The Cellular Level of Organization

Antonie van Leewenhoek - first person to see cells.

Robert Hooke coined the term “cell” and illustrated what he saw.

Cell Theory

1. All organisms are composed of one or more cells

2. Cells are alive, and are the basic living unit of organization of all organisms

3. All cells come from other cells

Every cell has three main parts:

1. Plasma (cell) membrane – a boundary that separates inside of cell from external environment.

2. A set of genetic instructions In eukaryotes this is in the Nucleus; in prokaryotes there is no membrane, stays in a section of the cell is called the nucleoid.

3. A cell body - the cytoplasm

• Cytosol – gel like substance not contained in organelles

Cells are measured in micrometers or microns.

Eukaryotic cells are bigger than prokaryotic cells.

Advantages of being made of cells:• Specialization

• Surface area

• Obtaining nutrients

• Separating chemical processes

• A multicellular organism can survive the loss of single cells.

Cytoplasm refers to cytosol plus organelles and inclusions. cytosol - contains proteins, enzymes, nutrients, ions, and other small moleculesorganelles -“little organs” - highly organized structures with characteristic shapes that are specialized for specific cellular activities. inclusions - are temporary structures in the

cytoplasm that contain secretions and storage products of the cell.

Nucleus

• Double membrane called the nuclear envelope

• Nucleoplasm

• Chromatin granules – unwound DNA

• Nucleoli – puts RNA and protein together to make ribosomes

• Nucleus is essential for cell survival

Ribosomes – made of ribosomal RNA and protein, these are the “work benches” where proteins are put together.

Made up of two subunits that come together when needed.

Found free in cells

Also in association with endoplasmic reticulum

Endoplasmic reticulum (ER)Can be rough or smooth

Smooth ER is the site of fatty acid, phospholipid and steroid synthesis. In certain cells also detoxifies chemicals, such as alcohol and pesticides.

Rough ER has ribosomes, makes proteins for export outside the cell.

May add sugars to proteins and make glycoproteins.

Gogli complex (apparatus, body)

Made of flattened sacs

Process, sort and deliver proteins and lipids to the plasma membrane and forms vesicles and lysosomes.

The carbohydrates of the glycoproteins may be modified to act as “shipping labels”

The “UPS” of the cell

Lysosomes

The cell’s “stomach” – vesicles that contain digestive enzymes.

Fuses with vesicles containing food or microorganisms.

Can be released outside the cell – “frustrated phagocytosis”

Mitochondria

Mitochondrion – singular

Two membranes – inner membrane folded. Main function is the use of oxygen to produce ATP – cellular (aerobic) respiration

These are the “power plants” of the cell.

Believed to once be free living organisms – have unique DNA and RNA

Plastids• Found in plants and some protista

• Surrounded by a double membrane

• Function photosynthesis – chloroplasts

– Contain chlorophyll

• Chromoplasts – contain red, orange or yellow pigments

• Amyloplasts – used to store starches

Cytoskeleton• Made of protein tubules

• Gives structure and support

• Also acts as muscle in moving the cell and transporting materials within it.

Musculoskeletal system of the cell.

Centrosomes and Centrioles

Centrioles are 2 cylinders of tubules arranged at right angles.

Form the microtubules of the mitotic spindle during cell division, and also make up a part of cilia and flagella

Flagella

Vesicles

Membrane sacs that transport substances within the cell.

Vesicle trafficking

Vacuoles

• Larger than vesicles

• Found in plant cells

• Can be storage areas

• Can contain enzymes or waste products

• Central vacuole helps plants maintain turgor pressure

Inclusions

Usually contain chemical substances produced by the cell, these are temporary structures that are not surrounded by a membrane.

Melanin, glycogen, triglycerides

ribosomes

Plasma MembraneThe plasma membrane does more than just separate the outside of a cell from the inside; it controls what enters and leaves the cell, and much of the activity within the cell.

Most of the cell membrane is made of a phospholipid bilayer. Amphipathic molecule – phosphate heads on the outside and inside, and fatty acid tails in the middle.

The membrane is selectively permeable – it allows fat soluble substances to pass through (such as steroid hormones) and some other small, uncharged molecules.

Cholesterol is a large molecule, and helps to stabilize the membrane in animals.

Plants use phytosterol.

Fluid mosaic model - proteins float like icebergs in a sea of phospholipids.

Proteins can be integral proteins – go all the way through the membrane, or may be peripheral proteins -bound to the inside or outside membrane.

Integral Proteins can be channels or transporters.

Peripheral proteins can be receptors, or can be cell identity markers or recognition proteins that identify a cell as “self” (like UPC codes). These are often glycoproteins.

They may also mark worn out red blood cells or cells that have been infected with a virus.

• All cells have a plasma membrane

• Some cells, such as bacterial, fungal and plant cells, also have a cell wall.

Cell Physiology

• Cell membrane function:– Are selectively permeable

• Lipids• Size• Electrical charge• Presence of channels and transporters

Movement of molecules depends on:

1. Kinetic energy

Temperature

2. Concentration

gradient – more of something in one area than another

Diffusion• Passive process

– Depends on concentration and kinetic energy– Does not require energy– Moves substances from an area of high

concentration to an area of low concentration• Down a concentration gradient

The most concentrated form of water is

pure water.

To make water less concentrated, we dissolve substances in it.

Tonicity

• Concentration of one solution relative to another

• Isotonic – equal concentrations

• Hypertonic – more concentrated

• Hypotonic – less concentrated

Tonicity – relative concentrations of solutions

• Isotonic – two solutions contain the same amount of substance dissolved in them- equal concentrations

• Hypertonic – a solution containing a greater amount of dissolved substance- more concentrated

• Hypotonic – a solution containing a lesser amount of dissolved substance – less concentrated

Osmosis• The movement of water across a

selectively permeable membrane, down a concentration gradient.

• Dialysis is the diffusion of a solute across a selectively permeable membrane.

Cell in an isotonic solution

Cell in a hypertonic solution

Cell in a hypotonic solution

Facilitated diffusion

• Passive process

• Substances can move through protein channels

• Substances may be shuttled across by carrier proteins

• Substances still move down a concentration gradient

Filtration• Movement of water and solutes across a

membrane due to physical pressure:

– Gravity

– Hydrostatic pressure (water pressure or blood pressure)

Active Transport

• Depends on the use of energy (ATP)

• Moves substances up a concentration gradient (up hill)

• These permease proteins are often called “pumps”

– Na+ / K+ pump

Vesicular Transport

Exocytosis – moves substances outside the cell

Endocytosis – takes substances into the cell

Phagocytosis – “cell eating”

Pinocytosis – “cell drinking”

Receptor mediated endocytosis

Human Immunodeficiency Virus

• Originally developed in apes or monkeys

• Probably transmitted to man in central Africa before 1931

• First cases reported 1980’s in male homosexuals

• Transmitted by sexual intercourse, sharing needles, blood products

• Incidence increasing most rapidly in

heterosexual females