Nucleus
The nucleus is separated from the cytoplasm by two membranes known as the nuclear envelope. The nucleus contains molecules of deoxyribonucleic acid (DNA), which store the hereditary information responsible for cell growth and reproduction. This information is divided into 46 chromosomes. Enzymes in the nucleus copy parts of the DNA into strands of ribonucleic acid (RNA). Each segment of a chromosome used to produce a strand of RNA is known as a gene. Some types of RNA (messenger) are used to produce new copies of DNA. Other types (ribosomal) enter the cytoplasm through the nucleic membrane. There they form protein-RNA structures known as ribosomes, used as blueprints for producing molecules that fuel and organize cell activity.
Cytoplasm
The cytoplasm is contained inside the plasma membrane. Nutrients and waste materials pass through this membrane by slowly dissolving through or by way of channels formed out of proteins. Similar internal membranes distinguish organelles, compartments in the cell that specialize in a particular function, from the cytoplasm. Ribosomes and organelles inhabit the cytoplasm as separate membrane-bordered chemical and molecular environments.
Ribosomes. Free-floating ribosomes use proteins to
41
make enzymes, structural supports, and elements used for movement. Other ribosomes attached to a cluster of flattened, membranous sacs (endoplasmic reticulum) build proteins that are stored, used in membranes, or relapsed from the cell altogether. The Golgi complex, another system of sacs, modifies proteins made in the endoplasmic reticulum.
Some Types of Organelles. The mitochondrion produces most of the chemical energy needed by the cell. Carbohydrates, fats, proteins, and nucleic acids provide fuel for reactions inside the mitochondria that release energy to spur cell growth and development.
The lysosome contains the cell's digestive enzymes. Substances engulfed by the cell may be directed to a lysosome for digestion. In case of disease or as part of cell division, lysosomes may release their enzymes into the cytoplasm, killing the cell.
Microrubules and microfilaments are used for cell locomotion. They also form the cytoskeleton, an internal structure that supports the cell.
Answer the questions:
1 Name the two major parts of a cell.
2 What are the general functions of these two parts?
3 Give an example to illustrate that the shape of a cell is related to its function.
4 What is the function of the Golgi complex?
5 Why are mitochondria sometimes called the “powerhouses of cells”?
42
6 Describe the functions of microfilaments and microtubules.
Read and translate the following sentences:
Active transport:
a) Active transport is responsible for movement of molecules or ions from regions of lower concentration to regions of higher concentration.
b) It requires cellular energy and involves action of carrier molecules in the cell membrane.
Endocytosis:
a) Pinocytosis is a process by which a cell membrane engulfs tiny droplets of liquid.
b) Phagocytosis is a process by which a cell membrane engulfs solid particles.
c) Receptor-mediated endocytosis is a process by which receptor proteins combine with specific kinds of molecules in the cell surroundings, and the combinations are engulfed.
Life Cycle of a Cell:
1 The life cycle of a cell includes mitosis, cytoplasmic division, interphase, and differentiation.
2 Mitosis:
a) Mitosis is the division and distribution of nuclear parts to daughter cells during cell reproduction.
b) The stages of mitosis include prophase, metaphase, anaphase, and telophase.
3 Citoplasmic division is a process by which cytoplasm
43
-
is divided into two portions following mitosis.
4 Interphase:
a) Interphase is the stage in the life cycle when a cell grows and forms new organelles.
b) It terminates when the cell begins to undergo mitosis.
5 Cell differentiation involves the development of specialized structures and functions.