Cell Structure and Function

2.1

Organelles and the Endomembrane System

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 4 — Systems Interactions Biological systems interact, and these systems and their interactions exhibit complex properties.	2.1.A Explain how the structure and function of subcellular components and organelles contribute to the function of cells.	2.1.A.1 Ribosomes are comprised of ribosomal RNA (rRNA) and protein. These non-membrane, subcellular structures are found in cells in all forms of life and reflect the common ancestry in all known life. Ribosomes synthesize proteins according to messenger RNA (mRNA) sequences. 2.1.A.2 The endomembrane system consists of a group of membrane-bound organelles and subcellular components (endoplasmic reticulum (ER), Golgi complex, lysosomes, vacuoles and transport vesicles, the nuclear envelope, and the plasma membrane) that work together to modify, package, and transport polysaccharides, lipids, and proteins intercellularly. 2.1.A.3 Endoplasmic reticulum provides mechanical support by helping cells maintain shape and plays a role in intracellular transport. i. Rough ER is associated with membrane-bound ribosomes, allows for the compartmentalization of cells, and helps carry out protein synthesis. ii. Smooth ER functions include the detoxification of cells and lipid synthesis. Exclusion statement: Knowledge of the specific functions of smooth ER in specialized cells is beyond the scope of the AP Exam. 2.1.A.4 The Golgi complex is a membrane-bound structure that consists of a series of flattened membrane sacs. Functions of the Golgi include: i. Correctly folding and chemically modifying newly synthesized cellular products ii. Packaging proteins for trafficking Exclusion statement: Knowledge of the role of Golgi in the synthesis of specific phospholipids and packaging of specific enzymes for lysosomes, peroxisomes, and secretory vesicles is beyond the scope of the AP Exam. Illustrative examples for 2.1.A.4: Glycosylation and other chemical modifications of proteins that take place within the Golgi and determine protein function or targeting 2.1.A.5 Mitochondria have a double membrane that provides compartments for different metabolic reactions involved in aerobic cellular respiration. The outer membrane is smooth, while the inner membrane is highly convoluted, forming folds that enable ATP to be synthesized more efficiently. 2.1.A.6 Lysosomes are membrane-enclosed sacs that contain hydrolytic enzymes that digest material. Lysosomes also play a role in programmed cell death (apoptosis). 2.1.A.7 Vacuoles are membrane-bound sacs that play many different roles. i. In plant cells, a specialized large vacuole maintains turgor pressure through nutrient and water storage. ii. In animal cells, vacuoles are smaller in size, are more plentiful than in plant cells, and store cellular materials. 2.1.A.8 Chloroplasts are specialized organelles that are found in plants and photosynthetic algae. Chloroplasts contain a double membrane and serve as the location for photosynthesis.

Source: College Board AP Course and Exam Description

A eukaryotic cell divides its work among organelles 细胞器. The endomembrane system 内膜系统 is a connected set that makes, modifies, and ships molecules: the nucleus 细胞核 (holds DNA), rough and smooth endoplasmic reticulum 内质网 (protein and lipid synthesis), the Golgi 高尔基体 (modifies and packages), lysosomes 溶酶体 (digestion), and vesicles that carry material between them. Mitochondria 线粒体 (respiration) and chloroplasts 叶绿体 (photosynthesis) have their own membranes.

A plant cell also has a cell wall, a large vacuole, and chloroplasts

A generalised animal cell and its organelles

Vocabulary Train

English	Chinese	Pinyin
organelles	细胞器	xì bāo qì
endomembrane system	内膜系统	nèi mó xì tǒng
nucleus	细胞核	xì bāo hé
endoplasmic reticulum	内质网	nèi zhì wǎng
Golgi	高尔基体	gāo ěr jī tǐ
lysosomes	溶酶体	róng méi tǐ
Mitochondria	线粒体	xiàn lì tǐ
chloroplasts	叶绿体	yè lǜ tǐ

