Cell Structure & Diversity

Cells are the building blocks of all organisms. They are extremely small and require a microscope to view. Unicellular organisms (e.g. bacteria) consist of one cell; multicellular organisms are made of many cells.

Eukaryotic vs Prokaryotic Cells

Eukaryotic cells contain a nucleus and organelles (animals, plants, fungi). Prokaryotic cells lack a nucleus - their DNA is in a circular chromosome called a nucleoid (bacteria, archaea).

Animal vs Plant Cells

Feature	Animal Cell	Plant Cell
Cell wall	No	Yes (cellulose)
Chloroplasts	No	Yes
Vacuole	Small/temporary	Large, permanent
Cell membrane	Yes	Yes
Nucleus	Yes	Yes

Electron Microscopes

Light microscopes magnify up to ~400×. Electron microscopes magnify up to 1,000,000×, revealing cell ultrastructure. Transmission EM (TEM) shows internal structure; Scanning EM (SEM) shows surface detail.

Cell Membrane

Made of a phospholipid bilayer with embedded proteins. It is selectively permeable, controlling what enters and exits the cell. Marker antigens on the surface identify the cell to the immune system.

Nucleus

Contains the cell's genetic material (DNA) in the form of chromatin (which coils into chromosomes during division). Surrounded by a double membrane with nuclear pores that allow mRNA to exit.

Mitochondria

The 'powerhouse' of the cell - site of aerobic respiration. Has a double membrane; inner membrane is folded into cristae (increases surface area for ATP production). Contains its own DNA and ribosomes.

Ribosomes

Tiny organelles where protein synthesis occurs. Found free in the cytosol or attached to rough endoplasmic reticulum.

Chloroplasts (Plant cells only)

Site of photosynthesis. Contains the green pigment chlorophyll in thylakoid membranes arranged in stacks called grana. The liquid between grana is the stroma.

Vacuole (Plant cells)

Large, permanent, fluid-filled sac. Maintains turgor pressure to keep the plant upright. Stores water, sugars, and waste products.

Cell Wall (Plant cells)

Made of cellulose. Provides structural support and prevents the cell from bursting. Adjacent cell walls are joined by the middle lamella.

Levels of Organisation

Cell → Tissue → Organ → Organ System → Organism

• Tissue: A group of similar cells working together (e.g. muscle tissue, nervous tissue)
• Organ: A structure made of different tissues working together (e.g. heart, leaf)
• Organ system: A group of organs working together (e.g. digestive system)

Specialised Cells

Cells become specialised (differentiated) for specific functions. Examples: red blood cells carry oxygen; nerve cells transmit impulses; root hair cells absorb water.

Stem Cells

Stem cells are undifferentiated cells capable of dividing and developing into different cell types. They replace cells that die. Types include embryonic stem cells (can become any cell type) and adult stem cells (more limited).

Endoplasmic reticulum

The endoplasmic reticulum is a network of flattened membrane sacs continuous with the nuclear membrane. Rough ER is studded with ribosomes; proteins made on those ribosomes pass into the ER where they fold into shape and are checked for errors, then leave in transport vesicles. Cells that export a lot of protein, such as pancreas cells and antibody-producing white blood cells, are packed with rough ER.

Smooth ER has no ribosomes. It makes lipids and steroids, stores calcium ions, and detoxifies substances, which is why liver cells contain so much of it.

Golgi apparatus

The Golgi apparatus is a stack of curved, flattened sacs. Vesicles from the ER fuse with one face of the stack; inside, the proteins are modified (for example by adding sugars to make glycoproteins), sorted by destination and packaged into new vesicles that bud off the other face.

The secretory pathway

Nucleus (gene transcribed) → ribosome on rough ER (protein made) → transport vesicle → Golgi (modified and packaged) → secretory vesicle → cell membrane, where the vesicle fuses and releases its contents by exocytosis. The Golgi also forms lysosomes, vesicles of digestive enzymes. Enzymes, hormones such as insulin, and mucus all leave the cell by this route, which shows how organelles work as a coordinated system rather than in isolation.

The cell is the basic structural and functional unit of all living things. All cells arise from pre-existing cells (cell theory).

Cell Theory (Schleiden, Schwann, Virchow):
1. All living things are made of one or more cells
2. The cell is the basic unit of life
3. All cells arise from pre-existing cells

Microscopy:
• Light microscope: magnification up to ~1,500×. Can view living cells. Uses visible light and glass lenses.
• Electron microscope: magnification up to ~500,000×. Much higher resolution. Uses electron beams. Cannot view living cells (specimen must be dead and in a vacuum).

