Cell Structure and Function

The key to every biological problem must finally be sought in the cell, for every living organism is, or at some time has been, a cell. E.B. Wilson, 1925

Why Study Cell Biology?

Cells are Us

Cells are Us

Cilia on a protozoan Sperm meets egg

Cells are Us

A person contains about 100 trillion cells. That’s 100,000,000,000,000 or 1 x 1014 cells.

There are about 200 different cell types in mammals (one of us).

Cells are tiny, measuring on average about 0.002 cm (20 um) across. That’s about 1250 cells, “shoulder-to-shoulder” per inch.

nerve cell

Red and white blood cells above vessel-forming cells.

Discovery of the Cell

• Englishman Robert Hooke in 1665 used an early compound microscope to look at cork

• What is cork?– Plant material – therefore has plant cells

• Called them “cells” because they reminded him of a monastery’s rooms, called cells

• Anton van Leeuwenhoek in 1674 observed pond life in a microscope

The Cell Theory

All organisms are composed of one or more cells.

Cells are the smallest living things.

All organisms living today are descendents of an ancestral cell.

Cells arise only by division of previously existing cells.

The cell theory (proposed independently in 1838 and 1839) is a cornerstone of biology.

Schleiden

Schwann

A Sense of Scale and Abundance – Bacteria on the Head of a Pin

Two Fundamentally Different Types of Cells

A prokaryotic cell

A eukaryotic cell

Us vs. Them -Eukaryotes and Prokaryotes

Eukaryotic Cells

• Eukaryotic cell can be likened to a factory

• There are many structure in eukaryotic cells.

• These are called organelles – “little organs”

• Two major parts of cell:– Nucleus– Cytoplasm – portion of cell outside nucleus

Major Divisions of the Eukaryotic Cell

An Idealized Animal Cell

The Nucleus

• The nucleus (plural: nuclei) is a large membrane-enclosed structure that contains the cell’s genetic material in the form of DNA.

• The nucleus controls many of the cell’s activities.

• Eukaryotes are cells that contain nuclei.

• Prokaryotes are cells that do not contain nuclei.

A rat liver cell (with color enhancement to show organelles)

It’s Crowded In There

An artist’s conception of the cytoplasm - the region of a cell that’s not in the nucleus or within an organelle.

A micrograph showing cytoskeleton (red), ribosomes (green), and membrane (blue)

It’s Crowded In There

Animal and Plant Cells Have More Similarities Than Differences

Cellular Anatomy

We’ll start by seeing what role these parts play in making and moving proteins.

The Nucleus Think of the nucleus as the cell’s control center.

Two meters of human DNA fits into a nucleus that’s 0.000005 meters across.

• Contains nearly all of cell’s DNA – the instructions for making proteins• Surrounded by nuclear envelope• Most cells have one nucleus• RBCs have 0• Skeletal muscle cells have many

Two Other Unique Features of Plant Cells

The central vacuole may occupy 90% of a plant cell.

The Nucleus• Granular material is called

chromatin – DNA bound to protein

• When cell divides, chromatin condenses to form chromosomes which pass genetic information on to new cells

• Nucleolus - Dense region of nucleus where ribosomes are assembled

Ribosomes and the Endoplasmic Reticulum

Ribosomes

• Ribosomes – sites of protein assemblage

• Follow instructions from nucleus

The Rough Endoplasmic Reticulum

Protein movement (trafficking)

Protein synthesis (about half the cell’s proteins are made here).

Protein “proofreading”

Functions:

Golgi Apparatus

• Proteins produced in ER then move into Golgi Apparatus.

• Function – modify, sort, package proteins

• Proteins then transported to elsewhere in the cell or outside the cell

The Lysosome

Cell suicide – programmed cell death

Recycling cellular components

Functions:

Digesting food or cellular invaders

Many Diseases are Caused by Lysosome Malfunction

Vacuoles

• Vacuole – saclike structure that stores materials such as proteins, salts, water, and carbohydrates

• Many plants have a single, large vacuole filled with liquid, allowing for turgor

The Central Vacuole Controls Turgor Pressure

flaccid

turgid

The Mitochondrion

Think of the mitochondrion as the powerhouse of the cell.

