Study guide: The Fundamental Unit of Life

📋 Course Outline

1. Cell Discovery and Cell Theory
2. Cell Size, Shape, and Organization
3. Preparing Temporary Cell Mounts
4. Plasma Membrane Structure and Functions
5. Transport Across the Plasma Membrane
6. Osmosis and Tonicity
7. Cell Wall and Cell Theory
8. Cell Wall Structure and Function
9. Nucleus Structure and Classification
10. Cytoplasm and Cell Organelles
11. Golgi Complex Structure and Functions
12. Mitochondrial Structure and Respiration
13. Mitochondria and Plastids
14. Vacuoles and Cell Types
15. Cell Organization and Arun Kumar Sharma
16. Diffusion and Osmosis
17. Cancer Development and Treatment
18. Advances in Cell Biology

📖 1. Cell Discovery and Cell Theory

🔑 Key Concepts & Definitions

• Cell: The basic structural and functional unit of living organisms.

★ Must-know

• Robert Hooke observed dead, honeycomb-like cells in cork in 1665 using a self-designed microscope and described them in Micrographia.

• M. J. Schleiden (1838) and T. Schwann (1839) formulated cell theory, stating that plants and animals are composed of cells and that the cell is the basic unit of life. - M.J. Schleiden

📌 Cell theory states that all living organisms are composed of cells and cell products, and that all cells arise from pre-existing cells.

Further detail

• Anton Van Leeuwenhoek first observed free-living cells in pond water in 1674 using an improved microscope.

• Robert Brown discovered the nucleus in 1831.

• J. E. Purkinje coined the term protoplasm for the living matter inside the cell in 1839.

• Rudolf Virchow expanded cell theory in 1855 by stating that all cells arise from pre-existing cells, expressed as “omnis cellula e cellula.”

📖 2. Cell Size, Shape, and Organization

🔑 Key Concepts & Definitions

• Unicellular Organism: Made of a single cell, which performs all life processes, including obtaining food, respiration, excretion, growth, and reproduction.
• Multicellular Organism: Made of many cells, which have a similar basic structure and perform specialized functions.
• Cell Inclusions: Non-living materials in the cytoplasm, such as pigment granules, starch granules, and fat droplets.

★ Must-know

• The shape of a cell mainly depends on the specific function it performs.

• The size of an organism depends on its cell number, not on the size of its cells.

Further detail

• A normal human cell measures 20–30 μm in diameter, and 1 μm equals $1 \times 10^{-6}\ \text{m}$.

• Cell-size extremes include: the ostrich egg as the largest isolated single animal cell, the nerve cell as the longest animal cell, PPLO (Mycoplasma), about 0.3 μm long, as the smallest cell

• Bacteria measure 3–5 μm, while viruses are smaller and do not have a cellular structure.

• Examples of cell shapes include:

  • elongated and branched nerve cells
  • discoidal or saucer-shaped red blood cells
  • spindle-shaped muscle cells
  • spherical eggs
  • branched skin pigment cells
  • slipper-shaped Paramecium
  • cuboidal gonad germ cells
  • polygonal liver cells

• Amoeba and white blood cells can change shape to move and engulf particles.

• Examples of unicellular organisms are:

  • Bacteria
  • Amoeba
  • Paramecium
  • Chlamydomonas

💡 Memory Hook

One cell does every task; many cells specialize

📖 3. Preparing Temporary Cell Mounts

★ Must-know

📌 Plant cells have a definite cell wall, plastids, and a well-developed vacuole, whereas animal cells lack a cell wall and plastids.

• 🔄 Prepare and examine the onion-peel mount:

  1. Separate a thin peel from the concave side of an onion scale.
  2. Place the peel in water.
  3. Transfer a small piece flat onto a glass slide with a drop of water.
  4. Add a drop of safranin.
  5. Drain excess stain and mount the peel in glycerine under a coverslip.
  6. Examine the slide under low and high microscope powers.

• Onion-peel epidermal cells appear as regularly arranged linear or rectangular compartments with rigid cell walls, and their nuclei are pushed towards the periphery by a large central vacuole.

• 🔄 Prepare and examine the cheek-cell mount:

  1. Wash the mouth.
  2. Gently scrape the inner lining with a clean toothpick, spatula, or ice-cream spoon.
  3. Transfer the material to a glass slide.
  4. Add methylene blue, wait a couple of minutes, and remove excess stain.
  5. Add glycerine and place a coverslip.
  6. Tap the coverslip gently to spread the cells.
  7. Examine the mount under low and high microscope powers.

Further detail

• The diagram identifies these cell structures:

  • cell membrane
  • protoplasm
  • cytoplasm
  • nucleus
  • cell organelles
  • nuclear membrane
  • nucleoplasm
  • nucleolus
  • chromatin threads

• The diagram groups organelles by membrane structure:

  • single-membranous: endoplasmic reticulum, Golgi body, lysosomes, and plastids
  • double-membranous: mitochondria
  • non-membranous: ribosomes and centrosomes

• Human cheek cells are flat and polygonal, with a distinct rounded nucleus in the middle, a thin cell membrane, and lightly stained cytoplasm.

📌 For onion-peel and cheek-cell mounts, spread the material uniformly, drain excess stain, and avoid air bubbles under the coverslip; place onion peel in water promptly to prevent folding and drying, and scrape cheek lining gently to avoid injury.

📖 4. Plasma Membrane Structure and Functions

🔑 Key Concepts & Definitions

• Plasma membrane: A flexible boundary enclosing the contents of every living cell.

★ Must-know

• The plasma membrane consists of a bilayer of phospholipids and proteins, with a small fraction of carbohydrates among its organic molecules.

📌 The selectively permeable plasma membrane allows nutrients and essential materials to enter and wastes to leave, while regulating passage of specific molecules.

Further detail

• Singer and Nicolson explained the structure of the plasma membrane through the fluid mosaic model.

• Oxygen and carbon dioxide pass freely across the plasma membrane by diffusion, whereas passage of larger molecules such as amino acids and sugars is regulated.

• The plasma membrane protects internal cell components, gives the cell shape, and permits materials to pass through tiny pores.

• The plasma membrane’s flexibility enables a cell to engulf food and other materials by endocytosis, as in Amoeba.

• Short sugar chains linked to proteins and lipids on the exterior of the plasma membrane help the cell interact with surface molecules of other cells.

💡 Memory Hook

The flexible, selectively permeable membrane controls exchange; the rigid cell wall provides support.

📖 5. Transport Across the Plasma Membrane

🔑 Key Concepts & Definitions

• Active transport: Moves substances across the plasma membrane against the concentration gradient using energy.
• Passive transport: Moves substances across the plasma membrane along the concentration gradient, from higher to lower concentration, without using energy.
• Diffusion: The movement of substances from a region of higher concentration to a region of lower concentration until they spread uniformly through a space.

📝 Essential Points

• During respiration, carbon dioxide accumulates inside cells and diffuses out, while oxygen diffuses from the external medium into cells where respiration consumes it.

• Endocytosis is the uptake of bulk material from the external environment into a cell and includes phagocytosis, or cell eating, and pinocytosis, or cell drinking.

💡 Memory Hook

Active transport moves against the gradient using energy; passive transport moves down it without energy.

📖 6. Osmosis and Tonicity

🔑 Key Concepts & Definitions

• Osmosis: The net diffusion of water across a permeable membrane toward the region with higher solute concentration.
• Hypotonic solution: An external solution more dilute than the cell contents and with a higher water content, causing net water movement into the cell by endosmosis.
• Isotonic solution: An external solution with the same concentration as the cell contents, so water enters and leaves at equal rates and cell size remains unchanged.
• Hypertonic solution: An external solution more concentrated than the cell contents and with a lower water content, so water leaves the cell by exosmosis.

★ Must-know

📌 Endosmosis is the osmotic entry of water into a cell or system, whereas exosmosis is the osmotic withdrawal of water from it.

Further detail

• An animal cell placed in a hypotonic solution swells and may burst, whereas a plant cell does not burst because it has a cell wall.

💡 Memory Hook

Picture water moving toward the more concentrated solute: into a cell in a hypotonic medium and out in a hypertonic one.

📖 7. Cell Wall and Cell Theory

🔑 Key Concepts & Definitions

• Cell: The basic structural unit of life; all life functions of an organism reside in its cells, making cells functional units of life as well.

📝 Essential Points

• Cells can specialize for specific functions, such as contraction in muscle cells and impulse transmission in nerve cells.

• Robert Hooke discovered cells in 1665 by observing a cork slice with a primitive microscope and naming its box-like structures cells, from cellula.

• The cell wall is a rigid, freely permeable outer layer outside the plasma membrane in plant cells, bacteria, blue-green algae, some protists, and all fungi.

📖 8. Cell Wall Structure and Function

🔑 Key Concepts & Definitions

• Plasmolysis: The shrinkage of a living plant cell's contents away from the cell wall after the cell loses water by osmosis.

★ Must-know

• In a hypotonic medium, a plant, fungal, or bacterial cell takes up water by osmosis and swells; the cell wall pushes back against the resulting pressure and helps prevent bursting.

• Cell-wall materials by organism are:

  • cellulose in plants
  • peptidoglycan in bacteria
  • peptidoglycan in blue-green algae
  • chitin in most fungi

• The cell wall protects and supports the cell and gives it a definite shape.

• In a hypertonic solution, water leaves a plant cell's central vacuole by exosmosis, causing the protoplasm to separate from the cell wall; a cell in this state is plasmolysed.

Further detail

• The cell wall is absent in animal cells.

• A Rheo leaf peel placed in a strong sugar or salt solution shows plasmolysis if its cells are living, but not if the peel was boiled first to kill the cells.

• Only living cells have selectively permeable membranes and can exhibit osmosis or absorb water by osmosis.

📖 9. Nucleus Structure and Classification

🔑 Key Concepts & Definitions

• Nucleus: Robert Brown, 1831 - The nucleus is a double-membrane-bound, dense protoplasmic body that controls cellular metabolism and contains the cell's genetic information.
• Nucleoplasm: The part of the protoplasm enclosed by the nuclear membrane; it contains chromatin threads and the nucleolus.

★ Must-know

• The four components of the nucleus are:

  • nuclear envelope
  • nucleoplasm
  • nucleolus
  • chromatin threads

• The nucleus performs these functions: controls cell metabolism, directs structural-protein synthesis for cell growth, participates in ribosome formation, regulates the cell cycle, carries hereditary information between generations

📌 Prokaryotes lack a nuclear membrane and have genetic material lying freely as a nucleoid, whereas eukaryotes have a well-organized nucleus enclosed by a nuclear membrane.

Further detail

• The nuclear envelope consists of two membranes separating the nucleoplasm from the cytoplasm and contains pores that exchange substances between them.

• Fontana is credited with discovering the nucleolus, which stores RNA and is usually present once per nucleus, though more than one may occur.

• Before cell division, the long, fine chromatin threads organize into chromosomes.

• Bacteria and blue-green algae are examples of prokaryotes, while plant and animal cells are eukaryotic.

📖 10. Cytoplasm and Cell Organelles

🔑 Key Concepts & Definitions

• Cytoplasm: The fluid content inside the plasma membrane but outside the nucleus, containing cytosol and cell organelles.
• Ribosomes: The smallest, membrane-less cell organelles, composed of RNA and proteins, that synthesize proteins.
• Cytosol: The aqueous, nearly transparent, structureless ground substance inside the cell.
• Cell organelles: Subcellular structures with characteristic forms, structures, and functions; almost all are membrane-bound.
• Endoplasmic reticulum: A complex network of membrane-bound channels, sheets, tubules, and vesicles found throughout the cell.