Key differences between plant and animal cells:
• Plant cells only: cell wall (cellulose), chloroplasts (photosynthesis), large central vacuole (turgor pressure)
• Animal cells only: centrioles (cell division), lysosomes (more prominent), small vacuoles
• Both have: cell membrane, cytoplasm, nucleus, mitochondria, ribosomes, ER, Golgi apparatus

Magnification is how many times larger the image is than the object. Resolution is the smallest distance between two points that can still be seen as separate. Increasing magnification without resolution just gives a bigger blur, which is why the electron microscope, using a much shorter electron wavelength, reveals detail the light microscope never can.

Calculations

Total magnification = eyepiece × objective. A ×10 eyepiece with a ×40 objective gives ×400.

Actual size = image size ÷ magnification. Keep units consistent: 1 mm = 1000 µm.

Worked example: a cell measures 20 mm on a photograph taken at ×400. Actual size = 20 ÷ 400 = 0.05 mm = 50 µm.

Preparing a slide of onion or cheek cells

1. Place a drop of water on a clean slide.
2. Peel a single layer of epidermis from an onion (or gently scrape the inside of the cheek) and lay it flat in the drop.
3. Add a drop of stain: iodine for plant cells, methylene blue for animal cells. Stain increases contrast so the nucleus becomes visible.
4. Lower the coverslip at an angle with a mounting needle. This avoids trapping air bubbles, which look like thick black circles.
5. Focus on low power first using the coarse focus, then switch to high power and use the fine focus only.

Typical sizes

Plant cell 50–100 µm, animal cell 10–30 µm, bacterium 1–5 µm, virus 20–300 nm, ribosome about 25 nm. Only the first three are visible with a school light microscope.

All cells share a cell membrane, cytoplasm, ribosomes and DNA. Everything else varies.

Feature	Animal	Plant	Bacterial	Fungal
Nucleus	Yes	Yes	No - free loop of DNA in the cytoplasm	Yes, often several per cell
Cell wall	None	Cellulose	Peptidoglycan	Chitin
Chloroplasts	No	Yes	No	No
Vacuole	Small, temporary	One large permanent vacuole	None	Small
Mitochondria	Yes	Yes	No	Yes
Ribosomes	80S (larger)	80S	70S (smaller)	80S
Nutrition	Heterotrophic	Autotrophic	Both types occur	Heterotrophic, saprophytic or parasitic

Prokaryotic or eukaryotic?

Prokaryotic cells (bacteria and archaea) have no nucleus and no membrane-bound organelles. Their DNA is a single circular chromosome, often with small extra rings called plasmids. They are small, they have 70S ribosomes and they divide by binary fission.

Eukaryotic cells (animals, plants, fungi, protists) have a true nucleus enclosed by a nuclear membrane, membrane-bound organelles, linear chromosomes wound around proteins, 80S ribosomes, and they divide by mitosis or meiosis.

Why bacterial ribosomes matter

Antibiotics such as streptomycin bind to the 70S bacterial ribosome and block protein synthesis, but leave our 80S ribosomes alone. That difference is what makes an antibiotic safe to take.

Onion epidermis (plant cells)

1. Peel a single thin layer of epidermis from the inside of an onion scale using forceps.
2. Lay it flat on a slide in a drop of water, avoiding folds and air bubbles.
3. Add a drop of iodine solution to stain the nuclei and cell outlines.
4. Lower a coverslip slowly at an angle using a mounted needle to avoid trapping air bubbles.
5. Examine first under low power, then high power. You should see regular brick-shaped cells with cell walls and stained nuclei.

Human cheek cells (animal cells)

1. Gently scrape the inside of your cheek with a clean cotton bud or sterile spatula.
2. Smear the sample onto a slide in a drop of water.
3. Add a drop of methylene blue to stain the nucleus dark blue.
4. Lower a coverslip at an angle and examine under low then high power. You should see irregular flattened cells, each with a dark stained nucleus and no cell wall.

Electron microscopy

Transmission Electron Microscope (TEM): electrons pass through an ultra-thin sliced specimen to reveal internal 2D ultrastructure such as mitochondrial cristae, nuclear pores and ribosome layout.

Scanning Electron Microscope (SEM): electrons bounce off a metal-coated specimen surface to generate high-resolution 3D images of external topography.

Common mistake

Iodine stains plant preparations; methylene blue stains animal cells. Mixing up the stains is a classic lost mark, and forgetting to lower the coverslip at an angle leaves air bubbles that students then draw as cell structures.