CELL: THE UNIT OF LIFE – Comprehensive Exam Notes

What You Need to Know

• Main topics to learn
  • The cell: how it builds and runs living things
  • Protoplasm (all living cell stuff) & cytoplasm (the jelly inside, but outside the nucleus)
  • Differences: simple cells (prokaryotic) vs. complex cells (eukaryotic), and plant vs. animal cells
  • The job of each tiny part (organelle) inside the cell: cell wall, cell membrane, nucleus, nucleolus, ER, ribosome, mitochondria, Golgi, plastids, lysosomes, centrosome, vacuole.

• Important for tests
  • Know the differences in cell walls, centrosomes, vacuoles, and plastids.
  • Remember important numbers (cell size, how many cells) and famous discoveries (Hooke, Leeuwenhoek).

What Is a Cell?

• The very basic building block and worker of all life.

• It's the smallest thing that can live on its own and do all the necessary life tasks.

• All living things start from one single cell (like an egg, a seed, or a fertilized egg).

• Cells are always being replaced: old ones die, and new ones are made by splitting.

Looking at Cells: Microscopes & History

• Antony van Leeuwenhoek (1632-1723)
  • Made about 400 simple microscopes (just one curved lens) that could magnify things up to 200\times.
  • You put your eye close to the lens and the specimen on a needle.

• Robert Hooke (1635-1703)
  • Invented a “compound” microscope with two lenses.
  • Looked at a thin slice of cork and called the tiny boxes he saw “cells” because they looked like small rooms where monks lived. These were dead cell walls.

• Modern Optical Compound Microscope (light microscope): Can magnify up to about 2000\times.

• Electron Microscope
  • Uses electron beams and magnets; can magnify more than 2\times10^{5}\times (over 200,000\times).
  • Showed us the tiny insides of cells (like ER, ribosomes) that light microscopes couldn't see.

Cell Theory (Schleiden & Schwann, 1839; Virchow, 1858)

1. The cell is the basic part that makes up living things.

2. The cell is the basic part that makes living things work.

3. All cells come from cells that already existed (Virchow added this part).

How Many Cells & How Big?

• Rule: bigger living things usually have more cells, not bigger cells.

• Types of organisms & examples
  • Single-celled: bacteria, yeast, Amoeba.
  • Few-celled (hundreds to thousands): Spirogyra, Volvox.
  • Many-celled (millions to billions): mango trees, humans.

• Human facts
  • About 10^{15} (a quadrillion) cells in the whole body.
  • About 10^{10} (10 billion) brain cells (neurons).
  • In one cubic millimeter of blood: 5–6\times10^{6} (5-6 million) red blood cells, 7\times10^{3} (7 thousand) white blood cells.

• Cell sizes, from smallest to largest
  • Bacteria: 0.3–5.0\,\mu m (micrometers, very tiny).
  • Human Red Blood Cell: about 7\,\mu m.
  • Longest cell: a single nerve cell (can go from your fingertip to your spinal cord).
  • Largest cell: an unfertilized ostrich egg (it's just one cell!).

Why Cells Are Small: Surface-Area-to-Volume Rule (SA:V)

• Small cells have a larger surface/volume ratio. This means they can take in food and get rid of waste faster and communicate better inside.

• Imagine a cube experiment:
  • If you have a cube with 2\,\text{mm} sides, its outside surface is 24\,\text{mm}^2.
  • If you cut it into 8 smaller cubes, each with 1\,\text{mm} sides, the total outside surface becomes 48\,\text{mm}^2 (twice as much!), but the total space inside (volume) stays the same.

• This helps cells efficiently exchange nutrients, oxygen/carbon dioxide, and wastes, and makes it easier to fix any damage.

Cell Shapes & Their Jobs

• Cells come in many shapes: flat discs, many-sided, long threads, branched, or even changing their shape.

• Shape matches job
  • Red Blood Cells (RBC): disc-shaped with dips in the middle to easily fit through tiny blood vessels and pick up lots of gas.
  • White Blood Cells (WBC): change shape like amoebas to squeeze through blood vessel walls and chase germs.
  • Nerve cell: has a long tail (axon) to send messages over long distances.
  • Muscle cell/fiber: long and can shorten (contract) to cause movement.
  • Guard cells: two bean-shaped cells that open and close pores (stomata) on plant leaves.

