IB Biology SL 2025 Exam - Unit 4 Study Guide

Covering topics from Unit 4 that could be on the 2025 test!

What is the cell theory?

All living organisms are composed of cells

Cells are the basic unit of structure & function

All cells come from pre-existing cells

What are cells?

Fundamental units of life, they are capable of living on their own and make up all living organisms and tissues of the body

Developments in Microscopy

Improved light microscopes in the second half of the 19th century allowed discovery of bacteria and other unicellular organisms

Electron microscopes enabled scientists to discover the complexity of organs and find chromosomes

Common structures in all cells

Plasma Membrane

Cytoplasm

DNA

Prokaryote Cell Structure (mostly small in size)

Has a plasma membrane, sometimes having a cell wall (made of peptidoglycan) to further protect the cell

Inside is only filled with cytoplasm, no nucleus

70S ribosomes

Usually only a single molecule of DNA that forms a loop or circle`

Eukaryote Cell Structure (compartmentalized)

Plasma membrane

Nucleus (holds the cell’s chromosomes), DNA is typically linear

80S Ribosomes

Mitochondria

Processes of life in unicellular organisms

Homeostasis, Metabolism, Nutrition, Excretion, Growth, Response to Stimuli, Reproduction

Differences in eukaryotic cell structure between animals, fungi, and plants

Only plastids exist in plant cells

Only cell walls exist in plant and fungi cells

Small, temporary vacuoles in animal cells, whereas large, permanent vacuole in plant and fungi cells

Centrioles construct spindle to move chromosomes in mitosis in animal cells and 9+2 microtubules in cilia and flagella

Cilia and flagella are present in many animal cells

Atypical cell structure in eukaryotes

Red blood cells - do not have a nucleus

Phloem sieve tube elements - dividing walls between adjacent cells are sieve-like. Nucleus and most other cell contents break down, but plasma membrane remains

Skeletal muscle - some large multinucleate structures are formed when groups of cells fuse together (syncytium)

Aseptate fungal hyphae - nucleus divides repeatedly without any subsequent cell division, resulting in a multinucleate structure

What are organelles?

Discrete structures in cells that are adapted to perform one or more vital functions

Advantage of the separation of the nucleus and cytoplasm into separate compartments

Keeping chromosomes inside the nucleus safeguards the DNA

mRNA can be modified after transcription to prepare it for translation

Advantages of compartmentalization in the cytoplasm of cells

Enzymes and substrates for a particular process can be concentrated more

Substances that could cause damage to the cell can be kept inside the membrane of an organelle

Conditions such as pH can be maintained at an ideal level for particular processes

Organelles with their contents can be moved around

Larger area of membrane available for processes that happen within or across membranes

Totipotent Stem Cells

Typically are early-stage embryos, and are capable of differentiating into any cell type

Pluripotent Stem Cells

Embryonic stem cells that have gradually committed to developing on certain pathways. It is still capable of differentiating into a range of cell types, but not every type

Multipotent Stem Cells

Stem cells that can still differentiate into several types of mature cell

Hematopoietic stem cells are multipotent

Surface area-to-volume ratios and constraints on cell size

If the ratio is too small, substances will not enter the cell as quickly as they are required, waste products will accumulate, and the cell could overheat

Lipid bilayers as the basis of cell membranes

Bilayer of phospholipids and other amphipathic molecules forms a continuous sheet that controls the passage of substances

Lipid bilayers as barriers

Membrane core has a low permeability to all hydrophilic particles

Solutes nearest to membrane surface might penetrate between the hydrophilic phosphate heads, but if they reach the hydrophobic core they will be drawn back to the aqueous solution outside

Larger the molecule size, lower the permeability

Simple Diffusion

Particles passing between phospholipids in the membrane, but can only happen if the bilayer is permeable to the particles

Particles will always move from higher to lower concentrations

Integral Proteins in Membranes

Hydrophobic and are embedded on the hydrocarbon chains in the center of the membrane

Many integral proteins are transmembrane proteins

Peripheral Proteins in Membranes

Hydrophilic and mainly attached to the surface of integral proteins

Osmosis

The net movement of water from a lower solute concentration to a higher solute concentration

Water, despite being hydrophilic, is small enough to pass through the phospholipid bilayer

Aquaporins

Water channels that can greatly increase the membrane permeability to water, allowing for more water to pass through the bilayer

At its narrowest point, the channel is only slightly wider than water molecules, so therefore they must pass through in a single file

Channel Proteins

Integral, transmembrane protein that connects the cytoplasm to the aqueous solution outside the cell

Ions and polar molecules typically pass through these channel proteins, typically down the concentration gradient