Exercise sheet

2.2

Why Cells Stay Small

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	2.2.A Explain the effect of surface area-to-volume ratios on the exchange of materials between cells or organisms and the environment.	2.2.A.1 Surface area-to-volume ratios affect the ability of a biological system to obtain necessary nutrients, eliminate waste products, acquire or dissipate thermal energy, and otherwise exchange chemicals and energy with the environment. Relevant equations: Volume of a Sphere: $V = \dfrac{4}{3}\pi r^3$ Volume of a Cube: $V = s^3$ Volume of a Rectangular Solid: $V = lwh$ Volume of a Cylinder: $V = \pi r^2 h$ Surface Area of a Sphere: $SA = 4\pi r^2$ Surface Area of a Cube: $SA = 6s^2$ Surface Area of a Rectangular Solid: $SA = 2lh + 2lw + 2wh$ Surface Area of a Cylinder: $SA = 2\pi rh + 2\pi r^2$ $r$ = radius $l$ = length $h$ = height $w$ = width $s$ = length of one side of a cube Illustrative examples for 2.2.A.1: SA/V Ratios and Exchanges Root hairs Guard cells Gut epithelial cells Cilia Stomata 2.2.A.2 The surface area of the plasma membrane must be large enough to adequately exchange materials. i. The surface area-to-volume ratio can restrict cell size and shape. Smaller cells typically have a higher surface area-to-volume ratio as well as a more efficient exchange of materials with the environment than do larger cells. ii. As cells increase in volume, the surface area-to-volume ratio decreases and the demand for internal resources increases. iii. More complex cellular structures (e.g., membrane folds) are necessary to adequately exchange materials with the environment. iv. As organisms increase in size, their surface area-to-volume ratio decreases, affecting properties like rate of heat exchange with the environment. Smaller amounts of mass exchange proportionally more heat with the ambient environment than do larger masses. As mass increases, both the surface area-to-volume ratio and the rate of heat exchange decrease. v. There is a relationship between metabolic rate per unit body mass and the size of multicellular organisms; typically, the smaller the organism, the higher the metabolic rate per unit body mass.

Source: College Board AP Course and Exam Description

Cells stay small because of the surface-area-to-volume ratio 表面积与体积比. As a cell grows, its volume rises faster than its surface area, so its membrane cannot exchange materials fast enough for the interior. Staying small (or being flat or folded) keeps enough surface to serve the volume.

The surface-area-to-volume ratio falls as a cell gets bigger

Worked example. A cube-shaped cell $2\ \mu\text{m}$ on a side has surface area $6\times2^2=24\ \mu\text{m}^2$ and volume $2^3=8\ \mu\text{m}^3$, so its surface-area-to-volume ratio is $\tfrac{24}{8}=3$. Double the side to $4\ \mu\text{m}$: surface area $=6\times4^2=96$, volume $=4^3=64$, ratio $=\tfrac{96}{64}=1.5$. Doubling the size halved the ratio – the larger cell has far less membrane per unit of interior, which is why cells stay small.

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See surface area vs volume as a cube grows

As a cell grows, volume rises faster than surface area, so the surface-area-to-volume ratio falls. A small cell keeps enough membrane to exchange materials fast enough.

Vocabulary Train

English	Chinese	Pinyin
surface-area-to-volume ratio	表面积与体积比	biǎo miàn jī yǔ tǐ jī bǐ

Exercise sheet

2.3

The Fluid Mosaic Model of the Membrane

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	2.3.A Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.	2.3.A.1 Phospholipids have both hydrophilic and hydrophobic regions. The polar hydrophilic phosphate regions of the phospholipids are oriented toward the aqueous external or internal environment, while the nonpolar hydrophobic fatty acid regions face each other within the interior of the membrane. 2.3.A.2 Embedded proteins can be hydrophilic (with charged and polar side groups), hydrophobic (with nonpolar side groups), or both. i. Hydrophilic regions of the proteins are either inside the interior of the protein or exposed to the cytosol (cytoplasm). ii. Hydrophobic regions of proteins make up the protein surface that interacts with the fatty acids in the interior membrane.
	2.3.B Describe the fluid mosaic model of cell membranes.	2.3.B.1 Plasma membranes consist of a structural framework of phospholipid molecules embedded with proteins, steroids (such as cholesterol in vertebrate animals), glycoproteins, and glycolipids. All of these can move around the surface of the cell within the membrane, as illustrated by the fluid mosaic model.