General Parts of a Cell

• Every cell has these three crucial parts:

  1. Plasma (cell) membrane: the outer skin.

  2. Cytoplasm: the jelly filling with tiny parts (organelles) and storage bits.

  3. Nucleus: the control center.

• Plants also have: a cell wall, a big central storage sac (vacuole), and plastids (like chloroplasts).

• Animals also have: a centrosome/centrioles (involved in cell division).

Plasma Membrane & Cell Wall

• Plasma membrane (cell membrane)
  • A thin, double layer made of fats and proteins (lipoprotein); it’s picky about what goes in and out (selectively permeable).
  • Has tiny holes or channels; helps animal cells keep their shape.

• Cell wall (plants only)
  • A stiff, non-living outer layer made of cellulose that lets everything pass through (freely permeable).
  • Gives the plant cell its strong shape, protects it, and is glued to other cell walls by a middle layer.
  • Natural fibers like cotton, jute, and coconut are actually the strong, dead cell walls of plants.

Cytoplasm: The Cell’s Inner Jelly

• A clear, jelly-like liquid that’s always moving; where most of the cell's chemical reactions happen.

• It holds all the tiny cell parts (“little organs” or organelles) and non-living stored materials.

Endoplasmic Reticulum (ER)

• A tangled network of double membranes that connects the nucleus to the cell membrane.

• Rough ER (RER): has tiny dots (ribosomes) on it, makes proteins that are sent out of the cell.

• Smooth ER (SER): makes fats, cleans up harmful stuff, and stores calcium (Ca²⁺).

• Works like an internal delivery system and helps support the cell (like a skeleton).

Ribosomes: Protein Builders

• Tiny granules (about 70\,\text{Å} in size) that float freely or are attached to the RER.

• They are the places where proteins are put together following instructions from messenger RNA (mRNA). They are made of rRNA and proteins.

Mitochondria: Powerhouses

• Have two membranes; the inner one is folded into ridges called cristae, which increases the surface area for energy-making reactions.

• They perform breathing (aerobic respiration) to release energy, which is stored in ATP (the cell's energy currency).

• They have their own DNA and ribosomes, meaning they can partly control themselves.

Golgi Apparatus / Dictyosomes

• Stacks of flattened sacs and small sacs filled with liquid, found near the nucleus.

• They change, package, and send out substances like enzymes and hormones; they also help form the head of sperm cells.

Lysosomes: “Suicide Bags”

• Sacs with a single membrane that contain more than 40 strong digestive enzymes.

• They break down foreign invaders, old cell parts, and stored food during starvation; they can also cause a cell to self-destruct if it's badly damaged (autolysis).

Centrosome & Centrioles (Animals only)

• A clear area in the cytoplasm with two centrioles, which are bundles of tiny tubes that sit perpendicular to each other.

• They organize the fibers that pull chromosomes apart during cell division (mitosis/meiosis).

Plastids (Plants only)

• Three types based on color
  • Leucoplasts – colorless, store starch (like in a potato).
  • Chromoplasts – yellow, orange, or red (due to pigments like xanthophyll, carotene) found in flowers and fruits.
  • Chloroplasts – green (contain chlorophyll) and perform photosynthesis (make food); they also have other hidden colors.

• Chloroplast structure: Has a double membrane, a fluid called stroma, and stacks of flattened sacs called thylakoids/grana; they have their own DNA and can copy themselves.

• Note: A chloroplast is not like a kitchen where food is cooked; it produces food using sunlight.

Non-Living Storage Parts (Inclusions)

• Vacuoles
  • Sacks filled with fluid, surrounded by a membrane (tonoplast); very large and central in plant cells; store water, salts, colors, and waste; help keep plant cells firm (turgor).

• Granules
  • Stored nutrients like starch (in plants), glycogen (in animals), and tiny fat droplets.

