Chapter 2 Cell Structure and Function

Cells

Basic unit of life that makes up living organisms

What are the substances cells need to survive made of?

• Cells require the 4 biomacromolecules, inorganic ions and water to survive

• They are composed of: Nitrogen, hydrogen, carbon and oxygen compounds

Nutrient

Essential chemical substances (EG biomacromolecules) that cells absorb to fuel metabolism, generate energy (ATP), repair structures, and support growth

Unicellular organisms

Living things made up of only one cell

Multicellular organisms

Organisms composed of multiple cells

Prokaryotes

Single-celled organisms (bacteria and archea) that are simple lack a nucleus and membrane bound organelles

Prokaryotic Cells

Cells without a nucleus, membrane-bound organelles with DNA in a singular circular chromosone and a cell wall

Eukaryotes

Single and multicellular organisms that have a nucleus and membrane-bound organelles

Eukaryotic cells

Larger, complex cells with a nucleus, membrane bound organelles with DNA linear and bound to histone proteins in chromosomes

Cell Theory

1. All living organisms contain at least one cell
2. New cells are produced from existing cells
3. Cells are the smallest structural unit of life

Common features for all cells

1. Plasma membrane: Contains cell contents and seperates cell from external environment
2. Cytoplasm: Everything inside plasma membrane (fluid in cytoplasm called cytosol)
3. Ribosomes: Protein factories
4. Nucleic acids: DNA and RNA

Bacteria (prokaryotic cells)

Single-celled organisms that lack a nucleus and membrane-bound organelles and have specialised structures that allow them to move around, attach and exchange genetic material

Specialised structures of Bacteria

Flagella: Tail-like extensions which allow for movement
Pili: Hollow cellular extensions that allow for genetic transfer
Fimbrae: Smaller, hair like type of pili important for surface attachment

SA:V

Concept that helps us understand the limitations of cell size and the need for internal organelles with specific functions

Surface Area

The total area of the cell membrane available for exchange of substances with the environment

Volume

The total space inside the cell (cytoplasm and nucleus)

Small cells

larger surface area to volume ratio, advantageous for exchanging waste and nutrients with external environment

Big cells

Smaller surface area to volume ratio, disadvantageous for exchanging waste and nutrients with external environment

Importance of organelles

- Provide separate compartments to allow for specialisation
- Reduce amount of energy and resources needed
- Complete processes essential for cell function

Organelles

Structures specialized that perform specific processes within a cell

Cell components in plant and animal cells

- Nucleus
- Endoplasmic return
- Golgi bodies
- Cytoplasm
- Cell membrane
- Mitochondria
- Ribosomes
- Vesicles
- Cytoskeleton

Plant cell only components

- Chloroplasts
- Plant vacuole
- Cell wall
- Plastids

Animal cell components

- Nucleus
- Endoplasmic return
- Golgi bodies
- Cytoplasm
- Cell membrane
- Mitochondria
- Ribosomes
- Vesicles
- Centrosome
- Cytoskeleton

Bacteria cell components

- Cell wall
- Ribosomes
- Plasma membrane
- Cytoplasm
- Flagella
- Fimbriae
- Pili

What is the structure of the plasma membrane?

- Selectively-permeable phospholipid bilayer
- Flexible, fluid structure
- Made up of phospholipids, cholesterol, proteins and carbohydrates

What is the function of the plasma membrane?

- Separate the cell's internal and external environment
- Control entry and exit of molecules based on size, chemical properties, and concentration
- Allows cell signaling and communication

Plasma membrane

- A selectively-permeable phospholipid bilayer that surrounds all cells
- Protects the cell from the external environment
- Enables nutrient and waste exchange

Phospholipids

- Arranges itself into bilayer due to chemical properties
- Forms the membrane, creates a selective barrier
Hydrophilic head: Polar head arranged to face extracellular environment and cytoplasm
Hydrophobic tail: Non-polar tails arranged to make the middle layer

How does cholesterol affect fluidity?

- Regulates fluidity and reduces permeability
- Embedded into the membrane
- Adapts to changing conditions EG Prevents membrane condensation during low temps

What do peripheral proteins do in the plasma membrane?

