Chapter 1

What is a Cell?By

Benjamin Lewin

1.1 Introduction

• Cells arise only from preexisting cells.

• Every cell has genetic information whose expression enables it to produce all its components.

• The plasma membrane consists of a lipid bilayer that separates the cell from its environment.

1.2 Life began as a self-replicating structure

• The first living cell was a self-replicating entity surrounded by a membrane.

1.3 A prokaryotic cell consists of a single compartment

• The plasma membrane of a prokaryote surrounds a single compartment.

• The entire compartment has the same aqueous environment.

• Genetic material occupies a compact area within the cell.

• Bacteria and archaea are both prokaryotes but differ in some structural features.

1.4 Prokaryotes are adapted for growth under many diverse conditions

• Prokaryotes adapted to many extreme environmental conditions

• This highlights the variations that are possible in constructing living cells.

1.5 A eukaryotic cell contains many membrane-delimited compartments

• The plasma membrane of a eukaryotic cell surrounds the cytoplasm.

• Within the cytoplasm there are individual compartments, each surrounded by a membrane.

• The nucleus is often the largest compartment within the cytoplasm – It contains the genetic material.

1.5 A eukaryotic cell contains many membrane-delimited compartments

1.6 Membranes allow the cytoplasm to maintain compartments with distinct

environments

• Organelles that are surrounded by membranes can maintain internal milieus that are different from the surrounding cytosol.

1.7 The nucleus contains the genetic material and is surrounded by an envelope

• The nucleus is the largest organelle in the cell.

• It is bounded by an envelope consisting of a double membrane.

• Genetic material is concentrated in one part of the nucleus.

• Nuclear pores provide the means for transport across the envelope for large molecules to enter or leave the nucleus.

1.7 The nucleus contains the genetic material and is surrounded by an envelope

1.8 The plasma membrane allows a cell to maintain homeostasis

• Hydrophilic molecules cannot pass across a lipid bilayer.

• The plasma membrane is more permeable to water than to ions.

• Osmotic pressure is created by ionic differences between the two sides of a membrane.

• The plasma membrane has specific systems for transporting ions and other solutes into or out of the cell.

1.8 The plasma membrane allows a cell to maintain homeostasis

• The transport systems allow the cell to maintain a constant internal environment that is different from the external milieu.

• Ion channels are proteinaceous structures embedded in membranes.– They allow ions to cross the membrane while

remaining in an aqueous environment.

1.8 The plasma membrane allows a cell to maintain homeostasis

1.9 Cells within cells

• Organelles bounded by envelopes probably originated by endosymbiosis of prokaryotic cells.

1.10 DNA is the cellular hereditary material, but there are other forms of hereditary information

• DNA carries the genetic information that codes for the sequences of all the proteins of the cell.

• Information can also be carried in cellular structures that are inherited.

1.11 Cells require mechanismsto repair damage to DNA

• The genetic material is continually damaged by:– environmental forces – errors made by cellular systems

• Repair systems to minimize damage to DNA are essential for the survival of all living cells.

1.12 Mitochondria are energy factories

• All living cells have a means of converting energy supplied by the environment into the common intermediate of ATP.

1.13 Chloroplasts power plant cells

• Plastids are membrane-bounded organelles in plant cells.

• They can develop into chloroplasts and other specialized forms.

1.14 Organelles require mechanisms for specific localization of proteins

• All organelles import proteins from the cytosol.

1.15 Proteins are transported to and through membranes

• Proteins are transported into organelles through receptor complexes embedded in the organelle’s membrane.

• Proteins are released into the cytosol after synthesis.

• For the endoplasmic reticulum, proteins are transferred into the receptor complex on the ER membrane during synthesis.

• Proteins then associate with the nucleus, or an organelle, such as:– Mitochondria– Chloroplasts

1.15 Proteins are transported to and through membranes

1.16 Protein trafficking moves proteins through the ER and Golgi apparatus

• All proteins that are localized in the – ER– Golgi apparatus– plasma membrane

initially associate with the ER during synthesis.

• Proteins are transported from one compartment to another by membranous vesicles.

• The vesicles bud from one membrane surface and fuse with the next.

• Proteins are transported into the cell from the exterior by vesicular transport in the reverse direction.

1.16 Protein trafficking moves proteins through the ER and Golgi apparatus

1.17 Protein folding and unfolding is an essential feature of all cells

• Protein conformation is a consequence of primary sequence.

• But often it cannot be achieved by spontaneous folding.– It requires assistance from chaperones.

1.18 The shape of a eukaryotic cell is determined by its cytoskeleton

• The eukaryotic cell cytoskeleton is an internal framework of filaments, including: – Microtubules– Actin filaments– Intermediate filaments

• It provides an organizing template for many activities, including anchoring organelles in place.

1.19 Localization of cell structures is important

• Localization of certain structures at specific positions in a cell may be part of its hereditary information.

• Positional effects are important in early development.

1.20 Signal transduction pathways execute predefined responses

• Events on the outside of the cell can trigger actions inside the cell by using receptor proteins embedded in the membrane.

• A receptor spans the membrane and has domains on both the exterior and interior.

• The receptor is activated when a ligand binds to the exterior domain.

• Ligand binding causes a change in the structure or function of the interior domain.

1.20 Signal transduction pathways execute predefined responses

1.21 All organisms have cells that can grow and divide

• The simplest form of division is shown by some organelles where the membrane is pinched inward.

• Bacteria often divide by growing a rigid septum across the cell as an extension of the cell wall.

• During mitosis, eukaryotic cells are extensively reorganized.

• They form the specialized structure of the spindle. – It partitions the chromosomes to daughter cells.

1.21 All organisms have cells that can grow and divide

1.22 Differentiation creates specialized cell types, including terminally differentiated

cells

• A multicellular organism consists of many different cell types that are specialized for specific functions.

• Many differentiated cells have lost the ability to divide and/or to give rise to cells of different types.

• Stem cells have the potential to divide to generate the many different types of cells required to make: – an organism – or a tissue of an organism

1.22 Differentiation creates specialized cell types, including terminally differentiated cells