★ Must-know

📌 Eukaryotic ribosomes are 80S, whereas prokaryotic ribosomes are 70S; Svedberg units indicate sedimentation coefficient and indirectly measure density and size.

📌 Smooth endoplasmic reticulum lacks attached ribosomes and is mainly composed of vesicles and tubules, whereas rough endoplasmic reticulum bears ribosomes on its outer surface and is mainly composed of cisternae.

• The endoplasmic reticulum transports materials, especially proteins, between regions of the cytoplasm and between the cytoplasm and nucleus.

• Smooth endoplasmic reticulum helps detoxify many poisons and drugs, while rough endoplasmic reticulum synthesizes proteins.

Further detail

• Free ribosomes make proteins used inside the cell, while ribosomes attached to rough endoplasmic reticulum make proteins used inside the cell or exported from it.

• Eukaryotic ribosomes have 60S and 40S subunits, while prokaryotic ribosomes have 50S and 30S subunits.

• The endoplasmic reticulum consists of cisternae, which are long, flattened, unbranched units arranged in stacks.

• Endoplasmic-reticulum tubules are irregular, often branched, membrane-bound tubes that may be free or associated with cisternae.

• Smooth endoplasmic reticulum stores substances including steroids and is important in lipid synthesis.

• The endoplasmic reticulum provides a cytoplasmic framework and a surface for some cellular biochemical activities.

• Proteins made by rough endoplasmic reticulum and lipids made by smooth endoplasmic reticulum help build the cell membrane, a process called membrane biogenesis.

📖 11. Golgi Complex Structure and Functions

★ Must-know

📌 The Golgi complex's convex cis face points toward the nucleus, while its concave trans face points toward the cell membrane.

• The Golgi complex modifies, stores, condenses, and packages materials for cell secretion.

Further detail

• Camillo Golgi discovered the Golgi complex in nerve cells of an owl in 1898.

• The Golgi complex is located near the nucleus and is a single-membrane-bound organelle.

• Materials move through the Golgi complex from the cis face to the trans face in vesicles.

• The Golgi complex forms lysosomes, secretory vesicles, and the sperm acrosome, a vesicle containing lytic enzymes that dissolve the egg membrane during fertilization.

• The Golgi complex is a site of glycolipid and glycoprotein formation, polysaccharide synthesis for the cell wall, and cell-plate formation during plant cell division.

💡 Memory Hook

Picture the Golgi as a post office that modifies, sorts, packages, and sends cell products.

📖 12. Mitochondrial Structure and Respiration

🔑 Key Concepts & Definitions

• Lysosome: A specialized, single-membrane-bound vesicle containing hydrolytic enzymes.

★ Must-know

• Lysosomes digest foreign materials and worn-out cell organelles, helping keep the cell clean.

• When a cell is damaged, lysosomes may burst and release enzymes that digest the cell itself, which is why they are called suicidal bags.

• Mitochondria are double-membrane-bound organelles found in eukaryotic cells except mature mammalian red blood cells, and they are absent from prokaryotic cells.

• Mitochondria have a small circular chromosome, some RNA, and 70S ribosomes, and they make some of their own proteins.

• Cristae increase the inner surface area of mitochondria and bear F1 particles, also called oxysomes, which are involved in ATP synthesis.

• Mitochondria are sites of aerobic respiration and the Krebs cycle.

Further detail

• Lysosomal enzymes are produced in the rough endoplasmic reticulum, packaged into vesicles, and processed by the Golgi complex before being released in lysosomes.

• Lysosomes may digest food or break down the cell after it dies.

• A mitochondrion has a smooth outer membrane with pore-forming proteins and an inner membrane that forms folds called cristae.