- Loosely attach to the edges of the membrane
- Important for speeding up reactions, cellular communication and signal transduction

What do integral proteins do in the plasma membrane?

- Some are transmembrane proteins
- All are embedded into the membrane
- Important for molecule transport and signal transduction

How do carbohydrates fit into the membrane?

- Chains of glucose molecules (carbs)
- Attach to form glyco proteins/lipids
- Important for cellular communication and maintaining the membrane structure

Nucleus

Structure:
- Has nuclear membrane (double membrane) with pores allowing the movement of substances
- Largest organelle centrally located in cells
(Plant cell nucleus moved outwards = vacuole)
- Outer membrane of nucleus is continuous with the endoplasmic reticulum
Function:
Contains genetic material (DNA)

Cisternae

Flattened, hollow membrane-bound sacks found in the golgi bodies and endoplasmic return

Endoplasmic return

- An extension of nucleus consisting of a network of cisternae
- Contains enzymes involved in synthesising and transporting different molecules
- 2 types: Rough and smooth

Rough endoplasmic return

- Contains embedded ribosomes
- Produces proteins that are modified, packaged and transported through the cisternae
- Then transported as vesicles to golgi bodies or other organelles

Smooth endoplasmic return

- Doesn't contain ribosomes
- Synthesises lipids and steroids which are modified and transported within the cisternae

Cytoplasm

- The entire contents of the cell including cytosol, cytoskeleteon and all organelles except the nucleus
- Site of chemical reactions, enables molecule transport and supports organelles and cell shape

Cytosol

- Fluid part of the cytoplasm
- Mostly water with dissolved ions, nutrients, and proteins
- Suspends organelle and site of chemical reaction

Ribosomes

- Small, freely floating organelles found in all cells that synthesise proteins and can't be seen under a micrcoscope
- Thousands in every cell either on rough ER or in the cytoplasm (free)

Golgi bodies

- Found in all eukaryotic cells
- Consists of a stack of flattened sacs called cisternae that collect and distribute substances
- Vesicles from the ER containing proteins, lipids or steroids fuse with the Golgi body
- It processes and moves those substances through the cisternae where the vesicles pinch off and move within the cell or are transported out of the cell

Mitochondria - DRAW AND LABEL

- Site of cellular respiration which produces ATP
- Has a double membrane with the inner membrane highly folded (into cristae) to provide large SA for production of ATP
- Contains a fluid filled centre called the matrix with the space between the two membranes being the intermembrane
- The more mitochondria a cell has, the more energy it produces

Cellular respiration

The process by which cells break down glucose and other organic molecules using oxygen to produce Adenosine Triphosphate (ATP)

Vesicles

- Small membrane-bound organelles that transport substances around the cell
- Formed from other organelles such as the ER or Golgi body
- Fuse with other organelles, vacuoles or plasma membrane to release contents

Centrosome

- Consists of 2 centrioles made up of microtubules
- Microtubules produce spindle fibres needed for cell division
- Present in all animal cells, usually not in plant cells
- If centrioles are absent or destroyed, cell division (mitosis and meiosis) can still occur but errors are more likely

Spindle

The temporary network of microtubules that forms for cell division to organise chromosomes and separate them to opposite poles of the cell, ensuring proper division

Centriole

One of the two structures made of microfilaments that forms the centrosome

Cytoskeleton

- Helps maintain the shape and internal structure of the cell
- Assists with cellular division and movement
- Consists of microtubules and microfilaments

Microtubles

A component of the cytoskeleton made of tubulin

Microfilaments

A component of the cytoskeleton made of actin

Plasmodesmata

- Channels that pass through cell walls and connect the cytoplasm of adjacent cells
- Allows for intercellular communication, nutrient transport (sugars, water, hormones, proteins, RNA) and coordinated growth and development

Cell wall

- Non-living, rigid component of all plant cells
- Sits outside the plasma membrane
- Contains cellulose in a framework of microtubules
- Provides structural support, maintaining shape of the cell and protecting the cytoplasm
- Contains plasmodesmata (channels) allowing movement of water and mineral salts between neighbouring cells
- Fully permeable to all molecules

Cellulose

Carbohydrate component of the cell wall providing structural support and rigidity to plant cell walls

Chloroplasts - DRAW AND LABEL

- Make energy from sunlight through photosynthesis
- Have a double membrane with stroma and grana
- Thylakoids contain chlorophyll, which absorbs red and blue light and reflects green
- Found mainly in leaf cells, not in roots since no light reaches them

Stroma

The fluid of the chloroplast surrounding the thylakoid membrane

Grana

The stacks of thylakoids embedded in the stroma of a chloroplast

Thylakoid

- Flattened membrane-bound sacs inside chloroplasts that form stacks of grana
- Contain pigment chlorophyll used to absorb sunlight and convert it into chemical energy

Plant vacuole

-Fluid-filled structure surrounded by tonoplast membrane
- Important for storing soluble substances: water, ions, sugars, proteins, pigments, and carbs
- Important for maintaining internal cell pressure
-Size depends on function, water availability and environmental conditions

Chlorophyll

- Green pigment found in the chloroplasts of plants
- Absorbs light energy (mainly red and blue wavelengths) and converts into chemical energy used to make glucose

Photosynthesis

Process by which plants and some organisms use light energy to convert carbon dioxide and water into glucose (sugar) and oxygen

Tonoplast

A membrane that encloses the central vacuole in a plant cell, separating the cytosol from the vacuole contents

Plastids

- Double membrane bound organelles that contain their own DNA
- Complete metabolic processes
- Store substances such as starches and pigments
- Can differentiate into different types of plastids (chloroplasts)
- Can create structures depending on the cell's needs

What molecules can be transported via simple diffusion?

- Must be able to dissolve into hydrophobic membrane
Includes: Small, non-polar, hydrophobic, uncharged
EG O₂, CO₂, Steroid hormones

What molecules can be transported via protein channels?

Req: Too hydrophilic to dissolve through, but small enough to fit through a pore (channel)
Includes: Small, charged OR polar, hydrophilic
EG Ions: Sodium, Potassium

What molecules can be transported via carrier proteins?

Req: Too large or polar for channels, must bind to carrier
Includes: Larger, polar, hydrophilic
EG Glucose, amino acids

What molecules can be transported via osmosis?

Passive: Water unassisted from lower concentration of solute to a higher concentration of solute to form an equilibrium

What molecules can be transported via protein pumps?

Active: Molecules assisted via protein pump against concentration gradient
Includes: Small, charged, hydrophilic, polar
EG Ions

What molecules can be transported via endocytosis?

Active: Molecules assisted via vesicles against concentration gradient to intracellular space
Includes: Macromolecules (large molecules)

What molecules can be transported via exocytosis?

Active: Molecules assisted via vesicles against concentration gradient to extracellular space
Includes: Macromolecules (large molecules)

Concentration gradient (in terms of transport)

The difference in the concentration of a substance across a selectively permeable membrane that drives the net movement of molecules, either along (passive) or against (active) the gradient, until equilibrium is reached

Diffusion

The passive movement of molecules from an area of high concentration to an area of low concentration

Equilibrium

The state where molecules are evenly distributed and there is no net movement of molecules in any particular direction

Passive transport

- Transport through the plasma membrane that requires no energy
- Molecules move along the concentration gradient from high -> low concentration
- 3 types depending on size and chemical properties

3 types of passive transport

Simple diffusion: Particle movement from high to low concentration
Osmosis: Water movement from low to high solute concentration
Facilitated diffusion: Movement of molecules through a membrane protein from high to low concentration

Simple diffusion

The passive movement of a substance across a selectively permeable membrane along the concentration gradient
Affected by:
- Membrane permeability
- Membrane surface area
- Concentration gradient
- Diffusion distance

Osmosis

The passive net movement of water molecules through a selectively permeable membrane from an area of low solute concentration to an area of high solute concentration

Osmotic pressure

Thee amount of force applied to a solution that prevents solvent (water) from moving across a semipermeable membrane
- Stops osmosis when equilibrium is reached

What is a hypotonic solution?

Hypo = HIppo = Big
Solution with solute concentration that is less than that inside the cell, water moves into the cell and enlarges
Plant: Optimal state for plant cells where they become turgid and the plant vacuole swells
Animal: Cell swells and may lyse due to lack of cell wall

What is an isotonic solution?

