Chapter 3 & 4 - Cells

Cells

This is the smallest unit of life.

Continually carry out metabolic activities essential for life. Are specialized and adapt to changing conditions. Actions and responses controlled by genes, and they vary in shape and function. Differences in shape and arrangement make different functions possible.

3 main parts of cells

1. Nucleus: contains the DNA and directs the cell’s activities

2. Cytoplasm: organelles and fluids that make up majority of the cell; between the nucleus and membrane

3. Cell membrane: boundary that encloses the cell

Cell membrane

This is selectively permeable, participates in transduction, and some cells have modified their shape in order to increase surface area. Composed of lipids, proteins, and some carbohydrates.

It consists of a double layer of phospholipids—hydrophilic heads and hydrophobic tails.

Also contains cholesterol because it is very rigid, so it provides stability to the cell. Cells actually change their cholesterol content.

Integral protein in the cell membrane

This type of protein in the cell membrane spans the width of the phospholipid layer and may protrude on one or both sides.

Transmembrane protein in cell membrane

This type of protein is an integral protein that protrudes on both sides.

Peripheral protein in cell membrane

These are temporarily attached proteins that associate with either the inside or outside of the bilayer.

Cell membrane protein functions

• Receptors and growth factors on the cell’s surface act as signal transductors

• Ion channels transport ions or molecules

• Glycoproteins have carbohydrates attached and used for cell identification (protecting them from attack by the immune system)

• Protruding into the cytoplasm to form the cytoskeleton with supportive rods and tubules

• Cellular adhesion molecules help determine a cell’s interactions with other cells.

The Cytoplasm

This makes up most of a cell’s volume and consists of a clear liquid (cytosol), a supportive cytoskeleton, and networks of membranes and organelles.

Organelles

In the cytoplasm, these perform specific functions for the cell. Include: ribosomes, endoplasmic reticulum, vesicles, golgi apparatus, mitochondria, lysosomes, and cytoskeleton.

Passive mechanisms

Mechanism of movement that requires no energy, examples include diffusion, facilitated diffusion, osmosis, filtration.

Active mechanisms

This is a mechanism of movement that requires energy (ATP) and includes: active transport, endocytosis and exocytosis.

Active transport

The Na⁺ -K⁺ ATPase Pump is an example of this, the process of moving substances from an area of low concentration to an area of high concentration through carrier molecules.

Endocytosis vs. exocytosis

This is when large substances are moved into or out of a cell without crossing the cell membrane. In ________, molecules that are too large to be transported by other means are conveyed into the cell inside a vesicle that forms from a section of the cell membrane. ________ is the movement of materials out of the cell in a vesicle that fuses with the cell membrane and opens up to the outside to secrete the material.

3 types of endocytosis

• Pinocytosis: process by which cells engulf liquids and the molecules dissolved in it and a small indentation in the cell membrane surrounds the fluid, creating a vesicle.

• Phagocytosis: process by which the cell takes in solid particles once the vesicle enters the cell it combines with a lysosome which digests its contents with enzymes.

• Receptor-mediated: process by which the cell takes in very specific molecules (ligands) that bind to specific receptors on the cell membrane to initiate vesicle formation.

The cell cycle

This is the series of changes a cell undergoes from the time it is formed until it divides.

Phases: interphase, mitosis, and cytokinesis (division of the cytoplasm).

Whether the cell progresses at certain times, checkpoints, is controlled by interaction of special proteins. A restriction checkpoint determines the fate of the cell—to continue cycle and divide, to enter a non-dividing stage as a specialized cell, or to die. Most cells do not divide continually and have a maximum number of times they can divide because of built-in “clocks” (telomeres) on the tips of chromosomes.

Interphase

This is the phase of the cell cycle before cell division begins, in which the cell grows and synthesizes new molecules, membranes, DNA, and organelles. During the G1 and G2 phases the cell grows, and structures other than DNA are duplicated. During the S phase the DNA of the cell is replicated in preparation for cell division.

Cell division

Meiosis

• Only used for egg & sperm cell production

• Ensures that mature gametes have half the normal number of chromosomes as normal body cells

Mitosis + Cytokinesis

• More common type of cell division, used to increase the number of cells for growth, development, and wound healing.

• M is the division of the nucleus

• C is the division of the cytoplasm

• Each of the two daughter cells receives an exact copy of the original cell’s DNA

Stages of mitosis

Prophase: DNA condenses into chromosomes, cytoskeletal microtubules organize into spindle fibers, nuclear membrane and nucleolus disassemble.

Metaphase: Spindle fibers move chromosomes to align midway between centrioles, which previously migrated to opposite poles.

Anaphase: Chromatids are pulled apart and now considered individual chromosomes. Cytoplasmic division (cytokinesis) begins here: constriction continues throughout telophase and pinches the newly formed cells apart.

Telophase: Chromosomes have completed moving to opposite sides of the cell. Nuclear envelope and nucleolus assemble for each forming cell.

Cell differentiation

Process by which a cell develops/specializes into a specific type of cell with specialized functions. This occurs by using different parts of the complete genome that is present in each cells, some genes are turned on and some are off.

Stem cells retain ability to divide without specialization; self-renewal. Progenitor cells are daughters of stem cells that are partially specialized. Stem and progenitor cells allow cells to retain ability to produce cells that will become differentiated cells in each area of the body.

Cell death

Some cells die :(

Apoptosis is a form of programmed cell death that is a normal part of development, rather than being derived from injury or disease. The point is to remove overgrown tissues, damaged cells, extra cells in the fetus.

Steps of apoptosis:

1. Cell becomes rounded and bulges

2. Nuclear membrane breaks down

3. Chromatin condenses and enzymes cut up the chromosomes

4. Cell shatters into pieces

5. Scavenger cells engulf and destroy fragments

DNA Deoxyribonucleic acid

This contains the genetic code, the instructions needed for the synthesis of each protein, including enzymes, required by the cell.

Gene

A _____ is a portion of a DNA molecule that contains the genetic information for making a single protein.

Exome

The portion of the genome that encodes proteins (~1.5%) is the _____. The rest of the genome regulates the activation of genes for protein synthesis.

DNA replication

This occurs during interphase. The double helix separates as the H bonds between base pairs break. Helix unwinds and strands come apart. DNA polymerase catalyzes the joining of new nucleotides to each strand, which form complementary pairs with the original strands.

Protein synthesis

DNA supplies genetic instructions for making _______. The genetic code is the sequence of base pairs in a gene that specifies the order of amino acids to make a particular ______. An amino acid is represented by a specific sequence of 3 nucleotides on DNA. There are two sequential steps to this: transcription and translation.

Transcription

The synthesis of mRNA molecules, directed by the enzyme RNA polymerase, occurs in the nucleus to create a sequence complementary to the DNA template strand. When RNA polymerase reaches the end of the gene, it releases the new mRNA strand, completing the process of ________.

Translation

mRNA with its codon moves out of the nucleus and associates with a ribosome where the protein will be constructed using the nucleotide sequence on mRNA in a process called ________. In the cytoplasm, tRNA has another 3-base sequence of nucleotides called the anticodon, which is complementary to mRNA codon. As the amino acids are joined, the new protein molecule folds into its unique shape—conformation.

A gene that has been transcribed and translated into a protein is said to be expressed.

Endoplasmic reticulum

This is an organelle which can be where ribosomes are found. It functions as a transport network in the cell, and there are two types:

• Rough - contains ribosomes and functions in protein synthesis

• Smooth - does not contain ribosomes and functions in lipid synthesis, absorption of fats, and metabolism of drugs.