Chapter 2: Cell: The Building Block of Life

• The human eye can see two points as separate only if they are about 0.1 mm apart. This is its limit of resolution.
• Most cells are too small for the unaided eye. Light microscopes (convex lenses) magnify them. Electron microscopes use a beam of electrons and show finer detail at the nanometre scale.
• Total magnification = magnifying power of the eyepiece x magnifying power of the objective.
• Robert Hooke first saw cells in 1665 in a thin slice of cork using a self-designed microscope.

The cell membrane is a thin boundary, about 7 to 10 nanometres thick, that surrounds and protects the cell. It is also called the plasma membrane. It is selectively permeable: it lets some substances pass and blocks others.

Fluid-mosaic model: the membrane is a lipid bilayer (fat molecules with heads facing outwards and tails inwards) with proteins embedded in it. The molecules can move sideways, so the membrane is fluid. The proteins act as gatekeepers.

• Diffusion: the net movement of particles from a higher to a lower concentration. It can occur even without a membrane.
• Osmosis: the diffusion of water across a selectively permeable membrane, from a dilute solution (more water) to a concentrated solution (less water), until the concentrations are equal.
• Isotonic: solute concentration outside equals inside. No net movement of water.
• Hypotonic: solute concentration outside is lower than inside. Water enters the cell.
• Hypertonic: solute concentration outside is higher than inside. Water leaves the cell.

The cell wall is an extra covering outside the cell membrane. It is rigid and permeable, and in plants it is mainly made of cellulose. It gives plant cells their shape, holds stems and leaves upright, and stops plant cells from bursting when they take in water. Animal cells have no cell wall.

• Prokaryotic cells (for example, bacteria): no well-defined nucleus. Their DNA is in a region called the nucleoid. They lack membrane-bound organelles. Usually unicellular. About 1 to 10 micrometres.
• Eukaryotic cells (plants, animals, fungi, protists): a well-defined nucleus and membrane-bound organelles. Can be unicellular or multicellular. About 10 to 100 micrometres.

The nucleus has a double-layered nuclear membrane with pores that let material pass between the nucleus and cytoplasm. The nucleolus is where ribosomal subunits are made. Chromatin is thread-like material that becomes chromosomes when a cell divides. Chromosomes are made of DNA and proteins; the segments of DNA that carry information are genes.

• Ribosomes: the protein factories, free in the cytoplasm or attached to the ER.
• Rough ER (RER): has ribosomes on its surface. Involved in protein synthesis and secretion, for example in gland cells.
• Smooth ER (SER): no ribosomes. Involved in making and storing fats (lipids) and hormones.
• Golgi apparatus: stacks of flattened sacs. It modifies, sorts and packages proteins and lipids into vesicles. It acts as the cell's post office.

• Lysosomes: single membrane-bound sacs of enzymes that break down unwanted proteins, carbohydrates, fats and worn-out organelles. They are the clean-up system.
• Mitochondria: the powerhouse. Double membrane; the inner membrane is folded into cristae. Glucose is broken down in cellular respiration, releasing energy stored as ATP. They have their own DNA and ribosomes.
• Chloroplasts: plastids with chlorophyll. Photosynthesis happens here; the stroma holds the sugars and starch made.
• Chromoplasts: plastids with yellow, orange or red pigments; colour flowers and fruits.
• Leucoplasts: colourless plastids that store starch, oils or proteins.
• Vacuoles: a large central vacuole in mature plant cells, filled with cell sap. It stores water, minerals, sugars and waste, and keeps the cell firm. When water is lost, the plant wilts. Animal vacuoles are smaller.

• Mitosis: produces two genetically identical daughter cells, each with the same number of chromosomes as the parent. It is used for growth, repair and replacement of cells.
• Meiosis: the parent cell divides twice, producing four daughter cells, each with half the number of chromosomes. Gametes (sperm, eggs, pollen) are made this way, which gives genetic variation. In animals it occurs in the testes and ovaries; in plants, in the anthers.

• Schleiden (1838) said all plants are made of cells; Schwann (1839) said the same for animals; Virchow (1855) said new cells come only from pre-existing cells.
• Normal cells stop dividing through contact inhibition. Cancer cells lose this control, divide uncontrollably and form tumours.
• Programmed cell death (PCD) removes cells in a controlled way; for example, it removes the cells between developing fingers.

Organelle: a specialised structure in the cytoplasm. Nucleoid: region of DNA in prokaryotes. Cristae: folds of the mitochondrial inner membrane. Totipotency: the ability of a plant cell to form a complete plant.