• The mitochondrial matrix contains respiratory enzymes.

📖 13. Mitochondria and Plastids

🔑 Key Concepts & Definitions

• Plastids: Semiautonomous organelles in plant and algal cells that manufacture and store important compounds, including photosynthetic pigments and starch.

★ Must-know

• Mitochondria are the main sites of cellular respiration, where respiratory substrates are completely oxidized into carbon dioxide and water.

• Mitochondria are called the powerhouse of the cell because their enzymes enable complete food oxidation and release energy as ATP, which powers chemical synthesis and mechanical work.

• Chromoplasts synthesize and store pigments; their red, yellow, or orange colours result from carotene and xanthophyll, and they occur in flower petals and fruits.

• Chlorophyll in green plant parts traps solar energy from sunlight, and chloroplasts contain the enzymes required for photosynthesis.

Further detail

• Plastids have their own circular chromosome and 70S ribosomes, and they are of prokaryotic origin.

• Leucoplasts are colourless or white plastids found in plant cells not exposed to light, such as roots and seeds, and they form starch grains and store oil, starch, and proteins.

• Chloroplasts are photosynthetic plastids usually located near the surface of sufficiently illuminated cells, such as leaf palisade cells; chlorophyll gives them their green colour, and they also contain yellow-orange pigments.

• A chloroplast has a double membrane enclosing protein-rich stroma, a continuous membrane system arranged as thylakoids in grana, and grana interconnected by lamellae; thylakoids contain chlorophyll a and b and carotenoids.

💡 Memory Hook

Mitochondria release energy; chloroplasts capture light energy.

📖 14. Vacuoles and Cell Types

🔑 Key Concepts & Definitions

• Vacuoles: Membrane-bound storage sacs containing non-living liquid or solid contents.

★ Must-know

• Mature plant cells contain a large central vacuole occupying 50–90% of cell volume, whereas animal and young plant cells have small vacuoles.

• Plant-cell vacuoles contain cell sap that provides turgidity and rigidity, store amino acids, sugars, organic acids, and some proteins, and serve as a dump house for excretory products.

Further detail

• The vacuole's covering membrane is called the tonoplast.

• In Amoeba, a food vacuole contains consumed food, while specialized vacuoles in some unicellular organisms expel excess water and wastes.

• Contractile vacuoles in unicellular freshwater organisms participate in osmoregulation and excretion.

📌 Animal cells lack a cell wall and chloroplasts and generally have a round or irregular shape, whereas plant cells have a cell wall and chloroplasts and generally have a fixed rectangular shape.

💡 Memory Hook

Picture a large central vacuole filling most of a mature plant cell.

📖 15. Cell Organization and Arun Kumar Sharma

🔑 Key Concepts & Definitions

• Cell division: The process by which new cells are formed for growth, replacement of old, dead, or injured cells, and production of gametes for reproduction.

★ Must-know

• Mitosis is the division by which a mother cell forms two identical daughter cells with the same chromosome number, supporting growth and tissue repair.

• Meiosis consists of two consecutive divisions that produce four cells, each with half the mother's chromosome number; it forms gametes in animals and spores in plants.

• Prokaryotes lack a nuclear membrane and nucleoli, whereas eukaryotes have a true nucleus with a nuclear membrane and nucleoli.

• Prokaryotes lack membrane-enclosed organelles, whereas eukaryotes have them, including lysosomes, Golgi complexes, endoplasmic reticulum, mitochondria, and chloroplasts.

• Prokaryotes have 70S ribosomes and single circular DNA, whereas eukaryotes have 80S ribosomes and linear DNA organized into multiple chromosomes.

Further detail

• Crossing over during meiosis produces genetically different gametes and creates variation in offspring.

• Prokaryotic cells are typically 0.2–10 μm in diameter, whereas eukaryotic cells are typically 5–100 μm in diameter.

