Cell Types & Organelles

Cells are classified into two major types: prokaryotic and eukaryotic. The features the two types share are evidence of a common evolutionary past — eukaryotes are believed to have evolved from ancestral prokaryotes. Every cell, of either type, contains the following components:

1. A cell membrane - defines the boundary of the cell and controls entry and exit.
2. Nucleic acids (DNA and RNA) - polymers of nucleotides that carry genetic information.
3. Proteins - polymers of amino acids that carry out cellular functions.
4. Ribosomes - small round structures composed of ribosomal RNA (rRNA); the site of protein synthesis. The ribosome is a specialised organelle with no outer membrane.
5. Cytoplasm - everything inside the membrane except the nucleus; the fluid that suspends the contents is the cytosol.

A prokaryotic cell carries a single circular chromosome free in the cytosol; a eukaryotic cell encloses multiple linear chromosomes in a nucleus, surrounded by membrane-bound organelles.

ENDOSYMBIOSIS

Scientific evidence suggests that prokaryotic cells existed first — the oldest fossils on Earth belong to primitive prokaryotic organisms — and that eukaryotic cells arose later through endosymbiosis: the incorporation of one organism into another for the benefit of both. Small prokaryotic cells, the ancestors of mitochondria and chloroplasts, were taken into larger cells. The host cell provided a favourable environment; the smaller residents supplied useful products such as ATP. Several lines of evidence support this theory:

1. Mitochondria and chloroplasts have their own circular DNA, like prokaryotes, suggesting they can replicate independently of the cell.
2. They have their own bacteria-like ribosomes, suggesting they can synthesise their own proteins.
3. They are enclosed by two distinct membranes — the outer one resembling the host's cell membrane, the inner one resembling a prokaryotic membrane.

COMPARTMENTALISATION

Eukaryotic organelles are compartments with specialised functions. Each set of reactions is regulated and controlled within its own organelle, so incompatible chemistry can run side by side in the same cell. In addition, the internal folded membranes of mitochondria, chloroplasts, the Golgi body and the endoplasmic reticulum pack a large surface area into a small space: more enzymes can bind to the membranes, so reactions proceed more efficiently.

EUKARYOTIC CELL (plant)

EUKARYOTIC CELL (plant) — callout labels identify the wall, membrane, nucleus with nucleolus, rough ER with ribosomes, Golgi body, mitochondrion, chloroplast and the large central vacuole.

ORGANELLES

Eukaryotic cells contain many types of organelles, including the nucleus, mitochondrion, chloroplast, vacuole, Golgi body, endoplasmic reticulum and ribosome. SACE requires both halves for each organelle: represent its structure and describe its function.

• Nucleus - under the microscope, the most conspicuous (typically dark) organelle. It stores the cell's genetic information, usually as loose chromatin, and is bounded by a double-membraned nuclear envelope pierced by nuclear pores, which let substances in and let others (such as mRNA) out.
• Nucleolus - a dense region inside the nucleus made of protein and RNA, occupying about 25% of the nuclear volume. It synthesises rRNA and the subunits that are assembled into ribosomes.

The mitochondrion is the site of the latter stages of aerobic respiration (glycolysis occurs outside, in the cytoplasm), and is often called the powerhouse of the cell because it is the site of ATP synthesis. It has a double membrane: the inner membrane folds inward to form cristae, which provide a large surface area for the reactions, enclosing the matrix — a fluid containing mitochondrial DNA, ribosomes, soluble enzymes, small organic molecules and ions. The enzyme ATP synthase is embedded in the inner membrane (cristae). Mitochondria are found in plant, animal and fungal cells.

A mitochondrion. The inner membrane folds into cristae, providing a large surface area for biochemical reactions; the matrix holds mitochondrial DNA, ribosomes and soluble enzymes; ATP synthase is embedded in the cristae.

The chloroplast (found in photosynthetic cells such as plants and some algae, but not animals) is the site of photosynthesis — the conversion of light energy into chemical energy, turning carbon dioxide and water into glucose. Inside its double membrane, chlorophyll-containing discs called thylakoids are stacked to form grana (each granum holds about 10–20 thylakoids). The fluid within the chloroplast is the stroma — a protein-rich fluid containing chloroplast DNA, ribosomes and starch granules.

A chloroplast. Thylakoids stack into grana, connected by membrane bridges and suspended in the stroma, all inside a double membrane.

The endoplasmic reticulum, ribosomes and Golgi body form the cell's production line. The endoplasmic reticulum (ER) is an extensive membrane network spanning from the nuclear membrane towards the cell membrane, with a large surface area for reactions:

• Rough ER - flattened sacs studded with ribosomes, located next to the nucleus. The ribosomes synthesise proteins, which are then folded inside the lumen (the space within the sacs).
• Smooth ER - connected tube-like structures without ribosomes; the site of lipid synthesis, including cholesterol and phospholipids.
• Golgi body - stacked, flattened pouches called cisternae, positioned next to the ER but structurally distinct from it (the Golgi body has definite ends and associated vesicles, while the reticulum is continuous). It processes, packages and exports proteins and other macromolecules in vesicles after they are synthesised on the ER.
• Ribosome - free in the cytosol or attached to the rough ER; the site of protein synthesis.

Storage and breakdown are handled by two membrane-bound sacs:

• Vacuole - a membrane-bound, fluid-filled sac found in both plants and animals. It stores water, salts, nutrients and wastes away from the cytosol, regulates turgor pressure (shape maintenance, especially in plant cells), and handles bulk intake (endocytosis) and export (exocytosis). Plants and fungi have one large vacuole; animal cells have several small ones. Contractile vacuoles are specialised vacuoles in unicellular freshwater organisms such as amoeba and paramecia that remove excess water.
• Lysosome - a phospholipid-bilayer sphere filled with digestive enzymes. It breaks down carbohydrates, lipids, proteins, nucleic acids and worn-out organelles for recycling, and helps destroy pathogens in the immune response. Lysosomes occur in animal cells, and in some plant and fungal cells as "acidic vacuoles".

In summary:

CYTOSKELETON

The cytoskeleton is a meshwork of protein filaments (actin and intermediate filaments) and microtubules spanning throughout the cell. It is not membrane-bound, so it is not classified as an organelle. The cell membrane is connected to it, and a red blood cell — which lacks most vital organelles, including a nucleus — still has an extensive cytoskeleton. Its functions are:

1. To provide shape and structural support for the cell.
2. To position organelles, transport substances and contribute to molecular transport (especially endocytosis and exocytosis).
3. To move chromosomes and organelles during cell division — the cytoskeleton gives rise to the spindle fibres (mitotic spindle).

PLANT, ANIMAL AND FUNGAL CELLS

All three are eukaryotes with the same core organelles. The differences lie in the cell wall, the mode of nutrition, and the arrangement of vacuoles.