Chapter 7: Cell Structure and Function

Honors Biology/Chemistry

The Discovery of the Cell• In 1665, Robert Hooke used an early

compound microscope to look at a thin slice of cork, a plant material.

• Cork looked like thousands of tiny, empty chambers.

• Hooke called these chambers “cells.”• Cells are the basic units of life.

• Overtime, the discoveries of multiple scientists were summarized in a single cell theory. The cell theory states:– All living things are composed of cells.– Cells are the basic units of structure and

function in living things.– New cells are produced from existing

cells.

Prokaryotes and Eukaryotes• Cells come in a variety of shapes and

sizes.• All cells:

– Are surrounded by a barrier called a cell membrane.

– At some point they contain the nucleic acid, DNA.

• Cells are classified into two categories, depending on whether they contain a nucleus.

• The nucleus 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.

• Prokaryotic cells:– Have genetic material that is not

contained in a nucleus.

• Do not have membrane-bound organelles.

• Are generally smaller and simpler than eukaryotic cells.

• Bacteria are prokaryotes.

• Eukaryotic cells:– Contain a nucleus in which their genetic

material is separated from the rest of the cell.

– Are generally larger and more complex than prokaryotic cells.

– Generally contain dozens of structures and internal membranes.

– Many eukaryotic cells are highly specialized.

– Plants, animals, fungi, and protists are eukaryotes.

Eukaryotic Cell Structure• Structures within a eukaryotic cell

that perform important cellular functions are known as organelles.

• Cell biologists divide the eukaryotic cell into two major parts: the nucleus and the cytoplasm.

• The Cytoplasm is the portion of the cell outside the nucleus.

The Nucleus• The nucleus is the control center of the cell.

– The nucleus contains nearly all the cell's DNA and with it the coded instructions for making proteins and other important molecules.

Nuclear envelope

Nuclear pores

Nucleolus

Chromatin

The nucleolus is inside the nucleus and synthesizes ribosomes.

Ribosomes• One of the most important jobs

carried out in the cell is making proteins.

• Ribosomes are the organelles where protein synthesis takes place.

• Ribosomes are small particles of RNA and protein found throughout the cytoplasm.

• Ribosomes can be either free (floating in the cytoplasm) or attached (found attached to the folded membranes of the endoplasmic reticulum.)

Golgi Apparatus• Appears of stacks of flattened

membranes.• The Golgi Apparatus modifies,

sorts, and packages proteins and other materials from the endoplasmic reticulum for storage in the cell or secretion outside of the cell.

Endoplasmic Reticulum• There are two types of Endoplasmic

Reticulum:– Rough: Covered in ribosomes

• Synthesis of proteins – Smooth: Lacking ribosomes

• Synthesis of lipids for the cell membrane

– Both types of Endoplasmic Reticulum help to transport materials throughout the cell.

Lysosomes

• Small organelles filled with enzymes.• Lysosomes function to digest

macromolecules, old organelles, and other unwanted material.

Vacuoles• Store materials such as water,

salts, proteins and carbohydrates.

Mitochondria• Organelles that convert the

chemical energy stored in food into compounds that are more convenient for the cell to use.

Chloroplasts• Organelles that capture the energy

from sunlight and convert it into chemical energy in a process called photosynthesis.

Cytoskeleton• A network of protein filaments that helps

the cell to maintain its shape. • Also involved in movement.• Centrioles: small structures found in animal cells

that assist in cell division.

Plant vs. Animal Cells• Eukaryotic cells can be divided into Plant and

Animal cells.• Plant cells:

– Have chloroplasts– Have a rigid cell wall outside of their cell membrane.– Lack centrioles– Have a single, large vacuole (central vacuole)

• Animal cells:– Lack chloroplasts– Lack cell walls– Have centrioles– Have several small vacuoles (if any)

Cell Boundaries• All cells are surrounded by a thin,

flexible barrier called a cell membrane.

• Plant and bacterial cells also have a strong protective layer surrounding the cell membrane called a cell wall.

Cell Membrane• Regulates what enters and leaves

the cell and also provides protection and support.

• Known as a lipid bilayer because it is made of 2 layers of lipids.

• In addition to lipids, cell membranes contain protein molecules that are embedded throughout the bilayer. – These proteins act as channels and pumps that

move materials across the membrane.• Carbohydrates are found on some proteins

and help cells to identify one another.

• We say that the cell membrane is “selectively permeable” or “semi-permeable” because it controls what can enter and leave the cell.

Phospholipids• The lipids in the cell

membrane are called phospholipids. – Contain a phosphate head

and fatty acid tails.

• The “head” of the phospholipid is polar, or hydrophilic, and the “tail” is non-polar, or hydrophobic.

Cell Walls• Provide support and protection for

the cell.• Found in plants, bacteria, and fungi.

Diffusion Through Cell Boundaries

• Measuring Concentration– A solution is a mixture of two or more

substances.– The substances dissolved in the solution

are called solutes. – The concentration of a solution is the

mass of solute in a given volume of solution, or mass/volume.

• If you dissolved 12 grams of salt in 3 liters of water, the concentration would be 12g/3L, or 4 g/L.

• If you dissolved 12 grams of salt in 6 liters of water, the concentration would be 2 g/L.

• The first solution is twice as concentrated as the second solution.

Food Coloring Demo!

• Diffusion– Particles in a solution tend to move from

an area where they are more concentrated to an area where they are less concentrated. (Down the concentration gradient

– This process is called diffusion. – When the concentration of the solute is

the same throughout a system, the system has reached equilibrium.

– Animation

–Diffusion happens randomly, and does not require energy. Therefore, diffusion is called a passive process.

Cucumber Demo

• Osmosis– Some substances are too large or too

strongly charged to pass through the lipid bilayer.

– When the solute cannot pass through the membrane (the membrane is impermeable to the solute), water passes though the membrane in order to reach equilibrium.

Osmosis is the diffusion of water through a selectively permeable membrane.

Sugar is too large to fit through the selectively permeable membrane.

In order to reach equilibrium, water moves from an area of high water concentration (low solute concentration) to low water concentration (high solute concentration)

• When the concentration of water and solute are the same on both sides of a membrane, we describe the two solutions as being isotonic.

• Before osmosis occurs, we describe one solution as being hypertonic (higher concentration of solute - sugar) and one solution as being hypotonic (lower concentration of solute - sugar).

• Water moves from a hypotonic solution (low sugar and high water) to a hypertonic (high sugar and low water) solution.

Which side of the beaker is hypertonic? Hypotonic? In which direction will water move across the membrane in order to reach equilibrium?

• Facilitated Diffusion– Some large molecules can actually pass

through the membrane.– Cell membranes have substance-specific

protein channels acting as carriers allowing (or...”facilitating”) certain molecules to pass through.

– This process moves solute from high to low concentration and does not require energy.

Learning Check

• Compare and contrast the 3 types of passive transport:– Diffusion– Osmosis– Facilitated Diffusion

• Active Transport– Sometimes cells must move materials

against the concentration gradient.• From low to high solute concentration

– This process usually utilizes protein pumps throughout the membrane and requires energy.

• Larger molecules or even clumps of material may be transported by movements of the cell membrane as a whole unit.

• Endocytosis: process of taking material into the cell by folding the cell membrane inward and forming pockets.

• There are two types of endocytosis:– Phagocytosis: “Cell eating”; taking in

solid materials.– Pinocytosis: “Cell drinking”; taking in

liquid materials.

• Exocytosis– The membrane of a vesicle filled with

material fuses with the cell membrane, forcing the material out of the cell.

Learning Check

• In what two ways does active transport differ from passive transport?