Some channels can be opened or closed, allowing for control of permeability of certain substances

Facilitated Diffusion

Channel proteins are required for the movement to occur

No energy is expended by the cell to cause this movement

Active Transport

Process where molecules need energy (ATP) to move across the membrane

Pump Proteins

Integral membrane proteins that only moves particles in one direction, typically against the concentration gradient (low to high)

Have two conformations: one for transporting the particle from one side to a central chamber, and another to bring across an ion to the opposite side

Selectivity in membrane permeability

Allows the passage of particular particles, but not others

Facilitated diffusion and active transport are examples of selective permeability

Glycoproteins

Conjugated proteins with carbohydrate as the non-polypeptide component

Component of plasma membrane of cells, with protein part embedded in the membrane and the carbohydrate part projecting out

Are responsible for cell-to-cell recognition

Glycolipids

Molecules consisting of carbohydrates linked to lipids

Carbohydrate part is usually a single monosaccharide or a short chain between 2-4 sugar units

Lipid part contains one or two hydrocarbon chains to fit in the hydrophobic core of membranes

Help the immune system to distinguish between self and non-self cells

Solvation with water as the solvent

Solvation is the combination of a solvent with the molecules and ions of a solute

Polar solutes dissolve due to attractions of the partially positive and negative charges on the water molecules

When this happens, water molecules form shells around many types of ion and charged molecule

Hypertonic

A solution that has a higher concentration of solutes

Hypotonic

A solution that has a lower concentration of solutes

Isotonic

A solution that is equal in the concentration of solutes on both sides

Water movement by osmosis into or out of cells

Water will always move from the hypotonic solution to the hypertonic solution to try and balance out the concentration gradient

There is always movement of water molecules, but if it’s in a isotonic solution, there is an equal amount of water going in and out

Effects of water movement on cells that lack a cell wall

If the cells that don’t have a cell wall are exposed to hypotonic solutions, the cells will easily burst

If the cells that don’t have a cell wall are exposed to hypertonic solutions, the cells will shrink and form indentations (crenations)

Effects of water movement on cells with a cell wall

Cells with a cell wall that are bathed in hypotonic solutions will expand and become turgid. This is the normal state of plant cells

Cells with a cell wall that are bathed in hypertonic solutions will shrink and become flaccid. If this happens, sometimes the plasma membrane will pull away from the cell wall (plasmolysis)

Medical applications of isotonic solutions

Isotonic sodium chloride solution is used (called normal saline), contains 9g of NaCl per cubic decimeter of solution

It can be safely introduced to a patient’s blood system

Used to rinse wounds and skin abrasions

Used to keep areas of damaged skin moistened prior to skin grafts

Used as the basis for eye drops

Frozen to the consistency of slush for cooling hearts, kidney, and other donor organs

Fluorescent Stains & Immunofluorescence

Fluorescent stains are used for substances to absorb light and re-emit it at a longer wavelength to make cells easier to identify

Immunofluorescence is a development of fluorescent staining. Antibodies that bind to particular chemicals are linked to fluorescent markers of different markers. Then, a multicolored fluorescent image can be produced showing where the chemicals are located

Freeze-Fracture Electron Microscopy

Sample is plunged into liquified propane at -190C to freeze the sample

Steel blade is used to fracture the sample

A vapor of platinum or carbon is fired onto the fracture surface at an angle of 35 degrees to form a coating, creating a replica

The replica can be analyzed through an electron microscope

Cryogenic Electron Microscopy

Thin layer of pure protein solution is applied to the grid and flash-frozen to create smooth vitreous ice

Grid with frozen protein solution is placed in an electron microscope and detectors record the pattern of electrons transmitted by individual protein molecules, producing many different patterns

Computational algorithms will combine patterns to produce a 3D image of the protein molecules

Plasma Membrane

Outer boundary of the cell and encloses all of its contents

Controls entry and exit of substances

Prevents entry of unwanted or toxic substances

Allows the cell to maintain concentrations of substances that are very different from those in the surrounding environment

Lysis

When the plasma membrane of the cell bursts due to excess pressure or by viruses

When this happens, the cell will always die

Autolysis

When the cell carries out the lysis itself

Cytoplasm

Water is main component of cytoplasm and there are many substances dissolved or suspended in the water

Enzymes in cytoplasm catalyze a multitude of chemical reactions (called the metabolism, which provides a cell with energy, produces proteins and other substances to make up the cell structure)

DNA

Contains the information needed for a cell to carry out all its functions

Many genes carry information for making proteins, with some of them used for growth and repair and others acting as enzymes

Nucleus

Has a double membrane with pores in it that holds chromosomes, which consist of one long DNA molecule wound around the outside of proteins (histones)