Solution with equal solute concentration inside and outside the cell, no net movement occurs
Plant: Loss of internal turgor, cell becomes flaccid
Animal: Cell maintains normal shape and volume, optimal

What is a hypertonic solution?

Solution with solute concentration that is greater than inside the cell
Plant: Lose water, plasmolysis occurs where the cytoplasm shrinks and the plasma membrane pulls away from the cell wall
Animal: Lose water, shrink and shrivel

Facilitated diffusion

- Passive movement of a substance through a protein channel or carrier protein along the concentration gradient
- Carrier protein change shape allowing the molecule to pass through the membrane and into the cell

Active transport

- Transport through the plasma membrane that requires energy (ATP)
- Molecules move against their concentration gradient, from low -> high concentration
- 2 types depending on size and chemical properties

3 types of active transport

Protein pumps: Use energy (ATP) to move molecules against the concentration gradient
Endocytosis (Bulk): Cell takes in large particles or liquids by forming internal vesicles
Exocytosis (Bulk): Cell releases substances packaged in vesicles to the extracellular space

ATP

- Adenosine triphosphate
- The main energy source that cells use

Protein pump (active transport)

- The active transport of a substance against the concentration via a protein pump
- The protein pump changes shape and uses energy (ATP) to move high-value molecules into the cell
- includes glucose as the body wants to absorb maximal nutrients as possible

Endocytosis (bulk transport)

Process by which a cell transports large molecules into the cell by engulfing them into a section of the plasma membrane to form a vesicle
- 3 main types/things transported in

Endocytosis (bulk transport): Phagocytosis

The transport of large, solid molecules into the cell such as food or bacteria

Endocytosis (bulk transport): Pinocytosis

The transport of small molecules and liquid substances

Endocytosis (bulk transport) :Receptor-mediated endocytosis

A form of pinocytosis (transport of small molecules and liquid substances) triggered by receptors on the membrane

Exocytosis (bulk transport)

- Enables the transport of macromolecules (large molecules) out of the cell
- Vesicles move to the inner edge of the cell and fuse with the membrane, releasing their contents into the extracellular space EG bloodstream

Protein secretory pathway

- Process by which proteins are made, processed and transported out of the cell
- Proteins are synthesised in the ribosomes of the Rough ER, further processed in the Golgi bodies before being packaged into vesicle and then secreted from the cell via exocytosis at the plasma membrane

Lysed cell

- Cell that has broken down due to membrane damage
- Cell lysis occurs in animal cells in hypotonic solutions
- Osmosis results in the cell swelling and rupturing

Plasmolysis

Occurs when a plant cell is in a hypertonic solution, causing the cytoplasm to shrink and the plasma membrane to pull away from the cell wall
- Cell wall is still intact

Turgid plant cell

- A swollen and firm plant cell in it's optimal state due to water entering via osmosis in a hypotonic solution
- Creates turgor pressure that helps maintain the cell's shape and keeps the plant upright

Flaccid plant cell

- Result of a plant cell in an isotonic solution
- The cell becomes soft due to the lack of water and the membrane is not pressed up against the cell wall but it still maintains its shape

What is binary fission?

Asexual reproduction used by prokaryotes (bacteria) and some unicellular eukaryotes to produce two genetically identical daughter cells from a single parent cell

How does binary fission occur?

1. Chromosomes replicate at the origin of replication 2. Cell elongates, moving DNA to opposite poles
3. Cell wall and plasma membrane begin to constrict
4. Cross-wall forms, DNA copies separated
5. Cells separate into two identical daughter cells

Asexual reproduction

Reproduction involving a single parent that produces genetically identical offspring without the fusion of gametes

Why can bacteria replicate so fast?

Simple: No nucleus, spindle fibres, or centromere
- Replicate chromosomes → divide → 2 daughter cells
Fast replication: doubles every 20 minutes leading to
rapid population growth
- 2n squared to calculate bacteria divisions/time taken for divisions

What are the purposes of the cell cycle?

Repair: Damaged cells
Replace: infected, old or dead cells
Growth: Allow for growth of cells
Reproduction: The production of new organisms