Both plant and animal cells contain many mitochondria.

(Mitochondria is the plural of mitochondrion)

Mitochondria• Mitochondria – convert

chemical energy stored in food into compounds the cell can use

• Inner and outer membrane• Inherited from mother• Cells that need more

energy will have more mitochondria (white meat vs. dark meat)

Cells In a Leaf

The Chloroplast

Think of the chloroplast as the solar panel of the plant cell.

Only plants have chloroplasts, but animals reap the benefits too.

Chloroplast• Chloroplasts – organelles

that capture the energy from sunlight and convert it into chemical energy in a process called photosynthesis

• Two membranes• Plants and a few other

organisms contain chloroplasts

Two Other Unique Features of Plant Cells

The central vacuole may occupy 90% of a plant cell.

A Consequence of Cell Walls – the Great Strength of Woody Plants

The name is misleading. The cytoskeleton is the skeleton of the cell, but it’s also like the muscular system, able to change the shape of cells in a flash.

The Cytoskeleton

An animal cell cytoskeleton

A Cytoskeleton Gallery

A white blood cell using the cytoskeleton to “reach out” for a hapless bacterium.

The Cytoskeleton in Action

The Cytoskeleton in Action

Cilia on a protozoan Beating sperm tail at fertilization

Smoker’s cough is due to destruction of cilia linking the airways.

The cytoskeleton is a network of protein filaments that helps the cell maintain it’s shape and aids in cell movement.

Type of filament Shape Purpose1. Microtubules hollow tubes maintain shape and

act as railroad tracks

2. Microfilaments long, thin fibers support/movement

Cell Membrane• Forms outer boundary

of cell• Separates contents from

environment• Made mainly of lipids

and proteins• Regulates movement

into and out of the cell

Membrane Structure• Lipids consist of

phosphate heads that are hydrophilic

• Tails consist of fatty acids that are hydrophobic

• Two layers in all

• Proteins imbedded in and on membrane

• Together they are called the “Fluid Mosaic Model”

Membrane Proteins

• Channel proteins – go through membrane and allow for passage into and out of cell

• Receptor proteins – receive information from other cells (hormones)

• I.D. proteins – identify whose cells and what type of cells

• Carrier protein – transmit material that is too large into and out of cell (facilitated diffusion)

Membrane Movements

• Equilibrium – when the concentration of a solute is the same throughout the system

• Diffusion – movement of solute from high to low concentration

• Heat, size, concentration, solubility all affect rate

• Osmosis – water diffusion through a selectively permeable membrane

Cellular Movements

• Facilitated Diffusion – still diffusion (w/o energy input) but must have a carrier protein to get through

• Think of needing to have a ticket to go to a concert

Cellular Movements• Active Transport –

carrier-mediated facilitated diffusion that requires ATP

• Goes against the conc. Gradient or from lo – hi

• Because particles are going the “wrong way” energy is required in the form of ATP

Sodium – Potassium Pump

• Simultaneously carries Na+ ions out of and K+ ions into the cell

• ATP provides energy to move 3 sodium ions out and 2 potassium ions in against gradients

• Allows for normal transmission of impulses by nerve cells

Membrane Movements• Endocytosis – engulf particle

into cell; nutrient acquistion• Phagocytosis – “cell eating”;

endocytosis of large particles such as bacteria; protective mechanism

• Pinocytosis – “cell drinking” gulping droplets of extracellular fluid; routine absorption

• Exocytosis – cell products and waste released from cell

• Biologists use levels of organization to make it easier to identify and classify the cells in a particular organism.

• Levels of Organization• 1 . Cells - division of labor for organism• 2. Tissues- similar cells performing a particular

function• 3. Organs- multiple tissues working together; • 4. Organ systems- multiple organs working

together• 5. Organism – living individual

Cells

Tissues

Organ

Organ System

Organism