Plastids

Family of organelles with two outer membranes and internal membrane sacs

Cell Wall

A rigid layer outside the plasma membrane to strengthen and protect the cell

Vacuole

Flexible fluid-filled compartment surrounded by a single membrane

Centrioles

Cylindrical organelles that organize the assembly of structures composed of microtubules

Undulipodia

Cilia and flagella used to generate movement of a cell or movement of fluid adjacent to a cell

Rough Endoplasmic Reticulum

Consists of flattened membrane sacs called cisternae

80S ribosomes attach to the outside of cisternae

Synthesizes protein for secretion from the cell, which passes to the cisternae and moved by vesicles to the golgi apparatus

Smooth Endoplasmic Reticulum

Consists of a branched network of tubular membranes, does not have ribosomes attached

Synthesizes lipids, phospholipids and steroids (one special one stores calcium ions in muscle)

Golgi Apparatus

Has cisternae, but not as long, often curved, don’t have ribosomes and have many vesicles nearby

Processes proteins brought in from vesicles from the rER

Most proteins are then carried in vesicles to the plasma membrane for secretion

Lysosomes

Formed from golgi vesicles and are spherical in shape

Contain high concentrations of protein

Contain digestive enzymes to break down ingested food in vesicles

Can break down organelles or cells

Mitochondria

Double membrane surrounds mitochondria, with inner membrane invaginated to form structures called cristae

Fluid inside is called the matrix

Shape of mitochondria is variable but usually spherical or ovoid

Produce ATP for the cell by aerobic cell respiration (fat can be digested here if it’s being used as an energy source)

Have their own DNA

Ribosomes

Appear as dark granules in the cytoplasm and not surrounded by a membrane, typically 80S ribosomes

Synthesize proteins, releasing it to work in the cytoplasm

Constructed in the nucleolus

Chloroplast

Double membrane surrounds the chloroplast with stacks of thylakoids inside

Shape of chloroplasts are typically spherical or ovoid

Produce glucose and other organic compounds by photosynthesis

Vesicles

Small vacuoles used to transport materials inside the cell

Cytoskeleton

Constructed from several types of protein fiber

Contains microtubules (made of tubulin) and microfilaments (actin)

Shape is dynamic since microtubules and microfilaments can be constructed and taken apart

Organelles with No Membranes

Ribosomes, Centrioles, Microtubules, Proteasomes, Nucleoli

Organelles with One Membrane

Vesicles & Vacuoles, rER, sER, Golgi Apparatus, Lysosomes

Organelles with Two Membranes

Nuclei, Mitochondria, Chloroplasts, Amyloplasts, Chromoplasts

Stem Cell Niche in Skeletal Muscle

These niches remain inactive until there’s injury, in which the changes will cause the stem cells to proliferate and differentiate rapidly

Stem Cell Niche in Bone Marrow & Hair Follicles

Two areas where the microenvironment enables the stem cells to have continuous proliferation and differentiation

Sperm

50 micrometers in length

Narrow shape and small volume allow it to swim to the egg easily

Egg

110 micrometers in diameter and spherical in shape

Allows large quantities of food reserves to be stored in cytoplasm

Red Blood Cells

6 to 8 micrometers but indented on both sides and only about 1 micrometer thick in the middle

Small size and shape allow passage along narrow capillaries and gives a large surface area to load and unload oxygen

White Blood Cells

Only about 10 micrometers in diameter but can enlarge up to 30 micrometers if they are activated and become antibody-secreting plasma cells

Extra volume comes from cytoplasm with rER and Golgi apparatus for protein synthesis

Cerebellar Granule Cells

Cell body is only about 4 micrometers in diameter

Twin axons extend about 3 millimeters in the cerebellar cortex

Small volume allows cerebellum to accommodate 50 billion of them

Motor Neurons

Cell body is about 20 micrometers in diameter

Large size allows enough proteins to be synthesized to maintain the immensely long axon

Can extend to a meter or more to allow it to carry signals from the central nervous system to a distant muscle

Striated Muscle Fibers

Diameter from 20 to 100 micrometers and lengths that exceed 100 mm

Dimensions allow the fiber to exert greater force and contract by a greater length

Magnification Triangle

Actual Size * Magnification = Image Size

Cell Size Chart

Differentiation

Process during development whereby newly formed cells become more specialized and distinct from one another as they mature

Morphogens

The process of early embryo differentiation is driven by the release of specialized signaling molecules
They play a critical role in the formation of different layers of cells (ectoderm, mesoderm, endoderm) in the embryo

Fluid Mosaic Model

Active Transport Visual