Source: College Board AP Course and Exam Description

The cell membrane is a phospholipid bilayer 磷脂双分子层 – polar heads facing the water, nonpolar tails inside – studded with proteins. The fluid mosaic model 流动镶嵌模型 pictures it as a fluid sheet in which lipids and proteins drift. Cholesterol and the degree of unsaturation tune its fluidity.

The fluid mosaic model of the cell membrane

Vocabulary Train

English	Chinese	Pinyin
phospholipid bilayer	磷脂双分子层	lín zhī shuāng fèn zǐ céng
fluid mosaic model	流动镶嵌模型	liú dòng xiāng qiàn mó xíng

Exercise sheet

2.4

What Can Cross the Membrane

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	2.4.A Explain how the structure of biological membranes influences selective permeability.	2.4.A.1 Plasma membranes separate the internal environment of the cell from the external environment. Selective permeability is the result of the plasma membrane having a hydrophobic interior. 2.4.A.2 Small nonpolar molecules, including $\mathrm{N_2}$, $\mathrm{O_2}$, and $\mathrm{CO_2}$, freely pass across the membrane. Hydrophilic substances, such as large polar molecules and ions, move across the membrane through embedded channels and transport proteins. 2.4.A.3 The nonpolar hydrocarbon tails of phospholipids prevent the movement of ions and polar molecules across the membrane. Small polar, uncharged molecules, like $\mathrm{H_2O}$ or $\mathrm{NH_3}$ (ammonia), pass through the membrane in small amounts.
	2.4.B Describe the role of the cell wall in maintaining cell structure and function.	2.4.B.1 Cell walls of Bacteria, Archaea, Fungi, and plants provide a structural boundary as well as a permeability barrier for some substances to the internal or external cellular environments and protection from osmotic lysis.

Source: College Board AP Course and Exam Description

The membrane is selectively permeable 选择透过性. Small nonpolar molecules ($\text{O}_2$, $\text{CO}_2$) and water cross easily; large or charged/polar particles (ions, glucose) cannot pass the nonpolar core without help. This selectivity lets the cell control its internal environment.

Three ways substances cross the membrane

Vocabulary Train

English	Chinese	Pinyin
selectively permeable	选择透过性	xuǎn zé tòu guò xìng

Exercise sheet

2.5

Passive and Active Transport

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	2.5.A Describe the mechanisms that organisms use to maintain solute and water balance.	2.5.A.1 The selective permeability of membranes allows for the formation of concentration gradients of solutes across the membrane. 2.5.A.2 Passive transport is the net movement of molecules from regions of high concentration to regions of low concentration without the direct input of metabolic energy. 2.5.A.3 Active transport requires the direct input of energy to move molecules. In some cases, active transport is utilized to move molecules from regions of low concentration to regions of high concentration.
	2.5.B Describe the mechanisms that organisms use to transport large molecules across the plasma membrane.	2.5.B.1 The processes of endocytosis and exocytosis require energy to move large substances or large amounts of substances into and out of cells. i. In endocytosis, the cell takes in large molecules and particulate matter by folding the plasma membrane in on itself and forming new (small) vesicles that engulf material from the external environment. ii. In exocytosis, internal vesicles release material from cells by fusing with the plasma membrane and secreting large molecules from the cell.

Source: College Board AP Course and Exam Description

Passive transport 被动运输 moves substances down their concentration gradient (high → low) with no energy – diffusion 扩散.
Active transport 主动运输 moves substances against the gradient (low → high), requiring energy (ATP), via protein pumps like the sodium–potassium pump.

Active transport moves particles against the gradient, using ATP and a carrier protein

Explore

Pump a solute against its gradient

Passive transport moves solutes down their gradient for free; active transport uses ATP to pump them the other way, from low to high concentration.

