chapter 04

A tour of the cell

Living organisms are made of cells , with or without a nucleus.
- Cell theory
  1. All living organisms are made up of one or more cells.
  2. All cells arise from pre-existing cells.
- Every cell on earth falls into one of two basic types:
  An eukaryotic cell typically
  - has a central organelle called a nucleus which contains the cell's DNA.
  - has other specialized organelles to perform various cellular functions.
  - is bigger than prokaryotes.
  A prokaryotic cell typically
  - does not have a nucleus to house its DNA.
  - does not have other specialized structures called organelles.
  - is smaller than eukaryotes
- Prokaryotic cells such as bacteria lack specialized structures called organelles.
  Like all cells, a prokaryote have
  - a plasma membrane surrounding the cell
  - a semifluid cytoplasm interior
  - the genetic molecule DNA
  - ribosomes that make proteins
The plasma membrane that encloses all cells consists of a phospholipid bilayer .
- All cells possess a plasma membrane.
  The membrane is a double layer (bilayer) of phospholipids.
  Hydrophilic "heads" on the exterior are attached to a hydrophobic interior made of hydrocarbon "tails".
  Proteins and other molecules embedded in the membrane provide unique functions.
- Various proteins embedded in the plasma membrane serve many functions; for example, carbohydrate chains may bind to a protein to form a glycoprotein that serves as an identification tag, unique to each type of cell.
  Cholesterol molecules are important components of animal plasma membranes, keeping the membrane fluid.
The nucleus contains the hereditary material DNA and serves as the genetic control center.
- The nucleus is composed of a double membrane and contains the hereditary material DNA, which is linear in form as opposed to the circular DNA of prokaryotes.
  The nucleus contains a structure called the nucleolus where ribosomes are made.
  Pores are openings through the nuclear membrane and allow for exchange of material between the nucleus and the cytoplasm.

The endomembrane system forms a connected network of membranous organelles within the cell.

The endomembrane system is a set of membranes spreading out from the nuclear membrane to the plasma membrane.
The rough and smooth endoplasmic reticulum and the Golgi apparatus play major roles in the production of macromolecules.
Vesicles transport molecules within the endomembrane system and may eventually fuse with the plasma membrane, secreting material by exocytosis.
Summary


The endoplasmic reticulum (ER) comprises folded `membranes` continuous with the nuclear membrane.

Rough ER has embedded ribosomes and is involved in protein synthesis.	Smooth ER has no ribosomes and is involved in lipid and carbohydrate synthesis as well as detoxification of chemicals.

The Golgi apparatus is a stack of membranes that processes and packages material made in the ER for transport elsewhere, such as the plasma membrane.
Molecules are transported into and out of the apparatus via vesicles that fuse with its membranes.
Vesicles can transport the material to other organelles, or fuse with the plasma membrane to secrete material outside the cell.

The mitochondrion is the site of cellular respiration , providing energy for the cell.
- Cells convert energy from food into the energy storage molecule ATP in a series of chemical reactions called cellular respiration.
  These reactions take place in mitochondria.
  A mitochondrion has a double membrane where enzymes involved in cellular respiration are embedded.
  Liver and muscle cells that require a lot of energy are rich in this organelle.
  Note that mitochondria possess their own circular DNA.
Plant cells have several structures not present in animal cells.
- The chloroplast is the site of photosynthesis .
  - The chloroplast is the site for photosynthesis: converting the energy of light into sugar.
    The chloroplast has a double membrane, within is a series of folded membranes called thylakoids where photosynthesis occurs.
    The interior is bathed in a semifluid stroma.
    Note that chloroplasts possess their own circular DNA.
- A central vacuole provides storage.
  - A vacuole (often called central vacuole) stores water and dissolved substances in plant cells.
    It is usually the largest observable organelle in a plant cell.
- Cell walls support and protect the cell.
- Plant cell walls are composed of the polysaccharide cellulose.
  The strong fibers provide protection and support for plant cells, and are difficult to digest for most organisms.
The interior of the cell is supported by a network of protein fibers called the cytoskeleton .
- The cytoskeleton is made of protein fibers that provide support and transport for the cell.
  The cytoskeleton contains microtubules, which form railroad-like "tracks" along which cellular material can be moved.
Materials pass into and out of the cell passively through diffusion and osmosis .
- Diffusion is the mixing process that spreads molecules across a fluid medium.
  Solute molecules move from areas of high concentration to areas of low concentration (a gradient).
  Diffusion down the concentration gradient continues until movement in both directions are equal (equilibrium).
  Review:
- Osmosis is the diffusion of water through a membrane.
  The plasma membrane is impermeable to some dissolved substances (solutes).
  Water will diffuse to the side of higher solute concentration.
  Review:
- An isotonic solution contains equal concentration of solutes outside the cell as inside the cell. There is no net movement of water.
  A hypotonic solution has lower concentration of solutes outside the cell than inside; water moves into the cell.
  A hypertonic solution has higher concentration of solutes outside the cell than inside; water moves out of the cell.
Active transport requires energy to move substances up (against) concentration gradients.
- Active transport
  A protein called a pump uses energy to move solutes up a concentration gradient across a membrane.
  A proton pump uses the energy of ATP molecules to move H⁺ ions (protons) into the stomach to keep its pH acidic.
Large substances move into and out of the cell through endocytosis and exocytosis without crossing the plasma membrane.
- Endocytosis is the cellular intake of material by folding plasma membrane around it, forming a vesicle.
  The material can be large molecules that cannot pass through the plasma membrane, or larger particles such as a bacterium, virus or organelle.
- Exocytosis is the discharge of material from vesicles at the cell surface.
  Molecules are packaged in transport vesicles whose membranes fuse with the plasma membrane.
  The contents (including hormones such as insulin) are then released (secreted) to the outside.
  Review: