CELL STRUCTURE
Science
INTRODUCTION
• Cell is the fundamental structural and functional unit of all living organisms.
• It is the smallest unit capable of carrying out all vital life processes.
• Each cell performs basic activities such as respiration, nutrition, excretion, growth and reproduction.
• With the invention of the electron microscope in 1940, scientists could study the detailed internal structure of cells.
• Cell shape and size vary depending on function. For example, Amoeba has a changing shape, while nerve cells are long and branched.
EUKARYOTIC VS PROKARYOTIC CELLS
Eukaryotic Cells:
• Possess a well-defined nucleus enclosed by a nuclear membrane.
• Contain membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi apparatus.
• Found in plants, animals, fungi and protists.
• Mostly multicellular, though some unicellular eukaryotes exist.
• Generally larger and more complex.
Prokaryotic Cells:
• Lack a true nucleus and membrane-bound organelles.
• Genetic material is present in an undefined region called nucleoid.
• Found in bacteria and archaea.
• Always unicellular, though some form colonies.
• Smaller and structurally simpler.
STRUCTURAL ORGANIZATION OF CELL
• Cells show division of labour where different organelles perform specialized functions.
• Despite functional diversity, basic organelles are common to most cells.
• Cell organelles together constitute the functional unit called a cell.
THREE MAIN COMPONENTS OF A CELL
A. PLASMA MEMBRANE
• Also known as cell membrane.
• Outermost boundary of the cell separating internal contents from the external environment.
• Selectively permeable – allows some substances to pass while restricting others.
• Made up of a lipid bilayer with embedded proteins.
• Flexible in nature.
Cell Wall (Plant Cells):
• Present only in plant cells, outside the plasma membrane.
• Composed mainly of cellulose.
• Provides rigidity, protection and structural strength.
• Absent in animal cells.
Plasmolysis:
• Shrinkage of cytoplasm away from cell wall due to loss of water in hypertonic solution.
• Occurs only in living plant cells.
B. NUCLEUS
• Acts as control centre of the cell.
• Contains genetic material (DNA).
• Controls growth, metabolism, reproduction and inheritance.
• Most human cells have one nucleus.
• Red blood cells lack nucleus.
• Some cells like liver and muscle cells are multinucleated.
Multinucleated Cells:
• Contain more than one nucleus.
• Allow faster and efficient protein synthesis.
• Example: Hepatocytes (liver cells).
Nucleoid:
• Found in prokaryotic cells.
• Poorly defined nuclear region without nuclear membrane.
• Contains genetic material.
Structure of Nucleus:
• Enclosed by double-layered nuclear membrane with pores.
• Nuclear pores allow exchange of RNA and proteins.
Components of Nucleus:
Chromosomes:
• Made of DNA and proteins.
• Carry hereditary information.
• Functional units of DNA are genes.
Chromatin:
• Present during non-dividing phase.
• Appears as thread-like material.
• Condenses into chromosomes during cell division.
Nucleolus:
• Dense body inside nucleus.
• Site of ribosome formation.
• Made of RNA and proteins.
C. CYTOPLASM
• Jelly-like fluid inside plasma membrane.
• Contains all cell organelles.
• Site of most metabolic reactions.
• Prokaryotes lack membrane-bound organelles.
• Eukaryotes possess membrane-bound organelles.
• Viruses lack cytoplasm and membranes and show life characteristics only inside host cells.
TRANSPORT OF SUBSTANCES ACROSS CELL MEMBRANE
Diffusion:
• Movement of substances from higher to lower concentration.
• Oxygen enters cells by diffusion.
• Carbon dioxide exits cells by diffusion.
Osmosis:
• Special type of diffusion involving water.
• Movement of water across selectively permeable membrane.
• Occurs only in living cells.
Types of Solutions:
Hypotonic Solution:
• Higher water concentration outside cell.
• Cell gains water and swells.
Isotonic Solution:
• Equal concentration inside and outside.
• No net movement of water.
Hypertonic Solution:
• Lower water concentration outside cell.
• Cell loses water and shrinks.
Endocytosis:
• Process by which cell engulfs material.
• Cell membrane folds inward.
• Seen in Amoeba for food intake.
KEY POINTS
• Cell membrane flexibility enables engulfing of materials.
• Osmosis is vital for water absorption in plant roots.
• Only living cells show osmosis.
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Subject: Science
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