Cell Structure and Organelles

There are two main types of cells:

Eukaryotic cells include animal and plant cells. They have a true nucleus enclosed by a nuclear membrane.

Prokaryotic cells (such as bacteria) are smaller and simpler. Their genetic material floats freely in the cytoplasm.

A typical animal cell contains the following organelles:

Cell Membrane

• A thin, flexible layer surrounding the cell
• Controls what enters and leaves the cell (selectively permeable)
• Made of a phospholipid bilayer with embedded proteins

Cytoplasm

• A jelly-like substance filling the cell
• Where most chemical reactions take place
• Contains enzymes that control metabolic reactions

Nucleus

• Contains the cell's genetic material (DNA) in the form of chromosomes
• Controls cell activities by regulating gene expression
• Surrounded by a nuclear membrane (nuclear envelope) with pores
• Contains the nucleolus, which makes ribosomes

Mitochondria

• The site of aerobic respiration
• Produces most of the cell's ATP (energy currency)
• Has a double membrane — the inner membrane is folded into cristae to increase surface area
• The equation for aerobic respiration:

$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$ $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

Ribosomes

• Tiny structures found in the cytoplasm or on the rough endoplasmic reticulum
• Site of protein synthesis (translation)
• Smaller in prokaryotic cells ($70S$) than in eukaryotic cells ($80S$)

Plant cells have all the organelles found in animal cells, plus three additional structures:

Cell Wall

• A rigid layer made of cellulose that surrounds the cell membrane
• Provides structural support and protection
• Freely permeable (allows all molecules through)

Permanent Vacuole

• A large, fluid-filled space enclosed by a tonoplast (vacuole membrane)
• Contains cell sap — a solution of sugars, salts, and pigments
• Maintains turgor pressure, keeping the cell firm and supporting the plant

Chloroplasts

• Contain the green pigment chlorophyll
• Site of photosynthesis — converting light energy into glucose
• Have a double membrane and internal thylakoid membranes (stacked into grana)

$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$

Bacterial cells are much simpler than eukaryotic cells:

• Cell wall — made of peptidoglycan (not cellulose)
• Cell membrane — controls entry and exit of substances
• Cytoplasm — where chemical reactions occur
• Circular DNA — a single loop of DNA (not enclosed in a nucleus)
• Plasmids — small, extra rings of DNA that can carry genes for antibiotic resistance
• Ribosomes — smaller ($70S$) than eukaryotic ribosomes
• Flagellum (plural: flagella) — a tail-like structure for movement (not all bacteria have this)
• Slime capsule — a protective outer layer found in some bacteria

Many cells are specialised — they have specific structures adapted for their function:

• Sperm cell: Streamlined shape, many mitochondria (energy for swimming), acrosome (enzymes to penetrate egg)
• Red blood cell: Biconcave disc shape (large surface area), no nucleus (more room for haemoglobin)
• Nerve cell (neuron): Long axon for carrying electrical impulses, branched endings (dendrites) for connections
• Root hair cell: Long hair-like extension (increases surface area for water absorption)
• Palisade mesophyll cell: Packed with chloroplasts near the top of the leaf (maximum photosynthesis)

Cells are measured in micrometres (μm):

$1 \text{ mm} = 1000 \text{ μm}$ $1 \text{ μm} = 1000 \text{ nm}$

Typical sizes:

• Most animal cells: 10–30 μm
• Most plant cells: 10–100 μm
• Bacteria: 0.2–5 μm
• Viruses: 20–300 nm (much smaller than cells — not visible with a light microscope)

Question: A student observes a plant cell under a microscope. The image of the cell measures 30 mm across, and the magnification used is $\times 500$. What is the actual size of the cell?

Solution:

Using the magnification formula:

$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$

Rearranging:

$\text{Actual size} = \frac{\text{Image size}}{\text{Magnification}} = \frac{30 \text{ mm}}{500} = 0.06 \text{ mm} = 60 \text{ μm}$

The actual cell is 60 μm across, which is within the typical range for a plant cell.

• Always use the correct terminology — say "cell membrane" not just "membrane"; say "permanent vacuole" for plant cells.
• Remember: Plant cells have ALL the organelles that animal cells have, PLUS cell wall, chloroplasts, and permanent vacuole.
• Don't confuse the cell wall with the cell membrane — the wall provides support; the membrane controls transport.
• Prokaryotic = no nucleus. Eukaryotic = has a nucleus.

• Cells are the basic units of life; eukaryotic cells have a nucleus, prokaryotic cells do not.
• Animal cells contain a cell membrane, cytoplasm, nucleus, mitochondria, and ribosomes.
• Plant cells have all of the above plus a cell wall, chloroplasts, and a permanent vacuole.
• Bacterial cells have no nucleus; DNA is in a circular chromosome plus plasmids.
• Specialised cells are adapted for particular functions through structural modifications.
• The magnification formula $M = \frac{I}{A}$ relates image size, actual size, and magnification.