ribosomes on the rough ER, where the protein is synthesised, powered by ATP generated in the mitochondria.
- The protein moves through the rough ER, folding into its 3D shape.
- They are packaged into transport vesicles that bud off and are carried to the Golgi apparatus via the cytoskeleton.
- Within the Golgi, proteins are modified (e.g., the addition of carbohydrate to form glycoproteins), then pinched off in secretory vesicles.
- These vesicles fuse with the cell-surface membrane, releasing the protein (such as extracellular enzymes) by exocytosis.
In complex, multicellular organisms, cells differentiate to become specialised for particular functions (e.g., red blood cells, sperm cells, epithelial cells).
Groups of specialised cells form tissues, different tissues combine to form organs, and organs coordinate within organ systems.
Tissues are groups of similar cells working together to perform a specific function (e.g., muscle tissue for contraction).
Organs are collections of different tissues working together (e.g., stomach contains muscle, glandular, and epithelial tissue).
Organ systems are groups of organs that work together (e.g., those involved in the digestive system).