Biology 101 Chapter 2: Cell Structure and Cell Organization

Vacuole

A vacuole is a fluid-filled sac which surrounded by tonoplast, a semi-permeable membrane. The fluid contained in a vacuole is call cell sap. A typical plant cell has a large central vacuole while a young plant cell has numerous smaller vacuoles. Freshwater microorganisms such as Paramecium sp. have contractile vacuoles and food vacuoles. The contractile vacuoles help to regulate water balance in these organisms. The vacuole acts as a storage place in a cell. The cell sap contains water, organic acids, sugars, amino acids, mineral salts, waste substances, pigments and metabolic by-products. The cell sap in a vacuole supports herbaceous plants so that they do not wilt easily.

Centriles

Centrioles are a pair of small cylindrical structures located just outside the nucleus. Centrioles are composed of a complex arrangement of microtubules. They form spindle fibers during cell division in animal cells. They are not present in plant cells.

Chloroplasts

Chloroplasts are typically lens-shaped. The membranous structures in the chloroplasts contain the green pigment chlorophyll. Chlorophyll traps sunlight and converts light energy into chemical energy during photosynthesis. The green color of plants comes from chlorophyll.

Endoplasmic Reticulum

Endoplasmic reticulum consists of a network of folded membranes forming interconnected tubules or sacs. The endoplasmic reticulum membrane s physically continuous with the nuclear membrane. Rough endoplasmic reticulum (Rough ER) has ribosomes attached to its surface while smooth endoplasmic reticulum (Smooth ER) does not have ribosomes attached to its surface. Most of the biochemical reactions of the cell take place on these membranes. The rough endoplasmic reticulum transports proteins made by the ribosomes. Proteins that depart from the rough endoplasmic reticulum are enclosed in vesicles. These proteins can be transported from one part of the cell to another. The smooth endoplasmic reticulum is the site of important metabolic reactions, including synthesis of lipids and detoxification of drugs and poisons.

Cell Membrane

It is a general term that refers to all membranes found inside the cell.

Lysosomes

Lysosomes are membrane-bound sacs that contain hydrolytic enzymes. These enzymes digest or break down complex organic molecules such as proteins, lipids, polysaccharides and nuclei acids. They also eliminate worn out mitochondria and other damaged organelles to enable a cell to renew itself. In the cells of certain organisms, lysosomes fuse with food vacuoles and dispense enzymes into these vacuoles to digest the contents of the vacuoles. This releases nutrients needed by the organisms. Lysosomes also digest bacteria by breaking down the bacterial cell wall.

Mitochondria

Mitochondria appear in electron micrographs as small spherical or cylindrical-shaped organelles. Mitochondria are involved in cellular respiration. The process releases energy when food substances such as glucose are broken down with the help of enzymes present in the mitochondria. The energy released is stored in the form of ATP (adenosine triphosphates) that can be readily used by cells.

Ribosomes

Ribosomes are compact spherical organelles found attached to the surface of rough endoplasmic reticulum or suspended freely in the cytoplasm. Each ribosome consists of two sub-units, one small and one large, each of which comprises a type of ribonucleic acid (RNA) and protein. Synthesis of proteins, such as enzymes, takes place in the ribosomes. Ribosomes use information carried by the chromosomes to make these proteins.

Golgi Apparatus

The Golgi apparatus consists of a stack of flattened membranous sacs. The Golgi apparatus functions as a processing, packaging and transporting centre of carbohydrates, proteins, phospholipids and glycoproteins. Transport vesicles carrying products such as proteins from the endoplasmic reticulum fuse with the membrane of the Golgi apparatus and empty their contents into the membranous space. These proteins are modified as they pass through the Golgi apparatus. The Golgi apparatus will then sort these products and package them into vesicles. Vesicles containing these products bud off from the Golgi membrane and travel to other parts of the cell or to the plasma membrane. These vesicles will then fuse with the plasma membrane before releasing their contents outside the cell.

Cell Wall

The cell wall is a rigid outer layer that surrounds the plasma membrane of plant cells. It is composed of cellulose, a tough and fibrous carbohydrate. The cell wall is permeable to all fluids because it has tiny pores that allow substances to move freely into and out of the cell. A cell wall gives shape to a plant cell and provides mechanical support for the plant cell. It also protects the plant cell from rupturing due to the excessive intake of water.

Cyoplasn

The cytoplasm is the region between the nucleus and the plasma membrane. It consists of a jelly-like matrix in which the organelles of the cell are suspended. Cytoplasm contains organic and inorganic substances and acts as a medium for biochemical reactions in the cell. IT also provides substances obtained from the external environment to the organelles.

Protoplasm

The living component of a cell. It consists of the cytoplasm and the nucleus. The protoplasm is surrounded by a thin layer of plasma membrane. In addition, plant cells have an outer boundary called cell wall, which completely surrounds the plasma membrane.

Plasma Membrane

The membrane at the surface which encloses the content of a cell. The plasma membrane is made of proteins and phospholipids. It forms a boundary which separates the content of a cell from the outer environment. The plasma membrane is semi-permeable and acts as a selective barrier. It regulates the movement of substances into and out of the cytoplasm.

Nuclues

The nucleus appears as a large, dense, spherical organelle enclosed by a nuclear membrane. It contains the nucleoplasm and a dark spherical structure called the nucleolus. Chromatin, the form that chromosomes take when the cell is not dividing, is contained within the nucleoplasm. Chromosomes carry genetic information which determines the characteristics and functions of a cell. The nucleus is of primary importance because it controls all the activities which take place in the cell.

Cells

Thet are the basic units of living organisms.

Organelles

They are contained inside of cytoplasm. They perform specific functions, which enable the cell to function as a unit of life.