Cell Structure and Function

• The cell is studied
• Lesson outline includes:
  • Eukaryotic vs prokaryotic cells
  • Plasma membrane
  • Cell size and microscopy
  • Organelles and cytoskeleton
  • Cell structure and function
  • Cellular respiration and fermentation in ATP generation

Introduction

• The cell theory is a guiding principle in biology
• The cell theory states that:
  • A cell is the smallest unit of life.
  • Cells make up all living things, from unicellular to multicellular.
  • New cells arise only from preexisting cells.
  • Eukaryotic and prokaryotic cells are the two basic types of cells

Prokaryotic Cells

• Structurally simpler and typically smaller than eukaryotic cells
• Limited to bacteria and archaea
• Some bacteria inhabit the body harmlessly, though others can cause illness
• Archaea include species that inhabit extreme environments

Eukaryotic Cells

• The cells of plants, animals, and all other organisms except bacteria and archaea
• All the cells that make up your body
• The key difference with prokaryotic cells is the presence of membrane-bound organelles
• An organelle, or "little organ", is a component within a cell performing specific functions
• Some organelles have membranes, others do not
• Non-membranous organelles like ribosomes and cytoskeletal elements are in both prokaryotic and eukaryotic cells.
• Membrane-bound organelles like mitochondria and endoplasmic reticulum are unique to eukaryotic cells

Cell Size and Microscopy

• Most eukaryotic and prokaryotic cells are measured in micrometers (µm), or 1/1000 meters (m)
• Cell size is dictated by the surface-to-volume ratio, where surface area increases more slowly than volume as the cell grows
• Nutrients enter and wastes leave a cell at its surface
  • A large cell would struggle to transport nutrients and waste across its surface area, and would die
  • A small cell has sufficient surface area for uptake and removal of substances
  • Most cells are small and require a microscope to see them
• Microscopic specimens are imaged using light or electron beams
  • Light microscopes are inexpensive and simple to operate
  • Electron microscopes are more complex and expensive, but can reveal finer details due to the shorter wavelength of electron beams

Cell Structure and Function

• A cell's structure reflects its functions
• Sperm cells are specialized for swimming and fertilizing eggs
  • They are streamlined with a whiplike tail
  • They have an enzyme-containing sac that digests a path to the egg
• Eggs are immobile, larger than typical cells, and packed with nutrients to initiate development
• Mature red blood cells extrude their nucleus and most organelles to make space for hemoglobin, the protein that transports oxygen
  • Mature red blood cells are an exception to the rule of eukaryotic cells having a well-defined nucleus

Plasma Membrane

• The plasma membrane, or outer surface, controls the movement of substances in and out of the cell
• As a cell's interior must remain balanced, molecules and ions cannot move randomly in and out
• Both prokaryotic and eukaryotic cells have a plasma membrane
  • Only eukaryotic cells also have internal membranes that divide the cell into compartments
  • Each compartment is specialized for particular functions and contains its own assortment of enzymes
  • The principles described for the plasma membrane also apply to the membranes inside the cell

Plasma Membrane Structure

• Made of lipids, proteins, and carbohydrates
• Molecules with hydrophilic (water-loving) heads and hydrophobic (water fearing) tails form a phospholipid bilayer
• Hydrophilic heads face both the extracellular fluid (interstitial fluid) outside the cell and the cytoplasm (jelly-like solution) inside the cell
• The cytoplasm includes contents between the plasma membrane and the nucleus
• Hydrophobic tails within the phospholipid bilayer point toward each other and hold the plasma membrane together
• Proteins interspersed in the phospholipid bilayer may be embedded in the membrane or span the bilayer completely
• Molecules of cholesterol are scattered throughout the bilayer
• Carbohydrates attach only to the outer surface of the plasma membrane
  • Most carbohydrates are attached to proteins, forming glycoproteins, which function in cell recognition
  • Other carbohydrates are attached to lipids, forming glycolipids
• The plasma membrane structure is described as a fluid mosaic
  • Proteins interspersed among the lipid molecules like tiles of different colors within a mosaic
  • Proteins can move sideways through the bilayer, giving the membrane a fluid quality

Plasma Membrane Functions

• Performa vital functions for the cell
• Maintains cells structural integrity by imposing a boundary between the internal and external environment, regulating the movement of substances into and out of the cell, permitting entry to some substances but excluding others
• Described as selectively permeable
• Functions in cell-cell recognition by recognizing molecules, often glycoproteins, on the cell surface
  • Membrane glycoproteins differ between species and among individuals of the same species