Vocabulary Train

English	Chinese	Pinyin
Passive transport	被动运输	bèi dòng yùn shū
diffusion	扩散	kuò sàn
Active transport	主动运输	zhǔ dòng yùn shū

Exercise sheet

2.6

Facilitated Diffusion

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	2.6.A Explain how the structure of a molecule affects its ability to pass through the plasma membrane.	2.6.A.1 Facilitated diffusion requires transport or channel proteins to enable the movement of charged ions across the membrane. i. Membranes may become polarized by the movement of ions across the membrane. ii. Charged ions, including $\mathrm{Na^+}$ (sodium) and $\mathrm{K^+}$ (potassium), require channel proteins to move through the membrane. 2.6.A.2 Facilitated diffusion enables the movement of large polar molecules through membranes with no energy input. In this type of diffusion, substances move down the concentration gradient. 2.6.A.3 Aquaporins transport large quantities of water across membranes.

Source: College Board AP Course and Exam Description

Facilitated diffusion 易化扩散 is passive transport through a membrane protein – a channel or carrier 载体 – for particles that cannot cross the lipid alone. It still goes down the gradient and needs no energy, but its rate can saturate when all the proteins are busy.

Diffusion: particles spread from higher to lower concentration, down the gradient

Vocabulary Train

English	Chinese	Pinyin
Facilitated diffusion	易化扩散	yì huà kuò sàn
carrier	载体	zài tǐ

Exercise sheet

2.7

Tonicity, Water Potential, and Osmoregulation

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	2.7.A Explain how concentration gradients affect the movement of molecules across membranes.	2.7.A.1 External environments can be hypotonic, hypertonic, or isotonic to internal environments of cells. Movement of water can also be described as moving from hypotonic to hypertonic regions. Water moves by osmosis from regions of high water potential to regions of low water potential. Equation (Water Potential): $\psi = \psi_p + \psi_s$ where: $\psi_p$ = pressure potential $\psi_s$ = solute potential Illustrative examples for 2.7.A.1: Contractile vacuole in protists Central vacuole in plant cells
	2.7.B Explain how osmoregulatory mechanisms contribute to the health and survival of organisms.	2.7.B.1 Growth and homeostasis are maintained by the constant movement of molecules across membranes. 2.7.B.2 Osmoregulation maintains water balance and allows organisms to control their internal solute composition and water potential. Water moves from regions of low osmolarity or solute concentration to regions of high osmolarity or solute concentration. Equation (Solute Potential of a Solution): $\psi_s = -iCRT$ where: $i$ = ionization constant $C$ = molar concentration $R$ = pressure constant $\left(R = 0.0831 \dfrac{L \cdot bars}{mol \cdot K}\right)$ $T$ = temperature in Kelvin (°C + 273)

Source: College Board AP Course and Exam Description

Osmosis 渗透 is the diffusion of water across a membrane. Tonicity 张力 compares solute concentrations: in a hypotonic 低渗 solution a cell gains water (may burst); in a hypertonic 高渗 one it loses water (shrinks); in an isotonic 等渗 one there is no net change. Water potential 水势 predicts the direction of water movement (water moves to lower water potential), and is the sum of a pressure term and a solute term: $\Psi=\Psi_p+\Psi_s$, where the solute potential $\Psi_s=-iCRT$. Osmoregulation 渗透调节 is how organisms control this balance.

How plant and animal cells respond to solutions of different water potential

Worked example. For a $0.1\,\text{M}$ sucrose solution ($i=1$) in an open beaker (so pressure potential $\Psi_p=0$) at $25\,°\text{C}$ ($T=298\,\text{K}$, $R=0.0831\ \text{L}\cdot\text{bar/mol}\cdot\text{K}$): $\Psi_s=-iCRT=-(1)(0.1)(0.0831)(298)\approx-2.48\ \text{bar}$, so $\Psi\approx-2.48\ \text{bar}$. A plant cell whose interior is $\Psi=-1.0\ \text{bar}$ sits in this solution: since the solution is more negative, water moves out of the cell into the solution, and the cell loses turgor.

Explore

Watch water move by osmosis

Water moves across the membrane from high water potential to low, toward the more concentrated (hypertonic) side. Set the concentrations and watch which way the cell swells or shrinks.

