Cell Biology

Nucleus

The nucleus is often referred to as the control centre of the cell, playing a crucial role in regulating cellular activities and storing genetic information. Enclosed by a double membrane called the nuclear envelope, the nucleus contains the cell’s DNA, which is organised into chromosomes. The nuclear envelope features pores that allow the exchange of materials between the nucleus and the cytoplasm, enabling communication and the transport of molecules such as RNA and proteins. Within the nucleus, the nucleolus is responsible for producing ribosomes, the structures that synthesise proteins.

Mitochondria

Mitochondria are responsible for producing the energy required for various cellular activities. These organelles generate energy through a process called cellular respiration, where they convert glucose and oxygen into adenosine triphosphate (ATP), the cell’s primary energy currency. Mitochondria have a unique structure, consisting of two membranes: an outer membrane that encloses the organelle and a highly folded inner membrane, known as cristae, where the enzymes involved in ATP production are located. This folding increases the surface area, allowing for more efficient energy production. Mitochondria have their own DNA, separate from the cell’s nuclear DNA, and can replicate independently within the cell. This feature supports the endosymbiotic theory, which suggests that mitochondria originated from free-living prokaryotes that were engulfed by early eukaryotic cells.

Rough Endoplasmic Reticulum

The rough endoplasmic reticulum (rough ER) is an organelle involved in the synthesis and processing of proteins. It is called “rough” because its surface is studded with ribosomes, which are responsible for protein production. These ribosomes translate mRNA into polypeptide chains, and the rough ER ensures proteins are correctly folded and functional before they are transported to other parts of the cell or secreted outside the cell. After synthesis, proteins are packaged into vesicles and sent to the Golgi apparatus for further processing and sorting.

Ribosomes

Ribosomes are tiny, spherical organelles can be found either floating freely in the cytoplasm or attached to the surface of the rough endoplasmic reticulum, giving it its “rough” appearance. Ribosomes consist of two subunits, one large and one small, both made up of ribosomal RNA (rRNA) and proteins. During protein synthesis, ribosomes read the messenger RNA (mRNA) sequences, translating the genetic code into polypeptide chains by linking together the appropriate amino acids.

Smooth Endoplasmic Reticulum

The smooth endoplasmic reticulum (smooth ER) is involved in lipid and steroid hormone synthesis, carbohydrate metabolism, and drug detoxification. Unlike the rough ER, it lacks ribosomes, giving it a smooth appearance. It is active in cells that produce steroid hormones, such as those in the adrenal glands, and in liver cells, where it detoxifies harmful substances. The smooth ER also synthesises phospholipids, vital for cell membranes, and assists in carbohydrate metabolism.

Golgi Apparatus

The Golgi apparatus, or Golgi body, modifies, sorts, and packages proteins and lipids. Composed of flattened, membrane-bound sacs called cisternae, it receives molecules from the endoplasmic reticulum in vesicles. Within the Golgi, proteins and lipids are modified, such as adding carbohydrate groups to form glycoproteins. The Golgi then sorts and packages these molecules into vesicles for delivery to their final destinations, including secretion outside the cell or integration into the cell membrane. It plays a key role in processing and dispatching cellular products, ensuring efficient cell function.

Lysosomes

Lysosomes are membrane-bound organelles that act as the cell’s digestive system. They contain a variety of powerful enzymes capable of breaking down macromolecules, such as proteins, lipids, and carbohydrates. These enzymes help degrade cellular waste, damaged organelles, and foreign substances that the cell has engulfed through processes like endocytosis. By digesting these materials, lysosomes recycle essential components back into the cell, maintaining its health and function. Lysosomes also play a role in programmed cell death, or apoptosis, by releasing their enzymes to help dismantle the cell when it is no longer needed.

Vacuoles

Vacuoles are membrane-bound sacs found in cells that serve various functions depending on the type of cell. In plant cells, a large central vacuole stores water, nutrients and waste products, contributing to turgor pressure, which helps maintain cell structure and rigidity. It can also contain pigments and defensive chemicals. In animal cells, vacuoles are smaller and typically involved in processes like nutrient storage, waste removal and maintaining cellular pH.

Chloroplasts

Chloroplasts are specialised organelles found in plant cells and certain algae that undertake photosynthesis, the process where light energy is converted into chemical energy. They contain the green pigment chlorophyll, which captures light energy and uses it to transform carbon dioxide and water into glucose and oxygen. Chloroplasts have a double membrane structure, with the inner membrane forming numerous thylakoid sacs stacked into structures called grana. These thylakoids are where the light-dependent reactions of photosynthesis occur. The space surrounding the thylakoids is filled with a fluid called stroma, where the light-independent reactions, or Calvin cycle, take place. Chloroplasts also have their own DNA, which supports the endosymbiotic theory suggesting they originated from free-living prokaryotes that were engulfed by early eukaryotic cells.

Animal and plant cells differ in several key ways. Plant cells have a rigid cell wall made of cellulose, providing structural support and a fixed shape, whereas animal cells only have a flexible plasma membrane and often exhibit a more varied shape. Plant cells contain chloroplasts for photosynthesis, enabling them to produce their own food from sunlight, while animal cells do not have chloroplasts and rely on external sources of nutrition. Additionally, plant cells usually feature a large central vacuole for storing water, nutrients and waste, which helps maintain cell rigidity, while animal cells have smaller vacuoles used mainly for storage and digestion. Animal cells also commonly contain lysosomes for breaking down waste and cellular debris, a feature less prominent in plant cells.