Human Biology: Cell Structure and Function

Cell Structure and Function

• All organisms are composed of one cell (unicellular) or more cells (multicellular)
• Cells are the basic structural and functional units of life
• Cytology is the branch of biology that deals with the structure and function of cells

Classification of Cells

• Cells are classified into two groups: prokaryotes and eukaryotes
• The primary difference between them is the presence or absence of a nucleus, a membrane-bound structure that houses the DNA
• Prokaryotes:
  • Unicellular microorganisms
  • No nuclear membrane or other membrane-bound organelles
  • DNA found in the central part of the cell (nucleoid)
  • Have a cell wall like plants
  • Examples: bacteria
• Eukaryotes:
  • Cells that contain a membrane-bound nucleus and other membrane-bound compartments called organelles
  • Organelles have specialized functions
  • Examples: animals, plants, and fungi

Cell Size and Shape

• Cells vary in size and shape
• Most cells are small and require a microscope to examine
• Light microscope can see objects as small as 200 nm
• Electron microscope can see objects as small as 0.2 nm
• Different shapes of cells:
  • Cube shape (e.g. cubical cells)
  • Long column shape (e.g. goblet cell)
  • Spindle shape (e.g. smooth muscle cells)
  • Cells with long projections (e.g. nerve cells)

Protoplasm

• The living part of the cell is called protoplasm
• Protoplasm is composed of water, proteins, lipids, carbohydrates, and electrolytes
• Protoplasm consists of two distinct regions:
  1. Cytoplasm: lies outside the nucleus
  2. Nucleoplasm: lies inside the nucleus
• Cells also contain inclusions, which consist of metabolic by-products, storage forms of various nutrients, and pigments

Three Main Components of Cells

• 1. Plasma membrane: an outer membrane that separates the cell's interior from its surrounding environment
• 2. Cytoplasm: the fluid present in the cell and surrounded by the cell membrane, which contains water, enzymes, salts, and various organelles
• 3. Nucleus: a large organelle that may or may not be centrally located within the cytoplasm

Plasma Membrane Structure

• Phospholipid bilayer: a "sandwich" made of two layers of phospholipids
• Proteins: scattered throughout the plasma membrane, playing important roles in allowing substances to enter the cell
• Carbohydrates: groups attached to some of the lipids (glycolipids) and proteins (glycoproteins)

Plasma Membrane Functions

• The plasma membrane isolates the interior of the cell from the external environment (physical barrier)
• The plasma membrane allows only certain molecules and ions to enter and exit the cytoplasm freely (selectively permeable)
• Small lipid-soluble molecules, such as oxygen and carbon dioxide, can pass through the membrane easily
• Small size of water molecules allows them to freely cross the membrane by using protein channels called aquaporins

Golgi Complex

• Located near the nucleus in most cells
• Consists of many smooth membranous saccules, some vesicular, others flattened, but all containing enzymes and proteins being processed
• Functions:
  • Completes posttranslational modifications of proteins produced in the RER
  • Packages and addresses these proteins to their proper destinations

Lysosomes

• Membranous sacs produced by the Golgi apparatus
• Contain hydrolytic enzymes
• Found in all cells of the body, but particularly numerous in white blood cells that engulf disease-causing microbes
• Functions:
  • Breaks down engulfed substances into simpler subunits
  • Participates in autodigestion, breaking down parts of a cell

Mitochondria

• Often called the powerhouses or energy factories of a cell
• Responsible for making adenosine triphosphate (ATP), the cell's main energy-carrying molecule
• Mitochondria are oval-shaped, double-membrane organelles:
  • Outer membrane encloses the intermembrane space
  • Inner membrane with many folds (cristae) enclosing a gel-like matrix
• Abundant in cells or cytoplasmic regions where large amounts of energy are expended

Mechanisms of Transport across the Plasma Membrane

Bulk Transport (Vesicular Transport)

• Cells use bulk transport to move large molecules, such as polysaccharides or polypeptides, across the membrane
• Processes:
  • Exocytosis: cells secrete substances, such as secretions or enzymes, packaged into vesicles
  • Endocytosis: cells take in large molecules or other materials, including particles, liquids, and specific substances

Diffusion

• Random movement of molecules from an area of higher concentration to an area of lower concentration, until they are equally distributed
• Passive way for molecules to enter or exit a cell
• Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane

Facilitated Transport

• Many solutes do not simply diffuse across a plasma membrane
• Transported by means of protein carriers within the membrane called a transporter
• Binds only to a particular molecule, such as glucose
• Passive transport, does not require energy to move substances down their concentration gradient

Active Transport

• Moves molecules from an area of lower concentration to an area of higher concentration
• Requires a protein carrier and uses energy to pump materials into the cell against the concentration gradient
• Proteins involved in active transport are often called pumps
• One type of pump moves sodium ions (Na+) to the outside and potassium ions (K+) to the inside of the cell

Osmosis

• Special case of diffusion
• Net movement of water across a selectively permeable membrane
• Direction of water diffusion is determined by the tonicity of the solutions inside and outside the cell
• Tonicity is based on dissolved particles, called solutes, within a solution
• Three terms: hypotonic, isotonic, and hypertonic, relate the concentration of solutes inside a cell to the concentration of solutes in the fluid that contains the cells