Cell And Tissue Biology

As more scientists examined plants and animals, a general pattern became clear. Schleiden, a botanist, proposed in 1838 that all plants are made of cells. In 1839, Schwann extended the idea to animals. Their work formed the classical cell theory: the cell is the basic structural unit of plants and animals, and living organisms are made of one or more cells. A single-celled organism, such as many pond microorganisms, performs all life processes within one cell. In a multicellular organism, such as a human or a tree, many cells work together.

Virchow added an important statement in 1855: “Cellula e cellula,” meaning that every cell comes from a pre-existing cell. This corrected the older belief that living cells could appear from non-living material under ordinary conditions. Modern cell theory can therefore be summarized in three ideas: all living organisms are made of cells, the cell is the basic unit of life, and new cells arise from existing cells. Cells may differ greatly in shape and function, but they share a basic organization.

A cell contains genetic information, carries out chemical reactions, obtains and uses energy, and responds to its surroundings. These activities are possible because different parts of the cell perform different jobs. The boundary of the cell controls exchange with the environment, while internal structures support energy release, protein production, storage, and transport. In larger cells, these tasks are often separated into organelles, which are specialized structures inside the cell.

Ask yourself: why is the cell called the basic unit of life rather than simply a small part of an organism? The answer is that a cell can perform the essential activities of life. A tissue or organ depends on cells, but a cell has the organization needed to maintain itself. The main takeaway is that cell discovery was not just the discovery of tiny objects; it revealed a common plan shared by living organisms.

Prokaryotic and Eukaryotic Organization

Cells are commonly grouped into two broad types: prokaryotic cells and eukaryotic cells. The word “prokaryotic” refers to cells that do not have a true nucleus surrounded by a membrane. The word “eukaryotic” refers to cells that possess a membrane-bound nucleus. This difference is not a small detail. It affects where genetic material is stored and how many internal structures a cell can organize.

Bacteria are examples of prokaryotic organisms. A typical bacterial cell has a plasma membrane, cytoplasm, ribosomes, and a region containing its genetic material. The genetic material is usually a circular DNA molecule found in a region called the nucleoid. The nucleoid is not enclosed by a nuclear membrane. Many bacteria also have a cell wall outside the plasma membrane, and some possess a capsule, flagellum, or small DNA rings called plasmids.

Prokaryotic cells are generally smaller and simpler in internal arrangement than eukaryotic cells, but “simple” does not mean “inactive.” A bacterial cell can take in nutrients, release waste, produce proteins, move, and reproduce. Ribosomes make proteins, while the plasma membrane controls exchange with the surroundings. Some bacteria use flagella for movement. Their compact structure allows them to grow and multiply rapidly when conditions are suitable.

Eukaryotic cells include the cells of plants, animals, fungi, and many single-celled organisms. Their DNA is enclosed within a nucleus. They also contain membrane-bound organelles, each suited to a particular task. Mitochondria release usable energy from food. In plant cells, chloroplasts capture light energy for photosynthesis. The endoplasmic reticulum helps make and transport proteins and lipids, while the Golgi apparatus modifies, sorts, and packages materials. Camillo Golgi identified the structure later named the Golgi apparatus, and Christian de Duve discovered lysosomes and peroxisomes, organelles involved in digestion and chemical processing.

A plant cell usually has a cell wall, a large central vacuole, and chloroplasts in addition to the structures found in animal cells. The cell wall gives support, the vacuole stores cell sap and helps maintain pressure, and chloroplasts contain chlorophyll for photosynthesis. Animal cells do not have a cell wall or chloroplasts. They often contain small vacuoles and have a flexible outer boundary that allows changes in shape.

The nucleus acts as a control centre because it stores DNA, the molecule that contains instructions for cell activities. The nuclear envelope separates the DNA from the cytoplasm, while nuclear pores allow selected materials to pass between the nucleus and the cytoplasm. Inside the nucleus, the nucleolus helps produce parts of ribosomes. This arrangement lets a eukaryotic cell protect its genetic information while managing many chemical processes at the same time.

A useful comparison is to think about organization rather than size alone. A prokaryotic cell keeps its genetic material in the cytoplasm and has no membrane-bound organelles. A eukaryotic cell separates tasks into compartments. This separation can make complex activities more controlled. However, both types still depend on a plasma membrane, cytoplasm, ribosomes, and genetic information.

