5.2. What is a Cell Made Up of?

We saw above that the cell has special components called organelles. How is a cell organised?

If we study a cell under a microscope, we would come across three features in almost every cell: plasma membrane, nucleus, and cytoplasm. All activities inside the cell and interactions of the cell with its environment are possible due to these features. Let us see how.

In every cell, there are three essential parts: the plasma membrane, the nucleus, and the cytoplasm. The plasma membrane serves as the protective outer layer of the cell, controlling what enters and exits. The nucleus is like the control center of the cell, containing the genetic material (DNA) and managing cellular activities. The cytoplasm is the fluid inside the cell where many cellular processes occur and where organelles are found. This division of structures allows the cell to function effectively and efficiently.

Think of a cell like a factory. The plasma membrane is the factory's walls, controlling what goes in and out. The nucleus is the manager’s office, where all decisions are made and information is stored. The cytoplasm is the workshop floor, where all the production takes place, filled with machines (organelles) doing various tasks.

5.2.1 PLASMA MEMBRANE OR CELL MEMBRANE This is the outer most covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents the movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane.

The plasma membrane is crucial because it controls which substances can enter or leave the cell. It is described as selectively permeable, meaning it only allows certain materials to pass through while blocking others. This is vital for maintaining the cell’s internal environment—like regulating nutrients and waste. For example, oxygen can enter the cell while carbon dioxide can exit through this membrane via diffusion.

Imagine the plasma membrane like a bouncer at a club. The bouncer allows only certain people (molecules) to enter based on specific criteria, ensuring that the club (cell) remains safe and well-regulated.

How does the movement of substances take place into the cell? How do substances move out of the cell? Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion.

Diffusion is a natural process where molecules move from an area of higher concentration to an area of lower concentration. In cells, this means that when there is more carbon dioxide inside the cell than outside, it will diffuse out of the cell, and when there is less oxygen inside the cell than outside, oxygen will diffuse into the cell.

Think of diffusion like a crowded room trying to empty out. If one side of the room (cell) is really crowded (high concentration of CO2), people (molecules) will naturally move toward the side with more space (low concentration), creating a more even distribution throughout the room.

What will happen if we put an animal cell or a plant cell into a solution of sugar or salt in water? One of the following three things could happen: [1] If the medium surrounding the cell has a higher water concentration than the cell... [2] If the medium has exactly the same water concentration as the cell... [3] If the medium has a lower concentration of water than the cell....

Cells react differently when placed in various solutions depending on the concentration of the solutions relative to the cell's interior. If placed in a hypotonic solution (more water outside), cells swell. In an isotonic solution (equal concentration), cell size stays the same. In a hypertonic solution (less water outside), cells shrink. This process is termed osmosis, specifically the movement of water.

You can liken it to a sponge in water. When put in fresh water (hypotonic), it absorbs water and swells. In salt water (hypertonic), it releases water, causing it to shrink. If placed on a towel (isotonic equivalent), it stays the same, not gaining or losing water.

5.2.2 CELL WALL Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose.

In plant cells, the cell wall provides additional support and structure. Made mostly of cellulose, it allows plants to maintain their shape and withstand various environmental conditions. It helps prevent the cell from bursting when it absorbs water and keeps the plant upright.

Think of the cell wall like the outer wall of a building. While the plasma membrane is like the door that controls access, the cell wall adds durable structure that keeps everything inside the building (cell) intact and standing, no matter the external conditions.

5.2.3 NUCLEUS ... The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells.

The nucleus is often referred to as the control center of the cell because it contains the genetic material (DNA) that directs cell functions, including growth, metabolism, and reproduction. The nuclear membrane protects the nucleus and controls the movement of substances in and out of it, facilitating communication with the cytoplasm.

Imagine the nucleus as a library filled with important books (DNA) that contain instructions on how to run a city (the cell). The librarian (nuclear membrane) controls who can access those books to ensure that the right information is used at the right time.

5.2.4 CYTOPLASM When we look at the temporary mounts of onion peel as well as human cheek cells, we can see a large region of each cell enclosed by the cell membrane. This region takes up very little stain. It is called the cytoplasm.

Cytoplasm is the jelly-like substance within the cell membrane that houses organelles and is the site for many biochemical reactions essential for life. It is made up of water, salts, and various organic molecules that support cellular processes.

You can think of cytoplasm like a swimming pool filled with water (liquid) where various activities occur. The pool is vast and contains different objects (organelles) that help with various games (cell functions) going on in that space.

5.2.5 CELL ORGANELLES Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function.

Organelles are specialized structures within cells that perform distinct functions, much like departments in a company. They help the cell carry out necessary processes, such as energy production, protein synthesis, and waste management. Notably, eukaryotic cells have many membrane-bound organelles that allow for compartmentalization of these processes.

Think of organelles as different departments in a factory. Just like each department has its specific job—some handle production (mitochondria), some packaging (Golgi apparatus), and some waste management (lysosomes)—every organelle has a unique role that contributes to the cell’s overall function.

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