Biology Chapter 5: Cellular Structure

The Fundamental Unit of Life

• In 1665, Robert Hooke discovered cells while examining a thin slice of cork with a self-designed microscope.
• Hooke called these units "cells," a Latin word for "a little room," which is still used today in biology.
• The discovery of cells marked a significant milestone in the history of science, revealing that living things are composed of separate units.

What Are Living Organisms Made Of?

• All living organisms are made up of cells.
• Cells are the basic building units of life.
• Even single-celled organisms like Amoeba, Chlamydomonas, Paramoecium, and bacteria are made up of cells.
• Multicellular organisms, such as plants and animals, are composed of many cells that group together to form various body parts.

What Is a Cell Made Up Of?

• A cell has three main features: plasma membrane, nucleus, and cytoplasm.
• The plasma membrane is the outermost covering of the cell, separating its contents from its external environment.
• The plasma membrane is selectively permeable, allowing or permitting the entry and exit of certain materials into and out of the cell.

Plasma Membrane

• The plasma membrane is flexible and made up of organic molecules called lipids and proteins.
• The flexibility of the plasma membrane enables the cell to engulf food and other material from its external environment through a process called endocytosis.
• The plasma membrane is also responsible for gaseous exchange between the cell and its external environment.

Osmosis

• Osmosis is the net diffusion of water across a selectively permeable membrane toward a higher solute concentration.
• If a cell is placed in a hypotonic solution (lower concentration of water), it will gain water by osmosis and swell.
• If a cell is placed in an isotonic solution (same concentration of water), there will be no net movement of water across the cell membrane.
• If a cell is placed in a hypertonic solution (higher concentration of water), it will lose water by osmosis and shrink.

Cell Wall

• Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall.
• The cell wall is composed mainly of cellulose, a complex substance that provides structural strength to plants.
• The cell wall allows plant cells to withstand very dilute external media without bursting.

Nucleus

• The nucleus is another important organelle of the cell.
• The nucleus was discovered by Robert Brown in 1831.
• The nucleus is an essential organelle found in all living cells.

Cell Organelles

• Each cell organelle performs a special function, such as making new material in the cell, clearing waste material from the cell, and so on.
• All cells have the same organelles, regardless of their function or the organism they are found in.
• The cell organelles together constitute the basic unit of life called the cell.### Cell Structure and Function
• Living plant cells lose water through osmosis, causing contraction of the cell contents away from the cell wall, known as plasmolysis.
• The nucleus is a double-layered membrane structure containing chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
• Chromosomes contain DNA molecules, which carry genetic information for inheritance of characters from parents to the next generation.
• DNA molecules are composed of DNA and protein, and functional segments of DNA are called genes.
• The nucleus plays a central role in cellular reproduction, determining the way the cell will develop and what form it will exhibit at maturity.

Cell Organelles

• Eukaryotic cells have membrane-bound cell organelles, including the endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and plastids, each performing specific functions for the cell.
• Prokaryotic cells lack most of these organelles and have a poorly defined nuclear region called a nucleoid.
• The endoplasmic reticulum (ER) is a network of membrane-bound tubes and sheets involved in protein synthesis, transport, and lipid synthesis.
• The Golgi apparatus consists of a system of membrane-bound vesicles (flattened sacs) arranged approximately parallel to each other in stacks called cisterns, involved in packaging and dispatching of products.
• Lysosomes are membrane-bound sacs filled with digestive enzymes, acting as a waste disposal system for the cell, digesting foreign materials and worn-out organelles.
• Mitochondria are known as the powerhouses of the cell, generating energy for the cell through ATP production, and having their own DNA and ribosomes.
• Plastids are present only in plant cells, involved in photosynthesis, and can be classified as chromoplasts (colored plastids) or leucoplasts (white or colorless plastids).
• Vacuoles are membrane-bound sacs containing cell sap, providing turgidity and rigidity to the cell, and storing substances of importance in plant cells.

Cell Division

• Cell division is the process by which new cells are formed in organisms to grow, replace old cells, and form gametes required for reproduction.
• The cell is the fundamental structural and functional unit of living organisms.### Cell Structure and Function
• The fundamental organisational unit of life is the cell.
• Cells are enclosed by a plasma membrane composed of lipids and proteins.
• The cell membrane is an active part of the cell, regulating the movement of materials between the cell and its environment.
• Plant cells have a cell wall composed mainly of cellulose, located outside the cell membrane, which enables them to exist in hypotonic media without bursting.

Nucleus and ER

• The nucleus in eukaryotes is separated from the cytoplasm by a double-layered membrane and directs the life processes of the cell.
• The ER functions both as a passageway for intracellular transport and as a manufacturing surface.

Golgi Apparatus and Plastids

• The Golgi apparatus consists of stacks of membrane-bound vesicles that function in the storage, modification, and packaging of substances manufactured in the cell.
• Most plant cells have large membranous organelles called plastids, which are of two types: chromoplasts and leucoplasts.
• Chromoplasts, which contain chlorophyll, perform photosynthesis, while leucoplasts function in storage.

Vacuoles

• Vacuoles are storage sacs for solid or liquid contents.
• Plant cells have very large vacuoles, with the central vacuole occupying 50-90% of the cell volume.
• Vacuoles help maintain the turgidity of the cell and store important substances, including wastes.

Cell Division

• Cells divide for growth of the body, replacement of dead cells, and formation of gametes for reproduction.
• There are two main types of cell division: mitosis and meiosis.
• Mitosis is the process of cell division that results in two identical daughter cells with the same number of chromosomes as the mother cell.
• Meiosis is a process of cell division that involves two consecutive divisions, resulting in four new cells with half the number of chromosomes of the mother cell.

Prokaryotic and Eukaryotic Cells

• Prokaryotic cells have no membrane-bound organelles, their chromosomes are composed of only nucleic acid, and they have only very small ribosomes as organelles.
• Eukaryotic cells have membrane-bound organelles and a true nucleus.