Vocabulary Train

English	Chinese	Pinyin
Osmosis	渗透	shèn tòu
Tonicity	张力	zhāng lì
hypotonic	低渗	dī shèn
hypertonic	高渗	gāo shèn
isotonic	等渗	děng shèn
Water potential	水势	shuǐ shì
Osmoregulation	渗透调节	shèn tòu tiáo jié

Exercise sheet

2.8

Mechanisms of Membrane Transport

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	2.8.A Describe the processes that allow ions and other molecules to move across membranes.	2.8.A.1 Metabolic energy (such as that from ATP) is required for active transport of molecules across the membrane and to establish and maintain electrochemical gradients. i. Membrane proteins are necessary for active transport. ii. The $\mathrm{Na^+}/\mathrm{K^+}$ pump and ATPase contribute to the maintenance of the membrane potential.

Source: College Board AP Course and Exam Description

Large materials move in bulk by vesicles: endocytosis 内吞 brings material in (the membrane engulfs it), and exocytosis 外排 sends material out (a vesicle fuses with the membrane). Both require energy and let cells import and secrete large molecules.

Endocytosis brings material in; exocytosis releases it out

Vocabulary Train

English	Chinese	Pinyin
endocytosis	内吞	nèi tūn
exocytosis	外排	wài pái

Exercise sheet

2.9

Compartmentalization Inside the Cell

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	2.9.A Describe the membrane-bound structures of the eukaryotic cell.	2.9.A.1 Membranes and membrane-bound organelles in eukaryotic cells compartmentalize intracellular metabolic processes and specific enzymatic reactions.
	2.9.B Explain how internal membranes and membrane-bound organelles contribute to compartmentalization of eukaryotic cell functions.	2.9.B.1 Internal membranes facilitate cellular processes by minimizing competing interactions and by increasing the surface area where reactions can occur.

Source: College Board AP Course and Exam Description

Membranes create separate compartments 区室 so incompatible reactions can run at once and conditions (pH, ion levels) can be tuned locally. This organization boosts efficiency – the internal membranes also add surface area for reactions.

Vocabulary Train

English	Chinese	Pinyin
compartments	区室	qū shì

Exercise sheet

2.10

The Origins of Cell Compartmentalization

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 1 — Evolution The process of evolution drives the diversity and unity of life.	2.10.A Describe similarities and/or differences in compartmentalization between prokaryotic and eukaryotic cells.	2.10.A.1 Membrane-bound organelles such as mitochondria and chloroplasts evolved from once free-living prokaryotic cells via endosymbiosis. 2.10.A.2 Prokaryotes typically lack internal membrane-bound organelles but have internal regions with specialized structures and functions. 2.10.A.3 Eukaryotic cells maintain internal membranes that partition the cell into specialized regions.

Source: College Board AP Course and Exam Description

The endosymbiotic theory 内共生学说 explains mitochondria and chloroplasts: they arose when a larger cell engulfed free-living prokaryotes that then lived inside it. The evidence – their own circular DNA, their own ribosomes, and double membranes – supports this shared evolutionary origin.

Vocabulary Train

English	Chinese	Pinyin
endosymbiotic theory	内共生学说	nèi gòng shēng xué shuō

Exercise sheet

2.10

Exam tips

Use the two AP-formula skills: surface-area-to-volume ratio (why cells stay small) and water potential $\Psi=\Psi_p+\Psi_s$ ($\Psi_s=-iCRT$).
Water moves toward the lower (more negative) water potential; predict swelling or shrinking from tonicity (hypo/hyper/isotonic).
Passive transport and osmosis need no energy (down the gradient); active transport needs ATP (against the gradient).
Distinguish diffusion, facilitated diffusion (a channel/carrier protein), and active transport.
Explain membrane selectivity from the phospholipid bilayer — small non-polar molecules cross, large/charged ones need help.

Organelles and the Endomembrane System

Why Cells Stay Small

See surface area vs volume as a cube grows

The Fluid Mosaic Model of the Membrane

What Can Cross the Membrane

Passive and Active Transport

Pump a solute against its gradient

Facilitated Diffusion

Tonicity, Water Potential, and Osmoregulation

Watch water move by osmosis

Mechanisms of Membrane Transport

Compartmentalization Inside the Cell

The Origins of Cell Compartmentalization

Exam tips

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