Cell Structure and Cell Organelles

2.6 The Generalised Cell

Although cells come in countless shapes and sizes, every eukaryotic cell shares the same basic plan: an outer covering, a jelly-like fluid filling its interior, and a set of small working "compartments" called organelles — each one carrying out a special task. Studying a "generalised" cell helps us see all these structures together.

2.7 The Plasma Membrane

The outermost living covering of every cell is the plasma membrane, also called the cell membrane. It is extremely thin (only a few nanometres thick) and is made of two layers of lipids studded with proteins.

How materials cross the membrane

• Diffusion: movement of gases (O₂, CO₂) from high to low concentration — no energy needed.
• Osmosis: movement of water across the membrane from a region of more water to a region of less water.
• Active transport: movement against a concentration gradient — uses ATP energy.

2.8 Cell Wall (Plant Cells Only)

Plant cells have an additional, thick, non-living covering outside the plasma membrane called the cell wall. It is mainly made of cellulose. The cell wall gives the plant cell a fixed shape, mechanical strength and protection. It also lets plant cells survive even when they lose water — without bursting.

2.9 Nucleus — The Control Centre

The nucleus is usually the largest, darkest organelle. It is bounded by a double-layered nuclear membrane with tiny pores. Inside, it contains a thin thread-like material called chromatin.

The nucleus also contains a smaller dense body called the nucleolus, which makes ribosomes.

2.10 Cytoplasm

The jelly-like fluid filling the cell between the plasma membrane and the nucleus is called cytoplasm. All the organelles float inside it. Many important reactions of life — like glycolysis (the first step of respiration) — happen here.

2.11 Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of membrane-bound tubes and sheets running through the cytoplasm. It comes in two types:

The ER also acts as a network of "highways" through which materials are transported inside the cell.

2.12 Golgi Apparatus

Discovered by Camillo Golgi, the Golgi apparatus looks like a stack of flattened, curved sacs called cisternae. It receives proteins and lipids from the ER, modifies them (e.g., adds sugars), packages them into tiny vesicles, and sends them either out of the cell or to other organelles. It also helps form lysosomes.

2.13 Ribosomes

Ribosomes are extremely small, dense particles found either floating freely in cytoplasm or attached to the rough ER. They are the protein factories of the cell — they assemble amino acids into proteins, following instructions carried from the nucleus.

2.14 Mitochondria — The Powerhouse

Mitochondria are sausage-shaped organelles with a double membrane. The inner membrane is folded inwards to form cristae, which provide a large surface area.

Interestingly, mitochondria have their own DNA and ribosomes — so they can make some of their own proteins! Many scientists believe mitochondria evolved from ancient bacteria that began living inside larger cells billions of years ago.

2.15 Plastids (Plant Cells Only)

Plastids are organelles found only in plant cells. There are three main types:

Chloroplasts, like mitochondria, have a double membrane and their own DNA and ribosomes — supporting the same theory of bacterial origin.

2.16 Lysosomes — The Suicide Bags

Lysosomes are small membrane-bound sacs filled with powerful digestive enzymes. They digest worn-out cell parts and any foreign material like bacteria. When a cell is damaged or dying, lysosomes burst and digest the entire cell. For this reason they are called the "suicide bags" of the cell.

2.17 Vacuoles

Vacuoles are storage sacs filled with water, food, salts, sugars or wastes.

• In animal cells, vacuoles are tiny and few.
• In plant cells, there is usually a single huge central vacuole that may take up 50–90% of the cell volume. It maintains turgor pressure, which keeps the plant firm and upright.