Biology Chapter: Cells and Body Fluids

Study Notes

The cell is the basic living unit of the body.
Organs are aggregates of many different cells held together by intercellular structures.
Each cell type is adapted to perform specific functions.
Despite differences, all cells have common characteristics, including reproduction.

About 60% of the adult human body is fluid, primarily water, with dissolved ions and other substances.
Most of this fluid is intracellular (inside cells).
A third of the fluid is extracellular (outside cells).
Extracellular fluid contains ions and nutrients needed by cells for life.

Cells can live, grow, and function if the proper concentrations of oxygen, glucose, ions, amino acids, fatty substances, and other constituents are available in the extracellular fluid.

Cells consist of two main parts: the nucleus and cytoplasm.
The nucleus is separated from the cytoplasm by the nuclear membrane.
The cytoplasm is separated from surrounding fluids by the plasma membrane.
Protoplasm is the collective name for the substances that make up a cell.
Protoplasm is mainly composed of water, electrolytes, proteins, lipids, and carbohydrates.

Important ions in cells include potassium, magnesium, phosphate, sulfate, bicarbonate, sodium, chloride, and calcium.
Ions provide inorganic chemicals for cellular reactions and are essential for electrochemical impulses in nerve and muscle fibers.

Structural proteins are filaments, which are polymers of individual protein molecules.
These filaments form micro-tubules crucial for the cytoskeletons of organelles, such as cilia and nerve axons and are important components of the mitotic spindles.
Extracellularly, they form collagen and elastin, essential for connective tissue, blood vessel walls, tendons, ligaments, etc.
Structural proteins form the main component of tendons, cartilage, and bones and include collagen and keratin.

Functional proteins are mainly enzymes that are mobile in the cell fluid and often attached to cell membrane structures.
Enzymes catalyze cellular reactions, like glucose breakdown to produce energy.

Lipids are another major cell component grouped by their fat solubility.
Important lipids include phospholipids and cholesterol, that comprise only about 2% of the total cell mass.
Phospholipids and cholesterol are primarily insoluble in water. Thus, they form cell membrane barriers separating cell compartments.
Some cells contain large quantities of triglycerides (neutral fat), which are major energy stores.

Carbohydrates have a minor structural role in the cell, but play a major role in cell nutrition.
Dissolved glucose is readily available to cells in the extracellular fluid.
Glycogen, an insoluble glucose polymer, is a reserve energy store in cells.

The cell membrane (plasma membrane) encloses the cell and is a thin, elastic structure (7.5-10 nanometers thick).
Its primary composition is proteins (55%), phospholipids (25%), cholesterol (13%), other lipids (4%), and carbohydrates (3%).
The membrane structure primarily consists of a lipid bilayer with hydrophobic and hydrophilic portions.
Hydrophilic phosphate portions form the two surfaces of the membrane in contact with intracellular and extracellular water.
The lipid layer in the middle is impermeable to typical water-soluble substances.
Fat-soluble substances (oxygen, carbon dioxide, alcohol) easily penetrate this layer.
Cholesterol molecules are fat-soluble and affect the fluidity and permeability of the membrane.

Most membrane proteins are glycoproteins (proteins with sugars attached).
Two protein types are: integral (penetrating the entire membrane) and peripheral (attached to one membrane surface).
Integral proteins often act as channels or carriers to move substances across the membrane.
Some integral proteins function as receptors for water-soluble hormones.

Membrane carbohydrates are usually combined with proteins (glycoproteins) or lipids (glycolipids).
These carbohydrates often extend outwards from the cell surface, forming a glycocalyx.
The glycocalyx plays roles including: negative surface charge repulsion, cell adhesion, receptor binding for hormones, immune reactions and tissue development.