• When present, prokaryotic cell walls are made of peptidoglycan, whereas eukaryotic cell walls are made of cellulose or chitin.

• Arun Kumar Sharma (1924–2017), known as the Father of Indian Cytology, headed the Botany Department at the University of Calcutta and studied chromosomes and their physical and chemical nature.

📖 16. Diffusion and Osmosis

🔑 Key Concepts & Definitions

• Diffusion: The movement of molecules or ions of a substance from a region of higher concentration to a region of lower concentration.
• Osmosis: The diffusion of water molecules from a dilute solution to a concentrated solution through a semipermeable membrane.

★ Must-know

• Arun Kumar Sharma’s contributions included: developing new techniques to study chromosomes, explaining their role in plant evolution and classification, proposing new ideas about plant speciation, introducing chromosome dynamism, which describes chromosomes as flexible and important in growth and evolution

Further detail

• Arun Kumar Sharma received the Shanti Swarup Bhatnagar Prize in 1967 and the Padma Bhushan in 1983.

📌 Diffusion can occur in solids, liquids, and gases, whereas osmosis occurs only in a liquid medium.

📌 Diffusion is rapid in gases and slow in liquids, whereas osmosis is slow.

💡 Memory Hook

Diffusion can cross many media without a membrane; osmosis is water crossing a semipermeable membrane.

📖 17. Cancer Development and Treatment

🔑 Key Concepts & Definitions

• Malignant tumors: Consist of rapidly multiplying cells that grow aggressively, invade and damage nearby normal tissues, and compete with normal cells for essential nutrients.
• Metastasis: The spread of cells from a malignant tumor through the bloodstream to distant body parts, where they form a new tumor.

★ Must-know

• 🔄 Cancer development proceeds as follows: cell-growth and differentiation controls break down, cell growth becomes uncontrolled, a mass of abnormal cells called a tumor forms

📌 Benign tumors usually remain confined to their original location, do not spread to other body parts, and generally cause minimal harm.

Further detail

• Cancer-causing agents may be: physical, including X-rays, gamma rays, and UV radiation, chemical, including substances in tobacco smoke, biological

• Cancer may be detected by biopsy, in which suspected tissue is removed, stained, and examined microscopically by a pathologist, or by imaging with X-rays, CT scans, and MRI.

• The main cancer treatments are:

  • surgery
  • radiation therapy
  • chemotherapy

💡 Memory Hook

Regulatory failure → uncontrolled cell growth → tumors; malignant cells may spread by metastasis.

📖 18. Advances in Cell Biology

★ Must-know

• Mitochondrial replacement therapy uses a donor egg whose nucleus is removed, places the mother's nucleus into that egg, and fertilizes the reconstructed egg with the father's sperm.

Further detail

• Researchers grow small organ models from stem cells to study organ function and test medicines.

• Gene-editing tools are being improved to treat genetic diseases, and powerful microscopes allow scientists to observe cells working in real time.

• Scientists are studying how cells communicate, particularly in diseases such as cancer, and stem cell techniques are helping develop regenerative treatments to repair damaged organs.

• Genetically modified plants can resist pests without chemicals, as in Bt Cotton, or have increased vitamin content, as in Golden Rice.

• Mitochondrial DNA is inherited only from the mother, and mutations in it can cause diseases such as Leigh Syndrome that severely affect energy-demanding organs, including a baby's brain and muscles.

• In 2016, scientists used mitochondrial replacement therapy to help a mother carrying a mitochondrial mutation have a healthy baby.

• In the described mitochondrial replacement therapy case, the baby had the genetic identity of the mother and father and healthy mitochondria from a donor; the mother's nucleus holds 99.8% of her traits.

💡 Memory Hook

Donor egg without nucleus → maternal nucleus transfer → fertilization → healthy baby.

📊 Synthesis Tables

Animal and Plant Cell Features

Diffusion and Osmosis Compared