Consider a bacterial cell and a leaf cell. Both need energy, proteins, and a boundary that controls substances. The leaf cell has chloroplasts because it carries out photosynthesis; the bacterial cell may obtain energy in a different way. Their structures differ because their needs differ. The practical takeaway is that cell structure is closely connected to cell function: each part is present because it helps the cell survive and work efficiently.

How Membranes Regulate Movement

Every living cell must control what enters and leaves. It needs water, oxygen, mineral ions, and nutrients, but it must also remove carbon dioxide and other wastes. The plasma membrane performs this control. It is thin, flexible, and selectively permeable, meaning that some substances cross easily while others are restricted.

Singer and Nicolson proposed the fluid mosaic model in 1972 to describe membrane structure. The membrane is mainly a double layer of phospholipids called a phospholipid bilayer. Each phospholipid has a water-attracting head and water-repelling tails. In water, the heads face the watery surroundings while the tails point inward, away from water. Proteins are scattered through this layer, creating a pattern like a mosaic. Because the phospholipids can move sideways, the membrane remains fluid rather than rigid.

Membrane proteins have different jobs. Some form channels through which particular ions or molecules can pass. Others act as carriers that change shape while moving substances across the membrane. Some receive chemical signals, while others help cells attach to neighbouring cells. Carbohydrate chains attached to membrane proteins or lipids can help identify the cell. Cholesterol also helps maintain suitable membrane flexibility, especially when temperature changes.

Passive transport does not require energy from the cell. Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. For example, if oxygen is more concentrated outside a cell than inside it, oxygen can diffuse through the membrane into the cell. The movement continues until the concentration becomes more balanced. The particles do not stop moving; there is simply no overall movement in one direction.

Some substances cannot pass directly through the phospholipid bilayer. They may be too large, electrically charged, or not soluble in lipids. Facilitated diffusion helps such substances cross through specific membrane proteins. For example, a channel protein may allow certain ions to pass, while a carrier protein may move glucose. Facilitated diffusion is still passive because the substance moves down its concentration gradient and no cellular energy is used.

Osmosis is the movement of water through a selectively permeable membrane from a region with more water to a region with less water. In everyday terms, water moves toward the side with a higher concentration of dissolved substances. If a plant cell is placed in pure water, water enters the cell by osmosis. The cell becomes firm because the cell wall resists excessive expansion. If an animal cell is placed in a very dilute solution, it may swell and burst because it has no cell wall.

Solutions can be described as hypotonic, isotonic, or hypertonic compared with the cell. A hypotonic solution has a lower concentration of dissolved substances outside the cell, so water tends to enter. An isotonic solution has a similar concentration inside and outside, so there is no overall movement of water. A hypertonic solution has a higher concentration of dissolved substances outside, so water tends to leave the cell. The direction of water movement depends on the comparison between the cell and its surroundings.

Active transport requires energy, usually from ATP, or adenosine triphosphate. It moves substances from a lower concentration to a higher concentration, against the concentration gradient. Carrier proteins act as pumps in this process. Root hair cells use active transport to take in mineral ions from soil even when the ion concentration is already higher inside the cell. This helps plants obtain minerals that are essential for growth.

Very large materials may cross the membrane through vesicles. Endocytosis brings material into the cell by forming a membrane pocket that closes into a vesicle. Exocytosis releases material when a vesicle joins the plasma membrane and empties its contents outside. Cells use these processes to take in large particles, release hormones, and export proteins. They show that membrane transport is not limited to tiny molecules.

A worked example helps connect these ideas. Suppose a cell contains less glucose than the fluid around it, and a membrane protein allows glucose to pass. Glucose will move into the cell by facilitated diffusion until the concentration difference becomes smaller. If the cell must move glucose from a lower concentration outside to a higher concentration inside, it must use active transport and energy. Always ask two questions: what is the concentration difference, and does the movement require energy?

The plasma membrane is therefore more than a covering. It is a living control system that maintains suitable conditions inside the cell. Its structure allows flexibility, communication, selective exchange, and energy-dependent movement. When you study any membrane transport example, connect the movement to the membrane’s main purpose: keeping the cell’s internal environment stable while allowing essential materials to move in and out.

End of chapter one. 4 more chapters in the full book.