Nucleus: The Cell’s Control Center

• Parts of the nucleus
  • Nuclear envelope: a double membrane with pores (holes).
  • Nucleoplasm: the jelly-like fluid inside the nucleus.
  • Chromatin network: made of DNA (our genetic material) and special proteins called histones. This network bundles up into distinct chromosomes during cell division.
  • Nucleolus: a dense area made mostly of RNA; it builds the tiny parts that make up ribosomes.

• Human DNA count: We have 46 chromosomes, which is 2\times23 pairs.

• Chromosome counts in other living things: peas have 14, onions 16, lions 38, potatoes 48, maize 20, and some insects have over 1000.

• Important note: Genes (specific parts of DNA) determine traits, not just the number of chromosomes (e.g., lions, tigers, and cats all have 38 chromosomes but are very different because of their genes).

• DNA fingerprinting: A technique used in crime solving or checking parentage by looking at unique patterns in a person's DNA.

Protoplasm: All the Living Stuff

• This is a general term for all the living material inside a cell: the cytoplasm plus the nucleus.

• It's chemically very complex; its main building blocks are carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, and iron.

• It includes water, proteins, carbohydrates, fats, and minerals.

• Once taken out of a cell, it quickly loses its “living” qualities, making it hard to study completely.

Plant Cell vs. Animal Cell: What’s Different?

• Cell wall: Present in plants, absent in animals.

• Plastids: Present in plants, absent in animals.

• Vacuoles: Large and permanent in plants, small or temporary in animals.

• Centrosome/centrioles: Absent in plants, present in animals.

• Cytoplasm: In plants, it’s a thin layer pushed to the side by the large central vacuole; in animals, it’s spread out evenly.

Simple Cells (Prokaryotic) vs. Complex Cells (Eukaryotic)

• Prokaryotes (like bacteria, blue-green algae)
  • No true nucleus (their DNA is just a loop floating in an area called the nucleoid).
  • No membrane-bound organelles (like ER, mitochondria); they only have small 70S ribosomes.

• Eukaryotes (like protists, fungi, plants, animals)
  • Have a proper nucleus with a membrane and multiple chromosomes.
  • Have a complex internal membrane system, mitochondria, chloroplasts (in plants), and larger 80S ribosomes.

How Cells Make Organisms Live

• Growth: Cells increase in number and get bigger by dividing.

• Repair & Healing: Cells divide to replace lost or damaged tissues (e.g., a lizard regrowing its tail).

• Movement: Muscle cells contract, tiny hairs (cilia) beat, cytoplasm moves inside the cell; plants move by growing differently in certain directions.

• Getting Food: Glandular cells make digestive juices; intestine cells absorb food; fat cells and liver cells store food.

• Circulation & Transport: Heart muscle cells pump blood; red blood cells carry gases.

• Immunity: White blood cells eat germs or make antibodies to fight disease.

• Feeling & Responding: Nerve cells (receptors) detect things; brain cells process information; other cells act on it.

• Temperature Control: Sweat gland cells release sweat, which cools us down as it evaporates.

• Reproduction & Passing Traits: Reproductive cells carry chromosomes/genes to babies; a new organism starts from one powerful stem cell.

Ethics & Real-World Uses

• DNA fingerprinting is very important for solving crimes and settling parentage disputes.

• Stem cell therapy offers great hope but also brings up discussions about using embryonic cells versus using iPS cells.

• Understanding the surface-area-to-volume ratio helps in designing tiny medical devices and delivering medicines more effectively.

Quick Review Points

• Plasma membrane: picky gatekeeper; Cell wall: stiff, lets everything through.

• Ribosome: makes protein; Mitochondrion: makes ATP (energy); Golgi: packages and sends out stuff; Lysosome: digests.

• Chloroplast: makes food with light; Leucoplast: stores starch; Chromoplast: colors flowers/fruits.

• Centrosome: organizes cell division; Nucleolus: builds ribosomes; Chromosomes: hold genes.

• Prokaryote: no true nucleus or many organelles; Eukaryote: true nucleus and many organelles.

• Plant cell marks: cell wall, plastid, big vacuole; Animal cell marks: centrosome.